Application of diffusion bonding model to superplastic γ-TiAl alloys

Abstract

In the present work, the superplastic characteristics of some γ-TiAl alloys were analysed on the bases of experimental data reported in previous work, and their suitability for diffusion bonding was examined. The constitutive equation ε = A DGb/kT (b/d)^p (σ?σ ₀ /G)ⁿ can be used to describe superplasticity in γ-TiAl alloys, and the equation predicts n=2, p=2, and activity energy Q=220 kJ mol^?1. It is suggested that the diffusion bonding model for γ-TiAl alloys should be considered two stages, i.e. a plastic deformation stage and a void shrinkage stage including both a diffusion controlled process and a plastic controlled process. Theoretical predictions for obtaining good bonding conditions were in good agreement with experimental results. Using a theoretical bonding model, a prediction map showing the relationship among temperature, applied stress and time for high quality diffusion bonding of superplastic γ-TiAl alloys was obtained.     

Characterisation of interfacial imperfections of TiAl and 40Cr diffusion bonding by ultrasonic amplitude and phase

Abstract

A characteristics extraction algorithm is proposed to characterise the interfacial imperfections in TiAl and 40Cr diffusion bonding. The algorithm is based on analysing the variation of the ultrasonic amplitude and phase after interacting with the bonding interface. Ultrasonic measurements were performed by an ultrasonic imaging testing system, and broadband transducers with central frequency of the 10 and 20 MHz were employed. Metallographic analyses and shear tests were also performed on the joints. It was found that the amplitude of the reflection coefficient is almost a constant, and the phase of the reflection coefficient is the same for the perfectly bonded interface; for the kissing bond interface, the amplitude increases with the ultrasonic frequency, and the phase is the same at the low frequencies and opposite at the high frequencies; the amplitude does not vary with the frequency, and the phase is opposite for the unbonded interface.     

Modelling behaviour of oxide film during vibration diffusion bonding of SiCp/A356 composite in air

Abstract

The vibration liquid phase diffusion bonding of SiC_p/A356 composite in air has been investigated. The surface of specimens to be bonded was treated with and without vibration under the bonding condition. It was found by atomic force microscopy analysis that some of the oxide film could be broken down when ridges on the surface of the matrix were ground down. Dissolution of the base metal by the filler metal occurred with removal of the oxide film during vibration liquid phase bonding, and SiC particles in the base metal entered the bond region. A removal process model for vibration bonding has been established with and without filler metal. Results show that shearing and impacting actions are the two main breaking mechanisms during vibration; the oxide film bulk is generally broken down by shear, and dissolution of the base metal by the filler metal promotes particle segregation from the matrix and their entry into the bond region.     

Vacuum hot roll bonding of titanium alloy and stainless steel using nickel interlayer

Abstract

Vacuum hot roll bonding of titanium alloy and stainless steel using a nickel interlayer was investigated. No obvious reaction or diffusion layer occurs at the interface between stainless steel and nickel. The interface between titanium alloy and nickel consists of an occludent layer and diffusion layers, and there are the intermetallic compounds (TiNi₃, TiNi, Ti₂Ni and their mixtures) in the layers. The total thickness of intermetallic layers at the interface between titanium alloy and nickel increases with the bonding temperature, and the tensile strength of roll bonded joints decreases with the bonding temperature. The maximum tensile strength of 440·1 MPa was obtained at the bonding temperature of 760°C, the reduction of 20% and the rolling speed of 38 mm s^–1.     

Technology and interface structure for diffusion bonding of Fe3Al/18-8 dissimilar materials

Abstract

The effects of technological parameters on the interface morphology and shear strength of Fe₃Al/18-8 diffusion bonded joints have been researched by means of a JXA-80 scanning electron microscope (SEM) and a digital tensile press test machine. Microstructural characteristics and fracture morphology near the Fe₃Al/18-8 diffusion interface, obtained under conditions of various technological parameters, have been analysed using the SEM. The test results indicate that a diffusion bonded joint with higher shear strength (σ_τ = 226 MPa) may be obtained with a heating temperature T = 1020 – 1040°C, holding time t = 45 – 60 min and pressure P = 12 – 15 MPa.     

Plane strain fracture toughness of modern ferritic structural steels

Abstract

The temperature dependence of the plane strain fracture toughness of a low carbon, fine grain, ferritic steel for structural applications is investigated. The ductile–brittle transition is found to occur in the interval between 160 and 184 K. The experimental results are interpreted by an analytical model which permits calculation of the plane strain fracture toughness K _1c in the brittle domain as a function of the tensile properties and the cleavage fracture stress, making use of a piecewise approximation for the distribution of tensile stress on the crack axis and applying a deterministic fracture criterion at the stress peak. A similar criterion, which consists of equating the severest strain on the crack axis to a critical strain for cavity nucleation, provides the upper shelf fracture toughness. A relatively simple figure for predicting the transition temperature of steels in this family as a function of material properties can be obtained in this way.     

Inverted blister test to measure adhesion energy of thermal oxide scales on metals or alloys

Abstract

A blister test experiment, used in an inverted configuration, has been developed in an attempt to quantify the interfacial fracture energy of oxide scales thermally grown on metallic materials. The practical use of this test is described for two systems: TiO₂ on Ti and chromia rich oxide on stainless steel. Provided that experimental problems of good oxide gluing to the sample holder and regular propagation of the interfacial crack could be controlled, adhesion energy values may be obtained with good accuracy. Values between 3 and 170 J m^-2 were measured, depending on the metal-oxide system and on the oxidation conditions.     

Characterization of Al/Ni multilayers and their application in diffusion bonding of TiAl to TiC cermet

The Al/Ni multilayers were characterized and diffusion bonding of TiAl intermetallics to TiC cermets was carried out using the multilayers. The microstructure of Al/Ni multilayers and TiAl/TiC cermet joint was investigated. The layered structures consisting of a Ni₃(AlTi) layer, a Ni₂AlTi layer, a (Ni,Al,Ti) layer and a Ni diffusion layer were observed from the interlayer to the TiAl substrate. Only one AlNi₃ layer formed at the multilayer/TiC cermet interface. The reaction behaviour of Al/Ni multilayers was characterized by means of differential scanning calorimeter (DSC) and X-ray diffraction. The initial exothermic peak of the DSC curve was formed due to the formation of Al₃Ni and Al₃Ni₂ phases. The reaction sequence of the Al/Ni multilayers was Al₃Ni → Al₃Ni₂ → AlNi → AlNi₃ and the final products were AlNi and AlNi₃ phases. The shear strength of the joint was tested and the experimental results suggested that the application of Al/Ni multilayers improved the joining quality.     

Creep kinetics in compression of sand cast commercial Zn–Al alloys

Abstract

The compressive creep behaviour of sand cast ZA8, ZA12, and ZA27 alloys was investigated. Primary creep contraction increased with aluminium content and stress and, with the exception of ZA27, decreased with temper ature. Secondary creep rates for ZA8 and ZA27 were similar and 33% less than for ZA12. Creep kinetics obeyed an empirical equation ln t = C′ — n ln (σ) + Q/RT where t is the time to a selected strain, σ is the nominal stress, T is the absolute temperature, R is the gas constant, and C′, n, and Q are material constants for each alloy. For total deformations up to 1%, the overall creep resistance increased in the order ZA27 > ZA12 > ZA8. In terms of secondary creep rate, the compressive creep resistance of ZA12 was similar to its tensile creep resistance as reported in the literature, but for ZA27 the creep rate in compression was generally slightly higher.     

Evolution of interface microstructure and strength properties in titanium – stainless steel diffusion bonded transition joints

Abstract

Diffusion bonding was carried out to produce transition joints between commercially pure titanium and 304 stainless steel at a temperature of 800°C for different times ranging from 30 to 180 min. in steps of 30 min under load in vacuum. The diffusion couples thus produced were studied using optical microscopy, scanning electron microscopy, and electron probe microanalysis to characterise the reaction layers formed in the diffusion zone. The chemical compositions of these layers indicate that intermetallics like σ phase, Fe₂ Ti, Cr₂ Ti, χ phase, FeTi, β-Ti, and Fe₂ Ti₄ O are formed in the reaction zone. The presence of these intermetallic compounds was also confirmed by the X-ray diffraction technique. Maximum bond strength of ~242 MPa was obtained for diffusion welded joints processed for 120 min. At this joining time, the plastic collapse of the surface asperities reaches near completion, favouring the interdiffusion of chemical species. Reduction in the bond strength of the transition joint processed for 180 min is due to the formation of a large volume fraction of voids in the reaction zone. Under tensile loading, failure takes place through α-Fe + χ phase mixture for transition joints processed in the time range of 30 – 90 min and through β titanium for joining times greater than 120 min.     

Thermal expansion of TiAl + TiB2 alloys and model calculations of stresses and expansion of continuous fiber composites

Thermal expansion values for three TiAl alloys with different additions of TiB₂ can be represented using a third-order equation at temperatures between 20 and 800°C. Expansion values were obtained on both heating and cooling temperature cycles. The total expansion at 800°C is between 0.917 and 0.931% for three different samples. The expansivity increases from about 10×10^–6°C^–1 at 80°C to 14×10^–6°C^–1 at 750°C. A five-coaxial cylinder elastic model for multizone-coated continuous fiber composites is developed for predicting stresses and thermal expansion of composites. Either isotropic or transversely isotropic material properties can be assigned to the various cylinder zones.Paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

Response of titanium alloys to high strain rate deformation

Abstract

The stress-strain response of samples of Ti64 and Ti550 at strain rates from 10^?1 s^?1 to 10³ s^?1 and samples of Ti811 and Ti153 at a strain rate of 10³ s^?1 have been assessed. It has been found that the influence of the imposed strain rate on the stress-strain response of Ti64 and Ti550 alloys is very similar – in both alloys the yield stress increases with increase of strain rate and the energy absorbed to fracture increases. At high strain rates localised deformation occurs in the form of shear bands in Ti64 and Ti550 but no shear banding was seen in Ti811 and Ti153. The fracture surfaces of Ti64 and of Ti550 show an increased tendency to brittle failure and an increase in necking with increase of strain rate. The influence of alloy microstructure and composition on the response to changes in imposed strain rate are discussed in terms of adiabatic heating and the factors controlling the flow stress in these alloys.     

Interface microstructures in diffusion bonding of titanium alloys to stainless and low alloy steels

Abstract

Commercial purity Ti and a Ti 6242 alloy have been diffusion bonded to an AISI 316L stainless steel and an AISI 4130 low alloy steel. The microstructures of the as processed products have been analysed using optical metallography, scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) techniques. The interdiffusion of the different elements through the interface has been determined using energy dispersive spectroscopy microanalysis in both a SEM and a STEM. For the combinations AISI 316L–commercially pure Ti and AISI 316L–Ti 6242 several regions surrounding the original interface have been observed. Starting from the 316L side, first a α phase is observed, followed by an Fe₂ Ti intermetallic, an FeTi intermetallic, and finally an Fe₂Ti₄O oxide just before the Ti and Ti 6242. Because the diffusion ofTi in Fe is faster than the diffusion of Fe in Ti, a Kirkendall effect is produced. In the AISI 4130–Ti 6242 combination a thin layer of TiC is observed at the interface, limiting the interdiffusion of elements.

MST/1746     

TiAl/40Cr钢扩散连接界面组织结构对接头强度的影响

扩散连接界面组织结构是影响连接性能的关键因素，不同的界面组织结构及生成相所决定的接合强度不同．本文研究了TiAl/40Cr钢的扩散连接，结果显示：连接温度过高及连接时间过长时，由于界面处形成了过多的TiC脆性层及Ti3Al FeAl FeAl2的金属间化合物混合层，接头拉伸强度低；当连接温度较低及连接时间较短时，界面紧密接触与元素扩散不充分，接头拉伸强度也较低．脆性TiC层的生成导致TiAl与40Cr钢之间的扩散连接性能较差，接头均破断于TiC层或TiC层与Ti3Al FeAl FeAl2的金属间化合物混合层之间．     

TiAl/40Cr扩散焊未焊合、弱接合和微孔缺陷的智能识别

为了解决异种材料扩散焊质量超声波检测时,从回波幅度无法判断界面是否存在微小缺陷的问题,采用支持向量机技术构建了扩散焊界面缺陷识别模型.以TiAl和40Cr扩散焊接头为研究对象,采用超声波水浸聚焦法采集扩散焊界面信号,从信号中提取4个特征值,优化样本数量和核参数后,训练扩散焊界面缺陷识别模型.扩散焊试样界面信号经模型识别后,根据C扫描图像的位置重构识别图像.结果表明,该模型有效地识别出未焊合、弱接合和微孔缺陷,3种缺陷的正确识别率分别为93%、90.5%和91.5%,识别图像直观地显示了界面的缺陷.TiAl/40Cr扩散焊界面缺陷识别模型实现了扩散焊试样界面缺陷的智能识别.     

热处理对TiAl/Ti2AlNb放电等离子扩散焊接头微观组织与力学性能的影响

采用放电等离子扩散连接方法,实现了TiAl/Ti2 AlNb合金扩散连接,对焊后的接头进行不同温度的热处理,分析热处理后接头显微组织,并检测接头抗拉强度和显微硬度.结果表明:热处理后Ti2 AlNb母材、TiAl母材和界面处显微形貌无明显变化;Ti2 AlNb热影响区发生B2相向O相转变,由于针状O相的析出,热影响区的显微硬度较焊态显著增加.随着热处理温度的升高,Ti2 AlNb热影响区的显微硬度逐渐减小,接头的室温抗拉强度逐渐增加.当热处理温度为900℃时,接头抗拉强度最大为376 MPa.热处理后接头的断裂方式为脆性断裂.     

TiB2.TiAl3/2024Al复合材料多向锻造数值模拟研究

目的 研究TiB2.TiAl3/2024Al复合材料多向锻造金属流动行为,以及锻造温度、锻造道次对复合材料再结晶行为及基体晶粒尺寸的影响。方法 将TiB2.TiAl3/2024Al复合材料的本构模型及再结晶动力学模型导入Deform-3D有限元模拟软件中,建立复合材料多向锻造的数值仿真模型。通过数值仿真方法分析锻造温度和锻造道次对复合材料多向锻造组织的影响规律。结果 多向锻造变形过程中,剧烈塑性变形和动态再结晶主要分布在材料试样内部的呈“X”形状的区域,单次下压最大等效应变为1.42。锻造1道次时,锻造温度从350 ℃升至500 ℃,再结晶体积分数从65.0%升至69.7%,平均晶粒尺寸由350 ℃的24.6 μm降至500 ℃的21.5 μm。在450 ℃锻造温度下,1道次锻造后,再结晶体积分数为69.2%,平均晶粒尺寸由铸态的45.0 μm减小到21.9 μm;2道次锻造后再结晶体积分数为89.5%,平均晶粒尺寸为16.3 μm;3道次锻造后坯料的再结晶体积分数为96.1%,平均晶粒尺寸为14.3 μm。与试验结果比较可知,模拟结果准确可靠。结论 提高锻造温度和增大锻造道次可以促进试样发生动态再结晶,从而达到细化晶粒的目的。     

Elevated temperature deformation of several quaternary L12Al3Ti based alloys reinforced with TiB2 particles

Concurrent with an investigation of quaternary cast and forged L1₂ modified Al₃Ti's containing 9 at% Cr, Fe and/or Mn, a similar series of four alloys was produced via XD^TM technology, whereby 20 vol% of 0.5 m TiB₂ particles was incorporated as a reinforcement. Following densification by hot pressing and slow isothermal forging, small diameter compression test samples were machined from each compact and tested. The 0.2% yield strength measurements revealed a strength advantage for the particulate reinforced materials over the unreinforced ingot metallurgy matrices to about 1175 K. Furthermore, 900 and 1100 K constant velocity testing indicated that the TiB₂ containing materials were stronger than the ingot metallurgy matrices down to strain rates of 10^–7s^–1. None of the quaternary L1₂ alloys + 20 TiB₂ consistently displayed a strength advantage over others. However, extrapolation of the present mechanical property data into slower strain regimes indicated that the composites will not possess any strength advantage overthe unreinforced versions. Such losses in strength appeared to be caused by small grains in the particulate reinforced alloys, which promoted grain boundary weakening mechanisms.     

Interface compounds formed during the diffusion bonding of Al2O3 to Ti

The interfacial reaction products of Ti/Al₂O₃ joints obtained in the context of real diffusion bonding technology were investigated by means of X-ray diffraction analysis, X-ray photoelectron spectroscopy, and transmission electron microscopy. Some Ti reacted with Al₂O₃ giving titanium oxides, but the main mass transport occurred into the bulk Ti due to Al₂O₃ dissolution. The formation of a Ti[Al, O] solid solution followed by a order/disorder reaction yielded Ti₃Al. Further Al enrichment at the interface could lead to the formation of TiAl, which was not observed in the present work due to either the short residence time at the bonding temperatures or to its lower oxygen solubility. For joints obtained at 800°C and shear test fractured it was ascertained that the crack always propagated within the Ti₃Al layer.     

Microstructure and refinement mechanism of TiB2/TiAl3 in remelted Al-5Ti-1B system

In this work, we propose a new method (by remelting Al-5Ti-1B) to investigate the grain refinement mechansim. It is found that the morphology and size of TiAl₃ phase had little effect on the grain refinement of pure Al. Therefore, further experimental studies were carried out to understand the potency of TiB₂ particles. The high-resolution transmission electron microscopy (HRTEM) observation has confirmed the existence of an atomic layer on the surface of (0001) TiB₂, which is possibly a two-dimensional (2D) (1-12) TiAl₃. Crystallographic study indicates that it is a more suitable nucleation sites for α-Al than other particles. The TiB₂ particle with TiAl₃ 2D acts as the best effective nucleation sites for α-Al.