Microstructural characteristics and gas content of rapidly solidified powders

The gas content and microstructural characteristics of rapidly solidified powders manufactured by different techniques were studied by Bureau of Mines researchers. Powders were screened and classified into size fractions. Powder characteristics including gas content and porosity were measured and related to powder particle size. Three different atomizing gases, argon, helium, and nitrogen were used in manufacturing the powders. In one series of experiments one gas was used to atomize the melt while a different gas was used in the melting and powder collection chambers. The gas content of the powders was shown to consist of three separate components: (1) solid solution, (2) physical entrapment associated with macroporosity, and (3) surface reaction such as surface oxide. The various components of gas content could be identified by the shape of the curve plotting gas content versus particle size. The identification of the presence of entrained gas as porosity from these curves is important because after consolidation, high-pressure bubbles of inert gas can result. This porosity can cause problems during subsequent heating or joining operations, seriously degrading mechanical properties. Analysis of gas content versus particle size represents a sensitive, technique to detect the presence of porosity.