Characterization of residual stress as a function of friction stir welding parameters in ODS steel MA956

Abstract

This thesis characterizes the residual stresses generated by friction stir welding of ODS steel MA956 as a function of heat index. The heat index of a weld is used to determine relative heat input among different friction stir welding conditions. It depends on a combination of the rotational speed and traverse, or welding speed of the tool through the metal. Higher rotational speeds and lower traverse speeds increase heat index and are expected to result in lower residual stresses than welds performed with lower heat index. Three conditions with varying heat indices were selected from high to low: 500 rpm/25 millileters per minute (mmpm), 400 rpm/50 mmpm and 400 rpm/100 mmpm. The full stress tensor was determined for points across these welds using x-ray diffraction. Longitudinal residual stresses approximately 90% of yield strength were observed for the lowest heat input condition. Stress profiles displayed the characteristic M shaped profile, and asymmetry between advancing and retreating stress peaks was limited, occurring mainly on the root of the weld. As expected, lowering the heat index of a weld increased the magnitude of residual stresses across the whole weld profile.