Study On Ultrasonic Welding Of NiTi Shape Memory Alloys

NiTi shape memory alloys are widely used in aerospace,medical,microelectronics and other fields due to their excellent functional properties,namely superelasticity and shape memory effect,and good biocompatibility.However,cracks and brittle intermetallic compounds such as Ti₂Ni and Ni₃Ti would probably formed in the joints of NiTi material during solidification of fusion welding,which seriously affects its application.As a solid-state welding technology,ultrasonic spot welding has attracted extensive attention in recent years due to its advantages of insensitivity to electrical conductivity,short welding time,low energy and pollution-free.In this work,effective joining of NiTi shape memory alloys was achieved by ultrasonic spot welding,and the joint performance was improved by adding copper foil interlayer.The effects of welding parameters on the mechanical properties and phase transformation behavior of the joints were studied.The interface morphology,element distribution and fracture morphology of the joints were analyzed and studied.The results show that:The effective bonding of NiTi shape memory alloy sheets can be achieved by ultrasonic spot welding.The welding energy has the most significant effect on the tensile properties of the joints.The phase transformation behavior of NiTi alloy ultrasonic spot welded joints changed,that was,austenite transformation temperature increased,martensite transformation temperature decreased,and R phase transformation existed in the joints during the cooling process.Because the weld interface presented partially bonded morphology,all joints showed interface fracture mode during tensile testing.The maximum tensile load increased with the increase of welding energy,and there were micro-cracks and plastic deformation areas at the fracture surface.The performance of ultrasonic spot welded joints of NiTi alloy was improved by adding copper foil interlayer.The orthogonal test results showed that the welding amplitude has the greatest influence on the mechanical properties of the joints compared with other process parameters.The indentation depth on the surfaces of joints increased with the increase of welding amplitude.The thickness of copper foil at the weld interface decreased gradually with increasing amplitudes.When the amplitude was too high,the copper foil at the weld interface brook.EDS and EBSD analysis showed that no intermetallic compounds were formed at the weld interface,and obvious dynamic recrystallization was observed on the side of copper foil.Compared with the base metal of NiTi shape memory alloys,the transformation temperature of joints had a slight deviation,that was,martensite transformation temperature decreased during cooling and austenite transformation temperature increased during heating,but the transformation temperature of joints obtained under different welding parameters was stable.The maximum tensile load of the joint increased firstly and then decreased with the increase of welding amplitude.There were four kinds of fracture modes in the joints during tensile tests,namely interface fracture,upper base metal fracture,base metal fracture and weld edge fracture.Under suitable welding parameters,the strength of the joint was higher than that of NiTi base materials.A large number of dimples existed on the fracture surface,which presented the typical ductile fracture characteristics.Excessive welding amplitude significantly reduced the effective thickness of NiTi thin sheets in the weld zone,and reduced the tensile strength of welded joints.