Improving arc joining of Al to steel and Al to stainless steel

Gas-tungsten arc welding (GTAW) with controlled heat input could avoid the formation of massive brittle Al–Fe intermetallics during solidification in dissimilar-metal joining of aluminum to steel. In the first part of the present study an Al–Mg alloy was joined to galvanized steel by GTAW with an Al–Si filler metal. The weld solidification microstructure was explained based on the Al–Mg–Si phase diagram. Ultrasonic vibration during GTAW increased the joint strength by 27%. Grain refinement, decreased Fe–Al intermetallics, and increased microhardness in both the heat-affected zone and the weld were observed, thus explaining the improved joint strength. In the second part, an Al–Mg alloy was joined to 304 stainless steel using a Zn-15Al filler metal with a noncorrosive flux core. The weld solidification microstructure was explained based on the Al–Zn–Mg phase diagram. Postweld heat treatment (PWHT) at 280 °C for 30 min doubled or even tripled the joint strength depending on the Al-sheet thickness. Dissolution of coarse Zn-rich particles at the weld/steel interface was observed after PWHT, thus eliminating these weak particles from the interface. The formation of Zn-rich particles near the interface and their dissolution by PWHT were explained based on the Al–Fe–Zn phase diagram.