Showing papers on "Equiaxed crystals published in 2019"

Insight into the mechanisms of columnar to equiaxed grain transition during metallic additive manufacturing

Abstract: The columnar to equiaxed transition (CET) of grain structures associated with processing conditions has been observed during metallic additive manufacturing (AM). However, the formation mechanisms of these grain structures have not been well understood under rapid solidification conditions, especially for AM of superalloys. This paper aims to uncover the underlying mechanisms that govern the CET of AM metals, using a well-tested multiscale phase-field model where heterogeneous nucleation, grain selection and grain epitaxial growth are considered. Using In718 as an example, the simulated results show that the CET is critically controlled by the undercooling, involving constitutional supercooling, thermal and curvature undercoolings in the melt pool, which dictates the extent of heterogeneous nucleation with respect to the grain epitaxial growth during rapid solidification.

Reducing arc heat input and obtaining equiaxed grains by hot-wire method during arc additive manufacturing titanium alloy

Abstract: Arc additive manufacturing technology has high deposition efficiencies and material utilization rates. However, the large heat input and high temperature gradient during arc additive manufacturing process lead to the formation of the coarse columnar grains. Due to the presence of the coarse columnar grains, the mechanical properties of the part have great anisotropy which limits the application of it. In this experiment, a method named hot-wire arc additive manufacturing was adopted to reduce the arc heat input and refine the columnar grains, and four thin-walled samples with the material of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si were manufactured. It is interesting to find that the coarse columnar grains have been greatly refined, and finally a part consisting of equiaxed grains and short columnar grains was obtained. At the same time, the width of the α-lath has also been refined. The mechanical properties are in accordance with the grain changes and the anisotropy almost disappeared. And it seems that a part with comprehensive mechanical properties can be obtained by the hot-wire arc additive manufacturing.

Effects of process parameters on microstructures and tensile properties of laser melting deposited CrMnFeCoNi high entropy alloys

Abstract: In this paper, a laser melting deposition (LMD) technique has been applied to fabricate CrMnFeCoNi high entropy alloys (HEAs). The microstructures and tensile properties of CrMnFeCoNi HEAs prepared under different laser power and scanning strategies have been investigated. It has been observed that the laser power and scanning strategy have significant effects on the columnar to equiaxed transitions (CET) of the microstructure of LMD CrMnFeCoNi HEAs because of their effects on heat flux direction and the temperature gradient. Due to small molten size and rapid cooling rate in LMD process which results in a significant solute-trapping effect and thus avoids component segregation, the elements distribution of LMD samples are more homogeneous than as-cast samples. Besides, tensile properties of the LMD samples can be adjusted by changing laser power and scanning strategy, which be corresponding to the changes of microstructure.

Gradient nanostructure evolution and phase transformation of α phase in Ti-6Al-4V alloy induced by ultrasonic surface rolling process

Abstract: The gradient nanostructure evolution and the mechanism governing this evolution of α phase in Ti-6Al-4V alloy induced by ultrasonic surface rolling process were investigated. A gradient nanostructure consisting of a roughly equiaxed nanograin layer, an elongated nano-lamellar layer, an elongated ultrafine lamellar layer, a refined grain layer, and a low-strain coarse-grained layer was formed with a thickness of more than 400 µm. The formation of gradient nanostructure of α phase was dominated by complex dislocation activities in hcp grains without twins occurring, supplemented by hexagonal close-packed (hcp) titanium (Ti) to face-centered cubic (fcc) Ti phase transformation. During the microstructural evolution, the coarse hcp-Ti grains were first elongated into lamellae. Then, these sub-micron lamellae were gradually transformed into roughly equiaxed nanograins via two deformation modes of longitudinal splitting and transverse breakdown, accompanied by dynamic recovery. The fcc-Ti grains were deformed mainly via twin-twin intersections and twin-dislocation interactions, accompanied by longitudinal splitting and transverse breakdown, resulted in refinement of the micron-scale fcc-Ti grains to roughly equiaxed nanograins. The interaction of hcp and fcc phases influenced and synergistically promoted the microstructural evolution process. In addition, the microhardness improvement in the surface layer of Ti-6Al-4V alloy was attributed to the increase of dislocation density, grain refinement and the occurrence of deformation twinning in fcc-Ti grains.

Selective laser remelting of an additive layer manufacturing process on AlSi10Mg

Abstract: AlSi10Mg samples were fabricated by selective laser melting (SLM) using meander strategy and remelting strategy to gain insight into the variation of the microstructures and microhardness as well as the surface morphology, characteristics of molten pool, relative density and phase identification. The results show that the remelting strategy in SLM improves the surface quality (9.94 μm) and relative density (99.3%) of AlSi10Mg, at the same time, the shallower melt pool also is attained by remelting step due to the higher thermal conductivity and the reflectivity of laser in solid than powder. The shallower melt pool for second scan remelts the columnar grain near the top of the original melt pool and the solidifies in the same way with as original melt pool, resulting in finer equiaxed grains. The finer microstructures with supersaturated α-Al improve the hardness performance of SLMed samples from 117.7 HV0.3 to 121.6 HV0.3.

Effects of deposition strategies on macro/microstructure and mechanical properties of wire and arc additive manufactured Ti6Al4V

Abstract: In this study, Ti 6Al 4V was fabricated via wire and arc additive manufacturing (WAAM) using different deposition strategies. The effects of the travel direction and interlayer dwell time on the evolution of the prior-β grain morphology, microstructure, microhardness, and room-temperature tensile properties of the as-deposited walls were systematically investigated. The thermal and stress/strain field during the deposition process were also simulated to elucidate the macro/microstructural evolution during the deposition. When the interlayer dwell time was less than 24 s, there was a transition from equiaxed prior-β grains (EBGs) to columnar prior-β grains (CBGs) from the bottom to the top of the deposited wall. Vertical and tilted CBGs were obtained with bidirectional and unidirectional travel directions, respectively. With a dwell time of 120 s and unidirectional travel directions, full-equiaxed prior-β grains were obtained within the wall. The EBGs were formed not from the solidification of the molten pool but from the recrystallization induced by the large heat accumulation and high stress/strain during the WAAM process. The recrystallization nucleation rate of the EBG zone was the highest with a dwell time of 24 s. There were tilted layer bands when bidirectional travel strategies were used. There was a fine lamellar α in the layer band-free region and a basketweave α in the layer band region. The increase in the interlayer dwell time led to a decrease in the width of the α lath in the layer band region, but little changed in the layer band-free region. In addition, the hardness, yield strength, and tensile strength increased with the increase in the dwell time from 0 to 120 s.

Laser solid forming additive manufacturing TiB2 reinforced 2024Al composite: Microstructure and mechanical properties

Abstract: The 2024Al alloy and 3%TiB2 reinforced 2024Al composite were fabricated using laser solid forming (LSF). Different from the LSF processed 2024Al with a large columnar grain and apparent preferential growth orientation, TiB2 reinforced 2024Al composite presents texture-less structure, consisting of the dendrite and fine equiaxed structures. Some TiB2 particles distribute within the Al matrix, others distribute along the grain boundaries and intertwine with the Al2Cu phase. The incorporation of TiB2 gives rise to significant grain refinement, which is one of the important reasons for the improvement of mechanical properties. Moreover, the TiB2 reinforced 2024Al sample exhibits 284 MPa high tensile strength, 163 MPa yield strength, 108.5HV microhardness, and 18.7% excellent elongation.

Varied heat treatments and properties of laser powder bed printed Inconel 718

Abstract: The effects of post-build heat-treatments on the microstructure, phase formation, recrystallization behavior, and mechanical properties of laser powder bed additively manufactured Inconel 718 superalloy were investigated. Several heat-treatment schedules were used, including simulated hot isostatic pressing (HIP), and variations of the standard double aging treatment with various soaking times and quench procedures. Their effects on the precipitation, grain morphology, grain size, texture sharpness and mechanical properties were all documented. For the as-built coupons, columnar grains with inter-dendritic micro-segregation were formed along the build direction (xz-plane) with equiaxed grains forming on perpendicular sections to the build direction (xy-plane). After prolonged soaking times during the simulated HIP process, the microstructure transitioned from heterogeneous columnar grains to homogenous recrystallized grains with MC-type carbide precipitates. This leads to changes of microhardness (281 HV2.0 to 171 HV2.0), Young's modulus (209 GPa–229 GPa) and texture intensity. However, aging treatments increased both hardness and Young's modulus possibly because of formation of γ″ and γ′ precipitates in the Ni-matrix and the small effective grain size. Phase analysis using XRD confirmed the evolution of the precipitate formation. The combination of additive manufacturing and post-build heat-treatments can result in optimized microstructures and mechanical properties for specific applications depending on part requirements and operating conditions.

Experiments and simulations on solidification microstructure for Inconel 718 in powder bed fusion electron beam additive manufacturing

Abstract: Previous research on the powder bed fusion electron beam additive manufacturing of Inconel 718 has established a definite correlation between the processing conditions and the solidification microstructure of components. However, the direct role of physical phenomena such as fluid flow and vaporization on determining the solidification morphology have not been investigated quantitatively. Here we investigate the transient and spatial evolution of the fusion zone geometry, temperature gradients, and solidification growth rates during pulsed electron beam melting of the powder bed with a focus on the role of key physical phenomena. The effect of spot density during pulsing, which relates to the amount of heating of the build area during processing, on the columnar-to-equiaxed transition of the solidification structure was studied both experimentally and theoretically. Predictions and the evaluation of the role of heat transfer and fluid flow were established using existing solidification theories combined with transient, three-dimensional numerical heat transfer and fluid flow modeling. Metallurgical characteristics of the alloy’s solidification are extracted from the transient temperature fields, and microstructure is predicted and validated using optical images and electron backscattered diffraction data from the experimental results. Simulations show that the pure liquid region solidified quickly, creating a large two-phase, mushy region that exists during the majority of solidification. While conductive heat transfer dominates in the mushy region, both the pool geometry and the solidification parameters are affected by convective heat transfer. Finally, increased spot density during processing is shown to increase the time of solidification, lowering temperature gradients and increasing the probability of equiaxed grain formation.

Microstructural evolution of post-processed Hastelloy X alloy fabricated by laser powder bed fusion

Abstract: Hastelloy X (HX) is a Ni-based superalloy which is employed to produce gas turbine and gas-cooled reactor sectors due to its outstanding oxidation resistance and high tensile strength at high temperatures. This alloy can be processed by laser powder bed fusion (LPBF) fabricating complex geometries in a single step. However, post-processing thermal treatments must be applied to generate a suitable microstructure for high-temperature applications. The investigation reports the microstructure evolution of LPBF HX samples under specific post-processing treatments. A hot isostatic pressing (HIP) treatment can close the internal cracks and reduce the residual porosity (less than 0.1%). Moreover, the HIP-triggered recrystallization generated equiaxed grains, while the slow cooling rate generated a film of intergranular carbides (Mo-rich M6C and Cr-rich M23C6) and intragranular carbides (Mo-rich M6C carbides). Therefore, a solution annealing was performed to dissolve the film of carbides which may reduce the ductility. The post solution annealed material consisted of equiaxed grains with ASTM grain size number mainly 4.5-5.5 and inter/intragranular Mo-rich M6C carbides. The microstructure is highly comparable with solution annealed wrought HX alloy. Finally, after simulating short thermal exposure at 745 °C for 6 h, a significant formation of Cr-rich M23C6 carbides was observed strengthening the LPBF HX alloy.