Formation And Mechanical Properties Of Al/Ti Dissimilalloys Of Ultrasonic Assisted Friction Stir Welding

Titanium and aluminum alloys have a wide application in the fields of aerospace and transportation to satisfy the high requirement of light weight.The combination of these two alloys can achieve the complementary advantages,which has a promising potential application.Friction stir welding(FSW)is a novel solid-state welding method which provides an effective way to join the dissimilar alloys.Ultrasonic assisted friction stir welding(UAFSW)is a new technique developed from the conventional FSW.In this investigation,UAFSW was used to join 6061-T6 aluminum alloy and Ti6Al4 V alloy.The formation,microstructure and mechanical properties of the joints by conventional FSW and UAFSW were investigated.Under the reasonable welding parameters,the application of ultrasonic can effectively improve the joint quality and increase the joint tensile strength.It can be attributed to the following reasons.Ultrasonic improves the material flow in the stir zone,which reduces the volume defects in the stir zone.Under the welding parameters with low heat input,the bonding of the Al/Ti interface relies on the diffusion bonding and the diffusion thickness is increased by ultrasonic.Ultrasonic leads to the formation of hook-like structure at the bottom of the joint interface,which is beneficial to increasing the bonding length of interface and results in a mechanical interlocking.Under the welding parameter with high heat input,the application of ultrasonic induces an adverse effect on the joint.Under the high rotating speed,the Al-Ti intermetallic compound(IMC)is formed at the joint interface and the distribution of IMC is enlarged by ultrasonic.When the pin offset is too big,the plastic flow property of Al alloy is extremely high due to the high input.Under this condition,the longitudinal vibration induced by the ultrasonic makes Al alloy flow out the weld along tool shoulder and leads to the lack of material in the stir zone,which results in the formation of tunnel defect.Moreover,thick IMC layer is formed at the interface under big pin offset and ultrasonic leads to the formation of crack in the IMC layer.