SIA OpenIR  > 工艺装备与智能机器人研究室
高性能金属零件激光冲击表面改性机理及实验研究
Alternative TitleResearch on the Mechanism and Experimental of High Performance Metal Parts Surface Properties by Laser Shock Processing
杨灏
Department工艺装备与智能机器人研究室
Thesis Advisor王天然 ; 赵吉宾
Keyword激光冲击强化 机械性能 微观结构 疲劳寿命
Pages122页
Degree Discipline机械电子工程
Degree Name博士
2020-04-09
Degree Grantor中国科学院沈阳自动化研究所
Place of Conferral沈阳
Abstract随着工业的快速发展,对机械设备中高性能金属零件的技术指标要求也越来越高。由于新型合金材料的研发周期较长、制造工艺复杂、生产成本较高,往往采用强化的方法对现有金属材料进行加工处理,以满足大量工业需求。激光冲击强化技术是一种高效的金属材料表面强化技术,这项技术主要是通过激光器发出的高能激光作用在金属材料表面产生高能冲击波,使金属材料表面发生塑性变形,同时在表面引入强大的残余压应力场,增加金属材料的抗拉强度,提高材料机械性能并延长使用寿命。本文主要针对不同性质的金属材料强化的影响及响应机制和相关的关键技术这两大方面进行研究分析,研究内容如下:1.激光系统采用钇铝石榴石系统,激光的波长为1064nm,冲击过程采用水约束层模型,此过程中所产生的冲击压力在空间内传播过程较为复杂,通过水约束层模型与广义胡克定律建立冲击波压力与表层应力变化过程的空间残余应力模型,并对材料表面冲击波压力的影响层深度以及峰值残余压应力进行估算,为后续研究中不同材料的冲击数据分析提供理论支撑。2.针对高塑性材料激光冲击强化表面特性进行分析。激光冲击高塑性材料的过程中,在低光功率密度阶段其力学性能表现出明显差异,而在高光功率密度阶段其力学性能趋于一致。引起这种现象的主要原因是高塑性材料的晶粒内部位错激活阈值与外部载荷之间的相互关系。低光功率密度激光作用在材料表面时,材料的塑性形变主要依赖于晶间滑移机制,此时外部载荷未达到位错激活阈值,晶粒尺寸不会出现明显变化。继续增加激光功率密度,当激光所产生的外部载荷达到并超过材料的位错激活阈值时,材料的塑性形变逐渐由晶间滑移机制为主转变为位错堆积晶间机制为主,由于位错结构大量出现,晶粒尺寸发生明显改变,微观织构的改变引起宏观性能改变,此时材料的各项性能结果趋于一致。3. 研究硬质金属材料激光冲击强化的响应机制。硬质金属材料激光冲击后其表面的分布呈现非均质现象,对其表面完整性造成较大影响。表面非均质现象主要是在塑性变形区域内产生反向凸起。其产生有两方面原因:1.硬质金属材料硬化过程往往是由滑移硬化系统演化形成。由于材料各向异性,导致各滑移系统中的应变响应不均衡,滑移过程中出现表面应变非均质的现象。在不同晶界间常常会产生晶界间的切应力,临界切应力随晶粒中的累积切应变而变化。2. 通常在静态微元体两侧的应力并不完全相同,在静力状态下处于应力平衡。当瞬时动态过程中两侧应力出现差值,瞬时动态过程结束时,微元所产生的内应力不断累积,当超过材料的屈服强度时产生反向形变,形成反向凸起。通过对其影响层深度的研究表明,与激光冲击影响层深度相比可以忽略不计,可以通过表面抛光或者电化学方式消除反向凸起,这样既能保证表面完整性也不会影响激光冲击效果。 4.分析金属材料表面涂层激光冲击强化过程。由于金属材料表面涂层的存在,激光冲击所产生的等离子体冲击波将不直接作用在材料基体表面,而是首先作用在涂层材料上,涂层材料吸收激光能量作用在材料基体上。当涂层材料获得较高冲击波压力向材料基体方向传播时,强大的冲击波压力使涂层与材料结合更为致密,此过程不产生热效应属于纯机械作用过程,不影响涂层与材料基体的性质。另外,涂层与基体结合过程中往往会伴随着基体的塑性形变,形变过程中基体残余应力以及硬度都有较为明显的增加。5. 针对应力调控提出两种新方法。首先从路径与应力的关系进行分析。目前传统激光冲击强化路径存在以下两方面问题:1.表面应力分布不均。2.单轴负载过高。因此提出一种新的激光冲击强化路径单元避免上述问题。从仿真与实验两方面对应力波的叠加以及金属材料边缘回弹等问题进行分析,研究改善路径单元的力学分布规律以及力学反馈过程。其次从冲击次数、光功率密度与应力分布关系进行分析。传统激光冲击强化理论模型中,激光所产生的冲击波使金属材料表面塑性形变达到最大值时,效果最好,多次冲击可以更好的增加冲击效果,但是也会增加局部应力集中,产生晶间断裂现象。针对这个问题,从激光功率密度以及冲击次数方面对传统模型的应力调控进行研究。
Other AbstractWith the rapid growth of industrialization, the quality of mechanical equipment metal parts need to be improved. Due to the long research cycle, complex process and high cost of new model metals, increasing the performance of metal in used is a better way to satisfy the large requirement in industry. Laser shock peening (LSP) is an effective surface processing technology. The laser launched high power laser beam and the huge shock wave product in the metal surface, plastic deformation occurred. The residual compressive stress field could resist the tensile stress from outside, increasing the material mechanical and fatigue properties. This manuscript research on the different kinds of materials LSP mechanism and optimize the key technology of LSP. 1. The laser system is Nd:YAG, the wavelength is 1064nm with water confined model. The dynamics process of shock pressure spread in the space is complex. To analysis the process, a shock pressure model was established with water confined model and generalized Hooke’s law. Estimate the depth of LSP affect layer and peak compressive residual stress. It provides the theory for the following experiments. 2. Analyzed the LSP characteristic of plastic metal material. In the process, the dynamic performance shows different in the lower laser power density, but the same in the high laser power density. The main reason is the dislocation activity threshold. When the laser power density is small, the internal slip plays a main role in the plastic deformation process, the outside pressure didn’t reach the dislocation activity threshold, the grain size didn’t change. With the laser power density increase, the pressure exceed the threshold, dislocation accumulation is the main part of plastic deformation. The grain size decrease, and the mechanical properties goes the same. 3. Discuss the cemented carbide LSP mechanism. After LSP, the cemented carbide surface shows heterogeneity, which damage the surface integrity. The heterogeneity is the reverse plastic deformation in the LSP region. There are two reasons of the heterogeneity: 1. the indurascent of cemented carbide evolved with slip hardening system, due to the anisotropy of the materials, the strain show different in the slip process. The shear stress occurs in the grain boundary, the critical shear stress changes depend on the accumulate shear strain. 2. The stress of the infinitesimal is different. The stress equilibrium in the infinitesimal in static state. However, in the dynamic state, the infinitesimal stress is different. There is the increment in the infinitesimal, the increment is on the opposite. To compare with the depth of LSP affect layer, the depth of increment could be ignored. 4. To research the LSP process with coating material. Due to the coating layer, the LSP pressure don’t impact on the material surface directly. The pressure impact on the coating layer, and the coating layer spread the laser power to the material. In the process, the coating layer achieve the pressure, which makes the coating layer embed in the material surface stable. It’s mechanical process which means no thermal effect. What’s more, the LSP pressure usually makes plastic deformation on the material surface, which could make the residual compressive stress and hardness increase. 5. Propose two ways to control the LSP stress. First of all, to analysis the laser path and the laser stress. There are two disadvantages with tradition laser path. 1. The stress distribution inhomogeneous. 2. High load on axis. To avoid the disadvantages, a new laser path unit is proposed. To discuss the stress wave overlay and spring-back from the edge, simulation and experiment are used. Research on the dynamics distribution rule and dynamics feedback process. Secondly, to analysis the LSP repetition and laser power density with stress distribution. The traditional LSP model could achieve good results, to increase the LSP repetition the results would increase. However, stress concentration would occur, which would crack the grain boundary. To solve the problem a new way is proposed to control the stress concentration.
Language中文
Contribution Rank1
Document Type学位论文
Identifierhttp://ir.sia.cn/handle/173321/27169
Collection工艺装备与智能机器人研究室
Affiliation中国科学院沈阳自动化研究所
Recommended Citation
GB/T 7714
杨灏. 高性能金属零件激光冲击表面改性机理及实验研究[D]. 沈阳. 中国科学院沈阳自动化研究所,2020.
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