本文關(guān)鍵詞： AZ31鎂合金 攪拌摩擦焊 疲勞斷裂 前進側(cè) 雙面攪拌摩擦焊　出處：《太原理工大學(xué)》2017年碩士論文　論文類型：學(xué)位論文

【摘要】：鎂合金材料由于其本身優(yōu)勢和節(jié)約能源的優(yōu)點,已經(jīng)廣泛應(yīng)用于汽車、高鐵以及航空航天等工業(yè)制造業(yè)中,這些產(chǎn)品結(jié)構(gòu)往往需要焊接技術(shù)的支持。常規(guī)熔化焊方法焊接鎂合金后,接頭的力學(xué)性能和冶金性能較差;攪拌摩擦焊技術(shù)用于鎂合金的焊接,其接頭力學(xué)性能有了極大的改善,焊接過程也更加環(huán)保。然而,這些焊接結(jié)構(gòu)在使用中往往承受疲勞載荷作用,很容易發(fā)生疲勞斷裂事故。而疲勞失效是金屬結(jié)構(gòu)件中常見的一種嚴重破壞形式,所以改善金屬結(jié)構(gòu)件的疲勞性能對其使用性和安全性的提高有著重要的意義。紅外熱像法以其便捷、無損等優(yōu)點可以用來表征試驗過程中試樣表面溫度的變化特征,從而分析疲勞過程中產(chǎn)熱溫度與變形的關(guān)系,因而采用紅外熱像法對鎂合金FSW接頭疲勞斷裂行為進行研究具有一定的實用價值。本試驗對AZ31鎂合金采用優(yōu)化的FSW參數(shù)進行對接焊接,結(jié)合金相組織、顯微硬度、EBSD、應(yīng)力-應(yīng)變曲線、掃描電子顯微鏡以及紅外熱像儀研究了鎂合金攪拌摩擦焊焊接接頭高周疲勞裂紋斷裂行為以及紅外熱像溫度特征,最終提出了改善AZ31鎂合金FSW接頭疲勞壽命的有效方法。試驗結(jié)果表明,AZ31鎂合金FSW焊接接頭疲勞循環(huán)次數(shù)達到107時,疲勞極限為55.11 MPa,焊接接頭的疲勞裂紋均沿著焊接接頭前進側(cè)(AS)發(fā)生疲勞斷裂。采用紅外熱像儀獲得FSW焊接過程溫度場分布,發(fā)現(xiàn)AS摩擦產(chǎn)熱高于RS。通過金相組織分析可以得出,熱影響區(qū)(HAZ)存在大量粗大晶粒和少部分孿晶,熱機影響區(qū)(TMAZ)相比于HAZ晶粒尺寸較小,并且存在部分等軸晶,表明該區(qū)域在FSW過程中存在不完全的動態(tài)再結(jié)晶現(xiàn)象,焊核區(qū)(NZ)所有晶粒都是細小的等軸晶粒,表明焊接過程NZ區(qū)域存在完全的動態(tài)再結(jié)晶現(xiàn)象。攪拌摩擦焊焊接接頭顯微硬度結(jié)果顯示,AS的HAZ區(qū)域顯微硬度最低,進一步證明了這一區(qū)域力學(xué)性能較差,為焊接接頭軟化區(qū)。在循環(huán)變形過程中,攪拌摩擦焊焊接接頭AS應(yīng)變大于后退側(cè)(RS)應(yīng)變。利用掃描電子顯微鏡(SEM)對疲勞斷口微觀形貌進行了研究,獲得結(jié)論為:AZ31鎂合金FSW接頭疲勞斷裂方式為脆性解理斷裂,同時斷口中可以觀察到撕裂棱和二次裂紋的存在。紅外熱像儀檢測結(jié)果表明,在循環(huán)載荷作用下AZ31鎂合金試樣表面存在五個區(qū)段的溫度演化:溫度極速上升、溫度下降、溫度水平、溫度二次升高和溫度陡降。AS側(cè)的HAZ溫度高于FSW接頭其他區(qū)域,進而驗證了此區(qū)域在相同循環(huán)載荷下應(yīng)變最大。雙面FSW接頭疲勞極限比單面FSW接頭提高了67%。雙面焊焊縫兩側(cè)熱輸入相等,微觀組織對稱分布,避免了由單面焊在AS部位粗大晶粒弱化焊接接頭的現(xiàn)象發(fā)生。相同循環(huán)載荷下,單面FSW接頭的AS應(yīng)變范圍大約為2.5%-5.25%,RS應(yīng)變范圍大約為1.5%-3.75%。雙面FSW接頭某一側(cè)應(yīng)變范圍大約為1.25%-3.75%,這與單面FSW接頭的RS應(yīng)變很接近,也說明了雙面FSW有較高的抗疲勞強度。利用掃描電子顯微鏡(SEM)對疲勞斷口微觀形貌進行了研究,獲得結(jié)論為:AZ31鎂合金雙面FSW接頭疲勞裂紋萌生于AS的表面或者近表面,微觀形貌為“人字型”花樣,最終在另一側(cè)焊縫的RS區(qū)域瞬斷,其微觀形貌特征為脆性解理形貌。
[Abstract]:The magnesium alloy material because of its own advantages and energy saving advantages, has been widely used in automobile, iron and aerospace industries in the manufacturing industry, the product structure often requires welding technology. The conventional fusion welding method of welding joints of magnesium alloy, mechanical and metallurgical properties of friction stir welding technology for the poor; the welding of magnesium alloy, the mechanical properties of the joints were greatly improved, the welding process is also more environmentally friendly. However, these are often subjected to fatigue load of welded structure is in use, it is easy to fatigue fracture accident. And fatigue failure of metal structures is a common form of serious damage, so improving the fatigue performance of metal structure one is of great significance to improve its usability and security. Infrared thermography with its advantages of convenient, nondestructive sample surface can be used to characterize the test process Temperature change characteristics, to analyze the relationship between the thermal fatigue process of middle temperature and deformation, and the use of infrared thermography to study the fatigue fracture behavior of FSW magnesium alloy joints has certain practical value. The experiment of AZ31 magnesium alloy by FSW parameter optimization of butt welding, metallographic structure, hardness, EBSD, should be stress strain curve, scanning electron microscope and infrared thermal imager on magnesium alloy friction stir welding joint of high cycle fatigue crack fracture behavior and infrared temperature characteristics, and finally puts forward some effective methods to improve the fatigue life of FSW joints of AZ31 magnesium alloy. The experimental results show that AZ31 magnesium alloy FSW welding joint fatigue cycle number reaches 107 and the fatigue limit is 55.11 MPa, the fatigue crack of welded joints of welded joints along the advancing side (AS). To obtain fatigue fracture of FSW welding using infrared thermography The temperature field distribution, found through metallographic analysis that AS friction heat is higher than RS., the heat affected zone (HAZ) the existence of a large number of coarse grains and a small part of the twin, TMAZ (TMAZ) compared to the grain size of HAZ is small, and there are some grains, indicating that the region in the FSW process in dynamic complete recrystallization in the weld nugget zone (NZ) of all grains are small equiaxed grains, shows that the welding process of NZ are completely dynamic recrystallization phenomenon. The microhardness of welded joints shows that friction stir welding, AS HAZ hardness is lowest, further proved that this region poor mechanical properties for welding joint softening zone. During cyclic deformation, welding joint of AS strain is higher than the retreating side friction stir welding (RS) strain. By using scanning electron microscopy (SEM) on the fatigue fracture morphology were studied, the conclusion is obtained AZ31 magnesium alloy FSW joint fatigue fracture mode is brittle cleavage fracture, while fracture can be observed in the tear ridge and two crack. Detection results of infrared thermography showed that evolution in the presence of five section specimen surface of AZ31 magnesium alloy under cyclic load temperature: the temperature rise speed, temperature drop, temperature level two, temperature rise and the temperature dropped.AS side of the HAZ FSW joint temperature is higher than that of other regions, and then verify the maximum strain in the same area under cyclic loading. The fatigue limit of double FSW joint single FSW improved 67%. double weld joint on both sides of the heat input is equal, symmetrical distribution of microstructure, avoid by welding at the site of AS coarse grain weakening of welded joints under cyclic loading. The same phenomenon, AS single FSW joint strain range is about 2.5%-5.25%, RS strain range is about 1.5%-3.75%. double face FSW The head of one strain range is about 1.25%-3.75%, the joint and the single FSW RS strain is very close, also shows that the fatigue strength of duplex FSW is high. By using scanning electron microscopy (SEM) on the fatigue fracture morphology were studied. The conclusion is: the surface of AZ31 magnesium alloy FSW joint double surface fatigue crack was adorable AS or near the surface, the micro morphology of "font" pattern, eventually in the RS area on the other side of the fault transient weld, its morphology is characterized by brittle cleavage morphology.

【學(xué)位授予單位】：太原理工大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TG453.9

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