(1. 清華大學(xué) 新材料國(guó)際研發(fā)中心，北京 100084；
2. 清華大學(xué) 機(jī)械工程系先進(jìn)成形制造教育部重點(diǎn)實(shí)驗(yàn)室，北京 100084)
摘 要: 通過(guò)對(duì)比實(shí)驗(yàn)的方法，利用多次拋光原位觀察和金相分析對(duì)近液相線鑄造Al-Si合金漿料非枝晶組織的形成及演變規(guī)律進(jìn)行研究，進(jìn)而從熔體結(jié)構(gòu)角度探討非枝晶組織形成和穩(wěn)定存在的機(jī)制。結(jié)果表明：對(duì)于Al-7.97Si工業(yè)用鋁合金，在390~400 ℃的熱環(huán)境中，常規(guī)近液相線鑄造半固態(tài)漿料的轉(zhuǎn)移及流變成形時(shí)間約為15 s；而在液相線以上不高溫度區(qū)間對(duì)熔體施加旋轉(zhuǎn)磁場(chǎng)作用，可延長(zhǎng)漿料轉(zhuǎn)移及流變成形的時(shí)間，其主要原因是旋轉(zhuǎn)磁場(chǎng)對(duì)過(guò)熱熔體產(chǎn)生的擾動(dòng)作用一直影響到凝固過(guò)程的形核與長(zhǎng)大，一方面使熔體中原子集團(tuán)的尺寸、形狀及分布發(fā)生改變，為提高形核率創(chuàng)造了條件，另一方面使固－液界面前沿的溶質(zhì)濃度梯度減小，抑制成分過(guò)冷，同時(shí)增大了晶核穩(wěn)態(tài)球形長(zhǎng)大的臨界半徑。
關(guān)鍵字: Al-Si合金；過(guò)熱熔體；旋轉(zhuǎn)磁場(chǎng)處理；非枝晶組織；演變；穩(wěn)定性；熔體結(jié)構(gòu)
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
