Study On Mechanical Properties And Interface Of Ultrasonic Welded Joints Of TiNi Alloys To TC4 Titanium Alloy

Ti Ni shape memory alloys and TC4 titanium alloy are used in medical device,micro-electronics,aerospace and other fields widely,with excellent physical and chemical properties.However,brittle intermetallic compounds,such as Ti₂Ni、Ni₃Ti,and cracks are formed in the weld joint easily,when joining TiNi to TC4 dissimilar metals by fusion welding.It affects the application seriously.In this paper,the ultrasonic spot welding（USW）has been performed on 0.2-mm-thick TiNi and TC4 dissimilar metals with pure Ni and Al interlayer by varying various welding parameters,aiming to identify the interfacial reaction,changes in microstructure,interface temperature and weld strength.The results show that:Due to the hardness and plasticity of the metal,the joints of TiNi to TC4 without interlayer are not produced by USW.Furthermore,the base material is fractured when the welding process parameters are too large.A well USW joint can be obtained by adding well-plastic interlayer.The interlayer reduce local stress concentration and produce severe plastic deformation in combination area during welding.The tensile shear strength of the joint increases at first then decreases with the increase of normal press and time.With the same parameters,strength of the joint with Al is better than the joint with Ni.The peak tensile shear load are 930.8N and 373.6N,which are obtained at 0.52 MPa of weld pressure,0.5 Sec of weld time and 0.52 MPa of weld pressure,0.6 Sec of weld time respectively.Dissimilar joints of TiNi to TC4 with Ni and Al were produced by USW.The interface is flat and without a jagged mechanical mosaic.The degree of plastic deformation near the interface of the joint with Al interlayer is much greater than that of the joint with Ni interlayer.There is a large number of Al at the edge of the weld which is extruded from the interface.The grain of the interface becomes small in the joint with Ni,which isn’t observed in the joint with Al.The fracture attributes to the mixed-mode of ductile and brittle fractures.In the joint with Ni interlayer,the brittle mode is given priority to the ductile mode,however,it’s the opposite in the joint with Al interlayer.The interface temperature of USW joint with Al is measured by thermocouple.It increases with the increase of normal press and time,and doesn’t reach the melting point of Al,which confirms that the USW is a solid-state joining process.The physical metallurgy and metal bonding are the main factor for the formation of USW joints of TiNi to TC4 dissimilar materials in the interface,which determines the quality of the welded joint.