Interfacial Microstructure and Strength of Steel to Aluminum Alloy Lap Joints Welded by a Defocused Laser Beam

Abstract

Lap joining was carried out using a defocused laser beam for 1.2-mm thick A6111 aluminum alloy plate and 1.0-mm thick low-carbon steel (SPCC) plate. The defocused laser beam was applied on the upper surface of the SPCC plate and the beam traveled under various welding conditions. When the depth of the molten SPCC pool was maintained at around 90% of the SPCC plate thickness, the selected area near the upper surface of the A6111 plate melted to form a semi-elliptical molten pool. Consequently, the solid SPCC and molten A6111 came in contact at the interface and joining was achieved through the solid–liquid reaction. The interfacial strength of the lap joint was controlled by the morphology of the Fe–Al intermetallic compound (IMC) layer. Improved strength was obtained when the weld interface was covered with a thin (about 1-μm thick) and continuous IMC layer. The estimated maximum shear stress of the lap joint was about 70% of that for A6111-T4. Welding conditions providing excessive heat input increased the thickness of the IMC layer and caused the formation of needle-like IMC and cracks in the A6111 molten pool. Such microstructures reduced the joint strength. X-ray diffraction analysis for the weld interface revealed that the IMC layer consisted of various kinds of Fe–Al base equilibrium phases (Al13Fe4, Al5Fe2, Al2Fe, FeAl, and Fe3Al) and a non-equilibrium Al6Fe phase.