| Silver-based brazing filler metals is the most important and typical hard solder, which has been widely used for brazing low-carbon steel, stainless steel, copper and copper alloys, high temperature alloy and refrigeration industry. However, some gaseous elements, especially oxygen and nitrogen will be introduced inevitably in silver-based or copper-based alloy smelting process. Its subsequent processing and wider application have been restricted seriously by the high oxygen and nitrogen content.In this thesis, the influences of oxygen and nitrogen on the microstructure, melting properties, wettability, spreadability, brazability and mechanical properties of silver-based brazing filler metals and brazed joints were investigated by optical microstructure(OM), scanning electric microscopy(SEM), energy dispersive X-Ray Spectrometer(EDS), X-ray diffraction(XRD), hydrogen/nitrogen/oxygen determinator, differential scanning calorimetry(DSC), tension test, and micro-hardness test. The formation mechanism of defect caused by oxygen and nitrogen were also be disscusssed, providing reference for the production of high quality solder.The results have shown that the oxygen and nitrogen content of the powderd silver filler metal which were prepared by atomization, reduction, oxidation, rised from0.0870wt.‰,0.0086wt.‰to4.264wt.‰,1.305wt.%o respectively. The main phase compositions of the AgCuZnCd powder filler metals are AgCdo.6Zn0.4, Cu2NiSn and Cu5Zn8phases. A small amount of NiO and CuO were produced during atomization process, and almost disappeared by hydrogen reduction. A large amount of high melting point oxides such as NiO, Ni0.7OZn0.3and Cu0.2Ni0.8O were generated because of oxidation treatment at300℃.With the increase of oxygen and nitrogen content, the solid and liquid phase line of AgCuZnCd solder raised32.1℃and32.4℃, respectively, and the corresponding crystallization temperature range increased from7.9℃to18.2℃. Spreading areas of AgCuZnCd solder decreased from395.6mm2to247.6mm2.In brazing process, liquid solder ability of clearance filling reduced significantly with the increase of oxygen, nitrogen content, and nodulation-shaped oxidation slag was formed on the brazed joints surface. Tensile strength of brazed joints declined sharply from334.1MPa to232MPa. Brazing filler metals with higher gas content required a long time local heating due to the higher melting point, portion of the substrate is melted and reacted with N generating Fe4N which is infiltrating brazing seam, resulting in a significant increase of hardness of the interface region, reached250.45HV0.1. The hardness of the brazing seam center has no obvious change, ranging between173HV0.1and183HV0.1.Brazing defects caused by oxygen and nitrogen mainly include blowhole, inclusions, cracks, etc. The microstructure of brazing seam is mainly composed of β1(AgCd phase),βZ (AgZn phase), y (CuZn phase) and trace amounts of Ag (Ag solid solution). The crack is mainly produced in βz (AgZn phase), y (CuZn phase) which is brittle phase area. With the increase of oxygen, nitrogen content in brazing filler metals, oxygen reacted with C in the base material producing CO in the brazing process, formating reactive pores, a large amounts of nitrogen was precipited in solidification, resulting in precipitate pores. |
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