Brazing Mechanism Of Graphite And Copper And Preparation Of New Lowtemperature Active Brazing Alloy

With excellent thermal shock and heat fatigue resistance as well as high melting point and heat conductivity, graphite has wide application field. For example, graphite commutators can be used in not only all direct current engine, but also aviation, space and army fields. In this paper, commercial and self-developed brazing alloy were used to join graphite and copper. Interface reaction mechanism of the brazed joints and preparation principle of braze alloy were analyzed. The investigation not only fills up the technique blank of producing graphite commutators in our country, but also supplies technique reservation for joining carbon-based materials with dissimilar alloys.Ag, Cu, Ti, Sn powder were selected to make braze alloy by mechanical alloying, in which Ag powder was added by two ways (pure Ag powder or AgCu eutectic powder). The effects of ball milling time, ratio of grinding media and adding way of element on pattern of powder, grain size, degree of alloying and melting point of braze alloy were studied. The model of mechanical alloying was built and a local model of pitting flaking was also introduced. It can be known from mathematical, kinetic and thermodynamic models that when the milling speed increases, Pmax is large and the alloy powder is crushed heavily. If the powder grain is small, the stability of powder is good and the powder is not be melted during ball milling. When the rotary speed is 260r/min and the ball milling time is 7h, the average diameter of powder is below 20μm after ball milling.When TiZrNiCu filler was used to braze graphite with copper, the interface structure is graphite/TiC/Ti-Cu, Cu-Zr, Ni-Ti intermetallic compound/Cu-based solid solution/Cu, in which TiC is the key factor for bonding braze alloy and graphite successfully. Brazing parameters have effects on the microstructure and property of joint interface. The maximum joint strength 26MPa can be obtained when the brazing temperature is 1193K and holding time is 900s. The average resistance of joint brazed at 1223K/900s and 1173K/600s is 3.3 m? and 3.2 m?, respectively, which can meet the requirements of application. The resistance stability of joint is optimum when the parameter is 1223K/900s, considering the resistance extreme difference, resistance difference and variance between adjacent blades.The interface structure of graphite/AgCuTiSn/Cu joint is graphite /TiC/Ti3Sn+Ag(s. s)+Cu-Sn compound+Cu(s. s)/Cu(s. s)/Cu. As the brazing parameters increase, the size and area of white Ag(s. s) decrease and the intergranular infiltration of braze alloy into Cu base metal intensifies. Meanwhile, black intermetallic compound grows up obviously. Whisker microstructure is found at graphite side, which becomes much, long, large and then decreases, even disappears as the brazing parameters increase. The change of microstructure leads to the corresponding variety of joint property. The results show that the maximum joint strength 24MPa is obtained when the brazing temperature is 1093K and holding time is 900s. On this situation, the joint fractures near graphite side. During joining graphite and copper by AgCuTiSn braze alloy, Sn becomes liquid and accelerates the diffusion and reaction among other elements as well as the melting of braze alloy, the result of which decreases the melting point of braze alloy and supplies active Ti atoms during brazing. By using AgCuTi and AgCuTiSn braze alloys, graphite/copper commutators were brazed. After trial test, the work state of the brazed commutators is stable and their service life is over 15000h, which is 5 times longer than the service life of traditional commutators.Kinetic equation of reaction layer growth of the brazed graphite/TiZrNiCu/Cu joint was built. Interface reaction mechanism during brazing graphite/AgCuTiSn/Cu is analyzed in detail. By studying the reaction mechanism of braze alloy and carbon-based materials systematically, Ti and C reacts and forms TiC when using Ti-based braze alloy. There is a CrmCn reaction layer on the interface when using Cr-based braze alloy. The kind of CrmCn is affected by brazing temperature, holding time and other elements in the braze alloy.