Effects of Gas Pressure during Electron Beam Energy Deposition in the EBM Additive Manufacturing Process

Abstract

Electron beam melting (EBM) is a metal powder bed fusion additive manufacturing (AM) technology that facilitates the production of metal parts by selectively melting areas in layers of metal powder. The electron beam melting process is conducted in a vacuum chamber environment regulated with helium (He) at a pressure on the scale of 10−3 mbar. One of the disadvantages of vacuum environments is the effect of vapor pressure on volatile materials: indeed, elements in the pre-alloyed powder with high vapor pressure are at risk of evaporation. Increasing the He pressure in the process can improve the thermodynamic stability of the pre-alloyed components and decrease the composition volatility of the solid. However, increasing the pressure can also attenuate the electrons and consequently reduce the energy deposition efficiency. While it is generally assumed that the efficiency of the process is 90%, to date no studies have verified this. In this study, Monte Carlo simulations and detailed thermal experiments were conducted utilizing EGS5 and an Arcam Q20+ machine. The results reveal that increasing the gas pressure in the vacuum chamber by one order of magnitude (from 10−3 mbar to 10−2 mbar) did not significantly reduce the energy deposition efficiency (less than 1.5%). The increase in gas pressure will enable the melting of alloys with high vapor pressure elements in the future.

Characterization of Spatter and Sublimation in Alloy 718 during Electron Beam Melting

“…Additionally, a study on compositionally complex Al0.5CrMoNbTa0.5 high entropy alloy showed a decrease in the content of Al to 8.4 at% in build sample as compared to 12 at% in mechanically alloyed powder [23]. Some studies have recently proposed metalization control mechanisms, including vacuum [24] and process parameter control [25]. Damri et al [24] suggested increasing the pressure in the build chamber by one order of magnitude (from 10 −3 to 10 −2 mbar).…”

“…Some studies have recently proposed metalization control mechanisms, including vacuum [24] and process parameter control [25]. Damri et al [24] suggested increasing the pressure in the build chamber by one order of magnitude (from 10 −3 to 10 −2 mbar). This did not significantly affect the energy efficiency of the electron beam but could help reduce the evaporation of elements.…”

The use of defects and compositional variations to elucidate physical phenomena in electron beam melted Ti-6Al-4V across scan strategies

“…Literature often carries the assumption that compositions are homogeneous in single-feedstock parts, as the powder remains the same throughout the build height [7,27]. However, during the initial and subsequent melting stages, melt pool interactions with the chamber environment include the evaporation and recondensation of elements, leading to local variations in composition [52][53][54][55][56] that are often neglected in the literature. In titaniumaluminum alloys specifically, including Ti-6Al-4V, high temperature processing, including various AM techniques such as EBM, is known to result in aluminum losses relative to the initial composition due to preferential vaporization during the processes [52,55,57].…”

“…However, during the initial and subsequent melting stages, melt pool interactions with the chamber environment include the evaporation and recondensation of elements, leading to local variations in composition [52][53][54][55][56] that are often neglected in the literature. The work presented…”

“…The variation in aluminum content can primarily be explained by preferential vaporization of Al during the build process. Preferential vaporization is a well-studied phenomenon, whereas, at high temperatures, elements with higher vapor pressures vaporize first and at a faster rate [52,55,56]. Elements such as zinc and gallium, as well as aluminum in titanium alloys and chromium in stainless steel or nickel-based alloys, both in conventional and additive manufacturing processes, are known to preferentially vaporize, among other examples [52,56,142].…”