Investigations of process by-products by means of Schlieren imaging during the powder bed fusion of metals using a laser beam

To produce geometrically complex parts with good mechanical properties, various industries increasingly apply the powder bed fusion of metals using a laser beam. Sufficient control of this laser-based additive manufacturing process is necessary to achieve a reliable development as well as reproducible part properties. Besides adjusting the standard process parameters, such as the laser power or the scanning speed, the process gas is an important parameter that influences the part characteristics. In addition to the inertization of the atmosphere, process by-products can be removed during the manufacturing with a constant gas flow across the build plate. Typically, argon or nitrogen is applied. This study investigated the effects of various argon-helium mixtures in comparison to pure argon gas on the heat balance of process by-products during the processing of a high-strength aluminum alloy by means of Schlieren imaging. The method enables visualizations of the process by-products and is further capable of studying evaporation phenomena. For this purpose, a Schlieren imaging setup was designed and installed in an AconityMINI machine. The experimental results were exploited to validate a simplified heat transfer model introduced in this work. The results indicate that the addition of helium to the process gas lowers the amount of fumes and incandescent spatters and thus could decrease evaporation. Also, it was found that the higher thermal conductivity of helium compared to argon leads to significantly higher cooling of process by-products.