Compressible Fluid Dynamics in Materials Processing

Steven P. Mates
National Institute of Standards and Technology, Metallurgy Division,

Two widely-utilized materials processing techniques, namely liquid metal atomization and thermal spraying, heavily involve compressible fluid dynamics. In liquid metal atomization (Fig. 1), a hot melt stream is disintegrated by a cold supersonic gas jet into small droplets. These atomized droplets then freeze in-flight into powder particles (Fig. 2), which are then collected. Atomized metal powders are used in powder metallurgy processing, rocket fuels, paints, dental amalgams, and many other applications. The characteristics of the atomized powder, including size distribution, microstructure and composition depend on the velocity, density and temperature distribution of the supersonic gas jet that governs liquid breakup and droplet solidification. In the thermal spray coating process, metallic or ceramic particles are injected into a hot, high-velocity jet formed by a nozzle driven with an electric arc driven plasma or hydrocarbon combustion. The injected particles are heated and accelerated towards the surface being coated. Thermal spray coatings, typically less than 1 mm thick, are formed by the impact and solidification of individual molten or semi-molten droplets. They are applied to provide wear, corrosion and/or thermal protection to the underlying materials. Important coating properties including microstructure, porosity and adhesion, among others, are strongly influenced by the properties of the hot fluid jet produced by the thermal spray torch, which governs the momentum and heat transfer to the thermal spray particles and determines their velocity and temperature on impact. Control and optimization of these materials processing technniques requires a multidisciplinary effort involving both compressible fluid dynamics and materials science.

Fig. 1

 

Fig. 2