Microstructure evolution and phase transition at the interface of steel/Al dissimilar alloys during Nd: YAG laser welding

Abstract

In this paper, we conducted fiber laser welding of steel/Al dissimilar alloys. The microstructural evolution and phase transition at the interface of laser welded joints were carefully investigated using an optical microscope, scanning electron microscope, energy diffraction spectrum and X-ray diffractometer. The forming mechanism of cracking was also studied by experimental and simulated methods. We determined that microstructural segregation at the interface of steel-Al joints was closely associated with the Al content in the steel side. Multiple Fe-Al intermetallic compounds (IMCs), such as Fe2Al5, FeAl3 and FeAl, were produced at the interface. Formation of these IMCs can improve the hardness of the interface layer dramatically. The continuous or semi-continuous distribution of Fe-Al IMCs with brittleness decreased the plastic and ductility of material itself, which greatly increased the susceptibility of solidification cracking in the fusion zone. The temperature gradient in the interface layer was a key factor affecting the residual stress. A method of reducing the heat input was proposed to control the occurrence of cracks. The simulated results were consistent with the experimental results, which proved the correctness and feasibility of the method.