深冷预处理激光喷丸强化TC6钛合金残余应力高温松弛特性研究

发布时间：2018-02-13 20:38

  本文关键词： 激光喷丸 深冷预处理 TC6钛合金 残余压应力 高温松弛　出处：《江苏大学》2017年硕士论文　论文类型：学位论文

【摘要】：随着TC6钛合金在航空工业中用量的增加,因其疲劳断裂引发的事故也日益突出,表面形变强化是抑制疲劳裂纹扩展,改善零部件使用寿命的有效方法。激光喷丸强化是一种新颖的表面形变强化技术,其可在材料表层诱导产生高幅值残余压应力分布,然而在高温服役条件下,残余压应力将产生松弛。为此,本文提出深冷预处理激光喷丸技术,探索深冷处理结合高应变率形变强化改善残余压应力高温稳定性的机理和工艺。主要工作如下:(1)基于位错强化理论、第二相强化理论、激光与材料相互作用诱导等离子体冲击波理论和细晶强化理论,阐述深冷处理与激光喷丸过程中材料的强化机制;根据位错滑移理论和蠕变理论阐释残余应力高温松弛的宏观机制,通过晶粒演变、位错密度演变和析出相演变分析残余应力高温松弛的微观机理。(2)开展深冷预处理激光喷丸的参数研究,进行不同工艺方法、参数下TC6试样的表面完整性和力学性能测试。结果表明,C-0h-LP试样和C-12h-LP试样表面出现了明显的晶粒细化,且断口处的韧窝更大更深,但C-12h-LP试样具有更高的表面硬度、残余应力及拉伸强度;而C-18h试样的硬度和强度虽然高于C-0h-LP试样,但延伸率却大幅下降,在其断口中发现大量微观组织结构由亚稳β相转变成的稳定α相,且在拉伸过程中被切割形成准解理面及解理台阶,表明C-18h试样为脆性断裂模式。分析确定了C-12h-LP为最佳工艺参数。(3)对C-12h-LP试样和C-0h-LP试样进行不同温度、不同时间的高温保持实验并测量表层残余应力,观察表面显微组织,随后进行高温拉伸试验。研究发现,400℃和600℃下,两种试样具有相当的残余应力稳定性;而500℃下,C-12h-LP试样的残余应力更稳定。500℃下C-12h-LP和C-0h-LP试样的晶粒长大缓慢,断口中均出现强化相被切割而形成的准解理面,且C-12h-LP试样中数量更多、分布更均匀。C-12h-LP工艺能在材料中诱导更高的位错密度,增加了析出相在位错线上的形核率,更多的析出相与位错相互作用导致更大的位错运动阻力,从而提高了残余压应力的稳定性。
[Abstract]:With the increase of the amount of TC6 titanium alloy in aviation industry, the accidents caused by fatigue fracture become more and more serious, and the surface deformation strengthening is to restrain the fatigue crack growth. Laser shot peening is a novel surface deformation strengthening technique, which can induce high amplitude residual compressive stress distribution on the surface of the material. However, under the condition of high temperature service, laser shot peening is an effective method to improve the service life of parts. The residual compressive stress will be relaxed. In this paper, a cryogenic pretreatment laser shot peening technique is proposed. To explore the mechanism and process of cryogenic treatment combined with high strain rate deformation strengthening to improve the high temperature stability of residual compressive stress. The main work is as follows: (1) based on dislocation strengthening theory and second phase strengthening theory, The theory of plasma-shock wave induced by laser-material interaction and the theory of fine grain strengthening are discussed, and the strengthening mechanism of materials in the process of cryogenic treatment and laser shot peening is described. According to dislocation slip theory and creep theory, the macroscopic mechanism of residual stress relaxation at high temperature is explained. Microcosmic mechanism of high temperature relaxation of residual stress. (2) study on parameters of cryogenic pretreatment laser shot peening and different processing methods. The surface integrity and mechanical properties of TC6 samples were tested under the parameters. The results showed that the surface of C-0h-LP and C-12h-LP samples had obvious grain refinement, and the dimples at the fracture surface were larger and deeper, but the surface hardness of C-12h-LP specimens was higher than that of C-12h-LP specimens. The hardness and strength of C-18h specimen were higher than that of C-0h-LP sample, but the elongation of C-18h sample decreased significantly. In the fracture surface, a large number of microstructures were transformed from metastable 尾 phase to stable 伪 phase, while the hardness and strength of C-18h specimen were higher than that of C-0h-LP specimen. The quasi cleavage surface and cleavage step were formed during the tensile process, which indicated that the C-18h specimen was brittle fracture mode. C-12h-LP and C-0h-LP samples were analyzed and determined as the optimum process parameters. The temperatures of C-12h-LP and C-0h-LP samples were determined. The residual stress of the surface was measured and the microstructure of the surface was observed at different time, and then the tensile test was carried out at high temperature. It was found that the two samples had considerable residual stress stability at 400 鈩，

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