GH4169合金电子束焊接头组织及疲劳裂纹扩展研究

发布时间：2018-07-29 20:23

【摘要】：本文以6 mm厚GH4169镍基高温合金为研究对象,采用电子束焊研究焊接接头组织和焊缝疲劳裂纹扩展。采用金相显微镜(OM)和扫描电子显微镜(SEM)对不同电子束流、扫描波形和热处理状态下的焊接接头显微组织进行分析;采用三维散斑动态应变测量仪(XTDIC)和拉伸试验机对焊缝拉伸变形过程进行分析;采用疲劳裂纹扩展试验机对焊缝疲劳裂纹扩展速率进行分析。研究的主要结论有:束流29 mA时,能获得成形最优的焊缝,成形美观无明显缺陷,接头横截面呈上宽下窄的钉形。电子束流增大,熔深显著增加,深宽比增大。加入扫描波形易导致焊缝背面未焊透,接头横截面中下部出现弯曲现象,呈不对称性。焊接接头热影响区小,上部组织为朝热源生长的γ相,同时伴随着二次枝晶形成,中部组织生长方式由胞状生长变为柱状晶生长直到等轴晶生长,下部组织为细小的枝晶臂。电子束流对组织影响不大,而扫描波形会改变γ相取向分布,并减少二次枝晶数量,焊后热处理焊缝晶粒细化,长条状枝晶数量减少。接头硬度呈母材高、焊缝低的趋势。电子束流增大,焊缝硬度升高。加入扫描波形不能提升焊缝硬度。热处理后母材硬度提升了96 HV,焊缝硬度提升了111.5 HV。最优工艺参数下焊缝抗拉强度达到母材的96.3%,热处理后母材和焊缝的抗拉强度增加了281 Mpa和275 Mpa,延伸率达到36%和31%,性能显著提升。XTDIC测试结果显示,热处理后母材和焊接试样变形断裂方式不同。热处理母材试样中心形成最大应变场且无偏移,中心晶粒拉长变形严重,裂纹于中心萌生,朝四周扩展最后发生断裂。焊后热处理试样初始最大应变场分布于试样左上角,随后偏移到试样右侧,整体应变场呈从右到左依次减小的趋势。发现右侧晶粒拉长变形严重,首先开裂,并朝左侧扩展最后发生断裂。热处理焊缝断口较热处理母材断口韧窝较浅,塑性更差。焊后热处理焊缝断口存在熔合不良,气孔等缺陷。GH4169合金母材、最优参数焊缝、S形扫描焊缝及圆形扫描焊缝的疲劳裂纹扩展符合Paris公式。不同焊缝抗裂纹扩展能力均优于母材,且最优参数焊缝抗裂纹扩展能力最好。估算母材和不同焊缝的疲劳裂纹扩展门槛值,母材抗裂纹萌生能力优于不同焊缝。加入圆形扫描波形有助于提升焊缝的抗裂纹萌生能力。GH4169合金疲劳裂纹扩展断口分为裂纹萌生区、裂纹扩展区和瞬断区。扩展初期存在解理小台阶,为准解理断口。扩展中期有明显的疲劳条带形成,同时伴随着撕裂棱。扩展快速区为撕裂棱、孔洞和二次裂纹构成的微孔聚集形韧窝断口。母材裂纹源萌生于中部区域,焊缝裂纹源主要萌生于近表面夹杂物及焊接缺陷处。最优参数焊缝疲劳条带之间有明显的二次裂纹出现,有助于释放裂纹尖端能量,提高抗裂纹扩展能力。圆形扫描焊缝裂纹扩展断口存在着更多的缺陷,裂纹扩展速率快,疲劳条带很浅。
[Abstract]:In this paper, the microstructure and fatigue crack propagation of welded joints were studied by electron beam welding (EBW) with 6mm thick GH4169 nickel-base superalloy. Metallographic microscope (OM) and scanning electron microscope (SEM) were used to analyze the microstructure of welded joints under different electron beams, scanning waveforms and heat treatment. The tensile deformation process of weld was analyzed by (XTDIC) and tensile testing machine, and the fatigue crack growth rate was analyzed by fatigue crack growth tester. The main conclusions are as follows: when the beam current is 29 Ma, the optimum weld can be obtained, and the shape of the joint has no obvious defects, and the cross section of the joint is a nail shape with the upper width and the lower width. When the electron beam current increases, the penetration depth and the aspect ratio increase significantly. The addition of scanning waveform can lead to the underside weld penetration and bending phenomenon in the middle and lower part of the cross section of the joint, which is asymmetric. The heat affected zone of the welded joint is small, the upper part of the joint is 纬 phase growing towards the heat source, accompanied by the secondary dendrite formation, the growth mode of the middle part of the joint changes from cellular growth to columnar crystal growth until equiaxed crystal growth, and the lower part of the joint is a fine dendritic arm. The electron beam has little effect on the microstructure, but the scanning waveform will change the orientation distribution of 纬 phase and decrease the number of secondary dendrites. The grain size of weld after heat treatment will be refined, and the number of long stripe dendrites will be decreased. The hardness of the joint shows the trend of high base metal and low weld seam. The hardness of weld increases with the increase of electron beam current. The addition of scanning waveforms does not improve weld hardness. After heat treatment, the hardness of base metal increased 96 HVV, the hardness of weld increased 111.5 HV. The tensile strength of weld metal reached 96.3Mpa. after heat treatment, the tensile strength of the base metal and weld increased 281 Mpa and 275Mpa. the elongation reached 36% and 31%. The deformation and fracture modes of base metal and welding specimen are different after heat treatment. The maximum strain field is formed in the center of the sample and no deviation is found in the center of the heat treatment sample. The center grain is elongated and deformed seriously, the crack initiation occurs in the center, and the fracture occurs at the end of the propagation. The initial maximum strain field was distributed in the upper left corner of the specimen and then shifted to the right side of the specimen. The whole strain field decreased from right to left. It is found that the right grain elongated and deformed seriously, first cracked, and then extended to the left side, finally fracture occurred. The fracture surface of heat treated weld is lighter and the plasticity is worse than that of base metal. The weld fracture surface of post-weld heat treatment has some defects, such as bad fusion, porosity and other defects. The fatigue crack propagation of the optimal parameters of the S-scan weld and circular scanning weld conforms to the Paris formula. The crack propagation resistance of different welds is better than that of base metal, and the optimum parameters are the best. The fatigue crack propagation threshold of base metal and different weld is estimated, and the resistance of base metal to crack initiation is better than that of different weld. The fatigue crack propagation fracture of alloy GH4169 can be divided into crack initiation zone crack propagation zone and transient fracture zone. At the beginning of expansion, there are small steps of cleavage and quasi-cleavage fracture. There are obvious fatigue bands in the middle stage of propagation, accompanied by ripping edges. The rapid propagation zone is composed of tear edges, holes and secondary cracks, which are formed by micropore aggregate dimple fracture. The source of the base metal crack originates in the middle region and the weld crack source mainly originates in the near surface inclusions and welding defects. There are obvious secondary cracks between the fatigue bands of weld seam with optimal parameters, which is helpful to release the energy of crack tip and improve the anti-crack propagation ability. There are more defects in the crack propagation fracture of circular scanning weld, the crack growth rate is fast, and the fatigue band is very shallow.
【学位授予单位】：南昌航空大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG456.3

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