CHARACTERIZATION OF POWDER AND THE EFFECTS OF POWDER REUSE IN SELECIVE LASER MELTING
by
BRADLEY K. BARNHART
Submitted in partial fulfillment of the requirements for the degree of
Master of Science
Materials Science and Engineering
CASE WESTERN RESERVE UNIVERISTY
August 2017
CASE WESTERN RESERVE UNIVERISTY
SCHOOL OF GRADUATE STUDIES
We hereby approve the thesis of
Bradley Kyle Barnhart
candidate for the degree of Master of Science.
Approved by:
Committee Chair: James McGuffin-Cawley School of Material Science and Engineering Case Western Reserve University
Committee Member: Frank Ernst School of Material Science and Engineering Case Western Reserve University
Committee Member: Badri Narayanan School of Material Science and Engineering Case Western Reserve University
Date of Defense: June 29, 2017
Table of Contents
LIST OF TABLES ...... v
LIST OF FIGURES ...... vi
ACKNOWLEDGEMENTS ...... ix
DISCLAIMER ...... x
LIST OF ABBREVIATIONS ...... xi
ABSTRACT ...... 1
Chapter 1: OVERVIEW ...... 2
1.1 Thesis Organization...... 2
1.2 Additive Manufacturing and Powder Recycling ...... 2
1.3 Powder Bed Fusion ...... 5
Chapter 2: Literature Review ...... 7
2.1 Prior Powder Recycling Research ...... 7
2.1.1 Polymer Powder Reuse Studies ...... 7
2.1.2 Metal Powder Reuse Studies ...... 10
2.2 Powder Production ...... 15
2.2.1 Polymeric Powder ...... 15
2.2.2 Metallic Powder ...... 15
2.2.3 Initial Surface Oxidation of Gas Atomized Stainless Steels ...... 20
2.3 Powder Characterization ...... 22
2.3.1 Sampling Powder for Analysis ...... 22
2.3.2 Methods for quantifying particle characteristics...... 23
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2.3.3 Particle Size Distribution ...... 26
2.4 Effects of SLM Thermal Gradients ...... 27
Chapter 3: Materials and Methods ...... 32
3.1 Materials ...... 32
3.1.1 Nylon 12...... 32
3.1.2 316L Stainless Steel ...... 33
3.1.3 17-4 PH Stainless Steel ...... 35
3.2 Examining Built 316L Stainless Steel Cubes ...... 36
3.3 Differential Scanning Calorimetry ...... 37
3.4 Bulk Powder Sampling...... 38
3.5 SEM Image Analysis ...... 39
3.5.1 Stainless Steels ...... 41
3.5.2 Nylon 12...... 43
3.6 Laboratory Emulation of SLM ...... 43
3.6.1 Laser Source...... 46
3.6.2 Atmospheric Control ...... 46
3.6.3 Powder Bed ...... 48
3.7 Particle Chemistry Characterization...... 51
3.7.1 X-Ray Energy Dispersive Spectroscopy ...... 53
3.7.2 Auger Electron Spectroscopy ...... 54
3.7.3 X-Ray Photoelectron Spectroscopy ...... 55
3.8 Particle Microstructure Characterization ...... 57
3.8.1 Electron Backscatter Diffraction...... 58
3.8.2 EBSD Sample Preparation ...... 59
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Chapter 4: Results ...... 60
4.1 Overview ...... 60
4.2 Pre-built 316L Stainless Steel Cube Inspection ...... 60
4.2.1 Density and Part Anisotropy ...... 60
4.2.2 Partially Melted Particles ...... 61
4.3 Nylon 12 DSC Results ...... 62
4.4 Particle Size Distribution and Shape Analysis ...... 64
4.4.1 Nylon...... 64
4.4.2 316L Stainless Steel ...... 66
4.4.3 17-4 PH Stainless Steel ...... 68
4.5 Particle Microstructure Characterization ...... 72
4.5.1 316L Stainless Steel ...... 72
4.5.2 17-4 PH Stainless Steel ...... 74
4.6 Particle Surface Morphology and Chemistry ...... 76
4.6.1 316L Stainless Steel ...... 77
4.6.2 17-4 PH Stainless Steel ...... 83
4.7 SLM Emulation Results ...... 87
Chapter 5: Discussion ...... 90
5.1 Overview ...... 90
5.2 316L Pre-Built Cubes ...... 90
5.3 Thermal Cycling Effects on Nylon 12 Process Window ...... 91
5.4 Reuse Effects on Particle Size Distribution and Particle Shape...... 92
5.5 Modeling Observations from PSD Changes ...... 98
5.6 SLM Thermal Effects on Particle Microstructure ...... 104
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5.7 Reuse Effects on Particle Surface Chemistry ...... 108
5.7.1 316L Stainless Steel ...... 108
5.7.2 17-4 PH Stainless Steel ...... 111
5.8 SLM Emulation Discussion ...... 113
Chapter 6: Conclusions and Future Work ...... 115
APPENDIX A ...... 118
APPENDIX B ...... 121
APPENDIX D ...... 128
APPENDIX D-2...... 130
APPENDIX E ...... 132
APPENDIX F...... 134
APPENDIX G ...... 135
BIBLIOGRAPHY ...... 136
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LIST OF TABLES
Table 4.1 Thermal transition temperatures from DSC analysis for Nylon 12 ...... 63
Table 4.2 Average CED and standard deviation for four different Nylon 12 powder samples...... 65
Table 4.3 Average CED and standard deviation for five different 316L stainless steel powder samples...... 67
Table 4.4 Average CED and standard deviation for five different 17-4 PH stainless steel powder samples...... 70
Table 4.5 Composition (wt%) of 316L virgin powder comparing surface oxide formations and base material...... 79
Table 4.6 Composition (wt%) of 17-4 PH stainless steel virgin powder...... 84
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LIST OF FIGURES
Figure 1.1: Schematic of the selective laser sintering (SLS) process [12]...... 6 Figure 2.1 Example DSC-Thermogram with the desired "process window" giving the SLM operating temperature [15]...... 8 Figure 2.2: Experimental results showing the effect of number of reuses on relative tensile strength [18–20, 22]...... 10 Figure 2.3: Schematic showing the secondary atomization of an individual particle with a Weber number passed the critical value of ~ 13. The particle starts as one large particle and creates several smaller particles [42]...... 17 Figure 2.4: Evidence of dendritic crystals: (a) cross-section of protrusion on particle, (b) protrusion on particle, and (c) particle surface [42]...... 18 Figure 2.5: High resolution FEG-SEM image of inert-gas-atomized 316L particle in the NMF showing a particular surface morphology and presence of oxide particles [57]. .... 21 Figure 2.6: Schematic illustrating the likely phase arrangement in the surface oxide layer of thickness E with outermost layer made of Fe2O3 and MnO and inner layer of Cr2O3 [50]...... 22 Figure 2.7: Schematic of a generic spinning riffler [60]...... 23 Figure 2.8: Method of projected area for particle size determination via microscopy. SEM micrographs of several images are converted to binary and the projected area is measured using the number of black pixels in the image [41]...... 24 Figure 2.9: Example of a number-frequency histogram [60] ...... 27 Figure 2.10: (a) Modeled temperature profile at the top surface of a 316L powder bed with the melting temperature isotherm lined in black. [71] (b) The Gaussian distribution of the laser irradiance intensity with a clear maximum at the center of the laser spot [69]...... 29 Figure 2.11: Schematic representation of heat transfer modes typical in powder bed fusion [69]...... 29 Figure 3.1 316L stainless steel cubes produced via SLM with varying process parameters ...... 35 Figure 3.2 The circle equivalent diameters are acquired from SEM image analysis are (a) plotted in a histogram and (b) the distribution is fit with a lognormal curve to represent the data...... 41 Figure 3.3: Example of making the scanning electron microscope image of 316L stainless steel into binary in ImageJ for analysis...... 42 Figure 3.4 SLM emulation box design...... 45 Figure 3.5: Experimental setup to control laser sintering parameters ...... 45 Figure 3.6: Comparison of powder bed heat profiles with (left) just the ceramic hot plate and (right) the addition of a copper plate obtained by a FLIR imaging camera...... 50 Figure 3.7: Hotplate configured to preheat and spread powder before sintering...... 50 Figure 3.8: A three dimensional surface profiling of 316L powder spread on the substrate surface using a 30 μm film applicator producing a 150 μm layer...... 51 Figure 3.9 Illustration of the process to sieve out the larger size fraction of the used powder...... 53 Figure 3.10 Schematic showing location and source of signal for different microscopy techniques. Adapted from Ref. [80]...... 57
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Figure 3.11 Cut-corner of a cube showing the location and presence of the partially melted particles...... 58 Figure 4.1 Relative density vs measured side lengths or pre-built 316L stainless steel cubes...... 61 Figure 4.2 Evidence of partially melted particles in a SEM image of the surface of a built cube (left) and an optical image of the etched cross-section (right)...... 62 Figure 4.3 Heat flow vs temperature plot output from DSC analysis for virgin Nylon 12 powder...... 63 Figure 4.4 Examples of Nylon 12 SEM micrographs used for determining PSD. From left to right is virgin, cake bake, and a 50/50 mixture of the two...... 65 Figure 4.5 Lognormal distribution fit of four different Nylon 12 samples...... 65 Figure 4.6 SEM images of 316L stainless steel samples from 1 use sieved (left) and 4 use retained (right)...... 66 Figure 4.7 Lognormal distribution fit of five different 316L stainless steel powder samples...... 67 Figure 4.8 Average circularity measurements for 316L stainless steel powder...... 67 Figure 4.9 SEM images of 17-4 PH stainless steel samples from single use sieved (left) and single use retained (right)...... 69 Figure 4.10 Lognormal distribution fit of four different 17-4 PH stainless steel powder samples...... 69 Figure 4.11 Mechanically disturbing 17-4 PH stainless steel powder which was retained after the sieving process after one use. The bond between joined particles is strong enough for agglomerate reorientation to occur with fracture of the shared surface...... 71 Figure 4.12 EBSD map of crystallographic orientation of the normals of (left) 316L stainless steel particles fused to the outside of a SLM built cube and (right) as-received virgin 316L stainless steel...... 73 Figure 4.13 SE images of virgin powder (a) and the large sieved powder (c). Corresponding BSE images of the virgin (b) and large sieved (d) cross sections for 316L stainless steel...... 73 Figure 4.14 (a) SE images of virgin powder, (b) BSE image of FIBed virgin powder, (a) SE images of virgin powder, (b) BSE image of FIBed virgin powder, (a) SE images of virgin powder, (b) BSE image of FIBed virgin powder. Magnifications vary...... 75 Figure 4.15 Higher magnification BSE images of particle to particle contact for the large sieved (a) and retained (b) 17-4 PH stainless steel...... 76 Figure 4.16 Example SEM images of 316L powder showing oxide formations and differences in surface roughness...... 77 Figure 4.17 Locations of localized XEDS spectra comparing the oxide formations to the base material of virgin 316L stainless steel. Used powder showed the same formations. 79 Figure 4.18 XEDS elemental mapping of the surface of a used 316L stainless steel powder, also shown in Figure 4.16 (left), which shows uniform distribution of elements except for silicon and oxygen...... 80 Figure 4.19 Electron image depicting the locations of the AES spectra of the 316L particle surface...... 81 Figure 4.20 316L stainless steel AES spectra overlay with corresponding compositions...... 82 Figure 4.21 XPS spectrum (500eV-900eV) overlay for 316L stainless steel...... 83
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Figure 4.22 XPS spectrum (500eV-900eV) overlay for 17-4 PH stainless steel...... 85 Figure 4.23 XPS Depth profile of Oxygen in 17-4 PH stainless steel...... 86 Figure 4.24 XEDS mapping of the external oxide formation on a non-spherical spatter particle of 17-4 PH stainless steel...... 86 Figure 4.25 XEDS mapping of the external oxide formation on a spherical spatter particle of 17-4 PH stainless steel...... 87 Figure 4.26 Optical images of powder fused in air (a) and argon (b). Powder from the gear fused in air at low magnification (c) and high magnification (d)...... 89 Figure 4.27 XEDS map of the oxidized surface of 316L stainless steel fused in air...... 89 Figure 5.1 Nylon 12 processing window trend as a function of reuse ...... 92 Figure 5.2 Example of an agglomerate made up of fused 17-4 stainless steel particles. . 93 Figure 5.3 Volume fraction of fine, average, and coarse particles in different 316L stainless steel samples...... 96 Figure 5.4 As measured average circularity for 316L stainless steel powder samples .... 96 Figure 5.5 Measured CED vs Circularity of virgin and reuse stainless steel powder as measured by SCM Metals (left) and via image analysis (right). The blue line is for comparison between samples...... 97 Figure 5.6 Histogram of random CED values (1) which are generated according a predetermined lognormal fit (2). Upper and lower particle bounds (3) and the powder layer thickness (4)...... 99 Figure 5.7 Adding the volumes of several individual particles to create a single new particle to represent the fusion process observed in the retained powder samples...... 100 Figure 5.8 Outputs of numerical model determine the effects of both preferential deposition of finer particles on the powder bed and the fusion of agglomerates on particle size distribution and shape...... 103
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ACKNOWLEDGEMENTS
I would first like to acknowledge my adviser, Jim McGuffin-Cawley, for all the
support and guidance that was provided through both this project and several others
throughout my time at CWRU. I would like to thank the engineers at CWRU’s SCSAM who were extremely accommodating when providing help with surface analysis methods and sample preparation. To my future boss, Badri Narayanan, for persuading me to continue my education in the field of Materials Science and Engineering which has been a great learning experience. I lastly want to thank my close friends, family, and significant other of 5 years for their endless support during the continuation of my higher education and during the production of this thesis.
This effort was performed through the National Center for Defense
Manufacturing and Machining under the America Makes Program entitled “Economic
Production of Next Generation Orthopedic Materials through Powder Reuse in Additive
Manufacturing (Project 4049)” and is based on research sponsored by Air Force Research
Laboratory under agreement number FA8650-12-2-7230. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon.
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DISCLAIMER
The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the Government. Distribution authorized to U.S. Government
Agencies and America Makes Members; Critical Technology. Other request for this document shall be referred to AFRL/RXMS, Wright-Patterson Air Force Base, OH
45433-7750.
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LIST OF ABBREVIATIONS
AES – Auger Electron Spectroscopy AM – Additive Manufacturing BSE – Back Scattered Electron CED – Circle Equivalent Diameter DED – Directed Energy Deposition DSC – Differential Scanning Calorimetry EBSD – Electron Backscatter Diffraction FIB – Focused Ion Beam PBF – Powder Bed Fusion PSD – Particle Size Distribution SE – Secondary Electron SEM – Scanning Electron Microscope SLM – Selective Laser Melting SLS – Selective Laser Sintering UND – University of Notre Dame XEDS – Energy-dispersive X-ray Spectroscopy XPS – X-ray Photoelectron Spectroscopy
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Characterization of Powder and the Effects of Powder Reuse in Selective Laser Melting
ABSTRACT
By
BRADLEY KYLE BARNHART
The ability to reuse feedstock in additive manufacturing powder bed processes is vital for cost control and widespread adoption of the technology. This requires an understanding of the effects of thermal cycling in the ambient conditions an additive machine has on the physical, chemical, and microstructural properties of the powder. Specifically, the work reported here centers on characterization of the changes that occur due to thermal cycling with 316L stainless steel, 17-4 PH stainless steel, and Nylon 12 powders. in a custom rig or an industrial selective laser melting (SLM) machine. All powders used in industrial machines exhibited shifts towards larger particle sizes as they were used. Of other properties measured, Nylon 12 showed no change. The surface chemistry of the used stainless steel powder is consistent with that of gas atomized powder with only the spatter having evidence of increased oxidation. The spatter was also used to determine the events leading particle to agglomeration and these observations were numerically modeled to represent the measured particle size distributions.
1
Chapter 1: OVERVIEW
1.1 Thesis Organization
The work reported in this thesis was completed for and funded by America
Makes, and is a subset of a larger project. The project title is the “Economic Production of Next Generation Orthopedic Materials through Powder Reuse in AM”. It was completed in conjunction with the University of Notre Dame (UND), SCM Metal
Products Inc., Zimmer Inc., and DePuy Synthes. Notre Dame was the project lead and was responsible for studying the mechanical response of AM-produced specimens from powder with qualitatively distinct histories of use, as well as constructing and validating a rig to replicate the thermal cycling powder experiences associated with bed preheating in typical SLS machines. The industrial partners provided additional mechanical testing and powder characterization in addition to access to industry SLS equipment. This thesis is based on Case Western Reserve University’s responsibility for characterizing the chemical and microstructural properties of virgin and recycled powder with an emphasis on the metallic materials.
1.2 Additive Manufacturing and Powder Recycling
Additive manufacturing (AM) has been defined by the ASTM F2792-12a
Standard [1] as “a process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies.” The standard also describes seven types of additive processes which include binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and vat polymerization.
2
The majority of early adopters of AM technologies were mainly interested in prototyping or only manufacturing a few piece parts at a time. This made material costs a relatively small factor when building a part additively because the cost of the machine is the largest fraction of the overall cost per part at small scales [2]. This trend of only using
AM for prototyping, however, is changing, as shown by manufacturing companies with larger throughputs investing heavily in AM technologies [3]. However, even with AM technologies now being increasingly used in mainstream manufacturing, it has become clear that AM is still often cost-prohibitive. A 2016 survey shows that AM is most competitive with traditional production in small to medium sized batches and is not currently suitable for large scale production [4].
Although the technology may not be ready for mass production now, the technology adoption by large companies with capital ready to advance the field bodes well for AM. This technology adoption has resulted in a growth in the 3D printing market with metal printing being the fastest-growing segment in 2016 with printer sales growing at 48% and material sales growing at 32% [5].
The two dominate process families for metal AM are directed energy deposition
(DED) and powder bed fusion (PBF). Several review papers have been written discussing the differences, benefits, and standard operations of each additive method [6–9]. The material feedstock for PBF is, unsurprisingly, powder and DED has utilized both powder and wire form for the feedstock. Both use a laser (e.g. CO2 or NdYAG) as the source of energy with the major difference between the two processes being that DED delivers the material to be joined at the focal point of the laser whereas PBF has the material already in place on the powder bed. A more detailed explanation of the PBF process is given in
3 section 1.3. Some significant advantages of PBF over DED from a manufacturing perspective include the ability to create higher complexity parts, better surface finishes requiring less post-machining, and the capability to build several parts at once so that the build chamber is fully utilized.
The initial users of AM in a manufacturing setting such as the aerospace and biomedical industries have favored quality over quantity due to their products having an exceptionally low tolerance of risk. Since the powder bed fusion processes are commonly used to create these typically complex parts, one way to provide consistency from build to build is to start with unused, virgin powder at the start of every build. However, a typical build can fuse as little as 5% of the powder in a build chamber leaving the remaining unfused powder to be discarded. Although material costs are the ultimate driving factor in determining the cost of an AM product for mass production [2] and recycling the unfused powder directly back into would save money, it is not commonly done to reduce risk. This results from uncertainty about the mechanisms and kinetics of changed powder. The remedy is adding depth to the research on the topic of powder recycling as a whole in the AM community, a topic that is touched on in section 2.1.
The magnitude of the reduction in cost per built-part via SLM associated with recycling material not incorporated into a part from previous builds is large [10], and needed since AM materials can cost 20 to 100 times more than the bulk materials used in conventional manufacturing [11]. In addition, the recycling approach allows for the recovery of significant embodied energy that is currently lost with discarding unused material.
4
1.3 Powder Bed Fusion
Powder bed AM processes offer the most potential for benefit associated with the reduction in wasted material. This is a result of the large volume of material initially forming the powder bed and that is ultimately not incorporated into the final product.
There are several variations of PBF processes that are similar overall, but which have unique process parameters/strategies that vary modestly – often associated with the manner in which the powder bed is built. Herderick reviews the different methods which include electron beam melting (EBM), laser cusing, direct metal laser sintering (DMLS), selective laser sintering (SLS), and selective laser melting (SLM) [6]. The materials utilized in PDF processes can include semi-crystalline polymers and metals of various types but will also vary depending on which exact process is used. From an additive perspective, SLS has become synonymous with SLM (selective laser melting) because the process is nearly identical with the only difference being that the material fully melts in SLM. It should be noted that the process will be defined as SLM throughout this work because the materials utilized are commonly fully melted.
The “selective” part of SLM is a result of selectively joining individual powder together using either a binder or a laser energy source. A computer aided design (CAD) model gives a well-defined fraction of the surface of the bed that is selectively illuminated with the laser. Beneath the beam the particles are homogeneously fused by solid state sintering or melting.
The SLM process as shown in Figure 1.1 consists of a laser source (CO2 for polymers or Nd-YAG, and increasingly Yb-fiber, for metals), scanning system, and the build chamber. The build chamber includes the powder supply, build platform, and a
5 powder-spreading mechanism. A common method of building a powder bed consists of loose powder being raised by the powder delivery piston and subsequent spreading into uniform layers (which can have thicknesses ranging from 20μm to 100μm) across the build platform, but exact powder delivery methods will vary between different machine manufacturers. The scanner system then directs the laser to fuse the material in pattern defined by the corresponding CAD slice. The build platform is then lowered down by the desired layer thickness. This step by step process repeats until the part is completely formed according to the CAD model, after which the part is removed from the loose, unfused powder that was surrounding the built part and also acting as a support structure during the build. Commonly controlled processing parameters are build layer thickness, laser power and beam diameter, and scan speed; all which ultimately control the volumetric energy input into the material.
Figure 1.1: Schematic of the selective laser sintering (SLS) process [12].
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Chapter 2: Literature Review
2.1 Prior Powder Recycling Research
The powder materials that have been studied as to how their reuse affects
subsequent builds has been focused on those with applications in the aerospace,
biomedical, and automotive markets. Specifically, Ti-6Al-4V has been a material of
interest because of its ability to lightweight as well as the biocompatibility [9]. Several
polymeric studies have also been conducted. Polymers are sometimes a model material
with relatively low cost when compared to metal powder, but are also of direct
importance in biomedical applications. The effect of reuse is typically measured
indirectly via physical properties of parts produced from powder with different thermal
histories.
2.1.1 Polymer Powder Reuse Studies
The polymer SLM process is different than that of metal SLM in a few different
ways, the largest difference being that the powder bed is kept above the glass transition
temperature and 2-4°C below the melting temperature of the material. The reason for this
being that after the laser (CO2) is used to provide enough energy to move past the melting point, the high ambient temperature will prevent the material from recrystallizing and causing shrinkage [13]. Several studies and material reviews have used differential scanning calorimetry (DSC), which outputs charts like that shown in Figure 2.1, to define the processing window for a given material [13–16]. Polyamide 12 (PA12, Nylon
12) and other semi-crystalline materials exhibit such a behavior. Schmid et al. [15] used operating temperatures near the upper and lower bounds of the process window to determine the effects on built parts. If the process temperature is too low, premature
7 crystallization occurs which causes curling and the parts are distorted once released from the powder bed. On the other extreme, if the process temperature is too high and the material has a chance to stay molten, there is a loss of surface definition, the possibility to melt powder away from the part creating a “hard cake” [13], and the “part growth” phenomenon occurs. Part growth occurs when neighboring particles adjacent molten material begin to laterally fuse to the outside of the build, decreasing dimensional resolution. It is therefore desired to have a wide process window where a metastable two- phase area exists and a larger variation in process temperature is acceptable.
Figure 2.1 Example DSC-Thermogram with the desired "process window" giving the SLM operating temperature [15].
The importance of temperature control is not only important for the processing window, but for mechanical properties as well. Work by Rennie [17] and Bourell [13] has also shown that minor fluctuations in temperature, as measured by IR cameras, can have 8
negative effects on both porosity and mechanical strength of the built parts as a function
of location in the powder bed. This is a result of operating only 2-4°C below the melting temperature of the material and a variation of only 2°C can either cause excessive or incomplete fusion between particles and/or layers.
For polyamide 12 (PA 12 and Nylon 12 are the same material), some of the
earliest work on the effects of powder reuse in selective laser sintering was completed by
Zarringhalam et al [18]. It was found that strength, ductility and elastic modulus all
increased with the reused powder, but a single reuse was studied. They also determined
that employing a blend of new powder with used powder produced lower values for
mechanical properties more than using a bed of either entirely new or entirely reused
powder. Conversely, Wudy et al. found that the ultimate tensile strength decreased
appreciably after the first reuse [19]. This drop in properties in the built parts was
attributed to the degradation of the polyamide powder and the reduced reflow associated
with increased molecular weight. Sewell, et al found that the strength reduction in a
similar Nylon was not seen until the third reuse, and that the ultimate strength dropped
22% after 11 reuses [20]. In addition to bulk reuse studies, Dotchev et al. presented
results that focused on local powder properties in relation to the build platform. Powder
which was exposed to higher temperatures for longer periods of time deteriorated more
quickly (e.g. the powder at the periphery and top layers were affected less than powder at
the bottom and center of the platform) [21]. These studies demonstrate a large variability
that is presented in the current literature.
A comparison of some of these experimental results is represented Figure 2.2
[18–20, 22]. Note the wide divergence of results between the researchers, especially for
9 the nylons. The difference is not surprising because different materials are being used but more importantly, the process parameters are not kept constant with respect to one another.
Figure 2.2: Experimental results showing the effect of number of reuses on relative tensile strength [18–20, 22].
2.1.2 Metal Powder Reuse Studies
The metallic powder studied most widely in powder reuse studies is Ti-6Al-4V due to the heavy use of the material in the aerospace and biomedical industries. Ti-6Al-
4V, and more recently Inconel [23], have been a popular materials to study because it is utilized heavily in the ARCAM electron beam melting process [6, 7, 24, 25]. EBM utilizes a vacuum environment to prevent highly reactive metals like titanium from oxidizing and operates at temperatures up to 750°C when preheating and can also experience prolonged thermal holding around 550°C [22]. The prolonged elevated temperatures will increase the probability of change in regards to the entire powder bed but the powder which is closer to the parts being built will experience even more severe conditions due to melting and solidification (release of latent heat) of the neighboring particles.
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Tang et al. investigated the effects of reuse on chemistry, particle shape and size,
as well as strength as a function of reuse in an e-beam melting process that occurs under vacuum [22]. The results showed increased oxygen content from around 0.08% (by weight) for virgin powder to around 0.21% after 22 reuses. Aluminum and vanadium concentration in the powder decreased, while not significantly in terms of expected composition, and the tensile strength was shown to increase over 22 reuses. The particle size distribution was measured using a laser diffraction analyzer (Malvern, MS2000) and the results showed a narrowed distribution and the mean size only changed by ~1 μm.
The particle morphology was only qualitatively examined over a small sampling of powder particles and they observed becoming less spherical and rougher with increasing reuse times and this was related to the reduction in the amount of satellites, tiny particles
(<5 μm) that are electrostatically connected to larger particles, which they proposed was due to gas flow carrying them away from the powder bed.
Ardila et al. published results from their study of the reuse of a nickel superalloy
(IN718), and found that it could be reused 14 times in selective laser melting without affecting metallurgical properties, but the only mechanical test they performed was a
Charpy V-notch test to obtain fracture toughness [23].
In 2012, a dissertation was written on a more in depth chemical analysis of reused
Ti-6Al-4V powder used in an e-beam melting process [26]. X-ray photoelectron spectroscopy (XPS) characterization was used to study the surface chemistry of virgin powders, some recycled for a single time, and others that had been repeatedly recycled.
Evidence was found of TiO2, Al2O3, and V2O5 on the powder particle surfaces with the recycled powder having been enriched with vanadium, oxygen and nitrogen in addition to
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thicker surface oxides. The increase in vanadium concentration contradicts the results
found by Tang.
A 2015 study by O’Leary presented the effects of Ti-6Al-4V powder reuse in a
SLM process which fused the powder into mechanical test pieces [27]. Virgin powder from the manufacturer was cycled 5 times inside a build chamber, sampling from the bulk powder after each sieving step. Repeated recycling showed that the mean particle diameter size increased after five reuses, but it had no effect on the mechanical properties
of the built parts. This increase in the particle size distribution is counter to results
reported by Tang [22]. His chemical analysis showed that the percentage of oxygen
contained in the SLM parts built with the reused powder was outside of the allowable
range for the specific grade of Ti-6Al-4V [27].
Another 2015 study by Sun et al. was focused on the change in powder
morphology as a function of reuse [28]. They quantitatively measured the shape, or
roundness, of individual Ti-6Al-4V particles from SEM micrographs following ASTM
F1877 which gives standard practices for the characterization of particles. High
magnification micrographs were used to quantify the area, perimeter, and roundness of
each particle (via ASTM F1877 [29]) and the roundness of the particles had a negligible
decrease with reuse. A change in average shape was determined via the Hall Flow Meter
test which suggested that the particles became less spherical after 30 reuses which
reduces the flow rate. The authors did not report the number of particles examined nor
produce error bars when comparing virgin to used powder.
Unlike Ti-6Al-4V and other highly reactive materials used in EBM systems,
stainless steel is relatively inert. The SLM process in which it is commonly used has
12 preheat temperatures of only 80°C [30] which is significantly lower than that of the Ti-
6Al-4V and which suggests that the likelihood of changing any material properties of the powder bed due to preheat temperatures will be lower, in fact unlikely. This and the lower relative price compared to titanium, which would correlate with the lack of urgency to determine recyclability, could explain the relative absence of prior literature on the effects of reusing stainless steels in powder bed processes.
Slotwinski et al. studied the PSD and chemistry of 17-4PH stainless steel as a function of reuse over eight uses [31]. Recycling of the 17-4PH led to an increase in the
PSD mean with progressive builds and there were no significant differences in chemical concentrations between the virgin and recycled powder. To determine the changes of powder close to the build the authors examined the sieve residue (i.e., the material that was too large to make it through the sieve). The sieve residue was made up of mostly fused or agglomerated particles and it was suggested that the high temperatures near the built part were responsible for the solid state sintering that was examined in the SEM.
The chemistry of the sieve residue was measured using XPS and the iron and silicon concentrations were larger than the virgin powder while the copper, chromium, and carbon decreased. The iron was also non-metallic which correlated with X-ray diffraction results showing a phase change, probably an oxide formation, at the near surface. During the particle size distribution (PSD) analysis, it was also examined that the powder bed
PSD pre- and post-sieving were different. This difference was suggested to be the result of the spreader arm preferentially transporting larger particles (those larger than the powder bed thickness but smaller than sieve diameter) past the build plate and into an excess bin. The larger particles would then not be allowed to be a part of the build and
13
would only rejoin the remainder of the powder population after the sieving process. That
is the distribution is reconstituted to near its original state but with a smaller fraction of
finer particles as these are preferentially incorporated in the built-part.
Although not designed as a reuse study, Simonelli et al. [32] and Liu et al. [33] examined 316L stainless steel spatter which is a byproduct of the SLM process. Spatter is created when the temperature in the melt pool is higher than the evaporation point of the metal. This results in the metal transitioning from a liquid metal phase to metallic vapor occurs and the sudden increase in volume generates a large recoil pressure on the molten pool. This recoil pressure creates a jet of molten metal from the melt pool which, while in flight, has the opportunity to break up into individual spheres. Both Simonelli et al. and
Liu et al. characterized spatter as any particle which did not make it through the final sieving step, typically a sieve size of 200 (75 μm). This is an arbitrary choice.
The spatter examined by Liu was mostly spherical with several particles adhering to the surface, a consequence of the spatter still being at high temperatures when impinging upon the powder bed. Energy dispersive x-ray spectroscopy (XEDS) analysis showed the spatter with larger oxygen contents than either the fused material and the virgin material. Simonelli [32] looked at the chemistry of virgin and spatter powder cross sections created with a focused ion beam (FIB). XEDS spectra of the virgin 316L stainless steel showed irregular grains with the alloying elements in full solid solution while the spatter particle had equiaxed grains with evidence of grain boundary segregation of Cr, Mo, and Mn. The reason for the difference is suggested to be the difference in cooling rates between gas atomization and the molten metal ejection with
14
gas atomization being substantially quicker. XEDS analysis also revealed that surface oxides made up of Mn and Si were present.
2.2 Powder Production
2.2.1 Polymeric Powder
Polymers powders have been produced via cryogenic milling [34], solid-state shear pulverization [35], immiscible blends [36], atomization [37], and precipitation methods [38]. Cryogenic milling and solid-state pulverization yield thin particles with large aspect ratios and are typically inadequate in SLS due to poor flowability characteristics. Poor flow can lead to low density parts with insufficient mechanical properties. Immiscible blends of PA12/Polyvinyl alcohol and PA12/Polyethylene glycol can produce spherical particles suitable for SLS applications, but these processes have low product yields [36]. Atomization is used for a wide variety of polymers but
Polyamide 12, a commonly used material in SLS, is not suitable for this method due to its inability to completely dissolve into a solvent. The precipitation method, since its patent in 1999, has become a widely used technique in PA12 powder production because of the ability to create rounder, more fluid particles with tighter distributions when compared to the milling processes [38]. This process creates potato-shaped particles similar to the powder examined in Figure 4.4 and, because the process history of the Nylon 12 is unknown, the powder used in this work is assumed to be precipitated.
2.2.2 Metallic Powder
The creation of metallic powders for AM typically starts with a melt of the desired composition being “atomized” or broken up into individual droplets because of the surface tension of the metal. Atomization, specifically gas atomization, is used
15 because the process can achieve nearly spherical particles with homogenous chemistry and relatively tight size distributions when compared to other atomization techniques
[39], [40]. Two other standard atomization techniques are water and plasma atomization.
They are not often used for AM because water atomization creates irregular particle shapes which would decrease flowability. Plasma atomization, although it makes highly spherical powder with no satellites, has a high cost of production and relatively low productions rates [41].
Yule and Dunkley wrote a detailed summary of the atomization process in
Atomization of Melts for Powder Production and Spray Deposition which is given here for convenience [42]. Gas atomization begins with the gravitational flow of the melt through an orifice after which it is impinged by either one or two streams of gas at predetermined rates. The liquid column is broken up and the individual droplets fall under gravity in an inert environment while being convectively cooled. The flow of inert gas is crucial for controlling both the cooling rates and powder chemistry.
The three stages of atomization include primary atomization, secondary atomization, and spheroidization. The primary atomization for metals occurs at Reynolds numbers at less than ~1000 [42], and is comparable to the commonly observed breakup of a thin stream of water column from a faucet. When the column of liquid reaches a critical Reynolds number value it becomes unstable and begins to break up into individual round droplets as a result of the surface tension. Secondary atomization, or droplets experiencing further breakup, happens when the gas Weber number is above a critical value of ~ 13. The Weber number is similar to the Reynolds number except it determines the critical value for droplet instability instead of column instability. When
16
the liquid metal droplets become dynamically unstable due to aerodynamic and surface
tension forces the particle will begin to distort [43]. Figure 2.3 shows the secondary
atomization process as depicted by Yule and Dunkley [42]. Metallic materials typically
do not reach their critical Weber number due to the high surface tension (~2 J/m2 for
metals compared to 0.07 J/m2 for water [44]), however, the presence of high velocity gas regions in the atomization process can temporarily raise the Weber number long enough to cause deformation or breakup. The spheroidization step is the result of surface tension
on individual particles reducing the overall surface energy and can occur either before or
after secondary atomization but must occur before solidification for spherical particles.
Figure 2.3: Schematic showing the secondary atomization of an individual particle with a Weber number passed the critical value of ~ 13. The particle starts as one large particle and creates several smaller particles [42].
Atomized particle solidification is assumed to be dominated by convective cooling and the time required for the temperature of the entire particle to fall below the solidus line will vary on the mean diameter. Gas atomization using N2 or Ar and creating
powder particles 5-30 μm in size could have cooling rates ranging from 103 – 105 k s-1
[42, 45]. Bauckhage et al. modeled the amount of time it would take for particles of
varying size to completely solidify when taking into account the droplet diameter,
relative gas-particle velocity, density and specific heats of both the solid and gas, gas
viscosity and thermal conductivity, latent heat of fusion, and temperature. It was found
that doubling the particle size from 20 μm to 40 μm would increase the time for
17
solidification by about five times longer (1.5 ms to 7.5 ms) [46]. This result could vary
slightly since different gas atomization process parameters can easily affect the cooling
rates.
Similar to basic solidification phenomenon, the solidification of atomized
particles begins in the form of dendritic crystals. Lavernia et al. determined that the
secondary arm spacing was a good representation of the homogeneity of the structure as
well as a tool for measuring the solidification rates based off the assumption that the
faster the cooling rates, the finer and more homogenous the dendrites will be; however,
there has been additional work showing that these methods may not yield accurate results
[47–49]. If the dendritic features are large enough resulting from slower cooling (i.e. particle was large), they will be easily viewed on the surface of the particle and will resemble those in Figure 2.4.
Figure 2.4: Evidence of dendritic crystals: (a) cross-section of protrusion on particle, (b) protrusion on particle, and (c) particle surface [42].
18
The smaller particles with the rapid cooling rates will have surfaces that look
comparatively smooth at SEM resolution with minimal surface features. This may be due
to the secondary dendrite arms being sufficiently fine or it may also be a result of massive
solidification where liquid droplets with large undercooling solidify volumetrically [50,
51]. When the fine stainless steel droplets, which should be austenitic by composition, reach a critical amount of undercooling, it causes the liquid to freeze in a single
crystalline δ-ferrite phase (BCC) as opposed to the cellular γ-austenite phase (FCC) of the larger particles with smaller undercooling. This phenomenon is explained by nucleation theory which derives the phase with the lowest activation energy to form a nucleus of critical size will nucleate first, whether it is a stable or metastable phases. The change is independent of kinetics and depends solely on the magnitude of the thermodynamic driving force. [45, 52].
These metastable BCC particles are typically single grained and lack microsegregation [51]. This yields two types of particles, i.e., both a magnetic and non- magnetic fraction, after atomization that is correlated to particle size. The mean diameter cutoff for the BCC to FCC transition is heavily dependent on chemistry and the Cr/Ni ratio (higher Cr values make it more susceptible to being BCC) with some studies showing the cutoff ranging from < 5 μm up to < 20 μm for varying stainless steels [50,
51, 53]. Since additive manufacturing powder suppliers such as EOS remove particles ≤ 5
μm and >75 μm and the powder used in AM has a mean diameter of ~30-50 μm, it is
expected to encounter particles exhibiting both single and polycrystalline, and magnetic
and nonmagnetic. The solidification mode has a significant effect on both the surface
morphology and oxide formation during gas atomization.
19
2.2.3 Initial Surface Oxidation of Gas Atomized Stainless Steels
Although the powder is formed and solidified quickly by impingement of nominally inert gas, the atmosphere is not oxygen free in most cases (titanium and other highly reactive materials have unique atomization processes [54, 55]) and the surface will be prone to oxidation. Individual particles regardless of size will have a relatively
homogenous internal chemistry due to the rapid cooling rates during atomization which
results in limited to no macrosegregation of the alloying elements [42, 56].
Microsegregation is expected to occur because of the visible dendritic surface formations
similar to those in Figure 2.4. The characteristic surfaces of individual particles have
been shown to vary, however, in regards to what type of oxide is present. Some of these
oxides were explored by Simonelli [32] which has already been discussed in section
2.1.2.
Prior work by Hedberg et al. [57] examined 316L stainless steel surface oxides on
atomized powder with X-Ray Photoelectron Spectroscopy (XPS), Auger Electron
Spectroscopy (AES), and Transmission Electron Microscopy (TEM). They measured the
oxide thickness of 2-5 nm on a 45 μm particle using both AES with ion etching and XPS.
The AES was also used to locally identify the surface particles shown in Figure 2.5.
They found the oxide nanoparticles to be enriched with Mn and a similar amounts of Fe
while the surrounding surface oxide layer contained mostly Fe and O, which they
suggested to be Fe2O3.
20
Figure 2.5: High resolution FEG-SEM image of inert-gas-atomized 316L particle in the NMF showing a particular surface morphology and presence of oxide particles [57].
Additional XPS work by Hedburg [58] has shown that the “metal release” rates of
316L in different pH solutions will yield Mn being preferentially released before Fe, Ni,
and Cr with the latter two being minimal suggesting that there is a layered oxide. It was
suggested since the oxygen affinity of Mn is high and it is known to diffuse to the surface
at high temperatures that it is a result of the atomization process. The XPS results after
the solution treatment also suggest that the Mn layer is just above a Cr enriched layer which agrees with literature findings of comparable austenitic stainless steels [50]. The
Mn enrichment on the surface layer was also found to increase as the particle size
decreased in both cases.
Early work on 303 and 304L stainless steels found that if the concentration of Mn
is large and the concentration of Si is low in a comparable austenitic stainless steel, a film
of liquid manganese oxide instantly covers the surface of the liquid particle and is highly
permeable to oxygen [51, 59]. This occurrence can be linked to the relatively low melting
temperature (1246°C) and density of Mn (7. 3 g/cm3). The oxygen, once diffused through
21 the outer layer, then reacts with the Cr to form an inner oxide. The iron oxide found at the surface of the particle is believed to form in the solid state by diffusion through cracks in the Cr2O3 layer. This phase arrangement was depicted by Bracconi in Figure 2.6 [50].
Figure 2.6: Schematic illustrating the likely phase arrangement in the surface oxide layer of thickness E with outermost layer made of Fe2O3 and MnO and inner layer of Cr2O3 [50].
2.3 Powder Characterization
2.3.1 Sampling Powder for Analysis
The “golden rules” of powder sampling are that a powder should be sampled when in motion and the whole stream should be sampled from short increments in preference to one part for the whole time. This compensates for the fact that powder segregates when in motion, due to either flow or vibration, and the larger particles move against gravity and appear on the surface [60]. The Metal Powder Industries Federation
(MPIF) standard 01 describes the simplest method of obtaining a representative sample by moving a container intermittently through a flowing stream of powder [61]. This is the premise of equipment such as a spinning riffler which removes samples from a stream of flowing powder originating from a hopper and moved via a vibrating feeder as depicted in Figure 2.7. It is collected by a series of rotating vials which act as intermittent collection bins. This is sufficient for lab scale powder samples (few hundred grams) but
22 larger piles may be sampled with a powder thief which can take powder from different depths simultaneously.
Figure 2.7: Schematic of a generic spinning riffler [60].
2.3.2 Methods for quantifying particle characteristics
ASTM F3049-14 gives methods of obtaining a quantitative value for the distribution of powder size [62]. It can either be done with mechanical sieves, automated equipment such as a laser diffraction particle size analyzer, automated image analysis
(e.g. Malvern Morpologi G3SE), or semi-manually using images obtained optically or with an SEM. The latter two methods allow for both particle size measurements as well as other size metrics and were used in this study due to the convenience of having an
SEM on-site in addition to a research partner having the automated image analysis equipment. The standard recommends not using the sieving method if particle sizes less than 45 μm or involved [62]. Comparisons made between the sieving, light scattering, and microscopic methods should be made tentatively because the results will vary [42].
23
Both the automated and manual image analysis methods follow the procedure of
creating a projection of the particle normal to the viewing direction. This projected area A
is then converted to an equivalent circle diameter:
= (4 / ) / 1) 1 2 𝐸𝐸𝐸𝐸 where represents the diameter of𝐷𝐷 a circle 𝐴𝐴with𝜋𝜋 the same cross-sectional area as the
𝐸𝐸𝑞𝑞 projected𝐷𝐷 particle. An example of the projected area of a particle is shown in Figure 2.8.
Figure 2.8: Method of projected area for particle size determination via microscopy. SEM micrographs of several images are converted to binary and the projected area is measured using the number of black pixels in the image [41].
Additional measurements that can be useful for complete characterization of particle size and shape include various other diameter equivalents, aspect ratio, and
sphericity. A few examples of additional diameter equivalents include perimeter
diameter, sieve diameter, and Feret’s diameter, but projected diameter gave the best
estimate of the true cross-sectional area of the particles [60, 63]. All of these
measurements are equivalents and do not measure the “true size” of one given particle but of a particular property. The most effective way of determining the impact of the measured particle size and shape and its effect on the AM process, however, is by measuring the flowability of the powder using a Hall flow meter [42] and this method is standard within the AM industry [62]. This measurement was completed because a project partner, Zimmer Biomet, did that experiment.
24
Sphericity for microscope analysis is defined as
= 2) 𝑜𝑜 𝐿𝐿 𝑆𝑆 𝛹𝛹 𝑝𝑝 where So is the surface area of a sphere with a diameter𝑆𝑆 equal to the equivalent diameter and Sp is the surface area of the particle computed from the measured surface area. The equation for circularity, C, is then the two-dimensional equivalent of sphericity and is calculated by:
4 A C = 3) P 𝜋𝜋 2 where A is the projected area of the spherical object and P is the perimeter of the same projection. For a perfect circle this value is unity and it begins to decrease as the projected area becomes further from spherical [60, 64, 65]. The minimum value of zero defines an acicular or needle-like shape (it equivalently is zero for a flake shape – any finite value for sphericity has at least two shapes). ASTM F1877 [29] has also adopted this description of particle shape as a quantitative description of the particles shape.
It is important to note some current limitations with measuring particle sizes from images, specifically SEM in this case, include the following:
(1) The image is a projection of the particles oriented preferentially with the long
side lying flat. This can make high aspect ratio particles seem larger than they
are. This can be seen if the particle in Figure 2.8. If it was to be stood on end
the two satellites increasing the total area could be no longer viewed.
(2) Due to the nature of gas atomized particles, it can often difficult to
automatically or even manually determine the partition line between
overlapping but discrete particles versus hard agglomerates. Furthermore, if
25
satellites or other fine particles are resting on the top surface of a larger
particle they will not be observed.
(3) Due to the statistical variance in the powder distribution, a large quantity
(typically many thousands in automated systems) of particles needs to be
measured to minimize error and validate results. Unfortunately, obtaining
such a sample size with an SEM is tedious, costly, and, fundamentally,
technically challenging due to fine particles dusting and coarse particles
rolling off of a pile to be sample. These reasons can limit the total number of
particles analyzed.
2.3.3 Particle Size Distribution
Yule and Dunkley show that the particle size distribution (PSD) is a commonly used method in characterizing a powder, specifically atomized, and can that it can affect powder bed properties such as density, packing, and rheology [42]. The data obtained from any given particle size measurement technique can be used to plot the number or volume fraction of mean diameters as a function of frequency. This data is typically presented as a histogram like Figure 2.9 where rectangles over a specified range or “bin” where the area is proportional to the number of particles in that interval. Depending on preference, the binning can either be set values (e.g. 1, 5, 10 μm) or with varied sizes with rectangles whose area is proportional to the number of particles in the intervals.
26
Figure 2.9: Example of a number-frequency histogram [60]
If a sufficient number of particles are measured and the variance in the data is reduced, a smooth line can be fit to the data, which is the probability density function
(PDF). The Gaussian or “normal” distribution is described by:
( ) = ( ) 4) 1 − 2 2 2 where the most probable value P𝑃𝑃 occurs𝑦𝑦 𝛼𝛼 at𝜋𝜋 y=0𝑒𝑒𝑒𝑒𝑒𝑒 and− the𝛼𝛼 𝑦𝑦chance of finding a given value higher or lower than the mean value is equally likely. Allen presents several additional distributions which have been shown to describe powder populations but particles on the scale of those used in PBF are typically well described by the log-normal distribution
[42].
2.4 Effects of SLM Thermal Gradients
The basic premise of selective laser sintering is adding enough energy via a laser source to melt a population of single particles together to form a solid mass. It is common in powder bed fusion technologies to calculate the amount of energy E input into the system by
27
= 5) 𝑃𝑃 𝐸𝐸 where E is the volumetric energy input, P is the𝑣𝑣ℎ 𝑑𝑑laser power, v is the scan speed, h is the hatch spacing or distance between fused lines of powder, and d is the layer thickness
[66]. Spears and Gold reviewed many papers on how these independent process parameters affect the energy input and subsequently the melting behavior of the powder
[67]. Most of the reviewed studies looked at changes in either laser and scanning parameters or powder bed properties and recoat parameters but there is still a lack of industry standard. In addition, many authors have only been able to report qualitative correlations and quantifying the differences will allow better statistical process control over the AM process.
The laser source profile often follows the symmetrical Gaussian model for distribution of irradiance (power per unit area) with the center at maximum [68]–[70] as shown in Figure 2.10(b). The large heat flux via irradiation from the laser produces a volumetric heat source that creates very high and localized temperature gradients [70],
[71]. The temperature profile modeled by Gusarov in Figure 2.10(a) shows the melting temperature of 316L (~1700K) as a black trace and the peak temperature is in excess of
3400K at the center of the melt pool while approaching the edge results in a significant drop-off in temperature. Hodge verified Gusarov’s work by doing similar volumetric heat source modeling [72]. The drop-off in temperature is due to a change in the thermal conductivity of the system as it transitions from the melt pool to loose-particles with limited surface area contact.
28
Figure 2.10: (a) Modeled temperature profile at the top surface of a 316L powder bed with the melting temperature isotherm lined in black. [71] (b) The Gaussian distribution of the laser irradiance intensity with a clear maximum at the center of the laser spot [69].
Figure 2.11: Schematic representation of heat transfer modes typical in powder bed fusion [69].
29
The energy that is absorbed from the heat source is utilized in either melting of
the powder or transfers through the powder bed. Figure 2.11 shows a schematic of a
SLM powder bed and includes the different modes of heat transfer that are present.
Approximately 10-15% of the initial laser power input is transferred via convection and thermal radiation at the surface layer of the powder bed [70]. The remainder of the absorbed energy is used to create the melt pool and is conductively transferred through the powder bed. Masoomi estimated the thermal conductivity of a powder bed, kpb, to be
= (1 ) & = 6) 𝑠𝑠 𝑝𝑝 𝑝𝑝𝑝𝑝 𝑠𝑠 �𝜌𝜌 − 𝜌𝜌 � 𝑘𝑘 𝑘𝑘 − 𝛷𝛷 𝛷𝛷 𝑠𝑠 where ks is the thermal conductivity material in a bulk form𝜌𝜌, Φ is the porosity of the
powder bed, is the density of the solid material, and is the density of the powder
𝑠𝑠 𝑝𝑝 bed. With an 𝜌𝜌estimated powder bed packing of 60%, the𝜌𝜌 value for thermal conductivity of
17-4 PH Stainless Steel used in the modeling was 12 W/(m K) [70]. This value is close to
that of bulk stainless steel which is not close to the true value for powder beds.
However, other studies [33, 71] use the work completed by Rombouts [73] which
argues that the effective thermal conductivity of a loosely packed metallic powder bed
(containing 5-85 μm sized powder) is essentially independent of material but is heavily
influenced by the size and morphology of the powder, the void fraction, as well as the
thermal conductivity of the surrounding gas. At room temperature the effective thermal
conductivity in air of a powder bed with sizes similar to that used in SLS ranges from
0.1-0.2 W/(m K) . Near the melting points of metals the estimated thermal conductivity
of the powder bed increases to about 0.3 W/(m K) since the thermal conductivity of gas
increases approximately with the square root of temperature [71]. The difference between
30 the thermal conductivities of the solid and the powder cause the steep temperature drop- off in Figure 2.10(a) when going from fused powder to unfused.
The low thermal conductivity of the powder bed and the steep temperature gradients have some negative effects both during and after the build. At the edge of the melt pool where the temperature begins to fall below the melting point of the material some particles may become partially melted and undesirably fuse to the outside of the laser CAD path. This occurrence has been called “part growth” in the additive manufacturing industry [74]. Since the thermal conductivity is so low and the build times are relatively long (many hours), the loose powder immediately surrounding the fused region is heated substantially and will not cool down to room temperature until the end of the build. For polymers such as Nylon this could cause degradation of continued part growth as shown by Zarringhalam due to solid state sintering [18]. Metals could start to exhibit time-temperature dependent phenomenon such as precipitation and diffusion of alloying species within individual particles.
The processing parameters while using Nylon 12 in SLS systems do not create a large thermal gradient because the powder bed is kept just under the melting temperature and the laser is used to just increase it enough to fuse the powder together. Issues particular to semi-crystalline polymer materials such as Nylon 12 include needing to operate inside of a narrow “sintering window” and maintaining a consistent build platform temperature. Due to the nature of semi-crystalline polymers they will have a melting point and recrystallization point. The temperature range between these events needs to be wide enough to give ample time for the two-phase melt to properly fuse to the previous layers [15, 36]. It has also been observed that variations in powder bed
31
temperature as a function of position on the build platform can yield standard deviations
of 6 MPa for UTS [17] and 147 MPa for the modulus [75].
Chapter 3: Materials and Methods
3.1 Materials
The materials chosen for this study are those frequently used in aerospace and biomedical applications. Each material under investigation was thermally cycled to a standard powder bed preheat temperature in either an industrial SLM machine or
University of Notre Dame’s (UND) thermal cycling rig. The rig at UND spread powder layer-by-layer, pausing long enough to sufficiently preheat the powder. It was designed to simulate the thermal and atmospheric conditions that the bulk powder would be exposed to, but the setup does not account for the energy input from the laser source.
3.1.1 Nylon 12
Nylon 12 or polyamide 12 (PA12) was studied in this report because it accounts for almost 90% of the commercially used powder in SLS [15]. The Nylon samples from this study were received from Zimmer Biomet via University of Notre Dame. It was not specified how the powder was produced, but from studying the morphology of the powder under the SEM in section 4.4.1 it was determined that our particles were similar to the “potato shaped” particles resultant of a precipitation method [15].
Standard operating procedures when using EOS Nylon 12 powder in SLS systems require the use a blend of virgin and pre-annealed powder during the production of parts
[76]. The virgin material was delivered from the manufacturer (EOS) and the pre- annealed powder or “cake bake” was conditioned by heating the powder in the machine.
The “cake bake” powder was Nylon 12 which was pre-annealed at a standard
32 temperature. The annealing process was completed by Notre Dame following Zimmer-
Biomet protocol – the atmosphere is nitrogen with a low oxygen partial pressure, the temperature is set at 175 °C (just below the melting temperature of Nylon 12 and the same at which the powder bed is maintained during the actual build), and for a total of four hours [76]. The blend of the two is a 50/50 mixture by volume.
After blending, the powder was thermally cycled in University of Notre Dame’s powder cycling rig (as described in section 3.1). The thermal cycling for the blended powder was at same temperature as the cake bake anneal, 175°C. The powder was spread one layer at a time and allowed to rest for 1 minute to equilibrate the temperature before another layer was added. The time required between layers was experimentally determined to produce similar melt flow index values to the cake bake powders produced in Zimmer Biomet’s SLS machines.
The complete list of Nylon powder samples includes:
1) Virgin Nylon 12 powder from EOS
2) Pre-annealed “Cake Bake” of Virgin Nylon 12
3) A 50/50 blend of 1) and 2)
4) A competing Nylon 12 powder from 3DSYSTEMS
5) The 50/50 blend thermally cycled 1x, 2x, 3x, 4x, 5x, 6x, 7x, and 8x
For a total of 12 batches of powder.
3.1.2 316L Stainless Steel
Commonly used in biomedical applications, 316L stainless steel is chosen for its biocompatibility and corrosion resistant properties. The main alloying elements include chromium (16.00 – 18.00 wt%), nickel (10.00 – 14.00 wt%), molybdenum (2.00 – 3.00
33
wt%), manganese (2.00 wt% max), silicon (0.75 wt% max), and carbon (0.03 wt% max).
The stable phase at room temperature is austenite because the atomic fraction ratio of
Ni/Cr exceeds 0.5 [45]. The 316L powder was received from Johnson and Johnson’s
DePuy Synthes company and was produced by EOS via gas atomization. Two different
batches were received; each delivered in sealed polyethylene bags:
1. The first bag of powder received was sampled from a batch used to build 25
solid cubes with 1cm x 1cm x 1cm dimensions and volume of 1 cm3. Both the
additively manufactured cubes and the bag of used powder which was used to
build the cubes were sent to Case Western Reserve University for analysis.
Figure 3.1 is a picture of a group of the AM built cubes.
2. The second shipment of received powder was sampled from multiple batches
used to build mechanical test pieces such as tensile bars and then reused in
additional builds. The powder samples included virgin powder, 1-time reused
powder that passed through post-build sieving, 1-time reused powder that was
retained after sieving, 4-time reused powder that passed through sieving, and
4-time reused powder that was retained after sieving. During each build the
process parameters (e.g. the volumetric energy density) was kept constant to
eliminate additional variables aside from number of reuses. The power used
during the build 195W and the laser speed was 1083 mm/s.
The 316L cubes produced from the first batch of powder were received from
DePuy Synthes for analysis. In total, 3 sets of 25 cubes were produced using varying process parameters including powder layer thickness, laser power, and scan speed. The first set of cubes was built using 20 μm layers for the cube contours while the bulk of the
34
cube was fused on every other layer (or at a 40 μm layer thickness). This setup yielded
cubes with sides having significantly higher densities compared to their interiors. The
second set of cubes was built using a constant 40 μm layer thickness while the third set used a constant 20 μm layer thickness.
Figure 3.1 316L stainless steel cubes produced via SLM with varying process parameters
3.1.3 17-4 PH Stainless Steel
17-4 PH stainless steel is alloyed with chromium (15.00 – 17.50 wt%), nickel
(3.00 – 5.00 wt%), copper (3.00 – 5.00 wt%), silicon (1.00 wt% max), and carbon (0.07 wt% max). It is a martensitic precipitation-hardening stainless steel used for its combination of high strength and corrosion resistance. As with the 316L stainless steel powder, the 17-4 PH stainless steel powder was created via gas atomization. Two separate powder sample lots were received:
1. Four samples from University of Notre Dame including two (2) virgin
samples, 1-use non-sieved powder (straight from the powder bed), and 1-use
sieved powder. The 1-use powder was thermally cycled in UND’s powder
cycling rig which exposed the powder bed to temperature of 80°C.
35
2. Three samples from Johnson & Johnson’s DePuy Synthes including two (2) of
1-use powder retained on the sieve and one of the sieved powder. The powder
which was retained on the sieve made up approximately 1-2% of the total
weight of the powder on the build platform.
Since there were duplicates in some of the samples provided by UND and J&J, only four samples were analyzed for their PSD: UND virgin, UND 1-use sieved, UND 1-
use non-sieved, and DePuy Synthes’ 1-use sieved powder. The CEDs were not obtained
for the 1-use retained powder due to the agglomeration of the majority of the particles as
shown in Figure 4.9.
3.2 Examining Built 316L Stainless Steel Cubes
The cubes of 316L stainless steel were measured for anisotropy and density to
determine which SLM process parameters created ideal built parts, meaning fully dense
and near net shape. While process parameters are not a part of this study, it was work
required for the encompassing project and determining the ideal parameters could benefit
this study by suggesting which parameters to use.
The cubes were designed to be 1cm x 1cm x 1cm and to determine how well the
cubes conformed to the cube design they were measured with calipers. Since the cubes
were cut off of the build plate with a band saw, only the x- and y- directions were
measured. For each cube, the width was averaged over four measurements for each
direction and the difference between the two was taken as how anisotropic the cube was.
Low differences or low anisotropic measurements are desired.
Another important property of parts made with a powder feedstock, including
those via SLM, is part density. If the particles do not fully melt or if there is an
36
irregularity in the powder bed, the amount of porosity in the part increases and
subsequently decreases the effective density of the built part. The density of the cubes was measured according to ISO 2738:1999, the Archimedes method for determining density of sintered metal materials [77]. The method is based on the principle that the
difference between the mass of a cube and the apparent weight of the cube submerged in water is equal to the mass of displaced water. Since the density of the water is known, the volume of the object can be calculated and thus the density. When the cube was measured in the water it rested on a platform and did not include the weight of water or that of the beaker. This method was effective in measuring the density for two of the three sets of cubes. The cube set built with 40 μm layer thicknesses and without the 20 μm contour was too porous for the method to work due to water penetrating the cube surface and increasing the apparent weight.
Some of the high relative density cubes were cross-sectioned orthogonal to the build direction to observe the porosity which lowered the relative density. The cross- sections were then polished and etched using Marbles Reagent for ten (10) seconds to observe the microstructure [78].
3.3 Differential Scanning Calorimetry
To determine if the thermal cycling of the Nylon 12 powder caused any change in the process temperature range, each cycle was tested using a DSC (TA Instruments,
Q100). A DSC measures the thermal transitions such as the melting point for a semi-
crystalline polymer. To do this, two separate pans, one with and one without the Nylon
12 powder, are placed on two separate heaters. The pans are heated at the same rate but
due to one pan having a polymer inside it takes additional power to keep the heating rate
37 constant. By measuring how much heat is needed to keep the rate constant the thermal transitions are apparent. For melting, the machine needs to overcome the latent heat of melting so there is an endothermic spike in the heat flow. Conversely, when the polymer begins to cool and subsequently begins to recrystallize, there is an exothermic peak which means that the heater does not need to continue to supply heat due to the latent heat in the liquid polymer.
The temperature range was set to run from 40°C to 240°C; chosen because it was
~ 50°C above the melting point. The temperature ramp rate was set to 10°C/second. This rate is faster than most SLM machines raise the powder bed temperature but slower than the rate experienced close to a part being fused with a laser, so this value acts as a middle value. The data was produced as a graph of temperature versus heat flow and values of melting onset, peak melting temperature, recrystallization onset, and peak recrystallization temperature were obtained using TA Instruments software. It should be noted that the exothermic peaks (recrystallization) and the endothermic peaks (melting) can be oriented differently along the y-axis of DSC plots such as the example in Figure
4.3. Depending on user preference, the exothermic peak could be oriented upwards or downwards which is why the orientation is typically specified. For all twelve nylon samples in this work the exothermic peaks will be pointing in the positive y-direction.
3.4 Bulk Powder Sampling
Since the powder samples were delivered through the mail and there was no recorded history of material handling, additional work was required to get uniform samples representative of the population from bulk form. For each material, the powder was sampled using a mechanical spinning riffler. The riffler uses vibrations to transfer the
38
bulk powder down a chute until it falls into one of eight rotating glass vials, each
collecting exactly 1/8 of the total powder. The process of removing one vial and riffling
its contents is repeated until the desired sample size is acquired, typically <5g. The riffler
and each used vial were emptied and cleaned prior to riffling to prevent any cross
contamination between samplings.
The final vial with a sufficiently small, but representative, sample was then
emptied into a petri dish and allowed to spread out under gravity. To extract a sample
suitable for the SEM, an aluminum stub with double-sided carbon tape was inverted and
pressed down into the powder. After the sample was obtained, a generic compressed
dusting gas was used to remove any loose particles from the surface that may become
dislodged and damage the SEM. The nylon powder, since it is non-conductive, was
sputter coated with approximately 5nm of palladium to be seen in the SEM without
excessive charging.
3.5 SEM Image Analysis
A scanning electron microscope (SEM) was used to examine both topographical features of the powder samples as well as chemical analysis. The SEM used for general imaging and chemistry analysis was a FEI Nova Nanolab 200 field emission SEM. The powders were imaged first with secondary electrons under lower magnifications to infer population characteristics such as particle size distribution and then under higher magnifications to examine surface specifics. To begin the analysis of the distribution of the powder samples, three (3) images of equal magnification were imported to an image processing program called ImageJ [63]. The exact image analysis process differed between materials, as will be discussed, due to differences in SEM image quality.
39
The image analysis produced a list of projected areas, calculated by pixel count,
of the particles. Those areas were converted to circle equivalent diameter (CED) values
using the equation for area, Area = Pi * (radius)2. The diameter values were first plotted
on a histogram as shown in Figure 3.2a and fit with a lognormal curve of the data as
shown in Figure 3.2b. The location and scale parameters of the lognormal fit were used
to determine the mean CED as well as the standard deviation. The mean is calculated
using the following:
6)
and standard deviation is the root of the variance which is calculated by
7)
where μ and σ are the location and scale parameters, respectively, or also termed the geometric mean and standard deviation. It is important to note that unlike Gaussian distribution, the peak of lognormal fit does not show the mean but rather the mode of the distribution. The mean value will be shifted towards the larger side of the distribution.
The plotting was done with a statistical software package called Minitab. The
distribution fits were used to graphically show differences in the distributions as opposed
to histograms because of the difficulty of visually distinguishing differences in the latter.
When comparing multiple histograms/distribution fits on the same graph, the y-axis in
Figure 3.2a, which is presented as the total number of particles within a given bin for a
given distribution, is changed to a relative volume percentage in order to normalize the
data. This is necessary because the number of particles measured in each sample varies
40 and affects the relative magnitude of each column. The results for each material are presented in this combined manner to ascertain any shifts in the PSD, if present.
a) 40
30 y c n e
u 20 q e r F
10
0 16 24 32 40 48 56 Circle Equivelent Diameter (um) Circle Equivalent Diameter (μm)
b) Lognormal
40
30 y c n e
u 20 q e r F
10
0 16 24 32 40 48 56 Circle Equivelent Diameter (um) Circle Equivalent Diameter (μm)
Figure 3.2 The circle equivalent diameters are acquired from SEM image analysis are (a) plotted in a histogram and (b) the distribution is fit with a lognormal curve to represent the data. 3.5.1 Stainless Steels
The first step was calibrating the software using the SEM scale bar provided in the image, after which any part of the image not containing powder is cropped out. It was
41
helpful to “smooth” the image by reducing the contrast before editing further. The image
was then converted to a binary image with the particles becoming black with a white
background. Any dark regions on the particles would also become white, so the “holes”
inside individual particles from the contrast are filled – these steps are shown in Figure
3.3. To distinguish one particle from another in the binary image a white line had to be
manually inserted at the boundaries since during the conversion there would be some
ambiguity where one particle stopped and another began. The standard water-shedding
procedure programmed into ImageJ, an image processing technique used to define
boundaries between high and low intensity pixels, could not be used due to the amount of
overlap between some of the particles in addition to the program causing incorrect
segmenting of the larger particles. This step introduced the most error due to the
estimation of overlaying particle boundaries, the exclusion of some smaller satellite
particles, the distinction of a fused particle versus a deformed particle, and the fact that
the particles will preferentially lay on the side with the greatest surface area. The ImageJ
software then used the total number of black pixels in a given particle to calculate the
area and other shape parameters of each particle.
Figure 3.3: Example of making the scanning electron microscope image of 316L stainless steel into binary in ImageJ for analysis.
42
3.5.2 Nylon 12
The nylon powder was imaged similarly to the metallic samples but was still
partially charging in the SEM after being coated with 5nm of palladium. This caused the
images to have poor contrast compared to the stainless steel images. The assumption is
that the coating process did not sufficiently cover the entirety of the particle surface
leaving minimal contact between the palladium and the carbon tape.
The lack of contrast yielded binary images with the boundaries between particles being obscured. In this case, a more manual method of measuring the particle size was used. After calibrating and cropping the image, a polygon selection tool was used to select multiple points around a particle and the individual particle size was measured and crossed off as to not be re-measured. This process repeated until every particle in the image was covered. Due to the amount of time required to manually select every particle, only one (1) image was used per sample but the magnification was reduced as to include more particles.
Only the particle sizes of the first four Nylon 12 samples listed in section 3.1.1 were measured since the remaining 8 samples were those from the thermal cycling in the
UND rig. Since the rig only thermally cycled the powder there was no reason to believe that the PSD of the subsequent cycles would vary because powder consolidation is
expected to occur as a result of the laser source in some manner.
3.6 Laboratory Emulation of SLM
The powder that was received for analysis was previously thermally cycled in a custom-built rig capable of reproducing the standard pre-heating temperatures powders typically experience in an SLM machine. This was completed at University of Notre
43
Dame and the system replicated the operation of every part of a standard SLM setup as shown in Figure 1.1 except for the laser source. However, since the metallic powders were heated to a temperature no higher than 80°C to simulate the preheating cycle, further studies were needed to account for the additional thermal energy provided by the laser. The energy input from the laser exposes the powder population at and around the beam spot to temperatures exceeding and just below the materials melting point. This would significantly raise the likelihood of the powder changing chemically or microstructurally due to increased kinetic activity. The ability to sample powder immediately adjacent to a fused part would allow for the determination of change as a function of distance away from the build. This test could be completed with an industrial
SLM system but there are financial restrictions in regards to purchasing one and the machines owned by the industrial partners on this project are typically already fully utilized. Therefore, a lab scale alternative was designed to emulate the environment provided by a SLM machine.
To emulate the laser conditions found in a commercial SLM machine a few key components had to be included: a controlled atmosphere, heated powder bed, powder layer thickness, and a laser to fuse the powder. The box in Figure 3.4 was designed to contain and control these parameters. How each component worked is further described in the following subsections and more detailed drawings of the box are given in
Appendix A. The complete setup of the SLM emulation setup is shown in Figure 3.5 and it is a similar setup to that created by Kusuma in a study of SLS parameters on the sintering of TI-6-4 powder [79]. The SLM emulation setup was positioned behind a lead curtain during use to prevent any stray IR radiation generated from the laser from striking
44
an operator. The SLM emulation setup was meant to work in conjunction with the
powder recycling rig at University of Notre Dame and it was decided that the experimental setup at CWRU did not need to incorporate every SLM component, such as a precision powder spreading mechanism, since that part of the process was occurring already at UND during the thermal cycling and the same powder would be used.
Figure 3.4 SLM emulation box design.
Figure 3.5: Experimental setup to control laser sintering parameters
45
3.6.1 Laser Source
A 100W pulsed Nd-YAG laser (1064 nm wavelength) from FOBA with pre-
packaged laser software controls was repurposed from an engraving project to fuse the
powder. The unit was mounted on top of a vertical knee-mill that allowed for very precise
laser focal length control. The laser software allowed for control of raster speed, pulse
frequency and duration, and the ability to load 2D graphics files with automatic
conversion to the systems laser path file. Unlike a standard SLS laser that contours the
desired profile before filling in the center, this raster pattern was a series of fixed parallel
lines a set distance apart, which were drawn in series from the top of the image to the
bottom. The laser spot diameter was also fixed at 100 μm, a parameter set by the physical
dimensions of the lens. The 2D image used for the melt profile was that of a 7-tooth gear;
a design chosen for its concave and convex features. Concave features should experience a larger heat flux than the convex features since the laser is in the proximity longer for the prior.
The laser parameters used in the SLM environment were based off of the settings used by DePuy Synthes in their SLM machines during the building of test parts as referenced in section 3.1.2. Using the volumetric energy input equation in section 2.4, a similar energy input was calculated for use in the in the SLM emulation – a value which
was calculated to be 100 J/mm3.
3.6.2 Atmospheric Control
The laser in Figure 3.5 was positioned on top of a polycarbonate (PC) box used to control the atmosphere in which the powder particles were fused. Between the box and laser was a metallic bellows that allowed for slight adjustments to the focal length if
46
necessary. The box was fabricated out of PC for a clear line of sight (via video camera) to
the powder as well as simple fabrication. The box has a volume of approximately 12,800
cm3 with one access point on the top side, electrical/thermocouple connections, and a 4-
way valve leading to vacuum, commercially pure argon, the SLM emulation chamber,
and ambient air. The topside entry point has two draw latches allowing easy access to the laser focal plane while still capable of drawing a vacuum seal down to 0.05 atm. To
obtain an inert argon environment the box was first pumped down to 0.05 atm after which
the valve was switched to argon to purge the system. It was noted that when the vacuum
was switched to the argon gas input the pressure doubled to 0.1 atm, most likely due to
insufficient sealing around the access lid. The argon would then fill the box and
pressurize it just enough for the door seal to leak at a constant rate.
Using the composition of room air, at this atmospheric pressure it was expected
that the oxygen content still present in the system before purging would be approximately
1%. A simple first order differential equation shows that with a flow rate of 280 cm3/s
into the box and an equal constant rate out of the box (assuming sufficient mixing with the remaining oxygen and no oxygen reincorporation into the system) it would take ~7
min for the oxygen level to reach 1 ppm. The flow rate of the argon was estimated by
timing how long it took to fill the volume of the box. To ensure the environment
remained inert as possible, a continuous flow of argon was admitted throughout the
experiments.
47
3.6.3 Powder Bed
Maintaining both a consistent powder bed temperature and layer thickness was
important for reproducibility between trials. Both variables change depending on the
material system being used. The research partners for this project specified normal powder bed preheat temperatures of 80°C for 316L stainless steel, 80°C for 17-4 PH stainless steel, and 175°C for Nylon 12 as these are standard operating process parameters for industry SLM machines [30]. Maintaining a consistent powder bed
temperature was accomplished by using the resistive heaters and digital controls of a
standard Fisher Scientific IsoTemp hot plate.
Figure 3.6 shows a FLIR image of the temperature profile provided by the two
centralized heaters. To make the profile uniform, a ¼” plate of copper was taped on to the
ceramic surface of the hotplate with high-temperature adhesive tape using the high thermal conductivity of the copper to distribute the heat evenly. Due to the large
percentage of wavelength reflectivity in the IR spectrum, the copper plate was coated
with a black high-temperature paint to allow for the thermal readings.
To better control the temperature of the hotplate, the input temperature reading
was changed from original thermocouple from inside the hotplate to an external
thermocouple reading from the copper plate. This helped to compensate for the effect of
heat loss between the resistive heaters and the copper surface. The temperature readings
from the thermocouple were then verified using the FLIR camera. When sintering on a
loose powder bed in the form of a relatively thick powder layer inside a Pyrex Petri dish,
similar thermal readings were taken to ensure the powder surface was also around 80°C.
48
The second challenge of recreating typical powder bed settings was the creation
of uniform and thin powder layers. Since there was not enough space for the typical SLS
automated spreader bar/roller, a simpler but less precise method for depositing powder
was used. First, an identically sized copper plate was placed on top of the first but with a
hole in the center resembling a picture frame. This would allow a similarly sized strip of
a desired material to be nested inside which would act as a substrate for the powder,
limiting contamination while in the liquid phase. These two copper plates were joined by
registration pins allowing for easy removal and cleaning. The powder was then manually
spread using a film applicator while inside the chamber and before the system is purged
with argon. The hot plate setup is provided in Figure 3.7.
The applicator was purchased from TQC-USA Inc. and has four eccentric sides, each with a varying gap size (30, 60, 90, 120 μm) between the cylinder and substrate.
During testing it was determined that the powder needed to be heated to drive off any moisture obtained during handling or from transport since the powder would be difficult to spread. Additionally, there was a small thickness mismatch between the copper plate and the desired substrate insert with the inserted material being slightly thinner. This resulted in the spread powder layer being deposited slightly higher than the pre-set value given on the film applicator. When attempting to create a 30 μm layer, the actual layer thickness was determined in Figure 3.8 to be approximately 150μm by using a Keyence
Olympus VHX 5000 optical microscope and its 3D surface analysis software. Typically, layer thicknesses in a SLS system will vary from material to material and strongly depend on the particle size distribution. Although 150μm is larger than the 40μm value used for
49
316L, the process produced consistent and repeatable powder layers and it is possible to adjust the laser settings and deliver a similar energy density to the powder bed.
Figure 3.6: Comparison of powder bed heat profiles with (left) just the ceramic hot plate and (right) the addition of a copper plate obtained by a FLIR imaging camera.
Figure 3.7: Hotplate configured to preheat and spread powder before sintering.
50
Figure 3.8: A three dimensional surface profiling of 316L powder spread on the substrate surface using a 30 μm film applicator producing a 150 μm layer.
3.7 Particle Chemistry Characterization
To determine if the thermal cycling or SLM process had an effect on the overall
chemistry, the samples were analyzed using Energy Dispersive X-ray Spectroscopy
(XEDS), Auger Electron Spectroscopy (AES), and X-ray Photoelectron Spectroscopy
(XPS). Each technique gives qualitatively and quantitatively different information regarding the chemical make-up of the sample. For both the 316L and 17-4 PH stainless steels a total of three samples for each material were targets of chemical analysis:
1. Virgin powder to obtain a baseline.
2. Powder retained after use which is removed from the population, representative
of the worst case scenario.
51
3. Powder sieved after use but selectively sampled from the larger end of the
distribution, classified as “large sieved” powder.
The illustration in Figure 3.9 shows the relative location the large sieved powder.
The large sieved powder sample is necessary because the retained powder represents the particles which will never be introduced back into a SLM process rendering the observed
changes harmless to the process. Observing only the particles at the large end of the
sieved powder PSD increases the statistical chance of analyzing a particle with the
chemistry and morphology similar to that of the retained powder but small enough to be
reincorporated into the reuse population. The retained powder sample consists of particles
>75 μm which is the opening size of a U.S. Tyler sieve size No. 200, the recommended size by SLM machine manufactures. To obtain the larger particles of the used sieved
powder, it was sieved through a U.S. Tyler sieve No. 270 with an opening of 53 μm and
the powder retained on the sieve was sampled while the smaller particles fell through.
The sieving was done by hand for 20 minutes with a series of lateral shakes and taps
which was done to replicate the movements of a Ro-Tap sieve shaker.
For the used 316L stainless steel powder sample, the powder in the 53-75 μm range made up 4.6% of the total weight sieved. The 17-4 PH stainless steel powder sample had
1.8% in the 53-75 μm range of the total weight sieved. Samples of the newly sieved
powder were collected using an aluminum stub with double-stick carbon tape for use in
the SEM.
52
Figure 3.9 Illustration of the process to sieve out the larger size fraction of the used powder
3.7.1 X-Ray Energy Dispersive Spectroscopy
XEDS relies on the principle of characteristic X-rays being released from an atom
when excited by a high energy source. Electrons from the inner shell are ejected which
causes outer shell electrons in a higher energy state to fill the remaining hole. In doing so,
the energy difference between the two shells is released in the form of an X-ray which can be measured. For each element there are characteristic X-rays for each shell from which the replacement electron originated from, therefore enabling the quantification of the elemental composition. The X-rays, as shown in Figure 3.10, can originate from as deep as 3 μm into the samples surface which gives good average composition and is not too sensitive to minor surface elements. During SEM imaging, XEDS maps of the particle surface were acquired to insure the virgin particles composition matched those of the standard composition.
53
To determine if the chemical composition varied as a function of composition,
individual particles that were representative of their population were cross-sectioned using
a focused ion beam (FIB). A different SEM, a FEI Helios 650 field emission SEM, was
used to FIB the particles because it had a large current range which allowed the particles
to be cut quicker. To obtain the desired cross section, the surface of the particle was first
covered with Pt to prevent damage to the sample surface. Large primary beam currents
were used to sputter away the majority of the particle surface/interior and was adjusted to
lower primary beam currents to smooth off the newly created surface. These surfaces were
also used in determining the microstructure of the particles in section 4.5.
3.7.2 Auger Electron Spectroscopy
A PHI 680 Scanning Auger Microprobe was to determine distribution of
elements. AES uses a beam of electrons as a source of radiation and the process is similar
to that of XEDS in the sense that the primary source is used to excite inner shell electrons to allow outer shell electrons to fall. This occurrence creates X-rays as mentioned above,
but the energy released can also be transferred electrons from the same higher energy
shell. In AES, the kinetic energy of these emitted electrons, or so called Auger electrons,
are measured and compared to the characteristic peaks for each material. The Auger
electrons have a much lower kinetic energy than that of the incoming electron beam, thus
only those at the near surface (<7.5 nm) will detectable and are easily distinguishable
from the high energy inelastically scattered secondary electrons. This technique surface
sensitive as shown in Figure 3.10, and can also have good lateral resolution (10-100nm)
which is useful for spot determination of composition. Spot analysis was used on the 4-
use retained powder received from Johnson and Johnson since it would be the most likely
54 to have experienced an environment promoting change to the surface features. AES was not used for the 17-4 PH stainless steel because there was no evidence of localized characteristic oxide formations unlike that of the 316L stainless steel.
3.7.3 X-Ray Photoelectron Spectroscopy
A PHI Versaprobe 5000 XPS was used for this paper. XPS, as opposed to XEDS and AES, uses X-rays (Al-Kα) as the source of primary radiation and the kinetic energy of the emitted photoelectrons is measured. The kinetic energy of the electron is directed associated to its binding energy to a given atom. Since each element has a characteristic binding energies for electrons in a given orbital, it will have a characteristic XPS spectrum from which the material and its chemical bonding state can be inferred. The ability of XPS to determine the chemical state of a material is extremely valuable in this case for determining how the powder oxide is composed. The data generated by XPS is typically presented as the number of counts per second at a given energy. As the X-ray source varies the energy over a given range (0 – 1100 eV in this experiment), the number of photoelectrons which are counted suddenly peaks above the surrounding values once the known bonding energy is reached. If an electron in the p, d, or f orbital is ionized a pair of peaks will be shown in the spectrum due to the spin-orbit interactions. This is the case for most metals and other heavy elements. For metals, the binding energy of the pure element is lower than that of its subsequent oxide forms but the peaks are relatively close to one another. In the case of powder reuse, repeated exposure to high temperatures suggests either the formation of a new oxide or the growth of a current one. This event would result in the metallic peak becoming weaker while the oxide peak strengthens.
55
Similar to AES, the electrons which are detected are those from the near surface.
However, unlike AES, the lateral resolution is much lower (200 μm spot size was used)
which results in a wider region of the sample surface being analyzed. For a powder
sample, this can be advantageous since composition will be acquired over a few particles
and is statistically more representative of the entire population than a single particle.
Six different XPS samples were created, three from both 316L and 17-4 PH stainless steel for each target group listed in section 3.7. To get sharper signals, the top 6 nm of the surface was sputtered using argon ions to remove any carbon contamination.
High resolution XPS was used for one sample to reduce the noise-to-signal ratio and the resulting more defined peaks matched up well with the lower resolution peaks, therefore it was decided to not continue with the extra high resolution scans for the remaining samples. The XPS results from the outermost surface for each sample were overlaid onto the same graph to make noticing differences easier. XPS was also used to do a depth profiling of the oxide by sputtering away the surface and stopping periodically to scan the surface. Since the slowest calibrated scan (3 keV over 5mm x 5mm area) removed 6.2 nm/min and the oxide thickness was reported to be 2-5 nm for stainless steels, a lower sputtering potential (1 keV over 2mm x 2mm area) was used. However, since this setting was not calibrated, the actual thickness could not be measured. Instead, the relative counts for a given element could be tracked as a function of sputtering time. To save time, only the oxygen counts were measured and the 17-4 stainless steel was assumed to be representative of both stainless steels since cross-sections from section 4.6 were thin.
56
Figure 3.10 Schematic showing location and source of signal for different microscopy techniques. Adapted from Ref. [80].
3.8 Particle Microstructure Characterization
Since some particles may be exposed to temperatures approaching their melting points as well as held at elevated temperatures for extended periods of time during the
SLM build, the grain structure or size may change. Two samples were analyzed using
Electron Backscatter Diffraction (EBSD), as described in section 3.8.1, to determine if there was a change in grain morphology after a build. The first was a sample of virgin
316L stainless steel which had no prior thermal history in a SLM machine. For the second, instead of guessing which powder particle in the retained population experienced the largest thermal cycle, a particle which was already partially fused to the outside of a pre-built 316L stainless steel cube was examined because the thermal history is known.
A high density cube (>99%), chosen because of the high volumetric energy input, had a corner removed and polished as shown in Figure 3.11. This provided a surface that
57 had cross-sections of partially fused 316L particles along the perimeter of the newly created surface. The top surface is the last layer of powder to be melted with the laser and the side surfaces are those in contact with the powder bed. Only the surfaces which are in contact with the powder bed can have partially fused particles to the outside of the part.
The magnified SEM image in Figure 3.11 shows an example of a partially melted particle inside the dashed circle.
Recoater Direction 150x Mag.
Figure 3.11 Cut-corner of a cube showing the location and presence of the partially melted particles.
3.8.1 Electron Backscatter Diffraction
EBSD is a technique used to characterize the microstructure of a crystalline or
polycrystalline material. The method uses a focused electron beam on the sample and
detects backscattered electrons that are ejected close to the Bragg angle and form Kikuchi
bands. These bands represent each of the lattice reflecting planes and three bands can be
used to determine the orientation of the crystal. A common way of graphically depicting
this information is with orientation mapping; pictorially showing the crystallographic
58 orientation of the plane normal in 2D space. In a polycrystalline material each crystal will be oriented in space differently, so by rastering the electron beam over a desired area a map can be produced with each grain colored according to its orientation. This map can also be helpful in determining grain boundaries, grain size, or processing history. The produced image will not convey rotation of grains within the plane which could be a problem in samples where directional solidification is dominant and grains are often in the same direction, but it is not expected for gas atomized powder.
3.8.2 EBSD Sample Preparation
EBSD sample preparation requires a surface with no surface defects for orientation data to be obtained. For the cube corner, the entire cube was mounted in a thermosetting epoxy mount and polished mechanically up to a 1 μm diamond suspension followed by vibratory polishing with 0.04 μm colloidal silica. The virgin 316L stainless steel powder sample was first dispersed in a curable epoxy after which it was polished using a Gatan Ilion ion polisher for 8 hours, at -40°C, using an accelerating voltage of 3 keV. The ion polisher uses a 1 mm argon ion beam which strips away a thin layer of the sample leaving a very flat and usually defect free surface. The ion polishing effectively cross-sectioned the powder particles. The sample needed was given a 5nm carbon coating for electrical conductivity, necessary for the SEM. After the ion polishing there was some evidence of titanium redeposition from the ion polisher mount, but enough particles were present to get a decent sample size.
59
Chapter 4: Results
4.1 Overview
The results in this section are presented according to how the results were obtained with subsets of individual materials (316L stainless steel, 17-4 PH stainless steel, and Nylon 12). The material came in the form of either the pre-built metal cubes, the thermally cycled powder from University of Notre Dame, powder used in an industry
SLM printer, or powder which was used in the SLM emulation environment. How the results relate to one another will be discussed further in Chapter 5:
4.2 Pre-built 316L Stainless Steel Cube Inspection
4.2.1 Density and Part Anisotropy
Figure 4.1 shows the measured density and dimensional measurements of the
316L cubes produced via SLM with varying process parameters. Each line represents the magnitude of the difference between edge lengths from a single cube. The longer the length of the line, the larger the differences in side length and the more anisotropic the part. The density, as measured by the Archimedes method, is represented as a relative density value compared to 7.99 g/cm3, the density of bulk 316L stainless steel. The blue
dashed lines represent the “perfect cube” which has perfect 1cm sides in addition to being
fully dense. The second of the two cube productions is not included because density
results could not be measured.
60
Figure 4.1 Relative density vs measured side lengths or pre-built 316L stainless steel cubes.
4.2.2 Partially Melted Particles
The pre-built 316L cubes provided evidence of partially melted particles on both the interior and exterior of the cube. The left image in Figure 4.2 is the vertical face of a
316L stainless steel cube which was in contact with the powder bed and had extra particles fuse to the outside. The right image in Figure 4.2 is the etched cross section of a cube with a relative density of 95%. The circled partially melted particles show a much different grain structure compared to that of the bulk material. Further examination of how these particles are bonded to the outside of the built part is presented in section 4.5.
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200 μm
Figure 4.2 Evidence of partially melted particles in a SEM image of the surface of a built cube (left) and an optical image of the etched cross-section (right).
4.3 Nylon 12 DSC Results
The heat flow as a function of temperature for virgin Nylon 12 powder was obtained using the DSC and plotted in Figure 4.3. The remaining plots are provided in
Appendix B. The temperature transitions and the process window for the virgin Nylon 12 in addition to the remaining eleven samples are compiled into Table 4.1 Thermal transition temperatures from DSC analysis for Nylon 12.
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Figure 4.3 Heat flow vs temperature plot output from DSC analysis for virgin Nylon 12 powder.
Table 4.1 Thermal transition temperatures from DSC analysis for Nylon 12 Melting Temperature Recrystallization Sample (°C) Temperature (°C) Processing Name Range (°C) Onset Peak Onset Peak 3DSystems 182 186 148 143 34 Virgin 181 187 150 147 31 Cake Bake 185 186 148 143 37 50/50 181 187 148 143 33 1 cycle 181 186 150 145 31 2 cycle 181 187 150 145 31 3 cycle 181 187 149 145 31 4 cycle 181 187 151 146 30 5 cycle 181 187 151 146 29 6 cycle 181 187 149 145 32 7 cycle 180 186 151 147 28 8 cycle 180 187 151 147 29
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4.4 Particle Size Distribution and Shape Analysis
The particle size was measured by using SEM images in an image processing program, ImageJ. The results of the image analysis gave lists of circle equivalent diameters which were plotted as a histogram and a lognormal distribution fit was acquired. Note that in this section the term “sieved powder” will refer to a powder sample which has been either thermally cycled in the UND rig or used in a SLM machine and contains particles which passed through the post-sieving process. The term “retained powder” refers to the powder that did not pass through the sieve and is typically discarded. The number of cycles/uses before the sieving process occurs will also be given.
4.4.1 Nylon
Examples of the SEM images used to analyze the Nylon 12 samples are shown
below in Figure 4.4 with the leftmost being virgin, the center being “cake bake”, and the
rightmost being a 50/50 mixture of the two. The distributions of the Nylon 12 samples
shown in Figure 4.4 as well as the powder supplied from 3DSystems are presented in
Figure 4.5. The calculated CED averages and standard deviations as calculated from the
fit parameters are gathered in Table 4.2. The mean CED for the virgin, cake bake, and
50/50 blend are very similar around 51 μm while the 3DSytems powder is slightly larger
at 57 μm.
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Figure 4.4 Examples of Nylon 12 SEM micrographs used for determining PSD. From left to right is virgin, cake bake, and a 50/50 mixture of the two.
Histogram of Virgin, Cake Bake, 50/50 Mix, 3DSystems Lognormal
0.04 Variable Virgin Cake Bake 50/50 Mix 3DSystems 0.03 Loc Scale N 3.852 0.3237 157 y
t 3.908 0.2287 135 i s 3.900 0.3047 144 n 0.02 e 3.998 0.3070 272 D
0.01
0.00 15 30 45 60 75 90 105 Circle Equivelent Diameter (um)
Figure 4.5 Lognormal distribution fit of four different Nylon 12 samples.
Table 4.2 Average CED and standard deviation for four different Nylon 12 powder samples. Sample Name Mean CED (μm) Standard Deviation (μm) Virgin 50 16 Cake Bake 51 12 50/50 Mix 52 16 3DSystems 57 18
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4.4.2 316L Stainless Steel
Examples of the SEM images used to determine the PSDs of the 316L stainless
steel samples are shown in Figure 4.6. The image on the left is powder that was used
once and passed through a sieve while the image on the right is from the 4 use retained
sample. The 4 use retained sample shows a significant increase in particles appearing to
be mechanically bonded to one another. The lognormal fits to the corresponding
distributions are compared in Figure 4.7 and the calculated averages and standard
deviations as calculated from the fit parameters are gathered in Table 4.3. The mean
circle equivalent diameter (CED) increases as a function of reuse for both the sieved and retained powder with significant differences occurring between the sieved and retained powder which is expected.
The SEM image analysis also produced results which described the average shape of the particles using the circularity value. The average circularity is shown for each of
the 316L stainless steel samples in Figure 4.8.
Figure 4.6 SEM images of 316L stainless steel samples from 1 use sieved (left) and 4 use retained (right).
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316L Virgin, 1 Use Sieved, 1 Use Retained, 4 use Sieved, 4 use Retained Lognormal
0.04 Variable Virgin 1 Use Residue 1 Use Sieved 4 Use Sieved 0.03 4 Use Residue
Loc Scale N y
t 3.284 0.4490 84 i
s 3.818 0.3476 181 n 0.02 e 3.510 0.3502 178 D 3.768 0.2747 214 3.995 0.5308 179
0.01
0.00 0 25 50 75 100 125 150 175 Circle Equivelent Diameter (um)
Figure 4.7 Lognormal distribution fit of five different 316L stainless steel powder samples.
Table 4.3 Average CED and standard deviation for five different 316L stainless steel powder samples. Sample Name Mean CED (μm) Standard Deviation (μm) Virgin 30 14 1 Use Sieved 36 13 1 Use Retained 47 12 4 Use Sieved 45 13 4 Use Retained 63 35
Figure 4.8 Average circularity measurements for 316L stainless steel powder.
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4.4.3 17-4 PH Stainless Steel
Examples of the SEM images used to determine the PSDs of the 17-4 PH stainless
steel samples are shown in Figure 4.9. The image on the left is single use sieved powder
while the image on the right is single use retained. Shown at the same magnification, the
powder particles of the sieved sample are obviously much smaller and lack the
agglomerates shown in the right image. The lognormal fits to the corresponding
distributions are compared in and the calculated averages and standard deviations as
calculated from the fit parameters are gathered in Table 4.4. The thermally cycled
powder from UND showed no sign of change in the mean CED or distribution and was
slightly larger than the single use sieved powder provided by DePuy Synthes. Since there
was no virgin powder provided by DePuy Synthes for comparison to the single use sieved
powder as distinction could not be made it the powder had changed inside the SLM
system similar to the 316L stainless steel.
Since the single use retained powder was not analyzed, the difference in circularity
between the virgin and sieved samples could not be quantitatively presented. However,
since the 17-4 PH stainless steel single use retained particles in Figure 4.9 show a larger fraction of agglomerations when compared to the 316L stainless steel 4 use retained particles in Figure 4.6, the average circularity is presumed to be significantly lower for the single use retained 17-4 PH stainless steel powder. To demonstrate that the agglomerates from the single use retained sample as shown in Figure 4.9 are truly mechanically bonded, the particle movement was observed using an optical microscope.
Figure 4.11 presents a series of video frames which show a needle entering a group
68 particles. Neighboring agglomerates are seen rotating in the plane of the image while the components of the agglomerate retaining their orientation with respect to one another.
Figure 4.9 SEM images of 17-4 PH stainless steel samples from single use sieved (left) and single use retained (right).
NDU 1-use Non-Sieved , NDU 1-use Sieved, NDU Virgin, J&J 1-use Sieved Lognormal
0.05 Variable NDU 1-use Non-Sieved NDU 1-use Sieved NDU Virgin 0.04 J&J 1-use Sieved
Loc Scale N 3.538 0.2787 414
y 0.03
t 3.521 0.2843 341 i
s 3.545 0.2789 242 n
e 3.389 0.3110 305 D 0.02
0.01
0.00 10 20 30 40 50 60 Circle Equivelent Diameter (um)
Figure 4.10 Lognormal distribution fit of four different 17-4 PH stainless steel powder samples.
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Table 4.4 Average CED and standard deviation for five different 17-4 PH stainless steel powder samples. Sample Name Mean CED (μm) Standard Deviation (μm) Virgin 36 10 UND Single 35 10 use Sieved UND Single 36 10 use Non-sieved J&J Single use 31 10 Sieved J&J Single use N/A N/A Retained
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Figure 4.11 Mechanically disturbing 17-4 PH stainless steel powder which was retained after the sieving process after one use. The bond between joined particles is strong enough for agglomerate reorientation to occur with fracture of the shared surface.
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4.5 Particle Microstructure Characterization
EBSD was used to determine if there were any microstructural changes in the stainless steel powder with respect to grain size and shape from the as-received state to a particle that had experienced partial melting. In addition to the ion-polished EBSD results, the FIB cross-sectioned particles, when imaged using back scattered electrons
(BSE), provided insight information on the size and shape of the grains.
4.5.1 316L Stainless Steel
Figure 4.12 shows crystallographic orientation maps of particles fused to the perimeter of the cube cross section (left) and loose virgin powder (right). The cube image has surface defects from mechanical polishing which were not fully removed and appear as black diagonal lines. The partially melted particles have comparable diameters and grains sizes to that of the loose powder. A third partially melted on the cube was not polished sufficiently enough to reveal the orientation but it is assumed that grain size is consistent to the observed ones. The BSE imaging of the 316L cross-sections for virgin and large sieved powder are presented in Figure 4.13. The cross section of the virgin powder was done on a particle of representative size, approximately 30 μm, and exhibited an irregular grain size similar to that shown via EBSD. The large sieved powder, chosen for it similarities in characteristics to the retained powder, has one particle with irregular grain size and the other with columnar grains originating at the point of contact.
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Interior
Particles
Figure 4.12 EBSD map of crystallographic normal orientations of (left) 316L stainless steel particles fused to the outside of a SLM built cube and (right) as-received virgin 316L stainless steel.
Figure 4.13 SE images of virgin powder (a) and the large sieved powder (c). Corresponding BSE images of the virgin (b) and large sieved (d) cross sections for 316L stainless steel.
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4.5.2 17-4 PH Stainless Steel
The same BSE method was also used for the 17-4 PH stainless steel and the images of the virgin, large sieved, and retained powders are presented in Figure 4.14. The grain size for the 17-4 PH stainless steel is much smaller than that of the 316L, but the virgin powder still shows relatively irregular grain size. The large sieved powder sample, an agglomerate of five particles, four of which were cut, was still small enough to make it through the sieve. The same irregular microstructure of the virgin material can be seen in three of the four particles with the third again having a columnar structure originating from points of contact. The retained powder was also cross-sectioned to check for
consistencies, but the structure looks relatively uniform. Under higher magnifications, the
microstructural boundaries for the large sieved and retained agglomerates can be seen in
Figure 4.15.
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Figure 4.14 (a) SE images of virgin powder, (b) BSE image of FIBed virgin powder, (a) SE images of virgin powder, (b) BSE image of FIBed virgin powder, (a) SE images of virgin powder, (b) BSE image of FIBed virgin powder. Magnifications vary.
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Figure 4.15 Higher magnification BSE images of particle to particle contact for the large sieved (a) and retained (b) 17-4 PH stainless steel.
4.6 Particle Surface Morphology and Chemistry
The surface of the stainless steel powders was characterized for both bulk composition using XEDS and surface composition using AES and XPS. AES was only used for the 316L stainless steel since the 17-4 PH stainless steel visually lacked sufficient oxide formations on the surface to warrant further examination.
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4.6.1 316L Stainless Steel
Several SEM SE images were obtained to exemplify the observed surface formations that contrasted with the bulk material below. In addition to surface oxide formations, the surface either showed the presence of dendritic grains with the appearance of surface relief during the atomization process or, conversely, had minimal
surface roughness. Examples of these features are shown in Figure 4.16 with several
additional unique features presented in Appendix C. The surface features were present in
both the virgin and 4 use retained powder which suggests that it is highly probable that
they originate from the atomization process and not during a SLM build. These surface
features were also found on the surfaces of internal pores during the cross-sectioning of the particle in Figure 4.13(b). Visual inspection of the retained powder shows a small fraction of particles with different colors ranging from yellow, brown, and blue but the large majority of the population was the same grey/silver color as the virgin powder.
Figure 4.16 Example SEM images of 316L powder showing oxide formations and differences in surface roughness.
Initial chemistry results of the 316L stainless steel powder were obtained using
XEDS mapping of the particle surface in addition to local spectra on specific surface
77 features. When the particle in Figure 4.16 (left) was scanned, the individual elemental maps in Figure 4.18 were created. The elemental maps how a distinct concentration of silicon and oxygen at the areas of dark contrast in the SEM image. The locations of the targeted composition spectra, targeting the expected oxide formations, are shown in
Figure 4.17 with the elemental composition given in Table 4.5. Spectrum 1, 2, and 3 were located on the base metal, a film-like oxide, and a protruding oxide, respectively. In addition to surface oxides, an XEDS mapping of both virgin and used powder showed the presence of internal oxides composed of Al and Si. An XEDS mapping of the cross section of a virgin particle revealed no oxide film that was discernable at the resolution of the SEM, which would be around the magnitude of around 10 nm or less. During the cross-sectioning, an XEDS map of the newly exposed base metal and a partial cross section of a protruding oxide again showed an increase in O and Mn, but this time Al was also measured in the same oxide.
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Figure 4.17 Locations of localized XEDS spectra comparing the oxide formations to the base material of virgin 316L stainless steel. Used powder showed the same formations.
Table 4.5 Composition (wt%) of 316L virgin powder comparing surface oxide formations and base material. Composition (wt%) Fe Cr Ni Mn Mo Si O 16.0 - 10.0 - 2.0 2.0 - 0.75 316L stainless steel Bal. N/A 18.0 14.0 max 3.0 max Spectrum 1 60.1 18.2 13.6 1.8 2.7 0.6 0.8 Spectrum 2 39.0 15.1 7.9 11.4 1.7 5.7 15.7 Spectrum 3 14.3 9.8 0.6 23.1 - 13.1 38.6
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Figure 4.18 XEDS elemental mapping of the surface of a used 316L stainless steel powder, also shown in Figure 4.16 (left), which shows uniform distribution of elements except for silicon and oxygen.
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To better understand the composition of the small oxide features without the interference of the base material they were analyzed for composition using AES. The locations of the AES spectra are shown by colored squares in Figure 4.19 with the composition results shown in Figure 4.20 by atomic percent. The locations of the obtained AES spectra were similar to the locations chosen for the surface XEDS spectra with locations 1, 2, and 3 being a protruding oxide, film-like oxide, and the apparent particle base metal, respectively. Locations 1 and 2 have significantly more silicon than the base metal in addition to elevated Mn levels.
33
22
11
2000 X 10.0 keV 10.0 µm F Figure 4.19 Electron image depicting the locations of the AES spectra of the 316L particle surface.
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4 x 10 S46SS..2.spe 4
2
0 Si2 Mn1 Mn2 Si1 Fe3 Fe1 Fe2
c/s -2 Red=Area 1 Mn3 Blue=Area 2 Green=Area 3 Atomic % Atomic % Atomic % -4 C1 54.6 C1 53.0 C1 61.4 O1 29.1 O1 28.4 O1 26.5 Fe4 Mn4
C1 Si2 10.0 Si2 10.2 Fe3 7.8 Fe3 3.6 Fe3 4.7 Si2 3.2 -6 O1 Mn1 2.7 Mn1 3.6 Mn1 1.2
-8 200 400 600 800 1000 1200 1400 1600 1800 2000 Kinetic Energy (eV) Figure 4.20 316L stainless steel AES spectra overlay with corresponding compositions.
To get a better understanding of the surface chemistry over several particles the samples were analyzed using XPS. The portion of the XPS spectrum for binding energies in the range of 500-900 eV are overlaid in Figure 4.21 and the element specific spectrum peaks are presented in Appendix D. Binding energy values for the metals and their oxides were obtained from Ref. [81]. Oxygen has a much larger peak compared to the used and virgin samples suggesting a greater magnitude of oxide. The iron Fe2p3 peak shift moved from more metallic iron for the virgin and used samples to mostly iron oxide in the retained sample. The Cr2p3 peak shows a similar shift but the used powder shows more oxide than metal which is similar to that of the retained. The used and retained powders show the presence of Si while only the used and virgin powder show Ni. All three samples have Mn in the oxide form.
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Figure 4.21 XPS spectrum (500eV-900eV) overlay for 316L stainless steel.
4.6.2 17-4 PH Stainless Steel
The initial chemistry of the 17-4 stainless steel powder was determined using
XEDS mapping of the surface and by the use of a spot scan inside the interior. The composition in weight percent obtained from each spectrum is given in Table 4.6. The values are close to the standard although the interior was Cr-rich and Mn was not detected on either the surface or the interior. AES was not used because SEM imaging did not provide any compelling surface features to look further into compared to 316L.
Unlike the 316L stainless steel, the retained 17-4 PH stainless steel had a clear difference in color between that of the virgin/used and retained powder. The used powder had a grey/silver color while the retained powder had a variation in color ranging from light yellow and rusty red-brown, to light blues and dark purples.
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Table 4.6 Composition (wt%) of 17-4 PH stainless steel virgin powder. Composition (wt%) Fe Cr Ni Mn Cu Si O 17-4 PH stainless 15.0 - 3.0 - 1.0 3.0 - 1.0 Bal. N/A steel 17.5 5.0 max 5.0 max Surface 68.5 15.4 4.1 N/A 4.3 0.1 N/A Bulk 69.9 18.5 3.9 N/A 3.5 0.2 1.0
The results of the XPS spectra for the 17-4 are again overlaid in Figure 4.22 with element specific spectrum provided in Appendix D-2. Similar to the 316L stainless steel, the oxygen peak suggests metal oxide formations with the retained powder having the largest signal. There is very little Mn and the Cu is the same for each sample. The virgin and retained powders have no discernable Si peak but the sieved powder has it. For both the Fe and Cr peaks, the virgin material has both a metal and metal oxide peak but with the metallic peak being stronger. The sieved materials also have both iron peaks but the oxide peak gets stronger while the metal peak weakens. The retained then shows the metal oxide peak and the metal peak disappears with the possibility of a slight shoulder in the spectrum representing the remaining metal at the surface.
The XPS depth profiling of the 17-4 PH stainless steel samples showed the oxygen counts decreasing at a much slower rate for the retained powder compared to the virgin and sieved. This suggests a thicker oxide on the retained powder. Each sample was sputtered for 30s and a new spectrum was obtained until a plateau in the oxygen counts was observed. Two retained powder samples in Figure 4.24 and Figure 4.25 were cross- sectioned and XEDS mapped to observe the suggested increase in oxide thickness. The first is the same particle from Figure 4.14(f) which was a non-spherical melt fused to additional particles. The second cross-section was a large, spherical spatter particle.
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There is a distinct oxide scale on the exterior of both particles which has an average thickness of approximately 50 nm for the non-spherical fused particle and 150 nm for the spherical. Both oxides did not show any other element aside from Fe in the internal oxide and both showed internal nanoparticle tantalum oxide formations ranging from 50-200 nm. Although not initially noticed in the virgin 17-4 PH samples, the same oxides were present in all the 17-4 PH stainless steel cross sections. It should also be noted that the XEDS map in Figure 4.24 is on the bottom side of the cutting surface, so beyond the particle-oxide interface is a random assortment of redeposited elements from the ion cutting opposed to the protective Pt coating in Figure 4.25.
Figure 4.22 XPS spectrum (500eV-900eV) overlay for 17-4 PH stainless steel.
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Figure 4.23 XPS Depth profile of Oxygen in 17-4 PH stainless steel.
Particle
Oxide
Redeposited Material
Figure 4.24 XEDS mapping of the external oxide formation on a non-spherical spatter particle of 17-4 PH stainless steel.
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Pt Coating
Oxides
Particle
Figure 4.25 XEDS mapping of the external oxide formation on a spherical spatter particle of 17-4 PH stainless steel.
4.7 SLM Emulation Results
The Nd-YAG laser was used to fuse powder on a loose powder bed in both
atmospheric and argon environments. The difference between the two environments can
be clearly seen in Figure 4.26 where the left image (a) is in air and the right image (b) is
in argon. The localized sampling, a technique which would give a powder sample with a better known thermal history, was done valley and peak locations specified in Figure
4.26(a). The only sample available for analysis was produced in atmospheric conditions
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and therefore not truly representative of what occurs in an industrial SLM machine.
However, in Figure 4.26(c), both the valley and peak locations when imaged in the SEM
exhibited incomplete fusion and balling for the powder exposed to the laser. The powder which did get fused showed a considerable amount of oxide, most of which Figure
4.26(d) shows spalling off. The spalling may also be a result of mechanical pressure applied to the oxide when the SEM stub was applied to the surface. The spalled particle was mapped using XEDS shown in Figure 4.27 which revealed an oxide heavy in iron and a Ni-rich metal under the spalled oxide.
The main reason for sampling these localized regions around the sintered shape was to look for any effects of partial melting of particles close to the melt due to solid state sintering. None of the observed particles were seen to have joined that were not part of the observed balling effect, but the since the gear itself was not completely fused it is difficult to determine. Apart from the particle fusion, there is also evidence of spatter. It is visible as larger, darker particles on the grey background surrounding the gear shape in
Figure 4.26(a,b) for both the air and argon cases; air causing significantly more. Another irregularity that was noticed had to do with the way the laser rastered compared to an industry SLM machine. The top of the gear shape showed considerably more fusion than the rest of the body, a result of the linear rastering pattern returning the laser to the next line quicker than the wider parts of the gear.
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Figure 4.26 Optical images of powder fused in air (a) and argon (b). Powder from the gear fused in air at low magnification (c) and high magnification (d).
Figure 4.27 XEDS map of the oxidized surface of 316L stainless steel fused in air.
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Chapter 5: Discussion
5.1 Overview
The discussion section gives an explanation of the physical, chemical, and microstructural experimental results in addition to how the different sections effect each other, if applicable. The different sections include the analysis of the 316L stainless steel cubes built via AM and the effects of thermal cycling or reuse for 316L, 17-4 PH stainless steel, and Nylon 12. A numerical model is created to describe the observed particle growth phenomenon and its effect in shifting the particle size distribution.
5.2 316L Pre-Built Cubes
While the cube production was performed strictly to determine process parameters used in the parent America Makes project, the results from this initial study still produced important results. As Figure 4.1 shows, there is not a trend with anisotropy and density since both high and low density cubes can provide uniform or irregular builds. It should be noted, however, that if one dimension was significantly longer or shorter than the 10 mm goal, the other side was correspondingly long or short. This suggests that even if some process parameters yield parts larger or smaller than the design, the remaining dimensions are likely to change comparatively.
For this work, the cubes with the highest relative density were selected for further examination in section 4.5 since the process parameters used were most probable in typical SLM machines during a build. The built cubes also served as perfect specimens for determining the effects of partial melting of powders since part growth was evident on the surface. The cross-sectioned and etched 316L cube in Figure 4.2, even with high
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relative density, also had partially melted particles. The evidence of these features led to
the further examination of the microstructure which is described further in section 5.6.
5.3 Thermal Cycling Effects on Nylon 12 Process Window
The thermal process window for Nylon 12 SLS is important for controlling mechanical properties of parts, but a varying window would also affect the consistency of the operating procedures. Ideally, a SLM machine is preheated to the same temperature for every build, regardless of what Nylon powder is used. If the processing window were to shrink, there is a possibility that the powder may no longer be held at the right conditions. The thermal cycling in the UND rig did not significantly affect the processing window of the Nylon 12 powder. The reuse data in Table 4.1 is graphed in Figure 5.1.
The initial annealing process during the creation of the cake bake increased the processing window from 31°C to 37°C, however, when the annealed powder was blended with additional virgin powder the window decreased again to a middle value of 33°C. The overall linear trend is downward with a slope of -0.4, meaning that for every time the powder is preheated and then allowed to cool that processing window may shrink by
0.4°C. While this value could be larger if the heat input from the laser is considered, the number of times a single powder population could be used repeatedly without the introduction of new blended powder would be relatively low.
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NDU Rig Effect on Processing Window 40 35 30 25 20 Linear Slope, m = -0.4167 15
Temperature (C) Temperature 10 5 0 0 2 4 6 8 10 Number of thermal cycles
Figure 5.1 Nylon 12 processing window trend as a function of reuse
5.4 Reuse Effects on Particle Size Distribution and Particle Shape
The samples measured for their distributions of Nylon (Figure 4.5) and 17-4
(Figure 4.10) stainless steel were both thermally cycled powder from UND or used in an industrial SLM machine. The Nylon 12 powder was strictly contained to UND’s thermal cycling rig so there was no expected shift in the PSD between samples except for the
3Dsystems powder which was from a different supplier. The cake bake sample, although it had a narrower distribution shifted slightly to the right from the virgin and 50/50 blend, the average size was very similar. Since the 50/50 blend and the virgin were nearly identical, the change in the distribution shape was attributed to low sample number instead of an actual variance. The 17-4 stainless steel, similar to the Nylon 12 powder, was thermally cycled in the UND rig and showed no variance between the samples besides that of the DePuy Synthes powder since it was from a different batch. The lack of
92 change in the PSD for both Nylon 12 and 17-4 PH stainless steel powder used in the thermal cycling rig was expected since there was no causation for change inside the rig.
The sieved and retained 17-4 PH stainless steel powders, with the latter being too difficult to get sufficient size analysis completed on, showed significant changes in both particle size and shape in Figure 4.9. A magnified example in Figure 5.2 shows one of the retained particles and the joining of multiple particles to form a much larger agglomerate. The necks which are formed between the individual particles are mechanically binding and increase the effective size, therefore no longer allowing the particle to fall through the sieve. The reason the thermal cycling rig at UND was ineffective at recreating the shifts observed in the PSD like those used in SLM machines could be the result of a lack of such particle fusion. The causes of this fusion was expected to be a result of either solid state fusion which takes place near the built part due to high local temperatures or molten spatter which is ejected from the melt pool. As the results from the SLM emulation and the microstructural characterization suggest, spatter is much likely to be the root cause.
Figure 5.2 Example of an agglomerate made up of fused 17-4 stainless steel particles.
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Since the 316L stainless steel powder was not cycled in the thermal cycling rig and was only studied during the use in a SLM machine, the measured distributions gave a better understanding on how and why the PSD could shift. Figure 4.7 shows that the
316L stainless steel powder, as its reused, shifts the PSD right and the mean diameter increase from virgin to 1 use to 4 uses. The retained powder, which is already shifted to the right because of the sieving process, shifts even further from 1 use to 4 uses in addition to becoming wider. Although a 75 μm sieve is used to remove the large particles from the population, smaller particles still make up parts of the retained powder which could be the result of inadequate sieving after a build. The measured PSDs for virgin, 1 use sieved, and 4 use sieved 316L material were also measured using automated particle size analyzers at SCM Metals and their results showed lower average diameters at values of 26.2 μm, 27.3 μm, and 34 μm, respectively. Although the manually measured CED values are larger, a result of probable particle segregation during sampling, the trends are consistent with those produced by SCM.
The trends in the PSD are reinterpreted in Figure 5.3 which shows the relative volume fraction of fines (CED < 15 μm), average (15 μm < CED < 50 μm), and coarse
(CED > 50um). As the powder is used the fines begin to disappear and the coarse particles begin to increase, especially those in the retained samples. The disappearance of the fines is especially intriguing because, if the powder is uniformly distributed on the powder bed, the volume fraction of each subclass of particles should decrease in a linear fashion. One possible cause of this, as reported by Sutton, is removal of fine particles from the population as a result of selectively melting particles of a given diameter [41]. If a build layer thickness is smaller than that of the maximum particle size, there will be
94 preferential deposition of finer particles as the larger particles will continue to be pushed off the build platform. As the fines become fused into a built part they are then removed from the distribution but the larger particles are reincorporated into the remaining powder.
As the PSD shifts to larger particle diameters which approach the build layer thickness, the larger particles will make the flowability of the powder decrease and reduce the uniformity of powder packing on the build platform.
In addition to the 316L stainless steel PSD measurements, the particle shape descriptor, circularity, was calculated from the CEDs produced from the SEM images.
Figure 5.4 shows the average circularity value as measured manually for each 316L stainless steel sample which shows no statistical difference between them. When the circularity and CED for each particle are plotted against one another as in Figure 5.5, it can be argued that a trend exists as a function of reuse where the particles are becoming less spherical overall in addition to having larger average diameters. The blue vertical lines through each set of graphs is at one diameter and the scatter plot can be seen to shift in reference to it as the powder is reused. The data in Figure 5.5 was obtained by SCM
Metals (left) as well for comparison which also exhibits a similar trend.
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Particle Classification of 316L Powder by CE Diameter Virgin 1 Use Sieved 4 Use Sieved 1 Use Retained 4 Use Retained
8.33% 2.25% CED < 15 um 0.00% 0.55% 2.23% 88.10% 85.96% 15 um < CED < 50 um 65.42% 58.01% 35.75% 3.57% 11.80% CED > 50 um 34.58% 41.44% 62.01%
Figure 5.3 Volume fraction of fine, average, and coarse particles in different 316L stainless steel samples.
Particle Circularity of 316L Stainless Steel Powder Virgin 1 Use Sieved 4 Use Sieved 1 Use Retained 4 Use Retained
1
0.8
0.6
0.4
0.2
0 Circularity (0-1)
Figure 5.4 As measured average circularity for 316L stainless steel powder samples
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Figure 5.5 Measured CED vs Circularity of virgin and reuse stainless steel powder as measured by SCM Metals (left) and via image analysis (right). The blue line is for comparison between samples.
As a result of the 17-4 stainless steel powder not being used in an SLM machine to the extent that the 316L stainless steel was, the only comparison between the two individual PSDs that can be made pertains to the retained powder. The retained 316L stainless steel powder differs from the 17-4 PH stainless steel powder in regards to the amount of agglomeration and, when comparing the retained images from Figure 4.6 and
Figure 4.9 which are representative of their respective powder populations, the 17-4 PH stainless steel had significantly larger volume fraction of these agglomerates. After discussing normal SLM process parameters with DePuy Synthes, they had observed the
17-4 stainless steel creating significantly more spatter than what normally occurs during
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316L stainless steel builds. Since the retained 316L stainless steel had a smaller number fraction of the retained powder with large agglomerations, this observation suggests that increased amounts of spatter can lead to a higher probability that particles will grow into agglomerates larger than the sieve size. If the spatter size is small, this could also affect the sieved powder population if the spatter were to only join a few particles together which would retain the ability to pass through the sieve and have subsequent detrimental effects on powder flowability.
5.5 Modeling Observations from PSD Changes
To determine if the observations and assumptions that were made while completing
the work regarding particle size distribution accurately described the results, the effects
of preferentially depositing fines and fusing particles either due to spatter or a solid state
sintering method were numerically modeled. The model was written in the Wolfram
Language which is the programming language of Mathematica, the software which was
utilized. The code in its entirety is shown in Appendix E but the process and results are
explained below.
An initial data set was created based off the lognormal distribution fit parameters
for the virgin 316L stainless steel which are given in Figure 4.7. The data set is
comprised of a list of randomly generated values representing CEDs that conform to the predetermined distribution (#2) which is represented by the histogram (#1) in Figure 5.6.
Powder production sets the upper and lower limits (#3) outside of which the generated data is discarded.
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Figure 5.6 Histogram of random CED values (1) which are generated according a predetermined lognormal fit (2). Upper and lower particle bounds (3) and the powder layer thickness (4).
To incorporate the effect of preferentially depositing particles smaller than the given layer thickness a limit for the powder bed threshold was set (#4), in this case at 40
μm, above which the particles are removed from the population and set aside for reincorporation. Only the group of values under this limit would be allowed to be affected by any subsequent procedures. The next step was to “fuse” a random number of particles that were next to each other in the data set. The maximum number of particles that would be part of a single agglomerate was set to five but was variable. The total number of particles that would actually be formed into an agglomerate was another variable which was set as a given number fraction of the population.
The particles which were chosen to fuse had their volumes added and a new characteristic diameter was derived from their sum. Due to the nature of random close packing of spheres, the packing efficiency of any powder bed will not exceed that 64%
99 for monodispersed spheres and 86% for an ideal bimodal distribution [82]. If the particle agglomerate can be assumed to have its own packing efficiency of approximately 70% to account for the randomness of particle fusion, a value somewhere in the middle of two packing densities, the diameter will be adjusted accordingly by about a 40% increase.
Figure 5.7 is an example of adding the relative volumes of particles to create a particle with a new effective diameter. During the fusion process, the circularity of the new particle was also recorded.
4 = [(1.0) + (0.3) + (0.5) + (0.6) ] / 0.70 = 5.73 3 3 3 3 3 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑃𝑃𝑃𝑃 3 = 1.6 3 4 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑁𝑁𝑁𝑁𝑁𝑁 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝑒𝑒𝑡𝑡𝑡𝑡𝑡𝑡 � ∗ ≈ Figure 5.7 Adding the volumes of several individual𝑃𝑃𝑃𝑃 particles to create a single new particle to represent the fusion process observed in the retained powder samples.
After the new diameter is derived, the diameters of the constituent particles are discarded and the new, larger diameter is put back into their place in the data set. This process continues until the predetermined fraction of the powder is fused. At this point the data set is filled with fused particles which may or may not have diameters larger than what is allowed through the sieving process which is represented by the upper bound.
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The particles above that limit are removed and the particles above the powder bed
threshold are reintroduced.
The results of this numerical model are produced in Figure 5.8 and show the
effect of preferentially depositing finer particles on the powder in addition to fusing given
fraction of the powder bed. The change in the PSD is in Figure 5.8a and the change in
the circularity is in Figure 5.8b. It should be noted that the circularity values are
inclusive of the agglomerates that are larger than the upper cutoff even though they are
not included in the PSD. The volume fractions of powder to be fused was set at
increments of 10% from 0% - 30%. Although it is unrealistic to have 10% - 30% of the
powder bed fuse into agglomerates which are discarded, the values were exaggerated to
emphasize the effect. A realistic value for the 17-4 PH stainless steel build, which had significant spatter, the amount of material which ended up being taken out of the process after a build at DePuy Synthes made up about 2.5% of the powder which was used but not incorporated into the part.
The model results show the emergence of a bi-modal-like distribution which does not match the distribution shifts measured in Figure 4.7. Since the model does not accurately represent the observed phenomena there must be an additional mechanism which is controlling the shift to larger particle sizes. The fusion of particles was visually observed increasing effective particle size, but the preferential deposition of fines is only assumed. To prove the presence of this, sampling should be done in a SLM machine during a standard build at locations before, on, and after a fused part.
The circularity results from the model are consistent with the results produced by both SCM and the circularity values calculated via image analysis regarding an increase
101 in irregular particles. However, the model assumes that every particle is initially a perfect circle which is never the case. An initial distribution of circularities like that shown by
SCM’s circularity graph in Appendix F would be needed for a more accurate representation.
102 a) b)
Figure 5.8 Outputs of numerical model determine the effects of both preferential deposition of finer particles on the powder bed and the fusion of agglomerates on particle size distribution and shape.
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5.6 SLM Thermal Effects on Particle Microstructure
The crystallographic orientation maps in Figure 4.12 are used to compare the microstructure of the partially melted particles to the side of a build to that of loose virgin
316L stainless steel powder. The particles on the surface of the cube were selected to be representative of the particles that are immediately adjacent to the cube but remain unattached. The reason for this comparison is that the partially melted particles would have experienced the highest temperature gradients as a result from being located closest to the laser source but without being in the molten pool. The particles partially melted to the surface of the cube have qualitatively retained their relatively small characteristic grain size and shape from the virgin powder. This can be seen by comparing the grain size of the partially melted particles to the interior of the SLM built part where the grains are columnar in appearance. The EBSD results on the virgin powder also determined that each particle had a FCC structure even down to a diameter of 10 μm. At that diameter, the particle was single crystalline but did not exhibit a BCC structure like some authors [50],
[51], [53] had observed for various stainless steel alloys. Particles less than 10 μm were not examined so there is still a chance that our samples had a small fraction of frozen-in
BCC structure, but the fraction would be extremely small. Since the 17-4 PH stainless steel is ferritic and BCC by composition with no literature to suggest otherwise, the powder was not analyzed for change using EBSD.
There was no significant change in the internal structure of the partially melted
316L stainless steel particles at the temperatures created by the laser so the microstructure of the remaining particles in the immediate vicinity should also be unchanged. However, while this statement is true for particles experiencing large temperature gradients but
104 staying below the melting temperature, the spatter, which was observed in both the retained and sieved powder, revealed a change in its microstructure. Spatter is a phenomenon caused by the evaporation of metallic elements under the laser which creates a recoil pressure in the melt pool. The force of the pressure ejects molten metal and when it elongates, surface tension causes the liquid metal stream to break up into smaller droplets [33, 83]. The spatter will vary in size and ejection direction, and depending upon the mass of the newly created droplet as well as the recoil pressure, each droplet will be in flight for a different amount of time.
This description of spatter helps to define the observations made in the BSE images of the virgin and sieved particles in Figure 4.13 and Figure 4.14. For both the virgin
316L and 17-4 PH stainless steel the grains are irregular. The fused particles chosen from the large sieved population show a microstructure similar to that of the virgin material on one half of the agglomerate with a columnar grain structure on the second half. The columnar grains in the 316L stainless steel sample (Figure 4.13d) are oriented such that they originated from the point of contact with the original grain structure. This suggests that spatter, in a molten state, come into contact with the solid particle on the powder bed which acted as a nucleation site. The particle also must have had a significant enough undercooling to quickly solidify and maintain its spherical shape during/after impact.
The 17-4 PH stainless steel sieved cross section shows the same directional solidification but in this case two competing fronts that meet in the center of the newly solidified particle. An argument could be made that these solidification mechanisms are resultant of the original atomization process, but the chance of one molten particle simultaneously touching two already solidified particles mid-flight is extremely low,
105 especially if the already solidified particles are larger than the newly formed particle since larger particles take longer to solidify during atomization.
To show that the grain formations in the sieved powder were not just an artifact of the atomization process, the retained 17-4 PH stainless steel particle in Figure 4.14 (e-f) was cross-sectioned. If similar formations were present in the retained powder, a population which removed prior to use in a SLM machine, they would have to be the result of spatter forming agglomerations. While there is a difference in the microstructure under higher magnification (Figure 4.15b), the retained powder is not a complete match.
While it is still expected that spatter created the agglomeration, the possible reasoning behind the lack of directional solidification is related to the temperature at which the molten particle hit the powder bed. If the molten drop was still at a relatively high temperature, the molten metal could spread out over a group of particles before solidifying at a normal rate resulting in a more uniform microstructure.
The type of spatter and where it should be located on the powder bed in relation to the laser path should be predictable depending on the mass and time of flight for a given molten particle. Smaller spatter particles should always solidify before fusing to the surrounding particles due the quick solidification time unless the ejection path is extremely short. The larger the particle the further away from the melt pool it can travel while still being in a molten state. If the, for example, the molten masses responsible for fusion in Figure 4.13(d) and Figure 4.14(e) were the same material and mass, Figure
4.14(e) must be located closer to the build than that of Figure 4.13(d) because it had not lost enough heat yet to prevent the fluid flow. This effect could be measured easily using the localized sampling technique and the SLM emulator.
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Some of the additional SE images in Appendix C show examples of virgin powder with fused particles showing evidence of more collision-like formations of a solidified and molten material, with the smaller particle always maintaining its shape. However, the majority of the particle agglomerates were seen in the powder samples that were previously used and did not originate from the atomization process.
The 316L spatter microstructure observed here differed from that observed by that of Simonelli [32] in the sense that there were no equiaxed grain sizes, only irregular or columnar. Liu [33] had similar findings of spatter, generally larger, falling onto a powder bed having smaller particles adhere to it. The particles in this study showed both large and small particles resulting in spatter. Liu also produced an image of 316L stainless steel spatter which looked similar to that of the examined 17-4 PH stainless steel; the 316L retained powder had relatively few agglomerations in this study. Liu did not state the parameters used during the creation of the spatter, but if larger energy inputs were used the volume of spatter created could be increased due to the increase in the amount of material exceeding the vaporization point.
The increase in spatter can also account for the increased visual color difference between the used and retained powders for 17-4 PH stainless steel. Using Figure 4.11 for a reference, the agglomerates are made up of particles of different colors which strengthens the argument for spatter leading to the agglomerated particles. If the particles were mostly joined via a solid state sintering method, the agglomerates should be, more or less, the same color since the thermal gradients should be closer. The spatter will vary in size and ejection direction from the melt pool in, so the longer the particle is in a molten state and in flight, the thicker the possible oxide film.
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5.7 Reuse Effects on Particle Surface Chemistry
The EBSD results determined that changes in the surface chemistry may be more
important regarding reuse since the interior structure remains unchanged for the majority
of the reused powder aside from the spatter. The surface chemistry would change with
regard to oxide composition and thickness due to the possible increased thermal
exposure. The results are discussed separately for both 316L and 17-4 PH stainless steels.
5.7.1 316L Stainless Steel
The majority of the particles in the larger end of the distribution for every sample tended to have dendritic surface features and oxide formations while the smaller particles were smoother and did not have discrete external oxide formations. The lack of surface features and oxides results from the increased cooling rates for the smaller particles kinetically hindering the chemical segregation responsible for the formations. In accordance with literature [47], the larger 316L stainless steel particles exhibited both the dendritic formations and oxides in the virgin, used/sieved, and retained powders. This is highly suggestive that the formation of these features are resulting from the gas atomization process. The retained powder shows an increase in the amount of particles exhibiting these features so the spatter, during its creation and solidification, acts similarly to that of the gas atomized powder.
The XEDS composition results of a particle surface in Table 4.5 show significantly increased oxygen, silicon, and manganese content at the discrete external oxidation locations with a respective decrease of the main 316L alloying elements. These
observations would support the rapid diffusion of Mn and Si to the surface as previously
shown [46]. The spectrum from the discrete oxide-free surface shows the composition of
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the particle falling within the standard ranges as expected. The film-like oxide has the same elemental changes as the protruding oxide, but due to the oxide being so thin the underlying base metal affected the spectrum results and larger amounts of Cr and Ni were detected. Also suggesting this is the Mo also being present in the thin but not the thick oxide. When converting the measured weight percentages of spectrum 3 to atomic percent, the protruding oxide had approximately 65 at% O, 12 at% Si, 5 at% Cr, 11 at%
Mn, and 7 at% Fe. The ratio of Mn:Si:O suggests that the oxide can be a manganese silicate, MnSiO2. Simonelli [32] cross-sectioned a similar protruding oxide on 316L
stainless steel spatter but found the observed oxide is mostly Mn in a non-oxide form
with a Si oxide coating on the exterior.
The XEDS map taken during the FIB cross-sectioning as mentioned in section
4.6.1 showed a similar Mn-rich oxide but this time with no apparent Si and the introduction of Al, possibly as aluminum manganate. The lack of Si is surprising since
general surface spectrum like that in Figure 4.18 show the only element to clearly be in
an oxide form is Si. All of these varying results signify that there may not be one
consistently forming oxide in terms of composition and could be directly related to
particle size and subsequently how long the particle is molten regardless if it is spatter or
gas atomized. The cross-section, again due to the thinness of the oxide, did not show the
layered oxide as suggested by prior work [50], [58] and confirmed the high cooling rate
of gas atomization stops macrosegregation of the elements inside the particle [42], [56].
The Auger spectra in Figure 4.20 show both measured oxides having ratios of
Si:O of about 1:3 with trace amounts Fe and Mn. The AES spectrum on the apparent
particle surface away from any external oxides had the largest concentration of Fe and O
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with traces of Si and Mn. If these oxides are of the same composition of those examined
by Simonelli [32], it is reasonable to believe that the Mn is low only because the top
surface is a Si oxide and on the order of a few nm.
The understanding of the composition of a single oxide formation on any given
particle is important because it only takes one unintended defect in the form of an
inclusion inside the AM built part to affect mechanical properties such as tensile strength
and fatigue life. However, the XPS provided a more general result in terms of the effect
of reuse and the formation of spatter for a given powder population. The near surface for
both virgin and retained material was composed of O, Fe, Mn, Ni, Cr, ad Si. The Mn and
Si were also picked up during the AES spot spectrum of the discrete oxides suggesting that was the origination of the signal. The Mn was present on all the samples, most likely since it is heavily present in discrete oxides but the distinction between a di-, tri-, or tetra- valent state could not be determined due overlapping peaks around 641 eV. While the virgin powder was known to have Si oxide on the surface, a peak not easily discernable from the signal noise while the used and retained material showed strong peaks. With an increasing amount of spatter, the likelihood of used powders obtaining a Si-rich particle is increased.
The observed peak shift in the Cr and Fe metal peaks both suggest that the surface oxide film that surrounds the Mn/Si discrete oxides is growing in thickness since the oxide peak strengthens while the metal peak weakens. The large sieved powder peaks resemble that of the virgin powder for the Fe and resembles the retained powder for Cr with respect to the peak locations. This suggests that Cr is more likely to oxidize first since the large sieved sample, while possibly still containing unaffected powder, has
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smaller diameter particles which would solidify at a quicker rate as described in section
5.6. The increase in the Cr and Fe oxides can also be the reason for the Ni peak
disappearing. Another reason for the disappearance of Ni is the fact that the virgin and
sieved powder has smaller, non-spatter particles which, if the particle solidifies quick
enough, could prevent significant segregation of the oxidation prone materials such as Fe,
Mn, and Cr while the particle is still liquid. The results of the XEDS cross-sections as
well as the XPS scans would indicate that the oxide thickness for the large sieved 316L
stainless steel, while additional Cr oxide was observed, it is still not detectably larger than
the 2-5 nm oxide described in literature [57]. Since the large particles from the sieved powder show insignificant oxide growth, the remaining population should be in a similar if not better condition.
5.7.2 17-4 PH Stainless Steel
The 17-4 PH stainless steel had the same variation of smooth and dendritic surface features exhibited by the 316L stainless steel which were resultant of the gas atomization process. The SE imaging of the surface showed no protruding oxides unlike the 316L and the quantitative XEDS spectrum of the surface and bulk in Table 4.6 show relatively consistent composition although the bulk scan showed a slight enrichment in
Cr. Since there was no distinct protruding oxide AES was not used, but the visual variation in color suggested that the oxide was changing.
The initial XPS scans of the 17-4 PH stainless steel surface showed the highest
concentrations of O, Fe, Cu, Mn, Si, and Cr. Both the Mn and Cu peaks do not shift
although the atomic fraction decreases as the powder is reused for both. The oxygen peak
is once again larger for the retained powder suggesting more oxide is present. The Fe and
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Cr peaks both show two peaks in the virgin powder with the metallic peak being larger, and as the powder is used, the area under the oxide peak grows and the metallic species shrinks. The specific chromium species could not be determined because of a wide peaks, but it is suspected to be trivalent which was confirmed by the literature for 316L stainless steel [57]. The large sieved powder was in a middle-state in terms of oxide thickness since the amount of virgin material being detected reduced as a function of reuse.
Although not completed for the 316L, the 17-4 PH stainless steel was used to determine if the change between the virgin, large sieved, and retained powder was significant in Figure 4.23. The XPS depth profiling results clearly showed a difference between the virgin, sieved, and retained powders with the retained powder having a considerably thicker oxide. A quantitative determination on the actual oxide thickness and relative oxygen composition could not easily be made. The reason for this being that the sputter rate was not calibrated in addition to the particles being spherical; the angle of
Ar+ ion incidence will vary and therefore sputter away different locations of the particle surface at different rates. Therefore a full width half maximum approach was used to compare the curves produced in Figure 4.23, the values representing how long the sputter took to reduce the oxygen value by half. The times as measured are approximately
4, 6 and 30 minutes for the virgin, used, and retained powders, respectively. Taking these values for relative oxide thickness, the retained powder is approximately 5-7x thicker.
The cross-section of the large sieved powder particle in Figure 4.15(a) showed no discernable of a visible oxide film at the given resolution. However, the cross-sections of the retained powder in Figure 4.24 and Figure 4.25 show visible oxide layers. The non- spherical cross-section had an oxide thickness of ~ 50 nm while the spherical particle has
112 an oxide thickness of ~ 150 nm. The oxides are both composed of mainly Fe and O with no other obvious element segregation. While both a substantially thicker than the virgin and sieved powder, the reason for the difference between the two is related to the time that the retained powder (spatter) was in flight after being ejected from the melt pool as mentioned in section 5.6. The spherical particle was molten long enough to both spheriodize and solidify before impacting the powder bed which is assumed because there are not particles fused to the surface. The non-spherical spatter particle hit the powder bed while it was still in a molten state which, due to the increased thermal conductivity of the now adhered particles, was solidified quicker and therefore not allowed to grow as thick of an oxide. The effect of this varying oxide thickness was previously discussed in section 5.6 due to the introduction of different interference patterns and the resulting variations in color for the retained powder.
While the oxide thickness on the retained powder is many times thicker than that of the virgin material, it removed from the population and is not a risk to further AM builds. The large sieved powder, although a shown to have a slightly thicker oxide via
XPS and the depth profiling, the difference in oxide thickness should not be enough to prevent sufficient melting of the particles during later uses. For both the 316L and 17-4
PH stainless steels, the change in surface chemistry is not considered to be sufficient enough to cause adverse effects in subsequent uses. The larger issue is the observed shift in the particle size distribution and its effect on flowability.
5.8 SLM Emulation Discussion
The principal reason for building an experimental setup to emulate the environment a SLM machine imposes on the powder feedstock was for the inability to freely access a
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real machine for this study and reasons regarding powder sampling. For the chemical,
physical, and microstructural analysis explained above, as issue of statistical accuracy is
present due to the nature of the powder and the process. If a build takes up approximately
20% of the volume in a powder bed, only around 0.5% of the remaining powder which is
not fused into the part will be thermally affected to the point where particle
characteristics may be expected to change (calculation in Appendix G). The chances of randomly selecting a given particle that has a known thermal history are quite small. For this reason, the ability to sample in locations of known thermal history is advantageous.
For both cases, fusion of 316L stainless steel in air and argon, the calculated process parameters were set to mimic the volumetric energy input of the SLM used powder and a balling effect was observed resulting in a non-uniform layer of fused material. The balling effect could have been worsened by not having a low enough oxygen partial pressure [84] or by having an insufficient volumetric energy input [85].
The SEM image of the fused particles in Figure 4.26c is similar to that produced by Gu
[85] when the powder layer thickness was too large. Unfortunately, due to complications with the computer which operated the laser controlling software, the material was not able to be fused to the base metal as planned which would have reduced the powder layer thickness to 150 μm, a thickness which produced reasonable continuous fusion. For the same reason, the only material which could be used in SLM emulator was the 316L stainless steel and the localized sampling could only occur on the part built in air.
The sampling of the powder as a function of position away from the fused particles was successful, but there was no evidence of partially fused particles aside from those created from the laser beam. Although there was no direct evidence of this occurring,
114 there may be two possible explanations. First, because the part under the laser did not create full continuous layer, smaller agglomerates which may be a result of particle fusion are masked by the balling affect. Secondly, since the fusion was done on a loose powder bed, the melted particles had no distinct lower boundary and also melted particles beneath the desires layer. This made the fused parts of the gear thicker and subsequently out the same plane as the surrounding powder bed which, when sampled, did not allow for easy sampling since the carbon tape was now at an angle and missing the particles closest to the fused material. This was not expected since the melting of the powder would reduce the overall density of the material and make it occupy less volume.
Although the SLM emulation was not able to fully function due to computer issues, there are several advantages to creating a SLM emulating environment as opposed to using an industry standard machine. First, the emulation is an extremely cost and time effective since there is no need to for ownership of a SLM machine or the need to purchase hours on an outsourced machine. Secondly, the ability to quickly change operating conditions including powder bed temperature, atmosphere, and volumetric energy input allows for extensive studying of additional variable effects beyond the scope of this work.
Chapter 6: Conclusions and Future Work
The importance of this work is strongly tied to the need for cost reduction of the additive manufacturing process. Recycling materials back into the supply chain reduces the need to purchase more materials but introduces the risk of incorporating residue from previous builds which can have detrimental effects on the AM process. The results of this work showed the following:
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- Thermal cycling to pre-heat temperatures show negligible effect on the Nylon
12 processing window and surface chemistry for the stainless steels.
- The particle size distribution for each material shifted to larger mean sizes as
powder gets used; a result of spatter from the melt pool joining smaller
particles from the powder bed. The spatter was the main factor for the shift
and the sieving process did not capture all the fused particles thereby allowing
some back into the population.
- The observations of the PSD shift were numerically modeled. The model
produced bi-modal-like distributions instead of a simple lateral shift to larger
particle sizes as in the experimental data. Another factor for a change in PSD
may be involved that was not determined or observed in these results. Unlike
the experimental data,
- The particle surface features and oxides, mainly Mn and Si-rich for 316L
stainless steel, originated from the gas atomization process and are reproduced
in spatter particles to a larger extent. The stainless steel oxide films became
thicker but were not large enough to be measured using XEDS for virgin and
sieved powder, therefore a change in composition should not be a factor
during the reuse of 316L and 17-4 PH stainless steels.
- The thermal gradients produced by the laser are not large enough to change
particle microstructure on their own and microstructural change only occurred
for resolidifying spatter particles.
Further XPS depth profiling of different sized particles can determine if only the spatter shows an increase in the oxide thickness or if the entire used population grows an
116 oxide. The use of localized sampling in the SLM emulation near a melt pool can give results as a function of distance away from the part. To verify/change the assumptions in the model, localized PSD measurements on a SLM machine should be taken to determine if there is truly preferential deposition of fines on top of the previously melted material.
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APPENDIX A – SLM Emulation Box Design
Bottom Panel
Top Panel
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APPENDIX A Cont.
Left Side Panel
Back Panel
Right and Front Panels Will Have NO Further Modifications.
119
APPENDIX A Cont.
Standoff for 4-way Valve
¾”
Brass Fitting
120
APPENDIX B – Nylon 12 DSC Results: 10°C/min, 40°C - 240°C
121
APPENDIX B Cont.
122
APPENDIX B Cont.
123
APPENDIX B Cont.
124
APPENDIX B Cont.
125
APPENDIX B Cont.
126
APPENDIX C – Secondary Electron SEM Images of Gas Atomized 316L Powders
Virgin powder showing in-flight Virgin 316L showing particles with “collision” of gas atomized powder. satellites and fused particles from atomization.
Single use particle with no distinct 4-use sieved particle with strong surface dendritic growth pattern and a discrete relief-like patterning on the surface. oxide formation. Oxide growth central on each island.
Virgin powder showing with an elongated Particle with smaller satellite particles on shaper due to solidification before the top side. spheroidization or before further break-up.
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APPENDIX D - Compared Element-Specific XPS Peaks for 316L Stainless Steel Virgin – Blue Used/Sieved (Large) – Red Cr Retained – Green
Metal Peak Oxide Peak
Fe
Mn
128
APPENDIX D Cont. O
Ni
Si
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APPENDIX D-2 - Compared Element-Specific XPS Peaks for 17-4PH Stainless Steel
Virgin – Blue Used/Sieved (Large) – Red Retained – Green
Metal Peak Oxide Peak
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APPENDIX D-2 Cont.
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APPENDIX E - Numerical Model for Particle Distribution Shift – Wolfram Language
132
APPENDIX E Cont. – NUMERICAL MODEL
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APPENDIX F - SCM Metals Circularity Results
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APPENDIX G - CALCULATING VOLUME OF EFFECTED PWODER
Calculation of thermally effected powder based of off parameters used by DePuy Synthes o Assumptions: Powder Bed Volume: 250mm x 250mm x 15mm = 937.5 cm3 o 3 o Built Parts Volume: 184.4 cm . 18 horizontal tensile bars: 150.65cm3, L = 82mm, r = 11.4mm . 5 cubes: 16.875 cm3: 15mm length, no bottom surface . 5 other: 16.875 cm3: ~50% more surface area than cubes o Stainless steel will not be affected below 700°C for short periods of time o Using Ref. [69], powder at temperatures between Tm and 700°C spans ~50 μm away from the melt pool into the powder bed o Calculating Volume of Thermally Affected Powder Around Each Part (Adding 50um to Dimensions of Considered Parts):
( . )( . ) ( )( . ) = = 157.87 Bar: / 2 2 𝜋𝜋 82 1𝑚𝑚𝑚𝑚 11 5𝑚𝑚𝑚𝑚 𝜋𝜋 82𝑚𝑚𝑚𝑚 11 4𝑚𝑚𝑚𝑚 3 Cubes:𝑤𝑤 𝑃𝑃𝑃𝑃𝑃𝑃/ 𝑃𝑃𝑃𝑃𝑃𝑃 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = (15.1 15.1 15.05 ) (15 ) = 56.55 𝑉𝑉 − 𝑉𝑉 4 − 4 𝑚𝑚𝑚𝑚 Other: = 56.55 1.5 = 84.83 3 3 𝑉𝑉𝑤𝑤 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 − 𝑉𝑉𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑚𝑚𝑚𝑚 ∗ 𝑚𝑚𝑚𝑚 ∗ 𝑚𝑚𝑚𝑚 − 𝑚𝑚𝑚𝑚 𝑚𝑚𝑚𝑚 3 𝑉𝑉𝑜𝑜𝑜𝑜ℎ𝑒𝑒𝑒𝑒 ∗ 𝑚𝑚𝑚𝑚 o Total Volume of Affected Powder:
18 157.87 + 5 56.55 + 5 84.83 = 3.55 3 3 3 3 o Powder𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 Remaining:∗ 𝑚𝑚𝑚𝑚 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 ∗ 𝑚𝑚𝑚𝑚 𝑜𝑜𝑜𝑜ℎ𝑒𝑒𝑒𝑒 ∗ 𝑚𝑚𝑚𝑚 𝑐𝑐𝑐𝑐
= 937.5 184.4 = 753 3 3 3 o 𝑃𝑃Fraction𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝐵𝐵of𝐵𝐵𝐵𝐵 Powder𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 Affected𝑉𝑉 − 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑐𝑐𝑐𝑐 − 𝑐𝑐𝑐𝑐 𝑐𝑐𝑐𝑐
/ = 3.55 /753 = 0.0047 3 3 𝐴𝐴𝐴𝐴𝐴𝐴0.47%𝐴𝐴𝐴𝐴𝐴𝐴 𝐴𝐴𝐴𝐴 𝑃𝑃𝑜𝑜𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑐𝑐𝑐𝑐 𝑐𝑐𝑐𝑐
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