Influences of grain orientation on high-cycle fatigue performance of laser powder bed fused Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy at room temperature and 500℃

Abstract

The fatigue performance of titanium (Ti) alloys fabricated by laser powder bed fusion (LPBF) is a crucial factor influencing the structural safety in service. The high-cycle fatigue (HCF) of LPBF Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy at the room temperature and 500 °C was tested. Compared with the room-temperature samples, the fatigue strength of fatigue samples tested at 500 °C is reduced by about 55.8 % after 106 fatigue cycles. Crack initiation on the surface is mainly related to fatigue after fewer cycles under high stress, while interior cracks initiate after a longer fatigue life under low stress. Electron back scattering diffraction (EBSD) was adopted to characterize the orientations of grains on the crack propagation paths and the activated slip systems. All prismatic oriented grains were found to activate prismatic slip systems, the activation of which achieves symmetric slips surrounding crack tips, forms ductile striations, and increases energy absorption during crack propagation. These characteristics are conducive to improved fatigue performance. About half of the basal oriented grains activate prismatic slip systems. These activated prismatic slip systems not only have a high Schmid factor (SF) but also need to have a high geometrical compatibility factor with slip systems activated by adjacent grains. The residual β phase and α phase are aligned according to the Burgers orientation relationship (BOR), so the β phase does not alter the direction of crack propagation. The results were used to predict the fatigue crack propagation paths in LPBF Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy during loading along building and transverse directions were predicted. This, to some extent, provides a basis for comparing fatigue performance of LPBF Ti alloys in different directions.