Interfacial microstructure and properties of diode laser brazed AZ31B magnesium to steel joints using Al-12Si and Ni interlayers

The brazeability of AZ31B-H24 magnesium alloy and steel sheet using two different types of micro-interlayers (Al-12Si and 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). The results of this study suggest that the feasibility of this process depends strongly on the pre-existing Al-12Si coating or Ni electro-plating layer on the steel sheet. Both interlayers promoted wetting of the Mg-Al filler alloy. Use of the Al-12Si interlayer resulted in the formation of a brittle layer of θ-Fe(Al,Mg) 3 intermetallic compound along the braze/steel interface, which was responsible for bonding. In comparison, using a Ni interlayer led to formation of a band of AlNi intermetallic compound with different morphologies along the steel-fusion zone interface. However, this phase was not directly responsible for bonding. 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 at the steel interface. The average fracture shear strength of the bond using the Al-12Si and Ni interlayers reached 49.3 MPa and 96.8 MPa, respectively, which gave joint efficiencies of 31% and 60% with respect to the AZ31B-H24 Mg alloy base metal. Thus, the Ni interlayer resulted in joints with significantly improved joint strength relative to those made using the Al-12Si interlayer. Copyright © 2013 ASM International® All rights reserved.