Microstructure and fracture analysis of TIG weld-brazed aluminum-steel T-joints

Dissimilar joining of aluminum (Al) and steel (St) is essential for lightweighting in automotive and aerospace industries. However, their distinct physical and thermal properties present considerable welding challenges. This study investigates the microstructure and fracture behavior of TIG weld-brazed AA5083 aluminum to hot-dip galvanized St37 steel T-joints, using ER4043 and ER5356 filler metals. Optical and scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) characterized the intermetallic compound (IMC) layers at the Al-St interface. Results show that silicon (Si) in the ER4043 filler reduced the IMC layer thickness to 2 µm and modified its structure, leading to a more desirable interface compared to the ER5356 filler, which produced IMC layers up to 16 µm. Although zinc coating on the steel improved initial wettability, high welding temperatures led to substantial zinc evaporation and subsequent porosity within the weld metal, impairing joint quality. Tensile tests indicated that both weld-brazed joints fractured predominantly in a brittle manner. Despite thicker IMC layers in some regions, the ER5356 joint achieved a slightly higher peak load (1342 N) than the ER4043 joint (1263 N), indicating intricate relationships between IMC morphology, porosity, and filler metal composition that influenced joint strength. These results suggest that a higher preheating temperature (300–400°C) could mitigate zinc evaporation and enhance overall joint performance.