Sonotrode design and spatial-temporal configuration strategy of ultrasonic vibration for aluminum/steel resistance spot welding

Exploring the rational and effective spatial-temporal configuration strategy of ultrasonic vibration for the ultrasonic longitudinal vibration-assisted resistance spot welding (UA-RSW) is significant important for the manufacture of high-strength and reliable aluminum/steel welded joints. However, systematic sonotrode design methods, spatial configurations (electrode-sonotrode geometric coupling, vibration application position), and temporal configuration (vibration duration) have been severely neglected in related work. This work proposes a systematic method for sonotrode design in UA-RSW process, achieving frequency matching between the ultrasonic excitation system and sonotrode. The effects of ultrasonic vibration application position and electrode-sonotrode combination on the macro-morphology and mechanical performance of aluminum/steel UA-RSW joints were systematically investigated. Experimental results revealed that directing ultrasonic vibrations toward the steel sheet increased energy transfer efficiency, which enhances sheet melting rate and thermal conductivity, ultimately increasing aluminum nugget diameters. Further studies demonstrated that the application of a radius-truncated electrode-sonotrode combination promoted radial Joule heat transfer and inhibited thickness reduction of the aluminum alloy sheet, achieving an average tensile-shear strength of 6.14 kN. The study on ultrasonic temporal strategy revealed that extending ultrasonic vibration duration from 350 ms to 550 ms effectively promoted grain refinement in both the aluminum melting zone and the heat-affected zone with the average grain size at the center region of the aluminum melting zone reduced by 54.3 %. Additionally, the regulation effect of ultrasonic vibration on the joint microstructure was significantly diminished after the solidification of the aluminum alloy. The experimental results demonstrated that ultrasonic vibration delayed by 100 ms post-current removal can effectively balance the IMC layer growth and grain distribution. These systematic investigations can provide both theoretical guidance and practical methodology for aluminum/steel UA-RSW in automotive applications.