Liquid metal solidification

We saw in Chapter 6 that diffusive transformations (like the growth of metal crystals from the liquid during solidification, or the growth of one solid phase at the expense of another during a polymorphic change) involve a mechanism in which atoms are attached to the surfaces of the growing crystals. This means that diffusive transformations can only take place if crystals of the new phase are already present. But how do these crystals - or nuclei - form in the first place ... 

Heat gains and losses for heating or cooling raw materials and parts brought into or taken out of the building, melted metal solidification, vapor condensation, or liquid evaporation... 

The microstructure of a metal resulting from the solidification of liquid metal such that two or more distinct solid phases are formed. 

K. A. Jackson, in "Liquid Metals and Solidification", (American Society for Metals, Metals Park, Ohio, 1958), p 174. 

To ensure that a liquid metal is completely disintegrated into droplets before the initiation of any solidification in the gap, the heat transfer between the liquid metal and the rolls must be minimized. Materials with low thermal conductivity and poor wetting properties have been found to have the most significant effect on reducing the heat transfer. Epoxy and phenolic resin coatings have been applied to... 

Droplet Formation in Water Atomization. In water atomization of melts, liquid metal stream may be shattered by impact of water droplets, rather than by shear mechanism. When water droplets at high velocities strike the liquid metal stream, some liquid metal fragments are knocked out by the exploding steam packets originated from the water droplets and subsequently contract into spheroidal droplets under the effect of surface tension if spheroidization time is less than solidification time. It is assumed that each water droplet may be able to knock out one or more metal droplet. However, the actual number of metal droplets produced by each water droplet may vary, depending on operation conditions, material properties, and atomizer designs. 

Figure 3.23. Schematic and micrograph of spreading, breakup and solidification modes of liquid metal droplets impinging on a cold surface. (a)-(d) Pancake pattern regularly shaped disks with or without corona, (e)-(h) Flower pattern irregularly shaped platelets with or without corona. (Micrograph reprinted with permission from Ref. 403 and 407. Courtesy of Dr. J. M. Houben, Eindhoven University of Technology, Netherlands and courtesy of Dr. Seiji Kuroda, National Research Institute for Metals, Japan.)...

The solution of the gas flow and temperature fields in the nearnozzle region (as described in the previous subsection), along with process parameters, thermophysical properties, and atomizer geometry parameters, were used as inputs for this liquid metal breakup model to calculate the liquid film and sheet characteristics, primary and secondary breakup, as well as droplet dynamics and cooling. The trajectories and temperatures of droplets were calculated until the onset of secondary breakup, the onset of solidification, or the attainment of the computational domain boundary. This procedure was repeated for all droplet size classes. Finally, the droplets were numerically sieved and the droplet size distribution was determined. 

It should be noted that it is difficult to obtain models that can accurately predict thermal contact resistance and rapid solidification parameters, in addition to the difficulties in obtaining thermophysical properties of liquid metals/alloys, especially refractory metals/al-loys. These make the precise numerical modeling of flattening processes of molten metal droplets extremely difficult. Therefore, experimental studies are required. However, the scaling of the experimental results for millimeter-sized droplets to micrometer-sized droplets under rapid solidification conditions seems to be questionable if not impossible,13901 while experimental studies of micrometer-sized droplets under rapid solidification conditions are very difficult, and only inconclusive, sparse and scattered data are available. 

The physical properties of bismuth, summarized in Table 1, are characterized by a low melting point, a high density, and expansion on solidification. Thermochemical and thermodynamic data are summarized in Table 2. The solid metal floats on the liquid metal as ice floating on water. Gallium and antimony are the only other metals that expand on solidification. Bismuth is the most diamagnetic of the metals, and it is a poor electrical conductor. The thermal conductivity of bismuth is lower than that of any other metal except mercury. 

G.H. Vineyard. The theory and structure of liquids. In Liquid Metals and Solidification, pages 1-48, Cleveland, OH, 1958. American Society for Metals. 

A process known as squeeze casting (solidification of liquid metal under pressure) contributes to producing defect-free eastings with improved metallurgical properties. Sec also Casting. 

The result of the solidification of a liquid metal is mostly an unperfect packing, which means a crystal lattice with imperfections. These imperfections have already been discussed in chapter 4. An imperfection is a deviation from the perfect packing. For instance a vacancy is an empty place in the lattice which should be occupied. It is also possible for a particle to end up in a place where it should not be. A foreign metal ion which is bigger than the own ions can upset the move of parts of the lattice in relation to each other. And then there are the faults which we call dislocations and which concern parts of the crystal lattice. For example when it seems as if a crystal is partly cleaved or both parts have shifted over one atom distance in relation to each other. Or the crystal is in fact partly cleaved, both halves move and the crack is filled with a layer of atoms. 

Liquid metal can be supercooled to temperatures considerably below their normal solidification temperatures. Solidification of such liquids takes place spontaneously, i.e., irreversibly. Now one mole of silver supercooled to 940°C is allowed to solidify at the same temperature. Calculate the entropy change of the system (silver). The following data are given ... 

The formation of mixed crystals results from limited solubility or insolubility of one solid metal in the other. Lead and tin and lead and antimony are examples of pairs of metals that form alloys consisting of intimate mixtures of tiny pure crystals of each metal. The formation of solid solutions results when the liquid metals are miscible in all proportions and are capable of solidification to compositions that are essentially the same as those of the melts. Many of the most common and useful alloys consist of homogeneous solid solutions of one metal in the other (e.g., alloys of copper and zinc, gold and silver, nickel and... 

Thermodynamics and Properties of Liquid Solutions. S. 56 in Liquid Metals and Solidification, Published by the American Society for Metals, Cleveland, Ohio 1958. 

Nachtrieb, N. H. Transport properties in pure liquid metals. S. 49 in Liquid Metals and Solidification, ASM (1958). 

But an actual metal is far from the ideal one. In the opinion of most expert metallurgists of our day, a liquid metal contains a significant amount of insoluble impurities. The cavitation strength in the liquid state and its structure in the solid state after solidification are mainly determined by the purity of the metal in such solid nonmetallic impurities. 

If homogeneous melts (containing no oxides) are able to form supersaturated hydrogen solutions during solidification, then heterogeneous formation of bubble nuclei can easily occur onto oxides within actual liquid metals containing oxides. 

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