Joining steels with electrodeposited nickel or copper barrier layers usingCu-P brazes

To prevent the formation of continuous brittle phosphides when steel is bra zed with Cu-P-Sn alloy, barrier layers were deposited on the steel: nickel on stainless steel and copper for mild steel. Their effectiveness is demons trated by the increase in the lap shear strength from <10 to 160 MPa for th e mild steel joints and from go to 136 MPa for those with stainless steel. The microstructures of the brazements were investigated by optical and scan ning electron microscopy (including microprobe analysis) to ascertain the r eactions and mechanisms occurring at the intermediate layer interfaces - bo th with the basis metal and with the braze alloy. Interactions invoicing th e barrier metal result in the formation of a solid solution with discrete p articles 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 i n (near) complete elimination of the phosphide particles in the joint zones adjacent to the patent metal. The suppression of the continuous brittle in terfacial phosphide layer enhances the mechanical properties of the joints.