高速列车用7XXX系高强铝合金焊接接头的疲劳行为研究

发布时间：2018-01-16 21:05

本文关键词：高速列车用7XXX系高强铝合金焊接接头的疲劳行为研究　出处：《上海工程技术大学》2015年硕士论文　论文类型：学位论文

【摘要】：A7N01高强铝合金为典型的7×××系高强铝合金,由于具有良好的性能,被广泛应用于高速列车的车身中,其疲劳行为的研究对高速列车的安全运行具有重要意义,通过对A7N01铝合金焊接接头及其各个区域进行疲劳试验,对焊接接头显微硬度和显微组织以及疲劳后断口表面和试样表面疲劳损伤形貌进行观察分析,得出了以下几点:A7N01铝合金焊接接头在单轴低周疲劳作用下产生了棘轮效应,结果表明,在循环初期,棘轮效应显著,特别是在第一循环内,材料发生明显的较大的塑性变形;在循环中期,棘轮应变速率减慢,棘轮应变较小,但棘轮应变仍是微裂纹增长的驱动力;在循环后期,材料内部空洞长大、裂纹扩展速度增加,材料内部的应力集中更加明显,导致材料的承载能力降低,最终导致了试样的疲劳失效。A7N01铝合金焊接接头在疲劳过程中主要为循环软化为主,应力大小不同,软化阶段占整个疲劳过程的比例不同,应力越小,达到循环稳定阶段所需的时间越长,当应力小于一定值时,整个阶段都处于循环软化状态,当应力大到一定程度时,没有循环软化阶段,反而在循环初始阶段表现出循环硬化现象。焊接接头的疲劳塑性变形主要发生在焊缝部位,焊缝的疲劳损伤主要以滑移带开裂为主,而热影响区疲劳损伤主要为含Fe、Si较多的粗大硬质化合物在疲劳作用下与基体分离或化合物破碎产生疲劳裂纹。当应力较大时,焊缝部位变形明显,很容易导致滑移开裂,熔合线部位应力也较为集中,热影响区硬质相与基体更容易挤压分离,所以各个区域可能为疲劳试样失效位置;当应力较小时,由于焊缝塑性相对较好,热影响区硬质相在疲劳过程中对试样的破坏作用相对较大,试样一般在热影响区断裂。由于受热循环作用的影响,热影响区内粗大化合物粒子里Fe、Si元素的含量为母材的3倍,使粒子更硬更脆,所以热影响区的疲劳性能远不及母材的疲劳性能。
[Abstract]:A7N01 high strength aluminum alloy is a typical 7 脳脳脳 series high strength aluminum alloy. Because of its good performance, A7N01 is widely used in the body of high-speed train. The study of fatigue behavior is of great significance for the safe operation of high-speed trains. The fatigue tests of A7N01 aluminum alloy welded joints and its various regions are carried out. The microhardness and microstructure of welded joints and the fatigue damage morphology of fracture surface and specimen surface after fatigue were observed and analyzed. The following points have been obtained: the ratchet effect of the welded joints of the aluminum alloy at the following points is produced under uniaxial low cycle fatigue. The results show that the ratchet effect is significant at the initial stage of the cycle, especially in the first cycle. Obvious plastic deformation occurs in the material; In the middle of cycle, ratchet strain rate slows down, ratchet strain is smaller, but ratchet strain is still the driving force of microcrack growth. In the later stage of the cycle, the cavity grows, the crack growth speed increases, and the stress concentration inside the material becomes more obvious, which results in the decrease of the loading capacity of the material. Finally, the fatigue failure of the specimen. A7N01 aluminum alloy welded joints in the fatigue process is mainly cyclic softening, the stress size is different, softening phase accounts for different proportion of the whole fatigue process, the smaller the stress. When the stress is less than a certain value, the whole stage is in the state of cyclic softening, and when the stress is large enough, there is no cycle softening stage. The fatigue plastic deformation of welded joints mainly occurs in the weld, and the fatigue damage of weld is mainly caused by slip band cracking. However, the fatigue damage in the heat-affected zone is mainly caused by the fatigue crack of coarse hard compound containing more Fe-Si under fatigue action. When the stress is high, the deformation of weld is obvious. It is easy to lead to slip and crack, and the stress in the fusion line is also concentrated. The hard phase and matrix are more easily separated by extrusion in the heat affected zone, so each region may be the failure position of fatigue specimen. When the stress is small, due to the relatively good plasticity of the weld, the hard phase in the heat-affected zone has a relatively large failure effect on the specimen during the fatigue process, and the specimen is generally fractured in the heat-affected zone, which is affected by the effect of thermal cycling. The content of Fe ~ (2 +) Si in the coarse compound particles in the heat affected zone is 3 times of that of the base metal, which makes the particles harder and more brittle, so the fatigue performance of the heat affected zone is far less than that of the base metal.
【学位授予单位】：上海工程技术大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG407

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