Microstructures And Mechanical Properties Of Friction Stir Welded 14mm-thick 7N01-T5 Aluminum Alloy Plate

7N01 aluminum alloy has a high strength,good welding performance and mechanical processing properties,is widely used in the structure materials of rail car,and aluminum alloy welding quality directly affects the manufacturing cost and traffic safety.In the body of the key load-bearing parts of the need to use aluminum alloy plate welded structure,and melting welding requires multi-layer and multi-pass welding process,the joints prone to grain boundary liquefaction,stomata and joint softening and other issues,limiting the application of the product.Friction stir welding is a new kind of solid phase joining technology,which avoids these shortcomings in melting welding,especially in the welding of aluminum alloy thick plate.However,compared with FSW of aluminum alloy sheet,the batch engineering application of FSW for aluminum alloy thick plate still needs a lot of experimental research to provide the production basis.From the perspective of microstructural performance,the study of microstructural evolution of weld microstructure and its influence mechanism is an important aspect of FSW research.And in this study,the microstructural characteristics and its effect on the mechanical properties of the friction stir welding joint of 7N01 aluminum alloy plate for high speed train beam bolster are studied systematically.It is critical to understand and reveal the special behavior(such as fatigue behavior and corrosion behavior)of the friction stir welding joint of 7N01 aluminum alloy.And it is of great practical significance to promote the application of 7N01 aluminum alloy plate friction stir welding in the welding bearing structure of high-speed train body.FSW joints of 14mm thick 7N01-T5 aluminum alloy can be divided into BM,HAZ,TMAZ and SZ.The grains in the base metal zone are typical hot deformed microstructures,and the grains are elongated.The texture types are composed of S texture {123}<634>and Goss texture {011}<100>and the {123}<634>texture is mainly distributed in the crystal.The small discoid ?' phases with the size about 5-15nm distribute densely in the intragranular.The short rod-like ? phases with size about 30-50nm distribute sparsely in the grain boundary.And the width of precipitates free zone is about 45-70nm.Because the heat-affected zone is affected by the welding heat cycle,the grain size becomes coarse.And the texture type is mainly composed of S texture {123}<634>,Goss texture {011}<100>,and cubic texture {001}<100>.The S texture ?123}<634>becomes weaker and the recrystallized texture becomes stronger than those of the parent metal.The ?'phases of the grain grow up,the ? phases in the grain boundary coarsen,about 40-100nm.And the width of precipitates free zone broaden,about 90-130nm.Heat affected zone due to the effect of mechanical deformation and thermal cycling,the grain arise significant deformation and dynamic recovery along the direction of metal plastic deformation.From the heat affected zone to the stirring zone,mainly composed by {111}<uvw>fiber texture,and though transitional texture to{001}<110>transformed to Goss texture {011}<100>.The peak temperature of the heat affected zone near the stirring zone is higher,the ? phase on the grain boundary and the grain interior ?' phase dissolved,there are a small amount of residual rod-like ? phase,which grain size is about 60-70nm in the grain boundary.And the distance of the phases increase.The stirring zone is dynamically recrystallized under the action of frictional heat and shearing force.The upper part is mainly affected by the shoulder and the lower part is mainly affected by the stirring pin.The heat input from the top to the bottom is gradually reduced,and the average grain sizes of the upper,middle,and lower parts are 8.3?m,7.1?m and 5.0?m.The texture of the stirring zone mainly consists of {111}<uvw>fiber texture,which is the typical shear texture in the aluminum.The grain boundary and the grain of the strengthening phase of the grain were dissolved obviously.The ?' phase,which was finer dispersed,was precipitated in the agitating zone after welding.The size is about 2-5nm.The microhardness test results show that the hardness distribution of the joints is typical "W",the hardness value of base metal is about 120HV,and the obvious softening zone is observed in the heat affected zone about 30mm from the center of the stirring zone.The hardness value is 90HV,and the hardness value of the heat-affected zone and the agitating zone are not different.The hardness value is low at 100-110HV.After the artificial aging,the hardness of the agitating zone and the heat-affected zone(near the stirring zone)is increased by about 17HV.The change of microhardness is mainly related to the morphology and distribution of the particles.The tensile strength of the joint is 316MPa,which is about 90%of the base metal.The elongation is 19.4%,which is slightly lower than the base metal.The fracture location is located in the softening zone of the heat-affected zone on the advancing side,which is consistent with the location of the softening zone determined by the microhardness test.Through T6 heat treatment,the tensile strength increased to 362MPa,elongation reached to 24.5%,and fracture located in the vicinity of the heat affected area on the back side.The transverse tensile strength of the joint is higher than that of the upper layer and the lower layer of the joint,and the transverse tensile strength of the middle layer is higher.The"over-aging" in the heat-affected softening zone is the main reason for the fracture of the welded joint at this position.