Microstructural evolution during friction stir welding of dissimilar aluminum alloy to advanced high-strength steel

One type of advanced high-strength steel, transformation-induced plasticity (TRIP) 780 steel, has been successfully welded to aluminum alloy Al 6061-T6 using friction stir welding (FSW) technique. The major Al-Fe interface in the steel side has been analyzed in detail under various welding conditions, where a thin layer of intermetallic compound (IMC) with a thickness of less than 1 mu m can be generally observed and the composition was identified to be either FeAl or Fe3Al. This thin IMC layer can be shown to be beneficial for joint strength. Optical microscopy and scanning electron microscopy showed the weld nugget was distributed with sheared-off steel fragments encompassed IMC layers or simply IMC particles. Finally, a stirred-over steel strip embedded in the aluminum matrix was revealed, which would determine the failure mode and is crucial to joint quality based on tensile test results.