Effect of laser power and powder feed rate on interfacial crack and mechanical/microstructural characterizations in repairing of 630 stainless steel using direct energy deposition

Abstract

Directed energy deposition (DED) can be used to repair damaged components. However, interfacial defects can occur between the repaired zone and substrate. Avoiding defects in repair processes using DED is critical because these defects significantly degrade the mechanical properties of the repaired parts. In this study, we investigated the occurrence of defects based on the laser power and powder feed rate. When experimenting with different powder feed rates (PFR), no defects such as lack of fusion and interfacial crack occurred under PFR 3.5 g/min conditions. However, repair deposition with a higher laser power resulted in cracks at the interface between the repaired region and the substrate. Microstructure observations showed that a mixture of lath martensite and retained austenite was formed in the repaired region. In the substrate, lath martensite was dominant due to solution annealing. Electron backscatter diffraction and X-ray diffraction analysis results confirmed that a large amount of the face-centered cubic (FCC) phase was formed in the deposited zone, and the heat-affected zone (HAZ) and substrate were mostly composed of the body-centered cubic (BCC) phase. Owing to these microstructural characteristics, the hardness of the deposited zone was lower than that of the substrate. Meanwhile, in the HAZ, the hardness increased significantly, which was attributed to the aging effect caused by continuous heat input during the DED process. The results of a tensile test indicated that the strength of a specimen with interfacial defects on the slope was 80.8% of that of the wrought material. However, compared with the cracked specimen, the repaired specimen exhibited improved tensile properties, even when there was a defect at the interface. Meanwhile, the strength of the specimen without interfacial defects was 97% of that of the wrought material. These results can provide guidelines for selecting the appropriate laser power and powder feed rate for defect-free repair using DED.