Research On The Process And Structure Properties Of Friction Stir Lap Welding For Dissimilar High-strength Aluminum Alloys

In this work, two extensively used high-strength aluminum alloys--2024-T3and7075-T6were chosen as the research object and friction stir lap welding experimentswere performed. The marco and microstructures, internal and surface defects, Vickershardness, tensile shear properties, fracture mode and mechanism of lap joints wereinvestigated in detail. The influences of lap sequence of upper and lower sheetmaterial, welding speed, rotational speed on welding defects and joint strength wereanalyzed.The2024-7075friction stir lap joints can be divided into four zones: Base metal(BM), Heat Affected Zone, Thermo Mechanically Affected zone (TMAZ) and StirZone (SZ). The SZ comprises fine equiaxed grains and the grains at upper SZ arelarger than those at lower SZ. When2024alloy acts as the upper sheet (2024/7075),the boundaries between HAZ and BM, TMAZ are obvious. At joints that7075alloyacts as the upper sheet (7075/2024), there is a distinct HAZ below the SZ. In addition,the weld surfaces of2024/7075joints are coarse, especially at300mm/min weldingspeed severe peeling occurs. Aggregated voids defects appear in2024/7075joints andsingle void defects appear in7075/2024joints.Hook defect appears in all lap joints in this experiments. The hook’s verticalextending reduces the effective sheet thickness (EST) while its horizontal extendingreduces the effective lap width (ELW). At low welding speeds, the hook extends alarge distance upwards and the morphology of hook on AS and RS is similar. At highwelding speeds, the hook on AS of7075/2024joint extends almost vertically whilethat on RS extends largely into the SZ. Both EST and ELW in2024/7075jointsincrease with the increase of welding speed. In7075/2024joints, the EST increaseswith increasing welding speed and the ELW decreases largely at high welding speed.The lap sequence of lower and upper sheet material as well as processingparameters affects the tensile shear properties of lap joints. At low welding speeds,7075/2024joints show high strength while2024/7075joints show higher strength athigh welding speed. The joint strength increases with the increase of welding speedand decreases with the increase of rotational speed as well as the value of rotationalspeed/welding speed (ω/υ). The lap joint attains the highest strength when the ω/υvalue is5. The2024/7075joint obtains the highest strength (138.86MPa) at1500rpm, 300mm/min and the7075/2024joint obtains highest strength (132MPa) at900rpm,180mm/min.There are three fracture modes in lap joints during tensile shear tests: tensilefracture TMAZ/SZ boundary, shear fracture from lap interfaces and mix mode of theabove two ones. With the increase of welding speed, the lap joints fracture in tensilemode, mix mode and shear mode in turn. All the failure joints show mix fractures ofmain ductile features and a few brittle features.