Direct metal deposition of IN625 on IN738LC superalloy: microstructure and crack analysis

Abstract

The direct metal deposition (DMD) of IN625 powder on the IN738LC substrate has been investigated in the present work. The samples were prepared by the DMD process with different scanning speeds and powder feed rates. The microstructure and phase characteristics of the specimens were investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-Ray diffraction analysis. The samples’ hardness was measured at room temperature, as well. The results showed that the solidification structure changed from columnar dendrites at the bottom to equiaxed dendrites at the top of the melt pool. This development of dendrites in these areas depends on the temperature gradient “G” and the growth rate of dendrites “R” (G/R ratio). As the scan speed and the powder feed rate decreased, the G/R rate increased, resulting in an expanded column dendrite region. Besides, by reducing the scan speed, the heat input increased and the cooling rate decreases. Therefore, the dendrites have enough time to grow, which leads to an increase in the primary dendrite arm spacing. Also, the dendritic structure varied from fine to thick. As the scanning speed increased, the thermal gradient between the upper part of the deposit and the heat-affected zone was high, leading to increased tensile stresses in the heat-affected zone. As a result, strain age cracks occurred in the heat-affected zone. The buildup of IN625 with scan speed 2 mm/s and powder feed rate 58.4 mg/s successfully deposited on the IN738LC substrate.