本文選題：新型鋁鋰合金 + 電子束焊��； 參考：《南京航空航天大學(xué)》2017年碩士論文

【摘要】：新型鋁鋰合金具有低密度、高比強(qiáng)度、抗疲勞性能優(yōu)良以及加工成型性能良好等優(yōu)點(diǎn),是航空航天、人造衛(wèi)星、核工業(yè)等高技術(shù)領(lǐng)域中應(yīng)用前景廣闊的候選材料。鋁鋰合金在工業(yè)生產(chǎn)中大多作為焊接結(jié)構(gòu)使用,采用常規(guī)的熔化焊方法對(duì)其進(jìn)行焊接,在焊接過程中易產(chǎn)生焊縫氣孔、熱裂紋和接頭軟化等問題,在很大程度上制約鋁鋰合金在工業(yè)生產(chǎn)中的應(yīng)用。電子束焊方法具有能量密度集中、熱輸入小和熱影響區(qū)小等特點(diǎn),用于鋁鋰合金的焊接具有較大優(yōu)勢(shì)。本文采用電子束焊方法對(duì)新型鋁鋰合金進(jìn)行焊接,通過分析工藝參數(shù)對(duì)接頭組織與性能的影響,優(yōu)化電子束焊接工藝;針對(duì)焊態(tài)下接頭存在軟化問題,對(duì)接頭分別進(jìn)行固溶+單級(jí)時(shí)效、雙級(jí)時(shí)效、三級(jí)時(shí)效焊后熱處理,以改善焊接接頭的微觀組織,從而提高接頭的力學(xué)性能;同時(shí)分析研究了不同焊后熱處理工藝對(duì)接頭耐蝕性能的影響。新型鋁鋰合金的電子束焊接工藝試驗(yàn)結(jié)果表明,如果焊接速度過快,熔池在高溫下的停留時(shí)間較短,不利于合金元素的充分固溶,同時(shí)也不利于氣體從熔池金屬中逸出,容易在焊縫中殘留氣孔缺陷,導(dǎo)致接頭的力學(xué)性能降低。金相組織觀察顯示,焊態(tài)下接頭區(qū)域的組織從熔合線至焊縫中心依次為細(xì)晶層、粗晶區(qū)、等軸晶和樹枝晶。熱影響區(qū)的微觀組織與母材區(qū)相似,都是長(zhǎng)條狀軋制晶粒。接頭拉伸斷裂發(fā)生在焊縫金屬處,斷口呈明顯的韌性斷裂特征。對(duì)接頭進(jìn)行不同的焊后固溶+時(shí)效熱處理,結(jié)果顯示,焊后先高溫后低溫的雙級(jí)時(shí)效接頭可獲得最優(yōu)的力學(xué)性能,顯著提高了接頭的強(qiáng)度系數(shù),接頭強(qiáng)度達(dá)到母材強(qiáng)度的89.7%。接頭微觀組織觀察表明,熱處理后焊縫組織轉(zhuǎn)變?yōu)榈容S晶,其中經(jīng)焊后單級(jí)時(shí)效接頭熔合線附近的細(xì)小等軸晶區(qū)保留,而經(jīng)焊后雙級(jí)時(shí)效、三級(jí)時(shí)效后熔合線附近的細(xì)小等軸晶區(qū)消失。焊后雙級(jí)時(shí)效熱處理促進(jìn)焊縫中強(qiáng)化效果較強(qiáng)的T1(Al2CuLi)相在晶界處大量形核、長(zhǎng)大,晶內(nèi)析出大量的彌散β′(Al3Zr)相顆粒;由于二級(jí)時(shí)效溫度降低,使焊縫再結(jié)晶過程進(jìn)行充分,從而細(xì)化焊縫組織晶粒,產(chǎn)生細(xì)晶強(qiáng)化作用;晶界數(shù)量增加的同時(shí)也提供了更多的T1相形核位置,促進(jìn)T1相在晶界處的二次析出,使得焊縫中T1相的體積分?jǐn)?shù)增加,從而使接頭的力學(xué)性能顯著改善。對(duì)熱處理前后的接頭分別進(jìn)行晶間腐蝕、剝落腐蝕及電化學(xué)腐蝕實(shí)驗(yàn),結(jié)果表明,焊接接頭的不同區(qū)域?qū)Ωg的敏感性不同。熱處理前后接頭焊縫都具有較好的抗晶間腐蝕和抗剝蝕性能。焊后熱處理會(huì)增大接頭母材區(qū)與熱影響區(qū)的晶間腐蝕敏感性,但焊后單級(jí)時(shí)效及三級(jí)時(shí)效可降低接頭母材區(qū)與熱影響區(qū)的剝蝕敏感性。經(jīng)過焊后熱處理,接頭焊縫的自腐蝕電位有所降低,電化學(xué)腐蝕傾向略增大。
[Abstract]:The new Al-Li alloy has the advantages of low density, high specific strength, good fatigue resistance and good processing and molding properties. It is a promising candidate in the field of high technology such as aerospace, satellite, nuclear industry and so on. Aluminum-lithium alloys are mostly used as welding structures in industrial production. They are welded by conventional melting welding method. During the welding process, the weld porosity, hot crack and joint softening are easy to occur. To a large extent, the application of Al-Li alloy in industrial production is restricted. Electron beam welding (EBW) has the characteristics of concentrated energy density, small heat input and small heat affected zone, so it has great advantages in Al-Li alloy welding. In this paper, a new type of Al-Li alloy is welded by electron beam welding. By analyzing the influence of process parameters on the microstructure and properties of the joint, the electron beam welding process is optimized. In order to improve the microstructure of the welded joints, the mechanical properties of the welded joints were improved by the solution aging, the two-stage aging and the three-stage post-aging heat treatment respectively. At the same time, the effect of different post-welding heat treatment processes on the corrosion resistance of joints was analyzed and studied. The experimental results of electron beam welding for the new Al-Li alloy show that if the welding speed is too fast, the residence time of the molten pool at high temperature is shorter, which is not conducive to the adequate solution of the alloy elements and the escape of gas from the molten pool metal. It is easy to residual pore defects in weld, which results in the mechanical properties of the joint reduced. Metallographic observation shows that the microstructure of the welded joints from the fusion line to the center of the weld is followed by fine grain layer, coarse crystal zone, equiaxed crystal and dendrite. The microstructure of the heat affected zone is similar to that of the base metal region, and all of them are long stripe rolled grains. The tensile fracture of the joint occurs at the weld metal, and the fracture surface shows obvious ductile fracture characteristics. The results show that the two-stage aging joint with high temperature and low temperature after welding can obtain the best mechanical properties, and the strength coefficient of the joint can be increased significantly, and the strength of the joint reaches 89.7 of the strength of the base metal. The microstructure observation of the joint shows that the microstructure of the weld is transformed into equiaxed grain after heat treatment, in which the fine equiaxed zone near the fusion line of the single stage aging joint is retained after welding, but the double stage aging is carried out after welding. The fine equiaxed region near the fusion line disappears after tertiary aging. The two-stage aging heat treatment after welding can promote a large number of T _ (1) Al _ (2Cu _ (Li) phase, which has a strong strengthening effect, nucleate at grain boundaries and precipitate a large number of dispersed 尾 _ (Al _ (3Zr) particles in the grain boundaries, and the recrystallization process of the weld is fully carried out due to the decrease of the secondary aging temperature. As a result, the grain size of weld is refined and fine grain strengthening is produced, and the increase of grain boundary also provides more T1 phase nucleation position, which can promote the secondary precipitation of T1 phase at grain boundary and increase the volume fraction of T1 phase in weld. Thus, the mechanical properties of the joints are greatly improved. The intergranular corrosion, exfoliation corrosion and electrochemical corrosion tests were carried out on the joints before and after heat treatment. The results showed that the sensitivity to corrosion was different in different regions of welded joints. Before and after heat treatment, the welds have better resistance to intergranular corrosion and denudation. Post-welding heat treatment will increase the intergranular corrosion sensitivity between the base metal zone and the heat-affected zone, but the single-stage aging and three-stage aging after welding can reduce the denudation sensitivity between the base metal zone and the heat-affected zone. After heat treatment, the corrosion potential of the weld decreases and the tendency of electrochemical corrosion increases.
【學(xué)位授予單位】：南京航空航天大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TG456.3

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