플라즈마 전자빔을 이용한 신개념 금속 적층 제조 공정의 개발을 위한 초합금 분말의 전자빔 예열 및 적층 특성에 관한 연구

Abstract

A powder bed fusion (PBF) process is one of representative metal additive manufacturing (AM) processes. The PBF process can be classified into laser and electron beam types according to the applied heat source. The power of the electron beam is greater than that of the laser. In addition, the electron beam is hardly influenced by the reflectivity of the applied material unlike the laser. The PBF process using electron beam is one of challengeable metal AM processes. However, only one PBF process using an electron beam, so called electron beam melting (EBM) process, has been developed. EBM process adopts a thermal electron beam using a hot cathode. The EBM process has several disadvantageous characteristics including a high level of vacuum environment, a short service life of the cathode, a high temperature of the cathode, a small diameter of beam, a high temperature of the building plate, etc.. The development of a novel PBF type AM process using an electron beam is needed to overcome demerits of the EBM process. The aim of this thesis is to investigate preheating and deposition characteristics of super-alloy powders for the development of a novel metal additive manufacturing process using a plasma electron beam. A plasma electron beam has several benefits, including a relatively low level of vacuum environment, a long service life of the cathode, a relatively large diameter of the beam, a low temperature of the cathode, as compared to the thermal electron beam. Due to these merits, a novel PBF type AM process using a plasma electron beam was proposed in this thesis. The proposed process consisted of preheating step to prevent spreading of powders, deposition step to create the deposited bead and re-melting step to improve surface roughness and density. Various experiments and numerical analyses were carried out to investigate the influence of process parameters on preheating and deposition characteristics of powders. The experiments were performed for two conditions, including no heating condition using the plasma electron beam (without heating of building plate) and heating condition using the plasma electron beam (with heating of building plate), according to heating of the building plate. The building plate was heated to 150 oC before loading of the building plate in the vacuum chamber for the case of the no heating condition. However, the building plate was heated using a plasma electron beam in the vacuum chamber before experiments for the case of the heating condition. The working distance, the probe current of the electron beam, the acceleration voltage of the electron beam, the travel speed of the table, and the length of the bead were chosen as process parameters. Stellite21 powders with 45-150 mm of diameter were selected as the used material. The experiments were carried out using a plasma electron beam finishing system. A commercial software ABAQUS was used to perform numerical analyses. Temperature dependent material properties including state and porosity of powders were used in the numerical analyses. Sintering and melting temperatures of the Stellite21 powders were estimated via sintering and melting experiments using a fiber laser and an infrared camera. Preheating experiments were performed for the case of no heating condition of the building plate. From the results of the experiments, the effects of the working distance, the probe current and the travel speed on spreading characteristics of powders and the formation of preheated beads were examined. In addition, appropriate working distance, probe current and travel speed were estimated to create the desired preheating bead. In order to investigate heat transfer characteristics during preheating of powders using a plasma electron beam, three-dimensional finite element analyses (FEAs) were performed. The heat source model of the plasma electron beam for preheating step were proposed through the measurement of intensity of the probe current and the regression method using normalized radius and intensity. Using the results of the FEAs, the effects of the probe current and the travel speed on the temperature distribution in the vicinity of the preheated region were investigated. The influence of the temperature distribution in the vicinity of the preheated region on the spreading phenomenon of powders was discussed. Deposition experiments were carried out for the case of no heating condition of the building plate. The influence of the probe current, the travel speed and the bead length on defects, morphologies, dimensions of deposited beads was examined. From the results of the examination, a proper deposition condition for the case of no heating condition of the building plate was estimated. In addition, a planner part with repeated deposition beads was fabricated using the proposed condition. FEAs for melting of powders during the deposition step were carried out to investigate the influence of the probe current and the travel speed on the temperature distribution in the vicinity of the melted region. The influence of the probe current and the travel speed on the formation of the melted region was examined using the results of FEAs. Through the comparison of results of experiments and those of FEAs, creation mechanism of the deposited bead for the case of no heating condition of the building plate was discussed. Using results of experiments and numerical analyses of preheating and deposition steps without heating of the building plate, multi-layer deposition experiments were carried out. A three-dimensional part with several defects was fabricated by the experiments. From the results of the experiments, it was noted that automatic successive building system is needed to reduce temperature changes during the deposition and delamination between successive layers. In addition, it was considered that the building plate should be heated in the vacuum chamber before feeding of powders to improve joining characteristics between the building plate and the deposited part. A PBF type experimental set-up with automatic feeding, deposition and specimen loading devices was developed to perform preheating and deposition experiments in a vacuum chamber. In order to investigate the probe current and focusing of the electron beam, a Faraday cage and a beam focusing tester were assigned in the experimental set-up. Using the experimental set-up, preheating and deposition experiments were carried out for the case of heating of the building plate. In those experiments, the building plate was heated during 10 minutes using the defocused plasma electron beam in a vacuum chamber. After heating of the building plate, powders were fed on the building plate using the automatic feeding device. Preheating conditions for the case of heating of the building plate were estimated through the examination of the influence of the probe current and the travel speed on the formation of the preheated bead. The effects of the acceleration voltage, the probe current and the travel speed on the formation, the ingredient variation and the surface roughness of the deposited bead were investigated through deposition experiments for the case of heating of the building plate. From the results of the investigation, the deposition window and the proper deposition condition for the case of heating of the building plate were estimated to fabricate the desired deposited bead. Multi-layers deposition experiments were carried out to investigate feasibility of the fabrication of three-dimensional parts using the proposed AM process. Estimated proposed preheating and deposition conditions for the case of heating of building plate were applied to the multi-layers deposition experiments. In multi-layers deposition experiments, a single layer was created from heating of building plate, feeding of powders, preheating of fed powders, deposition of bead, and surface re-melting of the deposited bead. Through the repetition of the creation method of the single layer, three-dimensional parts were fabricated. Planner part with a single layer, rectangular parts with 3 and 10 layers, cylindrical part with 5 layers, stepped part with 10 layers, and square net part with 11 layers were fabricated. The re-melting step was not applied to the fabrication of the square net part. From the results of multi-layers deposition experiments, it was noted that the proposed AM process can fabricate three-dimensional parts with a high density.