Synergistic and competitive mechanisms of plastic anisotropy in high-efficiency laser melting deposited TC11 alloy

Abstract

The layer-by-layer processing features of laser melting deposited (LMD) TC11 titanium alloy inevitably lead to microstructural inhomogeneity, and the grain morphology associated with micro-orientations induces plastic anisotropy, which causes uneven deformation and further performance deterioration. Understanding the main causes of these characteristics and how to distinguish their relative roles in inducing plastic anisotropy is critical. In this work, a high-efficiency LMD process is developed. Then, the individual factors causing the formation of elongated grains, microstructural inhomogeneity and micro-orientations are isolated to investigate the synergistic and competitive mechanisms of plastic anisotropy for high-efficiency LMD TC11 titanium alloy before and after various heat treatments. Initially, the TC11 titanium alloy is prepared by the LMD technique under high laser power and then heat-treated via three methods of double annealing in the α+β phase zone. Subsequently, the macro- and microscopic morphology, microstructure, and micro-orientations are characterized by optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The individual effects of grain shape, α grain boundary (αGB) and micro-orientations on tensile plastic anisotropy are systematically and thoroughly investigated via a hybrid method consisting of experimental and crystal plasticity finite element simulation methods. By introducing a redefined evaluation indicator of dislocation plugging coefficients, elongated grains are found to be the main source affecting the tensile strength in different loading directions. In addition, with increasing primary annealing temperature, the breakage degree of αGB parallel to the building direction (BD) is significantly higher than that perpendicular to the BD, which leads to an increase in tensile strength and elongation. Moreover, the preferred <0 0 0 1> orientation leads to different tensile strengths perpendicular and parallel to the BD, which is caused by the transformation between easy-to-slip and hard-to-slip orientations. The micro-orientations mainly affect the yield strength and elongation, while the tensile strength and elongation are simultaneously controlled by the grain morphology and αGB.