Optimizing the Process Parameters for TLP Bonding of AISI 321 Stainless Steel

To enhance the joint’s properties for AISI 321 steel, due to the limitations of conventional joining methods, transient liquid phase bonding was investigated for this steel in order to achieve the optimum process parameters. In this way, the effects of bonding temperature (1050 to 1200 °C) and time (30 to 120 min) on microstructure and shear strength of the AISI 321/MBF-20/AISI 321 and AISI 321/MBF-30/AISI 321 joints were studied. Results showed that while incomplete isothermal solidification of the samples results in the formation of Fe–B, Cr–B, Ni–Si, and Ni–B phases at the centerline of the joints and diffusion-affected zone, more homogenous joints with higher shear strength values were obtained with the increase in bonding temperature and time. In addition, it was found that the presence of Fe and Cr in chemical composition of MBF-20 interlayer increased the amount of eutectic compounds at the centerline of joints and the amount of precipitates at heat-affected zone. Furthermore, it was found that regardless of the type of interlayer, the increase in bonding temperature from 1050 to 1150 °C increased the shear strength of the joints. The highest shear strength occurred in AISI 321/MBF-30/AISI 321 system bonded at 1150 °C for 60 min that was 99% of that of the base metal. Also, it was found that further increase in the bonding temperature beyond 1150 °C reduced drastically the shear strength of the bonds due to the occurrence of partial melting, especially at the grain boundaries of base metal. In addition, the study of the fracture surface of the joints showed that samples with high shear strength values were fractured by ductile mode. However, features of brittle fracture were observed in the fracture surface of the samples with lower shear strength values.