The influence of heat input on microstructure and mechanical properties for dissimilar welding of galvanized steel to 6061 aluminum alloy in a zero-gap lap joint configuration

The microstructure of dissimilar joint between galvanized steel and 6061 aluminum alloy was studied by experiments and numerical simulation. Three types of experiments including full factor experiments to establish processing window, four chosen parameters to analyze macro profile of weld joints, and two parameters closing to upper and lower limits of processing window were carried out in this study. Besides two common intermetallic compounds (lamellar IMC and acicular IMC) which were reported before, other two new intermetallic compounds (IMC), namely stellate IMC and skeleton IMC were found at the joint under high linear energy density (LED). The chemical compositions and possible phase types of different IMCs observed were determined according to energy dispersive spectroscopy (EDS) results. The temperature histories during welding were simulated based on a finite volume model to further investigate the influence of LED on diffusion process which was thought to be the decisive factor for IMC growing. The growth orientation and phase morphology were determined by temperature histories, local iron atom fraction and constitutional supercooling. The increasing linear energy affected both macro morphology and microstructure of weld joints. It could be concluded that the increasing linear energy was harmful for mechanical properties because it increased thickness of lamellar IMC and brought some new brittle phases which led to some microcracks.