Clarification of Temperature Distribution for Metals with Different Thermal Conductivity in Large-area Electron Beam Irradiation

Abstract

High energy density of electron beam (EB) with diameter of about 60 mm is obtained in a large-area EB irradiation method. This method can be used for melting and evaporating material surface instantly, and highly efficient surface finishing of metals such as mold steels and biomaterials is possible. Furthermore, surface functions such as corrosion resistance, water repellency and wear resistance can be improved since a thin modified layer with fine microstructure is formed on the EB irradiated surface. It is expected that thick modified layer is effective to maintain their surface functions for long-term. However, the material surface is slightly removed in formation of modified layer. Therefore, control and prediction of removal and modified layer thickness are important in order to obtain appropriate modified layer thickness and to keep the shape accuracy for metals with different thermal properties. In order to predict the removal and modified layer thickness, it is necessary to clarify the temperature distribution in depth direction and its time variation in large-area EB irradiation. In this study, changes of removal and modified layer thickness on metal workpiece with different thermal conductivity are investigated by calculation of temperature distribution in large-area EB irradiation. It is made clear that the modified layer thickness of workpiece with low thermal conductivity is larger than that with high one, while the removal layer thickness is almost the same regardless of thermal conductivity. Furthermore, analytical removal and modified layer thickness obtained by our unsteady heat conduction analysis agree well with the experimental ones. Therefore, our unsteady heat conduction analysis model has high accuracy, and it is possible to predict variations of removal and modified layer thickness on workpiece with different thermal conductivity.