To develop high-strength Al alloys for selective laser melting (SLM) additive manufacturing, we designed a series of Al-Mg(-Si)-Sc-Zr alloys and additively manufactured them using atomized alloy ...powders. In the absence of Si, the developed Al-xMg-0.2Sc-0.1Zr (x = 1.5, 3.0 and 6.0 wt%) alloys are all susceptible to hot cracking and the average crack density increases with increasing Mg content. The addition of 1.3 wt% Si into the Al-6Mg-0.2Sc-0.1Zr alloys effectively inhibits hot cracking during SLM and simultaneously refines the microstructure, and thus leading to enhanced mechanical properties in the as-printed samples. By further fine-tuning the alloy compositions, we designed a new alloy Al-8.0Mg-1.3Si-0.5Mn-0.5Sc-0.3Zr. This new alloy demonstrates significantly refined microstructure consisting of submicron cells with coherent Al3(Sc,Zr) nano-particle (2–15 nm) residing in the cell and intergranular Al-Mg2Si eutectic (Mg2Si diameter 10–100 nm). High-density stacking faults and a unique 9R phase are formed in the as-printed sample. The tensile strength and elongation of the as-printed sample are up to 497 MPa and 11%, respectively. After the aging treatment, the tensile strength reaches 550 MPa, while the ductility ranges from 8% to 17%, depending on the aging conditions. In addition to solid solution strengthening, grain boundary strengthening and nanoparticle strengthening, the high-density stacking faults also contributes to strengthening.
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The selective laser melting (SLM) of an equiatomic CoCrFeMnNi high-entropy alloy (HEA) powder was studied, with emphasis on its non-equilibrium microstructural evolution and mechanical properties. ...The printed sample density increases gradually with increasing laser energy density and then decreases at an over-high laser energy density. The microstructure of SLM printed HEA exhibits a large number dislocation pile-ups and lattice distortion. Of particular intrigue are the existence of nanotwins and tetragonal σ phase in SLM printed HEA; such nanotwins and σ phase have never been observed in either cast or deformed HEA with true strain below 3.7%. The grain refinement by rapid solidification and σ phase synergistically improve the mechanical properties, as compared with conventionally solidified HEA. Hot isostatic pressing (HIP) was performed to improve the densification, yielding the increase of tensile strength from 601 MPa of SLM printed sample to 649 MPa after HIP.
•The equiatomic CoCrFeMnNi alloy was successfully printed by SLM.•Solidifying stress yielded a large number of dislocation pile-ups.•Nanotwins were detected in SLM printed sample.•Tensile strength of SLM + HIP processed sample reaches 649 MPa.
The influence of particle size on the densification kinetics of tungsten powder during spark plasma sintering was investigated. The densification rate of tungsten powder in the intermediate sintering ...stage decrease with increasing particle size, resulting in a delay in the sintering stages of coarse powder. The isothermal densification kinetic behaviors of tungsten powder show that the densification of tungsten powder can be divided into two kinetic stages: a low-stress exponent segment (n = 1.5) and a high-stress exponent segment (n = 3 or 4). With increasing of particle size, n increases from 3 to 4, and the activation energy decreases from 304 to 254 kJ/mol for the high-stress exponent segment. This is because the densification mechanism has a tendency to change from diffusion creep to dislocation creep or dislocation glide as the particle size increases. The evolution of the activation energy exactly matches the transformation of the deformation mechanism, indicating that the densification activation energy does not reflect a barrier to densification, but rather a barrier to deformation with different deformation mechanisms.
•The influence of particle size on the densification kinetics of tungsten powder was investigated.•The densification mechanisms of tungsten powder with different particle sizes were discussed.•The better sinterability of fine powder was explained from the aspect of densification kinetics.
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The atomic diffusion of a Fe–Al system under spark plasma sintering was investigated at temperatures ranging from 773 to 873K. A significant increase in the growth rate constant of ...Al13Fe4 and Al5Fe2 under the pulse current of spark plasma sintering was observed compared with the absence of current. The interdiffusion coefficient under pulse current was about 46 times higher than that without current at 873K. The activation energy for the interdiffusion was 332kJ/mol, which is higher than that without current of 155–190kJ/mol. Although the diffusion kinetics were enhanced, the thermodynamics state is not altered.
Magnesium alloys have superior mechanical property for industry applications as structural materials, but their poor corrosion resistance is still a bottleneck problem. Mg-Al-Zn alloys are one of the ...most extensively used Mg alloys. In order to study the effects of the second phases on corrosion property systematically, Mg-xAl-(15−x) Zn (x = 12.5, 5.6, 3.3, 1.0 wt%) two-phase alloys containing a certain amount of the different second phases, two binary compounds γ, MgZn, and two ternary second phases Φ, q, were prepared based on the Mg-Al-Zn phase diagram. On this basis, corrosion property of the alloys in the 3.5 wt% NaCl solution was studied by corrosion morphology observation and electrochemical tests. The role of the different second phases in the corrosion processes was investigated. It was revealed that the second phases precipitated both in the Mg matrix and along the grain boundaries and acted as micro-cathode, accelerating corrosion dissolution of the Mg substrate. The acceleration effect of the second phases is in the order of γ-Mg17Al12 > q- Mg44Zn41Al1 > Φ-Mg21(Zn,Al)17 > MgZn. The alloys were also investigated for their diversity in corrosion product characters. The corrosion mechanism was discussed terminally by equivalent circuit of the electrochemical impedance spectrum.
•Two ternary and two binary second phases were confirmed stable at 300 °C.•The roles of the second phases in corrosion processes were investigated.•Corrosion property effected by different second phases were compared.•Corrosion mechanism was studied by corrosion morphology and the equivalent circuits.
Additive manufacturing Al-Mg-Sc-Zr alloys offer significant advantages for lightweight application with complex shapes. The paper systematically investigated the effects of aging treatment on ...microstructural evolution and mechanical properties of SLM printed Al-3.02Mg-0.2Sc-0.1Zr alloy. The SLM printed sample with a relative density of 99.2% was achieved at an optimal laser parameters. After aging, the hardness was improved from initial 85 HV of as-printed sample to 120 HV. The strength and ductility trade-off of the printed sample can be tailored by regulating aging parameters; the maximal tensile strength of 400 MPa and yield strength of 327 MPa can be obtained at an optimal aging parameters. The XRD patterns show the secondary peak of Al3(Sc,Zr) after aging treatment due to Al3(Sc,Zr) particles separation from the α-Al matrix and the diffraction angles shifts to a higher value owing to the release of residual stress. Moreover, Nano-sized Al3(Sc,Zr) precipitates on the grains boundaries hamper grain growth, which was responsible for the grain sizes maintaining after aging treatment.
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•The strength and ductility trade-off of the alloy can be tailored by regulating aging parameters.•The diffraction peaks show the secondary peak of Al3(Sc,Zr) after aging treatment owing to the precipitation particles.•The segregation of Al3(Sc,Zr) particles on grains boundaries was characterized.
In this study, selective laser melting (SLM) of pure tantalum (Ta) was systematically investigated, with emphasis on densification, microstructure and mechanical properties of Ta specimen. The high ...laser scanning speed resulted in micropores and discontinuous scan tracks, owing to the elevated instability of the liquid induced by Marangoni convection and the balling effect. However, the interlayer thermal microcracks were produced at a low scanning speed, due to the thermal stress and balling effect. The microhardness and tensile strengths of the optimally SLM-processed Ta parts were improved to 445 HV and 739MPa, respectively, which were considerably higher than specimens processed by cast (110 HV and 205MPa) or powder metallurgy (120 HV and 310MPa) method, due to the fine-grain strengthening. The fracture morphology of the tensile-failed SLM-processed specimens showed that the porosities and incompletely melted particles are responsible for the fracture of porous sample. While for dense sample, cleavage fracture and minor ductile fracture both account for the fracture. And the failure mechanisms were discussed. The reduced coefficient of friction of 0.3 and lowest wear rate of 7.1×10−3mm3N−1m−1 in dry sliding wear tests were obtained for the optimally prepared Ta parts due to the formed adhesion of hardened tribolayers.
Selective laser melting (SLM) is a promising technique for manufacturing 2xxx series Al–Cu alloy parts. However, the metallurgical defects in SLM severely limits their applications. In this work, SLM ...of a Li and Zr modified Al–Cu–Mg alloy (Al–4Cu–1Li-0.4Mg-0.5Zr) was studied with emphasis on its microstructure, metallurgical defects and hardness. The results show that microstructure is composed of fine equiaxed crystals (0.5–1 μm) and columnar dendrites (10–12 μm in length and 1–2 μm in width) with pronounced pore and crack defects. The geometrically irregular micropores distribute at the boundaries of the molten pool while the regularly round micropores distribute randomly. Cracks were observed growing along the grain boundaries with high angle and almost parallel to building direction, which were mainly caused by local stress concentration in the liquid film. The texture of SLM-processed Al–Cu–Mg–Li–Zr alloy was found in {001} and {001} orientation. The microhardness improved as laser energy density increasing due to the higher cooling rate. This research provides useful guidance in the future for designing crack-free and high strength SLM printed 2xxx series Al alloys.
•2xxx series Al–Cu–Mg–Li–Zr alloys was successfully printed by selective laser melting.•The microstructure is composed of fine equiaxed crystals and columnar dendrites.•Cracks grow along the boundaries with high angle and almost parallel to the direction of dendrite growth.•Cracks are caused by local stress concentration in the liquid film.
The anisotropic microstructure and mechanical properties of Ti-13Nb-13Zr alloy via selective laser melting (SLM) are investigated for the first time in this study. Owing to the re-melting of the ...previous layer(s) and the thermal gradient, the prior β columnar grains grow in the building direction while scanning direction are mainly consist of finer equiaxed grains, which result in the higher nanohardness in horizontal section (5.18 ± 0.2 GPa) than in vertical section (4.63 ± 0.2 GPa). The as-fabricated longitudinal samples show slight higher ultimate tensile strength (UTS) (1020 ± 15 MPa) and yield strength (YS) (794.63 ± 15 MPa) than transverse samples (UTS of 996 ± 13 MPa and YS of 794 ± 15 MPa) because of the enhancement of twins in vertical section. The SLM processed samples show no element segregation because of the complete melting and repeatedly re-melting, and the refined microstructure due to the high cooling rate in SLM process, which make the tensile strength superior higher than those obtained by powder metallurgy (750 MPa). Owing to the columnar grain morphology, in building direction, the failure can be postponed, which results in the longitudinal samples (6.5 ± 0.3%) exhibit higher ductility than transverse samples (5 ± 0.3%).
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•The anisotropic behaviors of Ti-13Nb-13Zr via SLM are investigated.•The nano-hardness in horizontal section is higher than in vertical section.•The tensile behaviors in horizontal section are lower than in vertical section.•The SLM-processed parts exhibit superior performances than powder metallurgy.
The densification kinetics and grain growth behavior of an undoped tungsten powder during spark plasma sintering (SPS) were investigated under the pressure of 40MPa and constant heating rate of ...100°Cmin−1. Two stages of the sintering process were clearly identified: densification without grain growth at the low temperatures (1200–1450°C) and grain growth without much further densification at higher temperatures (1500–2000°C). A creep model was applied to determine the densification mechanisms involved in the densification stage, which can be elucidated by evaluating the stress exponent (n) and the apparent activation energy (Qd) from the densification rate law. It shows that a boundary diffusion governs the densification process at low effective compaction stresses (n=1.5, Qd=140.57±12kJmol−1), while grain boundary diffusion and dislocation motion both operate at higher effective compaction stresses (n=3, Qd=302.48±24kJmol−1), which is confirmed by transmission electron microscopy observation. During the final-stage of sintering, the fast grain growth mechanism was suggested as surface diffusion.
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•We consider pressure as one of the main driving force of densification.•The sintering process of tungsten during SPS contained two stages.•Creep model was applied to study the densification mechanisms.•Grain growth model were used to identify the grian growth mechanisms.
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