Effect of Ni on the wetting and brazing characterization of 304 stainless steel by Ag–Cu alloy

The effect of Ni in Ag–Cu filler on the wetting and brazing characterization of stainless steel was studied by means of wetting test and brazing test under high vacuum condition. The wettability of filler metal and the joint strength were improved after adding Ni in Ag–Cu filler. The filler penetrates into stainless steel and the topmost steel grains separate from the substrate, exposing the metal surface without oxide film after adding Ni, which leads to the reduction in contact angle. The interdiffusion between FM and SS is enhanced after adding Ni, which leads to further reduction in contact angle. At wetting temperature, large amount of Ni in liquid enriched at solid–liquid interface, increasing the Cu content in liquid near the interface to form Cu-rich liquid, which caused the penetration of liquid filler. Cu first penetrates into steel grain boundary, which provides a path for Ni diffusion into stainless steel. After the oxide film was removed, a large number of Ni directly diffused into stainless steel to form (γ-Fe, Ni) layer on stainless steel surface. Cu in liquid can directly diffuse into (γ-Fe, Ni). Stainless steel can dissolve in Cu-rich liquid with high Ni content. In the absence of Ni, FM/SS interface is the weakest location of the joint. After adding Ni in filler, the formation of Cu solid solution at FM/SS interface and the enhancement of interdiffusion between FM and SS strengthen the FM/SS interface, leading to the increase in joint strength. The results in this paper suggest Ni is beneficial for brazing stainless steel by Ag–Cu alloy.