Fatigue performance of surface ground and wire electrical discharge machined TiNi shape memory alloy

Abstract

Fatigue performance is a major consideration for critical aerospace components. The influence of surface grinding and rough/finish wire electrical discharge machining (WEDM) on the high cycle fatigue performance of a binary Ti50.8-Ni49.2 shape memory alloy was assessed. The effect of machined workpiece surface integrity in terms of surface roughness and subsurface microhardness on the fatigue results was also evaluated, in addition to fractography analysis. Testing was performed using a tensile-tensile regime at an elevated temperature of 150°C with specimens in the austenitic phase. Ground samples showed the highest fatigue strength of 390 MPa at run-out of 1.2 × 107 cycles, while finish and rough WEDM specimens were 21% and 57% lower respectively, despite the finish WEDM surfaces having significantly lower roughness. This was likely due to the presence of tensile residual stresses following WEDM. All of the S-N curves however exhibited a relatively flat response with no clear indication of endurance limits. This implies that the different machining processes/conditions affected the fatigue strength of the material, but not the overall trend/shape of the fatigue curves.