Study On Microstructure And Mechanical Properties Of Refill Friction Stir Spot Welded Joints For Aluminum Alloys

Refill friction stir spot welding is developed by GKSS in 1999, which is proposed based on the principle of friction stir welding. Refill FSSW is a hot area due to whose advantages in joining light material. In order to clarify the weldability, AA2024-T3, 2024-T4, 5052-H112 and 7075-T6 were selected to investigate the basic issue in refill FSSW. The joining mechanism, influence of welding parameters on microstructure, welding defects and mechanical properties were studied. It focuses on the formation mechanism of defects and interaction with mechanical properties.Metallurgical properties and thermoplastic liquidity play a dominant role in refill FSSW process for dissimilar aluminum alloys, followed by parameters. Welds with compact structure can be manufactured for AA7075-T6 using rotational speed of 2100 rpm and welding time of 3-3.5 s. For AA2024-T4, welds with compact structure can be manufactured using rotational speed of 1800 rpm and welding time of 6 s. However, for AA5052-H112, defects-free welds can not be manufactured using rotational speed of 1500-2100 rpm and welding time of 3-5 s. For welds of AA2024-T4/5052-H112, welds with metallurgical bonding and compact structure can be manufactured using rotational speed of 1200-1500 rpm and welding time of 3-5 s. For welds of AA2024-T3/7075-T6, defects-free weld can not be manufactured using rotational speed of 1400-2000 rpm and welding time of 3-5 s. Rotational speed should be higher than that in FSW(?1500 rpm).Microstructure refill FSSW welds was analyzed, which can be devided into stir zone(SZ), thermo-mechanical affected zone(TMAZ), heat affected zone and base metal(BM). For AA7075-T6 welds, TMAZ was formed in SZ due to particularity of material flow. For AA5052-H112, grain sizes are different in SZ and exhibits layered structure.Defects were formed in the weld, in which hook is the inherent defect, which can not be eliminated by optimized parameters. Annular weak link was formed at the retractable line. Bonding ligament only occurs in welds of alclad aluminum alloys, whose distribution plays a predominant in determining welds’ mechanical properties.Liquation crack was detected in AA2024 and 7074 welds, whose formation in AA7075 is associated to the formation of liquation film α-Al+η or α-Al+η+S, and liquation film α-Al+S in AA2024.Distribution of hardness in the weld was analyzed and discussed, which indicates that the hardness distribution in upper sheet exhibits a W-shaped appearance, W or U-shaped appearance in lower sheet. For AA7075 welds, the maximum hardness can reach 95% of BM, for AA2024 welds, the maximum hardness is almost the same as that of BM, for AA5052, the maximum hardness can reach 85.9% of BM.Weld strength and failure modes were studied in detail by tensile test. Overall, the weldability of solid solution strengthening aluminum alloy is more excellent than that of heat-treatable aluminum alloys, and the weldability of AA2024 is better than that of AA7075. The tensile strength of AA7075-T6 weld is 22.4% of BM, 37.9% for AA2024-T4 weld and 56.4% for AA5052-H112 weld. For dissimilar welds of AA2024-T4/5052-H112, the weld strength can reach 39.7% of AA2024, however, which can reach 68.9% of AA5052. For dissimilar welds of AA2024-T3/7075-T6, weld strength reaches 27.5% of AA7075 when AA7075 was set as upper sheet, however, it can reach 44.5% of AA2024 when AA2024 was set as upper sheet.Failure path mainly locates at interface and retractable line. The failure occurs as plug type fracture when crack propagates along the retractable line, shear fracture when propagates along interface, failure occurs as mixed fracture when crack propagates along both interface and retractable line. When weld is subjected to cross-tension loading, failure occurs in multifarious modes and inferior weld strength. Defects accumulate at interface, plug type fracture is the optimal failure mode.