结构用陶瓷接合述评

评述了制造高质量结构件接头的可行方法及其主要问题。论述了几种发展前景好的接合方法:熔焊、活性金属钎焊和间接扩散接合,并介绍了其它几种新颖特殊的接合方法:压铸法钎焊接合、电场辅助接合、超声波接合和微波接合。     

Modelling of diffusion bonding of metals

Abstract

Modelling of diffusion bonding has been carried out to quantify the kinetics of the bonding processes and to predict the time for achieving a sound bond. An alternative geometric assumption for the shape of the interfacial cavities to those considered previously was employed. Three subprocesses of bonding were introduced to simplify the modelling. These involved volume and interfacial diffusion coupled with creep, rigid collapse, and surface diffusion. The effects of grain size and phase ratio on diffusion bonding have also been considered. The predictions are compared with existing experimental data for copper and Ti–6Al–4V alloy and in general show good agreement.

MST/588     

Repair of air–cooled turbine vanes of high–performance aircraft engines – problems and experience

Abstract

Air–cooled turbine vanes made from nickel– and cobalt–base superalloys undergo distortion, cracking, burning, and material degradation in operation. The complex geometry of the parts and the compositional and microstructural heterogeneity of cast alloys impose limitations on the selection of repair methods. Selective chemical stripping of the diffusion coatings; elimination of cracks and restoration of dimensions by joining processes involving both welding and brazing techniques; and formation of coatings by pack cementation are the major processes employed. These processes may give rise to defects that significantly diminish the integrity of the parts.

MST/104     

New filler metals and process for fluxless brazing of aluminium engineering alloys

Abstract

Many high strength aluminium engineering alloys cannot be joined by brazing because they either degrade or melt at the temperature at which commercially available aluminium brazes are used. A brazing process suitable for joining aluminium engineering alloys has been developed employing two novel low melting point brazes. The brazes are available as ductile foil preforms. The process is fluxless and no post-joining cleaning treatments are necessary. The aluminium components and the brazing foil require a simple chemical treatment before use. The brazing process is tolerant and can be satisfactorily implemented by heating to a temperature of 510–550°C, maintained for a period of 5–45 min, in either vacuum or a furnace chamber that is purged continuously with nitrogen gas. The joints exhibit high strength and adequate resistance to corrosion for most applications. Some promising new applications for aluminium brazing technology based on this new process are described.

MST/3183     

Experience with repair of stationary gas-turbine blades –view of a turbine manufacturer

Abstract

Repair procedures for stationary gas–turbine blades, both established practices and techniques which have yet to find wide application, are described. Special attention is paid to quality assurance – making sure that repaired parts do not reduce reliability. Of the various repair techniques, only regenerative heat treatment (including hot isostatic pressing, or hipping) and recoating can be performed as standard procedures. Machining (e.g. grinding) changes the geometry of the blade and requires an exact knowledge of design tolerances. Straightening has to be done very carefully and should be followed by a hipping treatment. The greatest number of restrictions apply when fusion and deposition techniques such as brazing, welding, inserting, sintering, and plasma deposition are used. These techniques are used at present only if it is absolutely necessary to restore heavily worn blades.

MST/101     

Diffusion bonding of titanium alloy to stainless steel wire mesh

Abstract

The feasibility and appropriate processing parameters of diffusion bonding of titanium alloy to stainless steel wire mesh directly and with a nickel interlayer have been investigated. The microstructures of the diffusion bonded joints were observed by optical microscopy, scanning electron microscopy, X-ray diffraction, and electron probe microanalysis and the main factors affecting diffusion bonding were analysed. The maximum shear strengths of the joints were 72 and 148 MPa for direct bonding and indirect bonding using a nickel interlayer respectively. Atomic diffusion and migration between titanium and iron are effectively prevented by adding pure nickel as the interlayer metal, and a firm joint is obtained.     

Protective coatings – purpose,role, and design

Abstract

A wide range of coatings and coating processes is now available for protecting components in gas turbines operating in a variety of conditions. Coated aerofoils are less likely to fragment, and they keep their original shape – and hence their aerodynamic efficiency – for longer. Aluminide diffusion coatings perform well in relatively clean environments, especially on alloy bases which have a moderate resistance to the many forms of hot corrosion, and with additions of chromium or platinum they provide resistance to sulphate–induced hot corrosion. In hotter or more aggressive corrosive environments, overlay coatings based on MCrAlY (M = Fe,Co, Ni), CoCr, and MCrSi(Al), applied by electron beam evaporation or plasma spraying, are suitable for a wide variety of applications. It is not yet possible to design coatings from first principles for particular applications. Development still proceeds by repeated selection and testing, but modern technology has made this approach more effective than it was in the 1960s. The future is likely to see the rate of progress slow down; advances are expected to take the form of more efficient and flexible processes rather than a broadening of the range of applications.

MST/280     

Principles of superplastic diffusion bonding

Abstract

Superplastic diffusion bonding is now established as an important technique for solid state bonding. There are two types of superplastic diffusion bonding using either transformation or isothermal superplasticity. In this overview, the principles of these two types are summarised and the applications are discussed, including superplastic forming with concurrent diffusion bonding (the SPF-DB process), pressure welding, and the use of superplastic diffusion bonding in sintering.

MST/835     

Vacuum brazing of NiTi alloy by AgCu eutectic filler

Abstract

Joining of NiTi alloy to itself has been realised by vacuum brazing process using AgCu28 eutectic as filler metal. Microstructures, mechanical and shape memory behaviour have been investigated. The shearing strength of the brazed joint exceeds 100 MPa, and rupture occurs at the diffusion layer of parent metal beside brazing metal. The brazed joint will be stronger than parent metal on condition of the specimen with a joint of lap length 10 times of plate thickness. The brazed specimen shows a good shape memory behaviour. From the point of view of practice, the brazing joint design principle and brazing quality improvement have been discussed.     

Microstructure and strength of TiC cermet/cast iron brazed with Ag – Cu – Zn filler metal

Abstract

The brazing of TiC cermet to cast iron was carried out at 1223 K for 5 – 30 min using Ag – Cu – Zn filler metal. The formation phases, interface structures and shear strengths of the joints were investigated. The experiment result and analysis identify that three new phases, namely Cu base solid solution, Ag base solid solution and (Fe, Ni) have formed during the brazing of TiC cermet to iron. The interface structure of the joints can be expressed as TiC cermet/Cu base solid solution/Ag base solid solution + a little Cu base solid solution/Cu base solid solution + (Fe, Ni)/cast iron. The highest shear strength of the joints is 292.0 MPa, obtained with a brazing time of 20 min.     

Das Diffusionsschweißen von Titanlegierungen

Diffusion Bonding of Titanium Alloys. Diffusion bonding is a 4 stage process involving surface deformation atomic bonding interface diffusion and grain growth. These processes can proceed in the above order or more or less simultaneously. The effect of production parameters and material properties on the basic mechanisms is investigated. Several diffusion bonding techniques for industrial applications have been developed and are discussed in detail. Experience with the diffusion bonding of titanium alloys has shown that base material properties may be obtained in the weld zone. Actual parts fabricated by diffusion bonding sometimes show higher strength than made without welding. Some of the difficulties associated with diffusion bonding are presented. The economical and technical advantages of the diffusion bonding process are shown by reference to typical applications where this technique can be successfully applied.     

Crack analysis near vacuum brazing interface of Cu/Al dissimilar materials using Al–Si brazing alloy

Abstract

The present paper concentrates on the factors inducing cracking near the Cu/Al vacuum brazing interface using Al–Si brazing alloy. Various analysis and test methods were adopted to analyse the cause of the cracking. Experiment results indicated that two intermetallic compounds layers – δ phase and θ phase – were formed near the interface of Cu and brazing seam region. The microhardness of both the two intermetallic compounds phases reached 720 and 510 HM respectively. The brazing seam region consisted mainly of θ phase and α-Al (Cu) solid solution. Si presence had led to obvious zone liquation in the brazing seam region. Stress concentration appeared on the margin and edge region of the joint. The crack initiated on the θ phase layer, and then extended towards the brazing seam region and δ phase layer. The separation in the θ phase had an obviously brittle fracture surface.     

Solid state bonding of superplastic AA 7475

Abstract

Solid state (diffusion) bonds have been developed in AA 7475E sheet material using different regimes of bonding temperature, pressure, and time. The bonds produced have exhibited shear strengths in the range 30–150 MN m^?2 and have been found to be capable of withstanding peel during a slow high-temperature superplastic forming operation. However, the bonds formed under any one set of bonding conditions showed an extremely wide variation in both shear strength and shear fracture mode. The source of this wide variation has not yet been identified.

MST/601     

Vacuum diffusion bonding of TiB2 cermet to TiAl based alloys

Abstract

Vacuum diffusion bonding of TiB₂ cermet to TiAl based alloys was carried out at 1123 – 1323 K for 0.6 – 3.6 ks under 80 MPa. The microstructural analyses indicate that a compound Ti(Cu, Al)₂ is formed in the interface of the TiB₂ /TiAl joints, and the width and quantity of the Ti(Cu, Al)₂ compound increase with the increase of the bonding temperature and bonding time. The experimental results show that the shear strength of the diffusion bonded TiB₂ /TiAl joint is 103 MPa, when TiB₂ cermet is bonded to TiAl based alloy at 1223 K for 1.8 ks under 80 MPa.     

Interfacial microstructure and mechanical strength of WC–Co/90MnCrV8 cold work tool steel diffusion bonded joint with Cu/Ni electroplated interlayer

An important challenge in the design and processing of engineering materials is to combine incompatible properties of materials in the same component. One of the most common processes for joining dissimilar materials is brazing, but as a result of the poor resistance of the joints in service at high temperatures, diffusion bonding is proposed as the best suited alternative bonding process. In the present investigation, a cemented carbide (WC–15%Co) and a cold work tool steel (90MnCrV8) have been diffusion bonded in vacuum, using a ductile interlayer. Effects of the bonding time (30–60 min) and temperature (825–850 °C) on the quality of diffusion bonding interfaces were studied. The maximum tensile strength obtained confirms a very promising technology for industrial applications.     

Repair and rejuvenation procedures for aero gas-turbine hot-section components

Abstract

The general procedures for repairing aero gas-turbine hot-section components are described. A number of examples are given in order to illustrate particular repair processes: (i) the welding of nozzle guide vanes made from high–strength, nickelbase alloy, demonstrating the effect of microstructural refinement and welding techniques; (ii) the brazing of similar nozzle guide vanes, with emphasis on the advantages of using a halide cleaning process and of modifying the braze alloy by adding a powder having a composition close to that of the parent metal; (iii) the use of an improved coating system to upgrade components; (iv) the possibility of using MCr AlY coatings applied by low–pressure plasma spraying; and (v) hot isostatic pressing as a method of restoring creep life.

MST/103     

Growth mechanism of TiN reaction layers produced on AlN via active metal bonding

Interfacial reactions related to the TiN layer growth process between nanocrystalline epitaxial layers of AlN deposited on c-plane sapphire and a Ti-containing metal brazing or sintering layer using Ag–Cu–TiH₂, Ag–TiH₂ and Cu–TiH₂ pastes have been investigated. The brazed/sintered samples were heated in vacuum at 850 °C for 30 min. The TiN layer produced at the metal/AlN interfaces consists of TiN particles?<?50 nm in size and grain boundary phases including Al-containing Ag and Al-containing Cu. The Al concentration within the TiN layer decreases as the distance increases from the AlN epitaxial layer. These experimental observations all suggest that when AlN is used as a starting material in the active metal bonding method, interfacial reaction processes take place with the generation of a local Al-based eutectic liquid phase and elemental transport through this eutectic liquid phase.

     

Factors affecting bonding of metals

Abstract

Adhesives have several advantages over mechanical fastening, and the use of bonding is steadily increasing in many applications. Bonding of primary structures in aircraft has been used for many years and much more research has been directed towards aluminium than any other material. Joint performance depends on the alloy, pretreatment, primer, and adhesive. Many studies of these variables have been made, often making use of modern surface analytical techniques. Informative studies have also been carried out on titanium and steel. Hot, humid conditions, especially combined with stress, can be especially damaging to metal joints. A full understanding of the factors that provide the best resistance to this damaging effect has not yet been achieved. However, stable oxide layers with the correct topography appear critical and selection of the correct pretreatment is therefore of paramount importance.

MST/475     

Thermodynamic and kinetic analysis of interfacial reaction between CBN and titanium activated Ag–Cu alloy

Abstract

Thermodynamic and kinetic analysis was performed in order to study the interfacial reaction mechanism of cubic boron nitride (CBN) abrasive grains and Ti activated Ag–Cu filler alloy during high temperature brazing. Meanwhile, microstructure of the interfacial layer was experimentally detected using scanning electron microscope (SEM), energy dispersion spectrometer (EDS) and X-ray diffraction (XRD) in the present paper. The results indicate that according to the thermodynamic theory, the interfacial reaction has been proved feasible, and during brazing the special active element Ti concentrated to and reacted with the CBN abrasive to form TiB₂ and TiN, which joined hard the abrasive grain and steel substrate. Furthermore, the diffusion activation energy of the growing process in the interfacial reaction layer has discovered that the layer growth largely depends on the new formed TiN under conditions of 1153–1193 K and 5–30 min.     

Transient liquid phase bonding of magnesium alloy (Mg – 3Al – 1Zn) using copper interlayer

Abstract

The mechanism, microstructure and mechanical properties of transient liquid phase (TLP) bonded magnesium alloy (Mg – 3Al – 1Zn) joints using copper interlayers in an argon atmosphere have been investigated. The formation process of the TLP joint comprises a number of stages: plastic deformation and solid state diffusion, dissolution of the interlayer and base metal, isothermal solidification and homogenisation. The composition profiles and microstructures of the joint depend on the bonding time at a temperature of 530°C. With an increase in bonding time from 10 to 60 min, the concentration of copper and the amount of CuMg₂ compound in the joint decrease. For longer bonding times, the most pronounced features of the joint are composition homogenisation, grain coarsening and elimination of the bond line within the joint centre. The presence of brittle CuMg₂ and grain coarsening of the joint are the main reasons for impairing the joint shear strength. A joint shear strength of 70.2 MPa, which is 85.2% of the base metal strength (82.4 MPa), can be achieved by bonding at 530°C for 30 min.