Microstructure And Fatigue Properties Of 6061 Aluminum Alloy Laser Hybrid Welded Joint

With the gradual implementation of the policy of energy saving and emission reduction and the enhancement of people's awareness of environmental protection,the lightweight discretion of aluminium alloy has been more and more widely used in automobiles,high-speed railways and other transportation vehicles.Laser hybrid welding,a new light pollution welding method,has also been more and more used in various industrial production.In this paper,the microstructures,microhardness,tensile properties and fatigue properties of 6061 aluminium alloy laser composite welded joints are analyzed.The effect of pre-ratcheting test on fatigue properties of 6061 aluminum alloy laser clad welded joints and the initiation and propagation of fatigue cracks were studied.The weld microstructures of 6061 aluminum alloy laser composite welded joints can be divided into three types: long and slender columnar crystals formed in the fusion zone;coarse equiaxed grains formed in the upper part of the weld due to the large arc action at the center of the weld;and fine grains formed in the lower part of the weld due to laser action,rapid melting and cooling.Equiaxed crystal.The hardness of laser welded joint of6061 aluminium alloy presents certain regularity.In the horizontal direction of the weld,the hardness of the center of the weld is the lowest,and the hardness of the quenched zone is basically the same as that of the base metal,but there is obvious softening zone in the heat affected zone.In the vertical direction of the weld,the hardness of the lower part of the weld is obviously higher than that of the upper part of the weld.The laser clad welded joint of 6061 aluminum alloy exhibits high plasticity in tensile test.The yield strength is 48 MPa,the ultimate tensile strength is 236 MPa,and the total elongation is 12.6%.The ratchet strain shows that the ratchet strain of 6061 Al alloy laser-MIG composite welded joint increases with the increase of average stress and stress amplitude,and has the characteristics of cyclic softening first and then hardening.The fatigue limit of pre-ratchet and non-ratchet specimens is 87 MPa and 84.1 MPa respectively.When the stress level is lower than 200 MPa,the fatigue life of 6061 aluminum alloy laser composite welded joint can be improved by ratcheting test.In the low stress level ratcheting test,the surface of thespecimen is smooth without damage,but there are a lot of slip bands in the grain.In the high stress level ratcheting test,there will be a lot of cracks on the surface of the specimen,and the existence of cracks will greatly reduce the fatigue life of the welded joint.The fatigue crack mainly originates at the pore of the welding defect.After the fatigue crack initiation at the pore,the fatigue crack will enter a stable growth stage under the action of tensile stress.In this stage,a large number of river-like patterns and ridge-like patterns can be observed in the fracture surface,which indicates that the welded joint presents brittle fracture at this stage.However,there are a lot of equiaxed dimples in the fracture.The existence of dimples indicates that the fracture also presents the characteristics of ductile fracture.Therefore,the fracture mode of 6061 aluminum alloy laser composite welding joint is ductile-brittle mixed fracture.