3D Printing of Highly Pure Copper

Review 3D Printing of Highly Pure Copper

Thang Q. Tran 1,*, Amutha Chinnappan 1, Jeremy Kong Yoong Lee 1, Nguyen Huu Loc 2, Long T. Tran 2, Gengjie Wang 3, Vishnu Vijay Kumar 1, W. A. D. M. Jayathilaka 1 , Dongxiao Ji 1, Mrityunjay Doddamani 4 and Seeram Ramakrishna 1,*

1 Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, EA-07-05, Singapore 117575, Singapore 2 Faculty of Mechanical Engineering, Ho Chi Minh City University of Technology, VNUHCM, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City 740400, Vietnam 3 Institute of Materials Engineering, National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China 4 Lightweight Materials Laboratory, Department of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal 575025, India * Correspondence: [email protected] (T.Q.T.); [email protected] (S.R.)

Received: 14 June 2019; Accepted: 2 July 2019; Published: 5 July 2019

Abstract: Copper has been widely used in many applications due to its outstanding properties such as malleability, high corrosion resistance, and excellent electrical and thermal conductivities. While 3D printing can offer many advantages from layer-by-layer fabrication, the 3D printing of highly pure copper is still challenging due to the thermal issues caused by copper’s high conductivity. This paper presents a comprehensive review of recent work on 3D printing technology of highly pure copper over the past few years. The advantages and current issues of 3D printing methods are compared while different properties of copper parts printed by these methods are summarized. Finally, we provide several potential applications of the 3D printed copper parts and an overview of current developments that could lead to new improvements in this advanced manufacturing field.

Keywords: copper; additive manufacturing; selective laser melting; electron beam melting; binder jetting; ultrasonic additive manufacturing

1. Introduction During the last decades, 3D printing of metallic parts directly from 3D computer-aided design (CAD) models has drawn significant attention from both academia and industry [1–6]. One of the important advantages of 3D printing is the higher design freedom compared to conventional fabrication technologies [7–10]. In fact, complex 3D parts such as components with many internal structures or cellular parts can be fabricated by 3D printing without considering specific design rules. Moreover, the topological optimization of 3D CAD models with no additional cost in 3D printing provides many advantages such as complex shapes, short lead times, weight saving, and multifunctional integration [11]. Nowadays, many high-performance metallic alloys and pure metals can be processed successfully with achievable properties comparable to those fabricated by conventional methods such as forming or casting [1–4]. Additionally, parts with epitaxial growth and fine microstructures can be fabricated by process-inherent rapid and direct solidification in 3D printing. This offers a great potential in new design possibilities to achieve locally-desired material properties. Besides, the application of high cooling rates during 3D printing processes provides new possibilities in alloy and metal design [1–4]. 3D printing of highly pure copper with superior electrical and thermal properties has been studied extensively due to its broad potential in many applications including electronic devices [1,12–20], thermal management systems [4,12], and the aerospace industry [4,12,21–25]. Compared to conventional

Metals 2019, 9, 756; doi:10.3390/met9070756 www.mdpi.com/journal/metals Metals 2019, 9, x FOR PEER REVIEW 2 of 24

3D printing of highly pure copper with superior electrical and thermal properties has been studied extensively due to its broad potential in many applications including electronic devices [1,12– 20],Metals thermal2019, 9, 756management systems [4,12], and the aerospace industry [4,12,21–25]. Compared2 ofto 24 conventional fabrication methods such as metal casting, welding, and machining, 3D printing can fabricate more optimized and complex 3D copper parts without using additional tools [26–34]. Post- fabrication methods such as metal casting, welding, and machining, 3D printing can fabricate more processing methods such as abrasive polishing [35] and hot isostatic pressing (HIP) [36–39] could optimized and complex 3D copper parts without using additional tools [26–34]. Post-processing methods enhance physical properties of the printed copper parts further. However, 3D printing of pure copper such as abrasive polishing [35] and hot isostatic pressing (HIP) [36–39] could enhance physical properties has several significant processing challenges that need to be addressed. Due to copper’s high thermal of the printed copper parts further. However, 3D printing of pure copper has several significant conductivity, the melt area experiences rapid heat dissipation and high local thermal gradients, processing challenges that need to be addressed. Due to copper’s high thermal conductivity, the melt resulting in delamination, layer curling, and part failure [40,41]. Furthermore, the post-build powder area experiences rapid heat dissipation and high local thermal gradients, resulting in delamination, removal and recovery of the printed parts might be impeded by the high ductility of copper while layer curling, and part failure [40,41]. Furthermore, the post-build powder removal and recovery of the tendency of powder agglomeration could lower overall flowability and hinder powder the printed parts might be impeded by the high ductility of copper while the tendency of powder deposition. Additionally, special handling and storage of copper are required for 3D printing agglomeration could lower overall flowability and hinder powder deposition. Additionally, special processes due to the high sensitivity of copper to oxidation [40–42]. handling and storage of copper are required for 3D printing processes due to the high sensitivity of Although many papers have been published on 3D printing of highly pure copper copper to oxidation [40–42]. [12,21,26,40,43], no comprehensive review has been reported. Recent rapid growth in research on the Although many papers have been published on 3D printing of highly pure copper [12,21,26,40,43], 3D printing of pure copper has motivated this review. In Section 2, we present the main 3D printing no comprehensive review has been reported. Recent rapid growth in research on the 3D printing of methods for pure copper over the past few years. The advantages and challenges of each 3D printing pure copper has motivated this review. In Section2, we present the main 3D printing methods for method are discussed in Section 3. In Section 4, the potential applications of 3D printed copper parts pure copper over the past few years. The advantages and challenges of each 3D printing method are summarized, and a brief conclusion is presented in Section 5. are discussed in Section3. In Section4, the potential applications of 3D printed copper parts are 2.summarized, 3D Printing andMethods a brief of conclusion Highly Pure is presented Copper in Section5. 2. 3D Printing Methods of Highly Pure Copper 2.1. Selective Laser Melting 2.1.Selective Selective Laserlaser Meltingmelting (SLM) is a widely used powder bed-based process to fabricate metallic parts [44–48].Selective The laser building melting process (SLM) starts is a widely with spreading used powder metal bed-based powder in process thin layers to fabricate across a metallic work areaparts (Figure [44–48 1a)]. The [1,3]. building The metal process powder starts is withusually spreading fed by a metal hopper powder while ina recoater thin layers blade across is used a work to ensurearea (Figure uniform1a) powder [ 1,3]. The distribution metal powder (Figure is usually1b, Step fed I). byThen, a hopper a galvanometer while a recoater scanner blade is employed is used to toensure direct uniforma high energy powder density distribution laser beam (Figure across1b, Stepthe deposited I). Then, a layer galvanometer of the metal scanner powder is employed (Figure 1b, to Stepdirect II). aAccording high energy to the density CAD laser data beamof the acrossfabricated the depositedpart, only metal layer ofpowder the metal in the powder selected (Figure areas 1isb, exposedStep II). to According the laser tobeam the and CAD melted data of along the fabricated the part contour part, only and metal filling powder in the inXY the plane selected (Figure areas 1a, is B-B).exposed The tobuild the laserplatform beam is and lowered melted (Figure along the 1b, part Step contour III) subsequently and ﬁlling in and the XYa new plane powder (Figure layer1a, B-B). is depositedThe build for platform the next is loweredlaser scanning (Figure step.1b, Step These III) th subsequentlyree steps are and repeated a new powderuntil the layer required is deposited part is fullyfor thebuilt. next laser scanning step. These three steps are repeated until the required part is fully built.

Figure 1. (a) Schematics of a selective laser melting (SLM) machine and (b) the three-step process Figurerepeated 1. ( duringa) Schematics the build of [a1 ],selective with permission laser melting from Elsevier, (SLM) machine 2019. and (b) the three-step process repeated during the build [1], with permission from Elsevier, 2019. The build plate is usually connected to support structures which are necessary for the part’s ﬁxation in the powder bed and heat dissipation (Figure1a, Detail A). The distortion of the printed parts can be avoided by pre-heating the build plate to lower thermal gradients and reduce residual stresses

during the SLM process. Inert gas such as nitrogen or argon is continuously supplied to the building chamber to provide an inert atmosphere for protecting the metal powder and the heated metal parts Metals 2019, 9, 756 3 of 24 from oxidation. Once the printing process is completed, the substrate plate can be removed from the printed part. Important process parameters such as layer thickness, hatch spacing, laser power, and scanning speed need to be optimized to fabricate high quality printed parts [1,3].

2.1.1. Indirect Selective Laser Melting Badrinarayan et al. [49] first reported the indirect manufacturing of pure copper parts using the SLM method. A powder mixture of copper and poly(methyl methacrylate) (PMMA) was used as a feedstock for the printing process. Since PMMA acted as an intermediate binder, it was melted during the laser scanning process to bind the copper powders and form copper green parts. The green parts were debinded under reducing conditions afterwards and the resulting copper parts were sintered for several hours. However, the mixing ratio of PMMA in the fabrication process was too high (8 wt.%), leading to the low green density (under 27%) [49,50]. Consequently, the sintered copper parts had a density much lower than the theoretical density (about 77–88%) with the introduction of significant porosity. To address the issue, polymer-coated granules of small copper particles and bimodal mixed powders could be used to improve the powder packing density. Another drawback of the indirect SLM was that the impurities from the debinding process could be introduced to the final copper parts and might lower their properties [49,50].

2.1.2. Direct Selective Laser Melting Most commercial SLM systems use continuous wave and long-pulse lasers working in the near infrared radiation (IR) with a wavelength of about 1 µm [51,52]. Although SLM has been demonstrated for many materials ranging from metals to polymers and ceramics [53–58], the processing of materials with high thermal conductivities and high melting points such as pure copper is still challenging [59,60]. In addition to the rapid heat dissipation problems, reflectivity of copper to conventional laser light near IR is very high [61–63], resulting in low deposition of laser energy in the materials for the melting process [51]. Therefore, to achieve high laser energy density for fabricating dense copper parts, greater laser output power and lower scanning speed, layer thickness, and hatch spacing are required [52,59,64]. Generally, the minimal laser power for a successful SLM of pure copper is approximately 300 W, although some studies have reported the fabrication of copper parts at lower laser powers [65,66]. Lykov et al. [65] studied the SLM process of pure copper powder using a system equipped with a 200 W CO2 laser source. The process used a scanning speed ranging between 100 and 150 mm/s whereas the layer thickness and hatch spacing were fixed at 50 µm and 0.12 mm, respectively. Although the absorptance of pure copper powder for CO2 laser was very low (only 0.26), the copper powder could be selectively melted since the laser beam diameter was adjusted to as small as 35 µm to obtain sufficient power density for the melting process. Consequently, the printed copper parts had dense structures with good surface finish quality and no dimensional distortions were observed. Specifically, the relative density of the printed parts could be as high as 88.1% while their tensile strength could reach 149 MPa [65]. In another approach, Kaden et al. [52] reported a successful SLM process of pure copper by using ultrashort laser pulses with 500 fs pulse duration at a center wavelength of 1030 nm. At a 20 MHz repetition rate, the lasers could provide extremely high pulse energy (1–1.5 µJ) and line energy (100 J/m), which were sufficient for copper melting. The powder layer thickness and laser spot size were controlled at 30 µm and of 35 µm, respectively, to fabricate both bulky and thin-walled copper structures with thicknesses below 100 µm (Figure2). The scanning electron microscope (SEM) images in Figure2b suggested that the printed parts had porous structures with melting beads formed during the powder melting. This might stem from the large powder size (35 µm) compared to the laser spot diameter (35 µm) employed in the process. Therefore, the use of copper powder with small grain sizes was recommended to reduce porous structures of the printed copper parts and improve their relative density [52]. Metals 2019, 9, x FOR PEER REVIEW 4 of 24 Metals 2019, 9, x FOR PEER REVIEW 4 of 24 laserlaser spot spot diameter diameter (35 (35 µm) µm) employed employed in in the the process. process. Therefore, Therefore, the the use use of of copper copper powder powder with with small small grain sizes was recommended to reduce porous structures of the printed copper parts and improve grainMetals 2019sizes, 9 ,was 756 recommended to reduce porous structures of the printed copper parts and improve4 of 24 theirtheir relative relative density density [52]. [52].

FigureFigureFigure 2.2. 2. (( a(a)a) Sample) Sample Sample of of diof ﬀ differentdifferenterent thin thinthin wall wallwall structures structures structures and ( b andand) SEM ((bb) image ) SEMSEM of imageimage a single ofof wallaa singlesingle [52], wall withwall permission[52], with permission from Springer from Nature, Springer 2019. Nature, 2019. [52], with permission from Springer Nature, 2019. By using a laser source with much higher power, Ikeshoji et al. [64] could fabricate 3D copper ByBy using using a a laser laser source source with with much much higher higher power, power, Ikeshoji Ikeshoji et et al. al. [64] [64] could could fabricate fabricate 3D 3D copper copper parts with a relative density of up to 96.6% by SLM. In the fabrication process, the laser power was partsparts with with a a relative relative density density of of up up to to 96.6% 96.6% by by SL SLM.M. In In the the fabrication fabrication process, process, the the laser laser power power was was set at 800 W and the used hatch pitch was in a range of 0.025–0.12 mm. Additionally, the scanning setset at at 800 800 W W and and the the used used hatch hatch pitch pitch was was in in a a range range of of 0.025–0.12 0.025–0.12 mm. mm. Additionally, Additionally, the the scanning scanning speed and powder bed thickness were ﬁxed at 300 mm/s and 0.05 mm, respectively. As can be seen speedspeed and and powder powder bed bed thickness thickness were were fixed fixed at at 300 300 mm mm/s/s and and 0.05 0.05 mm, mm, respectively. respectively. As As can can be be seen seen in Figure3a, the surfaces of all the printed cubes had rough texture and ballshaped asperities with inin Figure Figure 3a, 3a, the the surfaces surfaces of of all all the the printed printed cube cubess had had rough rough texture texture and and ballshaped ballshaped asperities asperities with with no severe oxidation observed. These ball-shaped asperities might stem from the nonmolten residual nono severe severe oxidation oxidation observed. observed. These These ball-shaped ball-shaped aspe asperitiesrities might might stem stem from from the the nonmolten nonmolten residual residual powder particles while large asperities might be formed by splatter. Also, the surface texture suggested powderpowder particlesparticles whilewhile largelarge asperitiesasperities mightmight bebe formedformed byby splatter.splatter. Also,Also, thethe surfacesurface texturetexture the presence of voids within the copper structures [64]. suggestedsuggested the the presence presence of of voids voids within within the the copper copper structures structures [64]. [64].

Figure 3. (a) Cubes built by SLM of 99.9% pure copper powder and (b) relative density of SLM-built FigureFigure 3. 3. ( a(a)) Cubes Cubes built built by by SLM SLM of of 99.9% 99.9% pure pure copper copper powder powder and and ( b(b)) relative relative density density of of cubesSLM-built of pure cubes copper of [64pure], with copper permission [64], with from permission Springer Nature, from Springer 2019. Nature, 2019. SLM-built cubes of pure copper [64], with permission from Springer Nature, 2019. Due to the large asperities, smooth powder bed surfaces might not be achieved during the Due to the large asperities, smooth powder bed surfaces might not be achieved during the squeezingDue to process. the large Internal asperities, voids smooth could bepowder formed b ated thesurfaces shadow might of the not laser be irradiationachieved during under the squeezing process. Internal voids could be formed at the shadow of the laser irradiation under the squeezingball-like asperities process. Internal and, therefore, voids could lowered be formed the relative at the density shadow of of the the printed laser irradiation parts. As under shown the in ball-like asperities and, therefore, lowered the relative density of the printed parts. As shown in ball-likeFigure3b, asperities the relative and, density therefore, of the lowered printed partsthe re decreasedlative density at narrower of the printed hatch pitches, parts. As although shown the in Figure 3b, the relative density of the printed parts decreased at narrower hatch pitches, although the Figureasperities 3b, couldthe relative be exposed density to of the the laser printed beam parts several decreased times and at narrower melted down hatch to pitches, fill the although voids during the asperities could be exposed to the laser beam several times and melted down to fill the voids during asperitiesthe laser scanningcould be process.exposed Besides,to the laser transient beam seve heatral transfer times and including melted melting down to and fill solidification the voids during was the laser scanning process. Besides, transient heat transfer including melting and solidification was theanalyzed laser scanning by the finite process. element Besides, method transient (FEM) to heat study transfer the eff ectsincluding of hatch melting pitches and on thesolidification SLM process was of

pure copper. The various predicted dimensions of the melt pool with time indicated a slight overlap for Metals 2019, 9, x FOR PEER REVIEW 5 of 24

Metals 2019, 9, 756 5 of 24 analyzed by the finite element method (FEM) to study the effects of hatch pitches on the SLM process of pure copper. The various predicted dimensions of the melt pool with time indicated a slight overlapwider hatch for wider pitches hatch and pitches instability and forinstability narrower for hatch narrower pitches. hatch This pitches. result This suggested result thatsuggested the relative that thedensity relative of thedensity printed of the copper printed parts copper was strongly parts was related strongly to the related hatch to pitch the hatch [64]. pitch [64]. Later,Later, a afeasibility feasibility window window of the of theSLM SLM process process of pure of Cu pure powder Cu powder was investigated was investigated by Colopi by etColopi al. [59] et using al. [59 a] 1kWusing laser a 1kW source. laser A source. total of A 70 total experimental of 70 experimental conditions conditions were conducted were conducted by varying by thevarying laser power, the laser layer power, thickness, layer thickness,and scan speed and scan in ranges speed of in 200–1000 ranges of W, 200–1000 50–100 µm, W,50–100 and 1000–4000µm, and mm/s,1000–4000 respectively. mm/s, respectively. The process Theoutcome process was outcome classified was into classified three categories: into three not categories: fused (insufficient not fused solidification),(insufficient solidification), delamination delamination (insufficient interlayer (insufficient bonding), interlayer and bonding), acceptable and (no acceptable evident defect). (no evident As showndefect). in As Figure shown 4a, in the Figure process4a, the using process a layer using thickness a layer thicknessof 50 µm ofhad 50 theµm widest had the stability widest stabilityregion, whichregion, might which stem might from stem its fromhigher its process higher resolution. process resolution.

Figure 4. (a) Qualitative categorization of process outcome. Feasibility region for SLM of pure Cu Figure 4. (a) Qualitative categorization of process outcome. Feasibility region for SLM of indicated by dashed blue line, (b) cross-section of a cubic specimen representative of an “acceptable” pure Cu indicated by dashed blue line, (b) cross-section of a cubic specimen representative processing conditions, and (c) final demonstrator of pure Cu powder processability [59], with permission of an “acceptable” processing conditions, and (c) final demonstrator of pure Cu powder from Elsevier, 2019. processability [59], with permission from Elsevier, 2019. The “delamination” defect occurred in both layer thicknesses, suggesting the crucial role of inter-layerThe “delamination” bonding on obtaining defect occurred a stable powderin both layer bed densification. thicknesses, suggesting The occurrence the crucial of this role defect of inter- at low layerlaser bonding power indicated on obtaining the instability a stable ofpowder the process bed meltdensification. pool (Figure The4a). occurrence Since the previouslyof this defect deposited at low lasercopper power layer indicated had high thermalthe instability conductivity, of the thermal process cooling melt couldpool be(Figure elevated 4a). while Since melt the poolpreviously stability depositedmight be hindered.copper layer Besides, had high the formationthermal conductivity, of the inter-layer thermal defect cooling might could lead be to elevated thermally while insulating melt poolgases stability in between might the layersbe hindered. and, hence, Besides, limit the penetrationformation of thethe meltinter-layer pool to thedefect previous might interlayer lead to thermallydefect. Consequently, insulating gases the in delamination between the layers issue mightand, hence, be further limit worsenedthe penetration due toof thethe formationmelt pool to of thethermally-induced previous interlayer stresses, defect. resulting Consequently, in more the defect delamination formation issue and part might deformations be further worsened [59]. due to theAs formation shown in Figureof thermally-induced4b,c, the relative densitystresses, of resulting the printed in coppermore defect parts yielded formation upto and 97.8% part of deformationsthe separate measurement[59]. between the core volume of sections and the border. The porosity in the borderAs regionsshown in was Figure always 4b,c, higher, the relative which density might be of attributed the printed to copper the temperature parts yielded differences up to 97.8% between of thethe separate border and measurement the central between region. Duethe core to thermal volume accumulation, of sections and most the internalborder. The regions porosity could in reach the borderhigher regions temperatures was always compared higher, to which the border. might Furthermore,be attributed to the the border temperature was usually differences in contact between with thecooler border powder and the in the central powder region. bed Due during to thermal the scanning accumulation, process. Sincemost copperinternal has regions very highcould thermal reach higherconductivity, temperatures strong thermalcompared dissipation to the border. could beFurthe achievedrmore, in the border areas,was usually resulting in incontact their partial with coolermelting powder and higher in the porosity. powder The bed process during parametersthe scanning optimized process. theSince feasibility copper windowhas very which high thermal could be conductivity,used to produce strong complex thermal 3D copperdissipation parts, could as shown be ac inhieved Figure in4 cthe [59 ].border areas, resulting in their partial melting and higher porosity. The process parameters optimized the feasibility window which could be used to produce complex 3D copper parts, as shown in Figure 4c [59].

Metals 2019, 9, 756 6 of 24 Metals 2019, 9, x FOR PEER REVIEW 6 of 24

JadhavJadhav et et al. al. [51] [51] studied studied the influencesinfluences ofof didifffferenterent laser laser scan scan parameters parameters on on the the textural textural developmentdevelopment of of SLM-printed SLM-printed copper parts. TheThe fabricationfabrication process process used used a 1a kW1 kW laser laser source, source, with with a a beambeam diameter diameter of of 40 40 µm,µm, and and a layer thicknessthickness of of 30 30µ µm.m. At At optimum optimum processing processing conditions, conditions, printed printed partsparts with with a arelative relative density density of more thanthan 98%,98%, electrical electrical conductivity conductivity of of 88% 88% IACS IACS (International (International AnnealedAnnealed Copper Copper Standard Standard definition definition ofof 5858 MSMS/m/m as as 100% 100% IACS IACS for for electrical electrical conductivity), conductivity), and and thermalthermal conductivity conductivity of of up up to to 336 WW/m.K/m.K couldcould be be produced produced (Figure (Figure5a,b). 5a,b).

Figure 5. (a) Fabricated cube shaped parts along with the corresponding SLM process parameters—P in Figure 5. (a) Fabricated cube shaped parts along with the corresponding SLM process Watt, h in mm and v in mm/s, (b) relative density (%) of parts versus applied energy densities (J/mm3). parameters—Parts with a stableP in geometry Watt, h arein markedmm and with v ain circle, mm/s, (c) SEM (b) imagerelative of sampledensity top (%) surfaces of parts processed versus 3 appliedwith diff energyerent SLM densities scan parameters. (J/mm ). The Parts laser with scan a direction stable geometry is indicated are by themarked white dashedwith a arrows.circle, (c) SEM(d) Damages image of to sample the optical top mirror surfaces in the processed SLM machine with after different 12 h exposure SLM scan to laser parameters. back reflection The [51 laser], scanwith direction permission is from indicated Elsevier, by 2019. the white dashed arrows. (d) Damages to the optical mirror in the SLM machine after 12 h exposure to laser back reflection [51], with permission from The analysis of the crystallographic texture of the SLM copper parts suggested that the crystal Elsevier, 2019. orientation of the YZ and middle (XY-MID) plane was relatively random. This result could be attributed to theThe alteration analysis inof thethe heatcrystallographic gradient directions texture and of the subsequent SLM copper re-melting parts suggested which were that formed the crystal by applying a 90 scan rotation strategy in between the subsequent layers. However, the top surface orientation of ◦the YZ and middle (XY-MID) plane was relatively random. This result could be (XY-TOP) had strong crystallographic texture owing to the solidification morphology controlled by attributed to the alteration in the heat gradient directions and subsequent re-melting which were the temperature gradients within the melt pools. Due to the extremely high laser energy density and formed by applying a 90° scan rotation strategy in between the subsequent layers. However, the top laser power used in the process, damages of the optical coating could be formed on the laser mirror, surface (XY-TOP) had strong crystallographic texture owing to the solidification morphology as shown in Figure5d [51]. controlled by the temperature gradients within the melt pools. Due to the extremely high laser energy density2.2. Electron and laser Beam power Melting used in the process, damages of the optical coating could be formed on the laser mirror, as shown in Figure 5d [51]. Similar to SLM, electron beam melting (EBM) is also a common powder bed-based process used to fabricate highly-dense metallic parts [67,68]. In the EBM process, metal powder from a hopper is 2.2. Electron Beam Melting supplied and spread uniformly on a build plate by a rake (Figure6)[ 1,69]. The powder layer thickness Similar to SLM, electron beam melting (EBM) is also a common powder bed-based process used to fabricate highly-dense metallic parts [67,68]. In the EBM process, metal powder from a hopper is supplied and spread uniformly on a build plate by a rake (Figure 6) [1,69]. The powder layer thickness is usually in the range of 50–200 µm. According to the CAD data, the powder is melted selectively

Metals 2019, 9, 756 7 of 24 Metals 2019, 9, x FOR PEER REVIEW 7 of 24 isusing usually an electron in the range beam of as 50–200 a heatµ m.source. According The electron to the CADbeam data, is produced the powder by isan meltedelectron selectively gun and usingaccelerated an electron with a beamhigh acceleration as a heat source. voltage. The Then, electron electromagnetic beam is produced lenses are by used an electronto focus the gun beam and acceleratedand a magnetic with scan a high coil acceleration is employed voltage. to direct Then, it to electromagneticselected areas of lenses the metal areused layers to in focus the XY the plane. beam andImportant a magnetic EBM scan process coil isparameters employed tosuch direct as itelectron to selected beam areas power, of the scan metal speed, layers and in thefocus XY of plane. the Importantelectron beam EBM usually process depend parameters on suchthe speed as electron function beam, beam power, current, scan speed, and focus and focus offset, of respectively the electron beam[70]. usually depend on the speed function, beam current, and focus oﬀset, respectively [70]. AtAt ﬁrst,first, aa defocused beambeam pre-heatspre-heats thethe powderpowder bedbed byby scanningscanning thethe powderpowder bedbed surfacesurface severalseveral times.times. Then, a high beam current and proper scan speed are employed to heat and sinter sinter the the powder. powder. SimilarSimilar toto the the SLM SLM process, process, the the build build plate plate in EBM in EB isM then is then lowered lowered and metal and metal powder powder is fed andis fed spread and uniformlyspread uniformly across the across work the area. work The area. process The ofproc powderess of deposition,powder deposition, powder pre-heating,powder pre-heating, electron beamelectron scanning, beam scanning, and build and plate build lowering plate islowering repeated is untilrepeated thepart until printing the part is printing completed. is completed. The EBM The EBM process operates under a vacuum and helium is fed to the work area to prevent the metal process operates under a vacuum and helium is fed to the work area to prevent the metal powder from powder from electrical charging while improving the heat conduction and cooling of the melt [1,71]. electrical charging while improving the heat conduction and cooling of the melt [1,71].

Figure 6. Schematics of electron beam melting (EBM), 1: electron gun, 2: lens system, 3: deflection lens, Figure 6. Schematics of electron beam melting (EBM), 1: electron gun, 2: lens system, 3: 4: powder cassettes with feedstock, 5: rake, 6: building component, 7: build table [69], with permission fromdeflection Elsevier, lens, 2019. 4: powder cassettes with feedstock, 5: rake, 6: building component, 7: build table [69], with permission from Elsevier, 2019. Since electrons have different absorption and reflection mechanisms compared to photons, the EBM methodSince is not electrons affected have by the different optical reflectivityabsorption ofand materials reflection and mechanisms most of the energy compared in EBM to photons, is deposited the withinEBM method the materials is not [affected1,4,40,72 by]. Therefore,the optical this reflectivity method of has materials a high potential and most for of the the processing energy in ofEBM pure is copperdeposited [73 ,74within]. Furthermore, the materials the sintering[1,4,40,72]. step Therefore, of copper this during method the EBM has processa high canpotential be extremely for the shortprocessing due to of its pure high coppe electricalr [73,74]. conductivity. Furthermore, In the fact, sintering the copper step powderof copper might during not the be EBM removed process if preheatingcan be extremely is too high short or due too longto its because high electrical it has high conductivity. sintering activity In fact, [1 ,the4,40 copper]. Yang powder et al. [28 ]mightreported not thebe removed fabrication if ofpreheating pure copper is too with high orthotropic or too long re-entrant because auxetic it has structureshigh sintering using activity EBM. Because[1,4,40]. ofYang the highet al. thermal[28] reported conductivity the fabrication of copper, of pure the copper dissipation with oforthotropic thermal energy re-entrant during auxetic the meltingstructures process using wasEBM. severe, Because resulting of the inhigh large thermal variations conductivity in the dimensions of copper, and the mechanical dissipation performance of thermal energy of the printedduring copperthe melting parts. process Therefore, was more severe, process resulting optimization in large was variations required in to the improve dimensions the surface and qualitymechanical and accuracyperformance of the of printed the printed parts. copper parts. Therefore, more process optimization was required to improve the surface quality and accuracy of the printed parts. Since copper is quite sensitive to oxidation, its storage and handling should be considered properly [75]. Ramirez et al. [76] investigated the effects of Cu2O on the microstructure of the EBM- printed copper components with the use of a relatively impure Cu precursor powder (98.5%). The study results showed the formation of precipitation–dislocation architectures because of directional

Metals 2019, 9, 756 8 of 24

Since copper is quite sensitive to oxidation, its storage and handling should be considered properly [75]. Ramirez et al. [76] investigated the eﬀects of Cu2O on the microstructure of the MetalsEBM-printed 2019, 9, x FOR copper PEER REVIEW components with the use of a relatively impure Cu precursor powder (98.5%).8 of 24 The study results showed the formation of precipitation–dislocation architectures because of directional solidificationsolidiﬁcation during during EBM EBM (Figure (Figure 77a).a). Interestingly, thesethese precipitation–dislocationprecipitation–dislocation architectures architectures improvedimproved the the hardness hardness of of the the parts parts from from HV HV 57 57 for for a a base-plate base-plate to to HV HV 88 for EBM-printed products.products. SimilarSimilar phenomenon phenomenon was was also also observed observed in in their their la laterter study of the open-cellularopen-cellular coppercopper structuresstructures (Figure(Figure 7b)7b) produced produced by EBM of higher puritypurity coppercopper powderpowder (99.8%) (99.8%) [ 26[26].].

FigureFigure 7. 7. (a) TransmissionTransmission electron electron microscopy microscopy (TEM) (TEM bright-field) bright-field image showing image precipitate–dislocationshowing precipitate– dislocationarrays corresponding arrays corresponding to the horizontal to referencethe horizontal plane, asreference illustrated plane, in the as optical illustrated metallographic in the optical image shown in the inset [76]. (b) Comparison of non-isotropic geometry for reticulated mesh (A) and isotropic metallographic image shown in the inset [76]. (b) Comparison of non-isotropic geometry for stochastic, open cellular foam samples (B): at 90 (normal face view) and 45 (along sample diagonal), reticulated mesh (A) and isotropic stochastic, open◦ cellular foam samples◦ (B): at 90° (normal face all fabricated by EBM [26], with permission from Elsevier, 2019. view) and 45° (along sample diagonal), all fabricated by EBM [26], with permission from Elsevier, 2019A comprehensive. performance of pure copper parts fabricated by EBM using three diﬀerent copper powder conﬁgurations was reported by Frigola et al. [40]. The printed copper parts exhibited good propertiesA comprehensive with a relative performance density of 98.6%,of pure electrical copper conductivityparts fabricated of 97% by IACS,EBM thermalusing three conductivity different copperof 390 powder W/mK, andconfigurations yield strength was of reported 76 MPa. by More Frigol interestingly,a et al. [40]. the The copper printed cathode copper fabricated parts exhibited by the goodEBM properties process showed with gooda relative performance density duringof 98.6 high%, electrical power radio conductivity frequency (RF)of 97% testing, IACS, which thermal was conductivitycomparable toof other390 W/mK, cathodes and conventionally yield strength machined of 76 MPa. from More wrought interestingly, oxygen free the copper copper material. cathode fabricatedLater, by a muchthe EBM higher process relative showed density good (up toperformance 99.95%) of EBM-printed during highcopper powerparts radio was frequency achieved (RF) by testing,Lodes etwhich al. [77 was]. The comparable results in to Figure other8 cathodesshow that conventionally the samples fabricated machined at from higher wrought line energies oxygen were free copperdenser material. for all beam velocities. Part density was lower at the very low line energy input because the powderLater, could a much not behigher melted relative completely, density resulting (up to 99.95%) in layer of defects EBM-printed and porous copper printed parts parts. was However, achieved bythe Lodes use ofet tooal. [77]. great The an results energy in input Figure could 8 show lead th toat local the samples overheating fabricated and swelling at higher of line the energies sample. wereAdditionally, denser for overheated all beam velocities. copper powder Part density could wa sticks lower to the at melt the surfacevery low during line energy raking, input resulting because in thebinding powder defects could and not porous be melted areas. completely, At optimum resulting line energy, in layer the built defects sample and was porous almost printed fully denseparts. However,with no binding the use defects of too observed.great an energy input could lead to local overheating and swelling of the sample. Additionally, overheated copper powder could stick to the melt surface during raking, resulting in binding defects and porous areas. At optimum line energy, the built sample was almost fully dense with no binding defects observed.

Metals 2019, 9, x FOR PEER REVIEW 9 of 24 Metals 2019, 9, 756 9 of 24 Metals 2019, 9, x FOR PEER REVIEW 9 of 24

FigureFigure 8.8. Micro-8.Micro- Micro- andand and macro-photographs macro-photographs macro-photographs of ofof cube-shaped cube-shaped samples samplessamples built built built with with with di ﬀdifferent erentdifferent line line line energies energies energies [77 ], with[77],[77], permissionwith with permission permission from from Elsevier, from Elsevier, Elsevier, 2019. 2019. 2019.

RaabRaab et et al. al. [[78]78 [78]] fabricated fabricated 3D 3D copper copper partsparts withwith high relative relative densities densitiesdensities (up (up to toto 99.95%) 99.95%)99.95%) and and good good physicalphysical performanceperformance performance using usingusing EBM. EBM.EBM. At volume AtAt volumevolume energies energies higher than higherhigher 60 J /mm thanthan3, an 6060 electrical J/mmJ/mm3, 3, conductivityan an electrical electrical of upconductivityconductivity to 55.82 MS of/ mof up (96.24%up to to 55.82 55.82 IACS) MS/m MS/m and (96.24% (96.24% thermal IACS)IACS) conductivity and thermalthermal of 400.1 conductivity conductivity W/mK could of of 400.1 be 400.1 achieved. W/mK W/mK could The could study be be resultsachieved.achieved. also The suggestedThe study study results results that pores also also sugge andsugge impuritiesstedsted thatthat pores such andand as oxides impuritiesimpurities or carbides such such as as oxides inside oxides or the or carbides printedcarbides inside copper inside partsthethe printedmight printed lowercopper copper their parts parts electrical might might andlower lower thermal theirtheir performanceelectricelectrical andand due thermalthermal to the performance performance reduced conduction due due to to the cross-section.the reduced reduced Later,conductionconduction Guschlbauer cross-section. cross-section. et al. [79Later, Later,] reported GuschlbauerGuschlbauer the fabrication et al. [79][79] of reported EBM-printedreported thethe fabrication copperfabrication parts of of EBM-printed with EBM-printed excellent physicalcoppercopper parts propertiesparts with with excellent excellent at diﬀerent physical physical process properties properties parameters, at different as shown processprocess in Figureparameters, parameters,9. At as optimum as shown shown in conditions, inFigure Figure 9. 9. theAtAt optimum printed optimum parts conditions, conditions, could obtain the the printed aprinted relative partsparts density couldcould greater obtain thanin aa relative 99.5%,relative andensity density electrical greater greater conductivity than than 99.5%, 99.5%, of morean an thanelectricalelectrical 58 MS conductivity /conductivitym (>100 IACS), of of more more a thermal than than 58 58 conductivity MS/mMS/m (>100 ofIACS), more aa than thermalthermal 411 conductivity Wconductivity/mK, and of tensile of more more strengththan than 411 411 of W/mK, and tensile strength of 177 MPa. The uniform distribution of oxides in the copper matrix of 177W/mK, MPa. and The tensile uniform strength distribution of 177 ofMPa. oxides The in uniform the copper distribution matrix of of the oxides printed in partsthe copper and no matrix elevated of the printed parts and no elevated oxygen content observed at the grain boundaries suggested that oxygenthe printed content parts observed and no elevated at the grain oxygen boundaries content suggestedobserved at that the oxygen grain boundaries might have suggested no signiﬁcant that oxygen might have no significant role in crack formation during the tensile test. Instead, cracks might roleoxygen in crack might formation have no significant during the role tensile in crack test. Instead,formation cracks during might the tensile be generated test. Instead, by internal cracks stresses might be generated by internal stresses formed during the manufacturing process [79]. formedbe generated during by the internal manufacturing stresses formed process during [79]. the manufacturing process [79].

Figure 9. (a) Processing window. Green marks represent cuboids higher >99.5% relative density, blue Figure 9. (a) Processing window. Green marks represent cuboids higher >99.5% relative density, Figuremarks 9. display(a) Processing relative window. densities Greenlower <99.5%marks representand red marks cuboids are higherdense but>99.5% uneven relative cuboids density, due toblue blue marks display relative densities lower <99.5% and red marks are dense but uneven cuboids due markstoo muchdisplay energy, relative and densities (b) electrical lower conductivity <99.5% and of red the marks cuboids are in dense (MS/m). but Theuneven black, cuboids solid linesdue to torepresent too much constant energy, line and energies (b) electrical [79], with conductivity permission of from the cuboidsElsevier, in2019. (MS /m). The black, solid lines too much energy, and (b) electrical conductivity of the cuboids in (MS/m). The black, solid lines represent constant line energies [79], with permission from Elsevier, 2019. represent constant line energies [79], with permission from Elsevier, 2019.

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2.3. Binder Jetting Binder jetting (BJ) is a 3D printing process in which one or two inkjetinkjet printheads are used to selectively depositdeposit a liquida liquid binding binding agent agent on theon topthe of top a powder of a powder bed to joinbed powder to join materialspowder [materials21,80,81]. The[21,80,81]. schematic The ofschematic BJ process of isBJ shown process in is Figure shown 10 [in21 Figure]. In Step 10 [21]. 1, a roller In Step is employed1, a roller tois spreademployed metal to powderspread metal uniformly powder across uniformly the platform across tothe form platform a thin to layer. form Then, a thin an layer. inkjet Then, printhead an inkjet moves printhead along themoves X- andalong Y-directions the X- and toY-directions selectively to distribute selectively binder distribute droplets binder into droplets the powder into layer.the powder The binder layer. bondsThe binder the powder bonds particlesthe powder to produce particles a printedto produce cross-sectional a printed layer.cross-sectional Then, the layer. build platformThen, the lowers build (Zplatform direction) lowers for a(Z small direction) distance, for a the small roller distance, spreads the a newroller powder spreads layer, a new and powder the binder layer, injection and the processbinder injection repeats. process repeats.

Figure 10. Schematic of binder jetting process [36] [36],, with permission from Elsevier, 2019.2019.

When all the layers are deposited, the final final glued glued part, part, usually usually called called the the ‘‘green “green part’’, part”, is is formed. formed. Then, thethe binder binder is is cured cured at elevatedat elevated temperature temperatur to improvee to improve the green the partgreen strength part strength for easy handling.for easy Thehandling. printed The part printed experiences part experien furtherces sintering further to sintering achieve itsto finalachieve strength its final and strength density. and During density. the sinteringDuring the process, sintering the binderprocess, is removedthe binder and is theremoved loosely and packed the powderloosely ispacked bonded powder together is throughbonded powdertogether sinteringthrough powder and densification. sintering and In thedensification. BJ process, In many the BJ processing process, many and post-process processing and sintering post- parametersprocess sintering such as parameters the binder saturation,such as the spreader binder saturation, speed, layer spreader thickness, speed, and sintering layer thickness, profile a ffandect thesintering final densityprofile affect and propertiesthe final density of the and printed proper partsties [of82 the–87 printed]. Compared parts [82–87]. to SLM Compared and EBM, to BJ SLM has severaland EBM, unique BJ has characteristics several unique for fabricating characteristics complex for pure fabricating copper parts:complex (i) no pure requirements copper parts: of support i) no structures,requirements expensive of support energy structur sources,es, andexpensive enclosed energy chamber; sources, (ii) scalable and enclosed technology; chamber; (iii) no ii) concernsscalable overtechnology; a material’s iii) no reactivity, concerns melting over a point, material’s thermal reactivity, conductivity, melting and point, optical thermal reflectivity; conductivity, (iv) leveraged and understandingoptical reflectivity; of well-studied iv) leveraged powder understanding metallurgy of processes well-studied due powder to the separation metallurgy of partprocesses creation due and to powderthe separation sintering of part steps creation [21]. and powder sintering steps [21]. Bai et al. [[21]21] firstfirst studied the 3D printing of pure copper via BJ of copper powders with median µ µ µ diameter of 75.2 µm,m, 15.3 µm,m, andand 16.516.5 µm.m. Large powder size is usuallyusually preferred in the BJ process for powder spreading while small powder powder is is crucial crucial for for sinterability, sinterability, surface surface quality, quality, and resolution. resolution. PM-B-SR2-05, aa thermosetting polymer,polymer, waswas usedused asas aa bindingbinding agentagent inin the process duedue to its minimal ash residue after debinding. The crucial printing para parametersmeters in the green part production were binder saturation ratio and layer thickness. The binder satu saturationration ratio (the percentage of air space between the powderpowder particles occupied by the binder volume) needs to be susufficientfficient for the fabrication of a strong greengreen part.part. After sintering,sintering, green green parts parts with wi ath maximum a maximum relative relative density density of 85.5% of were 85.5% achieved. were achieved. The shrinkage The andshrinkage sintered and density sintered were density found were to befound proportional to be proportional to the temperature to the temperature in a range in ofa range 1060–1090 of 1060–◦C. A1090 purity °C. ofA 97.3%purity andof 97.3% improvement and improvement of 25.3% in of the 25 sintered.3% in the density sintered could density be achieved could bybe sinteringachieved theby printedsintering parts the inprinted a reduced parts atmosphere, in a reduced as shownatmosphere in Figure, as 11shown. The in printed Figure sintered 11. The copper printed parts sintered could onlycopper have parts a tensile could strength only have of 116.7 a tensile MPa duestrength to their of high116.7 porosity. MPa due One to maintheir challengehigh porosity. of the One BJ process main ischallenge to improve of the the BJ densification process is andto improve part sinterability the densification while satisfying and part the sinterability size requirements while satisfying of the copper the powdersize requirements [21]. of the copper powder [21].

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FigureFigure 11. (a) 11.SEM(a) image SEM image of the of sintered the sintered copper copper particles, particles, ( (b) complex-shaped copper copper made made via binder via binder jetting, and (c) sintered density/shrinkage vs. sintering time of 15 µm powder (60% and 80% binder jetting, and (c) sintered density/shrinkage vs. sintering time of 15 µm powder (60% and 80% binder saturation), both sintered in hydrogen/argon at 1080 ◦C[21], with permission from Emerald Publishing saturation),Limited, both 2019. sintered in hydrogen/argon at 1080 °C [21], with permission from Emerald Publishing Limited, 2019. In a later study, Bai et al. [80] reported the benefits of using bimodal powder mixtures in the BJ Inof a copperlater study, parts. Bai In theet al. process, [80] reported five powder the mixturesbenefits formedof using from bimodal copper powder powder mixtures with different in the BJ of copperdiameters parts. wereIn the employed process, as five the processpowder feedstock. mixtures Theformed study from results copper showed powder that the with bimodal different diameterspowder were mixtures employed could as improve the process the powder feedstoc packingk. The density study by 8.2%results and showed powder flowabilitythat the bimodal by 10.5%, compared to the monosized powder counterparts. More importantly, the printed green part powder mixtures could improve the powder packing density by 8.2% and powder flowability by and the sintered density could increase by up to 9.4% and 12.3%, respectively, while a decrease of 6.4% 10.5%, incompared sintering shrinkageto the monosized was observed powder due to counterp the use ofarts. the bimodal More importantly, powder mixtures. the Anotherprinted benefit green part and thewas sintered that less density energy inputcould might increase be required by up for to the 9.4% process and using12.3%, the respectively, bimodal powder while mixtures a decrease as it of 6.4% inwas sintering less sensitive shrinkage to the sinteringwas observed conditions. due to the use of the bimodal powder mixtures. Another benefit wasSince that coarse less powderenergy andinput loosely might packed be powderrequired beds for are the used process in the BJ using process, the the bimodal final printed powder mixturescopper as it parts was usuallyless sensitive contain highto the porosity sintering with conditions. low mechanical properties. One effective approach to Sinceaddress coarse this powder issue is to and apply loosely post-process packed heat powder treatments, beds suchare used as hot in isostatic the BJ pressingprocess, (HIP) the final [88,89 printed]. Kumar et al. [36] applied HIP to the final printed copper parts with three different dimensions (Type A: copper parts usually contain high porosity with low mechanical properties. One effective approach 16 mm 16 mm 4 mm type B: 32 mm 8 mm 4 mm, and type C: 16 mm 8 mm 4 mm). to address this× issue is× to apply post-process× heat treatments,× such as hot isostatic× pressing× (HIP) [88,89]. Kumar et al. [36] applied HIP to the final printed copper parts with three different dimensions (Type A: 16 mm × 16 mm × 4 mm type B: 32 mm × 8 mm × 4 mm, and type C: 16 mm × 8 mm × 4 mm). As seen in Figure 12a, the relative density of the sintered parts could reach 99.47% after HIP treatment. This result was supported by a significant decrease in pores observed in the sectioned parts shown in Figure 12b, suggesting that HIP could densify the printed copper parts to nearly full density.

Metals 2019, 9, 756 12 of 24

As seen in Figure 12a, the relative density of the sintered parts could reach 99.47% after HIP treatment. This result was supported by a signiﬁcant decrease in pores observed in the sectioned parts shown in MetalsFigure 2019 12, 9b,, x suggestingFOR PEER REVIEW that HIP could densify the printed copper parts to nearly full density. 12 of 24

Figure 12. (a) Density improvement upon hot isostatic pressing (HIP) of parts fabricated from powder Figure 12. (a) Density improvement upon hot isostatic pressing (HIP) of parts fabricated from powder types A, B, and C and (b) sample micrographs (type C) indicating density improvement upon HIP types A, B, and C and (b) sample micrographs (type C) indicating density improvement upon HIP of of the (left) sintered part, 1.88% porosity; (right) HIPed part, 0.13% porosity calculated from image the (left) sintered part, 1.88% porosity; (right) HIPed part, 0.13% porosity calculated from image analysis [36], with permission from Elsevier, 2019. analysis [36], with permission from Elsevier, 2019. In a later study, Kumar et al. [90] investigated the combined effects of bimodal powder mixtures andIn HIP a later treatment study, onKumar the BJ et printed al. [90]copper investigated parts. the In the combined process, effects the copper of bimodal parts were powder printed mixtures from andthree HIP powder treatment configurations: on the BJ printed two with copper 17 and parts. 25 µm In powders the process, and onethe copper with a bimodal parts were powder printed mixture from threeof 5 andpowder 30 µ mconfigurations: powders. The improvementtwo with 17 inand density 25 µm shown powders in Figure and 13onea indicatedwith a bimodal that the printedpowder mixtureparts with of 5 bimodal and 30 µm configuration powders. hadThe theimprovement best densification in density effects shown with in a relativeFigure 13a density indicated of 97.32% that theafter printed HIP. Theparts results with bimodal also suggested configuration that HIP had was the only best an densification effective post-treatment effects with when a relative the sintered density ofBJ 97.32% parts hadafter a HIP. minimum The results relative also density suggested of 90%, th whichat HIP could was only be achieved an effective by using post-treatment bimodal powder when themixtures. sintered The BJ relativeparts had density a minimum of 99.47% relative in their dens previouslyity of 90%, study which [36] could could not be be achieved replicated by due using to bimodalthe problems powder with mixtures. clogged The inkjet relative nozzles density and diofff 99.47%erences in in their the powder previously quality. study More [36] importantly,could not be replicateddue to the due reduction to the inproblems porosity afterwith HIPclogged application, inkjet nozzles both tensile and strength differences and ductilityin the powder of the bimodal quality. Moresamples importantly, increased due from to 144.9 the reduction to 176.35 MPa in porosity and from after 18.81% HIPto application, 31.28%, respectively. both tensile strength and ductility of the bimodal samples increased from 144.9 to 176.35 MPa and from 18.81% to 31.28%, respectively.

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Figure 13. (a) Density improvement upon HIP, (b) porosity in bimodal parts, and (c) samples of untestedFigure 13. (top) (a) andDensity tested improvement (bottom) bimodal upon tensile HIP, specimens(b) porosity [90 in], withbimodal permission parts, and from ( Elsevier,c) samples 2019. of untested (top) and tested (bottom) bimodal tensile specimens [90], with permission from Elsevier, The2019. e ffects of binders on the BJ process of pure copper were studied by Bai et al. [91] using two different nanoparticle binder systems: colloidal organic binder (a conventional polymer binder with suspendedThe effects copper of nanoparticles)binders on the and BJ process inorganic of nanosuspensionpure copper were (an studied inorganic by Bai copper et al. nanosuspension [91] using two ink).different They nanoparticle found that thebinder jetted systems: nanoparticles colloidal could organic produce binder denser (a convention green parts.al polymer However, binder there werewith nosuspended significant copper differences nanoparticles) in the sintered and density inorganic compared nanosuspension to copper parts printed(an inorganic by conventional copper organicnanosuspension binder. This ink). might They stemfound from that the the o ffjettedset of nanoparticles the reduced sintering could produce densification denser to green the density parts. obtainedHowever, in there the greenwere part.no significant However, differences the inkjetted in nanoparticlesthe sintered density could reducecompared the grainto copper size ofparts the sinteredprinted by parts conventional by hindering organic the grain binder. growth This during might the stem sintering from the process, offset leading of the reduced to their improved sintering tensiledensification strength. to Moreover,the density the obtained use of these in the metal green binders part. couldHowever, enhance the theinkjetted metal puritynanoparticles and structural could integrityreduce the of grain the sintered size of the parts. sintered Besides, parts aby higher hinder weighting the percentage grain growth of during copper the nanoparticles sintering process, in the printedleading copperto their parts improved could betensile achieved strength. at higher Moreover, binder the saturation use of these ratios, metal resulting binders in better could green enhance part strengththe metal as purity well asand higher structural green integrity part and of sintered the sinter parted density. parts. Besides, a higher weight percentage of copperIn anothernanoparticles study, in Bai the et printed al. [92] copper used metal-organic-decomposition parts could be achieved at higher (MOD) binder ink assaturation a binder ratios, in the BJresulting process in of better copper. green As part the MODstrength ink as is well particle-free as higher [ 93green,94] itpart can and avoid sintered the common part density. issues found in theIn BJ another process study, using Bai particle et al. [92] suspensions used metal-organic-decomposition such as particle oxidation, (MOD) sedimentation, ink as a binder and nozzle in the clogBJ process problems. of copper. To validate As the MOD this concept, ink is particle-free copper organometallic [93,94] it can avoid complex the common with a metal issues content found ofin 7.4the wt.%BJ process was synthesized using particle for BJsuspensions printing of such pure copperas particle (Figure oxidation, 14a). During sedimentation, the fabrication and nozzle process, clog an overheadproblems. heater To validate was used this toconcept, dry the copper MOD inkorganome betweentallic layers complex by removing with a metal the solvent. content Then, of 7.4 copper wt.% nanoparticleswas synthesized were for precipitated BJ printing and of sintered pure copper from organometallic (Figure 14a). complexDuring the to bondfabrication the copper process, powder an whenoverhead the printedheater was parts used were to dry cured the in MOD a reducing ink betwee atmospheren layers (Figureby removing 14b). However,the solvent. the Then, green copper parts usingnanoparticles MOD ink were were precipitated weaker than and those sintered printed fr withom organometallic the conventional complex polymeric to binder.bond the Although copper thepowder sintered when copper the printed parts printed parts were by MOD cured ink in had a reducing a denser atmosphere core section, (Figure their outer 14b). shell However, was more the porous.green parts This using porous MOD shell ink may were stem weaker from the than powder those printed loss due with to weak the powderconventional bonding. polymeric Consequently, binder. theAlthough relative the density sintered of thecopper copper parts parts printed printed by byMOD MOD ink ink had was a denser lower core than section, that of the their conventional outer shell binderwas more counterparts. porous. This porous shell may stem from the powder loss due to weak powder bonding. Consequently, the relative density of the copper parts printed by MOD ink was lower than that of the conventional binder counterparts.

MetalsMetals2019 2019, ,9 9,, 756 x FOR PEER REVIEW 14 ofof 2424 Metals 2019, 9, x FOR PEER REVIEW 14 of 24

Figure 14. (a) Comparison between jetting a nanoparticle suspension (with particles) and metal- Figure 14. (a)Comparisonbetweenjettingananoparticlesuspension(withparticles)andmetal-organic-decomposition Figureorganic-decomposition 14. (a) Comparison (MOD) between ink (particle-free) jetting a nanoparticle as the binder suspension and (b ) (withSEM imagesparticles) of andthe powdermetal- (MOD) ink (particle-free) as the binder and (b) SEM images of the powder particle formed by sintered copper organic-decompositionparticle formed by sintered (MOD) copper ink (particle-free)nanoparticles as[92], the with binder permission and (b) fromSEM Elsevier,images of 2019. the powder nanoparticles [92], with permission from Elsevier,2019. particle formed by sintered copper nanoparticles [92], with permission from Elsevier, 2019. 2.4.2.4. UltrasonicUltrasonic AdditiveAdditive ManufacturingManufacturing (UAM) (UAM) 2.4. Ultrasonic Additive Manufacturing (UAM) UltrasonicUltrasonic additive additive manufacturing manufacturing (UAM) (UAM) is is a manufacturinga manufacturing method method using using thin thin metallic metallic foils foils or tapesor tapesUltrasonic as feedstockas feedstock additive to fabricateto fabricatemanufacturing 3D 3D parts parts [(UAM)95 [95–98].–98]. is In aIn this manufacturing this method, method, an an ultrasonicmethod ultrasonic using wave wave thin and and metallic mechanical mechanical foils pressureorpressure tapes as are are feedstock applied applied onto on fabricate metallic metallic tapes3D tapes parts at at room room[95–98]. temperature temp In eraturethis method, to to bond bond an the theultrasonic interfaces interfaces wave of of the theand stacked stacked mechanical tapes tapes pressurebyby di diffusionﬀusion are [applied 99[99].]. 3D 3D on parts parts metallic can can be betapes built built at by roomby bonding bondin temp thegerature the stacked stacked to bond tapes tapes the layer-by-layer layer-interfacesby-layer of without the without stacked the the usetapes use of byanyof diffusionany heat source.heat [99]. source. The 3D metallicparts The canmetallic tapes be built are tapes usually by bondinare cut usually intog the designed stackedcut into shapestapes designed layer- before by-layershapes ultrasonic before without consolidation ultrasonic the use ofbonding.consolidation any heat To achievesource. bonding. goodThe To metallic ﬁnishing,achieve tapes good polishing arefinishing, usually is usually polishing cut appliedinto is designed usually during applied orshapes after during thebefore consolidation orultrasonic after the process.consolidationconsolidation Figure bonding. process. 15 illustrates FigureTo achieve the 15 working illustrates good processfinishing, the working of polishing an UAM process equipmentis usuallyof an UAM applied [95]. equipment during [95].or after the consolidation process. Figure 15 illustrates the working process of an UAM equipment [95].

Figure 15. Schematic of the double transducer-sonotrode system used in ultrasonic additive manufacturing Figure 15. Schematic of the double transducer-sonotrode system used in ultrasonic additive (UAM) [95], with permission from Elsevier, 2019. Figuremanufacturing 15. Schematic (UAM) of[95], the with double permission transducer-sonotrode from Elsevier, 2019. system used in ultrasonic additive manufacturing (UAM) [95], with permission from Elsevier, 2019.

Metals 2019, 9, 756 15 of 24 Metals 2019, 9, x FOR PEER REVIEW 15 of 24

TheThe building building process process startsstarts withwith the the laying laying out out and and stacking stacking of aof metallic a metallic tape tape on a on base a base plate. plate. Then, Then,a sonotrode a sonotrode applies applies ultrasonic ultrasonic vibrations vibrations laterally laterally to the to tapethe tape when when it rolls it rolls along along the tapethe tape length length [99 ]. [99].This This bonds bonds the metallicthe metallic tape totape the to previously the previously built partsbuilt toparts form to a form seam a weld. seam Duringweld. During this process, this process,the rapid the “scrubbing rapid “scrubbing action” action” between between the faying the surfacesfaying surfaces generates generates the bonding. the bonding. Typically, Typically, the first thetape first is weldedtape is welded onto the onto base the plate, base followed plate, foll byowed the bonding by the bonding of the next of tape.the next After tape. all theAfter layers all the are layersbuilt, are the built, final partthe final is cut part from is thecut basefrom plate the ba andse plate polished and forpolished desired for surface desired finishing. surface finishing. Therefore, Therefore,the UAM the process UAM can process produce can metallicproduce partsmetallic of complex parts of complex shapes and shapes designs. and designs. During theDuring bonding the bondingprocess, process, frictional frictional heat at the heat bonded at the interfaces bonded is interfaces produced, is leading produced, to the leading increased to temperaturethe increased of temperaturethe consolidated of the region. consolidated Therefore, region. a short Therefore, cooling a period short cooling between period the manufacturing between the manufacturing of each layer is ofrequired each layer to avoidis required thermal to avoid residual thermal stress residual [95]. stress [95]. WeldingWelding copper tapestapes areare usually usually di ffidifficultcult due due to materialto material hardening hardening and oxidation and oxidation issues, issues, leading leadingto the requirementto the requirement of high of power high power used inused the in process the process (up to (up 9 kW)to 9 kW) [95,96 [95,96,100].,100]. Sriraman Sriraman et al. et [al.96 ] [96]studied studied the the bonding bonding characteristics characteristics formed formed during during a very a very high-power high-power UAM UAM of copperof copper tapes. tapes. In theIn thestudy, study, C11000 C11000 copper copper tapes tapes (170 (170 mm mm ×25 25 mm mm × 0.150.15 mm) were successively bonded bonded via via ultrasonic ultrasonic × × seamseam welding welding at at room room temperature. temperature. The The welding welding process process was was conducted conducted at at an an amplitude amplitude of of 36 36 µm,µm, staticstatic force force of of 6.7 6.7 kN, kN, and and travel travel speed speed of of 30 30 mm/s. mm/s. The The final final built built part part was was comprised comprised of of 13 13 layers layers to to reachreach a aheight height of of about about 2 2mm. mm. Figure Figure 16a 16 ashows shows the the in interfacesterfaces of of the the built built sample sample with with wavy wavy features. features. ItIt might might stem stem from from the the violent violent plastic plastic flow flow of of the the material material which which was was formed formed along along the the ridges ridges and and valleysvalleys between between the the contacting contacting laye layersrs during during the the scrubbing scrubbing process. process. Hardness Hardness distribution distribution shown shown in in FigureFigure 16b16b suggestssuggests that that the the material material might might be uniformly be uniformly softened softened and the hardnessand the valueshardness throughout values throughoutthe built part the were built about part were 11–23% about lower 11–23% than lower those ofthan the those as-received of the as-received foils [96]. foils [96].

FigureFigure 16. 16. (a()a SEM) SEM image image of of the the C11000 C11000 Cu Cu built built sample sample etched etched with with 50% 50% HNO HNO3, (b3),( hardnessb) hardness map map of aof transverse a transverse section section of the of theC11000 C11000 Cu Cubuilt built sample, sample, (c) ( cgrain) grain size size distributions, distributions, and and (d (d) )grain grain misorientationsmisorientations [96], [96], with with permission permission from from Elsevier, Elsevier, 2019. 2019.

Metals 2019, 9, 756 16 of 24

As seen in Figure 16c, the grain size of the copper foils at the foil interface reduced from 25 to 10 µm after the processing [96]. Moreover, a larger fraction of grains with high-angle misorientations were observed in the built part (Figure 16d). The ﬁndings indicated that there was recrystallization followed by the movement of grain boundaries across the interfaces, resulting in metallurgical bonding between the layers. Additionally, the initial coarse grain structure might experience plastic deformation and recovery during the welding process. Their later study on the thermal transients during the process suggested a strong correlation between the interfacial temperature and plastic deformation heating [95]. Moreover, vibration amplitude had strongest eﬀects on interfacial temperatures as dynamic plastic shear strains at the asperities were higher with increasing amplitudes. Although the UAM process could produce fully dense parts, void defects might be formed within the material’s internal structure during the fabrication process, lowering its electrical and mechanical performance.

3. Advantages and Challenges of 3D Printing Methods of Pure Copper Table1 compares the characteristics of all 3D printing methods of pure copper. In both direct SLM and EBM, high energy power is required to heat the copper powder to its melting point for part fabrication. However, the required energy of direct SLM is much higher than that of EBM due to the low energy absorption of pure copper with conventional lasers (wavelength of 1 µm) [41,59,101–103]. Moreover, the high reflected laser radiation in direct SLM of pure copper can damage the system’s components. These issues can be addressed by (i) improving the optical absorption of copper by using a green laser with a shorter wavelength (515 nm) or chemically modifying the surface of the copper powder and melt; and (ii) applying advanced multi-variate ray tracings to eliminate the back focal points on the locations of the laser mirrors [51]. The required energy in the EBM of copper is lower than that in direct SLM because optical reflectivity of the materials do not affect the EBM process [4,40]. However, the EBM beam size is larger than the laser spot size in direct SLM, limiting the fine features size of the EBM-printed parts. Due to rapid melting/solidification of copper powder, the copper parts fabricated by both SLM and EBM have many issues in the full melting process, such as thermal residual stresses and thermally-induced deformation [2,27]. These issues can be minimized by properly designed anchors or support structures which dissipate heat and prevent part deformation [1,41].

Table 1. Comparison of diﬀerent methods for 3D printing of pure copper.

Process Requirements/ Indirect SLM Direct SLM EBM BJ UAM Part Characteristics Required power Low High High Low High Printing temperature Low High High Low Low Thermal residual stress No Yes Yes No No Post-processing Yes No No Yes No Part density/properties/purity Low High High High High

The printing temperatures in the indirect SLM, BJ and UAM processes are much lower than those in direct SLM and EBM (lower than 50% of copper’s melting temperature) [21,27,49,50,95,96]. This prevents many issues formed in full melting processes, such as thermally-induced deformation and thermal residual stresses. Both indirect SLM and BJ need quite low power to fabricate green parts, but post-processing such as green part curing, debinding, and sintering is required to achieve the final metal parts [21,36,49,50]. The final sintered parts usually have low density and high porosity, which require further post-processing such as HIP to obtain fully dense parts [36,90]. Residual impurities from the debinding process in indirect SLM are usually high, and this could affect the performance of the final printed parts [49,50]. The UAM can produce fully dense parts without post-processing. However, this process requires high power to process pure copper due to material hardening and oxidation issues [27,95,96]. Metals 2019, 9, 756 17 of 24

Table2 compares the relative density, electrical, mechanical and thermal properties of the printed copper parts fabricated by diﬀerent methods. Performance of the parts fabricated by UAM is not presented because no data are available in the literature. As can be seen, EBM can fabricate copper parts with the best value of relative density (99.95%), electrical conductivity (102% IACS), tensile strength (177 MPa), and thermal conductivity (411.89 W/mK). In contrast, copper parts from indirect SLM have the poorest performance with a relative density of 84.8% and strength of only 8 MPa. Interestingly, parts printed by JB have a high relative density and strength of 99.7% and 176.35 MPa, respectively, but no data for electrical and thermal conductivity are available. For direct SLM, the printed parts have a high relative density (99.6%) and good physical performance with an electrical conductivity of 88% IACS, tensile strength of 149 MPa, and thermal conductivity of 336 W/mK.

Table 2. Property comparison of copper parts fabricated by diﬀerent 3D printing methods.

Electrical Thermal 3D Printing Relative Density Tensile Strength Conductivity Conductivity Methods (%) (MPa) (%IACS) (W/mK) Pure copper 100 100 200 [21] 400 [79] Indirect SLM 84.8 [49] - 8 [50]- Direct SLM 99.6 [64] 88 [51] 149 [65] 336 [51] EBM 99.95 [77] 102 [79] 177 [79] 411.89 [79] JB 99.7 [36] - 176.35 [90]-

4. Applications and Opportunities

4.1. Complex Heat Exchanger

One of the common applications for 3D printing of pure copper is complex heat exchangers [12,13,104–106]. Heat transfer components such as micro reactors, radiators, exchangers, or coolers could have improved function and efficiency by additive manufacturing approaches. An effective heat transfer in a liquid cooling system usually has a reliable and complex structure with high surface-volume ratio so that the flowing liquid can have many contacts with the surface area of the components. Moreover, their structure needs to be reliable to provide high pressure and resistance against fluid crossing between channels or fluid leakage. Notably, heat exchangers fabricated by conventional processes such as soldering or diffusion bonding are costly and insufficiently reliable because of their complex assembly [12,13]. Due to the single-step process of 3D printing, the printed components have no joints and, therefore, lower the risk of leakage [12,13]. Moreover, higher efficiency of the heat transfer can be achieved by maximizing the surface–volume ratio because many complex internal structures or intricate channels in confined space can be designed and fabricated by 3D printing, as shown in Figure 17a [13]. Furthermore, many materials with high thermal conductivity such as copper or copper alloys can be used for 3D printing of heat exchangers. Figure 17b presents an example of a compact cooler unit fabricated by SLM. The component (46 mm 59 mm 15 mm) has many cooling fins with a × × thickness of 0.3 mm to ensure efficient heat exchange. A high surface–volume ratio and turbulent flow of the coolant are achieved by optimizing the ribbing within the component [13]. In another approach, open-cellular copper structures with densities ranging from 0.73 to 6.67 g/cm3 have been fabricated by EBM, as shown in Figure 17c [26]. The printed structures had hardness values of more than 75% of commercial Cu parts, exhibiting great potential for complex, multi-functional electrical and thermal management systems and heat exchange devices. Metals 2019, 9, 756 18 of 24 Metals 2019, 9, x FOR PEER REVIEW 18 of 24

Figure 17. (a) Heat exchanger designs with extremely intricate inner geometry [13], (b) miniature coolerFigure [1317.], ( anda) Heat (c) reticulated exchanger copperdesigns mesh with fabricatedextremely byintricate EBM [ 26inner], with geometry permission [13], from(b) miniature Emerald Publishingcooler [13], Limited,and (c) reticulated 2019. copper mesh fabricated by EBM [26], with permission from Emerald Publishing Limited, 2019. 4.2. Induction Heat Coil 4.2. InductionGenerally, Heat induction Coil heat coils have various shapes and sizes which are often highly customized for diGenerally,fferent applications induction [ heat27]. Moreover,coils have various they usually shapes have and complex sizes which structures are often with highly curved customized paths or helicalfor different shapes applications and hollow feature[27]. Moreover, for water they cooling. usually Therefore, have complex the production structures process with of curved induction paths coils or ishelical usually shapes tedious and and hollow time-consuming feature for water because cooling. they are Therefore, usually fabricatedthe production manually process with of customized induction design.coils is Theseusually features tedious make and induction time-consuming coils a good because candidate they forare 3Dusually printing fabricated due to its manually advantages with of fabricatingcustomized complex design. andThese highly features customized make induction parts with coils little a leadgood time. candidate Induction for 3D coils printing are usually due madeto its ofadvantages highly pure of fabricating copper because complex the coiland materials highly customized must have parts high with efficiency little fprlead conducting time. Induction electricity. coils Martinare usually et al. made [27] suggested of highly that pure direct copper SLM, because EBM and th UAMe coil arematerials suitable must for fabricating have high induction efficiency coils fpr withconducting higher precisionelectricity. compared Martin et to al. typical [27] sugges hand-madeted that coils. direct SLM, EBM and UAM are suitable for fabricating induction coils with higher precision compared to typical hand-made coils. 4.3. Radio Frequency Cathode 4.3. Radio3D printing Frequency can Cathode be used to fabricate photoinjector cathodes with high RF performance. Frigola3D et printing al. [40,107 can] testedbe used RF to performancefabricate photoinjecto of the EBMr cathodes printed with copper high cathode RF performance. in a Pegasus Frigola 1.6 cellet al. photoinjector. [40,107] tested No RF other performance heat treatment of the was EB performedM printed oncopper the printed cathode cathode in a Pegasus before the 1.6 final cell machining.photoinjector. The No study other results heat showedtreatment that was during performed high power on the RF printed testing, cathode the EBM before printed the copper final cathodesmachining. had The similar study performanceresults showed to that wrought during oxygen-free high power (OFE) RF testing, copper the cathodes EBM printed fabricated copper by conventionalcathodes had methods.similar performanc During twoe to hours wrought of RF oxygen-free conditioning, (OFE) a peakcopper electric cathodes field fabricated of 70 MV /m,by a photoelectron beam with a charge of 60 pC, energy of 3.3 MeV, and quantum efficiency of ~2 10 5 conventional methods. During two hours of RF conditioning, a peak electric field of 70 MV/m,× − a werephotoelectron achieved beam with stable with a operation. charge of 60 pC, energy of 3.3 MeV, and quantum efficiency of ~2 × 10−5 were achieved with stable operation. 5. Conclusions and Future Direction 5. ConclusionsThis paper presentsand Future a comprehensive Direction review of research on the 3D printing methods for fabricating highly pure copper, including SLM, EBM, BJ and UAM. The advantages, challenges, and performance This paper presents a comprehensive review of research on the 3D printing methods for of the copper parts fabricated by each method have been identified and compared. Additionally, fabricating highly pure copper, including SLM, EBM, BJ and UAM. The advantages, challenges, and potential applications of the 3D printed copper parts in thermal management systems, heat exchange performance of the copper parts fabricated by each method have been identified and compared. devices, RF cathodes, and induction heat coils have been demonstrated. Additionally, potential applications of the 3D printed copper parts in thermal management systems, heat exchange devices, RF cathodes, and induction heat coils have been demonstrated.

Metals 2019, 9, 756 19 of 24

In the near future, the 3D printing of pure copper will most likely grow further to address all the current issues of the printing methods while exploring more potential applications of the printed copper parts. For example, the TruPrint Laser Metal Fusion (LMF) 3D printer equipped with green laser source has been developed to overcome the low absorption of pure copper to conventional laser sources and component damages [108]. Heraeus, a German technology group, has addressed the technical issues and optimized the process of 3D printing highly-conductive copper components, which opens up new industrial ﬁelds of application such as in mobility, electronics, robotics, hydraulics, medicine and aerospace [109].

Funding: This research was funded by Lloyd’s Register Foundation, grant number R-265-000-553-597. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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