Electron Beam Coupling Efficiency in EBF

This study examines the coupling efficiency of the electron beam to a metal substrate in the Electron Beam Freeform Fabrication (EBF 3 ) system.EBF 3 is a type of additive manufacturing for metallic parts that uses an electron beam as an energy source.This type of additive manufacturing is of significant interest to NASA because of its ability to manufacture components for aircraft and spacecraft or repair parts while on long duration space missions.To make accurate models and computer simulations of the EBF 3 process, the electron beam coupling efficiency must be known.The coupling efficiency is defined as the percentage of energy from the electron beam that is transferred to the substrate material.By assuming a 100% coupling efficiency, the temperature change of a substrate material can be calculated based on material properties and the energy delivered from the electron beam.If the coupling efficiency is not 100%, a temperature change lower than this calculated value will be observed experimentally.This ratio of actual temperature change versus calculated temperature change will give the coupling efficiency of the electron beam to the substrate material.The initial intent of this study was to measure coupling efficiency for 316L Stainless Steel, Al 2219, and Ti-6Al-4V.After initial tests it appeared that processing parameters of beam focus, accelerating voltage, and beam power might have an effect on coupling efficiency.To further examine the effects of process parameters on coupling efficiency, tests focused on 316L Stainless Steel with additional testing done on Al 2219.Results of this study indicate a coupling efficiency of 60-70% in 316L Stainless Steel, a value that appears to be independent of process parameters.A simulation was done in COMSOL Multiphysics that validates for comparison to the results of the 316L Stainless Steel coupling efficiency tests.Coupling efficiency for Al 2219 was found to be 40-60% and appeared to depend on process parameters in some tests.However, no trends were observed when comparing all tests to support this observation.The experimentally obtained values of electron beam coupling efficiency are lower than values reported in other literature and the values currently being used in modeling and simulation of the EBF 3 process.Having more precise coupling efficiency values will lead to more accurate models and simulations and ultimately improve the EBF 3 system.