Assessing residual stresses in friction stir welding: neutron diffraction and nanoindentation methods

This study evaluates the distribution of residual stress in friction stir weld (FSW) joints in AA7075 alloy using a semi-destructive technique (depth-controlled nanoindentation), and compares the results to a non-destructive method (neutron diffraction). The FSW process offers many benefits in AA7075 alloys such as grain refinement, prevention of solidification cracking, and reduced heat-affected zone softening. To examine the effects of welding parameters on the development of residual stress in FSW joints, different tool rotation speeds were applied on 8-mm-thick sheets of Al-Zn-Mg-Cu alloy. Nanoindentation and neutron diffraction measurements reveal variations in the residual stress within the nugget zone, thermos-mechanically affected zone, and heat-affected zone as compared to the base metal. The trend in residual stress values obtained from nanoindentation agrees with that from neutron diffraction measurements. The application of higher travel speed was found to reduce the residual stresses to negligible values, most likely as the result of an auto-stress relieving mechanism caused by higher temperatures and slower cooling rates.