Study On Plating Preparation And Brazing Process Of Supersaturated AgCuZnSn Filler Metals

The surface plating technology was used to be in the field of brazing alloy manufacturing, and combination techniques of plating and heat diffusion process were applied to fabricate brazing filler metals, providing a new method to prepare supersaturated brazing filler metal, in order to breakthrough the traditional limit of melting alloyed method. Through the plating-heat diffusion process, to improve the Sn content and optimize the properties of silver brazing filler metals, preparing the AgCuZnSn supersaturated brazing filler metals. At the same time, induction brazing and flame brazing methods were used to join brass and stainless steel. The microstructure and properties of brazed joints with supersaturated brazing filler metals were studied, and analysis of the experimental results using thermodynamic principle. The properties of supersaturated brazing filler metals and its brazing process were compared with traditional brazing filler metals under the same condition of the Sn content. The conclusion of this paper mainly can be proposed as follows.(1) The optimization of plating preparation process was studied. In order to optimize the plating preparation process, the thickness control technology of coating was cooperative executed using current density, bath temperature, electrode distance and other parameters. The Sn content of supersaturated brazing filler metals were regulated by the thickness of the coatings.Optimum conditions of the plating-heat diffusion process were 1) current density of 4 A/dm2, electrolyte temperature of 40℃, electrode distance of 22 mm, ultrasonic power of 240 W, ultrasonic frequency of 45 kHz, plating time 7.5 min, and 2) diffusion process was temperature of 220℃ and time of 24 h. Then the diffusion limit value of Sn content in the supersaturated brazing filler metal was 7.2%, it was higher than that of traditional method about 31%.(2) Research on diffusion mechanism of brazing filler metals with tin coatings. The relationship between the microstructure and properties of brazing filler metals and diffusion temperature and time was established. It was found that under the same condition of composition and geometric distribution, the expected microstructure could be obtained by regulation of diffusion parameters.The temperature of solid-liquid phase of brazing filler metals was decreased and the thickness of diffusion transition zone was increased with the temperature rising and diffusion time prolonged. The microstructure of diffusion transition zone was mainly composed of Cu3Sn phase and Ag3Sn metastable phase. The distribution of Sn element in the transition zone was uniform. The formation mechanism of diffusion transition zone could be summarize as brazing, mutual diffusion, metastable, alloying.The kinetics equations for growth behavior of diffusion transition zone at 200℃ and 220℃ were established. The thickness of transition zone increased exponentially with the diffusion temperature and time increasing.(3) The properties of supersaturated brazing filler metals was discussed. The composition of brazing filler metals was regulated by the diffusion Sn content, in order to control the spreadability, melting temperature and mechanical properties of supersaturated brazing filler metals. The relationship between Sn content and properties of supersaturated brazing filler metals was obtained.The spreadability of the brazing filler metals was improved, the DSC endothermic peak shifted to the left, the melting temperature range of the supersaturated brazing filler metals was decreased, the tensile strength increased with the Sn content increasing. Compared with the traditional brazing filler metals under the same condition of the Sn content, the melting temperature range of supersaturated brazing filler metals decreased about 3.65%～14.5%, its wetting area increased about 7.8%～22.6%, the tensile strength of supersaturated brazing filler metals was slightly lower than the traditional brazing filler metals. When Sn content was 7.2%, the melting temperature and wetting areas of supersaturated brazing filler metals were 642～676℃ and 481 mm2.(4) Research on the microstructure and properties of the joints with supersaturated brazing filler metals. The induction brazing and flame brazing methods were applied to join H62 brass,304 and 316LN stainless steel, in order to obtain the relationship between the microstructure and properties of brazed joints and Sn content, and the properties of the joints with supersaturated brazing filler metals were compared with traditional brazing filler metals.When Sn content was less than 5.5%, the microstructure of the joints using supersaturated brazing filler metals were compose of Cu-rich phase, Ag phase, CuZn phase and CusZn8phase. But Sn content was higher than 5.5%, the microstructure of the joints by supersaturated brazing filler metals were compose of Cu-rich phase, Ag phase, CuZn phase, CusZn8 phase, Cu4iSnn phase and Ag3Sn phase. With the Sn content of supersaturated brazing filler metals increasing, the tensile strength of brazed joints were increased. The supersaturated brazing filler metals containing 5.5% Sn and 6.0%Sn were applied to braze the 316LN stainless steel and 304 stainless steel/H62 brass, respectively. Their tensile strength were 415 MPa and 395 MPa, which were slightly lower than the tensile strength of the traditional brazed joints under the same condition of the Sn content (430.5 MPa and 407 MPa). The reason was that the proportion of Cu3Sn and Ag3Sn phase in the supersaturated brazing filler metals increased with the Sn content increasing. It was lead to the tensile strength was slightly lower than the traditional brazing filler metals.(5) The thermodynamic properties of supersaturated brazing filler metals was studied using differential scanning calorimetry. The mathematical expression of the process entropy and performance entropy of the supersaturated brazing filler metals were proposed and to compare the theoretical results with the brazing filler metals spreadability and the tensile strength of brazed joints. Under the same condition of the Sn content, the process entropy value of supersaturated brazing filler metals was smaller than traditional brazing filler metals, and its performance entropy value was slightly higher than the traditional brazing filler metals. The experimental results indicated that the expression of process entropy and performance entropy could quantitative analyze the brazing process of supersaturated brazing filler metals and the mechanical properties of the brazed joints.(6) Evaluate on the corrosion resistance of brazed joints. The 3.5% NaCl solution was used to simulate seawater environment and evaluate the corrosion resistance of different brazed joints. When H62 brass/304 stainless steel brazed joint and 316LN stainless steel brazed joint were dipped in the solution of 3.5% NaCl for 2.5 h and 4 h, respectively. The former showed that serious corrosion phenomenon was occur at the interface of the 304 stainless steel and brazing seam, and corrosion groove was found on the interface region. The latter indicated that the interface of 316LN stainless steel and brazing seam would exhibit galvanic corrosion. Because the degree of crevice corrosion of 304 stainless steel in 3.5% NaCl solution was increased with the area of brazing seam decreasing. The potential difference was existed on the interface of 316LN stainless steel and the brazing seam.This thesis was supported by the national key basic research development program (973 program,2012CB723902) and the national high technology research and development program (863 program,2012AA040208).