| There are various kinds of solders and filler metals nowadays and new ones are invented endlessly. However, it is still lack of high-temperature solder to joining copper and steel alloys at 350-500℃. Recently, the development of innovative materials has been putting new request to the soldering technology and solders, especially fuctional materials whose microstructures and properties are sensitive to the changing of temperatures. Furthermore, the multi-step joining of complicated parts, e.g., heavy truck Cu radiator, also needs various melting temperature solders to avoid the remelting of seams. Therefore, the solders with melting temperature between 350-500 °C are urgently needed to joining these materials.Zn itself has a melting temperature of 419°C, and has a strong reaction with copper and steel. By alloying elements, the melting temperatures of Zn-based alloys can be adjusted to a suitable melting temperature between 350-500°C. Based on the principle of alloying, phase diagram and the research foundation,zinc(zinc), tin(Sn), copper(Cu) and bismuth(Bi) are picked as the basic composition. The melting temperature, wettability and shear strength of solder joints are chose as the objective function in uniform design method. By establishing the regression equation, the optimal composition of Zn-based high temperature solder was finally obtained. Results showed that Sn can significantly reduce the melting temperature and improve the spreading area. A small amount of Cu element can significantly increase the shear strength of solder joints. Both Sn/Cu and Sn/Bi exist interaction effect on the properties of solders, when Cu content is more than 1.8%, Sn has less effect on the wetting area. When Sn content is certain,the increase of Bi can significantly increase the wettability and shear strength. The optimal composition of Zn-based high temperature solder is as: 4.0 7.0 wt.% of Sn, 1.7-2.3 wt.% Cu, 1.0-2.0 wt.% Bi and balanced Zn.Systematically study the microstructures of Zn-5Sn-2Cu-1.5Bi(ZSCB) hightemperature solder and its interface as well as their properties. ZSCB high-temperature solder consists of η-Zn matrix, dendritic ε-CuZn5 phase, needle-flake like β-Sn phase and precipitate Bi phase, while the primary ε-CuZn5 is coarse and the β-Sn located at boundaty. Tensile strength of solder is 125.24 MPa, fractured at boundary and coarse dendritic phases, the joint of which is brittle. The microstructure of interface is formed by continous reaction layers of ε-CuZn5, γ-Cu5Zn8 and β-CuZn. The shear strength and elongation of joint is 41.98 MPa and 0.42%, respectively.With the method of modification and rapid cooling, the microstructure of solder was refined and the effect of rare earth metal, cooling rate and cooling-RE coupling on the microstructure and properties were studied. RE could modify the morphology, size and distribution of primary ε-CuZn5 and eutectic β-Sn/Bi phases. With the increase of RE, the primary ε-CuZn5 phase was transformed from petaloid to spindly dendritic phase, and the amount of ε-CuZn5 phase was increased, needle and flake-like β-Sn phases were changed to spherical. When the content of RE is 0.05wt%, the tensile strength of solder and elongation is 185.26 MPa and 1.21%, respectively. When compared with ZSCB high-temperature solder, tensile strength and elongation is increased by 60.02% and 70.42%, respectively. The density of corrosion product film was increased with the addition of RE, which improved the corrosion resistance of solder. Compared with RE modified solder, cooling rate has significantly effect on the morphology of η-Zn phases. With the increase of cooling rate, supersaturated solid solution η was formed and transformed from columnar crystal to equiaxed, when the cooling rate is 120 ℃/s, η-Zn solid solution columnar crystals is transformed into equiaxed with grain size of 5.70-8.65μm. The tensile strength is 193.56 MPa, creep stress index was 35.72, compared with natural cooling solder, tensile strength and creep stress index is increased by 54.55% and 34.49%, respectively. Cooling-RE coupling modified both η matrix, primary ε and β-Sn/Bi phases at boundary, the microstructure of which is consisted of equiaxed, spherical-like ε phase and fine β-Sn/Bi phases imbedded, with the tensile strength and elongation of 221.86 MPa and 1.51%, increased by 77.15% and 112.68%, respectively, compared with ZSCB high-temperature solder.Study the effect of process parameters on the interfacial microstructure and mechanical properties. The evolution of products at interface and the formation mechanism of solder joints was revealed by thermodynamics and dynamics analysis.With the increase of soldering temperature and holding time, the interfacial layer thickness is increased, and the number of layers is increased and then gradually decreased. When soldering parameters are T= 450℃ and t<45s, the interfacial microstructure consists of ε-CuZn5, γ-Cu5Zn8 and β-CuZn. With the increase of holding time, ε-CuZn5 reacted with Cu atoms as nuu ZC-5C-4 u ZC45 n?? ??, formed dendrite ε-CuZn4 phase. The interfacial microstructure is transformed into ε-CuZn5, ε-CuZn4, γ-Cu5Zn8 and β-CuZn four reaction layers, the layer number is increased. When soldering parameters are T= 450℃ and t> 90 s, ε-CuZn4 is consumed and reacts as854 Zn ??C-3C-2 ??ZnuCu u. With the continous increase of holding time, the layer number is decreased to γ-Cu5Zn8 and β-CuZn reaction layer. The entropy of generation and consumption of ε-CuZn4 was 92.90 J.K-1.mol-1 and 232.30 J.K-1.mol-1, respectively. Interfacial reaction products have a significant effect on the mechanical properties of solder joints. When the soldering parameters are T = 450 ℃ and t = 30 s, the ε-CuZn5 grain size is 2.32-3.51μm, with the whole reaction thickness of 26.56-32.43μm,the shear strength of which is 76.52 MPa. When the holding time is t> 45 s, the excess formation of ε-CuZn4 occurs, and the shear strength was reduced(38.25MPa). When soldering with cooling-RE coupling solder, the interfacial ε-CuZn5 phase was refined and the reaction activation of ε-CuZn5 and γ-Cu5Zn8 phase was increased, decreased the layer thickness of interfacial microstrcuture. The shear strength of solder joints was 92.63 MPa, improved by 120.76%, compared with ZSCB/Cu soldered at 430℃ for 30 s. |
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