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.
