Effect of the microstructure of IMCs and zinc accumulation on the mechanical properties of aluminum/galvanized steel joints in the VP-CMT process

Abstract

Abstract By employment of variable polarity cold metal transfer welding, 1.5 mm 6451 aluminum (Al) alloy sheets and 0.8 mm DC56 galvanized mild steel sheets were welded with ER1100 filler wire to clarify the correlation between the heat input, microstructure characteristics and tensile-shear strength. It was found Al-Zn hypoeutectoid dendrites formed in the weld toe, where shrinkage was evident at the dendritic clearances. Intermetallic compounds (IMCs) formed at the Al/steel interface were composed of a dominant Fe2Al5 phase on the steel side and a minor FeAl3 phase on the Al side. As the heat input increased from ∼82.8 J/mm to ∼443.7 J/mm, the maximum thickness of Fe2Al5 increased from ∼3.7 μm to ∼18.7 μm. The joint strength increased first with increasing heat input, then decreased as the heat input further increased. The joint strength was primarily determined by two competitive factors, the Zn accumulation and the IMC growth. The Zn accumulation tended to occur near the interface of weld with low heat input, leading to the harmful Al-Zn structure with interdendritic shrinkage. IMC overgrowth would be produced when excessive heat input was applied, resulting in embrittlement of Al/steel interface. It was suggested to improve the joint strength by the reduction of the Zn accumulation and the inhibition of the IMC growth through a moderate and evenly distributed heat input.