Mechanical properties improvement of thick multi-pass weld by layered ultrasonic impact treatment

The effects of layered ultrasonic impact treatment (LUIT) on the mechanical properties of thick multi-pass weld were investigated. Two 55-mm thick multi-pass welding joints were prepared, one was treated with LUIT, and the other was joined with conventional welding method. Shear-punch test and Charpy impact test were used to examine the mechanical properties of both joints. Results show that LUIT can increase the strength of weld metal (WM), resulting in more uniform shear strength distribution across the treated WM and higher impact toughness of the WM. The mechanical properties improvement of WM attributes to the oxidation of metals induced by ultrasonic impact treatment, which increases the oxygen content in the WM and causes more non-metallic oxide inclusions, and then affects the grain growth behaviour of the subsequent beads, resulting in a fine-grained microstructure and improved strength and toughness of the WM.