Binder jetting additive manufacturing of copper foam structures

Journal Pre-proof Binder Jetting Additive Manufacturing of Copper Foam Structures Hadi Miyanaji, Da Ma, Mark A. Atwater, Kristopher A. Darling, Vincent H. Hammond, Christopher B. Williams

PII:

S2214-8604(19)30951-0

DOI:

https://doi.org/10.1016/j.addma.2019.100960

Reference:

ADDMA 100960

To appear in:

Additive Manufacturing

Received Date:

18 July 2019

Revised Date:

17 October 2019

Accepted Date:

17 November 2019

Please cite this article as: Miyanaji H, Ma D, Atwater MA, Darling KA, Hammond VH, Williams CB, Binder Jetting Additive Manufacturing of Copper Foam Structures, Additive Manufacturing (2019), doi: https://doi.org/10.1016/j.addma.2019.100960

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Binder Jetting Additive Manufacturing of Copper Foam Structures Hadi Miyanaji1, Da Ma1, Mark A. Atwater2,3, Kristopher A. Darling3, Vincent H. Hammond3, and Christopher B. Williams1 1

Design, Research, and Education for Additive Manufacturing Systems Laboratory Department of Mechanical Engineering, Virginia Tech, VA, USA 2

US Army Research Laboratory, Aberdeen Proving Ground, MD, USA

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Graphical abstract

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Department of Applied Engineering, Safety, and Technology, Millersville University, PA, USA

Abstract

In Binder jetting additive manufacturing (BJAM), the part geometry is generated via a binding agent during printing and structural integrity is imparted during sintering at a later stage. This separation between shape generation and thermal processing allows the sintering process to be uniquely controlled and the final microstructural characteristics to be tailored. The separation 1

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between the printing and consolidation steps offers a unique opportunity to print responsive materials that are later “activated” by temperature and/or environment. This may allow a new paradigm in multi-scale, multifunctional materials. This concept is preliminarily demonstrated using a foaming copper feedstock, such that the copper is printed, sintered and then foamed via intraparticle expansion in separate steps. The integration of foaming feedstock in BJAM could allow for creation of ultra-lightweight structures that offer hierarchical porosity, graded density, and/or tailored absorption properties. This work investigates processing protocol for copper foam structures to achieve the highest porosity. The copper feedstock was prepared by distributing copper oxides through the copper matrix via mechanical milling, and that powder was then printed into a green geometry through BJAM. The printed green parts were then heat treated using different thermal cycles to investigate the porosity evolution relative to various heating conditions. The heat treated parts were then examined for their resulting properties including porosity, microstructural evolution, and volumetric shrinkage. Parts that were initially sintered in air and then annealed in a hydrogen atmosphere led to higher porosity compared to those sintered in hydrogen alone. It was also found that the annealing of parts at 600 °C for 2 hours resulted in the highest final porosity (59%) and the lowest volumetric shrinkage of 5%. Anisotropy in linear shrinkage in X, Y, and Z direction was also observed in the heat treated parts with the largest linear shrinkage occurring in the Z direction.

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Keyword: AERO process, Binder Jetting, Additive Manufacturing, Microstructure, Porosity,

1. Introduction

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Shrinkage.

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In binder jetting additive manufacturing (BJAM) technology, the powder material is spread layer after layer, and the green part geometry is generated via application of binder droplets in the selective areas of each layer identified from a computer-generated model. The green parts made by the BJAM possess limited strength and often require additional post-processing, such as sintering and infiltration, so as to impart mechanical strength and structural integrity [1, 2]. This separation between shape generation and thermal processing gives the BJAM process distinct advantages when working with nonequilibrium metallic powders that may be adversely affected by laser or electron beam melting techniques. Specifically, nanostructured alloys are often sensitive to elevated temperatures, and their distinctive microstructures may be completely reversed by even partial melting. The ability to decouple the build-up and sintering of complex geometries may be pivotal to generating unique structural and functional components. More importantly, by separating the printing and consolidation steps, responsive materials can be employed that are later “activated” by temperature and/or environment, and this may be tailored to fabricate multi-scale, multifunctional materials. As an initial test-case of this multi-functional capability, BJAM is investigated as a means to create porous structures with multi-scale porosity using a responsive copper feedstock powder that forms porosity when heated in a reducing atmosphere. 2

1.1. Porous Metals and the Additive Expansion by the Reduction of Oxides Process

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Metallic foam structures are of significant value to various applications due to their attractive physical and mechanical properties such as light weight, high energy absorptivity, high specific stiffness, and efficient heat transfer [3, 4]. Despite these advantages, widespread adoption of these materials is often hindered by (i) challenges in direct fabrication of near-net shape structures and (ii) inability to control and vary the cellular microstructure [5, 6]. There exist two major techniques to produce metal foam structures: liquid-state and solid-state foaming. In liquid-state foaming, porosity is introduced into a matrix material in a liquid or semi-liquid state, whereas solid-state foaming introduces porosity into a fully solid metal. Although liquid-state methods have been more commercially attractive to different industries, their application is usually limited to relatively low-melting point and nonreactive metals [7, 8]. From prior literature, solid-state foaming techniques, such as powder sintering [9], powder injection molding [10], gas entrapment [11], and fugitive templating [12], often suffer from process complexity and/or achievable final porosity [6, 7]. Despite the versatility of solid-state foaming, the rate of foaming and volume of the material produced by these methods are often smaller compared to liquid-state processing due to various technological challenges associated with this method [7, 8].

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To extend the capabilities of solid state foaming, Atwater et al. developed a new solid-state process, Additive Expansion by the Reduction of Oxides (AERO), which has shown promising outcomes for producing metallic foam structures [5-8, 13, 14]. In the AERO process, the formation of porosity in the microstructure of samples is accomplished by the reduction of oxide particles that are embedded in the metal powders. Therefore, the only steps required in the AERO process are (i) distribution of oxides into the metal powder via mechanical mixing/alloying, and (ii) reduction of embedded oxides at elevated temperatures. Once the metal matrix composite is heated in a reducing atmosphere (e.g., hydrogen), the reducing agent diffuses through the metal and reduces the oxide particles (MO) to the pure metal (M), which creates steam through the chemical reaction provided in Equation 1. The formation of steam at elevated temperatures leads to interconnected pores with size distribution from nanoscale to a few microns [5, 6]. As such, each individual metal particle develops porosity by the reduction of the oxide in a reducing atmosphere. MO(s) + H2(g) → M(s) + H2O(g)

(Eqn. 1)

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Using this process, cylindrical copper pellets made via powder compaction with 40% initial green porosity have been foamed to nearly 70% final porosity after annealing at 600 °C for one hour [6]. Although, fabrication of complex metallic geometries with hierarchical porosity for graded density would benefit many areas in energy management, lightweight structures, etc., the design freedom of these structures is often limited by their current traditional manufacturing processes. Due to the inherent design freedom and the ability to decouple thermal processing from geometry creation, BJAM technology offers great potential for creating such multi-scale porous structures using responsive powder feedstock. 1.2. Prior work in binder jet processing of porous structures 3

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BJAM technology is well known for its capability to fabricate complex porous geometries, process a wide range of material systems, and to be scaled up by merely increasing the size or number of printheads [2, 15-17]. Depending on the post-processes, the fabricated parts often possess varying amount of porosity in the microstructure. Although improving the densification of the parts has been thoroughly investigated in binder jetting [18-22], there exists limited research that explores the potential of BJAM to intentionally fabricate porous metallic structures in the literature [23]. Ziaee and co-authors investigated two feedstock powder materials (the agglomerates of fine 316 stainless steel and the mixture of steel with nylon powder) to fabricate porous stainless steel parts. While the agglomerated stainless steel powder achieved the same sintered density (>93%) as the raw powder (without agglomeration), adding nylon powder particles as fugitive agents to stainless steel powder reduced the sintered density below 70% [23]. The use of fugitive agents such as nylon for fabrication of graded porosity in metallic structures might also be limited by the contamination from the sacrificial material after pyrolysis. 1.3. Roadmap

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In this study, the BJAM and AERO processes are integrated to fabricate copper foam structures. The objectives of the current research are as follows: (i) to explore the fabrication of copper foam structures via processing AERO copper powder in BJAM, (ii) to experimentally investigate the effect of heat treatment processes on porosity formation so as to optimize the heat treatment parameters for higher porosity.

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Experimental methods, including characterization of powder made by AERO process, the process parameters used in BJAM to form the green part, and the different post-process heat treatment cycles explored, are described in Section 2. In Section 3, the effect of different heat treatment routes (Section 3.1) and different annealing parameters (Section 3.2) on the porosity and microstructure of the processed parts are discussed. The volumetric shrinkage of the heat treated parts is discussed relative to the different annealing parameters in Section 3.3. 2. Methodology

2.1. Cu/CuO powder material

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The feedstock powder material was prepared by planetary milling Cu powder (-100+325 mesh, 99.9%, Alfa Aesar p/n 42623) with CuO powder (<10 μm, 98%, Sigma-Aldrich p/n 208841) for 90 min in a Fritsch P5 mill operating at 370 rpm. During milling, two 500 mL tool steel milling jars were run together, each containing 35 g of powder and 75 ball bearings of 3/8” diameter made from 440C stainless steel. To prevent cold welding, 0.5 wt% of stearic acid (reagent grade, 95%, Sigma-Aldrich p/n 175366) was also added to each jar. The Cu and CuO powders were mixed to produce a 1.5 at% addition of oxygen through the CuO powder. Based on the manufacturer’s certificate of analysis, the as-received copper powder contained 0.5 at% oxygen, bringing the final content to 2 at%. This ratio corresponds to favorable findings in prior work on SPEX-milled oxide-

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dispersed copper powders [6]. After milling, powders were triple-rinsed in dichloromethane to remove any residual stearic acid.

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Fig. 1 displays the SEM image of the AERO powder used for printing of the copper parts. CuO powder consisted of particles with spherical and round flake shapes with a particle size distribution from 10 µm to 120 µm. Particle size analysis was conducted in isopropyl alcohol with a Malvern Mastersizer S. Five runs were performed on both the as-milled and annealed (600 °C, 1 hr, 5% H2) powders. The median particle size increased from 20.4 µm (± 0.2 µm) to 27.6 µm (± 0.1 µm) after annealing (a 35% increase in measured particle diameter).

Fig. 1 SEM image of the as-milled Cu/CuO powder materials

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2.2. Green part fabrication

Rectangular bars with dimensions of 9x6x3 mm3 were successfully printed using the Cu/CuO powder as feedstock materials on an ExOne R2 3D printer with the settings listed in Table 1. A standard ExOne solvent-based polymeric binder was used to print green parts.

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The amount of binder used in printing green parts is characterized by the binder saturation ratio. This key process parameter represents the amount of pores within a defined envelope that is filled with the binder [24, 25]. Different binder amounts (100%, 120%, 175%) were examined during green part fabrication trials; a binder saturation of 175% resulted in green parts with suitable structural integrity and sufficient strength for part handling and measurement via a digital caliper. The counter-rotating roller spread powder with a constant speed of 10 mm/s was used for layer creation. For powder bed additive manufacturing processes, it is recommended to use a layer thickness that is larger than the largest powder particle. Thus, a layer thickness of 150 µm was used for part printing given the maximum measured particle size of 120 µm. In addition, printing speeding of 100 mm/s was chosen for binder droplet deposition, as this speed has been shown to 5

improve droplet spreading isotropy and improve final part mechanical strength [26]. The powder bed temperature was maintained at 75 °C during printing, and an overhead heater (250 °C) was used to dry/cure each printed layer with a scanning speed of 2 mm/s.

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Powder spreading speed (mm/s)

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The density of the green parts was calculated using the dimensions and measured mass of the printed samples. The fractional density was obtained by comparing with the pore-free density of pure copper at room temperature (8.96 g/cm3). The average fractional density of 30 green parts was determined to be 36.3 (+/- 1.9%).

Overhead heater scanning speed (mm/s)

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Table 1. Process parameters used for green part fabrication

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2.3. Post-processing

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Two different heat treatment routes were examined in this study. For the first route, a batch of green parts were sintered in a pure hydrogen atmosphere according to the sintering profile shown in Fig. 2a. The green specimens were first heated at 450 °C for half an hour to pyrolyze the binder (based on thermogravimetric analysis), followed by an isothermal holding at 600 °C for 120 minutes. This dwell time of 120 minutes at 600 °C was aimed to reduce copper oxides distributed inside each individual copper particle [5, 14]. After the oxide-reduction step, the parts continued to be heated to the target sintering temperature (1075 °C) and held for 180 minutes, which has been successfully used for sintering of copper parts produced via the binder jetting process [16, 19, 27]. Heating/cooling ramp of 5 °C/min was used for all transitions between different stages. Using this heat treatment route, intra-particle porosity is created during the isothermal hold at 600 °C followed by part sintering at 1075 °C, which is aimed at improving the strength of the processed parts. To explore the effect of sintering on intra-particle porosity, an alternative thermal cycle route was employed to decouple particle sintering from intra-particle pore formation. For this thermal cycle route, a second batch of green parts was sintered in air using the same sintering profile as in Fig. 2a. Then, after complete sintering in air, the sintered specimens were annealed in hydrogen atmosphere at 600 °C for 120 minutes (as shown in Fig. 2b) for oxide reduction and porosity creation within each copper particle. 6

A full factorial experiment of two factors and three levels based on design of experiments (DOE) method was implemented to study the effect of different heat treatment settings (i.e. temperature and holding time) on air-sintered part characteristics. Three specimens were printed and heat treated for each heat treatment condition. To analyze the experimental data, Minitab software was employed to determine the correlations between the annealing parameters and part characteristics based upon Analysis of Variance (ANOVA) analysis.

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2.4. Part characterization

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Time Time Fig. 2 The heating schedules used for a) sintering the green parts b) annealing of parts sintered in air

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The density/porosity, volumetric and linear shrinkage, and microstructure of the processed specimens were evaluated to explore the effect of process conditions. Archimedes’ principle was used to measure the bulk density of the oil-impregnated samples [28]. The fractional density as well as the fractional porosity of the heat treated parts was obtained by comparing with the porefree density of the pure copper at room temperature. To conduct metallography, a number of specimens were sectioned, mounted and polished in successively finer polishing materials. A LEO (Zeiss) 1550 field-emission scanning electron microscope (FESEM) with backscattered electron detector was used to study the microstructure of these specimens. A digital caliper (+/- 0.01 mm) and high precision scale (+/- 0.0001 g) were used for measuring the dimensions and the weight of the samples, respectively. Green part dimensions were used as the basis for calculation of the volumetric and linear shrinkage of heat treated parts. All data points in subsequent plots represent average values for at least three measurements. 3. Results and discussion 3.1. Different post-printing thermal cycles The porosity measurements of the specimens in the green state, after sintering in pure hydrogen (Fig. 2a), and after sintering in air followed by annealing in pure hydrogen (Fig. 2b) are shown in Fig. 3. The parts that were initially sintered in air and subsequently reduced in a hydrogen 7

atmosphere (at 600 °C for 120 minutes) resulted in higher porosity, 58.1%, compared to those fully sintered in a hydrogen atmosphere.

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This significant difference between the final porosities of the parts with different heat treatment conditions is attributed to the formation and behavior of porosities within each particle under different heat treatment conditions. The basic mechanism for intra-particle pore generation in the AERO process is hydrogen diffusion into the metal matrix and consequently, reduction of oxide particles distributed in the metal structure [5, 6]. For the first scenario, where the green parts are fully sintered in a pure hydrogen atmosphere (Fig. 2a), some quantities of the pores generated in the intermediate stage of sintering process are later eliminated during final stage sintering in the peak temperature (1075 °C). This is mostly attributed to the shrinkage of the pores generated along the grain boundaries [29, 30], and a similar shrinkage process has been observed in these materials via dilatometry [31].

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It is noted that heat treatment of the parts without final sintering at 1075 °C resulted in specimens with poor structural integrity. Fig. 4 depicts specimens heat treated at 800 °C for one hour in the pure hydrogen atmosphere that were mounted for microstructural analysis. It is evident that the polishing process has led to disintegration of the particles due to the weak inter-particle bonding, which is caused by the elimination of the final stage sintering.

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In contrast, the oxide content of the metal matrix within each individual particle remains intact during sintering in air, which subsequently serve as pore formation nuclei during the oxide reduction step in a hydrogen atmosphere. As a result, the second scenario, where the oxide reduction and thus intra-particle pore creation step is separated from the sintering process, leads to higher porosity in the microstructure due to the larger amounts of pores within each individual particle as schematically illustrated in Fig. 5. 64

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Fig. 3 Porosity of the green parts and parts sintered under various heat treatment cycles

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Fig. 4 Parts heat treated at 800 °C for one hour (parts were mounted for microstructural analysis)

Fig. 5. Schematic of copper foam fabrication process

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Backscattered electron images of microstructures of copper samples that have undergone different heat treatment cycles are shown in Fig. 6. It is evident that sintering specimens in a hydrogen atmosphere (Fig. 6a) has resulted in both smaller intra-particle pore size and lower quantities of pores compared to the samples that were sintered in air and subsequently annealed in hydrogen atmosphere (Fig. 6b). The resulting intra-particle porosity in the specimens that undergo the airsintering and annealing thermal sequence appear to be in nanoscale to a few microns in size, as shown in Fig. 6b. The formation of such porosity within each particle is reported in the literature [5, 6, 13] and is attributed to the mechanism by which the metallic matrix expands due to the steam creation from oxide reduction. 9

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Fig. 6 Backscattered electron (BSE) images of copper specimens that have undergone different thermal cycles: a) fully sintered in hydrogen atmosphere using the profile depicted in (Fig. 2a); b) sintered in air and then annealed in pure hydrogen (Fig. 2b)

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Given these results, the thermal cycling route in which the green parts are initially sintered in air and subsequently annealed in a reducing atmosphere was chosen for further analysis due to the higher porosity amounts achieved using this heat treatment cycle. In the following sections (3.2 and 3.3), the porosity and volumetric shrinkage of the parts are studied relative to the different annealing parameters. 3.2. Porosity The effect of annealing temperature and dwell time on total porosity amount is shown in Fig. 7. Annealing of air-sintered specimens at 600 °C resulted in the higher porosity than 500 °C or 700 °C

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for each of the dwell times studied, and the 2 hr dwell time resulted in the highest amount of porosity (58.1%) in the microstructure overall.

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Fig. 7 The effect of the annealing settings on the total porosity of the samples

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To eliminate the errors introduced by the inherent variance of the initial porosities in the printed green parts, change of porosity with reference to the green porosity was used as a metric instead of total porosity of the heat treated parts (figures 8 and 9). The effect of the annealing settings (i.e., temperature and dwell time) on the porosity variation of the annealed parts is shown in Fig. 8. The negative values in this figure indicate reduction in porosity of heat treated specimens. From the ANOVA results, it is seen that the porosity of the parts is affected by the annealing temperature and the interaction of the temperature and dwell time. In other words, the results do not present strong statistical dependence of porosity variation to the dwell time. For all dwell times, increasing the annealing temperature from 500 °C to 700 °C seems to first increase and then decrease the final porosity in the samples.

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Fig. 8 Effect of annealing temperature and time on porosity of the parts sintered in air and heat treat in hydrogen

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The simultaneous effect of annealing temperature and dwell time on porosity variation in the microstructure of the final parts is illustrated in Fig. 9. At dwell times of 1 hour and 2 hours, nonlinear correlation appears to exist between the annealing temperature and porosity variation in the processed parts. Although an increase in porosity is observed with increase in the annealing temperature from 500 °C to 600 °C, further increases in the temperature results in lower final porosity. For a dwell time of 3 hours, increasing the annealing temperature from 500 °C to 700 °C is not of strong influence on the porosity of the specimens. From the results, isothermal heating of samples at 600 °C for two hours results in the maximum final porosity.

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The trends observed can be attributed to the effect of the thermal energy input on the rate of porosity formation within each particle and on the porosity between adjacent particles. In this process of metal foam fabrication, the total porosity achieved encompasses intra-particle (within each individual particle) and inter-particle (between particles) porosities with different length scales. While intra-particle porosity created within each copper particle ranges from nanometer to few microns in size, inter-particle pores between neighboring particles are in the order of tens of microns as depicted in Fig. 10. The images with higher magnifications in Fig. 10 illustrate porosity created within copper particles after annealing. From prior literature [5, 6, 8, 13], it is expected that changing heat treatment conditions would affect not only porosity formation within each copper particle but also inter-particle pore size distribution in the annealed samples. In the context of copper foam fabrication using the AERO process, it is known that the oxide reduction process begins at particle free surfaces and progresses inward with a temperature-dependent rate [6]. Therefore, the volume of the porosity generated inside each particle depends on the annealing conditions (e.g., annealing temperature and time) as well as oxide content distributed inside particles [6, 8]. On the other hand, porosity between adjacent particles (i.e. inter-particle porosity) are also affected by the thermal energy input imposed by the heat treatment settings [32]. As a result, the final porosity in the microstructure of each part are determined by the intra-particle and 12

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inter-particle porosity characteristics and their relationships to varying thermal energy dictated by different annealing settings. To identify the pore evolution (in each type of porosity network) with different heat treatments, further systematic investigation is needed to understand the correlations between the annealing parameters and the pore characteristics within each particle and between neighboring particles, which will be addressed in future works.

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Fig. 9 The effect of the annealing temperature and dwell time interaction on final porosity of the parts

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Fig. 10 Backscattered electron photographs of printed parts annealed at a) 600 °C for 1 hour and b) 500 °C for 1 hour

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3.3. Volumetric shrinkage

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Fig. 11 shows the effects of the annealing temperature and holding time as main effects (Fig. 11a), as well as the interaction of these two factors (Fig. 11b), on the volumetric shrinkage of the heat treated specimens. As expected, these results are in agreement with the results of the porosity variation with changing the annealing parameters. Based on ANOVA analysis, while the annealing temperature and temperature-time interaction are of significant influence on volumetric shrinkage of the parts, the dwell time does not appear to have statistical significance on part shrinkage. At dwell times of 1 hour and 2 hours, volumetric shrinkage decreases with increasing temperature from 500 °C to 600 °C and then increases with further increase in the annealing temperature to 700 °C. For 3-hour dwell time, increasing annealing temperature does seem to have statistically significant effect on volumetric shrinkage of the parts according to ANOVA analysis. Minimum volumetric shrinkage of ~5% is observed in parts annealed at 600 °C for 2 hours. To assess the anisotropy of porosity distribution, the average dimensional shrinkage in X, Y and Z directions is displayed in Fig. 12 for the annealing parameters that resulted in the smallest 14

volumetric shrinkage (600 °C for 2 hours). The results suggest that the linear shrinkage of the samples in Z direction (along the thickness of the samples shown in Fig. 5) is slightly larger as compared to the X or Y directions. Such anisotropy in linear shrinkage of copper parts made via BJAM is reported in prior literature for sintered parts, and is partially attributed to the different degrees of particle consolidation across layers and within layers [19, 27]. Further optimization in AERO powder properties (e.g. oxide content and particle size distribution) as well as BJAM process parameters would be promising in fabrication of parts with near-zero shrinkage.

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Fig. 11 The effect of the annealing settings on volumetric shrinkage a) annealing temperature and dwell time as main factors b) interaction of temperature and time

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Fig. 12 Dimensional shrinkage in parts after annealing in hydrogen at 600 °C for 2 hours

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Fig. 13 exhibits a part fabricated using the developed process chain in the green state and after thermal treatment with suitable structural integrity. Using Archimedes oil impregnation method, the final porosity of this part was determined to be 59%.

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Fig. 13 Copper foam part fabricated via AERO-Binder Jetting a) green part b) after sintering and annealing

4. Summary and future pathways

In this study, the feasibility of using Binder Jetting technology to fabricate metallic foam structures with multi-scale porosity using a responsive material was investigated. The proof of concept was demonstrated by the fabrication of copper foam structures containing multi-scale porosity with powder feedstock prepared via AERO process. This study indicated that using AERO-Binder Jetting and suitable post-printing heat treatment conditions would allow for fabrication of copper 16

foams with the final porosity up to 59%. The following are the main findings based on the experimental results of this work:  

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The results indicated that the parts that were initially sintered in air and then annealed in a hydrogen atmosphere led to higher porosity than those sintered in hydrogen. From ANOVA analysis, it was found that the dwell time for annealing of the fabricated parts does not have significant influence on part final porosity amount and consequently on volumetric shrinkage. The results indicated that annealing of parts at 600 °C for 2 hours resulted in the highest final porosity and the lowest volumetric shrinkage. The parts annealed at 600 °C for 2 hours exhibited anisotropy in linear shrinkage in X, Y, and Z direction, with the largest linear shrinkage being in the Z-direction.

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Prospective pathways for future works identified in this research include: (i) optimization of AERO powder characteristics (e.g., oxide content and particle size distribution) for greater intraparticle porosity, (ii) systematic study on the characteristics of inter- and intra- particle pores and their correlations to heat treatment conditions (iii) evaluation of mechanical performance and physical/thermal properties of fabricated metallic foams, (iv) fabrication of ultra-lightweight structures with hierarchical porosity for graded density and tailored absorption properties using the developed process chain, and (v) BJAM of other metallic responsive materials.

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Acknowledgment

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This material is based upon the works supported by the National Science Foundation under Grant No. 1555016 and No. 1254287. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The authors acknowledge technical supports provided by the ExOne Co.

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