직접 에너지 적층 공정을 이용한 316L 스테인리스강의 보수 공정 설계

Abstract

The metal additive manufacturing (AM) process is a technology that uses laser or electron beam to melt metal materials and rapidly solidify them, depositing layers of metal into three-dimensional shapes. One of the metal additive manufacturing processes, the direct energy deposition (DED) process can be used for product manufacturing, mold hardfacing, remodeling and repair. In particular, the laser-based DED process facilitates local deposition when parts are repaired, and has good quality advantages due to its lower thermal effect than conventional welding. Therefore, DED repair studies using various materials are actively carried out. However defects, such as pores and cracks, can occur at the interface between the substrate and the area to be repaired. In this study, the 316L stainless steel was repaired using the DED process, and in the process, it was found that defects occurred in the slope of the repair and substrate due to the temperature gradient and thermal stress of the melt pool. Thus, various methods such as groove shape, post heat treatment, and substrate heating during process were attempted to suppress defects. In this study, a basic experiment was conducted according to the process variables for analysis of the 316L stainless steel deposition characteristics, and the design height and the conditions for which no defects occurred were elected. These conditions were used for repair, and the possibility of repair was analyzed through the assessment of mechanical properties before and after. In addition, the results of experiments with the shape of the groove confirmed that the size of the cracks generated was reduced and the mechanical properties improved. Nevertheless, cracks occurred on the sloped surface due to lack of fusion. To this end, excess deposition was attempted such that the deposition volume is greater than the volume to be repaired, to cool down slowly. As a result, the 1mm repair depth specimen has reduced the size of the crack, increasing the tensile characteristics. In addition, the in-situ substrate heating was applied to reduce the cooling rate and thermal stress. This is commonly used in welding and is a method of heating the substrate before repair and during the process. As the temperature of the substrate heating increased, the crack size decreased, and the tensile characteristics were improved. In addition, the relationship between changes in microstructure characteristics and hardness was identified. Nevertheless, the tensile characteristics compared to the original materials were reduced due to cracks still present on the slope. This is because the energy density of melt pool formed on the slope decreases, resulting in a lack of fusion. Thus, it is planned to observe the relationship between the supplied energy density and deposition characteristics depending on the inclination of the sloped surface. Based on these findings, I would like to propose a variety of methods for controlling faults that can occur in the DED process that can be used in the component repair industry.