Effect of Solidification Mode on Microstructure and Mechanical Properties of AISI420 Steel to SAF2507 Steel Dissimilar Joint Produced by Transient Liquid Phase

In this investigation, the microstructure and mechanical properties of AISI420/SAF2507 dissimilar joint produced by TLP process were investigated at different bonding times. For this purpose, TLP process was carried out in a vacuum furnace at 1050 °C using BNi-2 interlayer. The microstructure of different areas of the joint was evaluated by optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Also, the mechanical properties of the bonded samples were studied using microhardness and shear-tensile tests. It was found that isothermal solidification was completed after 60 min of bonding time. The width of DAZ in SAF2507 steel side was greater than that of AISI420 steel side at all bonding times owing to the easier volume diffusion of boron in to the fully austenitic structure of AISI420 steel as compared with dual crystal structure of SAF2507 steel. At bonding time of 60 min, by regarding the more time for diffusion as compared with 1 min, boride compounds dissolved in the base metals and were discontinued. Thus, the aggregation of boride compounds decreased and as a result the microhardness decreased. In fully isothermal solidification sample, due to the lack of eutectic and intermetallic compounds and creating a homogenous joint (γ-Ni phase), the strength increased. In fact, transformation of athermal solidification to isothermal solidification led to the increase of the joint strength.

The OM images indicating the base metals and the bonding area and DAZ of the bonded samples at different bonding times at temperature of 1050 °C for TLP bonding of AISI420/BNi-2/SAF2507.