基于预孪晶诱导沉淀相调控镁合金板材弯曲中性层偏移机制研究

Due to the shorten behaviors and neutral layer shifting outside during bending which have an important effect on the stamping process and springback phenomenon sheet, pre-twinning and aging process are conducted in this project to achieve various precipitations so that controlling the offset of neutral layer and explore the precise forming theory of magnesium alloys sheets. In this project, the effect of pre-twinning on releasing of precipitations will be investigated, and then effects of precipitations on the evolution of microstructure during bending will be illuminated. Furthermore, the influence of precipitations on twinning, detwinning and slips during bending will be discussed. Besides, the interact relation among initial twinning lamellas, precipitation particles and magnesium matrix will be revealed and the mechanisms of neutral layer shifting will be obtained overall. At last, combining the finite element modeling, mathematical calculation and experimental results on various magnesium alloy sheets, the theoretical model about how to regulate springback and shorten behavior during bending through controlling offset of neutral layer based on precipitations induced by pre-twinning will be set up. This model will provide the method how to weaken and forecast springback precisely so that realizes the precision forming of magnesium alloy sheets. The results of the project will provide the engineering value about the controlling springback and baiting so as to realize the precise sheet forming.

针对镁合金板材弯曲后中性层外移这一反常现象将严重影响冲压下料及回弹的问题，本项目将借助预置孪晶时效处理诱导沉淀相析出的手段对中性层偏移量进行调控，探索镁合金板材弯曲精确成形新理论。研究预孪晶对沉淀相析出的影响规律，阐明析出相在弯曲过程中对不同微区组织演变的影响机制，探讨析出相粒子在弯曲过程中对孪生、去孪生、滑移等塑性变形机制的影响，揭示弯曲变形过程初始孪晶结构与沉淀相粒子、镁基体之间的相互作用关系，获得中性层偏移规律，结合有限元模拟、力学计算及不同状态镁合金板材弯曲实验结果拟合构建基于预孪晶诱导沉淀相析出调控中性层偏移进而控制宏观弯曲回弹及板材长度之间的理论模型，提出镁合金板材弯曲过程中回弹减弱、长度演变的精确控制及预测方法，实现镁合金板材精确成形。项目研究成果将为镁合金冲压和弯曲变形过程中板材下料和回弹控制奠定理论基础与工程应用依据。

本项目通过对AZ31及AZ80镁合金板材沿横向进行预压缩预置初始孪晶，随后时效热处理诱导第二相粒子在晶界及孪晶界处析出，弯曲变形后研究了其组织演变、中性层偏移机制及其与回弹之间的关系。研究结果表明：对低铝AZ31镁合金板材弯曲，内外两层呈现明显的组织差异，外层为均匀分布的等轴晶粒，滑移主导变形；内层为{10-12}拉伸孪晶，孪生主导变形，内外层变形机制的不对称性导致弯曲过程中性层外偏。无第二相粒子镁合金预置初始孪晶弯曲，其外层发生去孪生，内层孪生变形受到初始孪晶抑制，拉压不对称性减弱，内外层减弱的拉压不对称性导致中性层偏移量减小。对高铝AZ80镁合金进行预置不同体积分数初始孪晶后不同时间时效处理，第二相粒子在晶界及孪晶界处大量析出，反向拉伸变形，去孪生现象出现。然而由于孪晶界沉淀相的出现，反向拉伸过程去孪生机制受阻，反向断裂之后仍有大量初始孪晶保留，未时效预孪晶AZ80镁合金去孪生发生完全。弯曲之后，外侧受拉，去孪生发生，内侧受压，孪生主导变形。然而由于第二相粒子的出现，外侧去孪生变形机制受阻，内外侧拉压不对称性减弱，中性层偏移量减小。随着预孪晶体积分数增大，第二相粒子析出增多；同时随时效时间增长，第二相粒子在孪晶界及晶界处析出增多，弯曲过程中均抑制中性层向弯曲外侧偏移。沿轧制及横向不同方向不同变形量交叉预压缩预置不同路径拉伸孪晶随后时效处理，进一步0°，45°及90°三个方向反向拉伸。随着拉伸角度逐渐与预压缩方向一致，去孪生现象逐渐减弱，而在45°方向时去孪生屈服台阶没有出现，说明在此加载方向情况下主要是滑移主导变形，且三个方向各向异性明显减弱，中性层偏移量最小。通过进行弯曲过程微区力学分析，获得了中性层偏移量与回弹之间的理论模型，与实验数据相比较，该理论模型误差较小，具备适用性。项目研究成果将为镁合金弯曲变形过程组织演变、中性层偏移及回弹预测提供一个理论依据及实验指导。