Fracture toughness of superplastic formed/diffusion bonded interfaces

The present paper is concerned with the interfacial fracture problem of bi-material interfaces of superplastic forming/diffusion bonding (SPF/DB) components. It has been known that the interfacial fracture toughness is a strong function of the mode mixture of crack tip, which is the most important characteristic of interfacial problems. Micromechanical models have been proposed to understand this trend of interfacial toughness curves, based on which a modified criterion for fracture under mixed-mode loading is advanced in this paper to predict phenomenological functional forms of toughness curves. On the ground of linear elastic fracture mechanics (LEFM), mode I and mode II interfacial fracture toughnesses of SPF/DB components formed under different processing conditions have been measured in this paper using the double cantilever beam (DCB) and end-notched flexure (ENF) specimens, respectively. It is found that, just as is the general trend of interfacial fracture toughness curves, the experimental results of mode II toughness values are much greater than those of mode I toughness values, and the fluctuation of mode II toughness values under different processing conditions is much greater than that of mode I toughness values. On the basis of the experimental results, the four basic parameters in the modified criterion are obtained and the interfacial toughness curve of Ti-50A/Ti-6Al-4V SPF/DB components is determined using the modified criterion for fracture under mixed-mode loading. Comparison between the results shows that the agreement between experiment and theory is excellent.     

镍基合金超塑成形与扩散连接技术研究进展

本文对镍基合金超塑成形与扩散连接技术的研究概况及取得的研究成果进行了综述 ,并根据镍基合金在航空航天中的应用情况 ,对镍基合金超塑成形 /扩散连接 (SPF/DB)技术在航空航天工业中的应用进行了探讨与展望。     

Innovations in the Superplastic Forming and Diffusion Bonded Process

The Superplastically Formed and Diffusion Bonded (SPF/DB) titanium structure in production today for Boeing products, not including engines, are all diffusion bonded using matched metal tooling and are all fabricated using the common 6Al-4V alloy. The matched metal tooling concept presents a challenge in obtaining high-quality bonds over large areas due to tolerance build-up in the tools and the titanium sheets. Boeing Commercial Airplanes (BCA) is currently advancing the state of the SPF/DB process in several ways. One of these advances is using stop-off between the sheets and diffusion bonding the pack first and then superplastically forming the stiffening features. This generates a component that is very well bonded in the required locations. However, this process also has its challenges. One of these involves how to apply the stop-off material in the proper location using the most cost effective process. Historically, silk screening has been used to define the required pattern for the stop-off material. This process requires several pieces of equipment including a wash booth since the screen needs to be cleaned after each part. A paper maskant and laser scribing process has been developed for defining the stop-off pattern. Also, because diffusion bonding is performed first, when the component is superplastically formed, there is a tendency to form creases on the surface of the part. Methods have been developed to eliminate these surface creases on the unformed surface. Another advance in the SPF/DB process is in the titanium alloys being used for products. A fine grain 6Al-4V material has been developed that bonds and forms at 775 °C. The use of this material will minimize wear on the tools and presses as well as significantly reducing the amount of alpha case on the part surface. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Baltimore, MD, June 25-28, 2007.     

Developments in Design and Materials for Cast SPF Tooling

Large titanium and aluminum sheet parts can now be formed into very complex assemblies by the process of superplastic forming. This has resulted in the need for major work to develop new alloys, new design methods and new manufacturing routes to ensure the production of high quality tooling. SPF tooling represents a significant part of the total cost of producing the super-plastically formed part. Careful selection of the material and its manufacturing route is essential in order to obtain the “right quality at the least cost” commensurate with the conditions of use. For many years the optimization of the alloy composition, the melting and casting foundry techniques and the design of the tools was done in a semi-empirical way, being largely based on practical experience. This method meant that the development was slow and unwieldy, and therefore, not compatible with the rapidly changing constraints of the SPF industry today. The fact that extremely powerful modelling programmes such as ThermoCalc and DICTRA are available for the design of new alloys, or Thercast for the optimization of the foundry melting and processing, makes it possible to introduce innovative techniques faster and with more immediate certitude as to their success. Thus, the casting process has become the most competitive and appropriate method of manufacture of the high performance tools in use today. This paper will discuss how these current technologies have been developed, and coupled with the experiences in the foundry, have assisted in the production of new materials, that optimize the tooling required in SPF operations. This article was presented at the AeroMat Conference, International Symposium on Superplasticity and Superplastic Forming (SPF) held in Seattle, WA, June 6-9, 2005.     

钛/铜中间层/钢扩散焊复合管界面组织与性能

以铜箔为中间层,采用拉拔—内压扩散法制备钛/钢复合管.利用光学显微镜、扫描电子显微镜、X-光衍射仪和能谱仪对界面组织、断口形貌和成分进行分析,通过剪切试验测定界面的结合强度.结果表明,以铜箔作中间层,拉拔—内压扩散法实现了钛/钢的冶金结合;在钛/铜界面处发生了明显的原子扩散,并形成不同的扩散层;随着扩散温度和时间的增加扩散层的厚度逐渐增加;中间层的加入阻止了固相扩散中钛铁、钛碳脆性化合物生成;钛/钢界面的抗剪强度随着扩散温度的升高先增加后降低,铜层的加入使抗剪强度明显提高,最高可达310 MPa.     

电阻法用于扩散焊接头微孔缺陷评价的数值模拟

从扩散焊接头界面微孔实际形貌和分布特征出发,提出了界面微孔周期性分布的二维有限元模型,应用ANSYS电磁场分析模块,考察了界面焊合率、界面微孔宽度和长度对电阻增量的影响.结果表明,扩散连接接头的界面焊合率与电阻增量之间呈双曲线变化规律,界面微孔的宽度和长度对电阻增量均有影响.当界面焊合率在20%～80%范围内时,电阻增量与界面焊合率之间表现出更高的敏感性.基于Lodge等人提出的扩散焊界面焊合率与电阻增量关系式,建立了能够包含界面微孔几何尺寸影响的修正方法.     

Ti-17钛合金扩散连接界面特征及接头剪切强度(英文)

研究不同连接时间下Ti-17钛合金扩散连接界面特征及接头剪切强度。结果表明,随着连接时间的延长,连接界面平均空洞尺寸逐渐减小,空洞数量增加至最大值后逐渐减少。具有锯齿状边缘的不规则空洞逐渐转变为具有平滑表面的椭圆形空洞以及细小的圆形空洞。当连接时间为60 min时连接界面上形成了跨连接界面的晶粒。接头剪切强度随着连接时间的延长而增大,当连接时间为60 min时,接头剪切强度达到最大值,为887.4 MPa。尽管塑性变形的作用时间很短,但是对促进空洞闭合以及提高接头剪切强度的作用显著。     

TC21钛合金扩散连接接头的微观组织及界面质量

在780～980℃的温度下对TC21钛合金进行5～90min的扩散连接,研究接头的界面结合率、变形率、微观组织及微观硬度。结果表明,在连接温度为880℃、连接时间为15～30min的情况下,可以获得高质量的连接接头。接头的微观硬度随着连接温度的升高而增加,并在连接时间达到90min时取得峰值（HV367）。在连接温度为780~880℃、930℃和980℃时,分别观察到了全等轴组织、双态组织和全层片组织。α相的体积分数随着温度的升高和时间的延长而增加,并在880℃、60min时取得极值。     

钛-钢扩散复合界面组织与结合强度

将钛管、钢管利用冷拔-内压扩散法制备了内包覆钛-钢复管。用扫描电镜、能谱分析、X-光衍射和拉剪试验等方法，研究了扩散退火温度与时间对钛-钢扩散复合界面附近组织、成分和界面剪切强度的影响。结果表明，该制备方法可使钛～钢实现冶金结合；界面剪切强度随扩散温度升高先增加后减小；750—800℃×0．5h扩散退火界面剪切强度最高，可达210MPa左右；扩散退火中Fe、Ti原子发生了互扩散；界面上有TiC形成；750℃×0．5h扩散退火试样断VI未检测到TiFe、TiFe2相；900—950℃×0．5h扩散退火钢侧出现柱状晶区，钛侧出现无晶界晶区与针状马氏体晶区。     

The Effects of Stress State and Cavitation on Deformation Stability During Superplastic Forming

The current available models describing superplastic deformation do not account for a number of important characteristics, leading to the current limited predictive capabilities of deformation and failure. In this work, the effects of cavitation and stress state on deformation stability during superplastic forming are investigated using Finite Element simulations. The simulations are performed using constant strain rate forming and using a proposed optimization approach based on a multiscale failure criterion that accounts for stress state, geometrical necking, and microstructural evolution including grain growth and cavitation. The simulations are conducted for the superplastic copper-based alloy Coronze-638 and the superplastic aluminum alloy Al-5083 which are known to develop significant cavitation during deformation. The results clearly show the importance of accounting for microstructural evolution during superplastic forming, especially when the state of stress is biaxial. Furthermore, the results highlight the effectiveness of the proposed optimization technique in reducing the forming time and maintaining the integrity of the formed 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.     

Microstructure and strength of TiAl/40Cr joint diffusion bonded with vanadium-copper filler metal

1　INTRODUCTIONInrecentyears,considerableinteresthasfocusedonTiAlintermetallicsbecauseofuniquepropertiessuchaslowdensity,goodstiffness,highelevatedtemperaturestrength,andexcellentoxidationresistance[1～4].TiAlintermetallicshasbeenconsideredasidealnewhight…     

TiAl/40Cr扩散连接接头的界面结构及相成长

在 1173～ 1373K、0 .3～ 5 .4ks的接合条件下对TiAl金属间化合物与 4 0Cr钢进行了真空扩散连接。采用扫描电镜 (SEM )、电子探针微区成分分析 (EPMA)、X射线衍射分析 (XRD)等方法确定了反应相的种类和界面结构。研究结果表明 ,在 1373K的接合温度下 ,TiAl/ 4 0Cr接头生成了TiC ,Ti3 Al,FeAl和FeAl2 4种反应相 ,形成了 3个反应层 ,界面结构为TiAl/Ti3 Al+FeAl+FeAl2 /TiC/脱碳层 / 4 0Cr钢。界面总反应层的厚度随接合温度和接合时间按抛物线方程成长 ,成长的活化能Q为 2 11.9kJ/mol,成长常数k0 为 4 .6× 10 -5m2 /s。当脆性反应层厚度为 3μm时 ,TiAl/ 4 0Cr钢接头的室温拉伸强度达到 183MPa的最大值。     

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.     

Finite element modeling and optimization of superplastic forming using variable strain rate approach

Detailed finite element simulations were carried out to model and optimize the superplastic blow forming process using a microstructure-based constitutive model and a multiscale deformation stability criterion that accounts for both geometrical instabilities and microstructural features. Optimum strain rate forming paths were derived from the multiscale stability analysis and used to develop a variable strain rate forming control scheme. It is shown that the proposed optimization approach captures the characteristics of deformation and failure during superplastic forming and is capable of significantly reducing the forming time without compromising the uniformity of deformation. In addition, the effects of grain evolution and cavitation on the superplastic forming process were investigated, and the results clearly highlight the importance of accounting for these features to prevent premature failure. 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.     

Numerical simulation of stress distribution in Al_2O_3-TiC/Q235 diffusion bonded joints

The distributions of the axial stress and shear stress in Al2O3-TiC/Q235 diffusion bonded joints were studied using finite element method （FEM）. The effect of interlayer thickness on the axial stress and shear stress was also investigated. The results indicate that the gradients of the axial stress and shear stress are great near the joint edge. The maximal shear stress produces at the interface of the Al2O3-TIC and Ti interlayer. With the increase of Cu interlayer thickness, the magnitudes of the axial stress and shear stress first decrease and then increase. The distribution of the axial stress changes greatly with a little change in the shear stress. The shear fracture initiates at the interface of the Al2O3-TiC/ Ti interlayer with high shear stress and then propagates to the Al2O3-TIC side, which is consistent with the stress FEM calculating results.     

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.     

Reinforced ceramic dies for superplastic forming operations

Ceramic dies have been developed to meet the need for a dimensionally stable tool, which can withstand the temperatures (425 to 950 °C) and high forming pressures (up to 7 MPa) that are required for superplastic forming (SPF), superplastic forming with diffusion bonding (SPF/DB), and hot sizing of metal parts. With the improvements that have been made to strengthen fused silica based ceramics, the performance of ceramic tools is slowly closing in on meeting the same forming complexity as corrosion-resistant steel (CRES) dies can achieve. Boeing has successfully superplastically formed jet engine wide chord fan blades using ceramic dies, and many production aircraft parts are being built with Boeing’s patented ceramic die technology. 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.     

焊接温度对Mg/Al扩散焊接头微观组织和性能的影响

采用真空扩散焊在不同焊接温度下对AZ31B镁合金和6061铝合金进行连接。利用光学显微镜（OM）、扫描电镜（SEM）和能谱（EDX）观察Mg/Al异种金属接头的显微组织。结果表明：随着焊接温度的升高,扩散区各层的厚度增加,且组织发生明显变化。440°C时扩散层由Mg2Al3层和Mg17Al12层组成;460和480°C时由Mg2Al3层、Mg17Al12层和Mg17Al12与镁基固溶体的共晶层组成。随着加热温度的升高,高硬度区域显著增多,区域内不同位置的硬度存在明显差别。当焊接温度为440°C时接头的最大抗拉强度为37MPa,脆性断裂发生在Mg17Al12层。     

A prediction for fatigue crack growth rate in diffusion bonded Al/Ti interface

A melt diffusion bonding technique was developed to make a joint of Ti and Al, which had been regarded as almost impossible because of high activity. Fatigue crack growth rates, da/dN of the Al/Ti interfaces were measured experimentally at various stress ratios and compared with pure Ti and Al. The interface showed a resistance to fatigue crack propagation as good as that of aluminum alloy. The proposed universal equations on the basis of the crack closure concept made it possible to predict da/dN for various ΔK and R. Fatigue crack was observed to grow in the Al side adjacent to the interface along the direction parallel to the interface. It was observed to form the intermetallic compounds in the interface region.     

铝板/塑料混合成型中铝板大塑性变形区的有限元分析

采用弹塑性大变形更新的Lagrange有限元方法研究了铝板 /塑料混合成型过程中铝板的成形过程和变形特点。结果表明 ,当塑料熔体压力从 30MPa增大到 5 0MPa时 ,铝板凸缘区已基本不再参与变形 ,铝板上两个板厚减薄较严重的大塑性变形区在此阶段形成。模底接触区与自由变形区交界处的大塑性变形区依次处于板料曲面内双向伸长变形和平面应变状态 ;模腔入口圆角区与自由变形区交界处的大塑性变形区由两部分构成 ,其中与模壁接触部分依次处于板料曲面内双向伸长变形、拉伸变形和平面应变状态 ,另一部分依次处于板料曲面内双向伸长变形和平面应变状态。