Blended Elemental Powder Research Articles

Microstructural Characteristics of Ti‐Based Composites with Various Ceramic Reinforcements Manufactured via Hydrogen‐Assisted Blended Elemental Powder Metallurgy

Highly dense in situ reinforced titanium matrix composites are fabricated with hydrogen assisted two-stage press-and-sintering method as described in the article, number 2101689 by Yuchao Song, Tuo Cheng, Orest Ivasishin and co-workers. Type and size of starting ceramic additives have substantial impact on the microstructure formation, residual porosity, and hardness of the pre-sintered composites. Sintering of hydrogenated pre-alloyed powders at second processing stage ensures desirable microstructure, improved density and uniformity of composites.

Sustaining an excellent strength–ductility combination for Ti–6Al–4V alloy prepared from elemental powder blends

Sintered Ti–6Al–4V compacts prepared from TiH2/60Al40V powder blends were subjected to hot rolling and heat treatment under various conditions. The microstructural evolution and mechanical properties were investigated by X-ray diffraction, scanning electron microscopy, optical microscopy, electron backscatter diffraction, transmission electron microscopy, and universal testing. The hot-rolled Ti–6Al–4V samples with 88% deformation possessed a near-dense lamellar microstructure with ultrafine α lamellae (less than 1 μm). Systematic investigation of the heat treatment temperature and isothermal holding time enabled the evolution of a bimodal microstructure with precipitation of a secondary α phase, ultimately affording an excellent combination of strength and ductility: the yield strength and ultimate tensile strength reached 1248 and 1269 MPa, respectively, with an elongation to fracture of 16.05%.

High-strength lamellar high-entropy alloys in-situ synthesized by laser additive manufacturing

A high-strength lamellar high-entropy alloy (HEA) of Zr45Ti31.5Nb13.5Al10 with excellent ductility was fabricated by in-situ alloying of blended elemental powders via laser directed energy deposition (DED). Microstructure characterizations suggest that the molten pools with body-centred cubic (BCC) structure and heat affected zones with mixed structure of BCC + ordered BCC (B2) nanoprecipitates, are alternately distributed in the DED-processed HEA with a lamellar structure. During the deformation process, the molten pools are dominated by dislocation planar slipping, while in the heat affected zones, frequent cross-slip and dislocations pinning caused by dispersed B2 nanoprecipitates occurred, which endows a significant strain hardening capability and deformation uniformity in the DED-processed HEA. This research provides new options for the design and manufacturing of HEAs with outstanding mechanical properties for structural applications.

Double-phase refractory medium entropy alloy NbMoTi via selective laser melting (SLM) additive manufacturing

Elemental metals Nb and Mo are common refractory metals, performing good properties in a high-temperature working environment. While due to their high melting points, they are hard to be manufactured. Besides, elemental metal with a single BCC phase performs onefold properties, which promotes the design of medium/high entropy refractory alloys. This paper focuses on the formability analysis of refractory metals Mo, Nb and mixed MoNbTi powder printed by SLM. It successfully demonstrated for the first time that laser metal deposition can be used to produce MoNbTi high-entropy alloy from a blend of elemental powders by in-situ alloying with the assistance of the Flow 3D app. The densities of elemental Nb, Mo and MoNbTi manufactured by the corresponding optimized printed parameters were 99.41%, 99. 13% and 97.38%, respectively. As-built MoNbTi high-entropy alloy contained FCC+BCC double phase. The FCC phase is diffusively precipitated in the BCC phase, and the boundary is moist.

In situ Alloyed Medium Entropy Alloy Fabricated by Laser Powder Bed Fusion: Microstructure and Cryogenic Performance

An investigation is conducted into the feasibility to produce in situ alloyed medium entropy alloy containing four principal elements by laser powder bed fusion of the blended elemental powders. The effect of scan strategy, layer thickness, and laser power on in situ alloying is examined. It is revealed that the melting of Cr particles could be enhanced by increasing the remelting times of each building layer through the re‐scan strategy or reducing the powder layer thickness and by increasing the melting pool depth via large laser power. Besides, the combination of elemental uniformity and the low porosity in the printed alloy can be achieved. The accuracy of the nominal composition in the as‐printed sample could be ensured by the large remelting times combined with middle laser power, while an excessively large laser power could cause the loss of Cr element. Based on the high repetitive remelting process, the mechanism of pores elimination during in situ alloying is discussed. After the optimization, the as‐printed Fe60Co15Ni15Cr10 alloy fabricates with the re‐scan strategy or high laser power shows a tensile strength of ≈1140 MPa and an elongation of ≈0.39 under cryogenic loading. The as‐printed alloy overcomes the strength–ductility trade‐off at cryogenic temperature through the transformation‐induced plasticity.

Manipulating the strength and tensile ductility of a PM near α titanium alloy by adjusting the morphologies and volume fractions of α and βt domains

Powder metallurgy (PM) near α Ti−6Al−2Sn−4Zr−2Mo−0.1Si−0.5Y (wt.%) alloy with a low oxygen content and fabricated by thermomechanical powder consolidation of an elemental powder blend with a trace amount of Y addition was subjected to two heat treatments to obtain different microstructures which were free of grain boundary α (αGB) layers. One heat treatment (960°C/1h/AC, AC = air cooling) led to the formation of an α/βt microstructure consisting of 75.3 vol% primary α (αp) domains with globular and platelike morphologies, 21.5 vol% β transformed structure (βt) domains comprising ultrafine secondary α (αs) lamellae in a β matrix and 3.2 vol% thin β strips. The other heat treatment (960°C/1h/WQ+580°C/6h/AC, WQ = water quenching) led to the formation of a microstructure consisting of 38.2 vol% isolated αp plates, slabs, and globules dispersed in a βt matrix comprising nanometer-sized αs lamellae and residual β strips. The first microstructure rendered the alloy with high tensile strength of 1128 MPa and excellent tensile ductility of 15.6%, thanks to the activation of abundant <a> and <c+a> slips in all α grains and tensile twinning in the “hard” <0001> micro-textured α grains with the c-axis parallel to the tensile force direction which mitigated the strain localization and enhanced strain hardening. On the other hand, the harder nanostructure of the βt matrix in the second microstructure resulted in a significantly increased tensile strength of 1351 MPa while maintaining an excellent tensile ductility of 10.5%. This can be attributed to the activation of abundant <a> and <c+a> slips in thin α plates and a limited degree of tensile twinning in the “hard” α domains, as well as a large number of statistically stored dislocations (SSDs) within the micron-scaled α slabs/globules. Both microstructures rendered the alloy with favorable ductile fracture.

Optimization of biocorrosion resistance and mechanical properties of PM Ti–XAl–2Fe–3Cu alloys by response surface methodology

In the current study, the biocorrosion resistance and mechanical properties of the Ti–XAl–2Fe–3Cu (wt%) (X = 6, 4, 2 wt%) alloy produced by powder compact extrusion of blended elemental powders were optimized by using the response surface methodology in order to gain the best combination of processing parameters. To run the optimization process, central composite design with three equal levels of oxygen, hydrogen and aluminum was used and, by employing ANOVA, the analysis of variance was implemented. The results showed that the content of aluminum has a significant effect on the microstructure, mechanical properties, biocorrosion resistance and fracture behavior of the extruded rods. The highest aluminum content corresponds to the highest alloy strength and biocorrosion resistance. This was due to the reduction of the lamella thickness and of the material colonies as the aluminum content increases. Also, by running the experimental and optimization tests, the mechanical properties resulted 1245 MPa in yield strength and 1340 MPa in ultimate tensile strength with 0.1 µAcm−2 in corrosion current.

In Situ Alloying of Fe-Cr-Co Permanent Magnet by Selective Laser Melting of Elemental Iron, Chromium and Cobalt Mixed Powders

Fe-25Cr-15Co (wt.%) permanent magnets were fabricated via selective laser melting (SLM) and in situ alloying from a blend of Fe, Cr and Co elemental powders. Under the optimal laser scanning process, the as-built Fe-25Cr-15Co alloy has a homogeneous composition distribution without defects such as holes or un-melted particles, and presents a single α phase with the bcc crystal structure. The density of as-built samples was 7.705 g/cm3 (the relative density is 99.32%). The preferred magnetic properties of the sample in the isotropic state were obtained as Hc = 22.84 kA/m, Br = 0.86 T and (BH)max = 7.98 kJ/m3. The hardness and yield strength of Fe-25Cr-15Co permanent magnets are above 331.5 HV and 800 MPa, respectively. The results of this study verified the feasibility of fabricating Fe-Cr-Co permanent magnets by SLM in situ alloying and can be extended to a wide range of applications that require complex shapes with variable magnetic circuit characteristics or gradient structures.

Mechanical behavior of bilayer structures of Ti64 alloy and its composites with TiC or TiB under quasi-static and dynamic compression

Bilayer structures of titanium alloy Ti-6Al-4V (Ti64) and metal matrix composites (MMC) on the base of this alloy reinforced with 5, and 10 % (vol.) of TiC or TiB particles were made using press-and-sinter Blended Elemental Powder Metallurgy (BEPM). The mechanical behavior of these structures was studied under quasi-static and high strain rate compression. Dynamic tests with compression velocity in the range 4–15 m/s (820–2880 s−1) were done using the split Hopkinson pressure bar (SHPB), whereas the quasi-static tests were performed with compression velocity of 0.0005 m/s (10−3 s−1). Stress–strain curves were obtained and the effect of phase composition, microstructure, and strain rate was analyzed and discussed. Current results were compared with previous studies conducted on homogeneous MMCs and Ti64 alloy tested in similar conditions. The deformation mechanism evaluated from the microstructure analysis of the samples was verified using in-situ high-speed camera recordings. Numerical simulation based on Finite Element Analysis was conducted to further investigate the behavior of the samples during the SHPB trials. Generalized scheme of the deformation and damage process of two-layer alloy/composites hybrids as a function of compression rate was proposed.

Microstructural Characteristics of Ti‐Based Composites with Various Ceramic Reinforcements Manufactured via Hydrogen‐Assisted Blended Elemental Powder Metallurgy

The manufacturing of in situ reinforced titanium matrix composites by the sintering of titanium hydride and ceramic powder blends is an attractive and cost‐efficient process. Herein, a novel two‐stage (double‐compaction/double‐sintering) hydrogen‐assisted powder approach is suggested to produce uniform highly dense Ti‐based composites reinforced with TiB, TiC, and dual TiC + TiB phase particles. In this approach, the hydrogenation and milling of presintered products enable the initial porous materials to transform into hydrogenated prealloyed (PA) powders of controlled sizes, and a second press‐and‐sinter process further converts the PA powders into nearly dense composite microstructures with evenly distributed fine reinforcements. The adverse effect of the raw reinforcing powder size on microstructure uniformity, porosity, and mechanical characteristics of the presintered in situ composites becomes negligible in final sintering stage. The reduction of the porosity and enhancement of the hardness indicate that the hydrogen‐assisted double‐compaction and double‐sintering blended elemental powder metallurgy (BEPM) + PA approach enables the low‐cost fabrication of Ti‐based composites with suitable mechanical characteristics.

Damping Properties of Selective Laser-Melted Medium Manganese Mn-xCu Alloy.

In this work, selective laser melting (SLM) technology was applied to directly realize the in situ synthesis of medium manganese Mn-xCu (x = 30-40 wt.%) alloys based on the blended elemental powders. The effects of heat treatment on the microstructural evolution and damping properties of the SLMed Mn-xCu alloys were investigated. The metastable miscibility gap was studied by thermodynamic modeling and microhardness measurement. The results showed that γ-(Mn, Cu) phase with dendritic arm spacing (DAS) of 0.9-1.2 μm was the main constituent phase in the as-SLMed alloys, which was one to two orders of magnitude finer than those of the as-cast samples. Aging at 400-480°C for the Mn-30%Cu or 430°C for Mn-40%Cu alloys can induce spinodal decomposition, martensitic transformation, and α-phase precipitation, whose direct evidence was provided for the first time by transmission electron microscopy and 3D atom probe tomography in the work. The miscibility gap obtained from thermodynamics calculation was basically consistent with the microhardness results for the SLMed Mn-xCu alloys. Solution and aging (SA) treatment can improve the microstructure, tensile and damping properties of the SLMed Mn-xCu alloys more obviously than aging treatment. A 2.3-2.8 and 4.3-4.5 times increase was produced in damping capacity in the aged SLMed and SLMed+SAed Mn-xCu samples, respectively.

Operando X-ray diffraction study of thermal and phase evolution during laser powder bed fusion of Al-Sc-Zr elemental powder blends

Elemental powder blends are an emerging alternative to prealloyed powders for high-throughput alloy design via additive manufacturing techniques. Elemental Al+Sc(+Zr) powder blends were processed by laser powder bed fusion into Al-Sc and Al-Sc-Zr alloys, with operando X-ray diffraction at the Swiss Light Source extracting the structural and thermal history of the process. The pure Sc and Zr particles were found to react with the molten Al pool at 550–650 °C, well below their respective melting temperatures. Various scan areas (1 × 1, 2 × 2, 4 × 4, and 8 × 2 mm 2 ) were studied to compare (i) the base plate “preheating” effect caused by prior laser scans, (ii) the return temperature reached after the melting scan and before the following scan, (iii) the initial cooling rate immediately after solidification, and (iv) the time spent in the “intrinsic heat treatment range”, defined as 300–650 °C, where secondary Al 3 (Sc,Zr) precipitation occurs. Microstructural analysis of the as-built samples show 110–140 nm L1 2 -Al 3 (Sc,Zr) primary precipitates at the bottom of the melt pool. The 1 × 1 mm 2 samples exhibit the most elongated grains (long axis of 10 ± 5 µm), which correlates with the highest build plate temperature and the slowest initial cooling rate (3–5 × 10 5 K/s). In comparison, the 4 × 4 mm 2 samples exhibit the smallest equiaxed grains (2 ± 0.6 µm), corresponding to the lowest build plate temperature and the fastest initial cooling rate (6–7 × 10 5 K/s). These results indicate the need for establishing a minimum feature size during part design, or for modifying the laser parameters during processing, to mitigate microstructure and performance differences across features of different sizes.

Effect of Master Alloys on Synthesis and Mechanical Properties of Ti-6Al-4V Alloy Produced by Elemental Powder Blends

Effect of Master Alloys on Synthesis and Mechanical Properties of Ti-6Al-4V Alloy Produced by Elemental Powder Blends

Dispersion of reinforcing micro-particles in the powder bed fusion additive manufacturing of metal matrix composites

Understanding the complex multi-phase interactions are vital for defects reduction in additive manufacturing (AM) of metal matrix composites. In this study, we propose a high-fidelity model to reveal the dynamics of molten pool and reinforcing solid particles during the AM process, using the resolved Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) with bi-directional momentum and energy exchange. Our model is validated against the electron beam melting (EBM) experiments of tungsten-copper composites fabricated with elemental powder blends. The results demonstrate that the interface effect, including the dynamic wetting phenomena and Laplace pressure, play significant roles in the dynamics of reinforcing solid particles. On the other hand, the existence of reinforcing solid particles in the molten pool changes the molten pool size and alters the flow field during the melting process. Although the interface effect causes tungsten particle agglomeration at single track surface, the layer-wise deposition scheme with proper layer thickness eliminates the cluster and promotes the uniform tungsten distribution in the densified bulk sample, which shows the capability of AM to achieve spontaneous dispersion of reinforcing solid particles in the metal matrix. This work provides unprecedented details about the multi-phase dynamics in metal matrix composite AM process.

TiNi-Based Bi-Metallic Shape-Memory Alloy by Laser-Directed Energy Deposition

In this study, laser-directed energy deposition was applied to build a Ti-rich ternary Ti–Ni–Cu shape-memory alloy onto a TiNi shape-memory alloy substrate to realize the joining of the multifunctional bi-metallic shape-memory alloy structure. The cost-effective Ti, Ni, and Cu elemental powder blend was used for raw materials. Various material characterization approaches were applied to reveal different material properties in two sections. The as-fabricated Ti–Ni–Cu alloy microstructure has the TiNi phase as the matrix with Ti2Ni secondary precipitates. The hardness shows no high values indicating that the major phase is not hard intermetallics. A bonding strength of 569.1 MPa was obtained by tensile testing, and digital image correlation reveals the different tensile responses of the two sections. Differential scanning calorimetry was used to measure the phase-transformation temperatures. The austenite finishing temperature of higher than 80 °C was measured for the Ti–Ni–Cu alloy section. For the TiNi substrate, the austenite finishing temperature was tested to be near 47 °C at the bottom and around 22 °C at the upper substrate region, which is due to the repeated laser scanning that acts as annealing on the substrate. Finally, the multiple shape-memory effect of two shape-memory alloy sides was tested and identified.

The microstructure and compressive properties of a sintered Fe–Mn–Al porous steel produced by blended elemental powder mixture

In this study, powder compacts produced by mixed elemental Fe, Mn, Al and C powders without ball milling were sintered at 640∘C and 1200∘C, respectively. The results revealed that intermetallic phases belonging to Fe2Al5 and Al8Mn5 appeared after pre-sintered at 640∘C. When the temperature increased to 1200∘C, only α-Fe phase existed. The total porosities of the sintered samples were ∼60 vol.% at 640∘C and ∼70 vol.% at 1200∘C, and the peak compressive strength of the 1200∘C-sintered samples was ∼1.6 MPa at the strain of ∼11%.

Manufacturing of biomedical Ti alloys with controlled oxygen content by blended elemental powder metallurgy

Biomedical Ti-xNb-7Zr-0.7O (wt%) alloys were manufactured from elemental powders and controlled addition of TiO2 using field assisted sintering technique (FAST). First, sintering parameters were optimized on the benchmark Ti-29Nb-7Zr-0.7O alloy. Second, alloys with various content of Nb (20, 23, 26 and 29 wt%) were manufactured. FAST is capable to produce homogeneous material with controlled amount of oxygen. In addition, FAST is a valuable method for fast sampling of alloys with different compositions. Detailed XRD and SEM study proved that oxygen prevents formation of α” martensite phase. Presence of α phase, α” phase and especially ω phase causes an increase of elastic modulus and microhardness. Single phase β Ti-29Nb-7Zr-0.7O alloy can be manufactured by FAST from elemental powders and subsequent solution treatment. The alloy is proposed for biomedical use due to its low elastic modulus.

In Situ Ti6Al4V/TiB Composites Prepared by Hydrogen-Assisted Sintering of Blends Containing TiH2 and Ball-Milled Ti+TiB2 Powders.

In the present study, 98.6–99.5% dense in situ reinforced Ti6Al4V/TiB composites were manufactured with a newly developed approach based on hydrogen-assisted blended elemental powder metallurgy (BEPM). The approach includes the activation milling of titanium powder produced with hydrogenation-dehydrogenation (HDH-Ti powder) with finer TiB2 additives, following blending with TiH2 and master alloy (MA) powders, and final press-and-sinter operations. Scanning electron microscope (SEM) observations prove the formation of microstructures with improved density and homogeneous distribution of TiB reinforcements in a sintered Ti6Al4V matrix. Hardness and compressive tests validated the high mechanical characteristics of produced composites. The effect of preliminary milling time over 2–6 h and the ratio of hydrogenated and non-hydrogenated titanium powders used (TiH2 vs. HDH Ti) on microstructure and mechanical properties were studied to further optimize the processing parameters. Test results indicate the above approach can be regarded as a promising route for the cost-effective manufacturing of Ti6Al4V/TiB composite with reduced porosity, tailored microstructure uniformity, acceptable impurity level and, hence, mechanical characteristics sufficient for practice applications.

In situ alloying of Ti10Mo fused tracks and layers via laser powder bed fusion

Optimum process parameters for manufacturing a Ti10Mo alloy for biomedical applications via the laser powder bed fusion (LPBF) process were determined. Fused tracks were produced over a wide range of laser powers and scanning speeds, and layers were fused at varied hatch distances. The samples were analysed for continuity of the fused tracks, melting and distribution of the Mo powder particles in the Ti10Mo alloy layers, surface roughness, homogeneity of Mo in the alloy matrix and microhardness. The analysis revealed that the Mo powder particles melted completely in the alloy matrix with only pockets of Mo concentrations, mostly at the peripheries of the fused tracks due to the pushing effect. Complete melting of Mo in the Ti10Mo alloy matrix was due to the small size (1 μm) of the Mo powder particles used in the current experiment. The addition of Mo enhanced the wetting of the powder bed and prevented a pronounced balling effect. From this study, the parameter sets 150 W, 0.5 m/s and 200 W, 1.0 m/s both at a hatch distance of 80 μm, were obtained as the optimum process parameters. However, the Mo concentrations at the peripheries of the molten pool indicated that further research was required before a ‘completely’ homogenous sample could be manufactured via the LPBF process using elemental powder blends.

Cryogenic and Room Temperature ECAP Consolidation of Blended Elemental Powders of Aluminum and Copper

The effect of temperature was investigated on the consolidation of blended elemental powders of aluminum and copper by equal channel angular pressing (ECAP). Aluminum and Copper powders (1:1% vol.) were blended and consolidated in a 90° ECAP die at room (RT) and cryogenic temperatures (CT - ~77 K). ECAP samples were pressed until 4 passes at room temperature in route Bc. As a reference, a sample was obtained by conventional uniaxial pressing. The obtained results indicated a much denser (>99.5%) and harder structure by cryogenic ECAP. The hardness after one pass at CT was comparable with 4 passes at room temperature. Tensile tests performed at CT for materials with similar chemical composition showed a simultaneous increase in strength and ductility at CT, corroborating the results obtained by ECAP. The partial suppression of dynamic recovery and the activation and the transition between deformation mechanisms at CT, as well as stacking fault energies (SFE) of such metals, played an important role in these results. Copper presented a much higher capability of strain hardening than aluminum, due to its lower SFE and much lower homologous temperature. X-ray diffraction indicated a strong correlation between the variation of average microstrain and the variation of hardness on both metals. The results of this study demonstrated the great potential of the application of very low temperatures for the obtaining of deformation metal-metal composites.