Investigation Based On Strengthening Mechanism Of Thermo-echanically Affected Zone Of Aluminum Alloy Friction Stir Welded Joints

In this paper,the butt welding process of friction stir welding was used on 5 mm thick 7050 aluminum alloy.And then the surface shot peening and the surface rolling reinforcement test were carried out on optimized joint,respectively.The strengthening effect of the two methods on thermo-mechanically affected zone?TMAZ?and the joint was explored by combining comparative study of microstructure,mechanical properties,fatigue life and electrochemical corrosion test.?1?Through optimizing the FSWed joint of 7050 aluminum alloy,the welding optimization process parameters were selected,namely rotation speed 800 r/min and welding speed 100 mm/min.The average tensile strength of the optimized joint was greater than 81%of the strength of basic material.Then the joint surface integrity were examined and finding there was a certain difference a certain difference on the microstructure and mechanical properties between advancing side?AS?and retreating side?RS?.The gradient of organization transition in AS is larger than RS,distributing aggregations of insoluble impurity phase Al₇Cu₅Fe and Al₇Cu₂Fe.Otherwise residual tensile stress in AS is greater than RS.So its corrosion resistance of AS caused by microstructure above is poorer and Vicker microhardness value gradient of transition to nugget zone?NZ?is larger with a minimum of 93 HV.The high cycle fatigue life of the optimized FSW joint was 3.29×10⁵cycles.The fatigue fracture was more happened in the TMAZ and the fatigue fracture source was located in the joint surface of the TMAZ.?2?Based on the optimized FSW joint of 7050 alumium alloy above,Shot peening was applied by different shot time to strengthen the joint surface.The results showed that surface roughness of the joint was decreased and the surface roughness increased over 30 min of shot peening time.By XRD analysis on the surface of the joint after shot peening,the surface grain reached the nanometer forming a fine grain layer,and a certain amount of the second phase shear band were distributed in the subsurface deformation layer.With the increase of the shot time,the microhardness distribution of the joint surface is changed from the“W”to the zigzag shape,which tends to be homogenization.The tensile strength of the joint is increased within 4%of optimized joint comparing with the tensile strength 418.7 MPa of FSWed joint.Ultimately joint of shot peening 30 min with comprehensive performance was selected for next high cycle fatigue test.And the joint presented several“fish eye”fatigue sources and fatigue source transferred from the joint surface to the subsurface close to 100²00?m.In addition,fracture propagation area appeared“spine”transition region.Accordingly probability of fatigue fracture in TMAZ was decreased at the same time,average fatigue life of joint enhanced by shot peening increased to1.32×10⁶ cycles.?3?Due to the surface roughness caused by shot peening,which has the deteriorated effect on improvement space of joint fatigue life to some extent,so this paper attempts to adopt the rolling method used on joint surface.The results showed that the surface finish of the joint surface was improved by rolling,and during rolling the nano-layer was formed on the surface.So the similar“Sandwich”structure was formed on the upper and lower surfaces of the joints.The surface hardness tended to be homogenized,roughly 160²00 HV.Tensile strength of each parameter was larger than that of FSWed joint,and the elongation rate was decreased.Ultimately joint of rolling with comprehensive performance,which gained under rolling rotation speed350 r/min and advancing speed 50mm/min,was selected for next high cycle fatigue test.The fatigue crack source of rolled joint moved to subsurface about 200?m,fatigue crack propagation area appeared wider“spine”transition region.And fatigue fracture had a lower likelihood in TMAZ,the fatigue life was enhanced about 1.65times that of FSWed joint.On the basis of the above research,this experiment further explored the microscomic process of the fatigue source under certain life,the corrosion resistance and bending resistance of the joint before and after strengthening.The investigation found that the“Sandwich”structure formed on joint surface of the reinforced surface hinders or delays the initiation and propagation of fatigue cracks on the surface.There is also a difference in the fatigue source between the NZ and TMAZ in the joint.The NZ is the strain concentration while the fatigue source in TMAZ is the stress concentration.The surrounding grain boundary is distributed with continuous S phase?Al₂Cu Mg?and insoluble Impurity phase Al₇Cu₂Fe,this configuration becomes the intrinsic cause of fatigue fractures occurring in the TMAZ.The corrosion resistance of the joint before and after strengthening was compared with that of the electrochemical test,which showed that the corrosion resistance of the joint was gradually increased after strengthening,and the difference in the corrosion resistance of the joints was reduced.Among them,the corrosion resistance of the joints strengthened by rolling was the best.In addition,through the bending test,the joint surface strengthening properties were compared.The rolling load was close to the base metal about 3.04 k N,but the elongation was the lowest.The shot peening roughness had a negative effect on the bending performance,and the bending strength and elongation rate were reduced.