【摘要】：在工程結構和機械設備中,疲勞破壞現(xiàn)象遍及每一個運動的零部件,隨著服役時間的延長,會在表面產(chǎn)生微裂紋,最終發(fā)展成導致結構破壞的宏觀裂紋。然而裂紋修復方法很多,但都不盡如人意。本文提出一種新的思路用于裂紋的修復,即攪拌摩擦裂紋修復。針對2A12鋁合金表面裂紋實施攪拌摩擦修復,通過理論分析和試驗觀測相結合的方法得出了攪拌摩擦裂紋修復工藝區(qū)間,并研究了疲勞壽命和裂紋擴展行為。首先,通過修復工藝試驗確立合理的工藝區(qū)間。將修復后平板制成疲勞試樣和裂紋擴展試樣進行力學性能測試;利用OM(光學顯微鏡)、SEM(掃面電鏡)對修復區(qū)進行金相分析、斷口形貌觀察。通過疲勞試驗機測得不同工藝參數(shù)修復試樣的疲勞壽命和裂紋擴展速率。結果表明:試樣在ν=70-110mm/min、ω=700-1100rpm范圍內(nèi)可以獲得較好的修復效果,其中參數(shù)S1100-V110疲勞壽命最低達44800,參數(shù)S700-V110壽命最高可達632713,預制裂紋試樣壽命僅有5147。其次,將最優(yōu)工藝參數(shù)進行裂紋修復后進行熱處理,對比修復試樣與修后熱處理試樣的力學性能。進行疲勞壽命試驗和裂紋擴展試驗,結果表明修復件壽命達到632713,熱處理后疲勞壽命提升至853356,修復件與修后熱處理件疲勞壽命分別達到母材的36.98%和49.89%。斷口觀察發(fā)現(xiàn),未經(jīng)過熱處理的試樣,穩(wěn)定擴展階段由于攪拌摩擦打碎的晶粒而無法形成明顯的疲勞條帶,但在經(jīng)過熱處理以后晶粒、第二相、夾雜重新固溶于母材,修復區(qū)在穩(wěn)定擴展階段有大量疲勞條帶出現(xiàn),瞬斷區(qū)韌窩也比未經(jīng)熱處理試樣要大。最后,研究了裂紋修復位置對于疲勞壽命的影響。將裂紋置于攪拌頭的前進側(cè),返回側(cè)和中間分別進行修復,試驗結果:位于前進側(cè)時的疲勞壽命達14928,位于返回側(cè)時的疲勞壽命達到153927,裂紋位于返回側(cè)試樣的疲勞壽命是前進側(cè)試樣疲勞壽命的10.31倍,而裂紋位于中間時,修復試樣疲勞壽命為632713,是返回側(cè)修復試樣的4.11倍。斷口分析:看出裂紋位于前進側(cè)修復區(qū)分為上下兩層,由于沒有受到攪拌頭的直接作用,晶粒沒有被充分打碎再結晶,形成絮流狀材料,而亞表面充分打碎再結晶,于是形成雙層修復區(qū),裂紋位于返回側(cè)和中間試樣斷口相似。
[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.
【學位授予單位】：沈陽航空航天大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：TG146.21;TG453.9

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