Effects of ultrasonic shot peening on fatigue behavior of TA15 titanium alloy fabricated by laser melting deposition

Abstract

Due to the influence of tensile residual stress and pores, the fatigue resistance of titanium alloy fabricated by laser melting deposition (LMD) still needs to be improved. In this study, ultrasonic shot peening (USP) was utilized to strengthen the TA15 titanium alloy manufactured by LMD. The surface topography, microstructure, microhardness, residual stress and fatigue behavior were analyzed. After USP treatment, the root-mean-square deviation of the three-dimensional profile (Sq) and the local stress concentration factor (Kst) increased. High-density dislocations (dislocation tangles, dislocation walls, dislocation cells) and deformation twins were generated on the specimen subsurface. The existence of high-density dislocations improved the surface microhardness. Meanwhile, changes in microstructure affected the state and distribution of residual stress. The surface tensile residual stress was completely transformed into compressive residual stress. The fatigue behavior was significantly influenced. Under 720 MPa and 760 MPa stress levels, the safe fatigue life (reliability R = 0.95, confidence level C = 0.95) of TA15 titanium alloy manufactured by LMD increased by 200 % and 43 %, respectively. In the pore-profile valley competition, the pore showed greater fatigue sensitivity. The high-density dislocations and compressive residual stress accounted for the improvement in fatigue life. The high-density dislocations hindered the crack nucleation and short crack propagation at pores near the surface. The compressive residual stress reduced the crack propagation rate in the stable crack propagation stage. The results indicated USP as an effective method to improve the fatigue performance of the TA15 titanium alloy manufactured by LMD.