A new method for localization of the residual stress distribution and enhancement of wear resistance through underwater friction stir processing with stationary shoulder

A new method to achieve friction stir processed parts with higher wear resistance and lower residual stress values was presented. The underwater stationary shoulder FSP process (USSFSP) was investigated using experimental and numerical methods. Also, the presented process was compared with the conventional underwater FSP method (UCFSP). An improved CEL simulation method was used to prevent errors in the contact boundary between Eulerian and Lagrangian domains. Heat input was reduced significantly and resulted in a fine microstructure with higher microhardness and lower residual stresses. Temperature field was concentrated around the pin and caused a symmetrical residual stress distribution. The distribution area of Von-Mises and longitudinal stresses during the USSFSP was significantly narrower than the UCFSP process. The stress distribution around the UCFSP tool was compressive, and tensile stress was produced at further distances. However, the longitudinal stress distribution in the USSFSP was completely opposite. Tensile stress was generated in the lower part of the thickness and compressive stress in the upper part and in locations away from the centerline. This method caused the formation of a compressive residual stress region and considerably reduced the width of the tensile residual stress zone. By increasing the hardness and decreasing the residual stress level, the USSFSP proved to be instrumental in improving the material’s wear resistance. The wear rate in the stir zone undergoes a substantial reduction, decreasing from 5.6 × 10⁻³ mm³/m in UCFSP to 1.4 × 10⁻³ mm³/ m in USSFSP.