Effect of Ni on grain boundary penetration in vacuum brazing of copper and stainless steel

Ni plating is often required when copper and stainless steel are vacuum brazed with Ag28Cu filler. However, grain boundary penetration (GBP) of molten filler into Cu substrate occurs after adding Ni. In order to analyse the reason for GBP caused by adding Ni, the effect of Ni on GBP was investigated and the penetrating species in multi-component liquid filler was determined by experimental study and thermodynamic calculation. The penetration degree increases with the increase of the added Ni amount. SEM observations show the nanometric liquid metal film contains a lot of Ag. TEM observations show that there are only Ag and no Ni at the penetration front, indicating that it is Ag penetrating into copper grain boundary (GB) firstly. At brazing temperature, Cu content in liquid near solid–liquid interface where Ni content is high will increase by absorbing Cu in the surrounding liquid, leading to the formation of Ag-rich liquid in brazing seam centre. Ag in Ag-rich liquid tends to diffuse into solid copper where Ni amount is too low to form low melting point Ag28Cu eutectic. Ag has to diffuse into Cu substrate interior along GBs. Liquid-Ag28Cu film is firstly formed along GBs by eutectic reaction, resulting in GB decohesion. However, the effective measures for inhibiting GBP are not introduced in this paper and will be studied in the future.