Non-weld-thinning Friction Stir Welding And Keyhole Repairing Of 2195 Aluminum-lithium Alloy

Aluminum-lithium alloys have the attribute of lightweight and high strength,which have a great application prospect of the aerospace engineering.Nevertheless,the welding issues of Al-Li alloys limit their development.During welding,element loss of Li reduces the alloying degree of welds,which leads to the decreasing of joint strength.Friction stir welding is a low thermal input solid-state joining technology,which can not only suppress the loss of Li,but also avoid the defects of fusion welding,and it is suitable for the welding of Al-Li alloys.Despite that,there are some inherent issues of friction stir welding,such as weld thinning,keyhole and so on,which reduce the bearing capacity of joints.In this paper,numerical simulation models are adopted to solve the problems described above.The optimized design of welding tools is carried out from the aspects of reducing thermal input and promoting material flow,and the parameter scope is also determined by the models.Optimization of the repaired joints is also realized by experiments.Aiming to satisfy the high strength welding requirements of aluminumlithium alloy.In this work,three dimensional thermo-mechanical coupled analysis models based on the Coupled Eulerian Lagrangian fluid-structure method are established,which are adopted in the optimization of non-weld-thinning friction stir welding and keyhole repairing process.During the non-weld-thinning friction stir welding of conventional cone-shape tool,the material flow of the material around the welding tool is insufficient,and defects can be found in the weld.By using the pin with triple facets,Plastic deformation and heat generation of materials can be promoted,and the defects can be eliminated.Through the simulation of several parameters,the defect-free parameter scope is determined,which effectively improves the experimental efficiency.Furthurmore,design concept of filling bar angle during filling friction stir keyhole repairing is verified,and it is found that the filling bar with larger angle is suitable for improving the material flow on the interface of bar and keyhole,which mixes the material sufficiently to achieve the combination.By analyzing the flow behavior of the material in the repair process,the flow pattern of the materials is found,and the mechanism of the keyhole repairing is put forward.Comprehensive analysis of the numerical simulation results can provide guidance for the follow-up experimental process.According to the numerical simulation of non-weld-thinning friction stir welding and the experiments,a rotational speed of 600 rpm and a welding speed of 200 mm/min are determined,which can obtain the defect-free welds at low thermal input.There is no thinning of the weld after welding,and some areas are even slightly thickened,indicating that this technology successfully solves the problem of the weld thinning in conventional FSW.Since the thermal input is low,the phnonmenon of grain coarsening is not obvious,and the precipitates are dispersively distributed in the welds after welding.In the heat affected zone and the thermo-mechanically affected zone of the weld,parts of the precipitates are dissolved,which makes these two zones have lower microhardness and are considered as the weak region of joints.The tensile strength of joints reaches 395 MPa,equivalent to 75.2% of the base metal.In the heat affected zone,crack of joint with an angle of 45° appears,and its fractography consist of equiaxed dimples,indicating that the fracture of the joint qualifies ductile fracture characteristic.Keyhole repairing technologies such as filling friction stir welding and friction stir processing after filling by fusion welding are adopted to repair the keyhole left on the weld of non-weld-thinning friction stir welding.The joint strength of filling friction stir welding reached 358.1 MPa,equivalent to 90.7% of the welds.This technology has the advantage of low thermal input,sufficient friction and material flow and high strength of interfacial bonding.The technology of friction stir processing after filling by fusion welding is a high thermal input technology,which leads to the loss of Li and the coarsening of the grains.The joint strength of friction stir processing after filling by fusion welding reached 295 MPa,equivalent to 74.7% of the welds.As such,this technology of repairing has limitations,and the strength of joints obtained by it is relatively low.Combined with the numerical simulation and experimental results,the formation mechanism of the filling friction stir welding joint is analyzed.The root position of filling bar contacts to the keyhole firstly since the pin has larger angle,and at last,the top of the filling bar contacts to the keyhole when the filling bar is heated.This behavior not only prevents the top of the filling bar from cracking by impact,but also promotes the downward material flow of plasticized materials.As the volume of the filling bar is larger,high pressure can be applied to the interface during plunging stage,which provides interfacal friction and upward material flow.Joint obtains enough frictional heat and material flow for this reason,and exhibits considerable mechanical properties.