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Effect Of Scrap Returns On Microstructure And Properties Of Silver-based Brazing Filler Metals And Stainless Steel Brazed Joints

Posted on:2016-06-13Degree:MasterType:Thesis
Country:ChinaCandidate:Y YaoFull Text:PDF
GTID:2191330461451435Subject:Materials science
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Silver brazing filler metal has been widely used in precision instrument, aerospace and household appliances industry due to its excellent manufacturability, suitable melting point and favorable wettability. Actually, a certain percentage of scrap returns are used for raw material in order to reduce the cost of silver brazing filler metal in practical production owing to its containing high content precious metals(such as Ag, Cu and Zn), however, some impurity elements will be brought into unavoidably the silver brazing filler metal which will do harm to its microstructure and properties. Therefore, it’s necessary to maximize the dosage and explain the influence of scrap returns on the microstructure and properties of corresponding filler metal. In this paper, the influence of scrap returns on the microstructure, composition, melting characteristics, wettability of silver brazing filler metal, and mechanical properties of the brazed joints of stainless steel were investigated by OM, SEM, EDX, XRD, ICP, DSC, wetting test, tension test, and microhardness measurement. The results show that the miscrostructure of filler metal containing scrap returns and the welding seam is mainly composed of Ag based solid solution, Cu based solid solution, Ag Zn and Cu Zn compound. With the content increasing of scrap returns in the filler metal, the amounts of H and O in the filler metal have increased, while the amounts of Ag, Cu and Zn have changed little, and the particle-sizes of Ag based solid solution and Cu based solid solution has become uneven, the amount of lambllasome eutectic microstructure of Ag Zn and Cu Zn compound has increased.DSC and wetting test results indicate that, with the content increasing of scrap returns in the filler metal, the liquid temperature of the filler metal has increased, while, it’s silidus temperature has decreased, and the wettability has become worse.When the addition of scrap returns in the filler metal change from 25 wt.% to 50 wt.%, the liquid temperature of the filler metal increase 2.8℃ and the silidus temperature decrease 1.5℃, and the spreading areas reduce from 417.5mm2 to 336.2mm2.Brazing stainless steel joint analysis results show that the interface of joint which brazed by pure filler metal is mainly composed of Ag based solid solution and Cu based solid solution. while these brazed by filler metal containing scrap returns has no Ag based solid solution, and The thickness of the Cu based solid solution become thicker. The microshardness and tesile strength of the joints increase first and then decrease with scrap returns increasing. When the addition of scrap returns in the filler metal reach 25 wt.%, The microshardness and tesile strength of the joints are 175.2HV0.1 and 381.731 Mpa, respectively, which corresponding fracture surfaces have shown the characteristics of fracture toughness. These containing much more scrap returns have shown the characteristics of brittle fracture, and much more defects,such as hole and entrapped slag, were observed, which would weakening the tesile strength of joints.In summary, the comprehensive performance of Ag filler metal containing 25 wt.% scrap returns is as good as the pure-filler-metal’s, the addition of scrap returns in the Ag filler metal should be controlled no more than 25 wt.% in the producing process.
Keywords/Search Tags:silver based filler metal, scrap returns, miscrostructure, mechanical properties
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