Prediction of hot tearing tendency for multicomponent aluminum alloys
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HOT tearing is one of the most common and serious defects encountered during the casting of aluminum alloys. In general, it is defined by the formation of a macroscopic fissure in a casting as a result of stress and the associated strain, generated during cooling, at a temperature above the nonequilibrium solidus. It is also referred to as solidification cracking, hot cracking, hot shortness, supersolidus cracking, and shrinkage brittleness.[1,2,3] In most cases, the castings cannot be salvaged for further processing because of the hot cracking. Therefore, the control and elimination of hot cracking are very important in casting alloy, process, and product designs. Over the years, many efforts have been devoted to understand the phenomenon of hot cracking.[1–5,10] The main criterion applied to characterize the hot tearing tendency of an alloy system was based on the solidification interval. However, this criterion cannot explain the susceptibility-composition relation between the limits of the pure base metal and the eutectic composition. Several hot tearing criteria have been developed in past decades.[6–9,11] Eskin et al. presented a detailed discussion on different hot tearing criteria in a recent review.[2] Clyne and Davies[6] correlated the susceptibility-composition relationship in binary systems based on the concept of the existence of critical time periods during the solidification process when the structure is most vulnerable to cracking. The Scheil equation was used in their model using the constant partition coefficient and constant liquidus slope estimated from the binary phase diagram. This theory has also been adopted by other researchers[1,8] XINYAN YAN, Staff Engineer, Product Manufacturing Division, and JEN C. LIN, Technical Specialist, Alloy Technology Division, are with the Alcoa Technical Center, Alcoa Center, PA 15069. Contact e-mail: [email protected] This article is based on a presentation made in the John Campbell Symposium on Shape Casting, held during the TMS Annual Meeting, February 13–17, 2005, in San Francisco, CA. METALLURGICAL AND MATERIALS TRANSACTIONS B
and appears to be the most widely accepted theory, showing good agreement with experimental results for a number of binary alloys. Because commercial aluminum alloys are essentially all multicomponent systems, it is of practical importance to be able to predict the hot tearing tendency for multicomponent aluminum alloys. In order to extend Clyne and Davies’ model to higher order systems, it is necessary to have the detailed information about partition coefficients, liquidus slopes, and other thermal physical properties, such as latent heat and specific heat, of multicomponent systems. However, the partition coefficient as a function of temperature and concentration of alloying elements is usually unknown for multicomponent systems.[12] Many previous solidification simulation studies on multicomponent alloys often use phase equilibria data estimated from binary or ternary systems. The assumption of a constant partition coefficient is nor
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