On the Joint Formation and Interfacial Microstructure of Cold Metal Transfer Cycle Step Braze Welding of Aluminum to Steel Butt Joint

Abstract

Cold metal transfer cycle step braze welding process was modified to successfully produce an aluminum alloy/Zn-coated steel linear joint with a closed square butt joint configuration. The braze-welded joint shows wide-spreading and low wetting angle of molten filler alloy on both sides of the steel. The butt joint is formed by the overlapping of individual spot braze welds during cold metal transfer cycle step braze welding. The microstructure at the front, backside, and face side interface of the Al/steel joint were characterized in terms of the crystal structure, morphology, and thickness of intermetallic compounds by scanning electron microscopy, analytical transmission electron microscopy, and energy dispersive X-ray spectroscopy. The interfacial structure of Al/τ5(H)-Al8Fe2Si(film-like)/θ-Al3Fe(rod-like)/θ-Al3Fe(500 nm)/η-Al5Fe2(2.5 µm)/Fe is formed at the front interface associated with high-temperature interaction between liquid Al alloy and Zn-coated steel. On the contrary, Al/τ5(C)-Al8(Fe,Mn)2Si/τ5(H)-Al8Fe2Si/θ-Al3Fe(1.2 µm)/η-Al5Fe2(500 nm)/Fe is present at the backside interface associated with relatively low-temperature interaction between liquid Al alloy and Zn-coated steel. The intermetallic compounds and morphology at the face interface associated with medium temperature interaction are similar to those at the backside interface. However, the thickness of the η-Al5Fe2 layer is only 220 nm, which is caused as a result of the interaction of molten Al filler alloy with the bare steel sheared surface without Zn coating. The mechanism for the formation of varied intermetallic compounds at the different interfaces is discussed.