基于精准局域组织预控制的镁合金复杂冲压件的成形及组织性能演变规律

Owing to the poor ductility and formability at room and low temperature, the developing and application of Mg alloy stamping products with high quality are limited. In order to solve this problem, a new technology, width-limit bending and straightening, is developed to realize the pre-compression deformation on thin sheet along transverse direction. Based on this technology, the twins can be introduced on Mg alloy thin sheet with a large size. Besides, the larger volume fraction of tension twins can be obtained on the Mg alloy sheet with a strong basal texture. Consequently, the ductility and formability are improved essentially, and the comprehensive mechanical properties of Mg stamping products are enhanced at the same time . Aiming at the various requirements on the properties at different locations of complicated Mg alloys stamping parts, the strategy of local organization controlling is proposed out and the Local Multi-pass Bending Training Charged with Shape and Property technology (LMBT) is developed; the relationship between tension twinning and blank size, type of bending ripple, cycle length, amplitude, parameters of bending and straightening as well as cycling time is analyzed; the microstructure and mechanical properties evolution during LMBT and subsequent stretch deformation is explored so as to give play to the advantages of tension twins on microstructure modifying, texture weakening, deformation coordination during stamping processes. This will provide the approach and theory to the integrated forming of complicated Mg alloy stamping products.

镁合金在室温和较低温度下塑性和成形性能差，是制约高质量镁合金复杂冲压件生产和应用的关键所在。本项目提出通过弯曲限宽矫直实现金属薄板的横向压缩变形，不仅解决大尺寸镁合金轧制薄板难以预置拉伸孪晶的难题，且可借助镁合金原始轧板强的基面织构实现预置高体积分数的拉伸孪晶，从本质上改善镁合金薄板塑性和成形性能，并有效提高冲压件综合力学性能；针对复杂冲压件不同区域对板坯成形性能的不同要求，提出精准局域组织预控制思路，开发局域多道次控形控性弯曲训练方法；分析预置拉伸孪晶效果与镁合金板坯尺寸、弯曲波纹曲线类型、周期长度、振幅大小、弯曲变形和矫直变形条件及循环次数的关系，探索镁合金薄板在弯曲限宽矫直训练和随后冲压成形过程中的组织和性能的演变规律，以充分发挥预置拉伸孪晶在改善镁合金薄板组织、弱化基面织构、促进变形协调和有效提高其力学性能和成形性能方面的优势，为镁合金复杂冲压件的形性一体化成形提供方法和理论支撑。

镁合金作为实际应用中最轻的金属结构材料，其广泛应用对于节能减排意义重大。其中，镁合金壳体构件在汽车、航空航天等领域应用前景尤为广阔。然而，镁合金在室温和较低温度下的塑性和成形性能差，成为制约高质量镁合金复杂冲压件生产和应用的关键所在。为改善镁合金板的力学性能和成形性能，发展高性能镁合金壳体构件，本项目针对镁合金板具有强基面织构难以塑性变形和成形的组织性能特点，提出通过弯曲限宽矫直实现金属薄板的横向压缩和板厚增加，从而在镁合金板中高效预置拉伸孪晶，解决镁合金薄板中难以有效预置拉伸孪晶的难题，变镁合金板强基面织构的不利为有利，达到改善镁合金板材组织和性能的目的。项目针对不同厚度镁合金薄板，设计了不同波纹曲线的“弯曲限宽矫直”模具，研究了预置拉伸孪晶效果与弯曲限宽矫直变形条件的关系，实现了在镁合金薄板中高效预置拉伸孪晶。研究结果表明，预置拉伸孪晶可显著提高镁合金板的应变硬化指数，拉深性能得到改善，结合退火处理使板材的杯突值提高1倍以上，有效改善了镁合金板的塑性成形性能。项目针对镁合金板轧制边裂问题，提出了镁合金“限宽轧制”方法，有效防止了镁合金板的轧制边裂，并将“弯曲限宽矫直”和“限宽轧制”相结合，发展出可高效细化镁合金板晶粒改善其性能的“弯曲限宽轧制”方法，使镁合金板的塑性在RD和TD方向得到同步大幅度提高，制备出各向同性良好的高塑性高强度镁合金板。针对复杂壳体构件不同区域对板材成形性能的不同要求，提出了镁合金板局域弯曲训练方法，通过对镁合金板局域的拉深压平训练，可在局域组织中高效预置拉伸孪晶并显著细化局域组织，使板材局域的屈强比显著降低，应变硬化指数显著提高，从而有效防止成形过程中局部区域的过度减薄和开裂。基于以上方法和所阐明的镁合金冲压成形成性性能与组织预控制之间的关系，可以对镁合金板材整体和局部进行组织和性能调控，为镁合金复杂冲压件的形性一体化成形提供了方法和理论支撑。