The solidification of undercooled spherical droplets with a discrete melting temperature is analyzed using both a Newtonian and a non-Newtonian (Enthalpy) model. Relationships are established between atomization parameters, the growth kinetics, the interface velocity and undercooling, and other important solidification variables. A new mathematical formulation and solution methodology is developed for simulating the solidification process in an undercooled droplet from a single nucleation event occurring at its surface. The computational mesh used in the enthalpy model is defined on a superimposed bispherical coordinate system. Numerical solutions for the solidification of pure aluminum droplets based on the enthalpy model are developed, and their results are compared to the trends predicted from the Newtonian model. The implications of single vs multiple nucleation events are also discussed. In general, the results indicate that when substantial undercoolings are achieved in a droplet prior to nucleation, the thermal history consists of two distinct solidification regimes. In the first, the interface velocities are high, the droplet absorbs most of the latent heat released, and the external cooling usually plays a minor role. The second regime is one of slower growth, and strongly depends on the heat extraction at the droplet surface. The extent of “rapid solidification”, as determined from the fraction of material solidified at temperatures below a certain critical undercooling, is a function of the nucleation temperature, the particle size, a kinetic parameter, and the heat translow as 10~4.

Heat Flow during Rapid Solidification of Undercooled Metal Droplets

The generation and processing of fine powders in thermal plasmas has attracted increasing interest over the past years, precipitated by a growing awareness that conservation of materials is no longer an option but rather a necessity. Plasmaspheroidization, densification, fuming, metallurgical reduction, and the production of refractory oxides, carbides, nitrides, and borides in thermal plasmas are fast developing technologies which, in some cases, have already reached industrial production scale.

Generation and behavior of fine particles in thermal plasmas—A review

It is accepted that the cooling rates of alloys during solidification can strongly influence their microstructure and, consequently, their corrosion resistance and mechanical properties. However, t...

Secondary dendrite arm spacings in magnesium alloy AZ91D: from plaster moulding to laser remelting

Inverted Disk Centrifugal Atomization of AZ91 Magnesium Alloy

Steel-based composites with varying content of TiB 2 were manufactured by HP–HT process. The main objective of the study was to determine the effect of sintering conditions and TiB 2 content on the mechanical properties and corrosion behavior of steel–TiB 2 composites. The corrosion resistance of the composites was examined in H 2 SO 4 solution using two measurement techniques. The mechanical properties were determined in a compression test allowing also for the effect of elevated temperatures (600 °C and 800 °C). The results showed that both TiB 2 content and sintering temperature affected the properties tested. The highest compressive strength was obtained in the composites with 8 vol% TiB 2 . The corrosion properties of composites showed only insignificant variations. The microstructure was characterized by a homogeneous distribution of the TiB 2 reinforcing phase.

Corrosion behaviors, mechanical properties and microstructure of the steel matrix composites fabricated by HP–HT method

The development of a process of preparing spherical or flake powders in the size range 100–1500 μm is described. The process has been applied to a wide range of materials, including iron, nickel, cobalt and titanium alloys, refractory metals, carbides, and oxides.In one mode of operation the feed material is in the form of a consumable rod electrode (cathode) and is melted by a vertical arc struck between the electrode and a rotating crucible. The melted material falls into the crucible, which may be water-cooled, and is forced up and over the walls by centrifugal action. By selecting rotation speeds in the range 500–4000 rev/min, the mean size of the particles formed as the liquid tears off the crucible lip may be varied systematically. Alternatively, granules may be fed through a hollow non-consumable electrode or the crucible may be replaced by a consumable anode.The conditions controlling particle size and shape have been analysed theoretically in terms of the relevant alloy, atmosphere, and m...

p. W. Sutcliffe

Papers

Centrifugal shot casting: a new atomization process for the preparation of high-purity alloy powders*