Numerical simulation of solidification in an aluminum casting
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I.
INTRODUCTION
THE use
of aluminum castings allows for large weight reduction in many industrial fields (as in transports) resuiting in considerable energy and cost saving. Moreover, light alloy castings can replace complicated structures that have to be assembled by joining their many components by welding or adhesive bonding (as in the aerospace industry). However, only high quality and low cost products will allow a widespread use of aluminum castings. Design engineers are therefore faced with the difficult task of having to design increasingly complicated castings for defect-free and lowcost products. It is standard practice to optimize the mold casting system through careful experimental work: design, prototype production, X- or y-ray radiography, mechanical tests, and metallographic inspections. Once the defects of the prototype have been detected the mold is made over and over again until the required standards are met. Obviously this type of trial and error method can be very expensive, since many variables affect solidification of the metal inside the mold (thermal characteristics of materials, section thickness, pouring temperature, solidification range). Exact knowledge of the dynamic process of solidification would be, therefore, of great interest in predicting the metallurgical characteristics of the cast material and in avoiding the defects deriving from improper design of the mold (shrinkage, segregation, microvoids, etc.). As in other industrial fields, the use of computer methods can supply a suitable aid to solve these kinds of problems. The numerical methods implemented on high speed digital computers have been, in fact, widely used in many fields of technological interest, and the simulation of the behavior of complex systems is by now a commonly-adopted procedure, when performing structural analyses and optimizing industrial processes. Computer methods can, in fact, reduce cost design and improve quality level. Several high level software packages are now devoted to the solution of complicated nonlinear transient problems of structural analysis and are available on large machines. Many computer codes are also devised for the solution of nonlinear thermal transients, even if some difficulties can arise in treating phase changes.
The aim of this paper is to show how the solidification pattern of an actual aluminum casting can be predicted, with reasonable accuracy, by using computer methods in order to obtain useful information about material microstructure and to optimize mold design and/or initial conditions. A simple computer program has been implemented on a digital computer by which the characteristics of the solidification in an actual three dimensional AI 99.8 pct casting have been studied, To test the feasibility of the method, experimental work has been performed by pouring a casting under the same conditions as those described in the numerical analysis. The results from both sources corresponded and are hereafter described.
II.
MATHEMATICAL MODEL
Many transient field problems, s
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