Study On Dynamic Mechanical Properties Of Friction Stir Spot Welding In Overlap Joints Of Aluminum Alloy

The 6082-T6 aluminum alloy is characterized by high specific strength,excellent corrosion resistance and extrusion performance,which has been widely used in railway vehicles industries.Friction stir spot welding has been shown to be a new type of lap type solid connection method.So far,the research on joint dynamic mechanical performance of6082-T6 aluminum alloy FSSW joint is relatively rare.This article mainly aimed at 2.5 mm6082-T6 aluminum alloy to systematically study the quasi static mechanical properties,high cycle fatigue properties and dynamic impact properties of the FSSW joint under different welding process.This research has a important guiding significance on the application of spot welding joint.The result showed that the maximum tensile shear strength of RFSSW is up to 8.05 KN,while the FSSW and BFSSW are 4.75 KN and 6.0 KN respectively on Quasi static mechanical properties experimental.It's due to the effective welding area of RFSSW is more than the others,so is to the deformation resistance.It is also different in the way of fracture on the loading process about these three methods.The BFSSW is combined with FSSW and Bonding,therefore,the fracture mode of the BFSSW is consists of the glue layer and the spot joint.High cycle fatigue performance test results showed that: under the same load level,the fatigue performance of the RFSSW is of the best,BFSSW takes second place,and FSSW is the worst.The fatigue test data of different spot welding joints have good regularity,and the load and fatigue life span displayed the exponential type function relation.After cyclic loading was in stable stage,the cyclic hysteresis energy of RFSSW,FSSW and BFSSW in a single loop respectively are 0.72 MJ/m3 and 0.35 MJ/m3 and 0.36 MJ/ m3.From macro fatigue fracture mode,we can observe that fatigue crack extend into the thickness direction along the interface gap,and RFSSW exist secondary cracks in the macroscopic fracture.According to defects of FSSW and macro fracture fatigue mode,we set up a simple model of fatigue crack propagation to assess the approach of fatigue crack extension and the fatigue life of FSSW.Split Hopkinson pressure bar test on four fraction of RFSSW joint were performed respectively.The results showed that the deformation coordination ability of the base metal,nugget zone and thermo-mechanically affected zone is very good in the process of dynamic load,and the dynamic impact resistance is stronger,while the dynamic impact resistance of the heat affected zone is relatively poor.Materials in base metal,nugget zone,thermo-mechanically affected zone and heat-affected zone have an impact absorbing energy of 9.068 MJ·m-3,9.85 MJ·m-3,8.36 MJ·m-3,and 1.34 MJ·m-3,respectively.Four groups of adiabatic shearing sensitivity are smaller.To analyze the mechanism of quasi static tensile shear fracture and high cycle fatigue fracture,the tensile shear fracture and fatigue fracture was observed by SEM: there is quasi cleavage plane area in crack extension of FSSW and BFSSW joint,while "tongue plate pattern" in the spreading area of RFSSW,and a dimple in rupture region.So the tensile shear fracture is a mixture of quasi cleavage fracture and toughness fracture.The tensile shear fracture rupture region of FSSW joint exists large amounts of equal-axis ductile voids,explaining its joint plastic is good.Three kinds of joint fatigue fracture are also a mixture fracture of quasi cleavage fracture and toughness fracture.