Influence of interface microstructure on the strength of the transition joint between Ti-6Al-4V and stainless steel
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INTRODUCTION
TI-6AL-4V is an - alloy with a good combination of strength and toughness, satisfactory specific strength, high melting point, and excellent corrosion resistance for mediumtemperature applications. There is a strong interest to join this alloy with conventional structural steel and stainless steel to fabricate primary and secondary components for the aerospace, nuclear, and chemical industries.[1,2,3] The increasing trend in making monolithic joints between dissimilar materials put forward the need for a high-precision joining technique such as diffusion bonding, in order to exploit the individual properties of these materials. Mechanical joining or traditional fusion welding of dissimilar materials promotes different problems such as distortion of components, formation of stress-concentration sites, and development of chemical heterogeneities, which ultimately result in crevice corrosion or fatigue failure during service.[4] Major alloying elements in the alloy systems are Fe, Cr, and Ti. Isothermal sections of the ternary Ti-Cr-Fe diagram exhibit the presence of phase, Fe2Ti, phase, phase, FeTi, and -Ti for the bimetal system consisting of Ti alloy–stainless steel in the temperature range of 800 °C to 1000 °C, owing to the limited solubility of Fe and Cr in Ti, and vice-versa.[5] The formation of the intermetallic phases is also reported by researchers for the transition joint consisting of a Ti/Ti alloy and steel/stainless steel.[6,7,8] The existing literature reports that a shear strength of 148 MPa was obtained for diffusion-bonded Ti-6Al-4V and a type 304 SS joint when processing was carried out at 850 °C to 880 °C for 10 to 20 minutes under a 10 to 15 MPa load using a 30-m-thick Ni interlayer.[9] However, direct bonding M. GHOSH, Doctoral Student, S. KUNDU, Technician, and S. CHATTERJEE, Professor, are with B.E. College, Howrah, 711103 India. B. MISHRA, Professor, is with the Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] Manuscript submitted April 21, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
of these two dissimilar materials results in a lower bond shear strength of 72 MPa.[2] Reports concerning the tensile properties of the transition joints formed between these two dissimilar materials are scanty. In the present study, direct solid-state bonding was carried out between a Ti-6Al-4V alloy and type 304 SS to simplify the processing; the processing temperature was varied to obtain a good combination of strength and ductility. The present study focuses attention on the characteristic features of the diffusion zone of the bonded specimens as a function of processing temperature and their effects on the strength of the transition joints.
II. EXPERIMENTAL PROCEDURE The Ti-6Al-4V alloy (hereafter termed the Ti alloy) and the type 304 SS (hereafter termed 304 SS) used in the present investigation were in the form of 20-mm-diameter rods. The chemical compositions and room-temperature mechanical properties of the base metals are presented in Tables I and
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