Showing papers on "Equiaxed crystals published in 2021"

Effect of welding thermal treatment on the microstructure and mechanical properties of nickel-based superalloy fabricated by selective laser melting

Abstract: Strengthening the heat-affected zone in actual weld joint of Inconel 625 alloys was crucial for engineering applications in aerospace, petroleum, nuclear energy, and marine industries. In this study, Gleeble thermal simulations were employed to investigate the influence of welding heat on grain morphology, carbide evolution, and mechanical properties of Inconel 625 alloys manufactured by selective laser melting (SLM). Typical elongated columnar grains along the building direction and parallel columnar array with obvious layer boundaries along the scanning direction were observed in SLM-processed Inconel 625 alloys with hot isostatic processing treatment. After Gleeble thermal simulations, the prominent equiaxed grains appeared in the entire gauge region that was directly exposed to the simulated welding heat owing to the recrystallization process. Meanwhile, the size of equiaxed grains significantly increased with the increasing peak temperature. Particularly, only MC-type carbides were observed in the γ-Ni matrix before and after the thermal simulation owing to the fairly rapid heating (100 °C/s) and cooling (30 °C/s) process. Simulated welding heat produced a negative effect on the mechanical properties of the Inconel 625 alloys, especially for the peak temperature of 1350 °C. Then, the respective contributions of the four conventional strengthening mechanisms were successfully quantified for understanding the effect of the simulated peak temperature on the yield strength of the Inconel 625 alloys. Finally, the observations of microstructural evolution indicated that the cross-slip of dislocations was the dominant deformation mechanism to account for the large strain-hardening rate of the Inconel 625 alloys.

A novel crack-free Ti-modified Al-Cu-Mg alloy designed for selective laser melting

Abstract: A novel crack-free Ti-modified Al-Cu-Mg alloy for SLM was developed here, based on the thermodynamic calculations of the crack susceptibility index and growth-restriction factor. We found that the introduction of Ti into the Al-Cu-Mg alloy effectively promoted the grain refinement and columnar-to-equiaxed grain transition as a result of the heterogeneous nucleation provided by Al3Ti precipitates. The hot tearing cracks were eliminated after Ti modification due to the formation of the homogeneous and fine equiaxed microstructure. We created a new high-strength Al-Cu-Mg-Ti alloy with a tensile strength of 426.4 MPa, yield strength of 293.2 MPa and ductility of 9.1%. This novel Ti-modified Al alloy with fine equiaxed grains and highly-enhanced mechanical properties offers a new compositional space for the printable lightweight material categories specifically for the SLM technique.

Microstructure and mechanical properties of a TiB2-modified Al–Cu alloy processed by laser powder-bed fusion

Abstract: In this work, crack-free samples with a relative density of 99.5 ± 0.1% were produced from a gas-atomized Al–Cu–Ag–Mg–Ti–TiB2 powder via laser powder-bed fusion. The homogeneous equiaxed microstructure without preferred grain orientation shows the α-Al grains’ mean size to be 0.64 μm ± 0.26 μm TiB2 particles with sizes of several tens of nm up to 1.5 μm were observed in the as-built component. Small TiB2 particles of up to approx. 200 nm are located within the α-Al grains, which show a semi-coherent interface to the α-Al phase. Larger TiB2 particles of up to 1.5 μm accumulate in the liquid between the growing α-Al grains during solidification and inhibit grain growth. Al2Cu phase is precipitated at the α-Al grain boundaries. Coarse Al2Cu precipitates, which are slightly enriched with silver and magnesium, are also observed within the grains preferentially precipitated on small pores and TiB2 particles. The novel fine-grained microstructure results in the as-built state in a tensile strength of 401 ± 2 MPa and total elongation at fracture of 17.7 ± 0.8%.

Grain refinement and crack inhibition of selective laser melted AA2024 aluminum alloy via inoculation with TiC–TiH2

Abstract: AA2024 aluminum alloy easily forms coarse columnar grains and cracks during selective laser melting (SLM) Herein, a novel grain refiner composed of TiC nanoparticles and TiH2 powders was used to develop the microstructure of selective laser melted AA2024 aluminum alloy Due to the heterogeneous nucleation of α-Al promoted by the cooperation of TiH2 and TiC, the microstructure of the TiC–TiH2/AA2024 composite converted from columnar grains with an average area of 7290 μm2 into equiaxed grains with an average area of 34 μm2, and the morphology of the second phase changed from streamline to network-shaped Moreover, cracks in the TiC–TiH2/AA2024 composite were intrinsically eliminated because of grain refinement Benefitting from elimination of columnar grains and inhibition of cracks as well as the combined effect of refinement strengthening and Orowan strengthening, the TiC–TiH2/AA2024 composite exhibited excellent mechanical properties, the tensile strength and elongation of as-built samples rose from 240 ± 10 MPa and 03 ± 02% in AA2024 alloy to 390 ± 15 MPa and 120 ± 05% in TiC–TiH2/AA2024 composite After T6 heat treatment, TiC–TiH2/AA2024 composite exhibited a tensile strength of 490 ± 20 MPa and elongation of 160 ± 1%, which were comparable to those of wrought AA2024 alloys The results show that aluminum alloy with excellent mechanical properties can be prepared by SLM with effective microstructure control

On the microstructure and solidification behavior of new generation additively manufactured Al-Cu-Mg-Ag-Ti-B alloys

Abstract: Laser powder bed fusion (LPBF) was used to horizontally print A205 ( Al — Cu — Mg — Ag — Ti — B ) metal powder. The samples have been cut in various directions to analyze their phases, grain morphology, size, crystallographic orientation, and distribution of elements via X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and X-ray energy dispersive spectroscopy (XEDS) in a transmission electron microscope (TEM). To monitor deformation behavior and strain distribution, several uniaxial tensile tests accompanied by digital image correlation (DIC) technique were carried out. The results showed that the as-built microstructure fully contains fine equiaxed grains with no strong preferential crystallographic orientation. The XEDS elemental maps confirmed the solute trapping phenomenon taking place during the LPBF. The stress-strain curves obtained by the uniaxial tensile testing revealed a quasi-isotropic behavior in terms of building direction; moreover, the DIC strain contour maps confirmed yield-point phenomenon and Luders bands propagation during uniaxial tensile testing. Finally, a time/temperature interdependence grain growth model was successfully applied based on no diffusion in solid and partial mixing in liquid through the entire melt pool.

Effect of heat treatment on the microstructure and anisotropy of tensile properties of TiAl alloy produced via selective electron beam melting

Abstract: Selective electron beam melting (SEBM) is considered to be a promising method to fabricate TiAl alloy components with complex geometries. In this work, the effect of annealing on the microstructure and anisotropy of tensile properties of SEBM-produced Ti–47Al–2Cr–2Nb alloys was systematically investigated. The results indicated that the SEBM-produced TiAl alloy samples exhibited equiaxed and elongated grains in the transverse and longitudinal directions, respectively, and presented anisotropy of the tensile properties. For the sample produced at an energy density of 24.07 J/mm3, the microstructure in the vertical cross section presented an elongated lamellar structure consisting of (α2/γ) lamellar colonies, after annealing at 1310 °C for 3 h; on the other hand, for the same sample, elongated grains were almost invisible after annealing at 1370 °C for 2 h. Further, for the sample produced at an energy density of 35 J/mm3, the elongated grains remained after annealing at 1370 °C for 2 h and were destroyed by cyclic heat treatment. The texture intensity and anisotropy of the tensile properties of the two samples fabricated with different processing parameters were weakened by annealing at 1370 °C for a duration of 2 h and cyclic heat treatment between the α+β+γ and α phase regions, respectively. Both the tensile strength and elongation increased as a result of heat treatment. Moreover, the phase transformation during annealing was analyzed.

Elevated temperature mechanical properties of TiCN reinforced AlSi10Mg fabricated by laser powder bed fusion additive manufacturing

Abstract: This short communication presents 2–4 μm sized TiCN reinforced AlSi10Mg composites (TiCN/AlSi10Mg) fabricated by laser powder bed fusion (LPBF) with refined Al-Si eutectic microstructure consisting of equiaxed bi-modal α-Al grains and enhanced elevated temperature tensile strength. The formation mechanism of reported bi-modal structure is related to the modified temperature gradients and induced heterogeneous nucleation in LPBF. The enhancement in elevated temperature tensile strength is mainly attributed to refined bi-modal Al-Si microstructure and thermal stability of TiCN particles.

Microstructures evolution, texture characteristics and mechanical properties of Mg-2.5Nd-0.5Zn-0.5Zr alloy during the high strain rate hot-rolling

Abstract: The microstructures evolution, texture characteristics and mechanical properties of Mg-2.5Nd-0.5Zn-0.5Zr alloy processed by high strain rate rolling (HSRR) at different temperature were systematically investigated in the current study. The results showed that the initial material was approximately equiaxed grains (23.8 μm) with a large number of divorced eutectic Mg12Nd phases precipitated along grain boundaries in a continuous network. After HSRRed, the microstructure was obviously refined, and uniform distributed. The average grain size was refined to 3.63 μm (R673), 2.71 μm (R648) and 2.17 μm (R623), respectively, and corresponding percentage of recrystallization were 46.1%, 49.3% and 63.9%, respectively. R623, R648 presented a strong (0002) basal surface bimodal texture in the RD direction but this texture was in the TD direction in R673, all of which preferential orientation was at 66.3–90° region. An excellent combination of ultimate tensile strength (324 ± 2 MPa), the yield strength (298 ± 3 MPa) and the elongation (6.9 ± 1.5%) were achieved in a fine-grained (2.17 ± 1.17 μm) Mg-2.5Nd-0.5Zn-0.5Zr alloy prepared by HSRR at 623 K. The higher yield strength can be obtained by refinement strengthening, dislocation strengthening, precipitation strengthening and texture strengthening.