低压交流电脉冲对铝硅基合金共晶硅形态与富铁相形成的影响及调控机制

Since the addition of a grain refiner and modifier in Al-Si alloys can induce the impurities and undesired particle agglomerates, and the needle-like Fe-intermetallic phase is inevitably produced in industry production, a new approach needs to be proposed for those problems. Recently, we found that the morphology of eutectic silicon phase and Fe-intermetallic phase can be modified by a new low-voltage alternating electric current pulse (LVAECP) technique. It has open up a new way for solidification process of multicomponent Al-Si alloys. In this project, we aim to investigate the effect of a LVAECP on the morphology of eutectic silicon phase and the formation of Fe-intermetallic phase in multicomponent Al-Si alloys and their mechanisms. The alloying element as a function of modifier needs to be determined, and then the effect of alloying elements and LVAECP on the nucleation and growth of eutectic Si is studied. The corresponding mechanism which is related to the intrinsic effects of LVAECP is clarified. The characteristics of microstructure morphology, phase constituents and selected phase for the Fe-intermetallic phase in response to the LVAECP will be studied to reveal the phase transformation kinetics from β-Fe to α-Fe. Furthermore, we study the growth morphologies of eutectic Si and Fe-intermetallic phase in quinary alloys during directional solidification under LVAECP. Combined with the numerical simulation of electromagnetic force and flow field distribution, we analyze the effect of LVAECP on segregation and the effective distribution coefficient of elements. Subsequently, the model of solidification alloys is established under LVAECP, which can promote the development of the solidification theory under LVAECP and provide fundamental principles and technology accumulation for the application of the LVAECP technique.

传统的变质处理铝硅基合金会引起合金污染等问题，且生产中不可避免地生成有害的针状富铁相，因此迫切需要新方法来解决上述问题。低压交流电脉冲能改善铝硅基合金共晶硅及富铁相的形貌，为控制该类合金凝固过程开辟了新途径。本项目拟以铝硅基多元合金为研究对象，研究低压交流电脉冲对共晶硅形态与富铁相形成的影响及调控机制。首先确定起变质作用的合金元素，揭示其与电脉冲交互作用对共晶硅形核和生长的影响规律，探索元素向共晶硅生长界面附着的动力学，阐明电脉冲下共晶硅的变质机制；探究富铁相的组成、成分和形貌等对脉冲电流参数的响应规律，揭示富铁相形貌转变机制。进而采用定向凝固研究电脉冲下五元铝合金共晶硅和富铁相的生长规律，探明组织的优化工艺参数；结合数值模拟的流场，分析电脉冲对合金元素分布及分凝系数的影响，建立电脉冲下共晶硅和富铁相的生长模型。本项目的研究可为发展先进的电脉冲控制多元合金凝固新技术提供实验指导和理论依据。

高性能铝硅合金在工业上具有重要的应用价值，但传统的细化及变质剂添加法改善凝固组织会引起合金污染等问题，且生产中不可避免地生成有害的针状富铁相，影响了合金的性能。电脉冲复合合金元素能同时改善共晶硅组织及富铁相的形貌，为控制该类合金凝固过程提供了新思路。本项目研究了低压交流电脉冲复合合金元素作用下多元铝硅合金凝固组织中共晶硅及富铁相组织演化规律。并采用嵌入金属网铸型、不同凝固阶段施加电脉冲和非稳态定向凝固实验探究了共晶硅细化及富铁相形貌转变机理。项目的研究结果表明，电脉冲作用下铝硅合金中铜能起变质共晶硅的效果，铜含量不变时对于共晶硅显著细化存在一个最低脉冲电流强度值，且500ppm的铜含量使得细化最佳。凝固速率越慢，电脉冲复合Cu细化共晶硅越显著。电脉冲主要通过降低溶质扩散激活使Cu附着在共晶硅的异质结晶质点上，抑制其异质形核，增大共晶固液界面生长速率，从而细化共晶硅。另外，电脉冲引起的固液界面能及固液界面前沿溶质浓度降低也促进了共晶硅生长。电脉冲能使Al7Si1Fe合金中富铁相形貌由长针状β-Fe相部分转化为α-Fe相，且组织转变主要发生在液相线到共晶凝固结束阶段，归因于溶质扩散增强引起的Fe原子团簇周围Si原子浓度的增大，促进了α-Fe相的形成。电脉冲与合金元素Cr交互作用可使得合金组织内部富铁相为骨骼状α-Fe相，侧壁组织为规则多面体AlSiFeCr相。认为二次流导致了多面体相偏聚的形成。可以利用电脉冲复合Cr在改善合金中富铁相形貌同时，去除多余的铁相。本项目的开展将深入认知电脉冲与合金元素交互作用下合金的凝固行为，完善电流凝固理论，推进电脉冲控制多元合金凝固新技术的发展。