Strength Improvement Through Grain Refinement of Intermetallic Compound at Al/Fe Dissimilar Joint Interface by the Addit

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TRODUCTION

MULTIMATERIAL structures combining steel with lightweight materials are necessary to decrease weight in automotive applications to improve energy efficiency and driving performance.[1,2] The use of Al alloys may be effective for such multimaterial structures because they have high corrosion resistance and specific strength.[2] In commercial applications, the dissimilar joining of Al alloys to steel is necessary to produce multimaterial structures; however, obtaining sound joints is difficult because of the large differences in the thermophysical properties of the metals, such as the melting point, thermal expansion coefficient, and conductivity. Furthermore, Al-Fe intermetallic compounds (IMCs) are easily formed at the interface through mutual diffusion during dissimilar joining,[3–10] and brittle fracture and degradation of the joint strength can be caused by the IMC toughness.[3–5] In order to alleviate strength degradation at Al/Fe dissimilar joint interfaces, attempts to reduce the IMC layer thickness during dissimilar joining have been made, because the joint strength increases with the

H.S. FURUYA, Y.T. SATO, Y.S. SATO, and H. KOKAWA are with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, 6-6-02 Aramaki-aza-Aoba, Sendai, Miyagi 9808579, Japan. Contact e-mail: [email protected] Y. TATSUMI is with the Nippon Steel & Sumitomo Metal Corporation, 20-1 Shintomi, Futtsu, Chiba 293-8511, Japan. Manuscript submitted September 9, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

decreasing IMC layer thickness at the dissimilar joint interface.[3,4,11–14] Yasuyama et al.[4] showed that thinner IMC layers exhibited higher interfacial strength in Al-clad steel produced under various joining conditions, and that IMC layers thinner than 1 lm were required to obtain sound joints. Borrisutthekul et al.[13] also reported that the joint strength was gradually increased with the decreasing IMC layer thickness at Al/Fe dissimilar joint interfaces for IMC layer thicknesses < 10 lm. Therefore, joining processes to reduce the IMC layer thickness, such as brazing and solid-state joining processes, have widely been used to improve the strength at the Al/Fe dissimilar joint interfaces. Brazing processes, such as tungsten inert gas (TIG) arc brazing,[15–17] laser brazing,[13,18–20] and metal inert gas welding using cold metal transfer,[21,22] have been successful in producing Al/Fe dissimilar joints between molten Al and solid Fe. The IMC layers at the dissimilar joint interfaces were much thinner than those produced by conventional fusion welding processes. In solid-state joining processes, such as friction stir welding (FSW)[23–25] and ultrasonic spot welding,[26,27] the joining temperature can be lower than the melting temperature of Al, permitting the formation of IMC layers thinner than 1 lm. Previous studies have shown that brazing and solid-state joining processes are effective methods for improving the strength of Al/Fe dissimilar joints. The formation of an IMC layer a