Abstract:The influence of ultrasonic vibrations on microstructure and mechanical properties of the AZ91-C magnesium alloy after ultrasonic assisted friction stir welding (UaFSW) in comparison with conventional friction stir welding (FSW) was investigated. The FSW was applied at the rotational speed of 1400 r/min and welding speed of 40 mm/min and no defects were observed. Using the same welding parameters, the process was carried out with inducing ultrasonic vibrations to the weld line at the amplitude of 15 μm. The microstructure of the specimens was observed with optical and scanning electron microscopy. The results indicate that a very fine microstructure is obtained in UaFSW with respect to that of conventional FSW. Moreover, β-Mg₁₇Al ₁₂ coarse dendrites are segregated to very fine and partly spherical particles that homogeneously distribute in α-Mg matrix. This remarkably-modified morphology of microstructure attributed to severe plastic deformation comes from ultrasonic vibration and friction stirring effect. Tensile and hardness tests were performed to evaluate the mechanical properties of the welds. According to the results, the vibration greatly improves the mechanical properties of the conventional FSW joint. The tensile strength and hardness are increased from 195 MPa and HV 79 in conventional FSW to 225 MPa and HV 87 in UaFSW, respectively.

Key words: ultrasonic assisted friction stir welding; AZ91 magnesium alloy; microstructure; mechanical properties