Modeling and experimental analyses were carried out to investigate current and temperature distributions under typical spark plasma sintering (SPS) conditions. The simulation utilized two sample materials with widely differing electrical conductivities, alumina and copper. Current and heat generation gradients were evaluated in the radial and axial directions for both cases. Radial gradients result in significant differences in temperature between the surface of the graphite die and its center, in agreement with experimental measurements. Axial gradients are shown to influence the homogeneity of the product.

Fundamental investigations on the spark plasma sintering/synthesis process: II. Modeling of current and temperature distributions

Binder jet 3D printing—Process parameters, materials, properties, modeling, and challenges

The solid-state reactivity between layers of Si and Mo was investigated under spark plasma sintering (SPS) conditions. The primary product phase was MoSi2 with Mo5Si3 as a minor phase. The thickness of the primary phase showed no dependence on the direction of the current in the SPS. However, the growth rate of the product phase showed a strong dependence on the current. The activation energy for the formation of the product under the influence of the current was calculated to be 168 kJ mol−1, which was in general agreement with reported values for growth in the absence of a current, suggesting the absence of mechanism change due to the influence of the current. The results are interpreted in terms of mass transport enhanced reaction due to the current. Investigations of the temperature, power, and displacement parameters in the SPS were made for cases of simultaneous synthesis and sintering of B4C and MoSi2.

Fundamental investigations on the spark plasma sintering/synthesis process: III. Current effect on reactivity

The microstructures and mechanical behavior of simple, as-fabricated, solid geometries (with a density of 8.4 g/cm3), as-fabricated and fabricated and annealed femoral (knee) prototypes, and reticulated mesh components (with a density of 1.5 g/cm3) all produced by additive manufacturing (AM) using electron beam melting (EBM) of Co-26Cr-6Mo-0.2C powder are examined and compared in this study. Microstructures and microstructural issues are examined by optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD), while mechanical properties included selective specimen tensile testing and Vickers microindentation hardness (HV) and Rockwell C-scale hardness (HRC) measurements. Orthogonal (X-Y) melt scanning of the electron beam during AM produced unique, orthogonal and related Cr23C6 carbide (precipitate) arrays (a controlled microstructural architecture) with dimensions of ~2 μm in the build plane perpendicular to the build direction, while connected carbide columns were formed in the vertical plane, parallel to the build direction, with microindentation hardnesses ranging from 4.4 to 5.9 GPa, corresponding to a yield stress and ultimate tensile strength (UTS) of 0.51 and 1.45 GPa with elongations ranging from 1.9 to 5.3 pct. Annealing produced an equiaxed fcc grain structure with some grain boundary carbides, frequent annealing twins, and often a high density of intrinsic {111} stacking faults within the grains. The reticulated mesh strut microstructure consisted of dense carbide arrays producing an average microindentation hardness of 6.2 GPa or roughly 25 pct higher than the fully dense components.

Comparison of Microstructures and Mechanical Properties for Solid and Mesh Cobalt-Base Alloy Prototypes Fabricated by Electron Beam Melting

Compositions and methods for making a three dimensional structure comprising: designing a three-dimensional structure; melting the three-dimensional structure from two or more layers of a metal powder with a high energy electron or laser beam is described herein. The position where the metal is melted into the structure is formed along a layer of metal powder, wherein the location and intensity of the beam that strikes the metal layer is based on the three-dimensional structure and is controlled and directed by a processor. The instant invention comprises a novel dry state sonication step for removing metal powder that is not melted from the three dimensional structure.

Reticulated Mesh Arrays and Dissimilar Array Monoliths by Additive Layered Manufacturing Using Electron and Laser Beam Melting

The Stellite 6 hardfacing alloys with different Mo contents have been deposited on AISI 1045-carbon steel using a Plasma Transferred Arc (PTA) welding machine. The effect of Mo on the microstructures and wear resistance properties of the Stellite 6 hardfacing alloys were investigated using optical microscopy, scanning electron microscopy, electron probe microanalysis and X-ray diffraction. With an increase in Mo contents, the M 23 C 6 and M 6 C type carbides were formed instead of Cr-rich M 7 C 3 and M 23 C 6 type carbides observed in the interdenritic region of the Mo-free Stellite 6 hardfacing alloy. The size of Cr-rich carbides in interdendritic region decreased, but that of M 6 C type carbide increased as well as the refinement of Co-rich dendrites. The volume fraction of Cr-rich carbides slightly increased, but that of M 6 C type carbide abruptly increased. This microstructural change was responsible for the improvement of the mechanical properties such as hardness and wear resistance of the Mo-modified Stellite 6 hardfacing alloy.

Effect of molybdenum on the microstructure and wear resistance of cobalt-base Stellite hardfacing alloys

The growth kinetics of three Mo-silicide layers formed by chemical vapor deposition (CVD) of Si on a Mo substrate using SiCl 4 -H 2 gas mixtures were investigated at temperatures between 950 and 1200 °C. Three Mo-silicide layers (Mo 3 Si, Mo 5 Si 3 , and MoSi 2 ) grew simultaneously with a parabolic rate law after an initial nucleation period, indicating the diffusion-controlled growth. The activation energy (130 kJ/mol) for the MoSi 2 layer were in a good agreement with the previous results having low activation energy (130±20∼157 kJ/mol), but its growth rate was higher than the previous results with high activation energy (209∼241±25 kJ/mol). A possible explanation about this difference may be the detrimental effect of impurities such as oxygen on the growth rate of the MoSi 2 layer. The activation energy (350 kJ/mol) for growth of the Mo 5 Si 3 layer was consistent with the prior values (297∼360 kJ/mol) obtained by annealing of the MoSi 2 /Mo diffusion couples, but its growth rate was an order of magnitude lower than the rate measured in the MoSi 2 /Mo diffusion couples. The activation energy (223 kJ/mol) for the growth of the Mo 3 Si layer was similar with the value (199 kJ/mol) obtained from annealed Mo 5 Si 3 /Mo diffusion couple at temperatures between 1250 and 1350 °C. This value was lower than the value (326 kJ/mol) reported at higher temperatures from 1500–1715 °C. This suggests that the rate-limiting step for growth of the Mo 3 Si layer is the grain boundary diffusion-controlled process at low temperatures but volume diffusion-controlled process at high temperatures. The growth rates of the Mo 3 Si layer measured at condition of the simultaneous parabolic growth of three Mo-silicide layers were approximately two orders of magnitude lower than the rates measured in the Mo 5 Si 3 /Mo diffusion couples. The differences in the growth rates of the Mo 5 Si 3 and Mo 3 Si layers depending on the type of diffusion couples were well explained by the multiple layer growth model.

Growth kinetics of three Mo-silicide layers formed by chemical vapor deposition of Si on Mo substrate

Study on reaction and diffusion in the Mo-Si system by ZrO2 marker experiments

The effect of ammonia nitridation on the microstructure of MoSi 2 coatings formed by chemical vapor deposition (CVD) of Si on Mo substrates at 1100 °C was investigated. The microstructure of monolithic MoSi 2 coating formed without a prior nitridation of Mo exhibited a typical columnar structure perpendicular to the Mo substrate. On the other hand, a two-step deposition process (ammonia nitridation followed by CVD of Si) produced a MoSi 2 /α-Si 3 N 4 composite coating, which consisted of equiaxed MoSi 2 grains with an average size of 0.5 μm, where the α-Si 3 N 4 particles with an average size of 120 nm were mostly located at the grain boundaries of MoSi 2 . The shape of α-Si 3 N 4 particles was oblate-spheroidal type and its volume percentage ranged from 12.9 to 17.7%. The density and average width of cracks in MoSi 2 layer formed by the mismatch of thermal expansion coefficients between the MoSi 2 coating and the Mo substrate was smaller for the two-step deposition process than that for CVD process only. The columnar MoSi 2 coating grew by the diffusion of Si and conversion of Mo 5 Si 3 phase to MoSi 2 phase at the interface of MoSi 2 /Mo 5 Si 3 , while the MoSi 2 /α-Si 3 N 4 composite coating grew by successive displacement reaction of Mo 2 N phase and Si. The growth of MoSi 2 grains was inhibited by the nanosize α-Si 3 N 4 particles.

Effect of ammonia nitridation on the microstructure of MoSi2 coatings formed by chemical vapor deposition of Si on Mo substrates

The simultaneous growth mechanism of the Mo 5 Si 3 and Mo 3 Si layers in the MoSi 2 /Mo diffusion couple was investigated using optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray diffraction (XRD). The MoSi 2 /Mo diffusion couple was made by chemical vapor deposition (CVD) of Si on the Mo substrate at 1100°C for 5 h and annealed at temperatures of 1250 and 1600°C, respectively, in an argon atmosphere. Simultaneous parabolic growth of the Mo 5 Si 3 and Mo 3 Si layers was observed at an early annealing stage of the MoSi 2 /Mo diffusion couple. From the location of Kirkendall voids observed in the Mo 5 Si 3 layer and the change of columnar diameter and texture of Mo 5 Si 3 grains, simultaneous growth mechanism of the Mo 5 Si 3 and Mo 3 Si layers was found. The phase transformation of MoSi 2 into Mo 5 Si 3 and Si contributed to approximately 30% of the total thickness of Mo 5 Si 3 layer, but the remainder of Mo 5 Si 3 layer and the Mo 3 Si layer with small thickness were formed by the diffusion reaction of the released Si and Mo substrate. These results indicated that Si was the only diffusing element in the Mo 5 Si 3 phase.

Jae-Soo Kim

Papers

Effect of molybdenum on the microstructure and wear resistance of cobalt-base Stellite hardfacing alloys

Growth kinetics of three Mo-silicide layers formed by chemical vapor deposition of Si on Mo substrate

Study on reaction and diffusion in the Mo-Si system by ZrO2 marker experiments

Effect of ammonia nitridation on the microstructure of MoSi2 coatings formed by chemical vapor deposition of Si on Mo substrates

Simultaneous growth mechanism of intermediate silicides in MoSi2/Mo system