Study On Fatigue Properties Of 5052 Aluminum Alloy Joint By Friction Stir Joining

Aluminum alloy and other lightweight materials have been widely used in various industrial manufacturing fields,because of its particularity of the products,there are high demands to the fatigue property of the welded joints.At present,many scholars have done a lot of research on the process parameters,microstructure and mechanical,but seldom on the fatigue property.Therefore,strengthening the research on the fatigue properties of aluminum alloys is particularly urgent.In this paper,the 5052 aluminum alloy was used as the research object,and the microstructure,hardness,tensile strength and fatigue properties of friction stir welded joints were studied by the test method.The main research contents of this paper are as follows:1.The microstructure of the 5052 aluminum alloy friction stir joining area were studied.The results show that the metallographic tests demonstrated that FSW joints were composed of nugget zone(NZ),thermo-mechanical affected zone(TMAZ),heat affected zone(HAZ)and base material.Dynamic recrystallization occurs in the nugget zone,the grains was formed of fine equiaxial recrystallized grains.The TMAZ is subjected to the dual effects of heat engine and welding heat cycle.On the one hand,the grains are mechanically elongated with the stirring effect of the stirring pin,on the other hand,under the action of the welding heat cycle,the grains gradually grow and eventually exist as a deformed strip structure.The boundary between HAZ and TMAZ in AS was clear,but the one in RS was not clear.2.The mechanical properties of the 5052 aluminum alloy friction stir joining area were studied.The results show that when the welding speed is constant,the tensile strength and elongation of the joint show a trend of increasing first and then decreasing with the increase of the rotation speed.When the rotation speed is unchanged,the tensile strength and elongation of the joint show a trend of decreasing first and then increasing as the welding speed increases.In both cases,the fracture in the welded area is a ductile fracture.When the rotation speed is 800 rpm and the welding speed is 120 mm/min,the tensile strength of the welded area is the highest.The joint hardness is distributed in the shape of "W",which decreased from the center of the weld to both sides.In the transition zone of the HAZ and the TMAZ,the hardness was the lowest,and then gradually increased.When the rotating speed is 800 rpm and the welding speed is 120 mm/min,the maximum microhardness value is 62.6 HV,which is slightly higher than that of the base material.3.The fatigue life under different process parameters was studied.The results show that when the rotation speed is 800 rpm and the welding speed is 120 mm/min,the fatigue life is the largest.When the rotation speed is 600 rpm and the welding speed is 120 mm/min,the fatigue life is the smallest.When the welding speed is constant,the fatigue life increases first and then decreases with the increase of the rotation speed.When the rotation speed is constant,the fatigue life decreases first and then increases with the increase of the welding speed.The change trend of tensile strength under different welding process parameters is roughly the same as that of fatigue life,and the change trend of hardness under different welding process parameters is roughly the same as that of fatigue life.4.The characteristics of fatigue fracture under different process parameters were studied.The results show that the fatigue fracture of 5052 aluminum alloy can be divided into three regions:fatigue source region,crack propagation region and instantaneous fracture region.The morphology of fatigue fracture of 5052 aluminum alloy is not unique,and there are great differences.There is no obvious fatigue glow pattern in the crack propagation region,and most of the surfaces have tear ridges,which indicates that the fracture the propagation region is carried out in a manner similar to cleavage fracture.The number of dimples in the instantaneous fracture region is different,which is related to the properties of welded joints,and the fracture in the transient fracture zone is plastic fracture.