Formation of Fe5Si3 precipitate in the Fe2Al5 intermetallic layer of the Al/steel dissimilar arc welding joint: A transmission electron microscopy (TEM) study

Abstract

The further strengthening of the Al/steel dissimilar joint is significantly meaningful for car lightweight development in automotive industry. It has been experimentally proved that the use of Si containing aluminum alloy filler material such as ER4043 grade welding wire can effectively improve the joint strength by reducing the FeAl intermetallic interface thickness. However, the specific functioning mechanism of Si has not been clarified explicitly thus controversy still exists. In the present research, an Al/steel dissimilar joint is generated using the variable polarity cold metal transfer (VP-CMT) technique. To address the suppression function of Si on the FeAl intermetallic interface growth clearly, the focused ion beam (FIB) is conducted to extract the testing sample right at the interface, which is subsequently investigated under TEM. According to the obtained results, the intermetallics formed at the FeAl interface are generally Fe4Al13 phase on the aluminum side and Fe2Al5 phase adjacent to the steel side. Also, rod-shaped Fe5Si3 precipitates are confirmed in the Fe2Al5 matrix near the steel side, rather than Si solid solutions or ternary Fe-Al-Si phases stated in the previous researches. As indicated by the dislocation distribution in the Fe2Al5 matrix, the generation of Fe5Si3 precipitates considerably suppresses the brittle Fe2Al5 fracture. Such phenomenon clarifies the Si element functioning mechanism of improving the Al/steel dissimilar joint quality.