The Analysis on Working Efficiency of Highnergy Beam Machinery
Date Issued
2008
Date
2008
Author(s)
Ho, Je-Ee
Abstract
Material ablation rate, nonlinear penetration behavior and high aspect ratio constitute the special characteristics of high energy beam machinery, which are believed to have significant influence on the working quality of electronic beam (EB) or laser beam (LB) machining systems. The performance of the two machining systems and their respective characteristics form the main focuses in this study. The working efficiency estimated from relative parameters had been assessed by the analytical and numerical solutions developed in this study. Experiments were devised and conducted to compare with theoretical results derived. Three main areas are covered and discussed in this research. First, an investigation of the material removal rate using an impulse laser is made. In this study the incident energy intensity and wavelength of the laser beam are found to be the active parameters, which are the keys to improve the absorption of plasma, in turn inducing a strong ablating mechanism of evaporation. Te finding is also consistent with the experimental result made by Schmidt [1] and Tim [2]. The study followed is a setup of theoretical model corresponding a continuous and long pulse of laser or an electron beam (EB) to discuss its nonlinear penetration behavior, i.e. the instantaneous increase of the drilling velocity primarily concerned during the transitional energy density region. To identify the prior parameters responsible for the formation of the keyhole, a critical flow theory dealing with the balance of the vapor pressure near cavity base and the surface tension was then proposed. The related analysis not only defined the nonlinear penetration characteristic clearly, but the determination of incident energy density for the maximum material removal efficiency was made possible. By referring to the definition of critical flow theory, the vapor pressure induced by evaporation is required to compensate and support the keyhole if the melting velocity is slower than the critical flow velocity. It is found that the surface tension will otherwise be predominated in forming a new cavity again. The continuous formation of the keyhole can be confirmed by the both penetration mechanism mentioned above, which agrees with the analytical results obtained from Allmen and experiments carrying out in this study. The thesis is concluded by investigating an electron beam welding process to examine the variation of welding channels, where a higher aspect ratio appearing on the lower thermal conductivity metal, such as stainless steel 304, in which most of the irradiative energy on the cavity base will be used to penetrate into the material. An opposite phenomena observed from the aluminum, a higher thermal conductivity metal, shows a lower aspect ratio of depth to width as a contrast, which is nearly independent with the input energy density. This study shows that the heat absorbed by material will be dissipated in all direction evenly instead of increasing the welding depth only. The above theoretical analysis associated with experimental observation had been done and both were found to be compatible. Based on the principles of heat conduction, hydrodynamics and statistic distribution, a source code invoking the numerical method including central differential scheme and non-uniform spatial deposit of nodal grids was carefully made. The analytical package developed offers an easier and effective solution, and is found a useful tool to interpret the variation of physical characteristic observed in the high energy beam environments. The conclusions derived also provides useful and valuable directions for the advanced pico-laser or femto-laser machining applications.
Subjects
ablation rate
pulse laser
electron beam penetration
Type
thesis
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