Design and Manufacturing Guidelines for Additive Manufacturing

of High Porosity Cellular structures

A thesis submitted to the

Graduate School of the University of Cincinnati

In partial fulfillment of the requirements for the degree of

Master of Science

In the Department of Mechanical and Materials Engineering

of the College of Engineering and Applied Science (CEAS)

Nikhil Kabbur

Bachelor of Engineering (B.E.) – Mechanical Engineering

S.D.M College of Engineering and Technology, Dharwad, India. – 2011

Committee Chair: Dr. Sam Anand

Abstract

Additive Manufacturing (AM) is a layer by layer manufacturing approach for building

parts. Additive manufacturing as a technology provides immense design freedom, especially in

the field of medical implant design. Revolutionary technologies such as Direct Metal Laser

Sintering (DMLS) and Electron Beam Melting (EBM) have the potential to customize metal

implants to an individual patient. To take advantage of this technology it is important to develop

rules that can transform concepts to real world designs. There are many applications that can

potentially use additively manufactured cellular structures, but there is a need to provide design

guidelines for manufacturability of such structures. This thesis explores the opportunities and

challenges in manufacturing high porosity cellular structures, and provides a measure of

manufacturability of different cellular structures and design guidelines to improve

manufacturability. Cellular structures are defined as structures made from repeating a certain unit cell to form a block, and the characteristic length of each cell is in the range of 0.1 mm to 10 mm. The additive manufacturing technique considered in this work is laser powder bed fusion process (DMLS). This research studies and tests the effect of unit cell type, unit cell size, volume fraction, and orientation on the manufacturability of the cellular structures using experimental builds. Based on these parameters the manufacturability of each cellular structure is evaluated and design guidelines are provided to design and manufacture high porosity cellular structures.

iii

Acknowledgements

I would like to take this opportunity to express my appreciation to the people who contributed, in different ways, to the completion of this work.

First, I would like to thank my academic advisor Dr. Sam for his guidance and support for successful completion of this research. I also take this opportunity to thank Dr. Murali

Sundaram and Dr. Ashley Paz y Puente for serving as members on my master’s thesis defense committee.

I would also like to thank Mr. Dustin Lindley, Additive Manufacturing Center Manager,

University of Cincinnati Research Institute for his insightful comments during the course of this research.

I would like to dedicate this thesis to my parents and my family. Thank you for the love and support.

I would like to thank my past and present lab mates who were always there to help me. I would like to thank Rohit Vaidya, Rohan Vaidya, Sushmit Chowdhury, Vinay Jakkali, and

Kunal Mhapsekar for their support.

I also would like to thank my friends and roommates Swarup Zachariah, Kalayarasan

Seranthian, Hitesh Das, and Krishna Kenja for making this MS journey a ride to remember.

Table of Contents

Design and Manufacturing Guidelines for Additive Manufacturing of High Porosity Cellular

structures ...... i

Abstract ...... ii

Acknowledgements ...... iv

Table of Contents ...... v

List of Figures ...... x

List of Tables ...... xiv

Introduction ...... 1

1.1 Motivation for Research ...... 3

1.2 Objective and Impact of Research ...... 4

1.3 Thesis Outline ...... 4

2 Literature Review ...... 6

2.1 Support Structure Minimization ...... 6

2.2 Design for Additive Manufacturing Guidelines ...... 6

2.3 Cellular Structures Manufacturing using AM...... 7

3 Methodology ...... 10

3.1 Materials and methods ...... 10

3.1.1 Powder Material ...... 10

3.1.2 Manufacturing Machine Setup ...... 11

3.1.2.1. Concept Laser Mlab cusing R Process Parameters ...... 11

3.1.2.2. Scanning Pattern ...... 12

3.1.2.3. Part File Setup ...... 15

3.1.2.4. Post Processing ...... 17

3.1.3 Measurements ...... 17

3.1.3.1. Weighing Machine ...... 17

3.1.3.2. Optical Microscope ...... 18

3.1.3.3. Vernier Calipers ...... 18

3.2 Design of High Porosity Cellular Structures ...... 19

3.2.1 Unit Cell Types ...... 19

3.2.2 Unit Cell Type, Size and Volume Fraction Characterization ...... 20

3.3 Support Structure Minimization through Optimal Orientation ...... 20

3.4 Experimental Framework to Test Manufacturability Of High Porosity Cellular Structures 21

3.4.1 Development of CAD models to Test Manufacturability of High Porosity Cellular

Structures ...... 22

3.4.1.1. CAD model for Build 1 ...... 24

3.4.1.2. CAD model for Build 2 ...... 25

3.4.1.3. CAD model for Build 3 ...... 26

3.4.1.4. CAD model for Build 4 ...... 27

3.4.1.5. CAD model for Build 5 ...... 28

4 Results and Observations on Manufacturability of High Porosity Cellular Structures ...... 30

4.1 Results of Build 1...... 30

4.1.1 Observations And Discussions ...... 30

4.1.2 The Curling phenomenon ...... 31

4.1.3 Design Rules for Part Orientation and Placement ...... 34

4.1.4 Warping effect ...... 34

4.1.5 Design Rule for Interface of Support structures and Solid part ...... 35

4.1.6 Results of modified Build 1 ...... 35

4.2 Results of Build 2 and 3 ...... 36

4.2.1 Observations and discussions based on Build 2 and 3 ...... 38

4.2.2 Effect of Cell Size ...... 38

4.2.3 Dross formation ...... 39

4.3 Results of Build 4 and 5 ...... 40

4.3.1 Effect of Orientation ...... 41

4.3.2 Design Rule for Orientation of Parts ...... 42

4.4 Effect of Volume Fraction ...... 42

4.5 Effect of Unit Cell Type ...... 43

4.5.1 Laser Beam compensation error ...... 45

4.5.2 Balling effect ...... 46

4.5.3 Residual un-sintered powder ...... 47

vii

4.5.4 Topology of unit cell ...... 47

4.5.4.1. Nodes ...... 49

4.5.4.2. Strut Interactions ...... 49

4.5.5 Design Rules ...... 53

5 Experimental Framework to Predict Safe Ranges for Overhanging Struts ...... 54

5.1 Experimental Framework and Rationale ...... 54

5.1.1 Rationale for Strut Length Parameter and Range Selection ...... 55

5.1.2 Rationale for Strut Angle Parameter and Range Selection ...... 55

5.1.3 Rationale for Strut Diameter Parameter and Range Selection ...... 56

5.2 Development of CAD Models To Investigate Manufacturability Of Overhanging Struts .. 56

5.2.1 Design Intent ...... 57

6 Results and Observations on Manufacturability of Overhanging Struts ...... 58

6.1 Visual Inspection and Results Of Manufacturability of Overhanging Struts ...... 58

6.1.1 Evaluation of Length of Strut ...... 59

6.1.2 Evaluation of Angle of Strut ...... 60

6.1.3 Evaluation of Diameter of Strut ...... 61

6.1.4 Effect of Re-Coater Damage on The Manufacturability of Struts ...... 62

6.1.5 Design Space for Manufacturability of Overhanging Struts ...... 64

6.1.5.1. Methodology for The Creation of Design Space ...... 64

6.1.6 Design Rules ...... 65

viii

6.2 Dimensional Measurements and Results of Manufacturability of Overhanging Struts ...... 66

6.2.1 Results and Observations ...... 66

6.2.2 Characterizing the Effect Of Beam Compensation on Oversizing of Successfully Built

Struts 67

6.2.3 Characterizing the effect of dross formation on oversizing of successfully built struts

6.2.4 Design Rules ...... 72

7 Design and Manufacturing Guidelines Summary ...... 73

8 Conclusions and Future Scope ...... 74

References ...... 76

List of Figures

Figure 1: Setup for DMLS Process [2] ...... 1

Figure 2: Example of a 3 Dimensional cellular structure [4] ...... 2

Figure 3: Concept Laser Mlab cusing R [28]...... 11

Figure 4: Representation of the occurring stresses and deformation during sintering and cooling down as explained by the TGM and cool-down phase models [30] ...... 13

Figure 5: Uni-directional scanning strategy used in DMLS [31] ...... 14

Figure 6: Bi-directional scanning strategy used in DMLS [31] ...... 14

Figure7: Island scanning strategy[32] ...... 15

Figure 8: Effect of slice thickness on Additive Manufacturing ...... 16

Figure 9: Effect of beam compensation on manufacturing thin parts [34] ...... 16

Figure 10: Denver PI-214 Analytical semi-micro balance specifications and photograph [35] ... 17

Figure 11: Keyence Digital microscope specifications and photograph [36] ...... 18

Figure 12: Digital Vernier calipers specifications and photograph [37]...... 18

Figure 13: Types of unit cell (a) Basic Cubic structure (b) BCC structure (c) FCC structure (d)

BCC plus FCC structure ...... 19

Figure 14: BCC structure with handling platform and support structures ...... 23

Figure 15: CAD model of Build 1 ...... 24

Figure 16: CAD model of Build 2 ...... 25

Figure 17: CAD model of Build 4 ...... 26

Figure 18: CAD model of Build 3 ...... 27

Figure 19: CAD model of Build 5 ...... 28

Figure 20: Photograph of scorching defect observed during Build 1 ...... 31

Figure 21: Curling of side walls due to residual stresses induced by laser based process ...... 32

Figure 22: Interaction of re-coater with the side walls of the previously sintered layer ...... 32

Figure 23: Reduction in area of interaction between the re-coater and previously sintered layer with change in orientation of part ...... 33

Figure 24: Warping of thin walled support structures ...... 35

Figure 25: Redesign of CAD model of Build 1 ...... 35

Figure 26: Successful build after redesign of CAD model of build 1 ...... 36

Figure 27: Actual build result of build 2 with catastrophic strut failure defect highlighted ...... 36

Figure 28: Successful build of Basic Cubic structures for build 3 ...... 37

Figure 29: Actual build result of build 3 with catastrophic strut failure defect highlighted ...... 37

Figure 30: Catastrophic strut failure observed in a FCC structure ...... 39

Figure 31: Dross formation in overhanging structures [42]...... 39

Figure 32: Scorching of downward facing surfaces ...... 40

Figure 33: Actual build results of build 5 with catastrophic missing sections highlighted ...... 41

Figure 34: Comparison of error in manufacturing for all unit cell types and volume fractions for

1 mm unit cells ...... 42

Figure 35: Error in strut diameter for BCC structure ...... 43

Figure 36: Error in strut diameter for Basic Cubic, FCC, and BCC+FCC structure ...... 44

Figure 37: Effect of laser beam compensation on dimensional accuracy ...... 45

Figure 38: SEM images a) Balling defect observed on sintered surfaces b) magnified image of section highlighted in (a) to show balling [46] ...... 46

Figure 39: Laser beam error during sintering of contours with sharp corners ...... 48

Figure 40: Geometrical differences for 1 mm FCC structure a) Designed geometry of sharp

corners b) Manufactured geometry of sharp corners ...... 48

Figure 41: Node in a basic cubic structure...... 49

Figure 42: Strut interactions in a basic cubic structure ...... 49

Figure 43: Types of nodes and node interactions ...... 50

Figure 44: Summarizing the error due to unit cell topology for successfully built high porosity

cellular structures with volume fraction of 5% ...... 52

Figure 45: CAD model for 30° strut build experiment ...... 57

Figure 46: Orientation of the prismatic column to the re-coater movement direction ...... 57

Figure 47: Actual build results of overhanging struts ...... 58

Figure 48: Build results of overhanging struts built at 0° with respect to the build plane with strut

lengths of 3 mm and less highlighted...... 59

Figure 49: Build results of overhanging struts built at 30° with respect to the build plane...... 60

Figure 50: Build results of overhanging struts built at 0° with respect to the build with failed strut

diameters of 0.025, 0.05, and 0.075 mm highlighted ...... 61

Figure 51: The effect of re-coater damage on the experimental strut build ...... 62

Figure 52: Progressive strut deformation in 5 mm length struts with varying angles due to re- coater damage ...... 63

Figure 53: Unique configurations of successful built struts for 10° and 15° strut builds

highlighted ...... 64

Figure 54: Design space for manufacturability of overhanging struts ...... 65

Figure 55: Effect of over-sizing error due to sub-optimal laser scanning parameters v/s length of

strut ...... 67

xii

Figure 56: Effect of over-sizing error due to sub-optimal laser scanning parameters v/s diameter

of strut ...... 68

Figure 57: Effect of over-sizing error due to sub-optimal laser scanning parameters v/s angle of orientation of strut ...... 68

Figure 58: Effect of dross formation over-sizing error v/s length of strut ...... 69

Figure 59: Effect of dross formation over-sizing error v/s diameter of strut ...... 69

Figure 60: Effect of dross formation over-sizing error v/s angle of orientation of strut ...... 70

Figure 61: The relation between overhang surface area to change in diameter, length and angle of

orientation of strut ...... 70

xiii

List of Tables

Table 1: Chemical composition of CL 20ES as provided by Concept Laser [27] ...... 10

Table 2: Processing parameters used for the experiments ...... 12

Table 3: Optimal orientations for Basic cubic structure, BCC, FCC, and BCC plus FCC unit cell types ...... 21

Table 4: Experimental framework to test manufacturability of high porosity cellular structures 21

Table 5: List of structures modeled for Build 1 ...... 24

Table 6: List of structures modeled for Build 2 ...... 26

Table 7: List of structures modeled for Build 4 ...... 27

Table 8: List of structures modeled for Build 3 ...... 28

Table 9: List of structures modeled for Build 5 ...... 29

Table 10: Analysis of sharp corners for successfully built high porosity cellular structures ...... 51

Table 11: Experimental framework to test manufacturability of Overhanging struts ...... 54

Table 12: Measurements of diameters for successfully built struts ...... 71

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Introduction

Additive Manufacturing (AM) is the process of manufacturing a part by adding material layer by layer. Due to the immense design freedom offered by the process, it is gaining wide acceptance as the manufacturing process for custom and complex geometries of aerospace parts, automotive components, and medical implants [1]. To manufacture high strength parts, one of the leading additive manufacturing technologies adapted by the industry is Direct Metal Laser

Sintering (DMLS). DMLS is a process in which a layer of metal powder is deposited on a platform by a re-coater arm and a laser is used to sinter the layer of metal powder. After sintering, the platform is lowered by the predetermined layer thickness and a fresh layer of powder is deposited by the re-coater arm that is subsequently sintered by the laser. This process is continued until the complete part has been sintered layer by layer. Figure 1 depicts a schematic explaining the DMLS process [2]

Figure 1: Setup for DMLS Process [2]

Cellular structures are structures made up of interconnected network of solid struts or

beams that form the edges and faces of the structure [3]. The two common types of cellular

structures are stochastic cellular structures and non-stochastic periodic cellular structures. This

research focuses on 3-D non-stochastic periodic cellular structures. 3-D non-stochastic periodic cellular structures are a class of cellular structures based on repeating 3-dimensional unit cells along cartesian co-ordinates. Figure 2 depicts an example of a 3-D periodic cellular structure [4]

Figure 2: Example of a 3 Dimensional cellular structure [4]

1.1 Motivation for Research

With the recent advances in using biocompatible metals in additive manufacturing, there

is great potential to customize implants according to specific patient needs. Currently the medical

industry uses dense metallic implants that are heavier than bone, have a limited lifespan and causes the bone to weaken due to the stress shielding effect. Porous implants with tailored mechanical properties similar to bone would promote bone growth and would have longer lifespans. To mimic bone, metallic implants would have to be designed with porosities as high as

90% due to the relative difference in stiffness [5]. Periodic 3D cellular structures [3] with low volume fractions provide an elegant design solution. This study investigates the design and manufacturability of structures with volume fractions less than 25%.

Despite its many advantages, one of the biggest limitations of additive manufacturing is the need for support structures for overhanging surfaces. Due to the inherent layer by layer nature of the process, surfaces with angles less than 35° require support structures [6]. Support structures also dissipate heat from the newly created melt pool and prevent distortion due to the thermal stresses [7]. After the manufacturing is complete, these support structures have to be removed and hence the accessibility of support structures restricts the design freedom. In addition, support structures affect the part quality, build time, and cost of post processing.

The application of cellular structures in the medical field leads to small sized structures with high porosities [8]. Due to their design intent, cellular structures need to be manufactured without the use of support structures. To tackle the process limitation and promote the adoption of additively manufactured high porosity cellular structures, the focus of this research will be to investigate the manufacturability of these structures. The aim is improve the manufacturability

3 and provide design guidelines to build high porosity cellular structures without the need for support structures. The design guidelines would provide the designers with the confidence to create, use, and tailor high porosity cellular structures to specific applications.

1.2 Objective and Impact of Research

This research proposes investigating the manufacturability through multiple experimental builds of high porosity cellular structures with various cell types, sizes, volume fractions and orientations. In this study, high porosity cellular structures are defined as cellular structures with volume fractions less than 25%. The experimental builds examine the effect of cell type, size, volume fraction and orientation on the manufacturability of high porosity cellular structures without support structures. Based on the evaluation of the built high porosity cellular structures, design guidelines are developed to improve manufacturability of high porosity cellular structures. Additional experiments investigate the manufacturability of overhanging struts and provide a design space for safe overhangs. The design guidelines and design space would aid the designer to adapt or create high porosity cellular structures optimized for manufacturing using

AM.

1.3 Thesis Outline

The first chapter of this thesis briefly discusses DMLS technology, cellular structures, motivation of this research, and the objective of this research. The second chapter provides a brief literature review of the research carried out in the field of support structure minimization,

Design for Additive Manufacturing (DFAM) guidelines and the manufacturing of cellular structures using Additive Manufacturing. In the third chapter, materials and methods used for manufacturing are discussed. This chapter also explains the design of the high porosity cellular

4 structures and the experimental framework to test manufacturability of the designed high porosity cellular structures. The fourth chapter discusses the results and observations of manufacturability of high porosity cellular structures and based on the observations design guidelines have been suggested. This chapter also discusses the need for investigating the manufacturability of overhanging struts and identifying the process specific challenges. The fifth chapter details the experimental framework to identify geometric constraints for manufacturing self-supporting overhanging struts using AM. The sixth chapter discusses the results and observations of manufactured self-supporting struts and identifies the geometrical constraints.

Based on the geometrical constraints and process challenges, design guidelines have been proposed. This chapter also discusses the reasons for failures and defects observed during the manufacturing of overhanging struts without support structures. In the last chapter, conclusions and future scope for this research are highlighted.

2 Literature Review

2.1 Support Structure Minimization

Many attempts have been made to evaluate the criteria for support structures and

minimize the volume of support structures by changing the orientation of part.

Kulkarni et al. [7] investigated the need for support structures to stabilize the melt pool created during additive manufacturing using support structures. They also identified the effect of

support structures on the part quality. Cloots et al. [6] identified that surfaces which are aligned

at angle less than 35° with the horizontal build plane require support structures.

Allen and Dutta [9] developed and implemented an approach based on the convex hull to

identify the optimal orientation to manufacture parts with minimum support structure volume.

Paul and Anand [10] presented a graphical method to identify the optimal orientation of a part

with reduced overall support volume which satisfies the cylindricity tolerance for the part.

2.2 Design for Additive Manufacturing Guidelines

Thomas et al. [11] identified the process specific challenges for SLM and developed

design rules to relate part orientation and surface. They also proposed modifying straight

overhang surfaces with large chamfers and fillets to create self-supporting features.

Ranjan et al. [12] proposed comprehensive DFAM guidelines and algorithms that identify features that violated the DFAM guidelines. These algorithms identified critical features such as

sharp corners and thin features in the design that affects the manufacturability of that part.

2.3 Cellular Structures Manufacturing using AM

With the immense design freedom provided by additive manufacturing, several

researchers have discussed the idea of using cellular structures for a wide variety of applications and have conducted experimental studies to build cellular structures.

Wang et al. [13] designed graded cellular structures called mesostructured cellular

structures that are used in developing a porous acetabular cup for hip implants. Williams et al.

[14] manufactured the proposed porous acetabular cup design with mesostructured cellular

structures using 3D printing. Park et al. [15] investigated the mechanical properties of struts built

in multiple orientations using FDM and developed a model to predict the mechanical behavior of

cellular structures.

Kadkhodapour et al. [16] manufactured Ti6Al4V cylindrical lattices with unit cell sizes

of 100 µm using SLM and experimentally investigated the failure mechanisms based on porosity

and type of unit cell. Sercombe et al. [17] designed topologically optimized porous scaffold

structures with high stiffness and high strength-to-weight ratio and manufactured them using

Ti6Al4V with SLM. The designed structures had a solid fraction of 5%, 10% and 15% with a

unit cell size of 3.33 mm. They investigated the mechanical behavior of the manufactured

structures and concluded that there is a need for further refinement of the design and build

quality. Amirkhani et al. [18] designed cubic and hexagonal scaffolds with 42% volume fraction

and manufactured them using Polyjet 3D printing. They also simulated the failure mechanisms

using FEM analysis and experimentally investigated the stress-strain curve for these scaffolds at

different orientations [19].

Heinl et al. [20] proposed cellular structures with interconnected macro-porosity for bone

implants. Their work focused on generating different cellular structures based on a laser hatch

pattern with a porosity of 60%. This work did not include cellular structures with different

orientation or overhanging surfaces.

Harryson et al. [21] proposed the fabrication of titanium hip implants with tailored properties using rhombic dodecahedron unit cell. The hip implant in this study was populated using rhombic dodecahedron unit cellular structures with 6 mm unit cell size and 60% porosity.

All the struts are inclined at an angle of 35°. The hip implant was evaluated for its mechanical

property but this work does not explore the manufacturing and design limitations of cellular

structures. Mullen et al. [8] proposed the idea of using 3D periodic cellular structures to

manufacture porous implants for orthopedic applications using SLM. They used an octahedral

unit cell to populate shapes and CAD geometries of implants. Their work also included the

manufacture of a prototype hip augment with porous unit cells using SLM. However, they did

not characterize the manufacturing and design limitations of the SLM process.

Murr et al. [22] fabricated stochastic open cellular structures (foams) using EBM and

Ramirez et al. [23] manufactured non-stochastic open cellular structures (reticulated meshes)

using EBM. Both these studies focused on characterizing the stress response of these structures

but did not investigate the manufacturing limitations associated with the EBM process. Hussein

et al. [24] conducted several experimental studies to manufacture Triply Periodic Minimal

Surfaces (TPMS) cellular structures, characterize their mechanical behavior [25] and proposed the concept of using TPMS cellular structures as support structures to minimize energy consumption during manufacturing [26]. The TPMS cellular structures in this work were

manufactured on DMLS machines using 316L stainless steel, Ti-6Al-4V, and AlSi10Mg. This

work utilized self-supporting curved cell topologies but had several manufacturability issues.

This work did not investigate the manufacturability of 3D cellular structures with straight

prismatic struts and the effect of overhang on the manufacturability. The current work studies the

manufacturability of 3D cellular structures with prismatic struts of circular cross-section and provides design guidelines and design space to improve the manufacturability and increase the adoption of these structures.

3 Methodology

The methodology section is sub-divided into four different sections. The first section details the material used, manufacturing equipment setup, part file setup and the measurement approach. The second section explains the design of high porosity cellular structures. The third

section discusses the approach for support structure minimization and identification of optimal

orientation for each cell type. The fourth section describes the experimental framework to

manufacture the designed high porosity cellular structures and the development of the

experimental builds.

3.1 Materials and methods

3.1.1 Powder Material

All the parts for the experiments are built using Concept Laser’s CL 20ES stainless steel

powder. CL 20ES is chemically equivalent to 316L stainless steel [27]. It is an austenitic

stainless steel in fine powder form. This metal powder meets the chemical requirements of AISI

316L, EN 1.4404, and X2-CrNiMo 17-13-2 [27]. 316L is widely used for medical devices and implants due to its high strength and corrosion resistance.

The CL 20ES stainless steel powder for this work was procured from Concept

Laser GmbH and the chemical composition is detailed in table 1. The powder has an average particle size distribution from 15 to 50 µm, centered around 35 – 40 µm.

Table 1: Chemical composition of CL 20ES as provided by Concept Laser [27] Component Fe Cr Ni Mo Mn Si P C S

Indicative Balance 16.5 – 10.0 – 2.0 – 0.0 – 0.0 – 0.0 – 0.0 – 0.0 –

value (%) 18.5 13.0 2.5 2.0 1.0 0.045 0.030 0.030

3.1.2 Manufacturing Machine Setup

3.1.2.1.Concept Laser Mlab cusing R Process Parameters

All the experiments are conducted using Concept Laser’s Mlab cusing R (figure 3) – a

DMLS machine at the Additive Manufacturing center in collaboration with the University of

Cincinnati Research Institute (UCRI). This machine has a build envelope of 90 x 90 x 80 mm3

(x,y,z) and is equipped with an NdYAG Fiber laser with a continuous wavelength of 1064 nm.

The process parameters have been set according to the guidelines from Concept Laser GmbH

and extensive characterization and parameter optimization tests conducted by the Additive

Manufacturing Center. Table 2 details the processing parameters used for the experiments.

Figure 3: Concept Laser Mlab cusing R [28]

Table 2: Processing parameters used for the experiments Processing Parameters Value

Laser Power 90 W

Scanning Speed 1500 mm/s

Laser spot diameter 40 µm

Hatch spacing 0.056 mm

Layer thickness 25 µm

Inert Gas used Argon

Chamber pressure < 0.2 psi

Re-coater type Soft re-coater (Material: Rubber)

3.1.2.2. Scanning Pattern

DMLS introduces residual stresses due to the large thermal gradients inherent in the

process. Mercelis and Kruth [29] explained the mechanism for residual stresses using two

models. The temperature gradient mechanism (TGM) states that during the process of sintering

powdered material, the laser beam rapidly heats a zone on the top layer which as a result tends to expand. The expansion of the heated zone is partly restricted by the surrounding cooler zones leading to the introduction of thermal stresses. When the magnitude of the thermal stresses exceeds the material’s yield strength, the compressive strain induced is partially elastic and partially plastic. The cool-down phase model states that after irradiation, as the heated zone cools down, it tends to shrink. The shrinkage is restricted partially due to the plastic deformation, which induces residual stresses in the material. Figure 4 depicts the formation of residual stresses as explained by the TGM and cool-down phase models.

Figure 4: Representation of the occurring stresses and deformation during sintering and cooling down as explained by the TGM and cool-down phase models [30]

The magnitude of thermal stresses depends on part size, material and thermal properties of the metallic powder, the temperature of the lower layers (i.e. build plate or powder bed temperature), laser settings (i.e. laser power, laser spot size, scanning speed), and scanning pattern. To minimize the thermal stresses without limiting the part geometry, commensurate with the laser setting, material properties and machine specifications, optimal scanning strategies can be utilized. Mercelis and Kruth [29] identified that the stresses induced perpendicular to the scan direction are greater than the stresses induced along the scan direction. They also identified that the increase in the length of the scan vector leads to increase in the magnitude of the thermal

stresses induced. The typical scanning strategies used in DMLS are uni-directional and bi-

directional. Figure 5 and 6 illustrate the uni-directional and bi-directional scanning strategies

Figure 5: Uni-directional scanning strategy used in DMLS [31]

Figure 6: Bi-directional scanning strategy used in DMLS [31] The laser first scans the outer boundary contour of the layer and then hatches the inner

contour using either uni-directional or bi-directional scanning strategy. The next layer is either scanned in the same fashion or the scan is rotated at 90° to the previous layer to mitigate thermal

stresses. Concept Laser uses the “island scanning” strategy to ensure significant stress reduction

as depicted in figure 7. The island scanning strategy breaks down a block into grids or “islands”,

effectively reducing the length of the scan vector. It also scans the adjacent grid zones in

opposing directions. The sequence in which the islands are scanned is chosen randomly. This

work utilizes the island scanning strategy with island sizes of 5 mm and scan angle of 45°. For

every layer the islands are shifted 1 mm in x and y directions to further mitigate the residual

stress formation.

Figure7: Island scanning strategy[32] 3.1.2.3. Part File Setup

Once the parts have been modeled using a CAD (Solidworks, Siemens NX) software,

they are converted to an STL file. STL file is a triangular mesh representation of the part. This

STL file is sliced using Materialise Magics [33] software to generate a CLI file for the AM

machine. Slicing generates contour points at each layer. The important slicing parameters are

slice thickness and beam compensation.

Slice thickness affects the surface finish, design to part deviation and the build time of the

part. Due to the size of the cellular structures and high level of dimensional accuracy required,

the slice thickness is set to the minimum value (0.025 mm) per machine specifications. Figure 8

illustrates the effect of slice thickness.

Figure 8: Effect of slice thickness on Additive Manufacturing

Beam compensation is an offset inwards on the outer contours of the slice to offset the positive bias due to the finite dimensions of the laser beam. The zone around the laser spot in which the material is sintered by the laser’s energy is offset inwards to ensure accurate dimensions. For small features, a high value of the beam offset can lead to manufacturing errors.

Figure 9 depicts the effect of beam compensation on manufacturing errors in thin parts [34]. Due to the small dimensions of the high porosity cellular structures, the beam compensation is set to the minimum value (0.01 mm) according to machine specifications.

Figure 9: Effect of beam compensation on manufacturing thin parts [34]

3.1.2.4. Post Processing

After the parts are built, they are removed from the build platform. The sacrificial support

structures are removed using hand tools and parts are cleaned using air blasting to clear any

residual powder from the structure. Structures built without support structures (i.e. they are

directly bonded on to the surface of the platform) are removed from the base plate using Wire-

Electric Discharge Machine (Wire-EDM). All the parts built without support structures are

removed using Accutex SP-300i Wire-EDM in collaboration with the “Ohio Center for Laser

Shock Processing for Advanced Materials and Devices” located at the University of Cincinnati.

3.1.3 Measurements

3.1.3.1. Weighing Machine

The weight measurement of all the High Porosity Cellular structures in this work is

performed using an analytical semi-micro balance (Denver Instrument PI-214A) in collaboration

with the University of Cincinnati Research Institute (UCRI). Figure 10 depicts the instrument

specifications and photograph.

Denver PI-214 Analytical semi-micro balance Manufacturer: Denver Instruments Capacity: 210 g Readability: 0.1 mg Accuracy: ± 0.1 mg Operating temperature range: 10°C – 30°C Stabilization time: 3 sec.

Figure 10: Denver PI-214 Analytical semi-micro balance specifications and photograph [35]

3.1.3.2. Optical Microscope

The optical microscope - Keyence Digital Microscope (VHX-600) shown in figure 11 is

used for lateral measurements of the strut lengths, strut diameters and strut angles of the

experimental parts in collaboration with the “Ohio Center for Laser Shock Processing for

Advanced Materials and Devices” located at the University of Cincinnati. These optical

measurements are compared with the original CAD dimensions.

Figure 11: Keyence Digital microscope specifications and photograph [36]

3.1.3.3. Vernier Calipers

The digital vernier calipers shown in figure 12 is used for transverse measurements of the

strut diameters.

Digital Vernier Calipers specifications Manufacturer: Pittsburgh Accuracy: ± 0.03 mm Range: 0 – 150 mm Jaw depth: 39.6875 mm (outside jaws) 17.4625 mm (inside jaws)

Figure 12: Digital Vernier calipers specifications and photograph [37]

3.2 Design of High Porosity Cellular Structures

3.2.1 Unit Cell Types

The research focuses on 3D periodic cellular structures and the unit cell type is an important factor for manufacturability. The unit cells designed for these structures are based on the most common crystal unit cell structures found in nature i.e. Body Centered Cubic (BCC) structure and Face Centered Cubic (FCC) Structure. All the designed structures are made up of struts with circular cross-section, and the interaction of these struts defines the type of structure.

The unit cell types used in this study are depicted in figure 13.

Figure 13: Types of unit cell (a) Basic Cubic structure (b) BCC structure (c) FCC structure (d) BCC plus FCC structure

The selected cell structures have differences in their cell topology, which results in

difference in strut thicknesses, strut orientations, and strut interactions (i.e. type of strut

connections). As a result, the type and amount of overhanging struts vary based on the unit cell

type. This affects the manufacturability of the cellular structures in DMLS process.

3.2.2 Unit Cell Type, Size and Volume Fraction Characterization

To aid in the modelling of unit cells with different size and volume fractions, the relationship between the length of the struts, diameter of the struts and the obtained volume fraction for the unit cell type is characterized. Multiple models with different strut lengths and strut diameters are modeled using CAD (Solidworks) software and the volume fraction for these structures are recorded. The strut lengths and strut diameters are plotted against the volume fraction recorded for that structure, a curve is fit on these data points and an equation is derived that relates the strut diameter, strut length and volume fraction for that particular unit cell type.

The equations for the unit cell types developed for this research are as follows:

Basic Cubic Structure: Vol 1.8817

BCC Structure: Volumeume fraction = 120566.2 π * Length * Diameter 1.8492

FCC Structure: Volume fraction = 818.4 π * Length * Diameter 1.859

BCC plus FCC Structure: Volume fraction = 218.27π * Length * Diameterer 1.8484

π * Length * Diamet 3.3 Support Structure Minimization through Optimal Orientation

The focus of this research is to build high porosity cellular structures without support structures. To identify the best orientation to build these structures, the cellular structure part files are converted to STL files. All the facets of the STL are analyzed and the facets that need support structures are identified by applying the minimum angle criterion. Cloots et al. [6]

20 showed that overhang geometries which make an angle of 35° or more with respect to the horizontal can be manufactured without support structures. The structure is rotated in multiple orientations and analyzed for the need of support structures and the orientation which can be manufactured with the minimum surface contact area of support structures is chosen as the optimal orientation.

Table 3: Optimal orientations for Basic cubic structure, BCC, FCC, and BCC plus FCC unit cell types Unit cell type Orientation Orientation Percentage of surface area

w.r.t x-axis w.r.t y-axis that needs support structures

Basic Cubic Structure 45° 45° 0 %

BCC structure 0° 0° 0 %

FCC structure 45° 0° 8.34 %

BCC plus FCC structure 0° 0° 11.31 %

3.4 Experimental Framework to Test Manufacturability Of High Porosity

Cellular Structures

To experimentally investigate the manufacturability of different cell types, sizes and volume fractions efficiently the experimental framework detailed in table 4 is used.

Table 4: Experimental framework to test manufacturability of high porosity cellular structures Factors Levels

Type of Unit Cell 4 (Basic Cube, BCC, FCC, BCC plus FCC)

Volume Fraction 5 (5%, 10%, 15%, 20%, 25%)

Unit Cell Size 3 (1 mm, 5 mm, 10 mm)

Volume Fraction is defined as the ratio of the volume of struts to the volume of unit cube.

Volume fraction is inversely related to the porosity of the structures. As the volume fraction

increases, the strut thickness increases resulting in lowering of porosity. When the volume

fraction is increased, the pore size decreases which hinders efficient removal of the trapped loose

powder. As this research focuses on high porosity cellular structures, volume fractions are kept

lower than 50% for all parts.

Unit cell size is the size of the unit cube that houses the unit cell. Unit cell size is directly

related to the length of the struts and the size of the void space. As the unit cell size increases, the

strut length increases resulting in larger void space. Depending on the cell type, larger unit cell

sizes may lead to larger overhung struts, which limits their manufacturability. For smaller unit

cell sizes, efficiently removing residual loose powder is typically a challenge. All the unit cells

are analyzed for optimal orientation and manufactured in the best orientation to avoid using

support structures.

In order to optimize time and cost of manufacturing experiments commensurate with the

machine specifications, and to draw inferences based on the results for the parameters chosen,

this work focuses on unit cell sizes of 1 mm, 5 mm and 10 mm and volume fractions of 5%,

10%, 15%, 20%, and 25%

3.4.1 Development of CAD models to Test Manufacturability of High Porosity Cellular

Structures

All the cellular structures are built in blocks made up of repeating unit cells in the x, y,

and z direction. The time to manufacture all the parts per the experimental framework depends

on the size and height of the part. The size of the part determines the number of parts that can be accommodated on the build platform. The height of the part determines the build time as the

height of the part is directly related to the number of layers needed to build the part. To minimize

the number of builds to manufacture all the unit cells per the experimental framework, the largest

unit cell size i.e. 10 mm is built as a block of 3x3x3 unit cells, and the smaller unit cell sizes i.e.

1 mm and 5 mm are built as a block of 5x5x5 unit cells. All the structures are built on solid

platform to facilitate handling. To ensure that the parts are built in their optimal orientation and

for ease of removal of the structures from the build platform, the solid base/platform of the parts

is not directly built onto the build platform of the machine. Instead, it is raised by 2 mm and

supported using block support structures as illustrated in figure 14.

BCC cellular structure

Handling Platform Support structures

DMLS Machine build platform

Figure 14: BCC structure with handling platform and support structures

3.4.1.1. CAD model for Build 1

The design intent of this model is to maximize the number of structures that can be

accommodated on the build platform and investigate the manufacturability of

• All unit cell types with the small unit cell size and for all volume fractions

• BCC structures with the larger unit cell sizes and lower volume fractions

A CAD model developed using Solidworks is illustrated in figure 15. A comprehensive

list of the structures modeled for this build is detailed in table 5.

Figure 15: CAD model of Build 1

Table 5: List of structures modeled for Build 1 3 Type Size (mm ) Volume Fraction

Basic Cubic Structure 1 5% – 25%

BCC 1 5% – 25%

FCC 1 5% – 25%

BCC+FCC 1 5% – 25%

BCC 5 5% - 15%

BCC 10 5% & 10%

3.4.1.2. CAD model for Build 2

The design intent of this CAD model is to maximize the number of structures that can be

accommodated on the build platform and investigate the manufacturability of

• FCC structures with the largest size and lowest volume fractions

• BCC structures with the largest size and subsequent volume fractions

A CAD model developed using Solidworks is illustrated in figure 16. A comprehensive

list of the structures modeled for this build is detailed in table 6.

Figure 16: CAD model of Build 2

Table 6: List of structures modeled for Build 2 3 Type Size (mm ) Volume Fraction

BCC 10 15% and 20%

FCC 10 5% and 10%

3.4.1.3. CAD model for Build 3

The design intent of this CAD model is to maximize the number of structures that can be

accommodated on the build platform and investigate the manufacturability of

• Basic Cubic structure with 5 mm and 10 mm unit cell size and the lowest volume

fractions

• BCC plus FCC structures with the largest size and the subsequent volume fraction

• FCC structures with 5 mm unit cell size and the lowest volume fraction

A CAD model developed using Siemens NX is illustrated in figure 17. A comprehensive

list of the structures modeled for this build is detailed in table 7.

Figure 17: CAD model of Build 4

Table 7: List of structures modeled for Build 4 3 Type Size (mm ) Volume Fraction

Basic Cubic structure 10 5%

FCC 10 20% and 25%

BCC plus FCC 10 5%

3.4.1.4. CAD model for Build 4

The design intent of this CAD model is to maximize the number of structures that can be

accommodated on the build platform and investigate the manufacturability of

• FCC structures with the largest size and subsequent volume fractions

• BCC plus FCC structures with the largest size and lowest volume fraction

A CAD model developed using Solidworks is illustrated in figure 18. A comprehensive

list of the structures modeled for this build is detailed in table 8.

Figure 18: CAD model of Build 3

Table 8: List of structures modeled for Build 3 3 Type Size (mm ) Volume Fraction

FCC 10 15% - 25%

BCC plus FCC 10 5%

3.4.1.5. CAD model for Build 5

The design intent of this CAD model is to test the manufacturability of FCC structures at

different orientations. The FCC structures with the largest size and the smallest volume fraction

are analyzed using an adjusted minimum angle criterion of 15° (i.e. facets making an angle of

15° or less with respect to the build plane would need support structures) and orientations that do

not require support structures are chosen. A CAD model developed using Siemens NX is

illustrated in figure 19. A comprehensive list of the structures modeled for this build is detailed

in table 9.

Figure 19: CAD model of Build 5

Table 9: List of structures modeled for Build 5 Volume Orientation w.r.t Orientation w.r.t 3 Type Size (mm ) Fraction x-axis y-axis

FCC 10 5% 35° 45°

FCC 10 5% 35° 5°

FCC 10 5% 0° 15°

FCC 10 10% 5° 25°

4 Results and Observations on Manufacturability of High Porosity

Cellular Structures

In this section, the evaluation of the high porosity cellular structures manufactured per table 4 in section 3.4 is discussed in detail. The purpose of this experiment is to investigate and characterize the manufacturability of the designed high porosity cellular structures. Here the manufacturability refers to the ease of additive manufacturing high porosity cellular structures without support structures. The effect of ancillary parts viz. handling platform and thin walled block support structures, orientation of the unit cell, unit cell size, volume fraction, and unit cell type on the manufacturability of high porosity cellular structures is evaluated. The reasons for failures are discussed and based on the evaluations, design rules are suggested to improve manufacturability. The evaluations are conducted based on visual inspections, weight and dimensional measurements.

4.1 Results of Build 1

As mentioned in section 3.4.1, all the parts are built on a prismatic platform to facilitate handling. During the manufacture of Build 1, the build was stopped midway due to scorching of the surface. A photograph of this build is depicted in figure 20, which highlights the scorching error.

4.1.1 Observations And Discussions

Due to insufficient feed, there was excessive scorching of the surface and the build had to be stopped. The scorching of the surface is due to inadequate feed of a fresh layer of powder. As the laser sinters the top layer of the surface in the absence of fresh powder, the top layer suffers

30 excess heating. This excessive heating leads to scorching of the surface. The reason for inadequate feed can be attributed to insufficient amount of powder deposition and non-uniform deposition of fresh powder by the re-coater arm. These are caused due to the curling of the handling platform and the warping of the thin walled support structures.

Figure 20: Photograph of scorching defect observed during Build 1

4.1.2 The Curling phenomenon

“Curling phenomenon” is an unwanted effect associated with manufacturing square geometries using laser-based process. Curling is the bending of the side walls of a square component, such that the edges protrude above the sintered plane as shown in figure 21. Mercelis and Kruth [29] developed the Thermal Gradient Model (TGM) and the cool down phase model to explain the formation of residual stresses in components manufactured by a laser based process. They also conducted experiments on manufacturing square/boxy components by selective laser melting of stainless steel powder 316L to identify and characterize the factors that affect the amount of residual stresses induced. Soe [38] conducted a quantitative analysis on the

31 effect of part placement, geometry on the amount of curling and provided recommendations to minimize the degree of curling. In general, an increase in the height of the part results in an increase in the number of layers required to manufacture the part that leads to larger deformations due to accumulated residual stresses.

Re-coater arm movement

Figure 21: Curling of side walls due to residual stresses induced by laser based process

As the soft re-coater interacts with a deformed edge of the part, it flexes leading to flicking of the fresh powder being deposited. The flicking of the fresh powder to be coated results in insufficient feed on the surface. Figure 22 depicts the interaction of the re-coater with the edges of the side walls of the previously sintered layer. As the height of the platform increases, it induces greater deformation leading to greater amount of powder flicking.

Re- coater

Previously sintered layer Curling of side walls

Figure 22: Interaction of re-coater with the side walls of the previously sintered layer

The amount of flicking depends on the area of interaction between the re-coater and the

sintered layer at any given instant during the re-coater travel. To minimize the area of interaction between the re-coater without changing the geometry of the part, the part should be oriented at a

slight angle to the re-coater. Figure 23 depicts the top view to illustrate the reduction in area of

interaction between the re-coater and previously sintered layer by a change in the part

orientation.

Area of interaction between Area of interaction between the re-coater and previously the re-coater and previously sintered layer sintered layer Figure 23: Reduction in area of interaction between the re-coater and previously sintered layer with change in orientation of part

In addition, another factor in ensuring adequate feed is the placement of parts on the build

plate. The amount of powder required to completely cover a contour is directly related to the size

of the contour. As the contour size increases, a greater amount of powder is required to cover it.

To ensure that adequate feed is provided to large components, it is advisable to position big boxy

components towards the starting position of the re-coater. As the re-coater travels over the layer,

it ensures adequate feed to these components.

4.1.3 Design Rules for Part Orientation and Placement

• Design Rule 1: Big boxy components should be situated closer to the starting position of re-coater to ensure adequate feed.

• Design Rule 2: The base of the cellular structures should be oriented at a slight angle to the build platform to minimize powder flicking by the re-coater and ensure adequate feed.

4.1.4 Warping effect

Warping is an undesirable effect of manufacturing thin walled structures using DMLS.

All the parts for this work are built with thin walled block support structures, which affect the manufacturability of the part. Ranjan et al. [12] identified thin regions as critical features that affect manufacturability of a part and also proposed that greater number of thin regions result in reduced manufacturability of the part. Soe [38] and Clijsters et al. [39] conducted several experiments to identify the type of failures associated with thin regions and characterized the effect of laser scan parameters (laser power, laser scan speed and scanning strategy) on the quality of the thin walled structures. Clijsters et al. [39] proposed that failure in thin walled structures is due to the high energy input from the laser beam on thin contours. The small cross sectional area of the thin walled region experiences high thermal stresses and deforms in the form of cracks and irregular non-planar surfaces as depicted in figure 24 resulting in non- uniform feed. To minimize energy consumption, thin walled support structures for the experiments are built using a single scan strategy with a scan speed of 1500 mm/s and a laser power of 90W. The high energy input and scan speed causes deformation of the support structures. To mitigate the effect of warping with minimal energy consumption, the powder feed dosage was increased at the interface of solid part and support structures.

Ideal surface Deformed surface due to warping

Figure 24: Warping of thin walled support structures

4.1.5 Design Rule for Interface of Support structures and Solid part

• Design Rule 3: To minimize energy consumption as well as the effect of warping of thin

walled support structures, the powder feed dosage should be increased at the interface of

support structures and solid parts.

4.1.6 Results of modified Build 1

The model for build 1 was redesigned based on the design rules and the powder feed dosage was increased at the interface of the support structures and solid parts. These design rules were used to redesign the CAD model of Build 1 as illustrated in figure 25, which resulted in a successful build as illustrated in figure 26.

Figure 25: Redesign of CAD model of Build 1

Figure 26: Successful build after redesign of CAD model of build 1 4.2 Results of Build 2 and 3

During Build 2 and 3, FCC structures had catastrophic strut failures. As detailed

in table 3 (Optimal orientation of unit cells) 8.34% of the surface area of the FCC

structures in their optimal orientation require support structures. Due to the absence of

support structures, the FCC structures failed as depicted in figure 27.

Figure 47: Actual build result of build 2 with catastrophic strut failure defect highlighted

In addition, the Basic Cubic structures with 5% and 10% volume fraction and unit cell sizes of 5 mm and 10 mm built successfully as depicted in figure 28.

Figure 58: Successful build of Basic Cubic structures for build 3

Further, FCC structures with 5 mm unit cell size and 5% volume fraction failed to build successfully. BCC plus FCC structures with 10% volume fraction and 10 mm unit cell size did not build successfully without support structures as depicted in figure 29.

Figure 69: Actual build result of build 3 with catastrophic strut failure defect highlighted

Similar to FCC structures, as depicted in figure 29, BCC plus FCC structures had

catastrophic strut failures. As detailed in table 3 (Optimal orientation of unit cells)

11.31% of the surface area of the BCC plus FCC structures in their optimal orientation

require support structures. Due to the absence of support structures, the BCC plus FCC

structures failed.

4.2.1 Observations and discussions based on Build 2 and 3

• High porosity cellular structures that do not need support structures when analyzed with

the minimum angle criterion of 35° and built at the optimal orientation build successfully

without support structures.

• BCC and Basic Cubic structures built successfully without support structures at 5 mm and

10 mm unit cell size with volume fractions less than 25%.

• High porosity cellular structures that require support structures and have overhanging strut

lengths of 5 mm and beyond, do not build successfully.

• BCC plus FCC and FCC structures without support structures do not build successfully at

5 mm unit cell size or 10 mm unit cell size with volume fractions less than 25%.

4.2.2 Effect of Cell Size

As per the experimental framework, the high porosity cellular structures are designed

with varying unit cell sizes of 1 mm, 5 mm, and 10 mm. FCC and BCC plus FCC structures with

unit cell size of 1 mm built for all volume fractions, but for the larger unit cell sizes (5mm and 10

mm) catastrophic strut failure was observed. Catastrophic strut failure implies that a part of the

strut did not build or was not present at the completion of build as illustrated in figure 30. The

catastrophic strut failure occurred for struts inclined at an angle of 0° with strut lengths of 7 mm

and 14 mm. The reason for the failure is the absence of support structures to support overhanging

38 strut geometries. Calignano [42] observed structural failure in overhanging geometries and proposed that the main cause for catastrophic strut failure is dross formation. He conducted experiments on overhanging ledges and parts with concave and convex radii and suggested anchoring and thermal compensation as the possible solutions.

Figure 30: Catastrophic strut failure observed in a FCC structure

4.2.3 Dross formation

During sintering of an overhanging geometry without support structures, the heat conduction is lower due to the layer of loose powder underneath it. As the heat conduction is lower, the sintered zone absorbs a higher energy input leading to large melt pools. These large melt pools tend to sink into the powder due to the increased mass resulting in dross formation as illustrated in figure 31.

Figure 31: Dross formation in overhanging structures [42]

Strut failure was observed in FCC and BCC plus FCC structures with unit cell sizes of 5

mm and 10 mm, whereas BCC and Basic Cubic structures with the same unit cell sizes built

successfully. This is due to the difference in topology of the unit cell. For the FCC and BCC plus

FCC structures, strut failure was observed for struts inclined at an angle of 0° with respect to the

build plane, but struts inclined at 45° and 90° with respect to the build plane built successfully.

Kruth et al. [43], Vandenbroucke and Kruth [44], and Rehme and Emmelmann [45] have

conducted extensive investigations which suggest that geometries inclined at an angle of 45° or

more with respect to the build plane are self-supporting. Due to the difference in topology and

increased overhanging strut lengths, FCC and BCC plus FCC structures are not manufacturable

at unit cell sizes of 5 mm and 10 mm.

4.3 Results of Build 4 and 5

During build 4, scorching was observed on the downward facing surfaces of the FCC

structures with 10 mm unit cell size and volume fractions of 20% and 25% as illustrated in figure

32.

Figure 32: Scorching of downward facing surfaces

During build 5, the FCC structures analyzed with an adjusted minimum angle criterion of

15° failed to build without support structures. The FCC structures had large sections missing as depicted in figure 33.

Figure 73: Actual build results of build 5 with catastrophic missing sections highlighted

4.3.1 Effect of Orientation

Kulkarni et al. [7] investigated the need for support structures in a laser based powder bed fusion process and concluded that support structures stabilize the melt pool and provide conduits for heat dissipation. Clijsters et al. [40] investigated the behavior of melt pool for overhang geometries and concluded that the heat flow for structures built on loose powder is different and sub-optimal heat dissipation leads to scorching of such surfaces. Mertens et al. [41] classified the heat conductivity for different orientation and geometries and proposed scan parameter optimization methods for these geometries. Scorching is a phenomenon observed in downward facing surfaces due to inefficient heat dissipation pathways. As the laser irradiates a surface supported only by loose powder, the resulting melt pool is unstable. The unsupported melt pool has a lower cooling rate due to the lower and non-uniform conductivity of the loose powder. As this research explores the manufacturability of high porosity cellular structures without support structures it is advisable to avoid large surfaces inclined at an angle of 45° or less with respect to the build plane to avoid such defects.

4.3.2 Design Rule for Orientation of Parts

• Design Rule 4: For constant scan parameters, avoid orienting large surfaces at an angle

of 45° or less with respect to the horizontal build plane to avoid scorching of the

downward facing surfaces.

4.4 Effect of Volume Fraction

To compare the effect of volume fraction on manufacturability, the weight of the successfully built high porosity cellular structures with 1 mm unit cell size is measured. Based on the weight measurement, the manufactured strut diameters are compared against the designed strut diameters as shown in figure 34.

Error in manufactured strut diameters for 1 mm unit cells 0.4

0.35

0.3

0.25

0.2

0.15 Strut diameter (mm) 0.1

0.05

Type of unit cell / Volume Fraction

Designed strut diameter Calculated strut diameter based on weight measurement

Figure 34: Comparison of error in manufacturing for all unit cell types and volume fractions for 1 mm unit cells

It can be observed from figure 34, as the volume fraction increases, the manufactured structures better approximate the designed structures as increase in the volume fraction for a certain unit cell type results in increase in the strut diameters of that structure. The minimum feature that can be manufactured using DMLS depends on the particle size distribution of the powder used, laser spot diameter, beam compensation, and slice thickness. This experiment utilizes the minimum possible values for these parameters commensurate with the machine limitations. Further experimental investigation has been performed to characterize the limits of the minimum strut thickness that can be built using the current DMLS setup.

4.5 Effect of Unit Cell Type

To compare the effect of unit cell type on manufacturability, the strut diameters on the outer periphery are measured and compared for different unit cell types and volume fractions. In the absence of 3D scanning data of the structures, no visible strut failure, and due to their small overhanging strut lengths, all the 1 mm unit cell size structures are assumed to have built successfully. The comparison of strut diameters for different unit cell types and volume fractions is illustrated below in figure 35 and 36.

BCC 1 mm unit cell 0.35 0.3 0.25 0.2 Designed strut diameter 0.15 Measured Strut diameter 0.1

Strut diameter (mm) 0.05 Calculated strut diameter based on weight 0 5% 10% 15% 20% 25% Volume Fraction

Figure 35: Error in strut diameter for BCC structure

Figure 36: Error in strut diameter for Basic Cubic, FCC, and BCC+FCC structure

It can be observed that the calculated strut diameter values based on weight are greater

than the measured strut diameter values. In addition, it can be observed that the measured strut

diameters are greater than the designed strut diameter. The reasons for these errors are as follows:

1. Laser Beam compensation

2. Balling effect

3. Residual un-sintered powder

4. Topology of unit cell

4.5.1 Laser Beam compensation error

Laser beam compensation is a part setup parameter that offsets the laser pathway to account for the oversizing of a contour due to the melt pool. Figure 37 illustrates the phenomenon of laser beam compensation to account for melt pool dimensions. To avoid the loss of critical small-dimensioned parts, the laser beam compensation was chosen as 0.010 mm (the lowest value per machine specifications). This minimal value of compensation leads to oversizing of the strut diameters and hence is one of the major reasons that the measured strut diameters exceed the designed strut diameters of 1 mm unit cells.

Figure 37: Effect of laser beam compensation on dimensional accuracy

4.5.2 Balling effect

“Balling” is an unwanted defect that affects the dimensional accuracy of structures

manufactured using DMLS. Balling is the formation of discontinuous micro-sized metallic balls

on the surface of structures due to shrinkage of molten metal under the action of surface tension

and thermal effects of the melt pool and melt splashes. Figure 38 shows SEM photographs of the

balling defect observed in structures manufactured using DMLS [46]. Several researchers

(Tolochko et al. [47], Kruth et al. [48], Yadroitsev et al. [49], Gu and Shen [46], and Li et al. [50]

) have identified and characterized the cause of balling. Balling depends on the laser process

parameters and powder material properties and typically the size of the micro-sized balls range from 25µm – 200µm. Due to the balling phenomenon, the struts have a tendency to be over-sized

especially for small diameter struts as observed in 1 mm unit cell sizes. Balling is also a

contributor for the oversizing of the manufactured struts. The balling effect can be mitigated by

optimizing the scan parameters for different strut sizes [46].

Figure 38: SEM images a) Balling defect observed on sintered surfaces b) magnified image of section highlighted in (a) to show balling [46]

4.5.3 Residual un-sintered powder

Due to the small unit cell sizes of 1 mm with multiple strut interactions, it is difficult to

remove the un-sintered powder left after the build. The increase in volume fraction results in

struts with higher thickness. The increase in the thickness of the struts leads to smaller pore/void

size for a certain unit cell type. BCC plus FCC structures with 1 mm unit cell size and 25%

volume fraction have the lowest pore size of 0.1008 mm2. The particle size distribution of the powder material is 0.040 mm. As the particle size is smaller than the smallest pore for 1 mm unit cell size structures, the error in weight estimation due to residual powder is negligible. Hence, the effect of residual powder does not play a major role in the difference between measured strut diameter and calculated strut diameter.

4.5.4 Topology of unit cell

One of the major reasons for the difference in measured strut diameters versus calculated strut diameters for 1 mm unit cell sizes is the difference in topologies. The topology of the unit cells is based on the number of nodes, type of nodes, number of struts interacting at a node, and the type of strut interactions in the structure. All these factors affect the number of sharp corners in the structure. Ranjan et al. [12] identified that sharp corners affect the manufacturability of the structures and the number of sharp corners reduce the manufacturability. Several researchers

(Childs and Juster [51], Mahesh et al.[52], Kim and Oh [53], Castillo [54], and Fahad and

Hopkinson [55]) have examined the effect of sharp corners on dimensional accuracy of parts

built using laser powder bed fusion process and have identified the effect of laser process

parameters and powder size on the manufacturability of sharp corners. One of the key parameters

that affect the manufacturability of sharp corners is the laser spot shape. Figure 39 illustrates the

dimensional error induced by laser beam while sintering contours with sharp corners.

Figure 39: Laser beam error during sintering of contours with sharp corners

The dimensional accuracy due to the laser beam diameter coupled with laser beam compensation are the reasons for the oversizing of the structure. They account for the difference between measured strut diameter and calculated strut diameter. Figure 40 shows an actual photograph of the sintered profile of the sharp corner in an FCC structure of 1 mm unit cell size.

Figure 40: Geometrical differences for 1 mm FCC structure a) Designed geometry of sharp corners b) Manufactured geometry of sharp corners

The number of sharp corners for the designed high porosity cellular structures can be

calculated based on the cell topologies. Nodes and strut interactions define the cell topology.

4.5.4.1. Nodes

Nodes are defined as the points of connection for the struts in a particular structure.

Figure 41 depicts a node in a basic cubic structure.

Figure 41: Node in a basic cubic structure 4.5.4.2. Strut Interactions

Strut interactions can be classified by the number of struts interacting at a node and the

angle at which multiple struts interact with the node. Figure 42 depicts the strut interaction angle

for a Basic Cubic structure.

90°

90° 90°

Figure 42: Strut interactions in a basic cubic structure

To characterize the number of sharp corners for a unit cell type, it is analyzed based on

type and number of nodes, number of struts interacting at a node, and the angle of strut

interactions. An example of a high porosity structure made by repeating 3 instances of a basic

cubic structure in the x, y and z directions is shown in figure 43.

Figure 43: Types of nodes and node interactions

The different types of nodes are classified based on the number of sharp corners. All the successfully built high porosity cellular structures have been analyzed based on the cell type and

cell size for the number of sharp corners as detailed in table 10

Table 10: Analysis of sharp corners for successfully built high porosity cellular structures Size Type Number of struts Sharp corners of Type of Total number of sharp of Number of nodes interacting at a Angle of strut interactions per unit unit unit cell corners nodes node volume cell Basic 8x3x90°+24x4x90° cubic 4 8 40 24 8 n/a 3 4 5 6 n/a 90° 90° 90° 90° n/a 10.67 +24x5x90°+8x6x90° structure

27x8x45°+24x2x45° BCC 3 27 40 24 n/a n/a 8 2 4 n/a n/a 45° 45° 45° n/a n/a 13.33 +24x4x45° 1 mm 8x3x45°+24x5x45° FCC 4 8 40 192 8 n/a 3 5 4 12 n/a 45° 45° 90° 45° n/a +192x4x90° 37.33 +8x12x45° BCC 8x4x45°+24x7x45° plus 5 8 40 192 8 27 4 7 4 16 8 45° 45° 90° 45° 45° +192x4x90°+8x16x45° 48.59 FCC +27x8x45° Basic 8x3x90°+24x4x90° cubic 4 8 40 24 8 n/a 3 4 5 6 n/a 90° 90° 90° 90° n/a 5.34 +24x5x90°+8x6x90° 5 structure mm 27x8x45°+24x2x45° BCC 3 27 40 24 n/a n/a 8 2 4 n/a n/a 45° 45° 45° n/a n/a 6.65 +24x4x45°

Basic 8x3x90°+24x4x90° cubic 4 8 24 24 8 n/a 3 4 5 6 n/a 90° 90° 90° 90° n/a 1.07 +24x5x90°+8x6x90° 10 structure mm 27x8x45°+24x2x45° BCC 3 27 24 24 n/a n/a 8 2 4 n/a n/a 45° 45° 45° n/a n/a 1.33 +24x4x45°

To investigate and characterize the effect of sharp corners on manufacturability of high porosity cellular structures, a comparison of the error percentage versus unit cell topology for a constant volume fraction of 5% is depicted in figure 44.

Difference in weight measurement due to unit cell topology for volume fraction of 5% 5.9 % 16 8.2 % 14

12 19.7 % 32 % 10 8 6 Weight (gm) 562 % 4 204 % 338 % 390 % 2 0 FCC / 1 mm FCC BCS / 1 mm BCS BCS / 5 mm BCS BCC / 1 mm BCC / 5 mm BCS / 10 mm BCS BCC / 10 mm BCC + FCC / 1 mm FCCBCC + Type of unit cell / size of unit cell

Predicted weight Measured weight

Figure 44: Summarizing the error due to unit cell topology for successfully built high porosity cellular structures with volume fraction of 5%

From table 10 and figure 44, it can be clearly observed that as the number of sharp corners per unit volume decreases the manufactured structures better resembles the designed structures. The number of sharp corners per unit volume depends on the number of nodes per unit volume, number of struts interacting at a node, and the angle of strut interactions and hence these affect manufacturability.

4.5.5 Design Rules

• Design Rule 5: Avoid cellular structures involving multiple unit cell topologies of large

variations in strut length and diameter as the differences in sharp corners per unit

volume for these structures requires robust and complex laser beam compensation.

• Design Rule 6: Minimize the number of nodes in a cellular structure commensurate

with the loading conditions.

• Design Rule 7: Minimize the number of struts interacting at a node in a cellular

structure commensurate with the loading conditions.

• Design Rule 8: Utilize higher angle strut interactions in cellular structures

commensurate with the loading conditions.

5 Experimental Framework to Predict Safe Ranges for

Overhanging Struts

All the high porosity cellular structures discussed in this work are made up of struts of

circular cross-sections with different diameters, length and angles. The strut diameter, length,

and angle of orientation with respect to the build platform affect the manufacturability of the

cellular structure. This experimental framework is designed to investigate the manufacturability

of overhanging struts with different lengths, diameters and angles of orientation built without support structures.

5.1 Experimental Framework and Rationale

To explore a wide and feasible range of parameters, the experimental framework to study

the manufacturability of overhanging struts is detailed in table 11. All the struts are manufactured on the Concept Laser’s Mlab cusing R machine using CL 20ES metallic powder.

The scanning parameters used are detailed in Table 2 and the part file setup is detailed in section

3.1.2.3.

Table 11: Experimental framework to test manufacturability of Overhanging struts Factors Levels

Strut length 10 (1 mm - 10 mm)

Strut angle 11 (0° - 50°)

Strut diameter 13 (0.025 mm, 0.05 mm, 0.075 mm, 0.1 mm – 1.0 mm)

The rationale for the selection of these parameters and ranges in the experimental study is discussed below.

5.1.1 Rationale for Strut Length Parameter and Range Selection

The length of the overhanging strut affects the manufacturability of the structure as observed from sections 4.3. The length of the overhang directly relates to the area of unsupported melt pool. As the length of the overhanging strut increases, the chances of a catastrophic failure is higher. The range for this parameter is selected based on the scope of the unit cell size for this study (i.e. 1 mm to 10 mm). The step size is selected as 1 mm to investigate the possibility of manufacturing smaller overhanging strut lengths without the need for support structures.

5.1.2 Rationale for Strut Angle Parameter and Range Selection

The angle of orientation of the strut to the build plane affects the manufacturability of the structures. The experiments for the manufacturability of the high porosity cellular structures considers the minimum angle criterion of 35° [6]. Current industry standards utilize the minimum angle criterion of 45° [7]. Both these approaches are defined for solid parts with dimensions greater than the strut sizes used in this study. As observed from section 4.5 certain strut configurations can be built at angles less than 35°. To identify the minimum angle criterion for struts (i.e. angle made by the strut with respect to the horizontal build plane that do not require support structures), the range for this parameter is chosen to be 0° - 50°. The step size of

5° provides the possibility of investigating the effect of strut angle on the manufacturability of overhanging struts with greater detail and without increasing the number of strut builds drastically.

5.1.3 Rationale for Strut Diameter Parameter and Range Selection

The strut diameter affects the manufacturability of the high porosity cellular as observed in section 4.4. As the strut diameter increases, due to the constant slice thickness, the number of slices required to manufacture a surface increases thereby resulting in increased possibility of lower layers of sintered material supporting the upper layers to be sintered. To investigate the effect of strut diameter on the manufacturability of the overhanging struts, the range of this parameter is set between 0.025 mm – 1.0 mm. The lower limit of the range (i.e. 0.025 mm) represents a dimension smaller than the laser beam diameter and the upper limit of the range (i.e.

1.0 mm) represents the strut diameter of a BCC structure with a unit cell size of 10 mm and volume fraction of 5%. This represents the minimum designed strut diameter of a 10 mm unit cell size that built successfully and accurately. The step size of 0.1 mm is to accommodate the maximum number of struts within the minimum number of strut builds that can fit within the build envelope of the Concept Laser’s Mlab cusing R machine.

5.2 Development of CAD Models To Investigate Manufacturability Of

Overhanging Struts

This section details the use of design rules used to develop the CAD model and the design philosophy to facilitate measurement and accommodate the maximum number of struts within the constraints of the build envelope of the DMLS machine used.

5.2.1 Design Intent

Figure 45: CAD model for 30° strut build experiment Concept Laser’s Mlab cusing R has a build envelope of 90 x 90 x 80 mm3. To fit within

these dimensions, overhanging struts for a certain angle and varying strut length and diameter are

modeled (Siemens NX) between two prismatic columns with square cross section (cross

sectional area: 2mm x 2mm) as shown in figure 45. To facilitate measurement using vernier

calipers, the struts are aligned in ascending order of diameters from the top to the bottom. Per

section 4.2.1, the prismatic columns are aligned at a small angle to re-coater movement direction as depicted in figure 46.

Re-coater movement direction Figure 46: Orientation of the prismatic column to the re-coater movement direction

6 Results and Observations on Manufacturability of Overhanging

Struts

In this section, the evaluation of the manufacturability of overhanging struts manufactured per table 11 in section 5.1 is discussed in detail. The evaluations are conducted based on a visual inspection and dimensional measurements using an optical microscope and digital vernier calipers.

6.1 Visual Inspection and Results Of Manufacturability of Overhanging

Struts

In this section, the results of the visual inspection of the manufactured overhanging struts are discussed in detail. The build results of the overhanging struts are as depicted in figure 47.

Figure 47: Actual build results of overhanging struts

6.1.1 Evaluation of Length of Strut

The manufacturability of struts with different lengths is affected by their angle of orientation. However, the manufacturability of struts with lengths less than 3 mm with different diameters detailed per the experimental framework was not affected by the angle of orientation of the strut. This implies that struts less than 3 mm are manufacturable at all orientations. Figure

48 depicts the strut build results for a 0° degree strut build. It can be seen that struts with a length of 3 mm and less were manufactured at all diameters, but struts with lengths beyond 3 mm were not manufacturable at this orientation.

Figure 48: Build results of overhanging struts built at 0° with respect to the build plane with strut lengths of 3 mm and less highlighted.

6.1.2 Evaluation of Angle of Strut

As the strut length increases beyond 3 mm, the angle of the strut plays an important role in the manufacturability of the strut. 30° is the minimum angle at which struts with different lengths and diameters detailed per the experimental framework were manufactured successfully.

This implies that the minimum angle criterion to build overhanging struts without support structures is 30°. Figure 49 depicts the build results of 30° strut build.

Figure 49: Build results of overhanging struts built at 30° with respect to the build plane

6.1.3 Evaluation of Diameter of Strut

The manufacturability of struts with diameters greater than 0.1 mm was affected by the angle of orientation and the length of the strut. Whereas, struts with diameters less than 0.1 mm

(0.025 mm, 0.05 mm and 0.075 mm) failed to build at all different diameter and lengths as detailed in the experimental framework. The nominal laser spot diameter for the Concept Laser

Mlab cusing R is 0.04 mm (table 2). This implies that struts with diameters less than 2 times the laser spot diameter cannot be manufactured. Figure 50 depicts the build results of 0° with unsuccessful strut diameters – 0.025, 0.05, and 0.075 mm.

Figure 50: Build results of overhanging struts built at 0° with respect to the build with failed strut diameters of 0.025, 0.05, and 0.075 mm highlighted

6.1.4 Effect of Re-Coater Damage on The Manufacturability of Struts

The manufacturability of struts with 5 mm length was affected by re-coater damage. The re-coater was damaged by the curling and deformation of struts of length 5 mm built at an angle of 0°. This re-coater damage affected the consecutive struts in the build direction as well as the path of re-coater movement. This damage on the experimental build is illustrated in figure 51.

Figure 52 shows the progressive deformations due to the re-coater damage. To avoid the effect of re-coater damage causing build defects, it is advisable to isolate structures that have a higher tendency to fail such that the re-coater movement path for these structures does not intersect with the path of the other structures in a build.

Figure 51: The effect of re-coater damage on the experimental strut build

Figure 52: Progressive strut deformation in 5 mm length struts with varying angles due to re-coater damage

6.1.5 Design Space for Manufacturability of Overhanging Struts

The results of the manufacturability of overhanging struts shows that for a few unique

configurations of strut length and diameters, struts greater than 3 mm in length were built

successfully at angles less than 30°. Some unique configurations from 10° and 15° strut builds

are depicted in figure 53. To provide designers with the data of such unique configurations, a 3D

plot of strut length v/s strut diameter v/s strut angles for successfully built struts is created. The

methodology for creating a 3D design space, which details the results of successfully built struts,

is explained in the next section.

Figure 53: Unique configurations of successful built struts for 10° and 15° strut builds highlighted

6.1.5.1. Methodology for The Creation of Design Space

A 3D grid of strut length v/s strut diameter v/s strut angle based on the parameter ranges

and step sizes discussed in the experimental framework is populated with successfully built strut

data. For a certain combination of strut length and strut diameter, the lowest angle at which the

strut was successfully built is plotted and these points are extracted. The collection of these

points is used to generate a bi-harmonic 3D surface. This surface distinguishes the successful

configurations at which overhanging struts are manufacturable from the unsuccessful

64 configurations. This surface provides a design space for the designers to design and customize high porosity cellular structures for specific applications without the limitations of support structures. The generated design space is illustrated in figure 54.

Figure 54: Design space for manufacturability of overhanging struts

6.1.6 Design Rules

• Design Rule 9: For the process parameters selected, the minimum angle at which

overhanging struts can be manufactured successfully is 30°.

• Design Rule 10: Features with a tendency to curl or deform that may damage the re-

coater should be on an isolated re-coater pathway.

• Design Rule 11: For the process parameters selected, the maximum safe overhanging

strut length is 3 mm. Struts with length less than 3 mm can be built in any orientation.

6.2 Dimensional Measurements and Results of Manufacturability of

Overhanging Struts

In this section the results of the dimensional inspection of the manufactured overhanging struts are discussed in detail. The struts are measured using a digital vernier calipers and an optical microscope. It has been discussed in section 4.5.1 that struts are prone to oversizing. The factors for oversizing are as follows:

1. Oversized melt pool due to sub-optimal laser scanning parameters viz. laser beam

compensation

2. Dross formation on overhanging surfaces [42]

To identify the effect of both these parameters, the struts are measured using different techniques

• Transverse measurements – The strut diameter is measured in the direction of the

build axis using digital calipers. The transverse measurement will quantify oversizing

due to sub-optimal laser scanning parameters.

• Lateral measurement -The strut diameter is measured perpendicular to the build axis

using an optical microscope to quantify the “dross” formation.

6.2.1 Results and Observations

The diameters of successfully built struts were measured using an optical microscope and digital vernier calipers. This section illustrates the results of 842 lateral strut measurements using an optical microscope and 455 transverse strut measurements using a digital caliper. All the struts measured were over-sized due to dross formation and laser beam compensation.

6.2.2 Characterizing the Effect Of Beam Compensation on Oversizing of Successfully

Built Struts

To characterize the effect of sub-optimal laser scanning parameters (mainly beam compensation), the struts were measured transversely using a Digital vernier Calipers. A plot

(using Minitab 17.0) of over-sizing error of strut diameter in the transverse direction versus length, diameter and angle of strut is illustrated in figure 55, 56, and 57 respectively. The transverse measurements are conducted using vernier calipers with an accuracy of ± 0.03 mm.

The upper and lower limits for the trend lines for the transverse measurements are within the accuracy range of the calipers. Hence, the investigation shows that the oversizing of the strut due to sub-optimal laser scanning parameters is not related to the length of strut, angle of strut, or diameter of strut.

Trendline of transverse error value for diameter of strut vs length of strut

0.07

0.06 ) m m (

0.05 e u l a v

0.04 r o r r e

0.03 e s r e v

s 0.02 n a r T 0.01

0.00

4 5 6 7 8 9 10 Length of strut (mm)

Figure 55: Effect of over-sizing error due to sub-optimal laser scanning parameters v/s length of strut

Trendline of transverse error value for diameter of strut vs diameter of strut

0.07

0.06 ) m m (

0.05 e u l a v

0.04 r o r r e

0.03 e s r e v

s 0.02 n a r T 0.01

0.00

0.0 0.2 0.4 0.6 0.8 1.0 Diameter of strut (mm)

Figure 56: Effect of over-sizing error due to sub-optimal laser scanning parameters v/s diameter of strut

Trendline of transverse error value for diameter of strut vs angle of strut

0.07

0.06 ) m m (

0.05 e u l a v

0.04 r o r r e

0.03 e s r e v

s 0.02 n a r T 0.01

0.00

10 20 30 40 50 Angle of strut (degrees)

Figure 57: Effect of over-sizing error due to sub-optimal laser scanning parameters v/s angle of orientation of strut

6.2.3 Characterizing the effect of dross formation on oversizing of successfully built struts

To characterize the error due to dross formation, the struts were measured laterally using

an optical microscope. A plot (using Minitab 17.0) of over-sizing error of strut diameter in the lateral direction versus length, diameter and angle of strut is illustrated in figure 58, 59, and 60 respectively.

Trendline of lateral error value for diameter of strut vs length of strut

0.20 ) 0.15 m m (

e u l a v

r 0.10 o r r e

l a r e t

a 0.05 L

0.00

0 2 4 6 8 10 Length of strut (mm)

Figure 58: Effect of dross formation over-sizing error v/s length of strut

Trendline of lateral error value for diameter of strut vs diameter of strut

0.20 ) 0.15 m m (

e u l a v

r 0.10 o r r e

l a r e t

a 0.05 L

0.00

0.0 0.2 0.4 0.6 0.8 1.0 Diameter of strut (mm)

Figure 59: Effect of dross formation over-sizing error v/s diameter of strut

Trendline of lateral error value for diameter of strut vs angle of strut

0.20 ) 0.15 m m (

e u l a v

r 0.10 o r r e

l a r e t

a 0.05 L

0.00

0 10 20 30 40 50 Angle of strut (degrees)

Figure 60: Effect of dross formation over-sizing error v/s angle of orientation of strut

Figure 61 illustrates the relation between the overhanging surface area to the length,

diameter and angle of orientation of struts. The overhanging surface is significantly affected by

the angle of orientation of the strut. The overhanging surface area is directly proportional to the

potential for dross formation leading to oversizing of struts in the lateral direction. Hence the

lateral error decreases as the strut angle increases.

DMLS

CAD

Overhang surface area

Figure 61: The relation between overhang surface area to change in diameter, length and angle of orientation of strut

The amount of overhang surface is also related to the slice thickness used. As the slice thickness decreases, the overhang surface area decreases. The slice thickness was set to the minimum possible value of 0.025 mm (per machine limitations). Hence, for constant machine and laser parameters, the strut diameters of the manufactured struts have been recorded and detailed in table 12.

Table 12: Measurements of diameters for successfully built struts

Designed Transverse diameter measurements (mm) Lateral diameter measurements (mm)

Diameter Average Min. Max. Std. Dev. Average Min. Max. Std. Dev. (mm)

0.1 0.1847 0.1142 0.2780 0.03663 0.143 0.1 0.17 0.0159

0.2 0.2832 0.1772 0.3901 0.04393 0.237 0.21 0.26 0.0131

0.3 0.3784 0.2479 0.4890 0.04359 0.336 0.31 0.36 0.0143

0.4 0.4635 0.3615 0.5609 0.04183 0.436 0.41 0.47 0.0160

0.5 0.5591 0.3624 0.6625 0.05304 0.523 0.48 0.57 0.0188

0.6 0.6570 0.4058 0.7851 0.05688 0.624 0.59 0.66 0.0179

0.7 0.7520 0.5540 0.8718 0.05086 0.728 0.69 0.76 0.0160

0.8 0.8481 0.7274 0.9959 0.04270 0.828 0.8 0.87 0.0167

0.9 0.9426 0.8016 1.0869 0.04501 0.922 0.89 0.95 0.0142

1 1.0446 0.8470 1.1349 0.04680 1.019 0.99 1.06 0.0155

Per table 12, the strut with the least diameter and least dross formation has a transverse measurement of 0.11 mm and a lateral measurement of 0.1143 mm. The minimum manufactured

71 strut diameter depends on the laser spot size. From section 6.1.3 (Evaluation of Diameter of

Strut), it can be observed that strut diameters less than 0.1 mm could not be manufactured. The minimum strut diameter manufactured is a multiple (3x0.04 mm) of the laser spot diameter.

6.2.4 Design Rules

• Design Rule 12: For overhanging struts and the process parameters selected, the

minimum diameter that can be manufactured is 3 times the laser spot diameter.

7 Design and Manufacturing Guidelines Summary

Type Guideline Rationale Big and boxy parts should be aligned towards the starting To ensure adequate feed is imparted and to Part Placement position of the re-coater mitigate loss of feed due to powder flicking Big and boxy parts should be aligned at a slight angle to the To minimize powder flicking due to curling re-coater path Part Orientation Avoid large surfaces inclined at an angle of 45° or less with To minimize scorching due to insufficient heat respect to the build plane dissipation through un-sintered powder Laser beam compensation should be the least possible value To ensure critical feature information is Part setup and for thin walled or minute diameter cellular structures retained slicing Slice thickness should be equal to or less than the diameter To ensure accurate manufacturing of critical of the smallest strut features Powder feed dosage should be increased at the transition To ensure adequate feed is provided and to between thin walled supports and solid parts mitigate loss of feed due to powder flicking Manufacturing Features with a tendency to curl or deform that may damage To ensure successful builds by mitigating the the re-coater should be on an isolated re-coater pathway effect of re-coater damage due to deformations Avoid cellular structures with multiple strut configurations To ensure accurate builds and mitigate laser involving large differences in strut length and diameter parameter optimization for multiple contours Minimize the number of nodes in a cellular structure Minimize over-sizing due to node contours Strut interactions Minimize the number of struts interacting at a node in a To ensure accurate build by minimizing cellular structure complexity of the node To reduce oversizing due to accumulation of Utilize higher angle strut interactions in cellular structures balling at nodes The maximum safe overhanging strut length is 3 mm. Struts Maximum length of self-supporting struts built with length less than 3 mm can be built in any orientation at 0° angle of orientation Process The minimum angle at which overhanging struts can be Minimum angle at which self-supporting struts limitations manufactured successfully is 30° can be built successfully For overhanging struts, the minimum diameter that can be To account for the melt pool size and laser manufactured is 3 times the laser spot diameter beam compensation

8 Conclusions and Future Scope

This research has investigated the manufacturability of high porosity cellular structures using Additive Manufacturing (AM). This research has led to novel designs of 3D periodic high porosity cellular structures. Experimental tests were conducted to identify critical features affecting the manufacturability of designed high porosity cellular structures manufactured using

CL 20ES. Based on the evaluation of the manufacturability based on cell type, size and volume fraction, comprehensive design rules/guidelines have been established. This research has also led to the investigation of manufacturability of safe overhanging struts without support structures that can be utilized to design and customize new 3D periodic high porosity cellular structures.

Also, the limits for manufacturing overhanging struts at different configurations have been discussed. The manufacturing tolerances due to the process limitations of the DMLS process have been identified and optimization strategies have been discussed. This study proposes a safe design space to manufacture overhanging struts and contributes to the increased adoption of high porosity cellular structures in real life applications.

Future work includes the investigation of heat treatment on these structures to improve the dimensional accuracy and relax residual stresses built up within the structure during manufacturing. The evaluation of the cellular structures and overhanging struts have been performed via weight and dimensional measurement and does not take into account the possible deformations of the structures. Evaluation of these structures using a 3D scanning approach such as micro-CT scanning could lead to the development of a model and better optimization strategies to improve the manufacturability. All the structures manufactured in this work have used the machine parameters suggested by Concept Laser and the University of Cincinnati

Research Institute (UCRI). There is great potential to explore and optimize machine parameters specifically for high porosity cellular structures. This work focusses on manufacturability using

316L stainless steel powder, although it is bio-compatible, it has a greater tendency for balling due to its inherent nature to oxidize during sintering because of its chemical composition [46].

Future work includes investigating the manufacturability of these structures using different materials. There is also immense potential in developing a comprehensive and intelligent model that can identify critical features, predict failure possibilities, redesign structures and propose novel designs for high porosity cellular structures.

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