금속 3D 프린팅 공정으로 다층적층된 금형구조의 충격 및 전자빔 표면 후처리 특성

Abstract

Recently, several researches have been investigated the multi-layer hardfacing technology using a metal three-dimensional (3D) printing process to increase the service life of the hot forming die. In order to improve the surface roughness, the morphology and the stability of the hardfaced region, a post process is needed. However, the fatigue life and the hardness of the hardfaced region is reduced when the machining based post-processing technology is applied to the hardfaced region. Hence, a new post-processing technology for the metal 3D printed part is needed to decrease stress relief after post-machining. The objective of this thesis is to investigate characteristics of impact and electron beam post-processing for the multi-layer deposited tool structures using a metal 3D printing process. Charpy impact and repeated drop impact experiments were performed to investigate the effects of the design of the multi-layer deposited structure on impact characteristics of the specimen and crack initiation in the specimen. Candidated designs of the multi-layer hardfaced region were estimated from the results of the Charpy impact experiments. In addition, proper thickness and assignment of each layer for the multi-layer structure were obtained from the results of the repeated drop impact experiments. Several remelting experiments were carried out to examine the influence of process parameters of the electron beam on characteristics of the remelted region. The emission voltage and the number of irradiation of the pulsed electron beam were chosen as the process parameters. Through the experiments, variation of the surface roughness, the hardness, the microstructure, and the morphology of the remelted region according to the process parameters was investigated. From the results of the investigation, a proper combination of the process parameters was estimated. Finally, the hot forming die was fabricated using the multi-layer hardfacing technology. In addition, characteristics of the fabricated die were investigated through the observation of multi-layer formation in the fabricated die and the behavior of the die during the hot forming experiment.