脉冲磁场对合金凝固过程影响的研究进展

Research Progress of the Effect of Pulsed Magnetic Field on the Solidification Process of Alloys

磁场影响合金凝固过程的相关技术自20世纪70年代出现以来,发展至今已衍生出脉冲、稳恒、旋转等多种磁场形式。其中,脉冲磁场因其瞬时场强高、非接触、场强易于控制而引起了学界的大量研究。脉冲磁场对合金凝固过程的影响主要体现在:晶粒形貌转变与细化;合金内部相结构与成分发生改变;合金凝固后宏观缺陷得到改善。目前,学者们对上述现象的内在机理进行了探索。由脉冲磁场引发的熔体强制对流可以将凝固过程中在模具金属型内壁激冷形核的细小晶粒不断从金属型撕下并运往熔体内部,促进晶粒细化,从而抑制晶粒长大。同时,脉冲磁场能降低形核能垒,提升形核率。脉冲磁场引起的焦耳热效应一方面能延长凝固时间,防止熔体表面过快固化;但焦耳热的积累也会导致晶核熔化进而降低形核率。已有大量将脉冲磁场应用于高温合金、Mg合金以及Al合金等对生产生活影响重大的合金的报道。但该技术距离大规模工业化应用还有一段距离。本文分别从脉冲磁场对合金凝固组织的影响、脉冲磁场对合金凝固过程的动态影响机理和脉冲磁场在不同合金凝固过程中的具体应用情况共三个方面系统性地介绍脉冲磁场对合金凝固过程的影响,分析了脉冲磁场处理合金凝固技术的优势与不足,并对该技术的未来发展进行了展望。

Since the 1970s when magnetic field technologies were used in alloy solidification processes,several types of magnetic fields such as pulsed,steady,and rotating fields have been developed.Among them,the pulsed magnetic field has attracted considerable research attention owing to its high instantaneous field strength,non-contact nature,and ease of control.The effect of the pulsed magnetic field on the solidification of the alloy is reflected mainly in the changes in grain morphology and grain refinement.The phase structure and composition of the alloy are changed.The macroscopic defects of the alloy improve after solidification.In the present work,the internal mechanism of the above phenomena has been explored.The forced convection induced by a pulsed magnetic field can continuously tear off the fine grains that cool and cause nucleation in the inner wall of the metal mold during solidification.The field transports the torn off grains to the interior of the melt zone,thus promoting grain refinement and inhibiting grain growth.At the same time,the pulsed magnetic field can reduce the nucleation barrier and improve the nucleation rate.On the one hand,the Joule heating effect caused by the pulsed magnetic field can increase the solidification time and prevent the melt surface from curing too fast.However,the accumulation of Joule heat can also lead to nucleation melting and a decrease in the nucleation rate.There have been a number of reports about the application of pulsed magnetic fields to superalloys,Mg alloys and Al alloys,which have wide application in industry.However,the technology is not mature enough for large-scale industrial application.In this paper,the application of a pulsed magnetic field to alloy solidification is explored from three angles,namely,the effects of the pulsed magnetic field on the solidification structure of alloys,their underlying dynamic mechanisms,and the application of such magnetic fields in the different alloy solidification processes.Moreover,the relative advantages and disadvantages of this technology,as well as the future prospects are also explained.