Study On The Process And Mechanism Of Brazing Stainless Steel Using Modified Nickel-based Brazing Alloy

With the quality of life of people improve, demand for cars is also more and more high, however, effects of automobile exhaust pollution of our environment, especially presents a significant challenge to large and medium-sized city living environment. EGR cooler can improve the environment, reducing emissions of pollutants. Manufacturing cooler parts of the material usually choose 304、306、316 and 316 L stainless steel precision castings or profile, the 316 L stainless steel with good resistance to intergranular corrosion and become the preferred material. In the welding process of stainless steel, stainless steel with nickel base brazing filler is most common, however, during brazing alloying, brazing seam center generates brittle intermetallic compound phase, which makes the mechanical properties of the welded joint is reduced, therefore, need to study the interaction between base metal and filler metal and how to reduce the brittle compound brazing seam center. Based on the BNi-7 brazing alloy, adding alloying elements Cu, through the brazing alloy formula more reasonable design to optimize the welding joint.The research results show that: the melting point of the brazing alloy alloy elements added decreased after Cu, but the solid-liquid phase line temperature slightly increased. Improved wetting front are the precursor film exists, the precursor film plays an important role in brazing alloy wettability contact angle decrease and increase. With the increase of Cu content, the brazing temperature and holding time, the precursor film brazing alloy wetting front widens, brazing alloy spreading area increases, the wettability improvement. The addition of Cu organization is Ni(Cr, Cu) solid solution, Ni3P- Ni(Cr, Cu) eutectic structure and a small amount of Cr Ni P compounds. Compared with the BNi-7 brazing alloy organization, reaction phases did not change.Joint is mainly composed of stainless steel / brazing alloy interface of Ni(Fe, Cr, Cu) solid solution and the brazing seam center Ni3P-Ni(Fe, Cr, Cu) eutectic and Cr Ni P brittle metallic compound between phase composition. With the addition of Cu, brazing temperature, holding time increases and decreases the brazing gap, joint microstructure changed regularly, constant type brazing joint reaction phase, but the interface of Ni based solid solution thickness increases, the brazing seam center Ni3P-Ni(Fe, Cr) Eutectic eutectic group increased, toughness of Ni(Fe, Cr, Cu) solid solution amount increases, the decrease in the number of brittle compounds, joint shear strength increase.The structure evolution of brazed joint is divided into physical contact, solution diffusion, isothermal solidification and cooling crystallization of four stages. In the brazing temperature is heated to above the melting point of the liquid brazing alloy brazing alloy on 316 L stainless steel surface flow, diffusion dissolution and element and the occurrence of liquid brazing alloy. When the liquid brazing alloy in the local supersaturation, nucleation and formation of nickel based solid solution. In the island of nickel based solid solution together into one, dissolved stop, base metal and nickel based solid solution phase and solid state diffusion, forming a thin layer of phosphide layer at the interface. With the decrease of temperature, the liquid in the brazing seam center to meet the precipitation of Ni(Fe, Cr, Cu) phase, because the shape of relatively large Ni(Fe, Cr, Cu) phase around the liquid phase composition deviates from the original composition, at this time, in Ni(Fe, Cr, Cu) are formed around Cr Ni P compound, finally, the remaining liquid solidification, the formation of Ni3P-Ni reached the eutectic point(Fe, Cr, Cu) Eutectic phase. The fracture of the joint seam center in brazing alloying, brazing seam center containing brittle Ni3 P phase, the fracture mode is quasi cleavage fracture. The crack in the brittle phase Ni3 P initiation and propagation, in the face of nickel based stop extended solid solution, after a period of time, the crack in the shear stress under the action of will in the nickel based solid solution formed around again and continue along the brittle phase expansion.