In–situ Observation of Melting and Solidification Behaviors of Cu Powder Caused by Laser Irradiation

Abstract

In the additive manufacturing (AM) process for metallic materials, solidification takes place from solid/liquid interfaces after local melting. For controlling microstructures of materials processed by AM, it is important to know how the structures form. However, as molten metal is optically opaque, it is difficult to ascertain not only the solidification process, but also how metallic materials melt. In the present study, in–situ observation of melting and solidification behaviors of Cu powder induced by laser irradiation was carried out by time–resolved X–ray imaging using synchrotron radiography. In the growth process of the melt pool by laser irradiation, the liquid phase of the melt pool did not move to Cu powders, but they were attracted to the melt pool by such means as electrostatic attraction. A powder–free zone formed in areas near the melt pool, which was one of the causes of defects in metallic products processed by AM. Despite steady laser scanning, a continuous melt pool did not form； rather, single tracks formed by repeated formation of melt pool, unification and solidification. The growth direction of grains in the melt pool changed due to changes temperature distribution, gradient and heat flow.