气动悬浮冷速控制及Al-7.7Ca共晶合金的凝固组织

COOLING RATE CONTROL AND SOLIDIFIED MICROSTRUCTURE OF Al-7.7Ca EUTECTIC ALLOY DURING AERODYNAMIC LEVITATION

分析了气动悬浮金属样品在凝固过程中的激光能量吸收、热辐射以及对流换热特性,推导出激光功率调控下的冷速控制公式.根据气动悬浮Al-7.7Ca共晶合金样品的温度-时间曲线,验证了计算与实测结果符合较好.金相观察表明,气动悬浮Al- 7.7Ca共晶合金在低冷速下凝固时呈现规则层片共晶形态;随着冷速提高,过冷度增大,晶粒细化,层片间距减小,不规则粒状共晶的体积分数不断增加,证实不规则共晶形成于凝固初期的快速凝固阶段.规则共晶的层片间距与过冷度的关系符合经典的JH理论,说明其形成于再辉后的慢速凝固阶段.

The aerodynamic levitation process, which possesses the advantages of con- tainerless solidification, easy control and versatility to materials, is a kind of advanced materials processing technology. How to accurately control the cooling rate during aerodynamic levita- tion is quite important for studying the processin^structure-property relationship of metallic materials. In this study, the characteristics of laser absorption, thermal radiation and heat con- vection of an aerodynamically levitated sample were analyzed, and a formula for the control of the cooling rate by means of tuning the laser power has been derived, which is described by： AT^At ： -3[（r^（T4 - Tf4） q- h（Ts - To）]/cpr ＋ a（6Ls - 3kAt） /8cpTrr3. By recording temperature-time curves with cooling rates of 9, 49, 98 and 253 I2/s for the aerodynamically levitated Ai-7.7Ca （mass fraction, 70） eutectic alloy, the calculated values of cooling rates agree very well with the experimental data, verifying and validating the formula. Also, the microstructure of the aerodynamically levitated AI-7.7Ca euteetic alloy under different cooling rates was examined using OM and SEM. The met- allographic observation shows that, under a low cooling rate of 9 I~/s, the solidification structure of Ai-7.7Ca alloy during aerodynamic levitation exhibits lamellar regular eutectic. With increasing the cooling rate, the undercooling measured from the recorded temperature-time curves increases, resulting in the refinement of the grain size and interlamellar spacing of regular eutectic. Moreover,there appears granular anomalous eutectic under higher cooling rates of 49, 98 and 253 ~C/s. The volume fraction of anomalous eutectic is increased with increasing the cooling rate. The anomalous eutectic is attributed to the formation during the rapid solidification stage. By measuring the in- terlamellar spacing of regular eutectic from A1-7.7Ca metallographs, tile fitting data of interlamellar spacing and undercooling are in accordance with the classical JH model, showing that the regular eutectic is formed during slow solidification stage after recalescence.