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 degrees 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. (C) 2011 Elsevier B.V. All rights reserved.