Effects Of Ultrasonic Surface Mechanical Rolling Treatment On Fatigue Behavior Of 6061 Aluminum Alloy And Its Friction Stir Welded Joints

Aluminum alloy is widely used in many fields because of its good corrosion resistance,high specific strength,easy processing and high price.However,due to the factors such as good thermal conductivity,high thermal expansion coefficient and active chemical properties of aluminium alloy,the defects such as cracks and gas holes often occur during welding by traditional melt welding method,which have adverse effects on the welded joints of aluminium alloy.Friction stir welding(FSW)is a new solid-phase bonding technology.Compared with traditional welding methods,FSW has good forming effect and excellent mechanical properties of joints.These advantages make it have a very broad application space in aluminum alloy welding.Fatigue is one of the main failure modes of welded structures.In recent years,it has been found that taking certain measures to optimize the surface properties of materials and reduce surface defects can improve the fatigue properties of materials or structural parts.Ultrasound surface mechanical rolling(USMRT)is a new surface modification technology,which can form gradient nanostructures on the surface of materials.This unique surface structure can effectively improve the comprehensive properties of materials and expand the application prospects of materials.Ultrasound surface mechanical rolling treatment of aluminium alloy and its FSW joints is conducive to the formation of gradient layer on the surface of materials,thus improving the performance of aluminium alloy welded joints.Microstructure,microhardness and gradient layer morphology of aluminium alloy and its FSW joints were analyzed by metallographic microscope,scanning electron microscopy(SEM)and microhardness tester.Tensile and fatigue properties of welded joints after ultrasonic surface mechanical rolling were studied by using high frequency fatigue tester and uniaxial tension test equipment system,and fatigue behavior of welded joints was analyzed by USMRT.Influence.The results show that the FSW joints of AA6061-T6 aluminum alloy are divided into four zones:weld nugget zone(WNZ),thermomechanical affected zone(TMAZ),heat affected zone(HAZ)and base metal zone(BM).After ultrasonic surface mechanical rolling treatment,the grain size of surface layer of aluminium alloy and FSW joint is obviously refined,showing gradient structure,and the grain refinement degree is different in different regions of welded joint.In the direction of TMAZ,the grains are significantly elongated,while in the direction of WNZ,the grains are gradient refined,not elongated,and still equiaxed.In terms of mechanical properties,the surface hardness of aluminium alloy after USMRT treatment has been greatly improved.From the outer surface along the core direction,the hardness value gradually decreases until it approaches the hardness value of aluminium alloy matrix material.After USMRT treatment,the hardness law of rolling layer in each region of FSW joint is consistent with that of FSW joint.The hardness law of FSW joint is"W"type,and the hardness value law also shows gradient descent from the outer surface to the core.After five passes of USMRT treatment,the yield strength of AA6061-T6 aluminium alloy and its FSW joints increased by 30%and 18.5%respectively;the surface roughness of aluminium alloy was 0.7?m,which achieved good results;however,the rolling passes continued to increase,the surface quality of the material decreased,and the wear and peeling phenomenon appeared,and the strength enhancement effect weakened.The fatigue tests of AA6061-T6 aluminum alloy and its FSW joints treated by USMRT show that fatigue fracture occurs within 10~7 cycles.Compared with the samples not treated by USMRT,the fatigue life of the joints treated by USMRT is improved.The fatigue life of the joints treated by rolling is close to that of AA6061-T6 base metal.At low stress zone,the effect of surface treatment on the fatigue life of the samples is more obvious.The results of fatigue fracture show that the fatigue crack growth zone and instantaneous fracture zone can be distinguished by the joint and the fracture surface after rolling treatment,and there are obvious fatigue striation characteristics.The crack initiation location migrates from the surface to the interior of the material.However,the macroscopic location of the fatigue fracture is still between the thermo-mechanical affected zone and the heat-affected zone of the FSW joint.