Microstructures and Mechanical Properties of Dissimilar Al/Steel Butt Joints Produced by Autogenous Laser Keyhole Welding

Dissimilar Al/steel butt joints of 6.0 mm thick plates have been achieved using fiber laser keyhole welding autogenously. The cross sections, interface microstructures, hardness and tensile properties of Al/steel butt joints obtained under different travel speeds and laser beam offsets were investigated. The phase morphology and thickness of the intermetallic compound (IMC) layers at the interface were analyzed by scanning electronic microscopes (SEM) using the energy-dispersive spectrometry (EDS) and electron back-scattered diffraction (EBSD) techniques. The results show that travel speeds and laser beam offsets are of considerable importance for the weld shape, morphology and thickness of IMC layers, and ultimate tensile strength (UTS) of Al/steel butt joints. This proves that the IMC layers consist of Fe2Al5 phases and Fe4Al13 phases by EBSD phase mapping. Increasing laser beam offsets from 0.3 mm to 0.7 mm significantly decreases the quantity of Fe4Al13 phases and the thickness of Fe2Al5 layers at the interface. During tensile processing, the Fe2Al5 layer with the weakest bonding strength is the most brittle region at the interface. However, an intergranular fracture that occurred at Fe2Al5 layers leads to a relatively high UTS of Al/steel butt joints.