本文選題：鎂合金　切入點：電子束焊　出處：《太原理工大學》2015年碩士論文　論文類型：學位論文

【摘要】：鎂合金因其輕質(zhì)高強且易于回收等特性，在“陸�？仗臁钡阮I域作為結構材料將具有廣泛的應用前景。但其廣泛的應用離不開焊接技術的支持。而焊接接頭的力學性能，尤其是疲勞問題成為制約其安全應用的瓶頸。因此，研究鎂合金焊接接頭的疲勞行為及其斷裂機理將具有重要的理論意義和應用價值。 針對鎂合金化學性質(zhì)活潑、易氧化等特點，本文采用真空電子束焊接方法，結合紅外熱像法，重點研究了鎂合金電子束焊接接頭在靜載拉伸、疲勞裂紋萌生及擴展過程中試件表面溫度的演變規(guī)律及產(chǎn)熱機制，快速預測了焊接接頭的疲勞極限和疲勞裂紋失穩(wěn)擴展門檻值，，結合斷口形貌探討了其疲勞斷裂機理。 研究結果表明：鎂合金及其電子束焊接接頭在拉伸試驗過程中，試件表面溫度呈現(xiàn)4個階段變化規(guī)律：初始溫度下降階段（熱彈性效應占主導）；溫度緩慢升高階段（非彈性效應占主導）；溫度急劇上升階段（加工硬化現(xiàn)象）以及最后自然冷卻階段。 鎂合金電子束焊接接頭在疲勞試驗過程中，溫度演變規(guī)律呈5個階段：初始快速升溫階段（非彈性效應占主導地位）；溫度下降階段（熱傳導效應占據(jù)主導地位）；溫度平穩(wěn)階段（非彈性效應和熱傳導效應達到平衡）；溫度急劇上升階段（疲勞裂紋萌生）和最后溫度自然冷卻階段。 鎂合金電子束焊接接頭在疲勞裂紋擴展試驗過程中，溫度演變規(guī)律呈4個階段：初始快速升溫階段（裂紋處于快速擴展階段，da/dN=9.05×10-14(ΔK)9.13）；溫度平衡階段（裂紋進入穩(wěn)定擴展階段，da/dN=1.01×10-7(ΔK)2.49）；溫度急劇上升階段（裂紋快速處于失穩(wěn)擴展階段，da/dN=3.38×10-16(ΔK)10.17）以及最后溫度自然冷卻階段。 紅外熱像法預測得到鎂合金焊接接頭的疲勞極限和疲勞裂紋失穩(wěn)擴展門檻值分別為83MPa和17.73MPa m1/2，并同傳統(tǒng)試驗結果相比，二者均具有較高的吻合度。 紅外熱像法能夠快速、實時及全場地對鎂合金電子束焊接接頭在靜載拉伸試驗、疲勞試驗以及疲勞裂紋擴展試驗過程中的損傷演變進行監(jiān)測。試件表面顏色和熱點區(qū)域的變化可作為評定試件損傷演變的有效工具。 鎂合金電子束焊接接頭疲勞裂紋擴展過程歷經(jīng)三個階段：初期快速擴展階段，斷口呈現(xiàn)以解理為特征的脆性斷裂機制；穩(wěn)定擴展階段，斷口呈現(xiàn)以解理和撕裂棱為特征的混合斷裂機制；失穩(wěn)擴展階段，斷口呈現(xiàn)以撕裂棱為主的斷裂機制。
[Abstract]:Magnesium alloys are widely used as structural materials in the field of "land, sea and sky" because of their light weight and high strength and easy to be recovered. However, their wide application can not be separated from the support of welding technology, and the mechanical properties of welded joints. Especially the fatigue problem has become the bottleneck of its safety application. Therefore, it is of great theoretical significance and practical value to study the fatigue behavior and fracture mechanism of magnesium alloy welded joints. Aiming at the characteristics of chemical activity and easy oxidation of magnesium alloy, vacuum electron beam welding (VEBW) method and infrared thermal imaging (IR) method are used to study the static tensile properties of magnesium alloy welded joints by electron beam welding. In the process of fatigue crack initiation and propagation, the evolution law of surface temperature and heat generation mechanism of the specimen are studied. The fatigue limit and fatigue crack instability propagation threshold of welded joint are predicted quickly. The fatigue fracture mechanism is discussed in combination with fracture morphology. The results show that during the tensile test of magnesium alloy and its electron beam welded joint, The surface temperature of the specimen shows four stages of variation: the initial temperature decreases (thermoelastic effect dominates), the temperature rises slowly (inelastic effect dominates), and the temperature rises sharply (work hardening phenomenon). And the final natural cooling stage. During fatigue test of magnesium alloy electron beam welded joint, There are five stages of temperature evolution: initial rapid heating stage (inelastic effect dominates; temperature drop stage (heat conduction effect dominates); temperature stable stage (inelastic effect and heat conduction effect reach). At the equilibrium stage, the temperature rises sharply (fatigue crack initiation) and the final temperature is naturally cooled. During the fatigue crack growth test of magnesium alloy electron beam welded joint, There are four stages of temperature evolution: initial rapid heating stage (crack is in rapid growth stage) 9.05 脳 10 ~ (-14) (螖 K ~ (9.13)); temperature equilibrium stage (crack entering stable growth stage) 1.01 脳 10 ~ (-7) (螖 K ~ (2.49)); temperature rising stage (crack is fast in the stage of unstable growth). Da / DN = 3.38 脳 10 ~ (-16) (螖 K ~ (10) 10.17) and the natural cooling stage of the final temperature. The fatigue limit and fatigue crack instability propagation threshold of magnesium alloy welded joints predicted by infrared thermal imaging method were 83 MPA and 17.73 MPA m ~ (1 / 2), respectively. Compared with the traditional test results, the fatigue limit and fatigue crack instability propagation threshold of magnesium alloy welded joints were higher than those of conventional test results. The infrared thermal imaging method can be used to test the tensile properties of magnesium alloy welded joints by electron beam welding in real time and in real time under static load. The damage evolution during fatigue test and fatigue crack growth test is monitored. The change of surface color and hot spot can be used as an effective tool to evaluate the damage evolution of specimen. The fatigue crack propagation process of magnesium alloy electron beam welded joint goes through three stages: the initial rapid growth stage, the brittle fracture mechanism characterized by cleavage, the stable growth stage, The fracture surface presents a mixed fracture mechanism characterized by cleavage and tearing edge, while at the stage of instability extension, the fracture surface presents a fracture mechanism dominated by tearing edge.
【學位授予單位】：太原理工大學
【學位級別】：碩士
【學位授予年份】：2015
【分類號】：TG407

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