Research On Microstructure And Property Of NiTi Shape Memory Alloy/TC4 Titanium Alloy Electron Beam Welded Joint

As an advanced functional material,NiTi shape memory alloy（NiTi SMA）is widely used in aerospace field.Shape memory refers to the phenomenon that after the material is deformed,the material returns to the shape before the deformation by means of temperature driving.It has good deformation adjustment ability by unique shape memory function and can realize intelligent driving of parts.Due to some NiTi SMA parts have complex structure,which need effective join technology to realize the manufacturing.Its low yield strength cannot be used as the main bearing structure.So,it should be joined with different materials to form heterogeneous components to meet the practical requirements.Aiming at the join requirements of turbine engine noise reduction driving device,this paper strives to finish the high quality join of NiTi SMA/NiTi SMA and NiTi SMA/TC4 titanium alloy.However,NiTi SMA/NiTi SMA welded joint have the problem of reduced shape memory function.In response to the problem,the memory function of the joint was recovered by means of martensitic remodeling function recovery in this paper.In addition,severe crack defects were formed in the welded joint due to the large difference in physicochemical properties and poor metallurgical compatibility between NiTi SMA and TC4 titanium alloy.Based on this,an electron beam offseted barrier welding process method was proposed,which eliminated the crack defects and realized the reliable join.Firstly,the electron beam welding of NiTi SMA/NiTi SMA was carried out.It was found that Ti₂Ni and Ti O₂ precipitates were formed in the weld zone,which were discontinuous distributed at the grain boundary,forming the grain boundary pinning effect.As a result,the grain boundary was difficult to achieve uniform plastic deformation by the external force.Moreover,it wass easy to form large stress concentration at the precipitated phase position,and the crack was easy to nucleate and expand at this position,which reduced the grain boundary strength.It was an important reason for the reduction of joint strength and toughness.In addition,the microstructure evolution of NiTi SMA welded joint was studied.It was found that the faster welding speed will lead to the formation of crystallization line（CL）in the center of the weld zone.And there were coarse columnar crystals on both sides.The structure will also lead to the reduction of joint strength.The effects of different welding speeds on the structure of CL were analyzed.It was revealed that the faster cooling speed and larger temperature gradient were the root reason for the formation of the structure.The transformation from columnar to equiaxed crystal was promoted by adjusting the welding temperature field,which eliminated the CL structure.The tensile strength of the welded joint increased from 576MPa to724MPa,increased by 25.7%and reached 76.1%of the base metal.On this basis,the phase transformation temperature and shape memory function（deformation recovery ability）of NiTi SMA/NiTi SMA electron beam joint weld zone were studied.It was found that the martensite started transformation temperature（M_s）and the austenite finished transformation temperature（A_f）increased by 12K and 16K compared with the base metal.The shape memory function of the different joint zones reduced.The shape memory recovery of the weld zone and heat affected zone was 28.2%and 61.8%of the base metal.The essential reasons for the decrease of shape memory function were the martensitic phase content and size reduced,the increase of micro defect density caused by non-equilibrium solidification and local plastic deformation,the martensitic phase incomplete transformation caused by insufficient driving force and the coherent relationship destruction with parent phase.In order to increase the content and size of martensite phase,reduce the micro notch density and improve the interface relationship between martensite and parent phase.By means of martensitic remodeling function recovery,an appropriate treatment process interval was established,which the M_s and A_f of the weld zone were effectively reduced and the phase transition temperature was recovered.The shape memory recovery of the weld zone and heat affected zone reached 90.9%and 98.2%of the base metal.The softening phenomenon of the joint disappeared and the tensile strength can reach770MPa,up to 81.1%of the base metal.TC4 titanium alloy is often used to join with NiTi SMA to make heterogeneous components.By giving play to the bearing capacity of TC4 titanium alloy and the deformation adjustment performance of NiTi SMA,the noise problem of turbofan engine can be reduced to the greatest extent.Based on this,the electron beam welding research of NiTi SMA and TC4 titanium alloy was carried out.It was found that there were serious crack defects,which nucleated on the surface and expanded inward along the reaction layer at the TC4 titanium alloy side.A large number of Ti₂Ni brittle intermetallic compounds were formed,improving the crack sensitivity.It was the underlying reason for the cracking formation.The average hardness of the joint was HV621 and the tensile strength was 98MPa.In order to avoid the formation of Ti₂Ni phase,the electron beam offseted NiTi SMA welding was proposed to adjust the proportion of Ni and Ti elements in the molten pool.At this time,the main microstructure changed from Ti₂Ni to NiTi phase,which the hardness decreased to HV389 and the tensile strength of the joint can reach 155MPa,was58.2%higher than the direct butt welding.However,the reaction layer（α-Ti）+Ti₂Ni was formed at the fusion line of TC4,which the hardness can reach HV650,where the crack was easy to nucleate and expand.It was difficult to avoid the formation of Ti₂Ni phase due to the melting and direct mixing of base metals on both sides.In order to eliminate the crack defects,an electron beam offseted barrier welding process was proposed.The metal barrier layer was preset before welding.During the welding process,the barrier layer did not melt.It was melt-brazed with the base metal on both sides to realize the join of NiTi SMA and TC4 titanium alloy.Accroding to the working principle,Mo with high melting point was selected as the barrier metal.The effects of electron beam offseted distance,barrier layer thickness and height on joint forming and microstructure were studied,which the appropriate welding process was determined.The electron beam offset distance was 0.1mm on the offset TC4 titanium alloy side.The thickness of the barrier layer was 300μm and the height was 1.2mm（the thickness of base metal plate was 1mm）.By this process,the Mo had a good barrier effect on the liquid metal on both sides,inhibiting the formation of Ti₂Ni intermetallic compound and eliminating the crack defects.The melt-brazing effect of was formed on both sides of the barrier layer.The microstructure of Mo/TC4titanium alloy side was（Ti,Mo）_ss,and that of NiTi SMA/Mo side was（Ti,Mo,Ni）ss and Ti₂Ni.The tensile strength can reach 318MPa,which was 224%higher than that of the direct butt welding.The fracture position was at NiTi SMA/Mo interface,which the fracture mode was cleavage fracture.