复合场作用下高压流变成形耐热铝合金非平衡富铁相调控机制

The addition of iron to Al-Si alloys will lead to the precipitation of coarse needle-like or plate-like iron-rich phases. The iron-rich phases are relatively stable at high temperature. However, the coarse iron-rich phases are detrimental to the mechanical properties at room temperature because they have a relatively low bond strength with the matrix. Thus, modifying the morphology of iron-rich phases to improve the comprehensive mechanical properties of Al-Si alloys with iron has already become a focus in recent years. Among the existed methods, ultrasonic vibration treatment has the advantages of simple process and low cost. But the grain size of primary δ phase is a little coarse in high iron-containing hypereutectic Al-Si alloy by using USV alone. Moreover, the long needle-like β phase cannot be removed thoroughly. A new method named high pressure rheo-squeeze casting with compound field (HPRCF) is proposed to overcome the problem described above. In this method, the semi-solid slurry of the alloy is treated by a compound field which is a combination of electromagnetic field and ultrasonic field for a certain time. Then, the slurry is processed by high pressure rheo-squeeze casting immediately. With an enlarged influence range of cavitation effect and the followed solidification under high pressure, the coarse primary δ phase will be refined significantly and long needle-like β phase will be removed thoroughly. At the same time, the forming of fine non-equilibrium iron-rich phase will also be promoted. This project will study the nucleation and growth mechanism of non-equilibrium iron-rich phase in high iron-containing hypereutectic Al-Si alloy produced by HPRCF. The effects of V/Fe ratio, parameters of the compound field treatment and rheo-squeeze casting process on the evolution of the iron-rich phase will also be discussed. The control criterion of the non-equilibrium iron-rich phase constitution, volume fraction, morphology, grain size and distribution will be obtained accordingly. The strengthening mechanism of fine iron-rich phases will also be researched. The implementation of this project will lay theoretical foundation for the manufacturing of high iron-containing hypereutectic Al-Si alloy components with high performance.

铝硅合金中加入铁元素，能析出热稳定性好的富铁相，然而粗大针片状富铁相割裂基体，危害合金室温力学性能。故改善富铁相形态的研究备受关注。在现有方法中，超声振动法工艺简单、成本低，但单独采用该法处理高铁过共晶铝硅合金后，初生δ相仍较粗大，且长针状β相无法彻底消除。本项目提出用超声/电磁复合场处理半固态合金熔体，然后进行高压流变挤压成形的新方法，通过扩大超声场的作用范围以及高压下的凝固，细化合金中粗大针片状初生δ相、消除长针状β相，并促进细小非平衡富铁相的析出。拟研究复合场处理及高压流变挤压条件下非平衡富铁相的形核和生长机理；探明V/Fe比、复合场处理和流变挤压成形参数对富铁相组织的影响规律，获得复合场下高压流变挤压成形合金中非平衡富铁相的相组成、体积分数、形态、尺寸和分布等的调控原则；并揭示细小富铁相组织对合金基体的强化机理。本项目的实施将为实现高性能耐热高铁铝硅合金零件的制造奠定理论基础。

铝硅合金中加入铁元素可形成热稳定性好的富铁相，有利于合金高温性能的提高。然而传统铸造工艺下，富铁相呈粗大针片状，对合金室温性能不利。本项目采用超声/电磁复合场处理含铁铝硅合金熔体，然后进行高压流变挤压成形。研究了超声/电磁复合场处理和高压流变挤压联合作用对富铁相组织的影响，揭示了富铁相的形核和生长机理，探明了不同V/Fe比、流变成形压力等主要参数对复合场下高压流变挤压成形合金中富铁相组织和性能的影响规律，获得了富铁相组织的调控原则。主要获得了以下重要结果：1）Al-14Si-2Fe合金中加入Mn元素，α相会通过包晶反应δ+L→α而生成；Al-17Si-2Fe合金中加入V元素，会生成细小不规则多边形状的AlSiVFe相。2）随着V/Fe比从0增至1.5，重力金属型铸造不同V/Fe比的Al-8.5Si-3.5Cu-0.7Fe合金的力学性能先增大后减小；当V/Fe=1时，合金力学性能达到最大值。3）在Al-8.5Si-3.5Cu-0.7Fe-(0,0.7)V合金的凝固过程中分别单独对合金熔体进行超声处理、电磁搅拌、以及超声/电磁复合场处理，发现超声/电磁复合场对富铁相组织的细化效果最好，超声处理次之，电磁搅拌最小。4）在超声/电磁复合场作用下，电磁搅拌对熔体流场和温度场的影响占主导地位，比超声声流效应对熔体的影响更强。5）Al-8.5Si-3.5Cu-0.7Fe-0.7V合金经超声/电磁复合场处理然后经高压流变挤压成形，合金组织得到显著细化。随着挤压力从0增至400MPa，合金组织中的针状β相变为细小的纤维状，AlSiVFe相变为平均晶粒直径小于10μm的颗粒。相较于传统重力金属型工艺，合金室温抗拉强度和伸长率分别提高了23%和106%。本项目的研究成果可为耐热铝合金制备新技术的开发提供重要的理论指导和技术支持。