Abstract
In this study, a crack-free 7075 aluminum alloy composed of fine, randomly-oriented and equiaxed grains ∼1 μm in size was successfully prepared by laser powder bed fusion (L-PBF). TiH2 particles were introduced to achieve significant grain refinement, serving as a critical method to relieve the high residual stress and prohibit inherent hot cracking during L-PBF manufacturing. Multiscale microstructural characterizations were applied to reveal the mechanisms of grain refinement and elimination of hot cracking. On the one hand, in-situ produced L12-structured Al3Ti particles stimulated heterogeneous nucleation of α-Al. On the other hand, Ti solute atoms played an important role in restricting grain growth during solidification. A conventional T6 treatment was conducted to modify the microstructures and mechanical properties of the alloy. E-Al18Mg3(Ti,Cr)2 dispersoids were precipitated, playing a role in strengthening through pinning grain boundaries. Thanks to grain boundary strengthening and Orowan dislocation strengthening arising from η' precipitates, Al3Ti particles and E dispersoids, the yield strength, ultimate tensile strength and elongation of T6 treated 7075-Ti alloy are 496 ± 10.8 MPa, 537 ± 7.6 MPa and 7.2 ± 0.3 %, respectively. These findings provide a broader insight into the use of TiH2 to improve the microstructure and mechanical properties of L-PBF-fabricated high-strength aluminum alloy.
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