Ti-6Al-4V合金超塑性变形中的组织演变及变形机制

920℃、应变速率为1×10.3和2×10.4 s.1时,对不同初始晶粒尺寸(2.6、6.5和16.2 μm)的Ti-6Al-4V合金进行超塑性拉伸变形.采用光学显微镜、透射电镜观察变形后的显微组织.结果表明,初始晶粒尺寸的不同对超塑性变形中的组织演变及变形机制有着显著的影响.拉伸变形中晶粒明显粗化,变形诱发晶粒长大是超塑性变形组织的重要特征之一;随着变形程度的增大,应变诱发的晶粒长大显著增大,并且远大于静态长大的增幅.对于细晶粒材料(2.6和6.5 μm),位错运动协调的界面滑动是其变形的主要机制.而对于晶粒较粗的材料(16.2 μm),超塑变形机制是晶界滑动与晶内位错运动的共同作用.随着晶粒尺寸的增大,以晶界滑动为主的变形方式逐渐转向以晶内位错运动为主.     

Postprocessing effect on the ductility and flexural behavior of three titanium alloys under simulated superplastic forming conditions

Ductility of three titanium alloys was evaluated after exposure to time and temperature conditions representative of superplastic forming (SPF). Following exposure, flexural specimens were postprocessed to remove the α-case by one of three methods: no material removed, the standard amount of material removed by chemical processing, or a reduced amount of material removed also by chemical processing. Results include the evaluation of the specimens per ASTM E 290-97a and AMS-T 9046, springback analysis, and prediction of minimum bend radius criteria for the three alloys from finite element method simulation. It was found that results varied based on alloy and exposure temperature and the reduced postprocessing of titanium SPF parts produced acceptable results under certain conditions. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming, sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.     

激光参数对Ti6Al4V钛合金激光冲击成形的影响

研究在Ti6Al4V合金激光冲击成形过程中,不同激光参数对板料弯曲角及表层硬度的影响。结果表明:当激光功率密度小于3GW/cm2时,弯曲角随着激光功率密度线性增加,激光功率密度超过3GW/cm2时,由于表面熔化现象的出现,弯曲角出现减小的趋势;板料弯曲角随冲击次数的增加也呈线性增长,但弯曲阻力的增加使得弯曲角的增长速度逐渐减慢;随着激光功率密度的增加,材料表面冲击区的硬度增高,表面硬化层的显微硬度最高达HV490,硬化层厚度约为1.0mm。     

不同冷却速度对Ti-6Al-4V钛合金热模拟试样组织的影响

采用Gleeble3500热模拟试验机对Ti-6Al-4V钛合金进行了焊接热模拟试验,采用金相显微镜、扫描电镜和X射线衍射仪分析研究了不同冷却速度对热模拟区域微观组织及显微硬度的影响.结果表明,冷却速度较低时,β晶界清晰完整,沿晶界向晶内形成片状α,随冷却速度的提高,α相减少,得到短小、纵横排列的针状α＇组织;XRD标...     

Modeling of grain size in isothermal compression of Ti-6Al-4V alloy using fuzzy neural network

Isothermal compression of Ti-6Al-4V alloy was conducted in the deformation temperature range of 1093-1303 K,the strain rates of 0.001,0.01,0.1,1.0,and 10.0 s-1,and the height reductions of 20%-60% with an interval of 10%.After compression,the effect of the processing parameters including deformation temperature,strain rate,and height reduction on the flow stress and the microstructure was investigated.The grain size of primary α phase was measured using an OLYMPUS PMG3 microscope with the quantitative metallography SISC IAS V8.0 image analysis software.A model of grain size in isothermal compression of Ti-6Al-4V alloy was developed using fuzzy neural net-work(FNN) with back-propagation(BP) learning algorithm.The maximum difference and the average difference between the predicted and the experimental grain sizes of primary α phase are 13.31% and 7.62% for the sampled data,and 16.48% and 6.97% for the non-sampled data,respectively.It can be concluded that the present model with high prediction precision can be used to predict the grain size in isothermal compression of Ti-6Al-4V alloy.     

TC4钛合金激光焊对接接头超塑变形显微组织

采用金相显微镜观察了钛合金激光对接接头超塑性变形前后各区域显微组织,并分析其形成机理.结果表明,变形温度的增加或应变速率的降低有利于TC4合金接头超塑变形,母材晶粒发生一定程度的长大,且α相的数量相对减小,而晶间β相数量逐渐增加,两相都有等轴化趋势;焊缝超塑性变形时,针状组织增厚成为片层状.变形过程中片层组织被打断,片层长度变短,具有球化的趋势;超塑性变形后焊缝截面显微硬度最大为380 HV,与变形前焊缝相比降低约50 HV,满足实际承载需求.     

Examination of superplastic forming combined with diffusion bonding for titanium: Perspective from experience

Superplastic forming (SPF) combined with diffusion bonding (DB) has been used successfully for the fabrication of titanium aerospace hardware. Many of these applications have been for military aircraft, whereby a complex built-up structure has been replaced with monolithic parts. Several methods for applying the two- and four-sheet titanium SPF/DB processes have been devised, including the welding of sheets prior to forming and the use of silk-screened stop-off (yttria) to prevent bonding where it is undesirable. Very little progress has been made in the past few years toward understanding and modeling the SPF/DB process using constitutive equations and data by laboratory testing. Concerns that engineers face in designing for fatigue life, acceptable design loads, and damage tolerance are currently being studied, but the database is very limited. This is a summary of past work found in the literature and forms the foundation for additional research. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.     

酸性电解液对Ti-6Al-4V合金微弧氧化的影响

分别以H3PO4、H2SO4和HF为电解液对Ti-6Al-4V合金进行微弧氧化实验研究。利用扫描电镜(SEM)和X射线衍射仪(XRD)研究不同条件下钛板表面氧化膜的形貌、组成以及组织结构。结果表明,以10%的H3PO4为电解液,微弧氧化的起始电压为185V,表面形貌为火山口状,孔口大小为400~800nm;以10%H2SO4为电解液,微弧氧化的起始电压为80V,表面形貌同样为火山口状,孔口大小为100~200nm,电压升高到120V,氧化膜的形貌变为网状多孔结构;以0.2%的HF为电解液,微弧氧化的起始电压亦为80V,表面高低不平、分布着大小为100~200nm的小浅坑;氧化膜的相组成均为金红石型和锐钛矿型TiO2共存。     

Ti-6Al-4V钛合金型材电热拉弯成形极限仿真方法

针对电加热拉弯成形工艺,建立了电热拉弯过程顺序耦合的数值模拟方法,实现了电热转台式拉弯成形过程的多工序、多场耦合数值模拟。并选用动力显式热-力耦合分析算法,基于J-C断裂准则,建立了电热拉弯成形极限与断裂预测的三维实体模型。通过Ti-6Al-4V钛合金挤压T型材电热拉弯成形试验与仿真预测结果比较发现:电热拉弯成形过程中,材料的失效主要发生在预拉伸和补拉伸阶段,失效原因为拉伸力过大;电热拉弯成形极限的主要影响因素包括加热温度或电流密度、预拉力、补拉伸过程温度或冷却时间和补拉力;预拉伸和补拉伸极限应力的预测相对误差分别为19.7%和19.1%,验证了模型的有效性。     

Superplastic bulging capability of Ti-6Al-4V buttcover plate

The investigation on the superplastic bugling capability of 1.5 mm mill annealed Ti-6A1-4V buttcover plate by means of manual gas tungsten arc welding (M-GTAW) and fusion type plasma arc welding (F-PAW) have been evaluated respectivdy. The result shows that untreated Ti-6A1-4V buttcover plate by M-GTAW exhibits no superplasticity,while the same untreated plate by F-PAW, shows good superplastic ability because of extremely fine acicular martensite microstructure of weld metal. To the buttcover plate by M-GTAW, the microstructure of weld metal changed into streaky α structure which exhibits good superplasticity from the original//structure under the condition of the constant temperature of 940℃ with the deformation degree of 45 %, and changed into the fine equiaxed grain which possesses excellent superplastic ahilitv under the condition of the constant temperature of 800℃ with the deformation degree of 40 %.     

The effect of alloy grain size on the transient oxidation behavior of an alumina-forming alloy

In the early stages of alloy oxidation, diffusion of solute through the metal to the surface is important in determining the composition of the oxide scale that forms during the transient stage. Rapid solute diffusion to the interface will promote the formation of a protective scale, thereby suppressing the formation of base-metal oxide. The effect of alloy grain size on the formation of the transient oxide scale has been studied using a very fine grained NiCrAlY alloy produced by plasma spraying. The long-term oxidation behavior of this alloy was found to be independent of the grain size of the underlying alloy. However, the short-term, transient oxidation rate was found to decrease with decreasing alloy grain size. This is attributed to the rapid grain boundary transport of Al and Cr to the oxide/metal interface which promoted the formation of Cr₂O₃ and Al₂O₃.     

Ti-6Al-4V钛合金的激光弯曲成形特性

以Ti-6Al-4V薄板材料为研究对象,研究了激光弯曲成形技术在钛合金上的应用特征.通过实验描述了不同扫描参数对弯曲角的影响规律,并对多次扫描下的板材弯曲动态特性进行了研究.结果表明:弯曲角随扫描次数呈近似的线性增加关系;在扫描次数一定的情况下,存在一个最优的累积线能量使弯曲角达到最大;不同的材料状态在加热和冷却过程呈现出不同的弯曲动态特性.此外,对激光弯曲后的材料微观组织、力学性能进行了测量和分析.结果表明:扫描参数的选择对发生相变的区域形状与相变区组织有较大影响;多次扫描后,加热区材料的强度与硬度均增加,塑性降低.     

Heat-treated microstructure and mechanical properties of laser solid forming Ti-6Al-4V alloy

The effects of heat treatment on the microstructure and mechanical properties of laser solid forming (LSF) Ti-6Al-4V alloy were investigated. The influences of the temperature and time of solution treatment and aging treatment were analyzed. The results show that the microstructure of LSFed samples consists of Widmanstätten α laths and a little acicular in columnar prior β grains with an average grain width of 300 µm, which grow epitaxially from the substrate along the deposition direction (Z). Solution treatment had an important effect on the width, aspect ratio, and volume fraction of primary and secondary α laths, and aging treatment mainly affects the aspect ratio and volume fraction of primary α laths and the width and volume fraction of secondary α laths. Globular α phase was first observed in LSFed samples when the samples were heat treated with solution treatment (950°C, 8 h/air cooling (AC)) or with solution treatment (950°C, 1 h/AC) and aging treatment (550°C, above 8 h/AC), respectively. The coarsening and globularization mechanisms of α phase in LSFed Ti-6Al-4V alloy during heat treatment were presented. To obtain good integrated mechanical properties for LSFed Ti-6Al-4V alloys, an optimized heat treatment regimen was suggested.     

Ti-6Al-4V合金高温氧化行为及显微组织研究

分析了Ti-6Al-4V合金在900 _oC,930 _oC,960 _oC温度下的氧化行为及表层显微组织变化。在0.5-24 h时间内,氧化层不断增厚,越靠近表面氧化层越疏松。氧含量在氧化层/富氧层界面的5 μm内发生急剧下降,进入富氧层后缓慢下降直到稳定。氧化层中以TiO₂为主,同时也出现了Al₂O₃。富氧层中的a相含量远远高于基体内部,其晶粒尺寸也发生长大。富氧层深度与热暴露时间的关系可用对数函数描述,通过线性回归分析计算出了O在富氧层中的扩散激活能为206 kJ/mol。     

不同取向对激光快速成形Ti-6Al-4V合金力学性能的影响

采用万能材料试验机系统,对激光快速成形TC4钛合金不同取向圆柱形试样进行准静态压缩试验,使用光学显微镜、扫描电镜、XRD等分析手段,研究不同取向对其力学性能的影响。结果表明:不同取向激光快速成形TC4钛合金由于微观结构的差异,其准静态压缩力学性能也不同,其中,与激光扫描方向成45°角试样显示最高的强度,但塑性最差;而沿沉积方向试样则显示最好的塑性性能,其强度与沿激光扫描方向试样的强度相当。     

High-temperature corrosion of Ti and Ti-6Al-4V alloy

Pure titanium and Ti-6Al-4V were exposed at 750°C in an H₂/H₂O/H₂S P_{O 2}10^–18 Pa and P_{S 2}10^–1 Pa), H₂/H₂O (P_{O 2}10^–18 Pa) and air environments for up to 240 hr. The corrosion kinetics, obtained by the discontinuous gravimetric method, showed that the sulfidation/oxidation kinetics were linear for Ti and linear-parabolic for Ti-6Al-4V in the H₂/H₂O/H₂S environment. Both materials obeyed parabolic rate laws in the H₂/H₂O atmosphere after a transient period, and linear-parabolic rate laws in air. After exposure to the H₂/H₂O/H₂S atmosphere, the titanium specimen displayed a double scale of TiO₂ with an intervening TiS₂ film between the double-layered scale of TiO₂ and the substrate. Ti-6Al-4V also contained a double layer of TiO₂ together with a stratum consisting of Al₂S₃, TiS₂ and vanadium sulfide at the junction of the inner TiO₂ layer and substrate. Some Al₂O₃ precipitated in the external portion of the outer TiO₂ layer. Following oxidation in the low-P_{O 2} atmosphere a double-layered oxide of TiO₂ scale formed on both Ti and Ti-6Al-4V. The scale on Ti-6Al-4V also contained an -Al₂O₃ film situated between the outer and inner (TiO₂) layers. For both materials, multilayered-scale formation characterized air oxidation. In detail a multilayered oxide scale of TiO₂ formed on the air-oxidized Ti, while a multilayered oxide scale with alternating layers of Al₂O₃/TiO₂ developed on Ti-6Al-4V oxidized in air.     

异步热轧对钛合金Ti-6Al-4V微观组织及力学性能的影响

通过异步/同步热轧实验研究了异步热轧工艺对钛合金显微组织和力学性能的影响。实验表征了试样的显微组织、力学性能、断口形貌和微观取向。实验结果表明,复杂应变路径较之简单应变路径能更好的细化晶粒及同时提高强度和塑性,并且表层晶粒小于中心晶粒。异步轧制工艺相比同步轧制能更好获得细小晶粒。异步轧制试样的强度及塑性值高于同步轧制试样相应值,提高异步速比可提高强度及塑性值。异步轧制试样的塑性变形机制可能是滑移,而同步轧制试样塑性变形机制为滑移或孪晶。     

氧化时间对Ti-6Al-4V微弧氧化膜结构与性能的影响

在Na3PO4电解液中于恒流条件下对Ti-6Al-4V合金进行了微弧氧化处理,考察了氧化时间对微弧氧化膜表面形貌、生长速率、表面硬度和相组成的影响,并对不同氧化时间所得微弧氧化膜的摩擦学性能和耐蚀性能进行了表征.研究结果表明:随着氧化时间的延长,氧化膜表面微孔孔径逐渐增大,微孔数量逐渐减少,氧化膜生长速率逐渐减小,氧化膜的厚度逐渐增加,平均表面硬度呈先增大后减小的趋势;微弧氧化膜主要由金红石TiO2和锐钛矿TiO2相组成,随着氧化时间的延长,金红石相所占比例逐渐增大;氧化时间的长短对微弧氧化膜的摩擦系数影响不大,但对其磨损寿命有着显著影响,延长氧化时间可大大提高微弧氧化膜的磨损寿命,且随着氧化时间的延长,所得微弧氧化膜的腐蚀电位逐渐增大,腐蚀电流先减小后增大.     

Multi-rate Superplastic Forming of Fine Grain Ti-6Al-4V Titanium Alloy

Most parts made by superplastic forming (SPF) have been formed at an optimum strain rate. The rate is selected to give the best SPF properties of the material. However, it has been proposed that multi-rate forming, where an initial high strain rate is successively reduced as the part is strained, can be used to make high strain parts in a much shorter time than traditional SPF forming. This paper examines the performance of fine grain Ti-6Al-4V alloy at very high initial strain rates, from 10–30 times faster than usual, with step reductions at prescribed levels of strain that still enables a total strain of over 2.1 (800%) to be achieved without degradation of the material. The paper also shows that the forming time to 100% deformation can be reduced from 55 min to 9 min. This technique can be used by industry to enable faster flow times and lower production costs of SPF parts. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Seattle, WA, June 6–9, 2005.     

Manufacturing advantages of superplastically formed fine-grain Ti-6Al-4V alloy

Superplastic forming (SPF) of Ti-6Al-4V has traditionally been performed at 900 °C. Although SPF equipment and dies have been developed to withstand this high temperature, their life is limited and maintenance is high. In addition, the formed sheets need chemical milling after processing to remove a significant layer of alpha case, and parts require hand polishing due to a rough die surface caused by high temperature oxidation. The recent development of a Ti-6Al-4V alloy with a grain size of around 1 μm has led to the possibility of superplastic forming at a much lower temperature than regular Ti-6Al-4V. In addition, the forming speed can be increased. This work looks at some of the SPF attributes of fine-grain titanium, in particular, the optimum forming temperature, the thinning characteristics, alpha case formation, and forming speeds. The authors also address manufacturing advantages, such as die life, equipment needs, and operator comfort. Some prototype and preproduction aircraft components on Boeing airplanes are presented. The results show that the new fine-grain material could significantly lower the cost of SPF Ti-6Al-4V part production for the aerospace industry. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming, sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.