Solidification casting microstructures

Microstructure evolution in solids formed from a melt is important in processes such as prilling or casting. For example, in aluminum alloy shape casting the final microstructure depends directly on the as-cast microstructure since the only post-casting process is the heat treatment. The micro-porosity formed owing to the combined effects of the volumetric shrinkage upon the solidification of the melt and the precipitation of the dissolved hydrogen affects the final properties of the aluminum alloy. 

The purpose of the present work is to provide a systematic compilation of solidification data, describing the formation of the as-cast microstructures in steels of technical importance. The compositions have been chosen to cover a large part of the spectrum of steels in current production. Where a specific steel is not included, it should be possible to gain an outline of its solidification characteristics from related compositions present. 

Solidification. When the ingot or casting solidifies, there are three main possible microstructures that form (see Figure 7.5). We will describe here only the final structures the thermodynamics of the liquid-solid phase transformation have been described previously in Chapter 2. The outside layer of the ingot is called the chill zone and consists of a thin layer of equiaxed crystals with random orientation. 

Corrosion rates of atomized, rapidly solidified alloy cast and chill cast alloys are given in Table 4.73. The data show that atomized rapidly solidified alloy is comparable in corrosion resistance to the cast AZ ID alloys. Rapid solidification improves the corrosion resistance of the alloys since the homogeneous microstructures tend to disperse the elements and particles, which otherwise act as cathodic sites. Extended solubility of various elements may also shift the electrode potentials to more noble values. 

Microstructure after casting of Rodent consists of grains/dendrites sizes up to a few hundred micrometers. Inside the grains are imiformly distributed second phase precipitations that originated during alloy solidification as a result of a superfusion on growing dendrites boimdaries (Fig. la). 

Since C and Si are the alloying elements which dominate the solidification behavior and the resulting microstructures of cast irons, their phase equilibria need to be taken into account. Figure 3.1-120 shows a section through the metastable ternary Fe—C—Si diagram at 2wt%Si which approximates the Si content of many cast irons. Compared to the binary Fe—C system, the addition of Si decreases the stability of FesC and increases the stability of ferrite, as indicated by the expansion of the a-phase field. With increasing Si concentration, the C concentrations of the eutectic and the eutectoid equilibria decrease while their temperatures increase. 

Ductile Iron. This cast iron is characterized by the spheroidal graphite phase (SG) in its microstructure. Spheroidal graphite is formed during solidification if the melt has been treated by the addition of a component which promotes the particular nucleation and... 

Ares, A.E. Caram, R. Schvezov, C.E. (2006) Relation between As-Cast Mechanical Properties, Microstructure and Solidification Conditions for Zn-Al Alloys, Proceedings of MCWASP International Conference Modeling of Casting, Welding and Advance Solidification Processes - XI, Opio, France, June of 2006... 

Special aluminum products. In recent years, a number of new aluminum alloys have been developed. For example, the powder metallurgy route can be a cost-effective method for manufacturing components with conventional aluminum alloys, especially for small parts requiring close dimensional tolerances (e.g., connecting rods for refrigeration compressors). But this process is still relatively expensive. Rapid solidification and vapor deposition processes permit production of aluminum alloys with compositions and microstructures that are not possible by conventional cast or wrought methods. 

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