Interfacial microstructure of diode laser brazed AZ31B magnesium to steel sheet using a nickel interlayer

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 + Mg 2Ni 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.