新型铝锂合金的电子束焊及其接头的组织与性能

发布时间：2018-05-12 21:04

本文选题：新型铝锂合金 + 电子束焊　；参考：《南京航空航天大学》2017年硕士论文

【摘要】：新型铝锂合金具有低密度、高比强度、抗疲劳性能优良以及加工成型性能良好等优点,是航空航天、人造卫星、核工业等高技术领域中应用前景广阔的候选材料。铝锂合金在工业生产中大多作为焊接结构使用,采用常规的熔化焊方法对其进行焊接,在焊接过程中易产生焊缝气孔、热裂纹和接头软化等问题,在很大程度上制约铝锂合金在工业生产中的应用。电子束焊方法具有能量密度集中、热输入小和热影响区小等特点,用于铝锂合金的焊接具有较大优势。本文采用电子束焊方法对新型铝锂合金进行焊接,通过分析工艺参数对接头组织与性能的影响,优化电子束焊接工艺;针对焊态下接头存在软化问题,对接头分别进行固溶+单级时效、双级时效、三级时效焊后热处理,以改善焊接接头的微观组织,从而提高接头的力学性能;同时分析研究了不同焊后热处理工艺对接头耐蚀性能的影响。新型铝锂合金的电子束焊接工艺试验结果表明,如果焊接速度过快,熔池在高温下的停留时间较短,不利于合金元素的充分固溶,同时也不利于气体从熔池金属中逸出,容易在焊缝中残留气孔缺陷,导致接头的力学性能降低。金相组织观察显示,焊态下接头区域的组织从熔合线至焊缝中心依次为细晶层、粗晶区、等轴晶和树枝晶。热影响区的微观组织与母材区相似,都是长条状轧制晶粒。接头拉伸断裂发生在焊缝金属处,断口呈明显的韧性断裂特征。对接头进行不同的焊后固溶+时效热处理,结果显示,焊后先高温后低温的双级时效接头可获得最优的力学性能,显著提高了接头的强度系数,接头强度达到母材强度的89.7%。接头微观组织观察表明,热处理后焊缝组织转变为等轴晶,其中经焊后单级时效接头熔合线附近的细小等轴晶区保留,而经焊后双级时效、三级时效后熔合线附近的细小等轴晶区消失。焊后双级时效热处理促进焊缝中强化效果较强的T1(Al2CuLi)相在晶界处大量形核、长大,晶内析出大量的弥散β′(Al3Zr)相颗粒;由于二级时效温度降低,使焊缝再结晶过程进行充分,从而细化焊缝组织晶粒,产生细晶强化作用;晶界数量增加的同时也提供了更多的T1相形核位置,促进T1相在晶界处的二次析出,使得焊缝中T1相的体积分数增加,从而使接头的力学性能显著改善。对热处理前后的接头分别进行晶间腐蚀、剥落腐蚀及电化学腐蚀实验,结果表明,焊接接头的不同区域对腐蚀的敏感性不同。热处理前后接头焊缝都具有较好的抗晶间腐蚀和抗剥蚀性能。焊后热处理会增大接头母材区与热影响区的晶间腐蚀敏感性,但焊后单级时效及三级时效可降低接头母材区与热影响区的剥蚀敏感性。经过焊后热处理,接头焊缝的自腐蚀电位有所降低,电化学腐蚀倾向略增大。
[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.
【学位授予单位】：南京航空航天大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG456.3

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