Ti-6246钛合金热机械处理及电子束焊接性研究

发布时间：2018-05-24 13:01

本文选题：Ti-6246合金 + 显微组织　；参考：《中国科学技术大学》2017年博士论文

【摘要】：Ti-6246合金是一种高强高温钛合金,常用在燃气涡轮发动机的中温部位,其长时使用温度400℃左右,短期工作温度可达540℃,在该温度范围内具有优异的力学性能。本文以Ti-6246合金为实验材料,利用金相显微镜、扫描电镜、透射电镜、X射线衍射仪等测试技术,研究了热加工、热处理及焊接对合金显微组织和力学性能的影响,为其进一步应用提供依据。应用加工图理论研究了Ti-6246合金的高温变形行为和不同应变条件下的变形机制。Ti-6246合金真应力-真应变曲线受加工硬化和动态软化的双重作用影响。在热压缩过程中,流变应力峰值与温度和应变速率有关。流变应力随变形温度的升高而降低,随应变速率的增加而增大。应用Zener-Hollomon关系式以及双曲正弦函数关系式,计算合金α+β两相区的热变形激活能为429.9 kJ·mol-1,β单相区的热变形激活能为245.6 kJ·mol-1;建立了合金的高温变形本构方程,根据动态材料模型建立了合金的热加工图。通过对加工图进行分析可知,合金在895℃/0.5 s-1条件下发生动态再结晶,在动态再结晶区域内热变形后组织细小均匀。该合金的功率耗散效率的峰值区温度为860℃,ε=0.001 s-1。在此区间内合金发生动态回复与动态再结晶。在低温高应变速率区,合金发生失稳现象,其失稳形式为流变局域化。热处理工艺会显著影响Ti-6246合金的显微组织及力学性能,因此研究了固溶温度和冷却方式对合金的显微组织、相组成和室温拉伸变形行为的影响。在实验温度范围内,固溶处理后合金的相组成主要与冷却方式相关,在β单相区及(α+β)两相区固溶后水冷,β相均转化为α"马氏体和少量亚稳β相。空冷组织中β相转变为含有少量的次生α相的β转变组织,且随热处理温度的升高次生α相的含量增加,尺寸也逐渐增大。时效后组织中的亚稳相发生分解,生成细小的次生α相。固溶后水冷试样的拉伸曲线上出现"双屈服"现象,并且随固溶温度的升高合金第一屈服点逐渐升高。水冷和空冷试样经595℃/8h时效后,合金的强度提高,塑形降低。这种趋势在水冷试样中更明显。在两相区空冷,合金对热处理温度不太敏感,经595℃时效处理后合金室温拉伸性能可达到较好的强塑性匹配。Ti-6246电子束焊接接头由熔合区、热影响区和母材区三个区域组成,分析表明熔合区中由α相和β相组成,无α"马氏体和亚稳β相出现。焊接接头显微硬度呈不均匀分布,焊接接头各区域随距熔合区中心距离减少,显微硬度逐渐增加。焊后热处理后,熔合区和热影响区显微组织中析出细小的次生α相片层。随着焊后热处理温度由545℃增加至645℃,片层的厚度增大,数量逐渐增加。焊接接头熔合区和热影响区中的显微硬度值先升高再降低。焊接接头室温拉伸断裂均发生在母材区。焊接接头断口可观察到以韧窝为主的塑性断裂。焊后热处理会影响焊接接头熔合区、热影响区和母材区中α相的尺寸和形状,进而影响焊接接头的拉伸性能。焊后热处理条件下,接头拉伸强度的降低归因于母材中α相片层粗化导致的强度降低,接头拉伸塑性的改善归因于焊接接头各区域拉伸塑性的提高。Ti-624x合金焊接接头各区域的显微组织随Mo含量的变化而改变。Mo含量越低,Ti-624x合金焊缝熔合区中β柱状晶晶粒尺寸越大。熔合区柱状晶内部为全片层组织,且随着合金Mo含量的增加,柱状晶内部α相的尺寸逐渐降低。热影响区的组织由大量的等轴状β晶粒组成,且晶粒的尺寸随着与熔合区中心线距离的增大而减小。热影响区的显微组织为介于母材及熔合区之间的过渡组织,随着与熔合区中心线距离的减小逐渐由双态组织过渡到片层组织。焊接接头不同区域次生α相分布不同,导致焊态焊接接头的显微硬度呈不均匀分布。且随Mo含量的增加,焊接接头的硬度逐渐升高,其中Mo元素含量在4-5时硬度值的变化最大。随Mo含量的增加,熔合区的强度逐渐提高,室温塑性逐渐减低。
[Abstract]:Ti-6246 alloy is a high strength and high temperature titanium alloy, which is usually used in the medium temperature part of a gas turbine engine. It is used at a temperature of about 400 C for a long time, the short-term working temperature is up to 540 degrees C, and it has excellent mechanical properties in the range of temperature. This paper takes Ti-6246 alloy as the experimental material, and uses metallographic microscope, scanning electron microscope, transmission electron microscope, X ray. The influence of heat processing, heat treatment and welding on Microstructure and mechanical properties of alloy is studied by the diffraction instrument and other testing techniques, which provide the basis for its further application. The high temperature deformation behavior of Ti-6246 alloy and the deformation mechanism of.Ti-6246 alloy under different strain conditions are studied by using the theory of processing drawing. In the process of thermal compression, the peak value of the rheological stress is related to the temperature and the strain rate. The rheological stress decreases with the increase of the deformation temperature and increases with the increase of the strain rate. The thermal deformation activation energy of the alloy alpha + beta two phase region is calculated by using the Zener-Hollomon relation and the hyperbolic sinusoidal function formula. For 429.9 kJ. Mol-1, the activation energy of thermal deformation of a single phase region is 245.6 kJ. Mol-1, the constitutive equation of high temperature deformation of the alloy is established. A hot working diagram of the alloy is set up according to the dynamic material model. By analyzing the working diagram, the alloy is recrystallized at 895 C /0.5 S-1 and after the thermal deformation in the dynamic recrystallization region. The peak region temperature of the power dissipation efficiency of the alloy is 860 degrees C, and the alloy has dynamic recovery and dynamic recrystallization in this zone. In the low temperature high strain rate region, the alloy is unstable, and its instability form is the rheological localization. The microstructure and force of the Ti-6246 alloy will be significantly affected by the heat treatment technology. The effect of solid solution temperature and cooling mode on the microstructure, phase composition and tensile deformation behavior of the alloy is studied. In the experimental temperature range, the phase composition of the alloy is mainly related to the cooling mode in the experimental temperature range, and the phase of the beta phase is transformed into a "martensite and a small amount of submartensite" in the phase region of the beta single phase and (alpha + beta) two phase. The beta phase is stable in the air cooled tissue, and the beta phase changes to a small amount of secondary alpha phase, and the secondary alpha phase increases with the increase of the heat treatment temperature. The metastable phase in the tissue is decomposed to produce a small secondary alpha phase. The "double yield" phenomenon appears on the tensile curves of the water-cooled specimens after solid solution. With the increase of the solid solution temperature, the first yield point of the alloy increases gradually. The strength of the alloy is increased and the shape of the alloy decreases after 595 /8h aging. This trend is more obvious in the water cooled sample. The alloy is not very sensitive to the heat treatment temperature in the two phase air cooling, and the tensile properties of the alloy at room temperature after 595 C are better than that of the alloy. The strong plastic matching.Ti-6246 electron beam welding joint consists of three regions of fusion zone, heat affected zone and parent material area. The analysis shows that the fusion zone is composed of alpha and beta phase, without alpha martensite and metastable beta phase. The microhardness of welded joint is uneven distribution, and the distance of the welding joint is reduced with the distance of the distance fusion zone, and the microhardness is the same. After heat treatment, the fine secondary alpha phase layer is precipitated in the microstructure of the fusion zone and the heat affected zone. With the increase of heat treatment temperature from 545 to 645 C after welding, the thickness of the lamellar layer increases, and the number of the microhardness in the weld joint and the heat affected zone increases first and then decreases. The welding joint is stretched at room temperature. The crack occurs at the parent material area. The fracture of the welded joint can be observed as the plastic fracture mainly in the dimple. The post weld heat treatment will affect the size and shape of the alpha phase in the weld joint, the heat affected zone and the parent material area, and then influence the tensile properties of the welded joint. The reduction of the tensile strength of the joint is attributed to the alpha phase in the parent material after the welding heat treatment. The strength of the lamellar coarsening is reduced. The improvement of the tensile plasticity of the joint is attributed to the increase of the tensile plasticity of the welded joints in various regions. The microstructure of the.Ti-624x alloy welded joint varies with the content of Mo, the lower the content of.Mo, the larger the grain size of the beta columnar crystal in the fusion zone of the Ti-624x alloy welds. With the increase of the Mo content of the alloy, the size of the internal alpha phase in the columnar crystal gradually decreases. The microstructure of the heat affected zone consists of a large number of equiaxed beta grains, and the size of the grain decreases with the increase of the distance between the central line of the fusion zone and the transition tissue between the parent material and the fusion zone. The decrease of the distance from the center line of the fusion zone gradually from the double state to the lamellar tissue. The distribution of the secondary alpha phase in the welded joint is different, which leads to the uneven distribution of the microhardness of the welded joint. The hardness of the welded joint is gradually increased with the increase of the content of Mo, and the change of the hardness value of the Mo element is the largest when the content of the element is 4-5. With the increase of Mo content, the strength of the fusion zone gradually increases and the room temperature plasticity gradually decreases.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG456.3

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