Transient Liquid Phase Bonding of 17-4-PH Stainless Steel Using Conventional and Two-Step Heating Process

Abstract

Conventional and two-step heating transient liquid phase bonding (TLP) experiments of martensitic precipitation hardening stainless steel 17-4-PH were studied employing AMS 4777 braze filler at temperature of 1050 °C for different times (15 to 90 min) to accomplish the ideal brazing condition. The effect of TLP methods and bonding conditions on the microstructure and shear bond strength was investigated. The critical bonding time for isothermal solidification in conventional and two-step heating TLP was 60 min and 45 min, respectively. The microstructure of isothermal solidification zone consisted of Ni-rich solid solution phase. Eutectic constituents Cr-rich boride and Ni3Si were formed in the joint centerline in the athermally solidified zone. In the conventional method planar interface was formed, however in TLP bonding under temperature gradient non-planar interface was formed, high angel grain boundaries merged at the end of isothermal solidification and homogenous bond was produced. Due to the decrease of athermally solidified zone the shear strength increased by increasing the holding time and when using the two-step heating process because of better metal to metal contact, the shear strength was higher. The optimum condition was a temperature gradient TLP in 60 min bonding time, which led to shear strength of 435 MPa.