Recently research at the Ohio State University is developing methods that combine the benefits of ultrasonic welding and resistance spot welding. Y. Lu and co-workersL-10 developed the ultrasonic plus resistance spot welding process (U+RSW). In this process thin aluminium sheet was joined with steel sheet via ultrasonic welding, followed by RSW between the aluminium side of previously sheet and aluminium sheet as shown in Figure 1. In this study they used 1-mm-thick AA6061-T6 to 0.9-mm-thick AISI 1008 steel with 0.4-mm-thick AA6061-T6 as the insert.
The U+RSW method showed improved mechanical properties (Figure 2), tensile shear strength and energy absorbed prior to failure. It has shown a relatively thin intermetallic layer.
Another research group have developed ultrasonic resistance welding process (URW), that apply in situ ultrasonic waves during the resistance spot welding process.S-17 The schematics of the process is shown in the Figure 3. Al-6061T6 and electrogalvanized TRIP 780 with dimensions of 70Lx 25Wx 1.6T mm3 and 70Lx 25Wx 1.3T mm3 respectively were used in this study.
The URW process have shown improved mechanical properties and defect free interface compared to the RSW process. In situ ultrasonic vibration facilitated the breakdown of the surface oxide and contaminants, which modifies contact resistance and heat generation rate accordingly. If melting occurs, ultrasonic vibration promoted wetting of the molten aluminium over the steel interface and resulted in the formation of defect-free, fully bonded interface as shown in Figure 4. Figure 5 compares the mechanical properties between the RSW & URW process. Comparing URW with traditional RSW, up to 300% increase in strength and more than 150% increase in displacement to failure is reported. SEM images of the fractured surface revealed that ultrasonic waves eliminated the eggcrate morphology generally observed in the RSW welds fractured surface, which is a typical representative of solidification cracking.