Evolution mechanism of lamellar α and interlayered β during hot compression of TC21 titanium alloy with a widmanstätten structure

TC21 titanium alloy, as an important metal to fabricate the aircraft structural components, has attracted great attentions recently. A TC21 titanium alloy with widmanstätten structure was isothermally compressed. Based on the microstructure observation, the evolution of initial β grain, Grain Boundary α phase (α_GB), lamellar α and interlayered β was systematically investigated. The results showed that, with the increasing of height reduction, the α_GB underwent an evolution process from bending/kinking to breaking inducing the corresponding blurring of initial coarse β grain outline. Meanwhile, a significant phase transformation from α to β took place at the terminations of broken α_GB. The evolution of lamellar α and interlayered β in the colony was closely related to their deformation compatibility. In the α colony, the interlayered β experienced a larger deformation amount than lamellar α. The higher distortion energy promoted the occurrence of Dynamic Recovery (DRV) and Dynamic Recrystallization (DRX) to generate many Low Angle Boundaries (LABs) and High Angle Boundaries (HABs) in interlayered β, which induced an apparent grain refinement of β phase. On the contrary, the lower distortion energy and low deformation temperature suppressed the occurrence of DRV/DRX and restrained the globularization of lamellar α. Furthermore, the microstructure observation clearly revealed that the shearing separation mechanism dominated the evolution of the α phase from lamellar to short bar-like morphology.     

Substructure evolution in two phases based constitutive model for hot deformation of TC18 in α + β phase region

The α + β dual phase titanium alloys are key structural materials in aviation and aerospace industries, and the complicated flow behavior of these titanium alloys during hot deformation requires to establish a constitutive model incorporating physical mechanism for optimizing processing parameters and designing forming tools. This work aims to establish a constitutive model incorporating physical mechanism for hot deformation of TC18 in α + β phase region. Firstly, the flow behavior and microstructure evolution for hot deformation of TC18 in α + β phase region are characterized. The TC18 shows significant strain hardening rate and negative strain hardening exponent around and after peak flow stress, respectively. After peak flow stress, Dynamic Recovery (DRV) mechanism dominates the evolution of α and β phases according to the results of substructure evolution. Then, the internal state variables method is applied to establish a constitutive model incorporating physical mechanism for hot deformation of dual phase titanium alloys. The variation of dislocation density during the hot deformation of titanium alloys is modeled by considering the accumulation of dislocation due to the impediment to dislocation movement by substructure obstacles and the annihilation of dislocation due to the dynamic restoration effect. The interaction between dislocations, the subgrain boundaries and the grain/phase boundaries obstruct the dislocation movement in the α phase, and the first two obstructs the dislocation movement in the β phase during the hot deformation of TC18. The dislocation annihilation process in the α and β phases during the hot deformation of TC18 is dominated by DRV. Finally, the substructure evolution in the two phases based constitutive model for hot deformation of TC18 in α + β phase region is presented. This model is well applied to predict the flow stress and quantitively analyze the role of DRV effect in the evolution of α and β phases during the hot deformation of TC18.     

Ti-22Al-25Nb合金剪切旋压过程中显微组织演化

通过剪切旋压试验,研究了旋压温度(910℃和1000℃)和减薄率(0、10%和30%)对Ti-22Al-25Nb合金显微组织的影响,探讨了旋压过程中的显微组织演化规律。结果表明:Ti-22Al-25Nb(原子数分数)合金旋压组织主要由α2相、B2相和O相组成,在旋压过程中,B2晶粒沿旋轮进给方向被拉长,且伴随有动态再结晶现象发生;温度主要影响Ti-22Al-25Nb合金中α2相与O相的尺寸和形貌,随着旋压温度的升高,O相片层逐渐变短并粗化,α2相趋向等轴化;变形量主要影响α2相体积分数与O相形貌,随着减薄率的增大,α2相体积分数逐渐减少,O相从片层状转变为短棒状。因此,Ti-22Al-25Nb合金剪切旋压过程中不仅发生晶粒变形与动态再结晶现象,更涉及复杂的相变行为,组织控制困难。     

Ti6242合金微观组织差异及其演变对拉伸和疲劳性能影响

典型近α型钛合金Ti6242合金制备盘锻件的综合力学性能与其棒材原始微观组织特征密切相关。本研究选取具有明显微观组织特征差别的3种Ti6242钛合金棒材为研究对象,定量化评估不同Ti6242钛合金棒材锻造态、固溶态和固溶+时效态的微观组织差别和演变规律,并讨论棒材微观组织差别和演变对综合力学性能的影响。研究结果显示具有明显差别的不同棒材微观组织在经历相同的固溶和时效处理后,其初生α相体积分数差别趋于相近,而初生α相的尺寸分布和形状分布等微观组织特征仍保留差别;力学性能结果显示室温拉伸力学性能趋于近似,而低周疲劳和保载疲劳力学性能对微观组织特征较为敏感。对比不同特征组织和力学性能分析结果,具有细小非等轴状且没有明显取向集中分布初生α相的棒材原始组织有助于获得更好的疲劳性能。     

Microstructure control techniques in primary hot working of titanium alloy bars:A review

How to control the microstructure of titanium alloy bars is important to fabricating high-performance aerial forgings. This paper gives a thorough survey of the manufacturing meth-ods and microstructure control techniques for titanium alloy bars. It summarizes the effects of pro-cessing parameters on the mechanisms and laws of microstructure evolution during b working and (a+b) working, including the kinetics and grains size of dynamic recrystallization (DRX) during b deformation and the kinetics and grains size of spheroidization during (a+b) deformation. The trends in microstructure control techniques are presented for fabricating titanium alloy bars with high efficiency, low cost, and high quality by means of b/(a+b) working, and the puzzles and chal-lenges in the future are also pointed out.     

非耦合韧性断裂准则及其在航空金属材料中的应用

结构轻量化是航空航天和汽车领域的重要发展趋势,对以铝合金、镁合金和钛合金为代表的轻质高强金属材料的需求与日俱增。预测材料的损伤断裂行为是高性能航空构件成形工艺设计和服役性能评估的关键,而发展先进的韧性断裂准则是其主要途径。本文首先介绍了金属材料损伤断裂的微观机制,包括剪切和压缩应力主导的剪切型断裂、拉应力主导的拉伸型断裂及复合型断裂。回顾了韧性断裂准则的研究现状,传统非耦合韧性断裂准则的发展历程、特点和适用场合,重点论述了近年来几种典型的非耦合韧性断裂准则的特点和优势。传统的非耦合韧性断裂准则通常只考虑最大主应力或平均应力对损伤断裂的影响,忽略了偏应力的作用,不适合于低应力三轴度或复杂应力状态下的断裂行为预测;而新的韧性断裂准则综合考虑应力三轴度和罗德角参数对损伤演化的共同影响,适用于复杂的应变路径和应力状态。最后,评述了非耦合韧性断裂准则在铝合金、镁合金和钛合金等航空金属材料中的发展现状和典型应用,展望了韧性断裂准则的发展趋势和研究方向。非耦合韧性断裂准则需针对先进结构金属材料的变形特点,综合考虑应力状态、应变速率、温度及各向异性等对损伤断裂的作用,使其具有更好的普适性和预测精度。     

制备工艺对NiAl-30Fe-Y合金组织与性能的影响

研究了铸态、挤压态、热轧态和定向凝固结晶４种不同制备工艺对ＮｉＡｌ－３０Ｆｅ－Ｙ合金组织与性能的影响。结果表明,热轧和定向凝固结晶工艺明显改善了合金性能,特别是提高了合金室温塑性。在室温下不同制备工艺其拉伸断口特征明显不同。不同制备工艺仅对微观组织形态产生影响,其合金组织皆是由先共晶Ｂ２结构β和β与（γ＋γ'）共晶相组成,定向凝固合金成长方向为＜２１１＞β‖＜１００＞（γ＋γ'）。     

粉末冶金Ti-6Al-4V 的组织及形成机理

利用热等静压(HIP)技术及旋转电极粉,采用预合金粉工艺制备了全致密的粉末冶金Ti-6Al-4V合金,用光学显微镜对其金相组织进行了观察,分析其形成机理,并对粉末冶金Ti-6Al-4V合金的室温拉伸性能、弹性模量、冲击性能及断裂韧性进行了研究.结果表明:粉末冶金Ti-6Al-4V金相组织主要由条片α相+相间β相组成,有细小的等轴α相分布在晶粒界面处,包裹着条片α相,这种独特的组织状态是由制备工艺决定的,具有良好的综合性能.     

新型高温钛合金成形工艺探索

通过分析合金的材料特点、材料对塑性成形工艺条件的要求,研究了Ti750合金的热压、热旋压、超塑、焊接工艺性能。结果表明:该合金的热旋压成形工艺性较差;热压与超塑成形工艺性较好;合金具有较好的焊接性能。     

Experimental study of effect of post processing on fracture toughness and fatigue crack growth performance of selective laser melting Ti-6Al-4V

For Ti-6Al-4V, a titanium alloy increasingly used in aerospace structure, selective laser melting (SLM) is an attractive additive manufacturing technology, which is attributed to its complex construction capability with high accuracy and good surface quality. In order to obtain qualified mechanical properties, SLM parameters and post processing should be tailored for diverse service conditions. Fracture toughness and fatigue crack growth (FCG) behavior are critical characteristics for damage tolerance evaluation of such metallic structures, and they are affected by post processing technologies significantly. The objective of this study is to obtain the fracture toughness and fatigue crack growth behavior of Ti-6Al-4V manufactured by SLM, and to evaluate the influence of post-SLM thermomechanical treatment and surface machining. Fracture toughness and FCG tests were performed for SLM Ti-6Al-4V in three types of post processing status: as-built, heat treated and hot isostatically pressed (HIPed), respectively. Specimens with as-built and machined surface were tested. The microstructure and fractography were analyzed as well in order to investigate the relevance among manufacture process, microstructure and mechanical properties. The results demonstrate that as-built SLM Ti-6Al-4V presents poor ductility and FCG behavior due to martensitic microstructure and residual stresses. Both heat treatment and hot isostatic pressing improve the plane-stress fracture toughness and FCG performance considerably, while surface machining shows slight effect.     

B93铝合金热加工特性模拟研究

在Gleeble1500热模拟机上采用轴对称等温压缩实验,研究了B93铝合金在热加工中变形温度和应变速率对合金组织的影响,基于动态模型计算出B93铝合金加工图,分析合金热加工的流变失稳区并确定了合金热加工的最佳工艺参数。结果表明:变形速率和变形温度对B93铝合金的微观组织有影响;合金的峰值应力随变形温度的升高而降低,随变形速率的提高而增大;B93铝合金最佳的热加工制度为:变形温度420℃左右,应变速率0.001 s-1。     

TC4合金的新型本构关系

通过热态模拟试验 ,系统地研究了主要锻造热力参数 (变形速度、变形温度、变形程度 )与 TC4合金流动应力、微观组织之间的数值关系 ,采用数理统计原理 ,科学地分析并回归出了能综合反映锻造热力参数和微观组织演化过程对材料成形性能影响的本构方程 ,定性地探讨了各热力参数对 TC4合金组织和性能的影响规律 ,为TC4合金锻造过程的数值模拟和锻造热力参数的合理制订与控制提供了基础。     

工艺参数对Ti75 合金筒形件旋压成形的影响

建立了Ti75合金筒形件旋压三维有限元模型,分析了旋压过程中应力应变的分布规律.得到了进给速率、减薄率和旋轮工作角三个关键工艺参数对旋压过程的影响规律:随着旋轮工作角、进给速率和减薄率的增加,旋压力和隆起高度均增大;等效塑性应变随着减薄率的增加而增大,随进给速率的增大而减少,进给速率超过1 mm/s时,等效塑性应变基本保持不变;工作角小于20°范围内变化时,最大等效塑性应变几乎不变,当旋轮工作角超过25°,最大等效塑性应变迅速增大.     

挤压态喷射成形GH738合金热变形行为及组织研究

在温度950~1150℃、应变速率0.001~1 s–1及工程应变50%条件下,利用Gleeble-3500TM热模拟试验机对挤压态喷射成形GH738合金进行热压缩实验,研究合金的流变应力,建立合金热变形本构关系,利用EBSD分析合金组织演变。结果表明:合金流变应力随温度的升高和应变速率的减小而降低,在相同变形条件下,具有细晶组织特征的挤压态喷射成形GH738合金峰值流变应力低于粗晶组织的铸锻GH738合金;挤压态喷射成形GH738合金热变形激活能为651.08 kJ·mol–1,GH738合金的热变形激活能随着初始平均晶粒尺寸的减小而升高;形变温度的升高使挤压态喷射成形GH738合金初始被拉长的晶粒逐渐演变为等轴再结晶晶粒,在1000℃以上获得完全动态再结晶组织,再结晶组织随形变温度的进一步升高发生长大。     

AZ31B镁合金双道次热变形过程流变应力特征模拟研究

建立了一类镁合金双道次热变形过程的二维元胞自动机模型。考虑到镁合金具有六重对称的密排六方晶体结构,模型采用了六边形元胞网格。另外,通过计算位错密度演变和设定晶粒形核长大规则,模型将热变形过程中动态再结晶、静态回复、静态再结晶、亚动态再结晶和晶粒长大等单个物理过程耦合到一起。为了验证模型,将其应用到AZ31B镁合金单道次和双道次热压缩过程。探讨了不同变形温度、应变速率、道次间隔和预应变对应力-应变曲线的影响。将计算结果和实验结果进行了对比,吻合较好。     

Hot deformation behavior and microstructure evolution of the laser solid formed TC4 titanium alloy

Hot compressive experiments of the laser solid formed (LSFed) TC4 titanium alloy were conducted at a wide temperature range of 650–950 °C and strain rate of 0.01–10 s⁻¹. The Arrhenius-type constitutive models of the LSFed TC4 alloy were established at the temperature range of 800–950 °C and of 650–800 °C, respectively. The average relative error between the predicted stresses and experimental values in those two temperature ranges are 10.4% and 8.3%, respectively, indicating that the prediction models constructed in this paper are in a good agreement with experimental data. Processing maps were established by the principle of dynamic materials modeling on the basis of the data achieved from the hot compression experiments. The processing parameters corresponding to the stable and unstable regions of material deformation can be determined from the processing maps. The microstructure evolution of the stable and unstable regions of the samples after tests were observed. Finally, the effect of hot compressive parameters on the microstructure were investigated to research the dynamic recrystallization and the texture of the deformed LSFed TC4 alloy.     

热等静压对激光选区熔化成形GH4169合金持久性能的影响

采用激光选区熔化(SLM)成形技术制备了GH4169合金,运用金相显微镜(OM)、扫描电子显微镜(SEM)、电子背散射衍射(EBSD)和透射电镜(TEM)等分析手段分析了热等静压对SLM成形GH4169合金组织及持久性能的影响规律。结果表明:沉积态合金组织中,沿沉积方向的晶粒为柱状晶,晶粒内枝晶组织细小,枝晶间分布大量Laves相;热等静压处理可有效消除组织中气孔缺陷,溶解大部分Laves相,促使组织晶粒等轴化,并减少后续980℃固溶处理过程中δ相在晶内的析出;热等静压处理能提高合金的持久寿命,但会降低合金的持久塑性。     

激光熔化沉积TA15+Ti_2AlNb合金的组织与力学性能

将TA15和Ti_2AlNb合金粉末以不同比例预混,采用激光熔化沉积工艺,制备出40%TA15+60%Ti_2AlNb,50%TA15+50%Ti_2AlNb,60%TA15+40%Ti_2AlNb(质量分数)3种比例的合金薄壁,分析了薄壁的成分、组织及力学性能。结果表明:3种沉积薄壁的成分分布均匀,合金由α相、α2相及β/B2相组成,针状α相和α2相呈网篮状分布在初生β/B2晶粒内部;3种合金的抗拉强度分别为1108 MPa,1071 MPa,1105 MPa,断裂伸长率分别为3.0%,2.2%,3.8%;拉伸断口可见沿α2相撕裂产生的撕裂棱,断裂方式均为准解理断裂。     

基于显微组织演变的涡轮叶片损伤分析

涡轮叶片作为航空发动机和燃气轮机重要的热端部件,在复杂温度场、应力场及氧化腐蚀等环境下工作,面临多种损伤失效风险。为了阐明涡轮叶片涂层损伤模式,总结了现阶段涡轮叶片涂层工艺、特点及其显微组织构成。在结合叶片材料热力耦合试验中相的演变规律研究成果基础上,对服役不同时间和类型的涡轮叶片基体和涂层系统的显微组织进行分析,并与原始组织对比;确定了各种服役组织损伤形式,主要包括涂层系统退化、原始缺陷导致的裂纹扩展、过热损伤及γ′相的退化等;初步给出了涡轮叶片损伤机理和服役环境评估,提出后期涡轮叶片工程化应开展的研究工作和注意事项,从而实现由服役叶片失效后分析向使用前预防的转变,完善涡轮叶片正向设计体系。     

TC4合金等温成形过程模拟与组织预测

在试验研究的基础上，建立了能够反映锻造热力参数对材料成形性能影响的新型本构方程，并采用变形与传热耦合分析的刚塑性有限元数值模拟方法，全面系统地研究了TC4合金的等温成形过程，基于建立的该合金动态再结晶组织的演化模型，定量地预测了等温成形过程中TC4合金微观组织的演变情况，并分析讨论了工艺参数合金微观组织演变过程的影响，从而为TC4合金热成形工艺参数的优化设计和控制提供了基础。