Influence of bonding variables on transient liquid phase bonding behavior of nickel based superalloy IN-738LC

The effect of process variables on the microstructure and properties of transient liquid phase (TLP) bonded IN-738LC superalloy was investigated using AMS 4776 filler metal. Continuous centerline eutectic phases, characterized as nickel-rich and chromium-rich borides, were observed at the joints with incomplete isothermal solidification. The eutectic width decreased with the increase of holding time and the increase in initial gap size resulted in thicker eutectic width in the samples bonded at the same temperature and for equivalent holding times. In contrast to the conventional expectation of the increase in the rate of isothermal solidification with the increase of temperature, rate decrease was observed with the increase of temperature to 1150 °C. The investigations demonstrated that low isothermal solidification rate was not only due to the enrichment of liquid phase with the base alloying elements such as Ti but also because of the reduction of solid solubility limit of B in the base metal contributed to the reduction of isothermal solidification rate. Microhardness and shear strength tests were carried out in order to investigate mechanical properties of the bonded samples. In the bonding condition in which isothermal solidification did not completely occur, eutectic constituent with the highest hardness in the bond region was the preferential failure source. The results showed that homogenized joints had the highest shear strength.