Study On Friction Stir Spot Welding Of DP780Advanced High Strength Steel

In recent years, with the rapid development of the automobile industry, the problems, such as energy consumption, environmental pollution and traffic security become very serious. Automotive light-weight become the develop trend of auto industry. As for the combination of high strength and toughness, energy absorption, high hardening index, the advanced high strength steel (AHSS) wisely used in vehicle production. Dual phase steel is one excellent kind of AHSS, and wisely used because of it’s low price.In the application of AHSS,the welding problem became urgent to be solved.The resistance spot welding is the main connection technology in automobile industry. However, due to the massive alloy elements in AHSS and high cooling rate while resistance spot welding process, coarseMartensite will generate, which worsen the toughness of welding joints. As a kind of high efficiency, non-pollution solid connection technology, friction stir spot welding (FSSW) get more and more attention. In this paper, using friction stir spot welding technology welding high strength automobile steel DP780by various welding parameters,detailed discussed the DP780organization evolution law in the process of welding and the mechanical properties of welded joint,and finally optimize the weld parameters ofFSSW.Weld joints of DP780is made up of stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ)、heat affected inter-critical region(HAZIC) and base metal(BM). In the low heat input situation of500rpm, the temperature of SZ is above Ac3slightly. For the effect of deformation induced transformation, the TMAZ and SZ contain Martensite and Ferrite. While the rotate speed above1000rpm, the temperature of weld joint SZ is far high than Ac3, the TMZA and SZ contain Martensite only; The HAZIC contains Martensite、Ferrite and Bainite, the closer to the BM, the more content of the Ferrite. When the shoulder insert depth is0.7mm, the welding rotation rate of1000rpm welded joint’s mechanical property is the best, reach as18.1kN. Under the same rotate speed, appropriate increase of shoulder insert depth was beneficial to the improvement of the mechanical properties of weld joints. The mechanical property of weld joints is related to the insert depth of the tool and temperature.