Bonding Interface Formation between Mg Alloy and Steel by Liquid-phase Bonding using the Ag Interlayer
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MG alloys have been used to develop lightweight structures. However, their strength, rigidity, formability, and wear resistance are lower than those of other structural alloys,[1] which often limits the applicability of Mg alloys. Cladding Mg alloys with steels, which are more widely used structural materials, can improve these properties of Mg alloys while reducing the specific weight of steels. However, bonding between Mg alloys and steels is difficult because of the large difference in melting point and the low reactivity between Mg and Fe, and thus, limited numbers of studies about the bonding have been conducted.[2–4] Friction stir welding was applied to join mild steel and the Mg alloy AZ31[5–7] as well as spot welding with SUS304 foil inserted between layers of steel and AZ31.[8] However, in these studies, the joint strength did not exceed the yield strength of the base material, and thus, sufficient joint strength has not yet been achieved. Recently, we developed a new bonding method, reactive transient liquid-phase (r-TLP) bonding, for joining Mg alloys and steels.[9] Transient liquid-phase (TLP) bonding employs a metal insertion that forms a eutectic melt with the base metal to be joined, and bonding is achieved by the completion of the isothermal solidification of the melt.[10–12] Note that the solidification proceeds as the inserted metal element diffuses into the base metal and that the inserted metal element does not remain after the solidification. In our previous study,[9] we applied the concept of TLP bonding to a Mg MASAKI KOBA and TOSHIO ARAKI, Students, SHOICHI NAMBU, Assistant Professor, JUNYA INOUE, Associate Professor, and TOSHIHIKO KOSEKI, Professor, are with the Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan. Contact e-mail: [email protected]. Manuscript submitted December 7, 2010. Article published online September 15, 2011 592—VOLUME 43A, FEBRUARY 2012
alloy and selected Ag as the metal insert because it forms a eutectic melt with Mg but does not form any compound with Fe. After inserting Ag, a uniform, thin intermetallic compound layer was formed at the interface and high bonding strength was obtained. Without the insertion of Ag, a coarse, nonuniform intermetallic compound was intermittently formed at the interface and the bonding strength was considerably weaker. In our previous study, we assumed that the formation of a uniform, thin intermetallic compound layer over the bonding interface as a reaction product during the bonding process increased the bond strength between the two dissimilar metals. This could be achieved by either adding some alloying elements to the inserted metal or by including alloying elements in the Mg alloy supplied to the Mg-Ag eutectic melt. We attempted the latter in the study, and thus, we selected AZ31, a Mg alloy that contains Al, to form an intermetallic compound with Fe. Because Al supplied from the Mg alloy is assumed to have relatively low activity and to be uniformly distributed over the entire surface of th
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