(1. 清华大学 新材料国际研发中心，北京 100084；
2. 清华大学 机械工程系先进成形制造教育部重点实验室，北京 100084)
摘 要: 通过对比实验的方法，利用多次抛光原位观察和金相分析对近液相线铸造Al-Si合金浆料非枝晶组织的形成及演变规律进行研究，进而从熔体结构角度探讨非枝晶组织形成和稳定存在的机制。结果表明：对于Al-7.97Si工业用铝合金，在390~400 ℃的热环境中，常规近液相线铸造半固态浆料的转移及流变成形时间约为15 s；而在液相线以上不高温度区间对熔体施加旋转磁场作用，可延长浆料转移及流变成形的时间，其主要原因是旋转磁场对过热熔体产生的扰动作用一直影响到凝固过程的形核与长大，一方面使熔体中原子集团的尺寸、形状及分布发生改变，为提高形核率创造了条件，另一方面使固－液界面前沿的溶质浓度梯度减小，抑制成分过冷，同时增大了晶核稳态球形长大的临界半径。
关键字: Al-Si合金；过热熔体；旋转磁场处理；非枝晶组织；演变；稳定性；熔体结构
Al-Si alloy slurry fabricated by near liquidus casting
（1. National Center of Novel Materials for International Research, Tsinghua University, Beijing 100084,China;
2. Key Laboratory for Advanced Materials Processing Technology, Ministry of Education,
Department of Mechanical Engineering, Tsinghua University, Beijing 100084,China）
Abstract:By comparing different experimental schemes and using multiple polishing in situ observation and metallographic analysis, the evolution law of non-dendrite microstructure of Al-Si alloy slurry fabricated by near liquidus casting was researched. Further, from the perspective of melt structure, the formation mechanism and stable existence of non-dendrite microstructure was discussed. The results show that, for Al-7.97Si, at 390−400 ℃, the process of transfer and rheoforming of semi-solid slurry will cost 15s by conventional near liquidus casting. However, in the temperature range not high above liquidus, because of the effect of applied rotating magnetic filed, the time of transfer and slurry rheoforming is elongated. The main reason is that the disturbance caused by rotating magnetic filed for overheating melt always influences the nucleation and growth during solidification. On one hand, the disturbance changes the sizes, shapes and distribution of atomic clusters, which provides the conditions for increasing nucleation rate; on the other hand, it also decreases the solute concentration gradients of solid-liquid interface, which restrains composition overcooling and increases the critical radius of globular growth for crystal nucleus in stable state.
Key words: Al-Si alloy; superheat melt; rotating magnetic filed treatment; non-dendrite microstructure; evolution; stability; melt structure
