This paper investigates the efficacy of Friction Stir Diffusion Bonding (FSDB) for joining aluminum (AA 1050) to zinc-coated steel (galvanized carbon steel) in a lap-joint configuration. The process was conducted with a tool rotation speed of 950 rpm, a welding speed of 20 mm/min, and a pin penetration depth of 2.8 mm, ensuring the pin remained above the steel surface. FSDB, a variant of Friction Stir Welding (FSW), differs primarily in its nonpenetrative approach to harder materials, particularly steel. This unique FSDB approach minimizes tool wear and reduces thermal stresses. The zinc coating on steel plays a crucial role in promoting metallurgical bonding with aluminum through the stirring action of the tool. Scanning Electron Microscopy (SEM) and energy-dispersive Xray Spectroscopy (EDS) analyses of the interface layer revealed a continuous Al-Fe-Zn solid solution, which contrasts with Al-Fe-Si intermetallic layers (IMCs) typically observed in Al/St joints produced via Weld-Brazing (WB). The reduced heat input in the FSDB, which can be attributed to the lack of tool penetration into the steel, prevented the evaporation of zinc and thus minimized the porosity within the joint. Mechanical testing, including tensile shear strength and microhardness assessments. The heat-affected zone (HAZ) and stir zone (SZ) exhibit hardness values ranging from 44.9 to 55.5 HV, indicating that FSDB joints exhibit superior mechanical properties and less brittleness than WB-fabricated joints. FSDB exhibited superior joint quality (3830 N fracture load for 10 mm width), less brittleness (fracture from Al and not from the interface), elimination of IMCs layer, minimized porosity, and controlled thermal input making it a promising technique for dissimilar metal joining in lightweight applications.
