Brazing of graphite and stainless steel with Ag-Cu-Ti filler: Effects of brazing process parameters on microstructure and mechanical properties

Using Ag-Cu-Ti solder (an active filler alloy) system, the graphite and stainless steel substrates were successfully connected by vacuum brazing. The phase composition, microstructure results show the solder is completely melted during the temperature form 850–1000 °C, and part of the Ti has reacted in the matrix. It forms a reaction layer with the graphite layer to form TiC, and a TiFe diffusion layer with the stainless steel layer. The composition of the joint was found to be graphite/TiC/Ag(s,s)+Cu(s,s)+Ag-Cu eutectic/TiFe/stainless steel by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). As the brazing temperature gradually elevated in the range of 850–1000 °C, TiC reaction layer on graphite side and TiFe reaction layer on stainless steel side became thicker. Moreover, brazed joints showed the first increase and later decrease variation trend in terms of their shear strength, which peaked at the temperature of 950 °C. Besides, as holding time extended from 10 to 30 min at 950 °C, the thickened reaction layers of TiC and TiFe were observed, and the joint resistance showed the first increase and later decrease trendwhile. When holding time extended from 30 to 60 min, the holding time made no significant difference to shear strength. The temperature of 950 °C and 10 min holding time were determined as the optimal brazing temperature and holding time, and joints were dense and crack-free with shear strength of about 27.4 MPa and contact resistance of about 42.5 mΩ. It has excellent electrical conductivity while ensuring the high mechanical strength of the connection strength.