Micro-deformation behavior of laser melting deposited multi-gradient TC4/TC11 alloy under tension–tension cyclic loading

Abstract

The microstructure of additive manufacturing titanium alloy is closely related to its mechanical properties. In this study, a TC4/TC11 titanium alloy with 20 layers was prepared by laser melting deposition (LMD) technology. Microstructure analysis revealed a transformation of the columnar grains into equiaxed grains along the build direction (BD), accompanied by the presence of heat affected zone (HAZ) bands and pore defects. The defect families of specimens in different directions were characterized by X-ray micro-computed tomography (micro-CT). The porosity of BD specimens was 0.0021%, and the porosity of the transverse direction (TD) specimens was 0.0001%, indicating that the material was dense. Specimens with different directions were subjected to tension–tension cyclic loading, and their deformation was analyzed by digital image correlation (DIC). The results show that the presence of noticeable pore defects leads to crack initiation around these regions due to local strain concentration. Moreover, the dispersion and anisotropy of mechanical properties are attributed to the influence of porosity and microstructure deformation, the latter being the dominant factor. In addition, the deformation mechanism and failure mode of the microstructure in LMD TC4/TC11 are proposed in this paper. The deformation of the α laths of TD specimen is hindered by the columnar grain boundary, resulting in improved resistance against crack nucleation and propagation compared to the BD specimen.