Investigation On Microstructure And Properties Of Cadmium-Free Silver-Based Intermediate Temperature Filler Metals

The cadmium-containing silver-based filler metals were applied extensively in the industry field because of the good properties of filler metals, but the appliance of filler metals were limited recently, because the element of cadmium was poisonous and the silver was more and more expensive, so the silver-containing 20 percent and cadmium-free of silver-based filler metals were studied. In this paper, the composition of filler metals was optimized by the method of phase graph, and the result was discussed by the distribution of temperature of AgCuZn and the distribution of phase of AgCuZn, at last, thirteen filler metals were get. The microstructure of cast filler metals, the wettability of filler metals, the properties of joints brazed with filler metals and the microstructure of interface were studied experimentally and theoretically.The studies of the microstructure of filler metals indicate that the filler metals are mainly consist of Cu-rich phase, CuZn compound, tin bronze (Cu5.6Sn compound and Cu40.5Sn11 compound), Cu3P compound or Ni3P compound and Ag solid solution. With the increasing of Sn, the proportion of each phase of filler metals is changed, and Cu40.5Sn11 compound can be observed. With the increasing of Ni, the size of tin bronze becomes smaller and smaller, and the Ni3P compound is observed; with the increasing of La, the size of microstructure of filler metals becomes smaller than before apparently, all these can effect the tensile strength of filler metals. The results of the tensile strength of filler metals indicate that when the content of Sn, Ni, La are 6.5%, 2.0%, 0.5%, respectively, the maximum tensile strength of filler metals are attained.By the experiments of wettability of filler metals, we can know that the amount of 1Cr18Ni9Ti stainless steel, which is dissolved into the welding seam, becomes more and more with the increasing of Sn; the amount of that becomes less and less with the increasing of Ni; when La is added into the filler metal, 1Cr18Ni9Ti stainless steel can be restrained dissolving into the welding seam; the amount of copper, which is dissolved into the welding seam, becomes more and more with the increasing brazing temperature. From the results of the experiments of wettability, the maximum wettability area of filler metals is attained, when the content of Sn, Ni, La are 6.5%, 2.0%, 0.3%. The maximum wettability area of filler metals on substrates of copper and 1Cr18Ni9Ti stainless steel are 351mm2, 212mm2, respectively, which are bigger than the wettability area of BAg30CuZnSn filler metal.The analysis of microstructure of joints indicates that the size of the microstructure of welding seam becomes smaller and smaller with the increasing of Ni, La, when the element of La is added into the filler metals, black compounds of silver and lanthanum are observed; From the analysis of shear strength of joints, we can know that when the content of Sn, Ni, La are 4.5%, 2.0%, 0.5%, the maximum shear strength of joints brazed with the filler metals are attained, respectively. Compared with the BAg30CuZnSn filler metals, we can know that the shear strength of joints of 1Cr18Ni9Ti stainless steel and copper brazed with filler metals, in which the content of Sn,Ni,La are 6.5%,2.0%,0.5%, respectively, is stronger than the shear strength of joints brazed with BAg30CuZnSn filler metal.