Mechanism Of Refill Friction Stir Spot Welding Between Aluminum Alloy And Steel

Refill friction stir spot welding(Refill FSSW)is of great advantage to join light metals,due to its low-heat-input nature,grain refinement in weld and elimination of the keyhole inevitably leaving by the tool retreating in conventional friction stir spot welding process.Owing to the advantages,this technique has been applied by Kawasaki Heavy Industries in Japan and Helmholtz-Zentrum Geesthacht in Germany to join aluminum or magnesium alloys.On the other hand,the dissimilar metals welding between aluminum alloy and steel can achieve the lightweight of automobiles,improve the efficiency of energy,reduce the emission of waste gas and protect the environment.Based on the advantages of Refill FSSW and the wide application of aluminum alloy/steel hybrid structures,this thesis aims to investigate the effect of main welding parameters(sleeve plunge depth and rotation speed)and zinc coating on the mechanical property and microstructure of aluminum alloy/steel dissimilar metals refill friction stir spot welded joint,and reveal the joining mechanism.Not only does this research have theoretical significance,but also it can give a practical guidance to manufacture industries.In this thesis,two kinds of aluminum alloy sheets for automobile(1.0 mm thick Novelis AC 170 PX and 1.5 mm thick Aleris Superlite 200 ST aluminum alloy)were overlap welded to two types of low carbon steel sheets(1.0 mm thick ST16 steel and 1.2 mm thick ST06 Z galvanized steel sheets)to produce two-layer dissimilar metals joints by Refill FSSW.The diameter of spot weld is 9.0 mm,which is located at the center of 25 mm × 25 mm overlap area.The mechanical property was evaluated through tensile/shear tests and cross-tension tests.In addition,the microstructure,composition,fractogragh and fracture path of aluminum alloy/steel joints were analyzed with electron probe microanalysis(EPMA),field emission scanning electron microscope(FE-SEM)and X-ray diffraction(XRD).The maximum average tensile/shear fracture load and cross-tension fracture load of the Novelis AC 170 PX aluminum alloy/ST16 steel joints are 2495 N and 337 N,respectively.For Aleris Superlite 200 ST aluminum alloy/ST16 steel joints,the maximum average tensile/shear fracture load reaches 3745 N with the maximum average cross-tension fracture load of 1073 N.All samples failed through the interface of aluminum alloy/steel joints during tensile/shear tests and cross-tension tests.When the sleeve does not touch to the steel surface,the influence of sleeve plunge depth on tensile/shear fracture load is not obvious.The metallurgic bonding was achieved and a thin Fe-Al intermetallic compound(IMC)layer formed at interface of sleeve affected zone.However,the mechanical performance sharp increases when the sleeve plunges into steel sheet,attributing to metallurgic and mechanical bonding between the aluminum alloy and steel sheets.Meanwhile,this process also brings about severe tool wear and shortens its service life.Besides,the rotation speed has little influence on the mechanical performance of joint.The maximum average tensile/shear fracture load and cross-tension fracture load of the Novelis AC 170 PX aluminum alloy/ST06 Z galvanized steel joints are 3044 N and 301 N,respectively.For Aleris Superlite 200 ST aluminum alloy/ST06 Z galvanized steel joints,the maximum average tensile/shear fracture load is up to 4500 N with the maximum average cross-tension fracture load of 359 N.All samples failed at the interface of aluminum alloy/steel joint.Compared with the results of aluminum alloy/ST16 steel joints,the joints between aluminum alloy and ST06 Z galvanized steel exhibit better mechanical performance in tensile/shear tests,and all tensile/shear samples failed through the aluminum alloy/steel interface on aluminum alloy side at pin affected zone.The IMC layer as thin as 0.68 ?m was found at interface of sleeve affected zone and seemed to be Fe Al3.Zn-O rich zone was observed at interface on aluminum alloy side,where microfissures and segregation of Zn,O and Mg formed at the grain boundary.Based on the fracture analysis,it is assumed that the zinc and oxygen rich zone at the center of spot weld is the weakest part of joint.During tensile/shear test,the joints failed mainly through brittle fracture mode with intergranular fracture feature.According to the Zn distribution,the flow thermoplastic material in weld of Refill FSSW can be analyzed,and the spot weld can be distinguished as pin affected zone and sleeve affected zone.