本文關(guān)鍵詞： MB8鎂合金 對(duì)接接頭 循環(huán)超聲沖擊 疲勞性能 晶粒細(xì)化 殘余應(yīng)力　出處：《華東交通大學(xué)》2015年碩士論文　論文類型：學(xué)位論文

【摘要】：疲勞斷裂是引起焊接結(jié)構(gòu)失效的主要原因，占到結(jié)構(gòu)失效的70%~90%。鎂合金具有高強(qiáng)度比、高阻尼、易切削加工性能以及易回收再利用等一系列獨(dú)特的優(yōu)點(diǎn)。提高鎂合金焊接接頭的疲勞強(qiáng)度及改善其疲勞性能，能提高鎂合金焊接結(jié)構(gòu)的可靠性及適用性，從而對(duì)推廣鎂合金結(jié)構(gòu)的應(yīng)用具有深遠(yuǎn)的意義。本次研究中，通過循環(huán)超聲沖擊處理MB8鎂合金焊接接頭的焊趾及其附近區(qū)域，比較了超聲沖擊前后對(duì)接接頭的疲勞性能，并分析了超聲沖擊MB8鎂合金焊接接頭表面的納米化機(jī)理。試驗(yàn)結(jié)果表明： （1）循環(huán)超聲沖擊試樣S-N方程：lgN=21.13-8.67Δσ，焊態(tài)試樣的S-N方程：lgN=28.17-13.82Δσ，超聲沖擊態(tài)試樣S-N曲線方程：lgN=22.04-9.27Δσ；在試驗(yàn)基數(shù)為2×106下，沖擊態(tài)的疲勞強(qiáng)度為49.87MPa，比焊態(tài)試樣的疲勞強(qiáng)度38.24MPa大約提高了32%。循環(huán)超聲沖擊的疲勞強(qiáng)度為51.33MPa，比超聲沖擊的疲勞強(qiáng)度大約提高了3%。 （2）通過SEM發(fā)現(xiàn)沖擊前后的試樣斷口均沒有疲勞輝紋。瞬斷區(qū)存在大量的撕裂棱，同時(shí)存在解理小刻面、解理臺(tái)階。在斷口擴(kuò)展區(qū)都出現(xiàn)了大量的二次裂紋，說明超聲沖擊并不能改變MB8鎂合金焊接接頭的斷裂方式，都為解理斷裂。 （3）超聲沖擊后，在MB8鎂合金焊接接頭區(qū)域的殘余拉應(yīng)力都轉(zhuǎn)變?yōu)闅堄鄩簯?yīng)力。疲勞試驗(yàn)后，焊接接頭區(qū)域的殘余應(yīng)力都有很大程度的釋放。 （4）超聲沖擊后，MB8接頭焊趾區(qū)域表面的晶粒細(xì)化現(xiàn)象很明顯。在相同的沖擊時(shí)間下，隨著超聲沖擊電流的增大，，MB8焊趾區(qū)域表層所獲得的晶粒尺寸逐漸減小。 （5）超聲沖擊細(xì)化MB8鎂合金晶粒主要分為以下幾個(gè)過程：超聲沖擊初期，焊趾區(qū)域發(fā)生了嚴(yán)重的塑性變形，位錯(cuò)纏結(jié)及位錯(cuò)墻逐漸形成。隨后位錯(cuò)纏結(jié)及位錯(cuò)墻逐步轉(zhuǎn)變?yōu)樾〗嵌鹊膩喚ЫY(jié)構(gòu)，把原始的粗晶粒分割為多個(gè)亞晶結(jié)構(gòu)。隨著變形量的增加，亞晶在晶界處發(fā)生動(dòng)態(tài)再結(jié)晶，亞晶最終會(huì)轉(zhuǎn)變?yōu)榧{米晶。 （6）超聲沖擊后試樣的表面產(chǎn)生了一層塑性變形層，該層的晶粒得到細(xì)化。隨著沖擊時(shí)間的增加沖擊表面的耐磨損性、耐腐蝕性能以及材料表面的顯微硬度均有不同程度的提高。沖擊電流為1.2A，沖擊時(shí)間為6min的試樣比沒有擊的試樣，耐磨損性提高了49.1%、耐腐蝕性提高了52.2%、表面硬度提高了103%。
[Abstract]:Fatigue fracture is the main cause of failure of welded structures, accounting for 70% of structural failure. Magnesium alloys have high strength ratio and high damping. A series of unique advantages, such as easy cutting performance and easy recycling and reuse, can improve the fatigue strength and fatigue properties of the welded joints of magnesium alloys, which can improve the reliability and applicability of the welded structures of magnesium alloys. In this study, the weld toe of MB8 magnesium alloy welded joint and its adjacent area were treated by cyclic ultrasonic impact, and the fatigue properties of butt joint before and after ultrasonic impact were compared. The nanocrystalline mechanism of ultrasonic impact on the surface of MB8 magnesium alloy welded joint is analyzed. The experimental results show that:. (1) the S-N equation of circulating ultrasonic shock specimen is: 1: lgN = 21.13-8.67 螖 蟽, the S-N equation of welded specimen is: lgN = 28.17-13.82 螖 蟽, and the S-N curve equation of ultrasonic impact state sample is 1: lgN equation 22.04-9.27 螖 蟽, the test base is 2 脳 10 ~ 6. The fatigue strength of the impact state is 49.87 MPA, which is about 38.24 MPA higher than that of the welded specimen, and the fatigue strength of the cyclic ultrasonic impact is 51.33 MPA, which is about 3 times higher than that of the ultrasonic impact. (2) it was found by SEM that there were no fatigue ripples on the fracture surface of the specimen before and after impact. There were a large number of tearing edges, cleavage surface and cleavage steps in the transient fracture zone. A large number of secondary cracks appeared in the fracture propagation zone. The results show that ultrasonic impact can not change the fracture mode of MB8 magnesium alloy welded joint, which is cleavage fracture. 3) after ultrasonic impact, the residual tensile stress in MB8 magnesium alloy welded joint is transformed into residual compressive stress, and after fatigue test, the residual stress in welded joint area is released to a great extent. At the same impact time, the grain size of the weld toe surface decreases with the increase of ultrasonic impact current. 5) Ultrasonic impact refining of MB8 magnesium alloy grain is divided into the following processes: in the initial stage of ultrasonic impact, serious plastic deformation occurs in the weld toe area. Dislocation entanglement and dislocation wall are gradually formed. Then dislocation entanglement and dislocation wall are gradually transformed into subcrystalline structures at small angles, and the original coarse grains are divided into multiple subcrystalline structures. With the increase of deformation amount, dynamic recrystallization occurs at grain boundaries. Subcrystals will eventually be transformed into nanocrystals. (6) A plastic deformation layer was formed on the surface of the specimen after ultrasonic impact, and the grain of the layer was refined. With the increase of impact time, the wear resistance of the impact surface was increased. The corrosion resistance and microhardness of the material surface were improved in varying degrees, the impact current was 1.2 A, the impact time was 6 min, the wear resistance was increased by 49.1%, the corrosion resistance was increased by 52.2%, and the surface hardness was increased by 103%.
【學(xué)位授予單位】：華東交通大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2015
【分類號(hào)】：TG407

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