Grain refinement of Al-7Si alloys and the efficiency assessment by recognition of cooling curves
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NTRODUCTION
THE melt quality of liquid Al-Si alloys means the melting and later treatment soundness, mainly including such indices as chemical composition, temperature, efficiency of degassing, refinement, and modification. Among them, grain refinement and modification are two of the melt treatments commonly performed on the Al-Si melt for improving the quality of the castings, such as mechanical properties. It is very important and necessary for either researchers or workers to evaluate the refinement/or modification levels before pouring the melt into the mold. Generally, the thermal analysis method is used to evaluate the melt quality owing to its rapidness and higher accuracy. Here, the main evaluation principle can be summarized briefly. First, some characteristic parameters are extracted from a cooling curve or its derivative. Then, through regress analysis or the artificial neural network method, a relationship is built up between the characteristic parameters and quality indices such as grain size, eutectic structure, silicon morphology, and so on. For grain size prediction, the parameters often include the liquids undercooling (⌬T ), liquidus arrest temperature (Tu), and liquidus undercooling time (t) (Figure 1); because it was suggested that with the increase of ⌬T and t the nucleation potential of the melt decreases, which means the solidification structure would be coarse grained.[1] The relationship of other characteristic parameters with grain size was developed by Charbonnier and Kearney.[2] It was also found that the grain size is corrective with the peak height of the first derivative curve corresponding to the nucleation of a-Al.[3,4] For the method mentioned previously, there is an unsolved problem that most characteristic parameters of cooling or Q. WANG, Post Doctor, is with the Department of Mechanical Engineering, Tsingnua University, Beijing 100080, People’s Republic of China. Contact e-mail: [email protected] Y.X. LI, Professor, and X.C. LI, Graduate Student, are with the Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China. Manuscript submitted July 16, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
differential curve are obviously influenced by the experimental condition, such as cooling rate,[5] which sometimes does not affect the grain size notably.[6] In other words, the correlation between parameters of cooling curve and grain size or modification level of Al-Si alloys depends greatly on the set-up condition. So the regress function obtained in a fixed experimental condition cannot be applied in other test situations. In addition, by present, most thermal analysis methods only give qualitative results. Considering the importance of the on-line melt quality evaluation and the existing problem mentioned previously, it is very necessary to develop a new evaluation method, which can adapt itself to the set-up condition and can predict the grain size and modification level quantitatively. In this article, a totally new method has been bro
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