激光冲击微成形中的尺度效应研究

Microscale laser peen forming (μLPF) is a novel plastic micro-forming technology, while its application is limited due to the fact that the influence mechanism of size effects on the forming process remains unclear. In this topic, two representative technologies, micro-bulging and micro-punching, were chosen to investigate the influence of size effects of grain size and sheet thickness on μLPF under the condition of high strain rate impact loading. Quasi-static and dynamic tensile tests of pure copper foils and brass foils with various grain size and thickness were performed to study size effects on dynamic mechanical behavior. The dynamic constitutive equation considering size effects was established based on experimental data. Then the constitutive equation was employed in numerical simulation of μLPF. The influence rules of size effects on laser shock induced micro-bulging and micro-punching were summarized. The relationship between extrinsic deformation behavior and intrinsic evolvement of microstructure was investigated to understand the mechanism of size effects. A model considering size effects was developed which can be used in predicting the maximum height of foils in micro-bulging, and the technological parameters can also be optimized according to this model. The formation of adiabatic shear bands and the fracture surface morphology in micro-punching were also discussed. Through the research the essential reason and the influence mechanism of size effects on μLPF were revealed, which had deep meaning in theory as well as actual using value.

激光冲击微成形是一种新兴微塑性成形工艺，由于尺度效应对微成形过程的影响机制尚不明确，限制了该工艺在微细制造领域的广泛应用。本课题提出以激光冲击微成形中微胀形和微冲孔作为代表性工艺，探索在激光冲击诱致高应变率塑性变形和动态断裂过程中，晶粒尺寸和板材厚度的尺度效应对激光冲击微成形的影响机制。采用不同晶粒尺寸和板材厚度的紫铜和黄铜箔材，实验研究尺度效应对铜箔动态基础力学性能的影响机理, 建立考虑尺度效应的铜箔动态本构模型；实验检测结合数值模拟，材料动力学行为结合微观组织演变，明确尺度效应对微胀形时材料外在变形行为、内在微观结构演变、成形极值条件等要素的影响规律，阐明尺度效应对微冲孔时绝热剪切带形成、冲裁断面质量的影响机理。以期揭示激光冲击微成形中尺度效应的本质原因和影响机制，为增强微成形过程的可调控性、优化工艺方案和提高制品质量提供新思路。

当前微型化产品巨大的市场需求促使研究者开发新技术新工艺以快速高效制备微型零件。激光冲击微成形是一种新型微塑性成形工艺，兼有高应变率塑性成形和激光冲击强化工艺的技术优势。在该工艺中，强脉冲激光冲击诱导的等离子体冲击波压力被用以使金属箔材发生塑性变形，具有快速、柔性、可控性好、产品质量稳定等显著特点。.本项目以激光冲击微成形中微胀形和微冲孔作为代表性工艺，探索在激光冲击诱致高应变率塑性变形和动态断裂过程中，晶粒尺度效应、特征尺度效应和工艺条件尺度效应对激光冲击微成形的影响机制。此外，对块体非晶激光冲击强化中的尺度效应、金属箔材激光冲击微冲孔中的反冲塞效应等也开展了相应研究工作。研究工作主要内容和结果如下：.(1) 实验研究了金属箔材基础力学性能的尺度效应。研究发现：铜箔微拉伸力学性能存在明显的尺度效应，呈现“越小越强”的现象；Fe78Si9B13非晶箔材激光冲击微冲孔形成的断面由滑移区和最终断裂区组成；高应变率加载时非晶合金断面形貌由大量脉状花样、熔融后凝固形成的液滴、大片熔融带构成；块体非晶的尺寸大小显著影响激光冲击后材料的屈服强度，高径比为1的试样在激光冲击表面强化后获得的力学性能更优。.(2) 实验研究了金属箔材激光冲击微胀形工艺中的尺度效应。研究发现：在单/多脉冲激光冲击条件下，约束层厚度显著影响着激光冲击后吸收层和微胀形件的表面形貌，紫铜箔材的胀形变形量随约束层厚度不同而明显变化；随激光冲击能量增大，不同初始晶粒尺寸下的微胀形件最大胀形高度均增大，利用金相观察获得了微胀形件板厚方向晶粒分布特征。.(3) 实验和数值仿真研究了金属箔材激光冲击微冲孔工艺中的尺度效应。研究发现：在激光光斑尺寸d大于微冲孔凹模直径D时，箔材断面均呈现剪切断裂特征，由圆角区、剪切区和毛刺组成，随晶粒尺寸变大，箔材的微孔断面上毛刺增多且形状不规则；比值d/D与金属箔材和非晶箔材的断裂行为十分密切，随d/D变化，金属箔材断裂分别出现剪切断裂、混合断裂和拉伸断裂模式。.(4) 利用数值仿真方法研究了激光冲击微冲孔中的反冲塞效应。研究发现：金属箔材在一定条件下会发生反冲塞破坏，该反常破坏过程分为正向塑性变形、正向局部剪切断裂、反向最终断裂三个阶段；微细冲孔凹模深度和激光冲击波压力对箔材断裂行为有显著影响；调整冲孔凹模入口圆角半径未实质改变箔材的断裂行为，但对冲孔件的断面形貌有一定影响。