Showing papers on "Equiaxed crystals published in 2018"

Columnar to equiaxed transition during direct metal laser sintering of AlSi10Mg alloy: Effect of building direction

Abstract: In the current study, cylindrical samples of AlSi10Mg alloy were fabricated using direct metal laser sintering (DMLS) technique in vertical and horizontal directions. The microstructure of the samples was analyzed using scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. It was observed that, by changing the building direction from vertical to horizontal, columnar to equiaxed transition (CET) occurred in the alloy. While 75% of the grains in the vertical sample were columnar, by changing the direction to horizontal, 49% of the grains evolved with columnar shape and 51% of them were equiaxed. Moreover, the texture of DMLS-AlSi10Mg alloy changed due to CET. While {001} fiber texture evolved in the vertical sample, the direction tilted away from the building direction in the horizontal one. Using the fundamentals of solidification and constitutional undercooling, the solidification behavior of AlSi10Mg alloy during DMLS process was modeled. It was observed that, the determinant parameter in CET during DMLS of AlSi10Mg alloy is the angle between the nominal growth rate and h k l > direction of the growing dendrite, which is controlled by the geometry and building direction of the sample. Further TEM studies confirmed that, CET alters the shape and coherency of Si precipitates and dislocation density inside the α-Al dendrites in DMLS-AlSi10Mg alloy.

Effects of initial microstructures on hot tensile deformation behaviors and fracture characteristics of Ti-6Al-4V alloy

Abstract: The effects of heat treatment processing on the microstructures of Ti-6Al-4V alloy are systematically studied. Static globularization behavior of lamellar α phase is found when the alloy is cooled in air or furnace from 950–960 °C (below the β-transus temperature, about 975 °C). Three kinds of microstructures (basket-weave, globular-lamellar, and equiaxed microstructures) are obtained by different heat treatments. The uniaxial tensile tests of the studied alloy with different initial microstructures are conducted at the elevated temperature and different strain rates. It is found that the initial microstructures have obvious effects on the tensile properties and fracture mechanisms. The alloy with basket-weave microstructures exhibits the most obvious work hardening behavior and the highest strength. The alloy with globular-lamellar microstructures has the better ductility than that with basket-weave microstructures. Furthermore, the alloy with equiaxed microstructures has the best ductility, because the equiaxed α phases can delay the formation and coalescence of microvoids. Meanwhile, α phases are elongated, bent and spherized, which contributes to the flow softening during tensile deformation. Especially, the alloy with initial equiaxed microstructures finally transforms to bimodal microstructures after tensile fracture. Additionally, the alloys with three different initial microstructures all show a primary ductile fracture.

Microstructure and mechanical properties investigations of copper-steel composite fabricated by explosive welding

Abstract: This paper presented a systematic study of microstructure and mechanical properties of Cu/Fe explosive-bonded interfaces. The periodic wavy bonding structure with both vortex region and solid-solid bonding region was embedded in the interface. Typical annealing twin structures were observed in the texture and orientation analysis of Cu matrix. The ASBs filled with some much smaller size equiaxed grains and orientation variations in different areas were found in the Fe matrix. The transition layer consisted of nano-sized grains of 60 nm was formed between the copper and steel plate. The microindentation results showed that the hardness of the interface (330.9 MPa) and deformation area (Cu 100 HV and Fe 286.8 HV) was higher than the matrix regions. The higher hardness of the transition layer (15.707 GPa) determined by nanoindentation analysis was explained by the existence of nanocrystallines in this zone. The induced tensile tests showed that the cracks didn’t extent along the interface wave structure but inside the copper matrix, reflecting the high quality of the bonding.

Influence of texture and grain refinement on the mechanical behavior of AA2219 fabricated by high shear solid state material deposition

Abstract: Issues with rapid grain growth, hot cracking and poor ductility have hindered the additive manufacturing and repair of aluminum alloys. Therefore, this is the first investigation to spatially correlate the processing-structure-property relations of a precipitation hardened aluminum alloy 2219 (AA2219) material with respect to deposition orientations and build layers. The AA2219 material was processed by a high deposition rate (1000 cm3/h) solid-state additive deposition process known as Additive Friction Stir Deposition or MELD. An equiaxed grain morphology was observed in the three orientations, where Electron Backscatter Diffraction (EBSD) identified a layer-dependent texture with a strong torsional fiber A texture in the top of the build transitioning to weaker textures in the middle and bottom layers. Interestingly, the tensile behavior reflected the texture layer-dependence with tensile strength increasing from the bottom to the top of the deposition. However, there were no statistically significant differences in hardness measured from the top to the bottom of the deposition. Furthermore, no orientation dependence on mechanical properties was observed for compression and tension specimens tested at quasi-static (0.001/s) and high (1500/s) strain rate. Transmission Electron Microscopy (TEM) determined a lack of θ′ precipitates in the as-deposited cross-section, therefore resulting in no precipitation strengthening.

Microstructures and mechanical properties of spray deposited 2195 Al-Cu-Li alloy through thermo-mechanical processing

Abstract: A large-scale billet of 2195 Al-Cu-Li alloy was prepared by spray deposition technology. The hot extrusion, solution treatment, pre-stretching, and aging treatment were performed on the deposited billet. It was found that the yield strength, ultimate tensile strength and elongation of the T83-treated alloy reach 600 MPa, 632 MPa and 10%, respectively. The microstructures of the deposited alloy in the thermo-mechanical processing were studied by optical microscopy, scanning electron microscopy and transmission electron microscopy. The mechanical properties were examined by tensile test. It was found that the spray deposited alloy presents a microstructure with fine equiaxed grains and low segregation degree. After hot extrusion, a typical fibrous structure with incomplete recrystallization is formed. After solution and aging treatments, lots of T1 and δ ′ phases precipitate in the matrix, leading to significant increase of the tensile strength. Pre-stretch prior to aging treatment produces a fine homogeneous distribution of the T1 plates throughout the matrix, which lead to an excellent tensile performance. The ductility of aged alloys is affected by the microstructure of grain boundary. Narrow precipitate free zone and dispersed grain boundary precipitates with small size are beneficial to improvement of alloy's ductility.

Electron beam additive manufacturing of Inconel 718 alloy rods: Impact of build direction on microstructure and high-temperature tensile properties

Abstract: Inconel 718 alloy rods were fabricated by electron–beam melting (EBM), where the cylindrical axes (CAs) deviated from the build directions (BD) by 0°, 45°, 55°, and 90°. The microstructures and high-temperature tensile properties of the rods were investigated by taking into account the effect of the BD. Columnar grain structures or mixtures of columnar and equiaxed grains were obtained in the rods. The direction of the grains tended to be oriented not only in the BD but also in the two horizontal EB scanning directions (SDs), which were perpendicular to each other. As a result, the crystal orientation of the rods could be controlled by appropriate choice of the CA. For instance, the rods oriented along the , , directions could be fabricated by choosing their CAs parallel to the BD, face diagonal, and space diagonal of the cubic space defined by the BD and the two SDs, respectively. The highest strength was obtained for the oriented rod. The dependence of strength on the rod orientation could be explained in terms of the anisotropies in the crystal orientation, columnar grain structure, and arrangement of the precipitate particles.

Microstructure Evolution and Mechanical Behavior of 2219 Aluminum Alloys Additively Fabricated by the Cold Metal Transfer Process.

Abstract: In this research, four different welding arc modes including conventional cold metal transfer (CMT), CMT-Pulse (CMT-P), CMT-Advanced (CMT-ADV), and CMT pulse advanced (CMT-PADV) were used to deposit 2219-Al wire. The effects of different arc modes on porosity, pore size distribution, microstructure evolution, and mechanical properties were thoroughly investigated. The statistical analysis of the porosity and its size distribution indicated that the CMT-PADV process gave the smallest pore area percentage and pore aspect ratio, and had almost no larger pores. The results from optical microscopy, scanning electron microscopy, and fractographic morphology proved that uniform and fine equiaxed grains, evenly distributed Al₂Cu second phase particles were formed during the CMT-PADV process. Furthermore, the X-ray diffraction test ascertained that the CMT-PADV sample had the smallest lattice parameter and the highest solute Cu content. Besides, the tensile strength could reach 283 MPa, the data scattering was the smallest, and the strength scattering of the sample in the horizontal direction was the shortest. In addition, the strength properties were nearly isotropic, with only 5 MPa difference in the vertical and horizontal directions. The above mentioned results indicated that the mechanical properties of 2219 aluminum alloy was improved using the CMT-PADV arc mode.

Processing and tensile properties of A356 composites containing in situ small-sized Al 3 Ti particulates

Abstract: This research proposed an in situ casting method for preparing Al3Tip/A356 composites with high strength and good ductility by adding Ti powders into molten A356 at 800 °C. A sampling experiment was firstly carried out to explore the evolution process of Ti powders in the A356 melt. Based on which in situ Al3Tip/A356 composites with 1.5 and 3.0 wt% Ti additions were fabricated respectively. Both the microstructures and tensile properties of samples after T6 heat treatment were investigated. The results showed that blocky Si-substituted Al3Ti particulates were synthesized in the in situ reaction and the formation of which followed a reaction-peeling model. The utilization ratio of Ti powders in the fabricating process could research 70%. In situ Al3Ti particulates existed inside the α-Al crystals of A356 alloy, with the size smaller than 10 µm. Both the equiaxed transition from long columnar dendrite structure and refining of α-Al crystals occurred due to the nucleating effect of Al3Ti. Compared with T6-A356, the yield strength, ultimate tensile strength, and percent elongation of the T6-A356/3.0Ti were improved by 29.2%, 36.0% and 143.6%, respectively. The mechanisms of the improved tensile properties, including the strength and ductility, of the composites were also discussed in this research.

Microstructure and performance evolution and underlying thermal mechanisms of Ni-based parts fabricated by selective laser melting

Abstract: This work presented a comprehensive study of microstructural evolution, microhardness and quantitative thermodynamic analysis within the molten pool during Selective Laser Melting (SLM) of Inconel 718 parts. Microstructures and corresponding microhardness of different zones within the molten pool experienced the following evolution: fine cellular dendrites or equiaxed grains on the top surface (387HV); columnar dendrites with single direction of grain growth at the bottom (337HV); columnar dendrites with multiple directions of grain growth at the edge of the molten pool (340HV-350HV); microstructures between cellular and columnar grains around the center of the molten pool (363HV). The impact of Gaussian-distributed laser energy and relatively weak thermal conductivity and convection of Inconel 718 contributed to the variation of temperature gradient at different zones within the molten pool. The formation of different kinds of microstructures in the molten pool was controlled by the temperature gradient (which determined the direction of grain growth) and the cooling rate (which determined the size of grain growth). The variation of microhardness within the molten pool was ascribed to the number of grain boundaries and the stress characteristics of different kinds of microstructures under mechanical load. The zones with fine cellular grains had elevated mechanical performance due to the superior capability to endure the load. This work hopefully provides scientific and theoretical support for SLM-processed Inconel 718 parts with favorable properties.

Characterization of the Fe-Co-1.5V soft ferromagnetic alloy processed by Laser Engineered Net Shaping (LENS)

Abstract: Processing of the low workability Fe-Co-1.5V (Hiperco® equivalent) alloy is demonstrated using the Laser Engineered Net Shaping (LENS) metals additive manufacturing technique. As an innovative and highly localized solidification process, LENS is shown to overcome workability issues that arise during conventional thermomechanical processing, enabling the production of bulk, near net-shape forms of the Fe-Co alloy. Bulk LENS structures appeared to be ductile with no significant macroscopic defects. Atomic ordering was evaluated and significantly reduced in as-built LENS specimens relative to an annealed condition, tailorable through selection of processing parameters. Fine equiaxed grain structures were observed in as-built specimens following solidification, which then evolved toward a highly heterogeneous bimodal grain structure after annealing. The microstructure evolution in Fe-Co is discussed in the context of classical solidification theory and selective grain boundary pinning processes. Magnetic properties were also assessed and shown to fall within the extremes of conventionally processed Hiperco® alloys. Hiperco® is a registered trademark of Carpenter Technologies, Readings, PA.