ANALYSIS OF MICROSTRUCTURE AND MECHANICAL PROPERTIES OF ALUMINIUM ALLOYS USING HYBRID PROCESSING (GMAW+FSP)

Abstract

The dissimilar joining of AA6082 and AA7075 alloys addresses critical demands in aerospace and automotive sectors by combining AA6082’s corrosion resistance with AA7075’s high strength-to-weight ratio. However, conventional MIG welding of these alloys often induces porosity, hot cracking, and HAZ softening, particularly in AA7075 due to its elevated zinc content. Friction Stir Processing (FSP), a solid-state method leveraging severe plastic deformation and dynamic recrystallisation, offers a viable route to refine weld microstructures and mitigate such defects. In this study, AA6082 and AA7075 plates (160 × 80 × 5 mm) were joined via MIG welding using ER-5356 filler wire under argon inert gas and best parameters are 150 A, 15 cm/min, 22 l/min. Post-weld, specimens underwent one, two, or three FSP passes using H13 steel tool with optimal parameters are 1200 rpm, 20 mm/min traverse, 3° tilt angle, and all passes in the same direction to ensure uniform strain. Mechanical performance was evaluated through tensile tests and Vickers hardness, while microstructural evolution was characterized by Optical microstructure (OM), Field emission scanning electron microscopic (FE-SEM), and Electron backscatter diffraction (EBSD). Tensile testing revealed that one-pass FSP increased UTS with one-pass 190 MPa to 210 MPa, and with two-pass 210 MPa to 235 MPa, whereas with third-pass slightly reduced 235 MPa to 213 MPa but achieved the highest elongation (14%). FESEM fractography confirmed a shift from cleavage-dominated to dimpled ductile fracture with increased FSP passes. Hardness measurements showed HAZ softening at 91.2 HV in conventional welds; one, two, and three pass FSP treatments elevated nugget hardness to 110, 120, and 115 HV and maintained HAZ hardness at 115, 110, and 95 HV, respectively. vi Optical microscopy documented a progressive reduction of porosity and improved fusion, culminating in a defect-free, equiaxed grain stir zone after three passes. FESEM grain-size analysis demonstrated refinement from 21 μm in as-welded joints to 3 μm after three-pass FSP, with correspondingly uniform Mg2Si and MgZn2 precipitate distributions. EBSD confirmed increased high-angle grain boundary fractions and pronounced shear textures in multi-pass specimens, correlating with enhanced toughness. Multi-pass FSP markedly improves dissimilar AA6082 and AA7075 weld integrity by refining grains, stabilising precipitates, and reducing defects. Two-pass FSP optimises strength, while three-pass FSP maximises ductility and microstructural homogeneity, suggesting a tailored FSP strategy for high-performance structural applications.