脉冲电场下的7075双辊铸轧板热变形行为

Thermal Formability of 7075 Alloy Strip Under Electric Pulse Rheo-rolling Process

在变形温度250~350℃和应变速率0.001~0.01 s^(−1)的条件下进行热拉伸试验,研究了厚度为5 mm的7075合金传统铸轧板和电脉冲铸轧板的微观组织和热成形性能。实验用脉冲电流参数为:单周期脉宽0.005 s和峰值强度300 A。结果显示,脉冲电流能够促进凝固过程的枝晶细化,并诱发组织均匀化。由于电磁振荡增强了凝固过程中溶质混合能力,极压电流有效地促进了晶粒细化和析出相的均匀化。同时,面心立方(fcc)铝合金铸轧板织构具有{423}<144>取向,经脉冲电流改性后,取向为{421}<216>。脉冲电流处理后的合金峰值流动应力约为未处理合金的1.1~1.3倍。电子背散射衍射分析表明,脉冲电流通过促进动态再结晶的方式,有效地改善了晶粒的择优分布。在此基础上,分别建立了铝合金带材的双曲正弦方程和含Zener-Holloman参数的本构模型。此外,通过对断口形貌的观察发现,脉冲电流的引入有利于提高7075铝合金的高温机械性能。最后建立了优化的工艺流程图,并确定了适宜的热加工温度为290~330℃和应变速率为0.001~0.002 s^(−1)。

The microstructure and thermal formability of twin-roll cast 7075 alloy strips prepared with and without electrical pulse(EP)of 5 mm in thickness were investigated by hot tensile tests under the condition of deformation temperature of 250~350℃ and strain rate of 0.001~0.01 s^(−1).The related EP current parameters are as follows:the single cycle pulse width is 0.005 s;the pulse peak current is 300 A.The results show that the EP current effectively promotes the grain refinement and the precipitate homogeneousness,because the electromagnetic oscillation enhances the solute mixing ability during solidification.Meanwhile,the rolling textures of face-centered cubic(fcc)7075 Al strips have the{423}<144>orientation.After EP current modification,the orientation of 7075 Al strips is{421}<216>.The peak flow stress of the 7075 alloy strips prepared with EP current is about 1.1~1.3 times larger than that of the original 7075 alloy strips.The electron back-scattered diffraction analysis shows that EP current can effectively improve the preferred distribution of the grains by promoting the dynamic recrystallization.Furthermore,the hyperbolic sine equation and the constitutive model with Zener-Holloman parameter are established based on the analyses.Besides,according to the fracture morphology,the EP current is beneficial to improve the mechanical properties of 7075 alloys at elevated temperature.Finally,the processing maps are established and the proper hot working parameters are designed as the temperature range of 290~330℃ and the strain rate of 0.001~0.002 s^(−1).