压铸镁合金双面激光冲击强化残余应力和拉伸性能研究

发布时间：2018-04-30 18:37

本文选题：激光冲击强化 + AM50镁合金　；参考：《江苏大学》2016年硕士论文

【摘要】：镁合金因其密度小,比强度高、导热性好、尺寸稳定性高、阻尼特性高等特点,广泛应用于航空航天、运输、汽车等工业领域。由于镁合金表面强度和硬度较低、耐磨性差,严重阻碍了镁合金在工程上的应用。因此,提高镁合金的表面性能已经成为重要构件制造中的关键问题之一。表面处理技术能够通过改善镁合金表层组织结构,从而提高镁合金整体的机械性能。激光冲击强化(LSP:Laser Shock Processing)是一种新型的表面处理技术,被广泛应用于金属表面改性,其最大的特点是通过高能量激光脉冲诱导的高强度冲击波在金属表面产生较深的残余压应力层。实现长寿命关键件抗疲劳制造是使我国从机械制造大国迈向机械制造强国的必由之路,是我国的战略举措和紧迫需求。2015年我国提出的《中国制造2025》行动纲领明确将激光先进制造列为“前沿技术”。针对以上现状,本文开展压铸镁合金激光冲击强化应力场和拉伸性能的研究,获得的一些主要结论和创新性成果如下:(1)利用有限元软件ABAQUS分析研究不同激光冲击路径对AM50镁合金残余应力场的影响规律以及双面同时冲击与双面间隔冲击之间的差异。在建立三维拉伸试样分析模型后,对比分析不同模型厚度方向和表面的残余应力分布,得出最佳的激光冲击方式。结果表明:当激光参数保持不变时,激光冲击强化虽然能在AM50镁合金表层诱导高幅值的残余压应力,但也会在材料内部产生不好的残余拉应力;双面同时冲击诱导出的残余压应力场在板料沿厚度方向的中间区域不仅有残余拉应力存在,还有一定的压应力;激光冲击路径垂直于拉伸力方向所诱导的表面残余应力分布更均匀、波动性更小、平均残余应力值更大、强化效果更好。另外,虽然双面激光间隔冲击在试样上表面诱导的残余压应力最大值更大一些,但是双面激光同时冲击诱导出的残余压应力场在深度方向上结构对称,上下表面残余应力的一致性更好。综上可得最优的激光冲击强化AM50镁合金拉伸试样方式是:双面同时激光冲击处理,且冲击路径垂直于拉伸力方向。(2)采用高能纳秒激光束对镁合金拉伸试样进行变面积激光冲击强化处理,研究冲击面积对应力-应变曲线的影响,结合断口形貌分析,获得冲击面积与拉伸裂纹萌生位置的关系模型。对比分析不同激光冲击覆盖面积下试样的应力-应变曲线,基于断口形貌和裂纹位置系统研究激光冲击面积对试样拉伸性能的影响机理,得出双面激光冲击强化AM50镁合金平板拉伸试样的最佳覆盖面积。结果表明:大面积激光冲击强化使得拉伸试样的抗拉强度和屈服强度得到明显提升,且抗拉强度和屈服强度随着冲击面积的增加而增加。然而,拉伸试样的塑性(包括断后伸长率和断面收缩率)随着激光冲击面积的增加先增加后减小。当激光冲击长度从0 mm(LSP-0)增加到40 mm(LSP-40)时,裂纹萌生位置由中间向两边转移再回到中间,由于不同激光冲击覆盖面积下裂纹萌生位置的转移,试样断裂模式发生从脆性断裂向韧性断裂的转变。(3)微观表征和对比镁合金激光冲击前后表层微观组织,获得冲击面积对AM50镁合金试样拉伸性能的影响规律。通过对激光冲击区域微观组织的观测获得激光冲击AM50镁合金表层晶粒细化过程。研究微观结构对试样拉伸性能的影响机理,较好的解释试样拉伸性能变化的原因。结果表明,大面积激光冲击AM50镁合金上表层晶粒细化过程示意性地描述如下:(i)由于超高能量激光冲击波的机械作用,在平均晶粒大小为50μm的粗晶中产生了一些变形孪晶,两个相邻变形孪晶的间隔长度大约为5μm;(ii)沿一个方向的变形孪晶将原始粗晶划分为T-M薄片;(iii)T-M薄片随机演化成平均大小5μm的细化晶粒。随着激光冲击面积的增加,晶粒细化的区域也逐渐扩大,使得裂纹萌生位置发生转移,从而导致AM50镁合金试样拉伸性能随激光冲击覆盖面积的变化而变化。激光冲击强化作为新型的材料表面强化技术,不仅可以在材料表层诱导出高幅值的残余压应力,也能明显细化表层晶粒。由于残余压应力层和晶粒细化层的共同作用,AM50镁合金试样的拉伸性能得到显著提升,裂纹萌生位置随激光冲击覆盖面积的变化而发生转移。
[Abstract]:Magnesium alloy has low density, high strength, good thermal conductivity, high dimensional stability and high damping characteristics. It is widely used in aerospace, transportation, automobile and other industrial fields. Because of low surface strength and hardness and poor wear resistance of magnesium alloy, magnesium alloy has been seriously hindered the application of magnesium alloy in Engineering. Therefore, the surface properties of magnesium alloys have been improved. As one of the key problems in the manufacturing of important components, surface treatment technology can improve the mechanical properties of magnesium alloy by improving the structure of magnesium alloy surface, and laser impact strengthening (LSP:Laser Shock Processing) is a new type of surface treatment technology, which is widely used in the surface modification of metal. High intensity laser pulses induced by high energy laser pulses produce a deep residual compressive stress layer on the metal surface. It is the only way for China to make our country from a large machinery manufacturing country to a powerful mechanical manufacturing country. It is a strategic measure in China and an urgent need to seek China's 2025> line made in.2015. The dynamic program explicitly lists advanced laser manufacturing as "frontier technology". Aiming at the above situation, this paper studies the stress field and tensile properties of laser shock strengthening of die casting magnesium alloy. Some main conclusions and innovative achievements are obtained as follows: (1) the study of AM50 magnesium alloy residue by different laser impact paths by the finite element software ABAQUS The influence of the stress field and the difference between the double side impact and the double-sided spacer impact. After establishing the three-dimensional tensile specimen analysis model, the distribution of the residual stress in the thickness direction and the surface of the different models is compared and analyzed. The results show that the laser shock enhancement is not constant when the excitation parameters remain unchanged. The residual compressive stress of high amplitude can be induced on the surface of AM50 magnesium alloy, but the residual tensile stress is produced inside the material, and the residual compressive stress field induced by double side impact at the same time has not only residual tensile stress but also some compressive stress in the middle area along the thickness direction of the sheet material, and the laser impact path is perpendicular to the direction of tensile force. The induced surface residual stress distribution is more uniform, the fluctuation is smaller, the average residual stress is higher and the strengthening effect is better. In addition, although the double-sided laser spacing impact on the sample surface induces the maximum residual compressive stress, but the residual compressive stress field induced by double side laser at the same time is structured in the depth direction. It is said that the consistency of the residual stress on the upper and lower surfaces is better. The best way to obtain the best laser impact strengthening AM50 magnesium alloy tensile specimen is double-sided laser shock treatment, and the impact path is perpendicular to the tensile force direction. (2) high energy nanosecond laser beam is used to improve the tensile test of magnesium alloy. The relation model of the impact area and the initiation position of the tensile crack was obtained by the effect of the area corresponding to the stress strain curve. The stress strain curve of the specimen under the different laser impact coverage area was compared and analyzed. The influence mechanism of the laser impact area on the tensile properties of the specimen was studied on the basis of the fracture morphology and the crack position system. The results show that the tensile strength and yield strength of the tensile specimen are greatly enhanced by the large area laser shock strengthening, and the tensile strength and yield strength increase with the increase of the impact area. However, the plasticity of the tensile specimens (including the after fracture) is increased. When the laser impact area increases, the length of the laser impact increases first and then decreases. When the laser impact length increases from 0 mm (LSP-0) to 40 mm (LSP-40), the crack initiation position is transferred from the middle to the middle, and the fracture mode of the specimen occurs from brittleness due to the shift of the crack growth position under different laser impact coverage area. The transformation of fracture to ductile fracture. (3) microscopic characterization and comparison of microstructure of surface surface before and after laser impact of magnesium alloy, the influence of impact area on the tensile properties of AM50 magnesium alloy samples was obtained. The grain refinement process of laser shock AM50 magnesium alloy was obtained by observing the microstructure of laser shock region. The influence mechanism of the tensile properties of the sample is explained well. The results show that the grain refinement process on the surface of AM50 magnesium alloy with large area laser shock is described as follows: (I) some deformation twins are produced in the coarse grains with the average size of 50 mu m due to the mechanical action of the ultra high energy laser shock wave. The interval length of two adjacent deformed twins is about 5 m; (II) the original coarse grains are divided into T-M thin slices along the deformation twins in one direction; (III) T-M thin slices are randomly evolved into refined grains with an average size of 5 mu m. With the increase of the impact area of the laser, the region of grain refinement gradually expands, so that the crack initiation position is transferred, thus the transition of the crack initiation position is made. The tensile properties of AM50 magnesium alloy change with the change of the coverage area of laser shock. As a new material surface strengthening technology, laser shock strengthening can not only induce high amplitude residual compressive stress on the surface of the material, but also obviously refine the surface grain. Because of the joint action of the residual compressive stress layer and grain refining layer, AM50 The tensile properties of magnesium alloy samples were significantly improved, and the location of crack initiation shifted with the coverage of laser shock.

【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TG249.2;TG665

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