Study On Process And Mechanism Of Brazing Of SiC And Nb With Composite Filler Metal

Residual stress of high level usually exists in the Si C brazing joints due to the mismatch of physical properties between ceramics and filler metals. Given this,homemade Ag Cu Ti/B4 C composite filler metals were employed in Si C/Si C and Si C/Nb systems for the in situ synthesis of Ti B whiskers and Ti C particles in seam, and thereby adjusting the coefficient of thermal expansion(CTE) of braze and lowering the residual stress. The strength of Si C brazing joints was, therefore, improved. Also, the strengthening and formation mechanism of brazing joints with composite filler metal was studied by experimental results and analytical work.In Si C/Si C joining system, Ag Cu Ti filler metals containing 0wt.%, 1wt.%, 1.5wt.%,2wt.% B4 C were employed to achieve the joining of Si C. The effects of brazing process parameters on the microstructure and mechanical properties of joints were studied.Besides, fracture location and crack propagation path were analyzed, and thereby establishing the connection among process, microstructure and mechanical properties. It was found that the increase of brazing temperature or holding time, to some extent,exerted positive effects on the in situ synthesis of Ti B and Ti C, which helped to lower the mismatch of properties between ceramics and braze, thus released the residual stress and raised the strength. In this situation, the angle between crack path and brazing seam reached 45°, and fracture mainly occurred on the ceramics. However, improperly high parameters resulted in violent interfacial reaction and deteriorating brazing joints, and fracture mainly occurred on reaction layer. When under same brazing process, the joints corresponding to composite filler metal containing 1.5wt.% B4 C showed the most satisfactory performance. The optimal processing parameters were determined as 950℃,10 min, and the average strength reached 140 MPa in this situation, which was 52%higher than the maximum strength of joints corresponding to pure Ag Cu Ti filler metal.Ag Cu Ti/B4 C composite filler metals were also employed in Si C/Nb joining system and achieved reliable brazing joints. According to the effects of processing parameters on microstructures and mechanical properties of Si C/Nb joints, it was found that there existed Nb-based solid solution at Nb/braze interface, and therefore showed excellent toughness. So the quality of brazing joints were mainly determined by the reaction at Si C interface and the in situ synthesis in brazing seam. When dependable metallurgical bonding was achieved at Si C interface and there existed dispersive and multiple in situ synthesized Ti B and Ti C, the shear strength of Si C/Nb joints reached 98 MPa,corresponding to the optimal parameters 890℃, 10 min.In addition, Si C/Nb joints were studied corresponding to composite filler metals with different compositions. Under the same brazing processing parameters, the strength of joints corresponding to 1.5wt.% B4 C filler metal was 60% higher than that corresponding to pure Ag Cu Ti filler metal. The CTE of brazing seam for pure Ag Cu Ti filler metal and filler metal containing 1wt.% B4 C was still very high, and thus the fracture occurred on ceramics completely with arc path under high residual stress.While the composite filler metal containing 1.5wt.% B4 C significantly reduced the CTE of brazing seam, the plasticity and toughness of seam stayed fine. In this situation, the joints showed composite fracture, and the crack propagated from Si C to brazing seam.Obviously the plastic deformation and combination effects of Ti B whiskers and Ti C particles would hinder the cracks’ propagation. When it came to composite filler metal containing 2wt.% B4 C, the CTE of brazing seam was extremely low, but the elastic modulus was significantly enhanced. The plasticity also deteriorated and thus had negative influence on the release of residual stress, and the fracture ultimately occurred at the interface completely.