铝合金搅拌摩擦裂纹修复的疲劳与断裂行为研究

发布时间：2018-08-05 10:24

【摘要】：在工程结构和机械设备中,疲劳破坏现象遍及每一个运动的零部件,随着服役时间的延长,会在表面产生微裂纹,最终发展成导致结构破坏的宏观裂纹。然而裂纹修复方法很多,但都不尽如人意。本文提出一种新的思路用于裂纹的修复,即搅拌摩擦裂纹修复。针对2A12铝合金表面裂纹实施搅拌摩擦修复,通过理论分析和试验观测相结合的方法得出了搅拌摩擦裂纹修复工艺区间,并研究了疲劳寿命和裂纹扩展行为。首先,通过修复工艺试验确立合理的工艺区间。将修复后平板制成疲劳试样和裂纹扩展试样进行力学性能测试;利用OM(光学显微镜)、SEM(扫面电镜)对修复区进行金相分析、断口形貌观察。通过疲劳试验机测得不同工艺参数修复试样的疲劳寿命和裂纹扩展速率。结果表明:试样在ν=70-110mm/min、ω=700-1100rpm范围内可以获得较好的修复效果,其中参数S1100-V110疲劳寿命最低达44800,参数S700-V110寿命最高可达632713,预制裂纹试样寿命仅有5147。其次,将最优工艺参数进行裂纹修复后进行热处理,对比修复试样与修后热处理试样的力学性能。进行疲劳寿命试验和裂纹扩展试验,结果表明修复件寿命达到632713,热处理后疲劳寿命提升至853356,修复件与修后热处理件疲劳寿命分别达到母材的36.98%和49.89%。断口观察发现,未经过热处理的试样,稳定扩展阶段由于搅拌摩擦打碎的晶粒而无法形成明显的疲劳条带,但在经过热处理以后晶粒、第二相、夹杂重新固溶于母材,修复区在稳定扩展阶段有大量疲劳条带出现,瞬断区韧窝也比未经热处理试样要大。最后,研究了裂纹修复位置对于疲劳寿命的影响。将裂纹置于搅拌头的前进侧,返回侧和中间分别进行修复,试验结果:位于前进侧时的疲劳寿命达14928,位于返回侧时的疲劳寿命达到153927,裂纹位于返回侧试样的疲劳寿命是前进侧试样疲劳寿命的10.31倍,而裂纹位于中间时,修复试样疲劳寿命为632713,是返回侧修复试样的4.11倍。断口分析:看出裂纹位于前进侧修复区分为上下两层,由于没有受到搅拌头的直接作用,晶粒没有被充分打碎再结晶,形成絮流状材料,而亚表面充分打碎再结晶,于是形成双层修复区,裂纹位于返回侧和中间试样断口相似。
[Abstract]:In engineering structures and mechanical equipment, fatigue failure occurs all over every moving part. With the extension of service time, micro-cracks will appear on the surface and eventually develop into macroscopic cracks that lead to structural failure. However, there are a lot of crack repair methods, but they are not satisfactory. In this paper, a new idea is proposed for crack repair, that is, friction stir crack repair. Friction stir repair for surface crack of 2A12 aluminum alloy was carried out. By means of theoretical analysis and experimental observation, the process interval of friction stir crack repair was obtained, and the fatigue life and crack propagation behavior were studied. First of all, through the repair process test to establish a reasonable process interval. The mechanical properties of fatigue specimen and crack propagating specimen were tested, and the fracture morphology was observed by means of OM (optical microscope) SEM (scanning electron microscope). Fatigue life and crack growth rate of different process parameters were measured by fatigue testing machine. The results show that the sample can be repaired in the range of S1100-V110 70-110mm / min, 蠅 -700-1100rpm. The fatigue life of parameter S1100-V110 is the lowest 44800, the maximum of parameter S700-V110 is 632713, and the life of precast crack specimen is only 5147. Secondly, the optimum process parameters were heat treated after crack repair, and the mechanical properties of the repaired specimen were compared with that of the repaired sample. The fatigue life test and crack growth test showed that the life of the repaired part reached 632713, the fatigue life of the repaired part increased to 853356 after heat treatment, and the fatigue life of the repaired part and the heat-treated part reached 36.98% and 49.89% of the base metal respectively. The fracture surface observation shows that in the stable extension stage of the sample without heat treatment, the obvious fatigue band can not be formed due to the grain broken by friction stir, but after the heat treatment, the grain, the second phase, the inclusion are dissolved again in the base metal, A large number of fatigue bands appeared in the repair zone during the stable growth stage, and the dimples in the transient fracture zone were larger than those in the untreated specimens. Finally, the effect of crack repair location on fatigue life is studied. The crack is placed on the front side of the mixing head, and the return side and the middle side are repaired separately. The results show that the fatigue life at the forward side is 14928, the fatigue life on the return side is 153927, the fatigue life of the specimen on the return side is 10.31 times that on the forward side, and the crack is in the middle. The fatigue life of the repaired specimen is 632713, 4.11 times longer than that of the return side. Fracture analysis: it can be seen that the crack repair at the forward side is divided into two layers. Due to the direct action of the agitator, the grain is not sufficiently smashed and recrystallized to form the floc flow material, while the sub-surface sufficiently shatters the recrystallization. A double layer repair zone is formed, and the crack is similar to the fracture surface of the intermediate specimen at the return side.
【学位授予单位】：沈阳航空航天大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.21;TG453.9

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