The Impact Resistance of Highly Densified Metal Alloys Manufactured from Gas-Atomized Pre-Alloyed Powders

: With the increasing acceleration of three-dimensional (3D) printing (for example, powder bed fusion (PBF)) of metal alloys as an additive manufacturing process, a comprehensive character-ization of 3D-printed materials and structures is inevitable. The purpose of this work was to test highly densified materials produced from gas-atomized pre-alloyed metallic powders, namely 316L, Ti 6 Al 4 V, AlSi 10 Mg, CuNi 2 SiCr, CoCr 28 Mo 6 , and Inconel718, under impact conditions. This was done to demonstrate the best possible performance of such materials. Optimized spark plasma sintering (SPS) parameters (pressure, temperature, heating rate, and holding time) are applied as a novel technique of powder metallurgy. The densification level, impact site (imprint) diameter and volume, and Vickers hardness were studied. The comparison of 316L stainless steel (1) sintered by the SPS process, (2) manufactured by PBF process, and (3) coated by the physical vapor deposition (PVD) process (thin layer of TiAlN) was successfully achieved.


Introduction
In powder metallurgy (PM), minimizing the grain growth (in comparison to conventional PM methods) to control the densification/consolidation is possible using a spark plasma sintering (SPS) device to control the ramp rate, sintering temperature, pressure, and time. The main parts used for the consolidation of materials during SPS are schematically illustrated in Figure 1. SPS is a PM sintering technique including a uniaxial press, graphite punches, vacuum chamber (possible with flow of nitrogen), thermocouple or pyrometer controlling, pulsed direct current generator, and cooling system surrounding the chamber [1]. In the current study, the primary heating mechanism was Joule heating, which is based on heat generation in the case of conductive materials and metals due to resistance to electric current. The graphite sheet/foil rolled in between the graphite die/mold and powders (also applied in the top and down, between the punch and powders) was used to facilitate the sintering of the powders [2].
The powder characteristics are a vast area, at least including production technology, density, size, melting point, particle shape, flowability, and thermal/electrical conductivity. [3]. The most commonly used metal(s) and their alloys in the 3D printing field include Fe, Ti, Al, Cu, Co, and Ni. Since the selective laser melting (SLM) process has become a novel Addictive manufacturing (AM) and powder bed fusion (PBF) technology based on the repeated sweeping of, for example, 316L powder in the manufacturing chamber on top of previously melted layers in every sequence of recoater movement, the fabricated surface may have some defects and pores. The defective layer can be improved during sintering of the next layer of powders [4,5]. Ti6Al4V is an α + β titanium martensitic microstructure alloy widely used in the aerospace and biomedical industries, along with 316L, due to its excellent mechanical, chemical, biocompatible, and lightweight character [6][7][8]. The AM technique and its subset, three-dimensional (3D) printing, is an approach for the rapid prototyping of a wide range of metals and ceramics, complex geometries and structures, and small or large size objects in form of lattice or solid directly produced from the computer-aided model (CAD). The fabrication process includes layer-by-layer material deposition that is built as layer nth on top of layer (n−1)th [9]. Additive manufacturing is characterized by minor postprocessing, treatment by cutting tools, need for additional fixture preparation, intermediate process control steps, etc. [10]. During the SLM process, as the most popular laser powder bed fusion (L-PBF) technology (standardized as ASTM F2792), an object is melted by the laser beam and formed on top of the printer platform/baseplate by sweeping the successive layers of powders [11]. SLM is a repetitive process, with a supply of power feeding the manufacturing platform, the consolidation of an object by laser scanning, heating up over the surface resulting in layer deposition on previously consolidated layers, and cooling during the fabrication of the following layers [12]. Scanning speed, laser current, and layer thickness have a crucial role as PBF process parameters. The schematic of selective laser melting device for lattice/scaffold fabrication sketched by SolidWorks is shown in Figure 2. Al-Si alloys are known for their good weldability, castability, corrosion resistance, conductivity, and light weight, and can be highly interesting for bionic aerospace and automotive applications. The addition of Mg alloying element to this composition improves its strength and dynamic toughness significantly [13]. Due to light reflection and thermal/electrical conductivity of pure copper, both processing and postprocessing are challenging. CuNiSi alloys expect to satisfy adequate tensile strength and electrical conductivity because of Si content. However, alloying of copper with Sn, reduces the electrical conductivity and enhance the corrosion resistance (such as CuSn4, CuSn8, or CuSn10 powders alloys) [14].
CoCrMo alloys are commonly used for dental replacements and restorations, including crowns, screws, and bridges in the posterior region. The highest priorities in this field are related to life expectancy, corrosion resistance, impact resistance, and biocompatibility [15]. The extreme enhancement of corrosion resistance while, at the same time, keeping the biocompatibility and coating properties, can be supplied by adding gas-atomized and pre-alloyed tungsten to this alloy. For example, both Co-based Starbond CoS 55 [16] or Febased Rockit 701 [17] powders are available with 9.5 wt.%. W. Inconel718 is a known superalloy (such as Inconel625 and Inconel939), and nickel-based alloys are used for the turbines, blades, shafts, etc., as well as in SLM processes. Nickel-based alloys are suitable for welding, fatigue, creep, and high-temperature applications (due to the low content of titanium and aluminum) [18]. The CAD-based design for additive manufacturing (DfAM) and finite element analysis (FEA) is a time-and cost-efficient way to predict either mechanical behaviors of alloys or composites or provide an optimal selection of the material regarding the application by CAD design and/or FEA multiphysics software, e.g., Solid-Works, Ansys, and/or Comsol [19][20][21].
SPS-built objects can be introduced as a criterion for SLM-built solid samples. (however, it is feasible for simple shapes, e.g., disks and cubes; for complex or porous shapes, hot isostatic pressing (HIP) can be an option). During the 3D printing process, depending on powder character (metals, ceramics, cements, composites, etc.), some powder particles could be agglomerated (in the vicinity of the printed part) or their properties can be changed (due to overheating, for example). Powder particles can be introduced into the build zone during the next whipping that can reduce precision or deteriorate the properties of the produced part. Six bulk metal alloys encompassing Fe-, Ti-, Al-, Cu-, Co-, and Ni-based were manufactured by the SPS process and tested using the hammering impact test. Samples produced by the SLM technique sometimes have high porosity (in the case of ceramics and/or composites in comparison with subtractive manufacturing and modern powder metallurgy routes), but it can be controlled by layer thickness, optimized laser power, and scan speed in case of metal alloys. However, the controlled porosity/permeability can be applied as an advantage for the delivery of antibiotics and bone healing in the tissue engineering field.
The current study aimed to produce the samples from gas-atomized, pre-alloyed, and spherical-shaped powders (applicable for AM and PBF technologies) with high densification levels to demonstrate their possible best performance. Sintering parameters were applied to fabricate and compare samples with the same appearance of SLM production (dimension and densification). Considering the expensive and time-consuming processing of AM, the present approach allowed us to fairly estimate the specification of solid objects subjected to mechanical tests. Comparison of most used metallic alloys in PBF (Ti6Al4V, AlSi10Mg, CuNi2SiCr, CoCr28Mo6, and Inconel718) with stainless steel 316L (sintered by the SPS process, manufactured by the SLM/PBF process, and coated by the PVD process) under the dynamic impact test was successfully achieved in the present work. The impact resistance of alloys is defined by the measurement of mechanical characterization, such as densification level, deformation site, indented volume, acceleration value in the test region.

Materials and Methods
Densification of samples depend on many parameters, such as powder size and shape, distribution of size and shape, phase change and grain growth of powders in the sintering mold, etc. It is inevitable for a gas atomization process to reach high purity, quality, homogeneity, and flowability of spherical powder/particle production for AM/PBF applications. In this procedure, alloying elements were added to the molten balanced metal before the gas atomization process. SEM micrographs of gas-atomized pre-alloyed metallic powders Fe-based 316L, Ti-based Ti6Al4V, Al-based AlSi10Mg, Cu-based CuNi2SiCr, Co-based CoCr28Mo6, and Ni-based Inconel718 are shown in Figure 3. The supplier [22-24], powder size, and alloy density are shown in Table 1. The SPS device, supplied by FCT Systeme GmbH (Frankenblick, Germany) [25], was installed inside the nitrogen glovebox to avoid oxidation. The alloys were consolidated at 50 MPa pressure in 20 mm (diameter) graphite mold, with 100 °C/min heating rate and 5 min holding time at sintering temperature. The sintered/produced samples, after sintering and slight polishing (polished by P800 sandpaper to remove the graphite sheet from both sides of diskshaped samples), were 20 mm in diameter and 10 mm in height.  High flowability, similarity in size, spherical shape, and high thermal conductivity of powders are preferable for the SLM process. However, during the SPS process, electric currents passed through all of the particles, and these parameters were less significant. The SLM process can be controlled by printed layer thickness, laser power, laser current, and laser speed. However, the main SPS process parameters are pressure, ramp rate, sintering temperature, and holding time. In SPS program control, grain growth and oxidation of metallic particles must be prevented during the sintering, whereas the extent of cooling in the SLM route can be controlled by time spent between the printing of layers. Based on experiences, we held and preheated the powers at 300 °C for 5 min and 5 MPa pressure during the SPS process to reduce the possible humidity and other contaminations. Herein, the ramp rate (or heating rate) was 100 °C/min for all samples, e.g., increasing the temperature from 300 °C (dehumidification point) to 1000 °C (sintering point) in 7 min.
Note that the present paper focused only on SPS production. However, the method was validated here by comparing the consolidated 316L powders in both SLM and SPS processes. The aim was to present a "test method" for the assessment of SPS's or SLM's "highly densified" productions. The point that it was possible to increase the pressure of SPS higher than 100 MPa and have ≈ 99.9% densification level, but the currently applied pressure was selected to obtain the samples closer to SLM output (≈ 90-99% for metal alloys). It is possible to apply another PM technique instead of SPS (e.g., conventional pressing and sintering or metal injection molding (MIM)) and to substitute other PBF approaches instead of SLM (e.g., electron beam melting (EBM)).

Results and Discussion
The impact wear test with multiple dynamic strikes was designed and developed in the "Research Laboratory of Tribology and Materials Testing" at the Tallinn University of Technology [26,27]. The aforementioned produced rounded samples (disk shape with 20 mm diameter and 10 mm height), which were fixed rigidly to the platform and tested by applying 30 repetitive impacts with the energy of 5.6 J at 27.5 Hz frequency (at 1.1 s) using a Makita hammer-drill through a ZrO2 ball (with 10 mm diameter, stabilized by Y2O3; supplied by Tosoh, Tokyo, Japan). The test was in situ monitored by a vibration sensor attached to the main platform with the help of a PCH-1420 (Denmark) analyzer. The load (98.1 N, 10 kg) was provided by the deadweight system as shown in Figure 4. The impact sites (indents) of metallic alloys and vibration graphs (acceleration of platform to which the sample is fixed) are shown in Figures 5 and 6, respectively.   The impact test was used as a method to assess the behavior of materials (damage) and energy spent in the contact zone between the ball and the sample. If no impact energy is consumed in the contact zone, then the acceleration of the platform/sample will be the highest. If energy is spent on (1) plastic, (2) elastic deformation, or (3) fracture of material, then the acceleration will be lower. The implemented test method can be used for a wide range of material types, such as ceramics or bioceramics, metal alloys, hard materials, and hybrid composites produced by SPS and/or SLM or other methods as additional characterization criteria [28][29][30][31]. The size of the indent can be used as an indication of "dynamic hardness." The larger diameter, depth, or volume of penetration shows the higher ductility (extent of plastic deformation).
The sample behavior during the impact depends also on changes in the surface layer of the material taking place in the contact area with the contacting ball. The Al-and Cubased materials were ductile, with a wider diameter of the deformation site (see Figure  5C,D). In contrast, Fe and Ti alloys provided higher resistance against impact (see Figure  5A,B). The stated diameters are average ones, because in the case of stiff metal alloys, the deformed area may change from circle to oval shape (ball may slightly slide during the impact). Due to this phenomenon, deformation diameter should be considered along with the indented volume (see Table 2). The density of metallic alloys was measured by the Archimedes immersion technique. It was expected that the densification level of each individual material would lead to a higher acceleration value (y-axis, Figure 6 or Table 2). This can be achieved by higher pressure during the SPS process (100 MPa instead of 50 MPa, for example), with the expected enhancement of densification up to 99.9% and reduction of deformation site. The 316L is Fe-balanced stainless steel (known also as 1.4404), with 16 wt.% Cr, 10 wt.% Ni, and 2 wt.% Mo and excellent manufacturability. Ti-balanced Ti6Al4V (Grade 5), the most commonly used corrosion-resistant, high-strength, and high-toughness alloy, had 5.5-6.5 wt.% Al and 3.5-4.5 wt.% V. The widely used AlSi10Mg was Al-balanced, with 9-11 wt.% Si and 0.2-0.45 wt.% Mg. AlSi10Mg is proper for aerospace application due its lightweight structure and thermal/electrical application. CuNi2SiCr with Cu-balanced, 2 wt.% Ni, 0.5 wt.% Si, and 0.1-0.8 wt.% Cr is great for weldability and thermoelectrical conductivity. The CoCr28Mo6 alloy is Co-balanced, 27-30 wt.% Cr, and 5-7 wt.% Mo is great for ductility, biocompatibility, and binding with hard materials. Inconel718 is Ni-balanced, with 17-21 wt.% Cr, 2.8-3.3 wt.% Mo, 4.7-5.5 wt.% Nb, and 0.6-1.2 wt.%. Ti includes a wide range of metals and is good for high-temperature applications.
The SEM micrograph of the top (intact/unpolished) and bottom (polished after removing the support), schematic of 3D printing configuration, impact test result, and acceleration measurement of 316L SLM-manufactured (As most used PBF method) are shown in Figure 7 produced by powder that is specified in Table 1. The macrostructure of the SPS-sintered 316L is depicted in Figure 7A to show the similarity of sintered and melted zone in the consolidated sample. The Realizer SLM®280 devices (made by SLM solutions AG, Lübeck, Germany) with 35 µm layer thickness, 60 W laser power, and 1000 mm/s scan speed were applied for the fabrication of the solid sample illustrated in Figure  7A. Since the SLM/PBF process is supposed to be the near-net-shape (NNP) AM technology (no tooling/machining required), herein, samples were both polished by P800 sandpaper for the macrostructure comparison of SLM-manufactured and SPS-sintered 316L surface. The 316L SLM-built sample with 96% densification was deformed and subjected impact test, resulting in its 3120 µm diameter of indent, 3.1 mm 3 volume of indent, and 970 m/s 2 average maximum acceleration. The TiAlN coating with 3 µm thickness was prepared by the physical vapor deposition (PVD) method on the AISI 316L substrate at a temperature of 450 °C in a nitrogen atmosphere for 1 h and 850 V. The π-80 PVD unit was supplied by Platit (Swiss, Grenchen, Switzerland). The lateral rotating cathode arc (LARC) method was used ( Figure 8C) [32].
The diameter of the coated 316L indent was higher than that of the uncoated Ti6Al4V sample, and the indented volume of the coated 316L was lower than that of 316L without coating due to minor sliding of the ball during the impacting. The penetration diameter, penetration depth, and indented volume of 316L decreased from around 1700 µm, 500 µm, and 3.2 mm 3 for uncoated sample to around 1500 µm, 300 µm, and 1.4 mm 3 for the sample with TiAlN coating subjected to the multiple impact test, respectively, as shown in Figure 8D. Various process parameters (for example, scan rotation) on microstructure and mechanical properties of stainless steel 316L made by L-PBF are highly effective (regardless of the density) [33]. Also, at constant laser speed, laser power has an essential role in porosity so that number of pores rises with the reduction of laser power [34]. SPS is known as a fast, homogeneous, isotropic, low-porosity, and limited grain growth process, and exhibits that pressure from 50-75 MPa has an equal effect for 316L (or other metal) alloy [35]. Through the literature, large density values with ultrafine grain size for AISI 316L have been obtained by the SPS-sintered process from ball-milled powders [36], meaning that SPS-made (PM made) samples can be compared with metal alloys made by SLMmade (L-PBF made) with high densities and less pores. This method may not apply to other ceramics or composites. Figure 9 illustrates the average of max values of acceleration (positive peaks in Figure  6) versus indented volume. The graph shows that 316L and Inconel718 had the maximum consumption of energy under the multiple impacting conditions (lowest acceleration after impact), and AlSi10Mg had minimum energy consumption (highest acceleration values). Based on this dynamic hardness graph, coated 316L was the material which spent all the impact energy on plastic deformation.

Conclusions
Six solid spark plasma sintered materials were successfully manufactured from gasatomized pre-alloyed powders (Fe-based 316L, Ti-based Ti6Al4V, Al-based AlSi10Mg, Cubased CuNi2SiCr, Co-based CoCr28Mo6, and Ni-based Inconel718) and subjected to multiple impact tests. The aim was to present a mechanical test method for the assessment of the dynamic performance of materials. This method can be applied to materials manufactured by powder metallurgy (PM) or powder bed fusion (PBF) processes. The results show that: 1. The high densification rate of samples produced from spherical-shaped powders intended for PBF technology was achieved by the SPS method. The performance of these samples could be used as an indication of the best achievable properties for materials produced by the PBF approach. 2. In situ acceleration values recorded during multiple impact tests composed of ≈30 impacts illustrate that 316L and Inconel718 had the maximum consumption of energy in the contact zone during impacting (lowest acceleration after impact). Ti6Al4V, CuNi2SiCr, CoCr28Mo6 enabled average values, whereas AlSi10Mg had the minimum energy consumption. 3. The impact energy consumed did not result in the respective size of indent (extent of plastic deformation). The largest-to-smallest size indents were observed for AlSi10Mg-CuNi2SiCr-Inconel718-CoCr28Mo6-316L-Ti6Al4V. It was possible to identify two materials that behaved differently from others. AlSi10Mg consumed less energy during impacts while it had the largest imprint. On the other hand, 316L consumed the most energy while had the smallest imprint. 4. The application of PVD coating to the 316L significantly reduced the consumption of energy during the impact while the size of the indent was reduced slightly. The di-ameter and volume of the indent were reduced and improved by 15% and 56%, respectively, for the coated 316L (with 3 µm TiAlN coating) in comparison with uncoated 316L. 5. The wear impact result was approximately equal for PBF-manufactured (35 µm layer thickness, 60 W laser power, and 1000 mm/s scan speed) and SPS-sintered (50 MPa pressure, 100 °C/min heating rate, 1000 °C sintering temperature, and 5 min dwelling time) 316L stainless steel with identical appearance (96-97% densification, 20 mm diameter and 10 mm height). This means that we can apply these modern powder metallurgy methods for the assessment of novel additive manufacturing processes. Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: This study did not report any supporting data.