Research On Interfacial Behavior And Mechanical Properties Of Ultrasonic Assisted Friction Stir Welding For Al/Ti

The article is based on urgent demanding requirements for Al/Ti dissimilar metals.In order to resolve tool wear and poor material fluidity of traditional friction stir welding(FSW),ultrasonic assist FSW(Ua FSW)was carried for 3 mm thick 6061 aluminum alloy and 2 mm thick TC4 titanium alloy.The influence law of the welding process parameters and ultrasoinc on the microstructure and mechanical properties of the joint was analyzed.In addition,the change of the welding process feature parameters was analyzed to explore the mechanism of ultrasound promoting the growth of intermetallic compounds.The influence of ultrasonic on the weld surface finish was studied.The acoustic softening of ultrasonic can promote the formation of the joint surface finish.The microstruction of joint can be divided into the weld stir zone(SZ),the thermal mechanical affected zone(TMAZ),the thermal affected zone(HAZ)and the interface of Al/Ti connection zone(IZ).The grain size of the SZ,TMAZ and HAZ has been significantly refined with ultrasonic.The thickness of the intermediate layer of the Al/Ti interface is significantly uneven with ultrasonic.Under the thermal coupling of ultrasonic mechanical vibration and friction stir welding,there are fragments of titanium alloy and micropores on the aluminum alloy side of the joint interface zone,which l eads to an Al/Ti mixed area.The ragged and jagged characteristics of Al/Ti interfaces are mechanically fitted with each other.Under different welding process parameters,the above characteristics are different due to the difference coupling effects of ul trasonic,the intermetallic compounds present at the interface is the Ti Al₃ phase.The joint hardness test results show that the joint hardness presents a W-shaped distribution along the horizontal direction on the aluminum side.The hardness of each zone on the joint is lower than the hardness of the base metal,in which weld stir zone has the highest hardness except for base metal.The hardness of thermal affect zone is the lowest,and the hardness distribution is more uniform along the thickness direction.The tensile and shear test of the joint found that the maximum strength of the Ua FSW was 4.2k N,which was about twice the strength of the FSW joint.By joint failure analysis,the cracks expanded along the Al/Ti interface with FSW,however the cracks expanded along the Al/Ti mixed area on the side of the aluminum alloy with Ua FSW.Fracture analysis showed that the fracture mode of the joint is a tough-brittle mixed fracture.The measurement results of welding torque and welding temperature field show that the anti-friction effect of ultrasonic reduces the peak temperature of the thermal cycle of the welding process,and ultrasound-induced reduces the spindle torque slightly.The change of the weld tool rotation speed has a great influence on torque and heat input.Thermodynamic research shows that the first intermetallic compounds produced by the joint are Ti Al₃.Using the Arrhenius formula to calculate the atomic diffusion activation energy on the Al/Ti interface,it was found that the atomic diffusion activation energy on the Al/Ti interface was reduced from 27.229 k J/mol to 24.063 k J/mol,which makes the thickness of the Al/Ti intermediate reaction layer increase from 31nm to 54nm,thereby improving the mechanical properties of the joint.