Characteristics and Processing of Hydrogen-Treated Copper Powders for EB-PBF Additive Manufacturing

applied sciences

Article Characteristics and Processing of Hydrogen-Treated Copper Powders for EB-PBF Additive Manufacturing

Christopher Ledford 1, Christopher Rock 1, Paul Carriere 2, Pedro Frigola 2, Diana Gamzina 3,* and Timothy Horn 4,*

1 Center for Additive Manufacturing and Logistics, Edward P. Fitts Department of Industrial and Systems Engineering, North Carolina State University, Raleigh, NC 27695, USA; [email protected] (C.L.); [email protected] (C.R.) 2 RadiaBeam, Santa Monica, CA 90404, USA; [email protected] (P.C.); [email protected] (P.F.) 3 SLAC National Accelerator Laboratory, Technology Innovation Directorate, Menlo Park, CA 94025, USA 4 Center for Additive Manufacturing and Logistics, Department of Mechanical and Aerospace Engineering, Consortium on the Properties of Additive Manufactured Copper, North Carolina State University, Raleigh, NC 27695, USA * Correspondence: [email protected] (D.G.); [email protected] (T.H.)

Received: 16 August 2019; Accepted: 19 September 2019; Published: 24 September 2019

Abstract: The fabrication of high purity copper using additive manufacturing has proven difficult because of oxidation of the powder feedstock. Here, we present work on the hydrogen heat treatment of copper powders for electron beam powder bed fusion (EB-PBF), in order to enable the fabrication of high purity copper components for applications such as accelerator components and vacuum electronic devices. Copper powder with varying initial oxygen contents were hydrogen heat-treated and characterized for their chemistry, morphology, and microstructure. Higher initial oxygen content powders were found to not only reduce surface oxides, but also reduce oxides along the grain boundaries and form trapped H2O vapor inside the particles. The trapped H2O vapor was verified by thermogravimetric analysis (TGA) and residual gas analysis (RGA) while melting. The mechanism of the H2O vapor escaping the particles was determined by in-situ SEM heated stage experiments, where the particles were observed to crack along the grain boundaries. To determine the effect of the EB-PBF processing on the H2O vapor, the thermal simulation and the validation of single melt track width wafers were conducted along with melting single layer discs for chemistry analysis. A high speed video of the EB-PBF melting was performed in order to determine the effect of the trapped H2O vapor on the melt pool. Finally, solid samples were fabricated from hydrogen-treated copper powder, where the final oxygen content measured ~50 wt. ppm, with a minimal residue hydrogen content, indicating the complete removal of trapped H2O vapor from the solid parts.

Keywords: electron beam melting; copper; hydrogen treatment; high purity copper

1. Introduction Applications including particle accelerators and vacuum electronic devices (VEDs) require materials with the highest electrical and thermal conductivity,as well as ultra-high vacuum compatibility. The copper used in these applications approaches the theoretical maximum achievable quality in terms of the purity, density, and metallurgical properties, such as crystallographic texture and grain size. These applications also require intricate designs and extensive metallurgical processing routes, followed by the assembly and brazing or welding of multiple components into a ﬁnal part. The consolidation of the component assemblies into complex monolithic parts, and the potential for novel designs achievable through additive manufacturing (AM), is desired in order to reduce the cost and

Appl. Sci. 2019, 9, 3993; doi:10.3390/app9193993 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 3993 2 of 22 improve the component performance and reliability [1–5]. The underlying challenges associated with the AM of high-purity copper, particularly for electron beam powder bed fusion (EB-PBF) and laser powder bed fusion (L-PBF), have been discussed in several recent studies [1,6–11]. The high thermal conductivity of copper rapidly removes heat from the melt pool, promoting high local and global thermal gradients; residual stress accumulation; and distortion, which is exacerbated by the significant difference in thermal conductivity between the consolidated material and the surrounding powder bed [12–14]. The resulting rapid solidification of the melt pool, coupled with the low viscosity of molten copper, also tends to retain defects such as keyhole porosity [15]. Additionally, the low absorptivity of copper for the lasers used in many commercial L-PBF systems (~1060 nm wavelength) necessitates the use of high-power lasers increasing the recoil pressure, vaporization, spatter, and related defects [16]. However, even with these challenges, several promising results have been reported with densities ranging above 99.95% for non-electronic grade copper [1,7–9] for EB-PBF, and 96.6% for Nd:YAG fiber laser AM processing [17–23]. Despite the progress, achieving the high purity requirements for copper using AM has remained a significant challenge. For accelerator applications, copper typically needs to meet or exceed that of ASTM F68 for Class 1 oxygen free electronic (OFE) copper, which is 0.15176 ohms g/m2 or 101.0% of the minimum International Annealed Copper Standard (IACS) at 20 ◦C[24]. The maximum oxygen content for OFE copper is 0.0005 wt% (5 wt. ppm). The negative influence of oxygen contamination on the electrical, thermal, and mechanical properties of copper is small, but not insignificant. The incoherent Cu2O found along the grain boundaries has an effect similar to porosity, and less of an influence on electron scattering than the solute contamination that strains the copper lattice. For instance, electrolytic tough pitch (ETP) copper has an oxygen content of ~0.04% (400 ppm) and typically exceeds 100–101% IACS [24–26]. Much more significant is the adverse effect of embrittlement in the copper components caused by an excessive oxygen content during the downstream hydrogen brazing processes [27]. It should be noted that the copper AM literature has either resulted in specimens with a high oxygen content (similar to ETP Cu), or has not reported an oxygen content in the fabricated samples. Of chief concern is the powder feedstock purity. The powder feedstock commonly used in AM is subject to oxygen contamination during handling, screening, loading, and transport after atomization. At ambient conditions, the oxygen solubility in pure copper is less than 2 wt. ppm [24,28], where the excess oxygen reacts to form cuprous oxide, Cu2O, as a non-passivating surface film or along the grain boundaries. At higher temperatures (>300 ◦C), cupric oxide (CuO) and other variants may also form [29,30]. This poses a unique obstacle for the copper powder used in AM, which is typically produced by nitrogen gas atomization, and quickly oxidizes if exposed to ambient conditions at any point in the powder fabrication and handling process. Because of the very high surface area of these small diameter particles, the relative contribution of oxygen from the surface oxide film typically causes the feedstock to exceed 400–600 wt. ppm oxygen. For example, in their study on the EB-PBF fabrication of copper, Raab et al. [7], Lodes et al. [8], and Guschlbauer et al. [9] utilize copper powder feedstocks with reported purities of 99.90 wt%, 99.94 wt%, and 99.95 wt%, respectively, while Frigola et al. [1] and Ramirez et al. [10] used powders with 99.99 wt% and 99.80 wt% purity, respectively. In our preliminary studies, a chemical analysis conducted directly after the powder atomization process, and after the screening, indicates that much of the oxygen pickup in the copper powder occurs during the handling, screening, and packaging processes, which are typically carried out in ambient atmosphere. Preserving even these oxygen contents during the AM melting and solidification process is extremely challenging, and requires near perfect conditions, as imperfect inert or vacuum atmospheres, moisture absorption, complex thermal cycles, powder recycling, and storage exacerbate oxygen pickup. Frigola et al. [1] showed a significant oxidation of feedstock powders after several EB-PBF reuse cycles. Once the oxygen contamination is present in the powder feedstock as oxide films or grain boundary particles, the contamination is directly transferred to the fabricated AM components [11]. Appl. Sci. 2019, 9, 3993 3 of 22

This combination of factors hinders the fabrication of quality copper components by most powder bed AM processes. Reduction processes may be employed in order to improve the copper feedstock purity for AM processing. Hydrogen, carbon monoxide, and ethanol are commonly employed reducing agents of Cu2O and CuO in thin films [31–33]. El-Wardany et al. [6] treated copper powders for L-PBF in a heated forming gas atmosphere (4% H2 + Ar bal.), theorizing that the hydrogen would reduce the surface Cu2O films into Cu + H2O, however, no quantitative data on the composition of the powders or solid samples produced were provided [6]. In addition, this study only considered the reduction of oxides from the surface of the Cu powders, and did not address the possibility that hydrogen also reduces the internal oxides. Previous work has shown that the diffusivity of hydrogen in copper is high at elevated 2 temperatures (e.g., ~9.58 m/s at 400 ◦C) [34], and that at temperatures above about 400 ◦C, hydrogen reduces the grain boundary oxides, forming H2O gas. The larger H2O molecules do not diffuse, and the resulting pressure forms high pressure steam pores along the grain boundaries [28,35,36]. This well-known embrittlement mechanism is considered detrimental for most traditional copper powder processing routes because of their significant impacts on downstream processing, such as swelling, porosity, and embrittlement, through plastic deformation, work hardening, the reduction of grain boundary area, and material failure at elevated temperatures [28,36–38], as Lin and Hwang demonstrated with the swelling and cracking of copper powders during the sintering of copper heat pipes [36]. On the other hand, PBF processes utilize thin layers of powder, which are spread over a substrate and selectively melted by a focused energy source. In the present work, we hypothesize that this localized processing can be leveraged to liberate the retained H2O from the hydrogen heat-treated copper powder during melting on a layer-by-layer basis. This results in a significantly higher copper purity, without the detrimental effects of embrittlement observed in traditional powder metal processes. Here, the mechanisms for H2O removal from copper powders is identified through a series of experiments, one building upon the next. First, copper powders of varying initial oxygen contents were treated in a hydrogen atmosphere at elevated temperatures. The temperature that H2O escapes from these copper powders was determined using thermogravimetric analysis. The powder cracking during H2O released at these temperatures was observed directly using a scanning electron microscope (SEM) equipped with a heated stage. We confirm the release of H2O in a single layer melt in a custom EB-PBF setup using residual gas analysis (RGA), and the subsequent chemical analysis of the melted layers. The single melt pool wide thin walled wafers produced with multiple layers of varying hatch spacings confirm the H2O release from the copper powder in the heated affected zone of the melt pool, and high-speed videos of these melt pools also show that a proportion of this H2O escapes from the molten during melting. The efficacy of this approach is validated with the fabrication of fully dense parts, and multi-layer samples with a very low oxygen content, with RGA monitoring of H2O outgassing on a layer by layer basis.

2. Materials and Methods The experiments were performed in order to understand the inﬂuence of a hydrogen heat treatment on copper powder feedstocks of varying initial oxygen contents. We evaluated three levels of oxygen content, with and without hydrogen heat treatment. To generate these six conditions, the following methodology was used. Two separate batches of nitrogen gas atomized copper powder were acquired. For the purposes of this discussion, the powders were categorized based on the initial measured oxygen content. The lowest oxygen content powder received was between 100–200 wt. ppm oxygen, and is referred to as low oxygen copper (LO-Cu). The second powder batch with 400–700 wt. ppm oxygen is referred to as medium oxygen content powder (MO-Cu). A selected amount of the received MO-Cu powder was intentionally oxidized to above 1000 wt. ppm oxygen, and is referred as high oxygen content copper (HO-Cu). Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 22 Appl. Sci. 2019, 9, 3993 4 of 22 2.1. Feedstock Preparation

2.1. FeedstockSpherical Preparation MO-Cu powder was received with a nominal 15–53 µm particle size distribution (d10– d90), smaller than what is typical for other materials used in EB-PBF (e.g., Ti6Al4V). Nevertheless, it µ is consistentSpherical with MO-Cu previous powder studies was in receivedcopper AM with by a our nominal group 15–53[4], andm compared particle sizeto early distribution work by (d10–d90),our group smaller and others, than what also is resul typicalts in for a othersignificantly materials reduced used in EB-PBF surface (e.g., roughness Ti6Al4V). [1]. Nevertheless, In order to itmaintain is consistent a comparatively with previous low studiesoxygen incontent copper in AMthe LO by- ourCu powder, group [4 special], and comparedconsideration to early was given work byto the our packaging, group and handling others, also, and results screening in a steps significantly post atomization reduced surface. This powder roughness was [received1]. In order in the to maintainas-atomized a comparatively (−250 µm) condition low oxygen, directly content from in the the manufacturer LO-Cu powder,, and special screened consideration to a nominal was 15 given–53 toµm the particle packaging, size handling, distribution and within screening an steps argon post glove atomization. box. To generate This powder the wasHO-Cu received powder, in the a as-atomized ( 250 µm) condition, directly from the manufacturer, and screened to a nominal 15–53 µm methodology− similar to that of Nieh and Nix [28,30] was utilized. A portion of MO-Cu was treated at particle150 °C for size 1 h distribution in one atmosphere within an of argonair, and glove cooled box. at To a rate generate of 5 °C the per HO-Cu minute. powder, The temperature a methodology was similarkept lower to that in ofthe Nieh current and study, Nix [28 compared,30] was utilized. with the A portion800 °C used of MO-Cu by Nieh was and treated Nix, at to 150 prevent◦C for the 1 h insintering one atmosphere of the small of air,powder and cooled size distribution. at a rate of 5Given◦C per that minute. the diffusion The temperature of oxygen was in copper kept lower is 2e in−11 them2/s current at 400 study,°C [39] compared, the oxygen with penetration the 800 ◦C useddepth by exceeded Nieh and the Nix, particle to prevent diameters the sintering used in of this the study small 11 2 powderduring the size 2 distribution.-h treatment, Given which that is confirmed the diffusion by ofsubsequent oxygen in copperfocused is ion 2e− beamm / s(FIB) at 400 sectioning◦C[39], the of oxygenindividual penetration powder particles depth exceeded. the particle diameters used in this study during the 2-h treatment, whichPortions is confirmed of the bypowder subsequent from each focused of the ion three beam batches, (FIB) sectioning LO-Cu, MO of individual-Cu, and HO powder-Cu, were particles. then exposedPortions to hydrogen of the powder treatment from ateach 400 °C of thefor three4 h under batches, one LO-Cu,atmosphere MO-Cu, of flowing and HO-Cu, pure hydrogen were then, exposedand cooled to at hydrogen a rate of treatment5 °C per minute at 400 in◦C a fordry 4 hydrogen h under oneatmosphere atmosphere heating of flowing cover retort pure furnace hydrogen,. A andcustom cooled stainless at a rate-steel of 5fixture◦C per comprised minute in of a drystacked hydrogen circular atmosphere trays allowed heating the cover processing retort furnace.of 1 kg of A custompowder stainless-steel per treatment fixture cycle comprised in 2 mm of thick stacked layers circular, as shown trays allowed in Figure the 1processing. To avoid of oxygen 1 kg of powdercontamination per treatment after hydrogen cycle in 2 treatment, mm thick layers,the powders as shown were in Figurepacked1 .in To vacuum avoid oxygen-sealed contamination foil bags after afterpurging hydrogen with dry treatment, nitrogen. the The powders vacuum were bags packed were in then vacuum-sealed sealed in a foilsecond bags bag after filled purging withwith dry drynitrogen nitrogen.. All of The the vacuum copper powders bags were were then subsequently sealed in a second stored, bag handled filled, with and drysampled nitrogen. in an All argon of the or coppernitrogen powders atmosphere. were subsequently stored, handled, and sampled in an argon or nitrogen atmosphere.

Figure 1. HydrogenHydrogen brazing brazing furnace furnace at at SLAC SLAC National National Accelerator Accelerator Laboratory Laboratory Technology Technology Innovation Directorate (TID)(TID) (left),(left), and stacked-stainlessstacked-stainless steel ﬁxturefixture (right)(right) for treating multiple trays of copper powder in 2-mm2-mm thick layers.

2.2. Feedstock Characterization Each ofof thethe threethree powderpowder conditions conditions were were characterized characterized before before and and after after hydrogen hydrogen treatment, treatment for, sizefor size distribution distribution by laserby laser diff ractiondiffraction (Microtrac (Microtrac S3500, S3500 Microtrac, Microtrac Inc., Inc. Montgomeryville,, Montgomeryville, PA, USA),PA, USA and), densityand density by helium by pycnometry helium pycnometry (ASTM B923-16, (ASTM Quantachrome B923-16, Quantachrome MicroUltrapyc Micro 1200e,Ultrapyc Quantachrome 1200e, Instruments,Quantachrome Boynton Instruments Beach,, Florida,Boynton USA). Beach, The Florida, oxygen USA and). hydrogenThe oxygen content and hydrogen of all the powders content of were all quantifiedthe powders by were inert quantified gas fusion analysisby inert (LECOgas fusion OHN836, analysis LECO, (LECO St. OHN836 Joseph, MI,, LECO, USA). St. The Joseph, reported MI, valuesUSA). The herein reported were acquiredvalues herein immediately were acquired prior toimmediately the experiments. prior to The the powderexperiments morphology. The powder was characterizedmorphology was by a characterized JOEL 6010LA by scanning a JOEL electron 6010LA microscope scanning electron (JEOL USA, microsc Inc.,op Peabody,e (JEOL MA,USA, USA). Inc., AnPeabody, additional MA, powderUSA). A characterizationn additional powder was performed characterization on individual was performed powder particleson individual using powder focused ionparticles beam using (FIB) millingfocused for ion the beam cross-sectional (FIB) milling analysis for the bycross secondary-sectional electron analysis and by gallium secondary ion electron contrast onand a gallium FEI Quanta ion contrast 3D Field on Emission a FEI Quanta Gun (FEG) 3D Field (Thermo Emission Fisher Gun Scientific, (FEG) (Thermo Hillsboro, Fisher Oregon, Scientific, USA). Appl. Sci. 2019, 9, x 3993 FOR PEER REVIEW 5 of 22

Hillsboro, Oregon, USA). Scanning transmission electron microscopy (STEM) analysis and energy Scanningdispersive transmission spectroscopy electron (EDS) microscopywere performed (STEM) on analysisa FEI Talos and energy(Thermo dispersive Fisher Scientific, spectroscopy Hillsboro, (EDS) wereOregon, performed USA), to onassess a FEI the Talos chemical (Thermo composition, Fisher Scientific, size of surface Hillsboro,, and internal Oregon, oxide USA), species. to assess the chemicalA thermogravimetric composition, size analysis of surface, (TGA) and was internal performed oxide species.using a TA SDT 650 Simultaneous Thermal AnalyzerA thermogravimetric (TA Instruments, analysis New Castle, (TGA) DE, was USA performed) under usingflowing a TA nitrogen SDT 650, while Simultaneous the samples Thermal were Analyzerheated at 5 (TA °C /s Instruments, to 600 °C in Newan alumina Castle, sample DE, USA) holder under in order flowing to identify nitrogen, the while temperature the samples at which were heatedthe H2O at can 5 ◦ Cescape/s to 600 from◦C the in antreated alumina copper sample powders holder. in order to identify the temperature at which the H2O can escape from the treated copper powders. 2.3. In-Situ Heated Stage 2.3. In-Situ Heated Stage In-situ heated stage SEM degassing experiments were performed on a FEI Quanta 3D FEG so as to observeIn-situ the heated mechanism stage SEM of H degassing2O gas release experiments from a weresingle performed copper powder on a FEI particle. Quanta Both 3D FEGhydrogen so as toheat observe treated the copper mechanism particles of H and2O gas untreated release from copper a single particles copper were powder separated particle. from Both the hydrogen adjacent heatpowder treated feedstock copper with particles a single and fiber untreated brush copperusing a particleslight microscope were separated at 50× magnification. from the adjacent Individual powder feedstockpowder particles with a singlewere then fiber placed brush usingonto a a SiC light heated microscope stage (Protochips at 50 magnification. Fusion) for Individualcontrolled powderheating × particlesunder stan weredard then SEM placed operating onto conditions. a SiC heated The stage stage (Protochips was ramped Fusion) to a maximum for controlled temperature heating underof 400 standard°C at 100 °C SEM/s, and operating image conditions.sequences were The stagemade was at 30 ramped fps with to secondary a maximum electron temperature detectors. of 400 ◦C at 100 ◦C/s, and image sequences were made at 30 fps with secondary electron detectors. 2.4. Customized EB-PBF Platform 2.4. Customized EB-PBF Platform All of the electron beam melting (EBM) experiments were conducted using a customized Arcam A2 EBAll-PBF of thesystem electron (Arcam beam Buil meltingd Control (EBM) Software experiments V3.2, SP2 were, Arcam conducted AB, Sweden using a). customized While the EB Arcam-PBF A2operation EB-PBF and system process (Arcam parameters Build Control have Softwarebeen described V3.2, SP2, in detail Arcam elsewhere AB, Sweden). [40], Whileit is important the EB-PBF to operationhighlight that and in process this work parameters, the internal have components been described of the in commercial detail elsewhere system [40 have], it is been important removed to highlightand replaced that with in this a custom work, the304 internalstainless components steel build tank, of the platform commercial, and systempowder have feeder been system. removed The andcustomized replaced chamber with a custom was designed 304 stainless with viewports steel build that tank, are platform, oriented and radially powder about feeder the system.vertical axis The customizedof the electron chamber beam wascolumn designed at a 55° with angle viewports to the thatsurface are orientedof the experimental radially about substrate/fixture the vertical axis, ofin theorder electron to view beam the column entire buildat a 55 surface.◦ angle toThe the e surfacexperiments of the were experimental monitored substrate with by/fixture, an emissivity in order tocorrected view the single entire color build pyr surface.ometer (Fluke The experiments Process Endurance were monitored Pyrometer with, Fluke, by an Everett, emissivity WA, corrected USA), a singlevisible color light pyrometer camera (Nikon (Fluke D7000 Process, Nikon, Endurance Melville, Pyrometer, NY, USA Fluke,), a Everett, 200-amu WA, residual USA), a gas visible analyzer light camera(Stanford (Nikon Research D7000, Systems, Nikon, Sunnyvale, Melville, NY, CA, USA), USA) a, 200-amuand an IR residual thermal gas imaging analyzer camera (Stanford (FLIR Research A655sc, Systems,FLIR, Wilsonville, Sunnyvale, OR, CA, USA USA), ). and The an v IRiewport thermal window imaging materials camera (FLIR varied A655sc, as aFLIR, function Wilsonville, of the OR,transmittance USA). The requirements viewport window for each materials instrument. varied asFigure a function 2 shows of the a photograph transmittance of requirementsthe customized for eachsetup. instrument. Figure2 shows a photograph of the customized setup.

Figure 2. PhotographsPhotographs of of the the customized Arcam A2 electron beam melting system,system, showing the interior of of the the vacuum vacuum chamber, chamber, mechanical mechanical components components and in-situ and in backscatter-situ backscatter electron electron (BSE) detector (BSE) (detectorA), and the(A), exterior and the of exterior the vacuum of the chamber, vacuum showingchamber, the showing electron the beam electron column beam and column the location and the of instrumentationlocation of instrumentation and viewports and vi (Bewports). The line (B). of The sight line of of thesight visible of the light visible camera, light camera, pyrometer, pyrometer, and IR cameraand IR camera are illustrated are illustrated by the dashedby the dashed lines in lines 2A, from in 2A viewports, from viewports 1, 2, and 1, 3, 2, respectively. and 3, respectively.

2.5. Simulation and Validation of Variable Spacing Single Melt Track Width Wafers Appl. Sci. 2019, 9, 3993 6 of 22

Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 22 2.5. Simulation and Validation of Variable Spacing Single Melt Track Width Wafers We hypothesized that during melting, H2O vapor escapes from both the liquid melt pool and fromWe powder hypothesized in the heat that affected during zone. melting, For Hthe2O powder vapor escapes adjacent from to the both melt the pool, liquid the melt internal pool andpressure from powderof the H in2O the vapor heat overcomes affected zone. the Formechanical the powder strength adjacent of the to thegrain melt bo pool,undary the, internaland the pressurepore begins of the to Hexpand2O vapor in size. overcomes This leads the mechanicalto the fracturing strength of the of theparticle grain along boundary, the grain and theboundaries pore begins and to a expandrelease inof size.H2O vapor This leads into tothe the vacuum fracturing system of the. To particle explore along this, s theingle grain melt boundaries track width and scenarios a release were of H simulated2O vapor intoat various the vacuum hatch spa system.cings To, utilizing explore the this, 3D single trans meltient heat track transfer width scenarios model developed were simulated by Lee at et various al. [41] hatchat the spacings,Oak Ridge utilizing National the Laboratory 3D transient. C heatalculations transfer were model made developed using byFEniCS Lee et software al. [41] at (2019.1.0 the Oak, RidgeFEniCS National), utilizing Laboratory. python Calculations scripts, and were the thermal made using model FEniCS output software was (2019.1.0,analyzed FEniCS), using ParaView utilizing pythonvisualization scripts, software and the ( thermalv5.7, Kitware model Inc., output Clifton was analyzedPark, NY, using USA) ParaView. The simulation visualization used variables software (v5.7,including Kitware heat Inc., input, Clifton beam Park, speed, NY, thermal USA). conductivity, The simulation and used so on, variables to model including the EB- heatPBF input,process beam and speed,the temperature thermal conductivity, profile across and aso so on,lid to plate model with the EB-PBF the thermophysical process and the properties temperature of a profile comparable across acopper solid platepowder with bed the during thermophysical different operating properties conditions. of a comparable The simulation copper powder variables bed used during in this diff studyerent operatingare listed in conditions. Appendix The A. simulation variables used in this study are listed in AppendixA. Based on the model results, single melt track width wafers 13 mm long and 1010 mm tall were fabricated using hydrogen-treatedhydrogen-treated HOHO-Cu-Cu powder with 2.0 mm on center spacings,spacings, as shown in Figure3 3.. TheThe buildbuild substratesubstrate waswas aa machinedmachined OFEOFE coppercopper substratesubstrate thatthat measuredmeasured 9090 mmmm inin diameterdiameter and 30 mm mm in in thickness. thickness. Duri Duringng the the EB EB-PBF-PBF processing, processing, the the build build temperature temperature was was maintained maintained at 270 at 270°C , measured◦C, measured by a bythermocouple a thermocouple attached attached to the to bottom the bottom of the of plate the plate,, and the and partial the partial pressures pressures of H2 ofO Hvapor2O vapor and otherand other gas gasspecies species were were monitored monitored using using the the RGA RGA.. Each Each wafer wafer was was melted melted with with a a line energy of 0.8 JJ/mm/mm (600(600 mmmm/s,/s, 8 mA, 0 mA focus offset offset)) and layer thickness of 40 µµmm,, with the power analyze function (Arcam specificspecific thermal controls) disabled. The beam scan direction was constant (right(right toto left)left) forfor eacheach waferwafer andand eacheach layer.layer.

FigureFigure 3. 3. Illustration of the single melt track track width width geometry geometry with with 2.0 2.0 mm mm spacing. spacing. Powder was harvested from between the wafers in a serial fashion,fashion, as a function of the distance from the solid copper wall.

Upon the build completion, eacheach block of wafers was removed from the surroundingsurrounding sintered powder andand buildbuild surface surface using using a razora razor blade. blade. The The sintered sintered powder powder cake cake between between the solidthe solid wafers wafers was seriallywas serially sectioned sectioned with with a razor a razor blade blade as a as function a function of the of distancethe distance from from the the melt melt pool, pool and, and placed placed on carbonon carbon tape tape for theforSEM the SEM analysis. analysis. Additionally, Additionally, the top the surface top surface of the intact of the sintered intact cakesintered was cake analyzed was onanaly a JOELzed on 6010LA, a JOEL in 6010LA order, to in observe order to sintered observe powder sintered characteristics powder characteristics at specific at distances specific distances from the meltfrom track.the melt track. 2.6. EB-PBF Processing of Hydrogen-Treated Copper Powder 2.6. EB-PBF Processing of Hydrogen-Treated Copper Powder To directly observe and quantify the H2O release during EB-PBF, single layer discs were melted To directly observe and quantify the H2O release during EB-PBF, single layer discs were melted using the various hydrogen-treated and untreated powders. Figure4 illustrates the experiment. The using the various hydrogen-treated and untreated powders. Figure 4 illustrates the experiment. The fixture upon which the experiments were carried out was an annealed OFE copper cylinder, 100 mm in fixture upon which the experiments were carried out was an annealed OFE copper cylinder, 100 mm diameter and 50 mm thick. Six equally spaced cylindrical pockets, measuring 25 mm in diameter by in diameter and 50 mm thick. Six equally spaced cylindrical pockets, measuring 25 mm in diameter 3 mm deep were milled into the surface at a radial distance of 30 mm from the plate center, such that by 3 mm deep were milled into the surface at a radial distance of 30 mm from the plate center, such that each pocket is effectively isolated from the others, as seen in Figure 4. After the machining, the Appl. Sci. 2019, 9, 3993 7 of 22 Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 22 eachcopper pocket substrate is effectively was cleaned isolated using from thea standard others, as ultra seen-high in Figure vacuum4. After (UHV) the machining, protocol for the machined copper substratecopper. was cleaned using a standard ultra-high vacuum (UHV) protocol for machined copper.

FigureFigure 4. 4.Illustration Illustration of of the the in-situ in-situ H H2O2O vapor vapor out-gassing out-gassing experiment. experiment. The The electron electron beam beam melting melting (EBM)(EBM) vacuum vacuum chamber chamber is is outfitted outfitted with with a a residual residual gas gas analysis analysis (RGA) (RGA) and and IR IR camera, camera, and and the the surface surface temperaturetemperature is is monitored monitored by by a two-color a two-color pyrometer pyrometer and and thermocouple thermocouple (A). ( TheA). The base base fixture fixture contains contains six 3six mm 3 deep mm pockets deep pockets (B), which (B), arewhich filled are with filled hydrogen-treated with hydrogen and-treated untreated and untreated powders with powders varying with initialvarying oxygen initial content. oxygen A content. single layerA single disc layer is melted disc is on melted the top on surface the top of surface each powder of each type. powder During type.

melting,During Hmelting,2O is released H2O is from released the powder from the or powder melt pool, or and melt detected pool, and by thedetected RGA (byC). the Upon RGA completion, (C). Upon thecompletion, powder is the manually powder removed is manually from removed the solid from disc the of copper,solid disc which of copper is analyzed, which for is oxygenanalyzed and for hydrogenoxygen and content hydrogen by inert content gas fusion by inert (D). gas fusion (D).

EachEach of of the the pockets pockets in in the the substrate substrate were were filled filled and and leveled leveled to to the the top top surface surface of of the the plate plate with with ~5~5 grams grams of of LO-Cu, LO-Cu, MO-Cu, MO-Cu, or or HO-Cu HO-Cu powder powder before before or or after after hydrogen hydrogen treatment, treatment each, each having having a a replicate.replicate. The The oxygen oxygen and and hydrogen hydrogen content content of theof the powder, powder, as noted as noted previously, previously, was measured was measured by inert by gasinert fusion gas priorfusion to prior thisexperiment. to this experiment. A type K A thermocouple type K thermocouple was affixed was to affixed the bottom to the of thebottom substrate, of the andsubstrate the substrate, and the was substrate placed on was top placed of the onbuild top piston, of the which build allows piston,for which the thermal allows for isolation the thermal from theisolation system. from the system. 5 The vacuum chamber was evacuated to a pressure of 3.75 e Torr,−5 and the copper substrate was The vacuum chamber was evacuated to a pressure of 3.75× − × e Torr, and the copper substrate heatedwas heated by repeatedly by repeatedly scanning scanning the defocused the defocused electron electron beam in beam a circular in a circular area, 25 area mm, 25 in diametermm in diameter at the centerat the of center the substrate, of the substrate with a beam, with current a beam of ~2–4 current mA. of The ~2 substrate–4 mA. The temperature substrate was temperature measured was by themeasured thermocouple by the atthermocouple the bottom, andat the a two-colorbottom, and pyrometer a two-color through pyrometer a sapphire through window a sapphire positioned window in onepositioned of the upper in one viewports. of the upper viewports. TheThe substrate substrate heating heating was was manually manually modulated modulated until until both both of of the the temperature temperature readings readings agreed agreed withinwithin 5% 5% of of the the target target value value (300 (300◦ C),°C ) and, and then then this this temperature temperature was was maintained maintained for for a a 2 2 h h outgassing outgassing cycle.cycle. A A 200 200 amu amu residual residual gas gas analyzer analyzer (SRS-RGA200) (SRS-RGA200) was was used used to to measure measure the the partial partial pressure pressure of of thethe gas gas species species of of interest interest for for this this study study (O, (O, H, H, N, N, NO, NO, NO NO2,2 and, and H H2O).2O). During During the the outgassing outgassing cycle, cycle, 7 thethe background background H H2020 partial partial pressure pressure was was observed observed to to decrease decrease and and stabilize stabilize at at ~8 ~8 e×− e−7Torr. Torr. This This × outgassingoutgassing period period at at 300 300◦ C°C also also allowed allowed su sufficientfficient time time for for the the initial initial stages stages of of sintering sintering to to take take place place withinwithin the the powder. powder This. This was was observed observed with with the the IR IR camera camera by by a a uniform uniform emissivity emissivity shift shift across across all all six six powderpowder pockets. pockets. TheThe focused focused electron electron beam beam was was then then raster raster scanned scanned over over an areaan area measuring measuring 20 mm 20 mm in diameter in diameter on theon surface the surface of each of each one of one the of powder the powder pockets. pockets. A line A energy line energy of 0.8 Jof/mm 0.8 (600J/mm mm (600/s, mm/s, 8 mA, 08 mAmA, focus 0 mA offfocusset, 100 offset,µm hatch100 µm off hatchset) was offset) suffi cientwas sufficientto melt and to consolidate melt and consolidate a single layer a single of copper layer supported of copper underneathsupported byunderneath the sintered by powder. the sintered The melting powder. process The mel wasting recorded process by was the thermalrecorded imaging by the camerathermal throughimaging a germaniumcamera through port a at germanium 25 Hz, and byport in-situ at 25 monitoringHz, and by ofin- thesitu back monitoring scattered of electronthe back signal. scattered A electron signal. A thermal video of the melting process is available in the supplemental data. During the melting, the outgassing of the key gas species was monitored by the RGA at 1 Hz. This process Appl. Sci. 2019, 9, 3993 8 of 22 thermal video of the melting process is available in the Supplementary Materials. During the melting, the outgassing of the key gas species was monitored by the RGA at 1 Hz. This process was repeated on each powder pocket with approximately 10 min to pass between each raster melt in order to allow for the RGA partial pressures and substrate temperature to stabilize. The solids samples were then cooled under a vacuum to room temperature. The material in each pocket was removed from the substrate and the sintered powder was mechanically removed from the solid wafer with a wire brush in an inert gas glove box. The oxygen and hydrogen content of both the sintered powder and solid wafer from each pocket were measured by inert gas fusion (LECO Form 203-821-455, LECO OHN836, LECO, St. Joseph, MI, USA). The experiment was repeated several times, varying the position of each powder type and the order in which powders were melted.

2.7. High Speed Footage of Melt Pool

In order to directly observe the H2O vapor escaping from the melt pool, a Photron SA-X2 high-speed camera was utilized to capture the footage at 30,000 frames per second, at a focal length of 30 cm. The footage was captured for both a hydrogen-treated LO-Cu and HO-Cu melt pool using the same procedure for the EB-PBF processing as described in Section 2.6.

2.8. Solid Sample Fabrication To validate the process for fabricating the components of high purity copper with hydrogen heat-treated powers, solid cylindrical specimens measuring 15 mm in diameter were fabricated for the microstructural and chemical analysis, using both untreated and hydrogen-treated MO-Cu powder. The untreated powder had an oxygen content (~600 wt. ppm) similar to what was used in the studies by Frigola et al. [1], Lodes et al. [8], and Guschlbauer et al. [9]. The processing parameters used were 600 mm/s, 8 mA, 18 mA focus oﬀset, and 120 µm hatch oﬀset with a 40 µm layer thickness. This falls within the processing space (~166 J/mm3) used by both Frigola et al. [1] and Guschlbauer et al. [9]. The build temperature was maintained at 300 10 C for the duration of the fabrication. The gas species in ± ◦ the chamber were monitored during the fabrication process using the RGA, as described earlier.

3. Results

3.1. Powder Characterization The oxygen and hydrogen content were carefully tracked after the atomization, screening, and hydrogen treatment. Table1 shows the oxygen and hydrogen content for the untreated and hydrogen treated LO-Cu, MO-Cu, and HO-Cu screened powders. The LO-Cu powder was procured as-atomized and was sieved in-house under argon in order to maintain a low oxygen content of ~225 wt. ppm and ~1 wt. ppm hydrogen. After the hydrogen treatment of the LO-Cu powder, the oxygen content was reduced to ~50 wt. ppm, with minimal change in the hydrogen content. The untreated MO-Cu powder contained ~450 wt. ppm oxygen and ~2 wt. ppm hydrogen, and after hydrogen treatment, the oxygen content was reduced to ~280 wt. ppm, but the hydrogen content increased to ~30 wt. ppm, indicating that the hydrogen was retained within the powder after treatment, likely as trapped H2O, which is consistent with the reported literature by Nieh and Nix [28,30]. As a third condition, the HO-Cu was intentionally oxidized in order to exaggerate the effects of oxygen, and produced an extreme condition with oxygen measured at ~1500 wt. ppm and ~2 wt. ppm hydrogen. After the hydrogen treatment of the HO-Cu powder, the oxygen content was reduced to ~580 wt. ppm, but the hydrogen content increased to ~75 wt. ppm, again most likely trapping H2O inside the particles. No significant sintering is observed after the powders undergo the hydrogen treatment. Figure5 shows a plot of the atomic percent of oxygen versus the atomic percent of hydrogen for all of the hydrogen heat-treated copper powders used in this study. The 2:1 stoichiometric relationship is a key indicator that hydrogen is retained in the powder in the form of H2O vapor. It is also notable that the linear Appl. Sci. 2019, 9, 3993 9 of 22 relationship is offset by ~0.06% at. % oxygen, which can be accounted for volumetrically on the surface ofAppl. the Sci. powder 2019, 9, x particles. FOR PEER REVIEW 9 of 22

produced an extreme conditionTable with 1. Chemicaloxygen measured content of copperat ~1500 powders. wt. ppm and ~2 wt. ppm hydrogen. After the hydrogen treatment of the HO-Cu powder, the oxygen content was reduced to ~580 wt. Average Standard Average Standard Powder ID Condition ppm, but the hydrogen content increasedO2 (wt. ppm) to ~75 Deviationwt. ppm, (wt. again ppm) mostH2 (wt.likely ppm) trappingDeviation H2O (wt. inside ppm) the

particles.LO-Cu No significantUntreated sintering is observed 226.51 after the 11.36 powders undergo 0.87 the hydrogen 0.56 treatment. Figure(15–53 5 shows um) a plotHydrogen of the Treated atomic percent 54.76 of oxygen 8.34versus the atomic 1.56 percent of hydrogen 0.97 for all of the hydrogenMO-Cu heat-Untreatedtreated copper powders 462.17 used in 18.87this study. The 2:1 1.81 stoichiometric 0.66 relationship (15–53 um) is a key indicator thatHydrogen hydrogen Treated is retained 282.33 in the powder 3.79 in the form of 31.83 H2O vapor. It is 2.02 also notable that theHO-Cu linear relationshipUntreated is offset 1507.33 by ~0.06% at 12.70. % oxygen, which 1.95 can be accounted 0.99 for (15–53 um) volumetrically on Hydrogenthe surface Treated of the powder 586.97 particles. 35.28 76.44 5.77

Figure 5. Plot showing at at.. % % oxygen oxygen versus at at.. % % hydrogen hydrogen for for all all of of the the hydrogen hydrogen heat heat-treated-treated copper powderspowders usedused inin thisthis study.study.

Further evidence for the formation of H O vapor induced cavities in the powder is supported by Further evidence for the formation of H22O vapor induced cavities in the powder is supported by thethe heliumhelium pycnometry resultsresults forfor eacheach powderpowder conditioncondition (shown(shown inin FigureFigure6 6).). No No significantsignificant changechange inin densitydensity isis seenseen afterafter thethe hydrogen treatment of the LO-CuLO-Cu particles from the untreated condition. Without internal, grain boundary Cu O, the hydrogen does not react to form H O vapor. However, Without internal, grain boundary Cu22O, the hydrogen does not react to form H22O vapor. However, both the MO MO-Cu-Cu and HO HO-Cu-Cu particles particles sho showw a a decrease decrease in in density density after after hydrogen hydrogen treatment treatment,, which which is is likely caused by the formation of H O in the particles due to the significant difference in densities likely caused by the formation of H2O2 in the particles due to the significant difference in densities between H O (1 g/cm3) and copper (8.96 g/cm3). The decrease in density among the as received between H22O (1 g/cm3) and copper (8.96 g/cm3). The decrease in density among the as received powders can be attributed to the increasing contribution of Cu O with the increasing oxygen content, powders can be attributed to the increasing contribution of Cu22O with the increasing oxygen content, whichwhich hashas aa densitydensity ofof onlyonly ~6~6 gg/cc./cc. Because of its relatively high oxygen content, the MO-Cu powder was selected to observe the effect of hydrogen heat treatment on the surface oxides. Figure7 shows the representative low and high magnification scanning electron micrographs of (Figure7A,C) and the hydrogen-treated (Figure7B,D) MO-Cu powder. Figure7C shows the surface of the untreated MO-Cu powder, where bright spots were identified as oxygen-rich copper particles. Figure7D shows the surface at the same magnification after the hydrogen treatment of the MO-Cu powder, where the surface oxides have been largely removed. These results were typical for untreated and hydrogen-treated MO-Cu and HO-Cu particles.

Figure 6. Plot showing the skeletal density of the untreated and hydrogen heat-treated copper powders of varying initial oxygen content, as measured by helium pycnometry. Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 22 produced an extreme condition with oxygen measured at ~1500 wt. ppm and ~2 wt. ppm hydrogen. After the hydrogen treatment of the HO-Cu powder, the oxygen content was reduced to ~580 wt. ppm, but the hydrogen content increased to ~75 wt. ppm, again most likely trapping H2O inside the particles. No significant sintering is observed after the powders undergo the hydrogen treatment. Figure 5 shows a plot of the atomic percent of oxygen versus the atomic percent of hydrogen for all of the hydrogen heat-treated copper powders used in this study. The 2:1 stoichiometric relationship is a key indicator that hydrogen is retained in the powder in the form of H2O vapor. It is also notable that the linear relationship is offset by ~0.06% at. % oxygen, which can be accounted for volumetrically on the surface of the powder particles.

Figure 5. Plot showing at. % oxygen versus at. % hydrogen for all of the hydrogen heat-treated copper powders used in this study.

Further evidence for the formation of H2O vapor induced cavities in the powder is supported by the helium pycnometry results for each powder condition (shown in Figure 6). No significant change in density is seen after the hydrogen treatment of the LO-Cu particles from the untreated condition. Without internal, grain boundary Cu2O, the hydrogen does not react to form H2O vapor. However, both the MO-Cu and HO-Cu particles show a decrease in density after hydrogen treatment, which is likely caused by the formation of H2O in the particles due to the significant difference in densities between H2O (1 g/cm3) and copper (8.96 g/cm3). The decrease in density among the as received powders can be attributed to the increasing contribution of Cu2O with the increasing oxygen content, Appl.which Sci. has2019 a, 9density, 3993 of only ~6 g/cc. 10 of 22

FigureFigure 6. 6.Plot Plot showing showing the the skeletal skeletal density density of the of untreated the untreated and hydrogen and hydr heat-treatedogen heat copper-treated powders copper ofpowders varying of initial varying oxygen initial content, oxygen as content measured, as measured by helium by pycnometry. helium pycnometry.

Figure 7. RepresentativeRepresentative low low and and high high magnification magnification scanning scanning electron electron micrographs micrographs of untreated of untreated and hydrogenand hydrogen-treated-treated MO-Cu MO-Cu powder powder.. (A) Low (A magnification) Low magnification and (C) high and magnification (C) high magnification of untreated of hydrogenuntreated-treated hydrogen-treated MO-Cu powder MO-Cu; (B) powder; Low magnification (B) Low magnification and (D) high and magnification (D) high magnification of hydrogen- treatedof hydrogen-treated MO-Cu powder MO-Cu. powder.

Appl. Sci. 2019, 9, 3993 11 of 22

The representative particles were serial sectioned using a FIB to reveal the internal structure of the untreated and hydrogen-treated powder of each type using ion channel imaging, as shown in Figure8. Figure8A is an ion image of a sectioned untreated LO-Cu particle showing no obvious porosity or oxide-rich particles. Figure8B,C are cross-sectioned MO-Cu and HO-Cu particles, respectively, which have ﬁne equiaxed microstructures and submicron oxygen-rich particles along the grain boundaries. The distribution of oxide particles along the grain boundaries vary by size, where larger oxides tend to be located at triple points, and smaller oxides along the grain boundaries. Figure8D shows a hydrogen-treated LO-Cu particle with minimal change after hydrogen treatment because of its initially

Appl.low oxygenSci. 2019, content.9, x FOR PEER The REVIEW LO-Cu particles, produced with a special handling consideration, consistently11 of 22 showed few to no internal oxides (within the resolution of the instrument). A grain boundary is shown contrastin Figure between8A,D. However, the two the grains lack ofcan contrast be attributed between tothe the two similarity grains canof the be attributedgrain orientations. to the similarity Figure 8ofE the shows grain a orientations. hydrogen-treated Figure MO8E shows-Cu particle a hydrogen-treated with both a MO-Cumicron particle and submicron with both scale a micron porosity and throughoutsubmicron scale the grainporosity boundaries. throughout The the high grain oxygen boundaries. content The of high the oxygen HO-Cu content particle of leads the HO-Cu to the formationparticle leads of toa significant the formation interconnected of a signiﬁcant porosity interconnected at the grain porosity boundaries at the grain after boundaries hydrogen after heat treatmenthydrogen. heatSeveral treatment. large pores Several are largeevident pores after are hydrogen evident after treatment hydrogen in Figure treatment 8E and in Figure Figure8E,F 8F forfor MOMO-Cu-Cu and HO HO-Cu,-Cu, respectively respectively;; however however,, in both cases cases,, small dark features are evident along the grain boundaries. It It was was not not immediately immediately clear clear from from this this SEM SEM analysis analysis whether whether these these were were residual, residual, unreacted oxides or small water vapor induced pores. Videos of the serial sectioning process using FIB are provided in the Supplementarysupplemental data Materials for each for of each the ofcases the in cases Figure in Figure 8. The8 .HAADF The HAADF STEM imagingSTEM imaging and EDS and (Figure EDS (Figure 9) of9 ) a of grain a grain boundary boundary triple triple point point of of hydrogen hydrogen heatheat-treated-treated MO-Cu MO-Cu confirmsconﬁrms thethe presencepresence of of the the H H2O2O vapor vapor induced induced porosity, porosity consistent, consistent with with the the observations observations of Nieh of Nieh and andNix [Nix28,30 [28,30]]. .

Figure 8. Representative highhigh magniﬁcationmagnification ion ion channeling channeling contrast contrast micrographs micrographs of of pretreated pretreated LO-Cu LO- Cu(A), (A MO-Cu), MO-Cu (B ),(B HO-Cu), HO-Cu (C (C),) and, and postpost hydrogen treatment treatment LO LO-Cu-Cu (D (D), ),MO MO-Cu-Cu (E), (E and), and HO HO-Cu-Cu (F) focused(F) focused ion ionbeam beam (FIB) (FIB) sectioned sectioned Cu powder Cu powder particles particles showing showing the presence the presence of grain of grainboundary boundary Cu2O

inCu the2O untreated in the untreated condition condition (A–C), (whichA–C), reacts which with reacts hydrogen with hydrogen to form toH2 formO pores H2 O(E,F pores). LO (-ECu,F). powder LO-Cu lackspowder significant lacks signiﬁcant grain boundary grain boundary oxides, and oxides, therefore and thereforedoes not doesexhibit not H exhibit2O vapor H 2inducedO vapor porosity. induced Noteporosity. that Notescale thatbars scale vary barsin the vary images. in the images.

Figure 9. HAADF STEM micrograph and energy dispersive spectroscopy (EDS) of hydrogen-treated

MO-Cu powder grain boundary triple point showing submicron scale H2O vapor induced porosity.

Figure 10 shows the results of the thermogravimetric analysis (TGA) of untreated and hydrogen- treated powders for the three initial oxygen contents. A TGA was carried out to ascertain the temperature at which the internal pressure of the H2O vapor pores exceeds the grain boundary strength of the copper powder particles. All of the untreated powders demonstrate no significant change in weight loss after heating to 600 °C at 5 °C /s. However, both hydrogen-treated MO-Cu and Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 22

contrast between the two grains can be attributed to the similarity of the grain orientations. Figure 8E shows a hydrogen-treated MO-Cu particle with both a micron and submicron scale porosity throughout the grain boundaries. The high oxygen content of the HO-Cu particle leads to the formation of a significant interconnected porosity at the grain boundaries after hydrogen heat treatment. Several large pores are evident after hydrogen treatment in Figure 8E and Figure 8F for MO-Cu and HO-Cu, respectively; however, in both cases, small dark features are evident along the grain boundaries. It was not immediately clear from this SEM analysis whether these were residual, unreacted oxides or small water vapor induced pores. Videos of the serial sectioning process using FIB are provided in the supplemental data for each of the cases in Figure 8. The HAADF STEM imaging and EDS (Figure 9) of a grain boundary triple point of hydrogen heat-treated MO-Cu confirms the presence of the H2O vapor induced porosity, consistent with the observations of Nieh and Nix [28,30].

Figure 8. Representative high magnification ion channeling contrast micrographs of pretreated LO- Cu (A), MO-Cu (B), HO-Cu (C), and post hydrogen treatment LO-Cu (D), MO-Cu (E), and HO-Cu (F)

focused ion beam (FIB) sectioned Cu powder particles showing the presence of grain boundary Cu2O in the untreated condition (A–C), which reacts with hydrogen to form H2O pores (E,F). LO-Cu powder Appl. Sci.lacks2019 significant, 9, 3993 grain boundary oxides, and therefore does not exhibit H2O vapor induced porosity.12 of 22 Note that scale bars vary in the images.

Figure 9. HAADF STEMSTEM micrograph and energyenergy dispersive spectroscopy (EDS) of hydrogen-treatedhydrogen-treated

MO-CuMO-Cu powder grain boundaryboundary tripletriple pointpoint showingshowing submicronsubmicron scalescale HH22O vapor induced porosity.porosity.

Figure 1010 shows shows the the results results of the of thermogravimetric the thermogravimetric analysis analysis (TGA) of (TGA) untreated of untreatedand hydrogen and- hydrogen-treatedtreated powders powdersfor the three for the initial three initialoxygen oxygen contents. contents. A TGA A TGA was was carried carried out out to to ascertain ascertain the temperatureAppl. Sci. 2019 at, 9 which, x FOR thePEER internal REVIEW pressure of the H O vapor pores exceeds the grain boundary strength12 of 22 temperature at which the internal pressure of the2 H2O vapor pores exceeds the grain boundary ofstrength the copper of the powder copper particles. powder particles. All of the All untreated of the untreated powders demonstratepowders demonstrate no signiﬁcant no significant change in HO-Cu powders experience weight loss beginning at 375 °C , which then stabilizes at approximately weightchange lossin weight after heatingloss after to heating 600 ◦C to at 5600◦C °C/s. at However, 5 °C /s. However, both hydrogen-treated both hydrogen MO-Cu-treated andMO- HO-CuCu and powders525 °C . experienceHydrogen- weighttreated loss LO- beginningCu powder at also375 ◦ showsC, which minimal then stabilizes mass change at approximately while heating 525 when◦C. Hydrogen-treatedcompared to the LO-Cuhydrogen powder treated also MO shows-Cu and minimal HO-Cu mass particles. change The while weight heating loss when can be compared correlated to to thethe hydrogen initial oxygen treated content MO-Cu of the and powder HO-Cu before particles. hydrogen The weight treatment loss, indicating can be correlated an increased to the presence initial 2 oxygenof H O content in the hydro of thegen powder-treated before particles hydrogen at higher treatment, oxygen indicating contents. an increased presence of H2O in the hydrogen-treated particles at higher oxygen contents.

Figure 10. Thermogravimetric analysis of untreated and hydrogen-treated LO-Cu, MO-Cu, and HO-Cu powderFigure under 10. Thermogravimetric ﬂowing nitrogen. analysis of untreated and hydrogen-treated LO-Cu, MO-Cu, and HO- Cu powder under flowing nitrogen. To directly observe the mechanism for the H2O release during the heating of the treated powders, treatedTo and directly untreated observe particles the were mechanism heated to for 400 the◦C, H and2O then release to melting during atthe 1200 heating◦C, using of thean in-situ treated SEMpowders heating, treated stage (Protochipsand untreated Fusion). particle Its iswere important heated to 400 note °C that, and the then reported to melting temperature at 1200 °C is, thatusing ofan the in surface-situ SEM of heating the heating stage elements, (Protochips not Fusion necessarily). It is important the equilibrium to note temperature that the reported of the temperature particles. Theis temperature that of the surface of the stage of the is heatingcalibrated elements, with a pyrometer not necessarily and correlated the equilibrium to the input temperature current.Here, of the itparticle is assumeds. The that temperature the mass ofof thethe particlestage is iscalibrated small compared with a pyrometer with the inputand correlated power, and to thatthe input the equilibriumcurrent. Here, temperature it is assumed is reached that the quickly. mass of Thisthe particle assumption is small is corroboratedcompared with by the the input data power acquired, and fromthat multiple the equi sourceslibrium in temperature this study (TGA, is reached in-situ quickly. EB-PBF, etc.).This Aassumption temperature is ofcorroborated 400 ◦C was chosenby the fordata imaging,acquired based from on multiple the onset sources temperature in this stud of weighty (TGA, loss in- observedsitu EB-PBF, by TGA. etc.). FigureA temperature 11A shows of 400 a typical °C was hydrogen-treatedchosen for imaging HO-Cu, based particle on the at onset room temperature temperature. of Additional weight loss treated observed and untreatedby TGA. Figure particles 11A areshows included a typical as Supplemental hydrogen-treated video ﬁles. HO-Cu This particle particle at was room placed temperature. on the SiC Additional chip in the treatedSEM, and and untreated particles are included as supplemental video files. This particle was placed on the SiC chip in the SEM, and heated in-situ to 400 °C at 100 °C /s. Figure 11B is an SEM image of the particle surface at 400 °C immediately after fracture lines are observed, as H2O vapor escapes from the particle, illustrated by the arrows. Subsequently, as shown in the supplemental videos, the fracture lines close and heal through diffusion mechanisms. Figure 11C shows images of another particle surface at 400 °C immediately after the onset of the grain boundary fracture. In this case, the particle was quenched from this condition, and subsequently cross sectioned with a FIB, which reveals interconnected porosity along the grain boundaries inside the particle (Figure 11D). Here, the release of H2O vapor at 400 °C causes the interconnected porosity along the grain boundaries underneath the particle surface, with an additional porosity towards the center of the particle. Appl. Sci. 2019, 9, 3993 13 of 22

heated in-situ to 400 ◦C at 100 ◦C/s. Figure 11B is an SEM image of the particle surface at 400 ◦C immediately after fracture lines are observed, as H2O vapor escapes from the particle, illustrated by the arrows. Subsequently, as shown in the Supplemental videos, the fracture lines close and heal through diAppl.ﬀusion Sci. 2019 mechanisms., 9, x FOR PEER REVIEW 13 of 22

Figure 11.11. ScanningScanning electronelectron micrographs micrographs of of a HO-Cua HO-Cu particle particle after after hydrogen hydrogen treatment, treatment before, before (A) and(A)

afterand after (B–D ()B in-situ–D) in- SEMsitu SEM heating heating on a on SiC a chipSiC chip to 400 to◦ 400C at °C 100 at◦ 100C/s °C under/s under a vacuum. a vacuum.

3.2. ElectronFigure 11 BeamC shows Fabrication images of another particle surface at 400 ◦C immediately after the onset of the grain boundary fracture. In this case, the particle was quenched from this condition, and subsequently EBM involves the selective melting of an elevated temperature powder bed for part fabrication. cross sectioned with a FIB, which reveals interconnected porosity along the grain boundaries inside In this study, the powder bed temperature for part fabrication in copper is below the critical the particle (Figure 11D). Here, the release of H2O vapor at 400 ◦C causes the interconnected porosity temperature for H2O release in hydrogen-treated powder. The reported powder bed temperatures along the grain boundaries underneath the particle surface, with an additional porosity towards the for the EB-PBF of copper range from 380 °C (Lodes et al. [8]) to 530 °C (Guschlbauer et al. [9]), with center of the particle. oxygen contents comparable to the MO-Cu used in this study (although with a larger powder size 3.2.distribution). Electron Beam It was Fabrication observed early in the development of this study that the removal of surface oxides with hydrogen heat treatment resulted in difficulty in spreading the powder at elevated EBM involves the selective melting of an elevated temperature powder bed for part fabrication. In powder-bed temperatures (>350 °C ). Without the presence of surface oxides, the activation energy this study, the powder bed temperature for part fabrication in copper is below the critical temperature for sintering is greatly reduced. In some cases, sintering was observed during spreading, resulting in for H O release in hydrogen-treated powder. The reported powder bed temperatures for the EB-PBF homogenous2 and uneven layering. A lower powder bed temperature alleviates this problem to some of copper range from 380 C (Lodes et al. [8]) to 530 C (Guschlbauer et al. [9]), with oxygen contents extent, and, from a practical◦ standpoint, facilitates the◦ removal of powder from closed cavities. Unlike comparable to the MO-Cu used in this study (although with a larger powder size distribution). It was other common materials utilized in EB-PBF (e.g., Ti6Al4V, TiAl, and 718), copper powder is less observed early in the development of this study that the removal of surface oxides with hydrogen heat susceptible to charge-induced scattering, because of its high conductivity, which also enables EB-PBF treatment resulted in difficulty in spreading the powder at elevated powder-bed temperatures (>350 C). processing at lower temperatures, with minimal sintering. ◦ Without the presence of surface oxides, the activation energy for sintering is greatly reduced. In some The single melt track width experiments, described in Section 2.5, were carried out to determine cases, sintering was observed during spreading, resulting in homogenous and uneven layering. A whether the powder adjacent to the melt-pool front reaches a sufficiently high temperature for a lower powder bed temperature alleviates this problem to some extent, and, from a practical standpoint, sufficiently long duration, in order to induce the powder cracking H2O vapor release mechanism facilitates the removal of powder from closed cavities. Unlike other common materials utilized in observed in the heated stage experiments. The thermal input conditions were modeled using the 3D EB-PBFtransient (e.g., heat Ti6Al4V, transfer TiAl,model and developed 718), copper by Lee powder et al. is [41 less] at susceptible the Oak Ridge to charge-induced National Laboratory scattering,, to becauseguide the of experimental its high conductivity, design. A which 2D contour also enables plot of EB-PBF the temperature processing profile at lower during temperatures, melting with with a minimal2000 µm sintering.hatch spacing is shown in Figure 12. The heat affected zone in the powder bed surrounding the meltThe pool single extends melt track several width hundred experiments, microns described below and in to Section the sides 2.5, of were the carriedmelt pool out, ranging to determine from whether500 °C at the150 powderµm from adjacent the center to of the the melt-pool melt pool front, to 350 reaches °C at a distance sufficiently of 2 high50 µm. temperature for a sufficientlyThe powder long duration, samples in were order harvested to induce from the powder between cracking each of H the2O vapor EB-PBF release wafers mechanism from the edge/wall wafers and in ~0.1 mm increments so as to determine the influence of the heat affected zone of the melt pool on the surrounding powder bed particles. Figures 12A–C are the SEM images of the sintered powder adhered to the fabricated part (Figure 12A), ~0.1 mm distance (Figure 12B) and ~0.3 mm distance (Figure 12C) from the part where the surface characteristics of the particles correlate to their respective associated temperature. The sintered powder at the edge of the melt pool was estimated to reach temperatures of 700 °C , where the subsurface H2O is expected to be expelled Appl. Sci. 2019, 9, x FOR PEER REVIEW 14 of 22

Appl.from Sci. the2019 grain, 9, 3993 boundaries, and the particle surface is smoothed due to diffusion at these relatively14 of 22 high temperatures. Powder images at 0.1 mm from the edge of the wall experienced temperatures of observedapproximately in the 400 heated–500stage °C , according experiments. to the The thermal thermal model input, conditionsand had features were modeled similar t usingo the thein-situ 3D transientSEM heating heat (Figure transfer 11 model), where developed obvious grain by Lee boundary et al. [41 cracking] at the Oakoccurred Ridge, but National no surface Laboratory, smoothing to guidewas visible. the experimental The collected design. powder A 2D imaged contour at plot0.3–0.5 ofthe mm temperature from the melt proﬁle pool during showed melting no obvious with a 2000surfaceµm changes hatch spacing because is of shown the relatively in Figure low 12. temper The heatature aﬀected (<350 zone °C ) e instimated the powder by the bed thermal surrounding model, theand melt still poolcontained extends the several characteristic hundred H2 micronsO vapor belowpores. andThese to data the sides support of the the melt hypothesis pool, ranging that H from2O is released in the heat affected zone ahead of the melt pool in EB-PBF. 500 ◦C at 150 µm from the center of the melt pool, to 350 ◦C at a distance of 250 µm.

Figure 12. Contour plot of the temperature profile in the XZ plane during melting, with a 2000 µm hatch Figure 12. Contour plot of the temperature profile in the XZ plane during melting, with a 2000 µm spacing, showing SEM images of particles located at the edge/wall (A), ~0.1 mm (B), and ~0.3–0.5 mm hatch spacing, showing SEM images of particles located at the edge/wall (A), ~0.1 mm (B), and ~0.3– (C) from the wafer. 0.5 mm (C) from the wafer. The powder samples were harvested from between each of the EB-PBF wafers from the edge/wall wafersTo anddetermine in ~0.1 the mm extent increments at which so H as2O to is determinereleased from the hydrogen influence- oftreated the heat copper affected powders zone during of the meltmelting pool as on a function the surrounding of the initial powder oxygen bed content, particles. single Figure layer 12A–C disc ares w theere SEMfabricated images from of the each sintered of the powdertreated and adhered untreated to the powder fabricated conditions part (Figure. Figure 12 13AA),~0.1 shows mm an distance image of (Figure the melted 12B) wafers and ~0.3 on mm the distanceOFE copper (Figure fixture 12C) after from removal the part from where the theEB- surfacePBF system. characteristics Figure 13 ofC shows the particles an optical correlate image to of their the respectivesurface of associatedthe hydrogen temperature.-treated LO The-Cu sintered melted powderdisc, which at the exhibits edge of a the uniform melt pool surface was texture estimated with to evenly spaced melt pools. However, a non-uniform surface texture is observed on the hydrogen- reach temperatures of 700 ◦C, where the subsurface H2O is expected to be expelled from the grain boundaries,treated HO- andCu meltedthe particle disc surface, where is exposed smoothed powder due to candiffusion be seen at these in circular relatively pores high on temperatures. the surface Powder(Figure 13 imagesB). The at oxygen 0.1 mm and from hydrogen the edge content of the of wall the starting experienced powder temperatures, and the resulting of approximately solid discs are tabulated in Figure 13D for the treated powders. The untreated powders showed no change in 400–500 ◦C, according to the thermal model, and had features similar to the in-situ SEM heating (Figureoxygen 11content.), where A obviousminimal grain change boundary in the oxygen cracking content occurred, was but detect no surfaceed in the smoothing treated LO was-Cu visible. solid Thediscs collected from the powder starting imagedpowder. at However, 0.3–0.5 mm the from MO- theCu and melt HO pool-Cu showed solid discs no obvious show an surface approximately changes 50% reduction in the oxygen content from the starting powder. In both cases, a reduction in the because of the relatively low temperature (<350 ◦C) estimated by the thermal model, and still contained hydrogen content is also observed as being approximately proportional to the amount of reduction the characteristic H2O vapor pores. These data support the hypothesis that H2O is released in the heat ainff ectedthe oxygen zone aheadcontent. of the melt pool in EB-PBF. To determine the extent at which H2O is released from hydrogen-treated copper powders during melting as a function of the initial oxygen content, single layer discs were fabricated from each of the treated and untreated powder conditions. Figure 13A shows an image of the melted wafers on the OFE copper fixture after removal from the EB-PBF system. Figure 13C shows an optical image of the surface of the hydrogen-treated LO-Cu melted disc, which exhibits a uniform surface texture with evenly spaced melt pools. However, a non-uniform surface texture is observed on the hydrogen-treated Appl. Sci. 2019, 9, 3993 15 of 22

HO-Cu melted disc, where exposed powder can be seen in circular pores on the surface (Figure 13B). The oxygen and hydrogen content of the starting powder, and the resulting solid discs are tabulated in Figure 13D for the treated powders. The untreated powders showed no change in oxygen content. A minimal change in the oxygen content was detected in the treated LO-Cu solid discs from the starting powder. However, the MO-Cu and HO-Cu solid discs show an approximately 50% reduction in the oxygen content from the starting powder. In both cases, a reduction in the hydrogen content is also Appl.observed Sci. 2019 as, 9 being, x FOR approximately PEER REVIEW proportional to the amount of reduction in the oxygen content.15 of 22

FigureFigure 13. 13. PhotographPhotograph showing showing the the oxygen oxygen free free electronic electronic (O (OFE)FE) copper copper fixture ﬁxture after after EB EB-PBF-PBF of of single single layerlayer discs discs on on top top of of hydrogen hydrogen-treated-treated powder powder cakes cakes (A (A). ).Optical Optical microscope microscope image imagess of ofthe the surface surface of singleof single layer layer disc disc produced produced with with hydrogen hydrogen-treated-treated HO HO-Cu-Cu showing showing pitting pitting and and porosity porosity ( (BB)),, and surfacesurface of of single single layer layer disc disc produced produced with with hydrogen hydrogen-treated-treated LO LO-Cu,-Cu, showing showing a a smooth smooth surface surface texture texture ((C).). Tabulated oxygenoxygen and and hydrogen hydrogen contents contents of of the the precursor precursor powders powders and and the solidthe solid single single layer layer discs discsfor each for treatedeach treated powder powder condition condition (D). (D).

TheThe residu residualal gas gas analysis analysis was was measured measured as asa function a function of time of time during during the EB the-PBF EB-PBF of HO of-Cu, HO-Cu, MO- Cu,MO-Cu, and LO and-C LO-Cuu single single layer layer discs discs,, and and is shown is shown in inFigure Figure 14 14. Each. Each disc disc was was melted melted as as a a single single step step andand the the H H22OO partial partial pressure pressure was was allowed allowed to to stabilize stabilize before before moving to the next step. The The HO HO-Cu-Cu and and MOMO-Cu-Cu powders show an increase in partial pressure of the H2O vapor vapor during during the melting melting of the the disc disc with the intensity of the peak being indicative of of the the relative relative amount amount of of hydrogen hydrogen in in the the powder powder,, as seenseen in in Table 11.. TheThe peakspeaks graduallygradually diminishdiminish asas thethe HH2OO vapor vapor is is removed removed from from the the system system via the vacuum pumps. A A minimal minimal peak peak is is witnessed witnessed during during melting melting for for the the LO LO-Cu-Cu powder because of the absenceabsence of appreciable H 2OO vapor vapor trapped trapped in in the the particles. particles. The presence of porosity on the surface of the discs produced from hydrogen-treated HO-Cu and MO-Cu powder is indicative of excessive spatter, and suggests that a portion of the H2O vapor escapes from the liquid copper, in addition to the cracking mechanism observed in both the EB-PBF wafer tests and the heated stage experiments. To examine this possibility, high speed imaging (30 kfps) was conducted during the melting process. Full videos are included as part of the Supplementary Materials in this report. Selected frames of high-speed footage of the EB-PBF of hydrogen-treated LO-Cu powder (Figure 15) show a relatively stable melt pool with minimal spatter. However, Figure 16 shows selected frames from the high-speed footage of hydrogen treated HO-Cu particles during melting, and in this case, signiﬁcant spatter is observed, caused by the escape of H2O vapor and the expansion in the molten copper. Large bubbles, on the order of 1–2 mm in diameter, are observed expanding and collapsing from liquid copper several mm behind the EB spot. Figure 16B shows a bubble forming in the molten pool, which then collapses and ejects molten metal from the pool shown, in Figure 16C (indicated by arrows). Figure 14. Residual gas analysis of HO-Cu, MO-Cu, and LO-Cu powder during the electron beam powder bed fusion (EB-PBF) of single layer discs.

The presence of porosity on the surface of the discs produced from hydrogen-treated HO-Cu and MO-Cu powder is indicative of excessive spatter, and suggests that a portion of the H2O vapor escapes from the liquid copper, in addition to the cracking mechanism observed in both the EB-PBF wafer tests and the heated stage experiments. To examine this possibility, high speed imaging (30 kfps) was conducted during the melting process. Full videos are included as part of the supplemental data in this report. Selected frames of high-speed footage of the EB-PBF of hydrogen-treated LO-Cu powder (Figure 15) show a relatively stable melt pool with minimal spatter. However, Figure 16 Appl. Sci. 2019, 9, x FOR PEER REVIEW 15 of 22

Figure 13. Photograph showing the oxygen free electronic (OFE) copper fixture after EB-PBF of single layer discs on top of hydrogen-treated powder cakes (A). Optical microscope images of the surface of single layer disc produced with hydrogen-treated HO-Cu showing pitting and porosity (B), and surface of single layer disc produced with hydrogen-treated LO-Cu, showing a smooth surface texture (C). Tabulated oxygen and hydrogen contents of the precursor powders and the solid single layer discs for each treated powder condition (D).

The residual gas analysis was measured as a function of time during the EB-PBF of HO-Cu, MO- Cu, and LO-Cu single layer discs, and is shown in Figure 14. Each disc was melted as a single step and the H2O partial pressure was allowed to stabilize before moving to the next step. The HO-Cu and MO-Cu powders show an increase in partial pressure of the H2O vapor during the melting of the disc with the intensity of the peak being indicative of the relative amount of hydrogen in the powder, as seen in Table 1. The peaks gradually diminish as the H2O vapor is removed from the system via the vacuum pumps. A minimal peak is witnessed during melting for the LO-Cu powder because of the Appl. Sci. 2019, 9, 3993 16 of 22 absence of appreciable H2O vapor trapped in the particles.

Appl. Sci. 2019, 9, x FOR PEER REVIEW 16 of 22 Appl. Sci. 2019, 9, x FOR PEER REVIEW 16 of 22 shows selected frames from the high-speed footage of hydrogen treated HO-Cu particles during showsmelting selected, and in this frames case, from significant the high spatter-speed is footageobserved of, caused hydrogen by the treated escape HO of-Cu H 2Oparticles vapor andduri theng meltingexpansion, and in inthe this molten case, coppersignificant. Large spatter bubbles is observed, on the, ordercaused of by 1– the2 mm escape in diameter of H2O ,vapor are observed and the expanexpandingsion inand the collapsing molten copper from .liquid Large copperbubbles several, on the mm order behind of 1– the2 mm EB inspot diameter. Figure, are16B observed shows a expandingbubble form anding collapsing in the molten from poliquidol, which copper then several collapses mm behind and ejects the EB molten spot . metal Figure from 16B theshow pools a Figure 14. Residual gas analysis of HO-Cu,HO-Cu, MO-Cu,MO-Cu, and LO-CuLO-Cu powder during the electron beam bubbleshown, formin Figureing in 16 theC (indicated molten po byol arrows, which). then collapses and ejects molten metal from the pool powder bed fusion (EB-PBF)(EB-PBF) ofof singlesingle layerlayer discs.discs. shown, in Figure 16C (indicated by arrows). The presence of porosity on the surface of the discs produced from hydrogen-treated HO-Cu and MO-Cu powder is indicative of excessive spatter, and suggests that a portion of the H2O vapor escapes from the liquid copper, in addition to the cracking mechanism observed in both the EB-PBF wafer tests and the heated stage experiments. To examine this possibility, high speed imaging (30 kfps) was conducted during the melting process. Full videos are included as part of the supplemental data in this report. Selected frames of high-speed footage of the EB-PBF of hydrogen-treated LO-Cu Figure 15. Representative frames of high-speed video footage showing the EB-PBF of a single layer powder (Figure 15) show a relatively stable melt pool with minimal spatter. However, Figure 16 Figuredisc on 15.hydrogen Representative-treated framesLOframes-Cu ofpowder of high-speed high -atspeed timestamps video video footage footage -172.333 showing showing ms (A the), the- EB-PBF170.200 EB-PBF ofms a of( singleB a), single-167.687 layer layer disc ms ondisc(C). hydrogen-treated on hydrogen-treated LO-Cu LO powder-Cu powder at timestamps at timestamps172.333 -172.333 ms (A ms), (170.200A), -170.200 ms ( Bms), (B167.687), -167.687 ms (msC). − − − (C).

Figure 16. Representative frames of high-speedhigh-speed video footage showing the EB EB-PBF-PBF of a single layer discFigure onon 16.hydrogen-treatedhydrogen Representative-treated HO-Cu HOframes-Cu powder ofpowder high at-speed at timestamps timestamps video footage 195.433 195.433 msshowing ms (A ),(A 197.700 ),the 197.700 EB ms-PBF (msB ),of ( 198.733B a), single 198.733 ms layer (Cms). disc(C). on hydrogen-treated HO-Cu powder at timestamps 195.433 ms (A), 197.700 ms (B), 198.733 ms Despite(C). the presence of porosity in the single layer melt scenario caused by the escape of H2O vapor fromDespite the liquid the zone, presence it was of speculatedporosity in thatthe single the layerwise layer melt remelting scenario of caused a material by the typical escape in of additive H2O manufacturingvaporDespite from the the wouldliquid presence zone, serve of it toporosity was reduce speculated in or the eliminate single that layerthe this layerw porositymelt scenarioise inremelting the caused fabrication of aby material the of escape components. typical of H in2O In ordervaporadditive tofrom determinemanufacturing the liquid the zone, e ﬀwouldect it ofwas serve successive speculated to reduce melts that or on eliminatethe the layerw removal thisise porosityremelting of H2O, in as of the well a materialfabrication as the solidtypical of sample in density,additivecomponents. cylindersmanufacturing In order were to fabricated woulddetermine serve using the to effect reduce hydrogen-treated of orsuccessive eliminate MO-Cumelts this porosity on powder. the removal in the fabrication of H2O, as ofwell as components.the soliInd the sample EB-PBF In order density, validation to determine cylinders runs, werethe the effect fabricated precursor of successive using untreated hydrogen melts powders on -thetreated removal had MO an- ofCu oxygen H powder.2O, as content well as of ~450the soli ppmd sample and a density, hydrogen cylinders content were of ~2 fabricated ppm, and using the hydrogen solid copper-treated components MO-Cu powder. fabricated with untreatedIn thepowder EB-PBF showvalidation no change runs, the inthe precursor oxygen untreated content. Hydrogenpowders had heat an treatment oxygen content of the MO-Cuof ~450 powderppmIn and the reduces a EBhydrogen-PBF the validation oxygen content content ofruns, ~2 ppm,the to precursor ~280 and ppmthe soliduntreated because copper ofpowders components the volumetric had an fabricated oxygen elimination content with of of surface ~450 oxides,ppmuntreated and and a powder hydrogen additionally, show content no the change hydrogen of ~2 ppm,in the content and oxygen the of solid thecontent. powder copper Hydrogen iscomponents increased heat totreatment fabricated ~33 ppm. of with Solidthe MO samples-Cu fabricateduntrpowdereated reduces powder with EB-PBFthe show oxygen no using change content hydrogen-treated in to the ~280 oxygen ppm becausecontent. MO-Cu ofHydrogen powder the volumetric results heat treatment inelimination an oxygen of the of content surfaceMO-Cu of ~50powderoxides, ppm, and reduces and additionally a negligiblethe oxygen, the hydrogen content hydrogen to content. ~280 content ppm The of because the in-situ powder of RGA the is duringvolumetricincreased both to elimination trials~33 ppm. shows Solid of thesurface partial pressureoxides,samples and fabricated of theadditionally layerwise with ,EB the outgassing-PBF hydrogen using of hydrogen content H2O vapor, of-treated the which powder MO while- Cuis increased powder present onresults to all~33 AM inppm. an powders, oxygenSolid is two orderssamplescontent of of fabricated magnitude ~50 ppm with, and higher EBa negligible-PBF for theusing treated hydrogen hydrogen powders content.-treated compared The MO i-nCu-situ with powder RGA the during untreatedresults bothin an powders. trials oxygen shows These datacontentthe partial are of reported ~50pressure ppm in, Figureofand the a layerwisenegligible17. Figure outgassing hydrogen18 shows thecontent. of representativeH2O vapor,The in -whichsitu microstructures RGA while during present both of theon trials all solid AM shows samples producedthepowders, partial is withpressure twoboth orders of treated the of magnitudelayerwise and untreated outgassing higher powders.for ofthe H treated2O Invapor, both powders which cases, comwhile a columnarpared present with grain on the all untreated structure AM is observed,powders,powders. isThese aligned two dataorders with are theof reported magnitude build direction, in Figure higher and 17. for Figurea the high treated density18 shows powders is the achieved. representative compared with microstructures the untreated powders.of the solid These samples data pr areoduced reported with in both Figure treated 17. Figure and untreated 18 shows powders. the representative In both cases microstructures, a columnar ofgrain the structuresolid samples is observed produced, aligned with bothwith treatedthe build and direction untreated, and powders. a high density In both is cases achieved., a columnar grain structure is observed, aligned with the build direction, and a high density is achieved. Appl. Sci. 2019, 9, x FOR PEER REVIEW 17 of 22

Appl. Sci. 2019, 9, 3993 17 of 22 Appl. Sci. 2019, 9, x FOR PEER REVIEW 17 of 22

Figure 17. Oxygen and hydrogen content of precursor powders and solid copper cylinders produced with the EB-PBF of untreated and hydrogen-treated MO-Cu powder, and (inset) in-situ residual gas Figureanalysis 17. of Oxygenboth EB and- PBF hydrogen runs, showing content the of layerwise precursor outgassing powders andof H solid2O. copper cylinderscylinders produced with the EB-PBFEB-PBF of untreateduntreated and hydrogen-treatedhydrogen-treated MO-CuMO-Cu powder, and (inset)(inset) in-situin-situ residual gas

analysis of both EB-PBFEB-PBF runs, showingshowing thethe layerwiselayerwise outgassingoutgassing ofof HH22O.

Figure 18. Optical micrographs showing polished anandd etched microstructures of copper samples fabricated with with EB-PBF EB-PBF using using hydrogen hydrogen heat-treated heat-treated MO-Cu MO powder-Cu powder (A) and (A untreated) and untreated MO-Cu powderMO-Cu (powderFigureB). The 18. build(B ).Optical The direction build micrographs direction for both samplesfor showing both aresamples polished indicated are anindicated byd etched the arrow by microstructures the in arrow B. in B. of copper samples fabricated with EB-PBF using hydrogen heat-treated MO-Cu powder (A) and untreated MO-Cu 4. Discussion 4. Discussionpowder (B ). The build direction for both samples are indicated by the arrow in B. The oxygen content of each of the three startingstarting powder conditions results from surface oxide filmsfilms4. Discussion and oxides along the grain boundaries. The copper oxide will continue to grow when exposed to oxygen, as it does not passivate [42]. The surface oxide thickness as a function of time at room to oxygenThe oxygen, as it does content not ofpassivate each of [42]the .three The startingsurface oxidepowder thickness conditions as a results function from of surfacetime at oxideroom temperature was empirically derived by White and Germer [43]. A typical oxide thickness would fall temperaturefilms and oxides was alongempirically the grain derived boundaries. by White The and copper Germer oxide [43] .will A typical continue oxide to growthickness when would exposed fall between 3–5 nm, but could be much thicker. The overall contribution of this oxide layer becomes betweento oxygen 3,– 5as nm it ,does but couldnot passivate be much [42] thicker.. The Thesurface overall oxide contribution thickness asof athis function oxide layerof time becomes at room a a significant portion of the chemistry for a powder with a high specific surface area [44]. Figure 19 significanttemperature portion was empirically of the chemistry derived for by Whitea powder and withGermer a high [43] .specific A typical surface oxide thicknessarea [44]. wouldFigure fall 19 shows the calculated wt. ppm of oxygen as a function of the particle diameter for cuprous oxide showsbetween the 3 –calculated5 nm, but couldwt. ppm be muchof oxygen thicker. as aThe functio overalln of contribution the particle of diameter this oxide for layer cuprous becomes oxide a surface films of various thicknesses. In the case of a 5 nm coating, and a typical Gaussian powder size surfacesignificant film portions of various of the thicknesses chemistry. In for the a casepowder of a 5with nm coating,a high specific and a typical surface Gaussian area [44] powder. Figure size 19 distribution centered at a 35 µm diameter, the oxygen content is estimated to be roughly 75 wt. ppm distributishows theon calculated centered atwt. a 35ppm µm of diameter, oxygen theas a oxygen functio contentn of the is particle estimated diameter to be roughly for cuprous 75 wt. oxide ppm oxygen, assuming all oxygen neglecting the 2 wt. ppm in solid solution at room temperature [39]. oxygensurface ,film assumings of various all oxygen thicknesses neglecting. In the the case 2 ofwt. a ppm5 nm incoating, solid solutionand a typical at room Gaussian temperature powder [39] size. The experimental data gathered in this study suggest that the oxide thickness on the LO-Cu particles Thedistributi experimentalon centered data at gathered a 35 µm indiameter, this study the suggest oxygen that content the oxide is estimated thickness to beon roughlythe LO-Cu 75 particleswt. ppm used in this study is ~15–20 nm, consistent with our STEM observations. Volumetrically, this accounts usedoxygen in ,this assuming study is all ~1 5oxygen–20 nm neglecting, consistent the with 2 ourwt. STEMppm in observations solid solution. Volumetrically at room temperature, this accounts [39]. for the measured 200 wt. ppm oxygen, which was mainly surface oxides in the LO-Cu material for forThe the experimental measured 200data wt. gathered ppm oxygen in this ,study which suggest was mainly that the surface oxide oxide thicknesss in the on theLO -LOCu- Cumaterial particles for particles in the range of 15–53 µm. particlesused in this in thestudy range is ~ 1of5– 1520– nm53 µm., consistent with our STEM observations. Volumetrically, this accounts for the measured 200 wt. ppm oxygen, which was mainly surface oxides in the LO-Cu material for particles in the range of 15–53 µm. Appl. Sci. 2019, 9, 3993 18 of 22 Appl. Sci. 2019, 9, x FOR PEER REVIEW 18 of 22

FigureFigure 19.19. Wt. ppm oxygen as a function of the particle size,size, assuming a 5 nm cuprous oxideoxide ﬁlm.film.

TheThe MO-CuMO-Cu andand HO-CuHO-Cu particles particles greatly greatly exceed exceed this this oxygen oxygen content content and and show show a largea large number number of submicronof submicron oxides oxides along along the the grain grain boundaries, boundaries as, seen as seen in Figure in Figure7B,C. 7B Naturally, and Figure the 7 HO-CuC. Naturally, particles the showHO-Cu the particles largest number show the of submicron largest number oxides of along submicron the grain oxides boundaries, along with the grain several boundaries larger oxides, with at tripleseveral points larger of oxides the microstructure at triple points because of the of microstructure its exposure to because air at elevated of its exposure temperatures. to air at elevated temperatures.To remove the oxide particles, a hydrogen atmosphere heat treatment is employed to reduce the copperTo remove oxide the to oxide copper particles, and H2O. a hydrogen This was firstatmosphere observed heat in thetreatment MO-Cu is powderemployed in to Figure reduce7C,D, the wherecopper the oxide surface to copper oxides and were H2O. removed This was by first hydrogen observed treatment. in the MO As- hydrogenCu powder di inffuses Figure rapidly 7C and in 2 copperFigure 7 (900D, whereµm /s the at 400surface◦C[ 34oxides]), it allowswere removed for the reduction by hydrogen of oxides treatment. present As inside hydrogen the particles diffuses inrapidly a relatively in copper short (900 amount µm2/s of at time. 400 °C The [34]) hydrogen, it allows treatment for the reduction temperature of oxides and time present was choseninside the to allowparticles for suinffi a cientrelatively hydrogen short di amountffusion through of time. the The particles, hydrogen while treatment not reaching temperature sintering and temperatures time was forchosen the fineto allow powder for sufficient size distribution. hydrogen Although diffusion not through directly the reported particles herein,, while itnot is worthreaching noting sintering that, nottemperatures since the completion for the fine ofpowder this study, size distribution. we have subsequently Although not demonstrated directly reported the hydrogen herein, it reduction is worth ofnoting copper that powder, not since in severalthe completion furnace of types this (includingstudy, we have batch subsequently and continuous) demonstrated in both pure the hydrogenhydrogen andreduction in forming of copper gas (5% powder h, 95% in Ar),several and furnace we have types observed (including consistent batch resultsand continuous) in all scenarios in both for pure the equivalenthydrogen and treatment in forming times. gas After(5% h, hydrogen 95% Ar), and treatment, we have the observed MO-Cu consistent and HO-Cu results powders in all scenarios exhibit a micronfor the scaleequivalent porosity treatment at the triple times. points After of hydrogenthe grain boundaries, treatment, the and MO a smaller-Cu and submicron HO-Cu powdersporosity alongexhibit the a micron grain boundaries. scale porosity These at the pores triple have points previously of the grain been boundaries shown in the, and literature a smaller to besubmicron trapped Hporosity2O vapor along [28, 30the,35 grain]. boundaries. These pores have previously been shown in the literature to be trappedAccording H2O vapor to Ito [28,30 and, Hayashi35]. [35], the formation of H2O vapor in closed pores is as a result of the reductionAccording of to copper Ito and oxide Hayashi during [35], hydrogen the formation heat treatment.of H2O vapor The in H closed2O vapor pores pressure is as a insideresult of a closedthe reduction pore is higherof copper than oxide that of during the surface hydroge stressn ofheat the treatment. pore, which The inhibits H2O thevapor pore pressure from collapsing. inside a However,closed pore the is H higher2O vapor than pressure that of is the not surface great enough stress to of overcome the pore, thewhich mechanical inhibits strength the pore of from the graincollapsing. boundaries However, of the the particle H2O atvapor room pressure temperature. is not great enough to overcome the mechanical strengthAt around of the gra 350in◦C, boundaries the strength of the of the particle copper at isroom significantly temperature. reduced, and the increasing internal pressureAt a ofround the H3502O °C vapor, the strength overcomes of the the copper mechanical is significantly strength ofreduced the grain, and boundary the increasing and the internal pore beginspressure to of expand the H in2O size. vapor This overcomes leads to thethe fracturingmechanical of strength the particle, of the seen grain as anboundary extensive and network the pore of interconnectedbegins to expand pores in size. along This the leads grain to boundaries, the fracturing and aof release the particle of H2,O seen vapor as intoan extensive the vacuum network system, of asinterconnected seen in the SEM pores images along inthe Figure grain 11boundariesC,D, and, the and RGA a release data of from H2O Figure vapor 14 into. Notably, the vacuum our worksystem is, presentedas seen in forthe aSEM given images size distribution in Figure 11 ofC gasand atomized Figure 11 powder.D, and the While RGA it data is apparent from Figure that the 14. reduction Notably, ofour internal work is Cu presented2O and thefor formationa given size of distribution steam pores of is, gas for atomized all intents powder. and purposes, While it independentis apparent that of particlethe reduction size (owing of internal to the Cu rapid2O an diffdusion the formation of hydrogen of steam in copper pores at 400is, for◦C), all the intents practical and upper purposes, and lowerindependent limits on of particle particle size, size ( forowing which to the observedrapid diffusion phenomena of hydrogen hold true, in copper is not clear at 400 from °C ), this the research.practical upper It is likely and lower that the limits combined on particle influences size, for of which grain the boundary observed strength, phenomena grain hold size, true radius, is not of curvature,clear from andthis research. oxygen content It is likely will that contribute the combined to the steaminfluences embrittlement of grain boundary/fracture strength, mechanism grain in disize,fferent radius ways. of curvature, and oxygen content will contribute to the steam embrittlement/fracture mechanism in different ways. The observations shown here suggest that H2O is released by two mechanisms during EB-PBF. A chemical analysis of the sintered powder bed shows a similar oxygen and hydrogen content as the Appl. Sci. 2019, 9, 3993 19 of 22

The observations shown here suggest that H2O is released by two mechanisms during EB-PBF. A chemical analysis of the sintered powder bed shows a similar oxygen and hydrogen content as the starting powder, as seen in Table1, and suggests that the H 2O vapor is not released during the preheating step, which is measured in the range of 250–270 ◦C. While not explored in this study, it is anticipated that higher preheat temperatures, similar to those used by Frigola [1] and Guschlbauer [9], would be sufficiently high to elicit the effect observed by the TGA and heated stage SEM experiments. In the present case, the simulation results and experiments of a single electron beam melt line in a copper powder bed show that as the electron beam scans across the sample during the melting step, the highly localized heat in front and to the sides of the beam causes the particles in the heat affected zone to fracture because of the H2O vapor expansion at 400 ◦C. The scanning electron microscopy of the particles surrounding the single wafer tracks show fracture surfaces similar to that of Figure 11C. The chemical analyses of the single layer melt solid discs of MO-Cu and HO-Cu show that some amount of H2O vapor is trapped during the solidification of the melt pool, as seen in Figure 14. We theorize that as the particles become molten, the volume of the H2O vapor rapidly expands because of the significant pressure drop due to the vacuum environment, and become bubbles of H2O vapor in the melt pool. According to Boyle’s law, a pressure of 5.7 MPa confined to a sphere with a diameter of 1 um 9 will expand to a sphere with a diameter of 1 mm at a pressure of 5 e− MPa. This is approximately 9 × a 10 increase in the volume of the trapped H2O vapor from within a closed pore. The H2O vapor bubbles are able to escape the melt pool, which is evident in the decrease in oxygen and hydrogen content of the single melt layer discs. However, because of the rapid solidification of the melt pool 4 6 (10− to 10− ◦C/s) and the high viscosity of molten copper, and the large amount of spatter associated with bubble collapse, a portion of these H2O vapor bubbles may become trapped in the solid single layer discs as pores. An observation of the multiple layer build validation runs suggest that these pores are largely eliminated by the subsequent remelting of the previous layers, which allows repeated opportunities for the H2O vapor to escape to the surface. This is in stark contrast to the traditional copper processing routes (powder metal, sintering, annealing etc.) for which oxygen containing copper powder exposed to a high temperature hydrogen atmosphere results in deleterious embrittlement, cracking, swelling, and porosity. This is clearly shown in the density and microstructures of Figure 18. Also evident is an apparent difference in grain size. This was not investigated in this study, but it is reasonable that the differences in oxygen content and the turbulent liquid pool/spatter brought about by the escape of water vapor during solidification could lead to the disruption of grain growth observed. Further analyses would be required in order to verify this hypothesis. While the present study has focused on the EB-PBF processing of copper, it is entirely plausible that a comparable methodology could be repeated for the LPBF processes. However, the poor absorptivity, lower ambient temperatures, and lack of a vacuum atmosphere may contribute to a significantly different outcome. In the work by El-Wardany et al. [6], a relatively high oxygen copper powder was heat treated in a forming gas environment in order to remove the surface oxides and subsequently be coated with a polydimethylsiloxane (PDMS) polymer. No further data on the chemistry, properties, or microstructure of this material were reported, other than the observation of the formation of unexpectedly large pores in single track welds that were not present in the untreated powders. These results, and the results of the current study suggest that further research is warranted, which also accounts for the potentially important effects of chamber atmosphere and the partial pressures of the relevant gas species during the AM processing of copper.

5. Conclusions By utilizing a hydrogen furnace treatment to reduce the oxygen content of atomized copper powder feedstock for use in EBM processing, we have shown the ability to produce pure copper parts with a lower oxygen content than that of the initial feedstock. Depending on the initial oxygen content, the mechanism for reduction shifts from the removal of surface oxides of a low starting oxygen content, to a combination of surface oxide removal and internal H2O vapor formation as the starting oxygen Appl. Sci. 2019, 9, 3993 20 of 22

content increases. The release of internally trapped H2O vapor is shown to occur in the heat aﬀected zone to the front and side of the melt pool, in addition to release from the melt pool. Successive remelting allows suﬃcient time for H2O removal from hydrogen-treated powder, to produce solids resulting in an oxygen content of 50 wt. ppm.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/9/19/3993/s1. Author Contributions: Conceptualization, C.L., C.R., D.G. and T.H.; methodology, C.L., C.R. and T.H.; software, C.L.; validation, C.L. and T.H.; formal analysis, C.L.; investigation, C.L.; writing—original draft preparation, C.L., C.R., D.G. and T.H.; writing—review and editing, C.L., C.R., P.C., P.F., D.G. and T.H.; supervision, T.H.; project administration, C.L., C.R., D.G. and T.H.; funding acquisition, D.G. and T.H. Funding: This research was partially funded by the Navy Sea Systems Command Contract Number N0025316P0261. This research was also partially funded by DARPA INVEST, program N66001-16-1-4044, and the Center for Additive Manufacturing and Logistics, North Carolina State University. Acknowledgments: This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). High speed videos were acquired with the assistance of Dr. Mark Pankow and the Ballistic Loading and Structural Testing Lab (BLAST), Department of Mechanical and Aerospace Engineering, NC State University. We would also like to thank Dr. Yousub Lee at Oak Ridge National Laboratory’s Manufacturing Demonstration Facility for assistance with transient heat input models using FEniCS software. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Appendix A

Parameter Value Units Density 5376.0 kg/m3 Speciﬁc Heat 390 J/kg*K Thermal Conductivity 50 w/m*K Heat Transfer Coeﬃcient 0.1 w/m2 Emissivity 0.5 Preheat Temperature 543 K Plate Thickness 1 mm Heat Input 480 watts Beam Speed 0.6 m/s Beam Radius 100 µm

References

1. Frigola, P.; Harrysson, O.L.A.; Horn, T.J.; West, H.; Aman, R.; Rigsbee, J.M.; Ramirez, D.A.; Murr, L.; Medina, F.; Wicker, R.B.; et al. Fabricating Copper Components. Adv. Mater. Process. 2014, 172, 20–24. 2. Gamzina, D.; Kozina, M.; Mehta, A.; Nanni, E.; Tantawi, S.; Welander, P.; Horn, T.; Ledford, C. Copper Reconsidered: Material Innovations to Transform Vacuum Electronics. In Proceedings of the 2019 International Vacuum Electronics Conference (IVEC), Busan, Korea, 29 April–1 May 2019; pp. 1–2. 3. Gamzina, D.; Luhmann, N.C.; Ledford, C.; Horn, T.; Karakaut, I.; Lin, L.; Frigola, P. Additive vacuum electronics: Electron beam melting of copper. In Proceedings of the 2017 Eighteenth International Vacuum Electronics Conference (IVEC), London, UK, 24–26 April 2017; pp. 1–2. 4. Horn, T.; Karakaut, I.; Ledford, C.; Gonzalez, M.; Gamzina, D.; Luhmann, N.C.; Lin, L. Additively manufactured WR-10 copper waveguide. In Proceedings of the 2018 IEEE International Vacuum Electronics Conference (IVEC), Monterey, CA, USA, 24–26 April 2018; pp. 409–410. 5. Ives, R.L. Additive Manufacturing of Klystrons. In Proceedings of the 2018 Nineteenth International Vacuum Electronics Conference (IVEC), Monterey, CA, USA, 24 April 2018. 6. El-Wardany, T.I.; She, Y.; Jagdale, V.N.; Garofano, J.K.; Liou, J.J.; Schmidt, W.R. Challenges in Three-Dimensional Printing of High-Conductivity Copper. ASME J. Electron. Packag. 2018, 140, 020907. [CrossRef] Appl. Sci. 2019, 9, 3993 21 of 22

7. Raab, S.J.; Guschlbauer, R.; Lodes, M.A.; Körner, C. Thermal and Electrical Conductivity of 99.9% Pure Copper Processed via Selective Electron Beam Melting. Adv. Eng. Mater. 2016, 18, 1661–1666. [CrossRef] 8. Lodes, M.A.; Guschlbauer, R.; Körner, C. Process development for the manufacturing of 99.94% pure copper via selective electron beam melting. Mater. Lett. 2015, 143, 298–301. [CrossRef] 9. Guschlbauer, R.; Momeni, S.; Osmanlic, F.; Körner, C. Process development of 99.95% pure copper processed via selective electron beam melting and its mechanical and physical properties. Mater. Charact. 2018, 143, 163–170. [CrossRef] 10. Ramirez, D.A.; Murr, L.E.; Li, S.J.; Tian, Y.X.; Martinez, E.; Martinez, J.L.; Machado, B.I.; Gaytan, S.M.; Medina, F.; Wicker, R.B. Open-cellular copper structures fabricated by additive manufacturing using electron beam melting. Mater. Sci. Eng. A 2011, 528, 16–17. [CrossRef] 11. Ramirez, D.A.; Murr, L.E.; Martinez, E.; Hernandez, D.H.; Martinez, J.L.; Machado, B.I.; Medina, F.; Frigola, P.; Wicker, R.B. Novel precipitate–microstructural architecture developed in the fabrication of solid copper components by additive manufacturing using electron beam melting. Acta Mater. 2011, 59, 4088–4099. [CrossRef] 12. Bala, K.; Pradhan, P.R.; Saxena, N.S.; Saksena, M.P. Effective thermal conductivity of copper powders. J. Phys. Appl. Phys. 1989, 22, 1068–1072. [CrossRef] 13. Smith, C.J.; Tammas-williams, S.; Hernandez-nava, E.; Todd, I. Tailoring the thermal conductivity of the powder bed in Electron Beam Melting (EBM) Additive Manufacturing. Sci. Rep. Nat. Publ. Group Lond. 2017, 7, 1–8. [CrossRef] 14. Wei, L.C.; Ehrlich, L.E.; Powell-Palm, M.J.; Montgomery, C.; Beuth, J.; Malen, J.A. Thermal conductivity of metal powders for powder bed additive manufacturing. Addit. Manuf. 2018, 21, 201–208. [CrossRef] 15. Martin, A.A.; Calta, N.P.; Hammons, J.A.; Khairallah, S.A.; Nielsen, M.H.; Shuttlesworth, R.M.; Sinclair, N.; Matthews, M.J.; Jeffries, J.R.; Willey, T.M. Ultrafast dynamics of laser-metal interactions in additive manufacturing alloys captured by in situ X-ray imaging. Mater. Today Adv. 2019, 1, 100002. [CrossRef] 16. Mahamood, R.M. (Ed.) Laser Basics and Laser Material Interactions. In Laser Metal. Deposition Process. of Metals, Alloys, and Composite Materials; Springer International Publishing: Cham, Switzerland, 2018; pp. 11–35. 17. Colopi, M.; Caprio, L.; Demir, A.G.; Previtali, B. Selective laser melting of pure Cu with a 1 kW single mode fiber laser. Procedia CIRP 2018, 74, 59–63. [CrossRef] 18. Constantin, L.; Fan, L.; Mortaigne, B.; Keramatnejad, K.; Zou, Q.; Azina, C.; Lu, Y.F.; Silvain, J.-F. Laser sintering of cold-pressed Cu powder without binder use. Materialia 2018, 3, 178–181. [CrossRef] 19. Ikeshoji, T.-T.; Nakamura, K.; Yonehara, M.; Imai, K.; Kyogoku, H. Selective Laser Melting of Pure Copper. JOM 2018, 70, 396–400. [CrossRef] 20. Kaden, L.; Matthäus, G.; Ullsperger, T.; Engelhardt, H.; Rettenmayr, M.; Tünnermann, A.; Nolte, S. Selective laser melting of copper using ultrashort laser pulses. Appl. Phys. A 2017, 123, 596. [CrossRef] 21. Lykov, P.; Safonov, E.V.; Akhmedianov, A. Selective Laser Melting of Copper. Mater. Sci. Forum 2016, 843, 284–288. [CrossRef] 22. Trevisan, F.; Calignano, F.; Lorusso, M.; Lombardi, M.; Manfredi, D.; Fino, P. Selective Laser Melting of Chemical Pure Copper. In Proceedings of the Euro PM 2017 Congress & Exhibition, Milan, Italy, 1–5 October 2017. 23. Zhong, H.Z.; Li, C.G.; Zhang, X.Y.; Gu, J.F. The graded microstructures evolving with thermal cycles in pure copper processed by laser metal deposition. Mater. Lett. 2018, 230, 215–218. [CrossRef] 24. Davis, J.R. Copper and Copper Alloys; ASM International: Cleveland, OH, USA, 2001. 25. Andrews, P.V.; West, M.B.; Robeson, C.R. The effect of grain boundaries on the electrical resistivity of polycrystalline copper and aluminium. Philos. Mag. J. Theor. Exp. Appl. Phys. 1969, 19, 887–898. [CrossRef] 26. Heuer, R.P. The Effect of Iron And Oxygen on the Electrical Conductivity of Copper. J. Am. Chem. Soc. 1927, 49, 2711–2720. [CrossRef] 27. Belkin, E.; Nagata, P.K. Hydrogen Embrittlement of Tough Pitch Copper by Brazing. Weld. Res. Suppl. 1975, 54–62. 28. Ward, T.G.; Nix, W.D. The formation of water vapor bubbles in copper and their effect on intergranular creep fracture. Acta Metall. 1980, 28, 557–566. 29. Hallstedt, B.; Risold, D.; Gauckler, L.J. Thermodynamic assessment of the copper-oxygen system. J. Phase Equilibria 1994, 15, 483–499. [CrossRef] Appl. Sci. 2019, 9, 3993 22 of 22

30. Nieh, T.G.; Nix, W.D. Embrittlement of copper due to segregation of oxygen to grain boundaries. Metall. Trans. A 1981, 12, 893–901. [CrossRef] 31. Goldstein, E.A.; Mitchell, R.E. Chemical kinetics of copper oxide reduction with carbon monoxide. Proc. Combust. Inst. 2011, 33, 2803–2810. [CrossRef]

32. Poulston, S.; Parlett, P.M.; Stone, P.; Bowker, M. Surface Oxidation and Reduction of CuO and Cu2O Studied Using XPS and XAES. Surf. Interface Anal. 1996, 24, 811–820. [CrossRef] 33. Ren, D.; Deng, Y.; Handoko, A.D.; Chen, C.S.; Malkhandi, S.; Yeo, B.S. Selective Electrochemical Reduction of Carbon Dioxide to Ethylene and Ethanol on Copper(I) Oxide Catalysts. ACS Catal. 2015, 5, 2814–2821. [CrossRef] 34. Magnusson, H.; Frisk, K. Diffusion, Permeation and Solubility of Hydrogen in Copper. J. Phase Equilibria Diffus. 1993, 38, 65–69. [CrossRef] 35. Ito, H.; Hayashi, K. Detection of Gases in Closed Pores of Incompletely Densified Copper Sintering. In Proceedings of the 1993 Powder Metall, Kyoto, Japan, 12–15 July 1993. 36. Lin, Y.-J.; Hwang, K.-S. Swelling of Copper Powders during Sintering of Heat Pipes in Hydrogen-Containing Atmospheres. Mater. Trans. 2010, 51, 2251–2258. [CrossRef] 37. Collet, R.; le Gallet, S.; Charlot, F.; Lay, S.; Chaix, J.M.; Bernard, F. Oxide reduction effects in SPS processing of Cu atomized powder containing oxide inclusions. Mater. Chem. Phys. 2016, 173, 498–507. [CrossRef] 38. Louthan, M.R.; Caskey, G.R.; Donovan, J.A.; Rawl, D.E. Hydrogen embrittlement of metals. Mater. Sci. Eng. 1972, 10, 357–368. [CrossRef] 39. Magnusson, H.; Frisk, K. Self-Diffusion and Impurity Diffusion of Hydrogen, Oxygen, Sulphur and Phosphorus in Copper; Swerea KIMAB AB: Stockholm, Sweden, December 2013; p. 33. 40. Murr, L.E.; Gaytan, S.M.; Ramirez, D.A.; Martinez, E.; Hernandez, J.; Amato, K.N.; Shindo, P.W.; Medina, F.R.; Wicker, R.B. Metal Fabrication by Additive Manufacturing Using Laser and Electron Beam Melting Technologies. J. Mater. Sci. Technol. 2012, 28, 1–14. [CrossRef] 41. Lee, Y.S.; Kirka, M.M.; Raghavan, N.; Dehoff, R.R. Simulation of Spot Melting Scan Strategy to Predict Columnar to Equiaxed Transition in Metal Additive Manufacturing. In Proceedings of the International Solid Freeform Fabrication Symposium, Austin, TX, USA, 7–9 August 2017; p. 13.

42. Lee, S.-K.; Hsu, H.-C.; Tuan, W.-H. Oxidation Behavior of Copper at a Temperature below 300 ◦C and the Methodology for Passivation. Mater. Res. 2016, 19, 51–56. [CrossRef] 43. White, A.H.; Germer, L.H. The Rate of Oxidation of Copper at Room Temperature. Trans. Electrochem. Soc. 1942, 81, 305–319. [CrossRef] 44. Zhou, Y.; Liu, J.; Liang, D.; Shi, W.; Yang, W.; Zhou, J. Eﬀect of particle size and oxygen content on ignition and combustion of aluminum particles. Chin. J. Aeronaut. 2017, 30, 1835–1843. [CrossRef]

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