Sponsored by the American Welding Society and the Welding Research Council Interfacial Microstructure of Diode Laser Brazed AZ31B Magnesium to Steel Sheet The formation of a nano-scale Fe(Ni) transition layer on the steel during laser brazing was found to be responsible for the formation of a metallurgical bond Introduction SUPPLEMENT TO THE WELDING JOURNAL, JANUARY 2013 Using a Nickel Interlayer between the steel and magnesium BY A. M. NASIRI, D. C. WECKMAN, AND Y. ZHOU Prized for its excellent strength-toweight ratio, magnesium and its alloys are currently under intense investigation for use in many applications in the automotive and aerospace industries (Refs. 1–3). However, steel sheet is still the most commonly used material in the automotive industry for fabrication of autobody structures. The ability to make hybrid structures of magnesium alloy and steel sheet would be desirable for many applications in the automotive industry, because the overall weight of the autobody could be reduced resulting in better fuel efficiencies 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 (Refs. 3–9). Joining magnesium alloys to steel by conventional fusion welding technologies is difficult due to the large difference in the melting points between Mg (649°C) and Fe (1538°C). In addition, the boiling point of magnesium is only 1091°C, so direct contact with molten steel causes catastrophic vaporization of the magnesium (Refs. 3–8). Moreover, the maximum solid solubility of Fe in Mg is estimated to be only 0.00041 at.-% Fe (Ref. 6) and wetting of steel by molten magnesium is very poor (Ref. 8). The weldability of magnesium to steel using the hybrid laser-arc welding (Refs. 5, 6, 8, 9) and resistance spot welding (Refs. 4, 7) processes has been examined. Zhao et al. (Ref. 5) used a hybrid laser-gas tungsten arc welding (GTAW) process to join AZ31B magnesium alloy and 304 stainless steel. However, oxides that formed at the interface were found to cause joints with poor tensile strength. Using the same welding technique, Liu et al. (Refs. 8, 9) studied lap joining of AZ31B Mg alloy to Q235 steel with Sn and Cu interlayers. Mg2Sn and Mg2Cu intermetallic compounds were found to form along the grain boundaries of the Mg alloy when using the Sn and Cu interlayers, respectively. The use of Sn and Cu interlayers was reported as the main reason for the elimination of gaps along the steel-fusion zone interface and the improvement of wetting properties of the steel by molten magnesium alloy (Refs. 8, 9). Finally, in a recent study, Liu et al. (Refs. 4, 7) used resistance spot welding to join AZ31B magnesium alloy to DP600 Zn-coated steel. They found that a preexisting transition layer of Fe2Al5 between the Zn coating and the steel improved wetting and bonding between the steel and the magnesium alloy. Review of the literature suggests that joining Mg alloys to steel will be possible provided the temperatures required for joining are kept below the boiling point of the magnesium alloy (1091°C) and provided another interlayer element is used that can interact and promote wetting and bonding between both immiscible alloys. For this reason, brazing can be a superior choice in joining dissimilar metals such as magnesium and steel because brazing KEYWORDS Laser Brazing AZ31B Mg Sheet Steel Sheet Dissimilar Intermetallic Compound A. M. NASIRI(amnasiri@uwaterloo.ca), D. C. WECKMAN, and Y. ZHOU are with Department of Mechanical & Mechatronics Engineering, Centre for Advanced Materials Joining, University of Waterloo, Waterloo, ON, Canada. ABSTRACT The brazeability of AZ31B-H24 magnesium alloy and steel sheet with a microlayer of electro-deposited Ni in a single flare bevel lap joint configuration has been investigated. The macro- and microstructure, element distribution, and interfacial phases of the joints were studied by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Wetting of the steel by the Mg-Al brazing alloy was improved significantly through the addition of a Ni electroplated interlayer. Bonding between the magnesium brazing alloy and the steel was facilitated by the formation of a transition layer composed of a solid solution of Ni in Fe on the steel followed by a layer of α-Mg + Mg2Ni eutectic. A band of AlNi intermetallic compound with different morphologies also formed along the steel-fusion zone interface, but was not directly responsible for bonding. Ni electroplating was found to significantly improve the brazeability and mechanical performance of the joint. The average fracture shear strength of the bond reached 96.8 MPa and the joint efficiency was 60% with respect to the AZ31B-H24 Mg alloy base metal. In all cases, failure occurred in the fusion zone very close to the steel-fusion zone interface. WELDING JOURNAL 1-s WELDING RESEARCH
Welding Journal | January 2013
To see the actual publication please follow the link above