Effects of ultrasonic vibration on resistance spot welding of transformation induced plasticity steel 780 to aluminum alloy AA6061

Highlights: • In situ ultrasonic vibration enhances resistance spot welding process. • The ultrasonic resistance welding process greatly improves AlFe weld properties. • AlFe bonding area increases with superimposed ultrasonic. • Ultrasonic vibration removes solidification cracking at AlFe interface. • Ultrasonic vibration breaks off surface layer and promotes wetting. -- Abstract: A newly developed joining technique, ultrasonic resistance spot welding (URW), where high frequency ultrasonic vibration is effectively integrated into resistance spot welding (RSW) process, has been applied for joining transformation induced plasticity (TRIP) steel 780 to aluminum alloy Al 6061. Comparing URW with traditional RSW, up to 300% increase in strength and more than 150% increase in displacement to failure is achieved. Light optical microscopy (LOM) and scanning electron microscopy (SEM) revealed cracks and wide interfacial debonded regions in conventional RSW AlFe welds. With the assistance of ultrasonic vibrations, these interfacial cracks are effectively removed and a thin AlFe intermetallic layer (IMC) of around 3 um thickness is formed. SEM images of the fractured surface revealed that ultrasonic waves eliminated the eggcrate morphology generally observed in RSW welds fractured surface, which is a typical representative of solidification cracking. In situ high speed videos showed that ultrasonic vibration can help break off surface contamination and oxides as well as improving wetting of melted aluminum over the AlFe interface.