Joining Of TiNi Shape Memory Alloy And Stainless Steel

TiNi shape memory alloy (TiNi SMA) is a new kind of functional materialused in many fields, such as aviation and space-flight, atomic energy, oceandevelopment, instruments and medical device, etc., due to its special shape memoryeffect (SME) and superelasticity (SE) as well as excellent erosion resistance andoutstanding biocompatibility. However, it is required urgently to solve problems ofjoining TiNi SMA for spreading its range of application. At present, great attentionhas been given to the effects of joining methods and process on structure andproperties of the homogeneous joint (joining TiNi SMA to TiNi SMA) while scarework has been carried on joining TiNi SMA to other materials, especially TiNiSMA to stainless steel (SS) and no mature technique for joining TiNi SMA to othermaterials has been reported. In this paper, the joining of TiNi SMA and SS usingmicro-beam plasma-arc welding, capacitor discharge welding and laser brazing wasinvestigated systemically, including microstructure and properties of the TiNiSMA-SS joints using different joining methods, effects of temperature onmicrostructure and properties of TiNi SMA, obtaining of the optimizedcomposition of Ag-based filler metals for brazing TiNi SMA and SS by means ofthe quadratic regression combination design process and genetic algorithm,preparation successfully of the composite archwire composed of TiNi SMA and SSusing the optimized filler metal and laser brazing technique and carrying on theclinical appliance tests of the composite archwire.TiNi SMA heat affected zone (HAZ) of TiNi SMA-SS joint using micro-beamplasma-arc welding was still wide (more than 4 mm) after welding, thoughmicro-beam plasma-arc welding benefited from high-energy density, concentratedheating and quick welding. Parts of base metals (TiNi SMA and SS) were meltedduring welding and the weld zone consisting of two molten base metals wasformed during cooling. The tensile strength of the joints is very low (127~159 MPa)and even the joints could not bear bending strength, which was probably caused bythe segregation of the solute and impurity elements and the form of coarse-grainand dendrite structure of the weld metal during solidifying. Fracture propagatedalong the interface of columnar crystals in the center of the weld metal, owing tothe boundaries of twin columnar crystals vertical with loading as well as theprecipitation of oxidizing impurities along the grain boundaries. Energy spectrumanalysis further confirmed that TiNi SMA strongly reacted with O during welding,leading to oxygen contents in excess of 16.04 at.% in the weld zone. Due to theinvasion by O, Ti4Ni2O would precipitate in the weld, which was one of the mainreasons to form the brittle joint. As a result, superheating of TiNi SMA should beavoided and measures should be taken to decrease the base metal melting too muchand to prevent the weld metal from the invasion by N, O, H during welding inorder to improve further the properties of TiNi SMA-SS joint.The weld metal of capacitor discharge welding (CDW) consisted of part of thebase metals, which were heated up quickly and melted. TiNi SMA HAZ of TiNi-SSjoint by CDW was narrower compared with the joint obtained by micro-beamplasma-arc welding. However, the tensile strength of the joint was still low(128~171 MPa) and the joint was very brittle. The weld metal mainly consisted ofB19′, B2, FeTi and Fe0.2Ni4.8Ti5 compounds. The brittle compounds and theasymmetry distribution of the composition and microstructure in the weld were themain reasons why the joint tensile strength was so low. The results showed thatsome oxide, little holes and incomplete fusion defects emerged in the interface ofTiNi-SS joint, which was mainly related to asymmetry distributing upsetting forceduring butt welding. When TiNi SMA and SS were welded, instability of the jointappeared during giving a high upsetting force caused of the small dimension andthe superelasticity of TiNi SMA while micro-vibration, angular misalignment andaxial misalignment of the joint emerged due to the release of the elasticity of TiNiSMA after welding. So when TiNi SMA was capacitor discharge welded, theexcessive molten metal and oxide could not be extruded out entirely caused by thelimited upsetting force and the excessive molten metal solidified and the aboveddefects formed after welding. The incomplete fusion defects in the weld and thecoarsened structure in the HAZs were one of the reasons deteriorating the jointproperties.Microstructure and properties of TiNi SMA are extremely sensitive totemperature changes. The microstructure of TiNi SMA base metal was cold-drawnfibrous structure. When TiNi SMA was short-time heated, the fibrous structure ofcold-drawn TiNi SMA was gradually translated to the plate martensitic structure.The microstructure of TiNi SMA coarsened and the dislocation density in TiNiSMA decreased while the tensile strength, shape memory effect and elasticity ofTiNi SMA decreased but the plasticity increased with an increase of the heatingtemperature. The tensile strength of TiNi SMA almost held the line (about 1300MPa) when the heating temperature was below 650℃ but the tensile strengthdecreased conspicuously when keeping to increase the heating temperature and thetensile strength was only 65% of TiNi SMA base metal when the heatingtemperature was 750℃. The elasticity recovering ratio of TiNi SMA was 97.9%when the heating temperature was 100℃. The elasticity recovering ratio of TiNiSMA decreased conspicuously when the heating temperature was more than 400℃and the elasticity recovering ratio of TiNi SMA was below 15% when the heatingtemperature was 750℃. However, the plasticity of TiNi SMA increased withincreasing the heating temperature and the fracture strain of TiNi SMA was morethan 2.5 times of that of TiNi SMA base metal when the heatin tempature was700℃.Laser brazing possesses with the characteristics of high-energy density, centralheating, strong penetrability, quick welding and narrow HAZ, etc., which isbeneficial to welding TiNi SMA and SS. The tensile strength of the laser-brazedjoint of TiNi SMA and SS was more than 300 MPa and the fracture strain was morethan 10% using two Ag-based filler metals sold in market. However, TiNi SMAHAZ was broad after brazing and the loss of shape memory effect of TiNi SMAwas big and it was less than 70% of TiNi SMA base metal. The meltingtemperature of the filler metal brazing TiNi SMA and SS should be reduced todecrease the loss of shape memory effect and elasticity in TiNi SMA HAZ.The composition of the optimized Ag-based filler metal for brazing TiNi SMAand SS was as follows: 51~53%Ag, 21~23%Cu, 17~19%Zn and 7~9%Sn by meansof the quadratic regression combination design process and genetic algorithm. Thefiller metal consisted of α-Ag solid solution, α′-Cu solid solution surrounded bythe α-Ag and eutectic structures containing α-Ag, α′-Cu, AgZn, Ag3Sn, Cu5Zn8 andCu41Sn11. When TiNi SMA and SS were joined using the filler metal and laserbrazing technique, all bonds exhibited good wetting between the filler metal andboth TiNi SMA and SS and the joint interfaces were smooth and compact. TheHAZ of TiNi SMA was relatively small (about 3 mm) and the loss of shapememory effect and elasticity of TiNi SMA was relatively low, the bending andtensile strengths of the joint were relatively high, which means that the optimizedfiller metal might be suitable for brazing TiNi SMA and SS.The microstructure and the tensile strength of the joint and shape memoryeffect and elasticity in TiNi SMA HAZ depended on brazing technologicalparameters (laser output power (P) and brazing time (t)). With increasing P and t,the α′-Cu phase in the brazing seam coarsened and tended to segregate in thevicinity of the interfaces between the brazing seam and the base metals, thethickness of reaction layers between the filler metal and the two base metalsincreased as well as the width of the HAZs increased and the microstructures inboth HAZs coarsened. The effect of brazing technological parameters on the tensilestrength of the joint was different from that on the loss of shape memory effect andelasticity in TiNi SMA HAZ. With increasing P and t, the loss of shape memoryeffect and elasticity in TiNi SMA HAZ and the fracture strain increased while thejoint tensile strength increased firstly but decreased when P and t were more than60 W and 15 s, respectively. The joint tensile strength could amount to 360 MPa,the fracture strain was 8~10% while the loss of shape memory effect and elasticityin TiNi SMA HAZ were relatively low when P and t were 60 W and 15 s,respectively.The corrosion resistance of the laser-brazed joint in artificial saliva wassignificantly better than that of the filler metal and was nearly close to that of TiNiSMA base metal but it was still lower than that of SS base metal. Both SS and TiNiSMA emerged the characteristics of pitting corrosion while the filler metal showedthe characteristics of uniform corrosion. The brazing seam and the interfacesbetween the base metals and the filler metal of the joint were the preferredlocations for corrosion. The corrosion resistance of the specimens in artificial salivadepended on the PH values of the solutions. The lower the PH values of thesolutions, the higher the corrosion mass loss rate, the lower the corrosion potentialand the higher the current density of the specimens.Composite archwires (CoAW) of TiNi SMA and SS were developedsuccessfully using the optimized filler metal and laser brazing technology. Themovement-teeth segment of CoAW was composed of TiNi SMA wires while thesupport-teeth segment was composed of SS wires, which overcomed the problemswhen TiNi SMA archwires or SS archwires were used solely in orthodontictreatment. CoAW has advantages in treating cases with malocclusion and crowding,individual torsional and malposed teeth, etc. CoAW with simple operatingtechnologies could improve the treatment efficiency, shorten the period oftreatment (about 5~9 months), lighten the pain of the sufferers, reduce the treatedexpenditure (about 600~1100 yuan) and predigest the orthodontic procedure, etc.and so it has good prospects.