| In recent years,because of the advantages of mechanical and physical properties of aluminum alloy,the applications of aluminum alloy in aerospace,rail transportation and navigation are becoming more and more widely.However,the standards for the performance requirements of aluminum alloys in different industries are not the same.Friction stir welding is a new solid phase joining technology developed by the British Welding Institute,its unique working principle makes it particularly suitable for welding low melting point metals such as magnesium and aluminum.This paper mainly explores the application of 7A04 aluminum alloy friction stir welding in rail transit.Due to the cyclicity of vibration,there is a certain requirement for the strength and hardness of materials,it also requires workability and fatigue resistance of materials.The main content of this paper is to explore the friction stir welding process of 3mm thick 7A04 aluminum alloy,and to study the strength,hardness and fatigue strength of friction stir welding joint;Yttrium(Y)element is added to 7A04 aluminum alloy,and study on the Effects of Y content on grain size,mechanical properties and weld ability of 7A04 aluminum alloy.The experimental results show that the friction stir welding joint of 7A04 aluminum alloy consists of four parts:weld zone,mechanical heat affected zone,heat affected zone and base metal zone.The grains in the weld zone undergo dynamic recrystallization under the action of frictional heat,and forms fine equiaxed grains.The grains of the heat affected zone are on the edge of the shoulder of the stirring head,which is little affected by friction heat,and there is no dynamic recrystallization.The grain size is obviously elongated and the size is larger.The grains in the heat-affected zone are not stirred by the stirring head and are only subjected to frictional heat,grain grows and HAZ is the area where the neutral energy of the joint is poor.Hardness test showed that the hardness of the joint along the cross section was "W",the hardness of the base material is about 170HV200g,the micro hardness of the nugget zone is slightly lower than that of the base metal,but it is obviously higher than that of the mechanical heat affected zone and the heat affected zone,reaching 152HV200g,the lowest hardness value in the joint is in the heat affected zone of the advancing side,and the hardness is only 110HV200g.The experiment of friction stir welding(FSW)found that the rotation speed was 1000r/min,welding speed was 120mm/min,and the friction welding joint had the highest tensile strength 452MPa which is 77.3%of the tensile strength of the base material.The elongation rate of the joint is also high,which is 4.24%.Fatigue tests on welded joints of 1000/120 show that the fatigue strength of joints is 141MPa,and there is dimple in the fracture,which is ductile fracture.In the joint EBSD experiment,it was found that the ratio of small angle grain boundaries in the heat affected zone and the mechanical heat affected zone was 40%,which was much higher than that in the base metal and nugget zone.The polar texture and reverse pole diagram show that the texture strength of the joints is not very high,and there has no obvious directivity.In the optimization experiment of 7A04 aluminum alloy,it is found that the grain size of the as cast material is obviously refined and the tensile strength is increased after adding yttrium element.With the increase of yttrium content,the grains become smaller and smaller,and the material strength is higher and higher.When the yttrium element is 0.25%,the grain size is the smallest and the tensile strength is the highest.When welding 7A04 aluminum alloy containing 0.25%yttrium,it is found that the joint strength is lower than that of the 7 A04 friction stir welded joint without Y addition.The experimental results show that the Y element can refine the grain and improve the microhardness and tensile strength of the material,but the material containing yttrium element is not suitable for welding or heat treatment. |
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