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QUANTIFICATION STUDY ON DENDRITE FRAGMENTATION IN SOLIDIFICATION PROCESS OF ALLUMINUM ALLOYS |
Cheng BI1,Zhipeng GUO1( ),E LIOTTI2,Shoumei XIONG1,P S GRANT2 |
1 School of Materials Science and Engineering, Tsinghua University, Beijing 100084
2 Department of Materials, University of Oxford, Oxford OX1 3PH, UK |
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Cite this article:
Cheng BI, Zhipeng GUO, E LIOTTI, Shoumei XIONG, P S GRANT. QUANTIFICATION STUDY ON DENDRITE FRAGMENTATION IN SOLIDIFICATION PROCESS OF ALLUMINUM ALLOYS. Acta Metall Sin, 2015, 51(6): 677-684.
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Abstract Alloy solidification is an important process to control the mechanical properties of engineering products. During solidification, dendrite fragmentation occurs commonly as a key phenomenon to determine the microstructure and to obtain fine grain size. Recently, in situ synchrotron X-radiography technique was developed and applied to observe thermodynamic behaviors such as dendrite growth and fragmentation during solidification. External forces such as mechanical and electromagnetic stirring, and thermal shock were added into the solidification process to investigate their effects on the fragmentation behavior. However, most work conducted in literature focused on qualitative aspects e.g. morphology transition or solute distribution and quantitative investigation such as determining the specific relationship between fragmentation and solidification conditions was rather limited. In this work, the third generation synchrotron X-radiography technique was used to observe the solidification process of an Al-15%Cu (mass fraction) alloy. Experimental conditions including the strength of the pulsed electromagnetic fields, dendrite growth direction and the temperature gradients were varied and the subsequent effect on fragmentation was studied and quantified. A computer program was developed based on Matlab to perform the image processing and measurement. The fragmentation number according to experiments was counted and correlated to the mushy zone depth and local solid fraction. Results showed that a stronger electromagnetic field, growing against gravity and growing at higher velocity would significantly increase the fragmentation number. Furthermore, the fragmentation number followed a Gauss distribution as a function of either mushy zone depth or local solid fraction, and the maximum fragmentation occurred when the solid fraction was about 0.45. In the end, the extent to which caused those statistic results above were analyzed as the necking process due to the velocity field, the cumulative solid due to the gravity field and the liquid flow due to the electromagnetic field.
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Fund: Supported by National Natural Science Foundation of China (Nos.51275269 and 51205229) |
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