Effect Of Root Weak Connection On Mechanical Properties Of Friction Stir Welded Joint Of 2195 Al-Li Alloy

With the wide application of Al Li alloy in aerospace and other fields,people pay more and more attention to the connection problem.Traditional fusion welding can realize the connection of Al Li alloy,but the joint is easy to produce welding defects such as pores and cracks.Friction stir welding(FSW)is a kind of solid phase bonding technology,which has incomparable advantages compared with traditional fusion welding.It has been widely used in the bonding of light alloy metals.At present,the research direction of friction stir welding of Al Li alloy is more focused on optimizing the welding process to obtain the best welding joint.There are few researches on improving welding efficiency.Increasing the welding speed is one of the most direct methods to improve the welding efficiency,but in friction stir welding,the increase of welding speed will make the joint vulnerable to weak root connection defects,which will affect the joint performance.In this study,2195 Al Li alloy with 7.5mm thickness was used as the research object.With the continuous improvement of welding speed,the effect of root weak connection on the mechanical properties of the joint was studied by thinning the root weak connection.It can not only improve the mechanical properties of the joint,but also get the best bottom thinning thickness of the joint under different welding speed,and improve the welding speed.In addition,the applicability of this method to other alloy materials is verified.The main conclusions are as follows:(1)When the welding speed is gradually increased from 100 mm/min to 500mm/min,no obvious welding defects are found in the weld,but at each welding speed,root weak connection defects with different thickness appear at the bottom of the weld,with the maximum thickness of 0.64 mm.If there is no other internal defect in the weld,the mechanical properties of the joint decrease with the increase of the thickness of the weak connection defect at the root;When there are other welding defects in the weld,the influence of weak connection defects on the mechanical properties of the joint will be reduced.(2)When the welding speed is less than 400mm/min,the tensile strength and elongation of the joint first increase with the thinning thickness of the root weak connection defects,until the root defects are completely removed,the mechanical properties of the joint reach a stable state.When the welding speed is 300 mm/min,the tensile strength of the joint is 433 MPa,which is 69.6MPa higher than that without thinning,and the welding coefficient is 72.3%.When other welding parameters are fixed,the welding speed can be increased to 400mm/min without reducing the mechanical properties of the joint.At this time,the tensile strength and elongation of the joint reach424 MPa and 9.8% respectively,which is still higher than that of the joint with complete penetration,and the performance is stable,which significantly improves the efficiency of friction stir welding.(3)When the welding speed is above 100 mm/min and the bottom is not thinned,the joints are broken in the nugget area corresponding to the joint surface.According to the macro and micro fracture analysis,the fracture source is at the bottom of the weld,the crack first starts from the bottom defect and gradually extends to the middle and upper part of the weld,and finally realizes complete fracture.After subtracting the root weak connection defect,the fracture position and the fracture source of the joint transfer.When the welding speed is 100 mm/min,the fracture position and the fracture source transfer to the heat affected zone;when the welding speed is more than 100 mm/min,the fracture position is still in the nugget Zone,but the crack source transfers from the bottom joint to the inside of the nugget zone.(4)The mechanical properties of FSW joints can be improved greatly by thinning the weak connection defects at the root.The tensile strength and elongation of 7075 aluminum alloy joint can be increased by 154 MPa and 9.8% respectively,and the welding coefficient can be increased to 85%.However,the improvement of mechanical properties of AZ31 magnesium alloy joint by this method is relatively limited.