Joining steels with electrodeposited nickel or copper barrier layers using Cu–P brazes

To prevent the formation of continuous brittle phosphides when steel is brazed with Cu–P–Sn alloy, barrier layers were deposited on the steel: nickel on stainless steel and copper for mild steel. Their effectiveness is demonstrated by the increase in the lap shear strength from <10 to 160 MPa for the mild steel joints and from 80 to 136 MPa for those with stainless steel. The microstructures of the brazements were investigated by optical and scanning electron microscopy (including microprobe analysis) to ascertain the reactions and mechanisms occurring at the intermediate layer interfaces – both with the basis metal and with the braze alloy. Interactions involving the barrier metal result in the formation of a solid solution with discrete particles of complex compositions in the interfacial region. These reactions are analysed and the resultant phases identified. Optimisation of brazing time and temperature and of the layer thickness resulted for both systems in (near) complete elimination of the phosphide particles in the joint zones adjacent to the parent metal. The suppression of the continuous brittle interfacial phosphide layer enhances the mechanical properties of the joints.