Research On Friction Stir Lap Welding Process And Joint Microstructure And Properties Of 2A12-T42 Aluminum Alloy

2-Series aluminum alloys are widely used in the aerospace field,but there are numerous shortcomings in using conventional riveting and bolt connection technology to connect components together.For example,the weight of the body structure is increased;It can generate multiple stress sources in the main structure,reduce the structural strength and shorten the service life of the aircraft.As a solid-state welding method,friction stir welding has the advantage of preventing the defects such as porosity,segregation and solidification cracks in fusion welding.Nonetheless,friction stir welded joints also have their own defects,such as internal holes in the weld and thinning of the top plate.Friction stir lap welding,as one of the forms,has its own Hook and cold lap defects.In view of the problems of friction stir lap welded joints,this paper takes 2A12-T42 aluminum alloy,which can be used as aircraft skin,as the material to explore the effects of process parameters on the forming,microstructure and mechanical properties of friction stir lap welded joints,establishes a numerical model of the welding process,uses the model to predict the temperature field of the joint during the welding process,analyzes the material flow,and reveals the formation mechanism of joint defects,It provides theoretical guidance for joint quality control.The numerical model of friction stir lap welding process of 2A12-T42 aluminum alloy was established by utilizing ABAQUS finite element software.The temperature field and material flow of the joint were studied by applying the model.A three-dimensional model of the material flow around the welding tool during the welding process is proposed by studying the material flow behavior of the weld with typical parameters.The influence of process parameters on the flow characteristics of joint material was analyzed.It was obtained that with the increase of welding speed,rotation speed,plunge depth and tilt angle,the fluidity of weld material decreases,increases,increases and increases first and then decreases,respectively.The material in the material concentrated zone below the lap interface flows to the weld surface and squeezes the lap interface,resulting in the upward movement of the interface on the advancing side and the retreating side.The formation of voids in the weld is caused by insufficient material flow and excessive plunge depth.Increasing the material in the material concentrated zone below the lap interface or increasing the depth of the material concentrated zone below the lap interface will increase the height of the Hook and cold lap.Increasing the upsetting pressure will reduce the height of the Hook and cold lap.The effective lap width is closely related to the weld heat input and the transverse pressure of the material concentrated zone at the bottom of the weld on the interface.Increasing the heat input and the transverse pressure on the interface will increase the effective lap width of the weld.The grains in the nugget zone undergo complete dynamic recrystallization,the grains are fine equiaxed grains,and the precipitate phase is only excess phase T phase;In the thermo-mechanically affected zone,some grains are recrystallized,some are deformed grains,some are fine equiaxed grains,and there is a small amount of S phase in addition to T phase;The grains in the heat affected zone are coarsened,and the grain morphology is the same as that of the base metal,but the size is larger than that of the base metal,and the content of S phase also increases.With the increase of welding speed,plunge depth and tilt angle,the average grain size in the nugget region increases progressively,increases first and then decreases,decreases first and then increases respectively.With the increase of welding speed,plunge depth and tilt angle,the average grain size in the thermomechanically affected zone increases first and then decreases,increases first and then decreases,and decreases first and then increases respectively.With the decrease of heat input,the microhardness distribution of the upper sheet changes from "W" shape to "U" shape,and the microhardness of the lower sheet changes from "W" shape to inverted "V" shape or inverted "U" shape.With the increase in welding speed,rotation speed,plunge depth and tilt angle,the tension shear failure load of the joint increases first and then decreases.There are three modes of joint fracture: tensile fracture near the interface between thermo-mechanically affected zone and nugget zone of the lower plate on the forward side,tensile fracture at the highest point of cold lap on the retreating side and shear fracture with fracture path through the nugget zone.Internal voids,redistributed alclad and small effective sheet thickness and effective lap width severely weaken the tensile load of the joint.The dimple in the fracture morphology of the position where alclad exists is small,and the bonding strength between aluminum clad and matrix is low.The microstructure has a great influence on the microhardness of the joint,but has little influence on the tensile load.