Infrared Brazing Ti 50 Ni 50 and Invar Using Ag-Based Filler Foils
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quiatomic Ti50Ni50 shape memory alloy (SMA) is well known for its excellent shape memory effect, corrosion resistance, damping capacity, and pseudoelasticity. Ti50Ni50 SMA exhibits high energy output per unit volume per cycle and rapid response time. Therefore, Ti50Ni50 SMA is regarded as an excellent candidate for the microactuators and micropump applications in micro-electro-mechanical systems.[1] The chemical composition of Invar alloy is Fe-36 pct Ni in wt pct. It is featured with good formability, high toughness, and extremely low coefficient of thermal expansion (CTE). Accordingly, Invar has been used in various applications for its dimensional stability, such as high precision mechanical systems, glass-to-metal seals, integratedcircuit lead frames, and bimetal strips.[2,3] The potential application of the Invar alloy is used as an interlayer to join Ti50Ni50 and ceramics due to its closely matched CTEs with ceramics. Both brazing and welding are possible approaches to join Ti50Ni50 and Invar alloy. It was reported that the Invar alloy is prone to solidification cracking and reheating cracking when fusion welding is used.[4–6] Once the filler metal is alloyed with Ti, Mn, and C, cracking of the weldment is decreased. However, the CTE of filler metal is increased. Besides, fusion welding of Invar and Ti50Ni50 leads to
R.K. SHIUE, Professor, Department of Materials Science and Engineering, Y.H. CHANG, Graduate Student, Department of Mechanical Engineering, and S.K. WU, Professor, Department of Materials Science and Engineering and Department of Mechanical Engineering, are with National Taiwan University, Taipei 106, Taiwan R.O.C. Contact e-mail: [email protected] Manuscript submitted April 14, 2013. Article published online August 13, 2013 4454—VOLUME 44A, OCTOBER 2013
reaction(s) among Ti, Fe, and Ni; thus, a great number of brittle intermetallic compounds are formed in the weldment. The presence of intermetallic compounds in the joint is detrimental to its bonding strength. Under this circumstance, brazing is expected to be an alternative approach in joining Ti50Ni50 and Invar alloy. Infrared brazing makes use of infrared energy to heat the brazed specimen, and it is featured with a rapid heating rate up to 50 K/s.[7–10] With the aid of precise thermal cycle control, it is highly suitable to evaluate the mechanism of early stage reaction kinetics in the brazed joint. Accordingly, it is applied in the experiment. Ag-based filler foils are widely used in brazing stainless steels, titanium alloys, and nickel-based alloys.[11–14] Based on previous studies, a minor Ti addition into the Ag-Cu eutectic braze significantly improves the wettability on the Ti50Ni50 substrate.[15,16] Two commercially available braze alloys, BAg-8 (Ag-Cu eutectic) and Cusil-ABA (63Ag-35.25Cu-1.75Ti in wt pct), were applied in infrared brazing Ti50Ni50 and Invar alloy. Interfacial reactions, microstructural evolution, and shear strength of infrared brazed joints were evaluated in the experiment. Both Ti50Ni50 and Invar plates with the dimensions of
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