Thermochemical Analysis of Phases Formed at the Interface of a Mg alloy-Ni-plated Steel Joint during Laser Brazing
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MAGNESIUM and its alloys are currently under intense investigation for use in many applications in the automotive industry.[1,2] As steel sheet is still the most commonly used material for fabrication of auto body structures, the ability to make hybrid structures of magnesium alloy and steel sheet would be desirable for many applications. This would allow the overall weight of the autobody to be reduced, resulting in better fuel efficiency and lower environmental impact. Therefore, there is increasing interest in identifying and developing new techniques and processes that can be used to make dissimilar joints between magnesium alloys and steel sheet.[2–7] It is well known that it is difficult to join magnesium alloys to steel by conventional fusion welding technologies.[2] This can be attributed to the large difference in their melting temperatures and the nearly zero solubility of magnesium and iron.[2] The melting point of steel [1823 K (1550 C)] is well above the boiling point of magnesium [1380 K (1107 C)], and this can cause catastrophic vaporization of the molten magnesium during a fusion welding process. In addition, the ALI M. NASIRI, Ph.D. Candidate, and DAVID C. WECKMAN and NORMAN Y. ZHOU, Professors, are with the Department of Mechanical & Mechatronics Engineering, Centre for Advanced Materials Joining, University of Waterloo, Waterloo, ON, N2L 3G1, Canada. Contact: e-mail: [email protected] PATRICE CHARTRAND, Professor, is with the CRCT—Center for Research in Computational Thermochemistry, Department of Chemical Engineering, E´cole Polytechnique, Box 6079, Station Downtown, Montreal, QC H3C 3A7, Canada. Manuscript submitted July 23, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
maximum solid solubility of Fe in Mg is only 0.00041 at. pct Fe.[2] There is also clear evidence that magnesium and steel do not react with each other and do not mix in the liquid state at ambient pressure.[2] Thus, metallurgical bonding between these two metals will only be possible provided another element that can interact and bond with both of them can be applied between the Mg and Fe and act as an intermediate interlayer element. The feasibility and benefits of using Cu, Sn, and Ni interlayers to join Mg and steel by various fusion welding processes have been investigated experimentally with varying degrees of success.[5–7] More recently, Nasiri et al.[8,9] have studied the use of Al and Ni interlayers when laser brazing these dissimilar metals using a Mg-Al alloy filler wire. However, identifying and evaluating an appropriate interlayer between steel and Mg alloys experimentally can be costly and time consuming. This process can be shortened with the use of computational thermodynamics, which provides a convenient way for predicting the phases most likely to form in complex multi-element alloy systems at the interface as a function of alloy composition and temperature with greater accuracy than is possible using the binary or ternary phase diagrams of the constituent elements. Prior knowledge of these phases ca
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