Interface characteristics and reaction mechanism of steel/Al welds produced by magnetic field assisted laser welding-brazing

Abstract In this paper, we describe experimental laser welding-brazing aluminum alloy to galvanized steel using an external magnetic field. The morphology, microstructure and phase transition at interface was carefully investigated using scanning electron microscope, energy diffraction spectrum and X-Ray Diffractometer. The reaction mechanism of steel/Al welds was also discussed. The results showed that the banded structures with zigzag pattern morphology were produced near the steel matrix. The thickness difference at interface was closely associated with characteristics of laser heat source. The introduction of Zn element in filler metals induces the presence of η-Al5Fe2Zn0.4 phase and δ-FeZn10 phase at the steel interface. These non-brittle phases have good ductility, which can effectively reduce the susceptibility of hot cracking of weld metal and improve the behavior of interface bonding. Meanwhile, fine columnar crystals and eutectic structures formed near the Al matrix as the result of dissolution of Al matrix and regulation of the ampere force. In addition, it has been also found that the metallurgical reaction at interface can be mainly divided into three stages: the solute transfer process, the initiation and the growth of non-brittle phases. The driven effect of magnetic field on molten pool can effectively control the size and distribution of non-brittle phases such as Zn-Al eutectic phases, greatly improving the interfacial performance.