铝青铜平面微弹簧冷成形时组织演变及裂纹形成扩展机制研究

A plane micro-spring with a high fatigue life is favorable to MEMS reliability. An extruded aluminum bronze plane micro-spring with fatigue life of more than ten thousands time is fabricated in our prior study. However, decreasing or removing of cracks and non-perfect fibrous texture in the spring will improve the fatigue life. In this project, microscopic damage mechanics modes and ductile fracture criterions of an aluminum bronze alloy are going to be built, and the size effects in these modes and criterions will be analyzed. Nucleation and propagation of cracks in the spring during extrusion processing will be studied. Primitive fiber texture orientation evolution during the micro-extrusion processing and its mechanism, and the effects of fiber texture on fatigue life of aluminum bronze plane micro-spring will also explored. Crack propagation of the micro-spring under tension-compression cyclic loading is going to be researched. Extrusion processing of the micro-spring will be optimized based on the minimum damage value and the optimum orientation of fibrous texture. Outcomes of the program are important to master the rules and mechanisms of crack nucleation and propagation, and primitive fiber texture orientation evolution during a micro-extrusion processing, and the effects of crack and fiber texture on fatigue life of aluminum bronze plane micro-spring. If so, this project will create a new start for fabricating plane micro-springs with higher fatigue life through extrusion processing.

长寿命平面微弹簧对于保证微机电系统可靠性意义重要。针对微纳米加工金属基平面微弹簧疲劳寿命不足数十次的现状，在成功挤压成形疲劳寿命达上万次的铝青铜平面微弹簧基础上，结合该弹簧的组织特征与裂纹缺陷，提出采用纤维组织坯料小挤压比冷挤压成形无裂纹缺陷的平面微弹簧以进一步提高疲劳寿命。为此需要从机理上深入把握纤维组织铝青铜坯料平面微弹簧挤压成形时组织演变及裂纹形成与扩展。项目研究内容包括：建立纤维组织铝青铜合金细观损伤力学模型、韧性断裂准则，分析尺寸效应；研究铝青铜合金平面弹簧挤压成形时裂纹形成与扩展规律、机理及尺寸效应；探索坯料纤维组织在微弹簧挤压成形时取向的演变规律、机理及对裂纹缺陷的影响；基于最小损伤与最优纤维组织分布优化平面微弹簧挤压成形过程，同时疲劳实验验证。本项目的开展有助于掌握铝青铜平面微弹簧挤压成形时原始纤维组织演变及裂纹形成扩展规律与机制，为制备长疲劳寿命平面微弹簧提供理论指导。

长寿命平面微弹簧对保证微机电系统可靠性意义重要。项目研究了铝青铜合金细观损伤力学模型、韧性断裂准则及其尺寸效应；铝青铜平面弹簧冷挤压成形时裂纹演变规律、机理及其尺寸效应；纤维状组织铝青铜合金冷挤压平面微弹簧时纤维取向的演变；基于最小损伤值和最优纤维组织取向，优化了铝青铜平面微弹簧冷挤压成形。QAl7铜合金最佳固溶工艺为850℃+5h。随晶粒尺寸D和试样厚度t减小，QAl7铜合金强度增加。随t减小，Lemaitre损伤模型抗损伤因子S、抗拉强度Rm、抗拉应变εth和临界损伤值D1c总体减小，即存在尺寸效应。随轧制压下率RR增加，S和Rm增加，εth和D1c减小。随应变速率增加，Rm增加，S、εth和D1c减小。RR、拉伸应变速率不影响损伤模型尺寸效应。裂纹集中工作带区和模角区，易出现100μm以下裂纹。应力增加后，裂纹可扩至500μm以上。弹簧表面和内部均可萌生裂纹。裂纹数目随摩擦系数增加而减少；挤压速度影响不明显；裂纹数目与材料延伸率成负相关。平面微弹簧冷挤压优化工艺为：热处理800℃-4h，30%RR，挤压速度2mm/min，干摩擦，线宽1.0mm。弹簧线宽由1mm减至0.3mm，金属流动难度加大，塑性变形更集中于外表面区域，组织均匀性变差。线宽减小，预热处理温度、D和摩擦系数增加，组织均匀性变差，影响微弹簧疲劳性能。RR增大到一定程度可成功挤压出密集连续的纤维组织，消除变形程度小的等轴晶。纤维组织初始角度θ增大，在镦粗填充阶段将往水平方向偏转直至与该方向平行，在初始挤压阶段将形成均匀性和连续性更好的弧形流线区，稳定挤压阶段时纤维组织的连续性和密集程度持续降低。增大θ可提升微弹簧疲劳性能，θ=90°时工作平面的纤维组织连续性最好，疲劳性能最佳。本项目有助于掌握铝青铜平面微弹簧挤压成形时原始纤维组织演变及裂纹形成扩展规律与机制，为制备长疲劳寿命平面微弹簧提供理论指导。