Influence of pressure on bonding of alumina with molten aluminium as insert

Abstract

Alumina cylinders of 91% purity were bonded at 1073 K with aluminium foil of different thicknesses (5 and 50 μm) under different pressures (0–2 MPa) to examine the effect of pressure on the reaction of molten aluminium with silica contained in alumina as a binding agent. The reaction process at the bonding interface was also examined. The thicknesses of the interlayer and the reaction zone decreased with decreasing foil thickness. The interlayer was composed of silicon crystals and hypereutectic Al–Si alloy melt. Cavities were also observed. The constitution of the interlayer changed with time following the migration of silicon from the alumina: the amount of Al–Si alloy decreased with time, and the amounts of crystallised silicon and the total cavity volume increased with time. The bonding pressure reduced the thicknesses of the interlayer and the reaction zone. The amount of silicon contained in the interlayer was also reduced by pressure.

MST/3035     

Diffusion bonding of alumina reinforced 6061 alloy metal matrix composite using Al–Li interlayer

Abstract

This article describes a study of the application of a solid state diffusion welding technique to an aluminium alloy (6061) matrix composite reinforced with alumina particles in two different percentages (10 and 20 vol.-%) using an Al–Li alloy interlayer. The influence of bonding parameters on joint formation and the effect of the reinforcement in the bond line were determined by microstructural study of the joints. Shear tests using single overlap joints were used to evaluate the strength of these bonds.     

Fabrication of 6061 aluminium alloy/Al18B4O33(w) composite by using sol–gel alumina binder

Abstract

For fabrication of aluminium borate whisker (Al₁₈B₄O_33(w)) reinforced 6061 aluminium alloy composites, a sol–gel alumina binder instead of conventional silica binder was used for preparing the whisker preforms of the squeeze cast composites. The results show that a sound whisker preform and a uniform composite can be made by this method. Unlike the reactive silica binder, the sol–gel alumina binder is rather stable throughout the entire high temperature fabrication process. Under appropriate conditions, the sol–gel alumina binder can also serve as a thermal barrier for minimising interfacial reactions between aluminium borate whiskers and the matrix alloy. With a binder concentration of 0.6 mol L^-1, the ultimate tensile strength of the composite is as high as 277.6 MPa at room temperature and moderate at elevated temperatures. The tensile fracture of the alumina bound composite shows a mixed mode of dimple fracture and interface debonding.     

Effect of Mechanical Activation on the Packed-Bed, High-Temperature Behavior of Hematite and Graphite Mixture in Air

The present study reports the effect of mechanical activation on the reaction behavior of the Fe₂O₃/C powder mixture at high temperature under air atmosphere. Hematite and graphite were ground up to 150 hours using a ball mill with an alumina vial. The mixture was heated isothermally in the temperature range of 1173-1373°K using an electric furnace. The degree of reaction was determined by weight-loss measurement using a high accurate balance. It was found that low-energy mechanical milling at room temperature increases (the degree and) the rate of reaction at constant temperature. However, this effect was more significant at temperatures above 1273°K. At temperatures below 1273°K, the main reaction is oxidation of graphite and the total reaction process is controlled by diffusion of gases, whereas above 1273°K both chemical reaction (gasification reaction) and diffusion were controlling mechanisms. However, increasing milling time would shift the controlling mechanism from diffusion toward pure chemical reaction above 1273°K. It was observed that the mechanical milling might cause the mechanism to be changed at lower temperatures. This could be attributed to the increase of the rate of reaction due to mechanical milling. It was also observed that milling of powder mixture would decrease the difference in the average reaction rates at various degrees of reaction.     

Thermo-compression bonding of alumina ceramics to metal

Alumina ceramics and Kovar with aluminum interlayer are pressed together under vacuum at temperatures around 600°C for joining. This process produces mechanically strong ceramic to metal bonds in one step in an economic manner. In order to arrive at the optimum conditions for solid-state bonding, effects of bonding temperature, pressure and time on the bond strength have been studied. Bonding kinetics is also elucidated. Irradiation of 99% Al₂O₃ ceramics by 4–5 MV X-rays has been found to increase the bond-strength sharply from 33 to 60 MPa with a dose of 15 k Rads for bonding temperatures around 540°C. The apparent activation energy for the bonding process (Q _B) depends strongly on the type of alumina ceramics. Irradiation of alumina ceramics (99%), prior to joining with Kovar, accelerates the solid-state bonding by reducing (Q _B) from 209 to 76 kJ/mole.     

Transient liquid phase bonding of 2124 aluminium metal matrix composite

Abstract

The transient liquid phase (TLP) bonding of particle reinforced aluminium metal matrix composites (MMCs) using copper interlayers often results in the segregation of SiC particles to the bond region, and this has the effect of producing bonds with poor mechanical strengths. In this preliminary study, the TLP bonding of a 2124 aluminium alloy MMC is investigated using nickel interlayers, and the initial results show that good bonds are produced with no effect on the SiC dispersion in the matrix. The absence of segregation is attributed to the high diffusivity of the nickel in the aluminium MMC, which produces rapid isothermal solidification at the bonding temperature. Bond shear tests show that near parent metal strengths are possible when thin nickel interlayers are used, and failure occurs at the MMC/bond interface. When thick interlayers are used, failure is predominately through the centre of the bondline.     

Reactive brazing of ceria to an ODS ferritic stainless steel

This research study shows that a ceria ceramic can be bonded to an ODS ferritic stainless steel (MA956) by reactive brazing using a Ag₆₈-Cu_27.5-Ti_4.5 interlayer. The ability to join these materials provides an alternative to the current ceramic interconnects used in the development of solid oxide fuel cells. Initial results show that the ceramic-metal bonds survived the bonding process irrespective of the degree of porosity within the ceria ceramic. Metallographic analyses indicate that a reaction zone formed along the ceria/braze interface, which was not only titanium rich, but also consisted of a mixture of copper oxides. When the ceramic-metal bonds were exposed to high bonding temperatures or when subjected to thermal cycling at 700°C, this reaction layer increased in thickness and had a detrimental affect on the mechanical strength of the final joints.     

Solid-state bonding of alumina to austenitic stainless steel

A low temperature and low pressure bonding process for alumina and 316L austenitic stainless steel has been developed using a titanium/molybdenum laminated interlayer. The intermetallic compounds of Ti₃Al (or Ti₂/Al) and TiAl were formed at the alumina/titanium interface on bonding at above 1273 K. The activation energy of the layer growth was about 142 kJ mol^–1. The construction of Al₂O₃/Ti/Mo/steel gave the most stable joints. The highest tensile strength was above 60 MPa with a titanium 0.4 to 0.6mm thick/molybdenum 0.4 to 0.5 mm thick interlayer on bonding at 1273 K for 3 h under pressure of 12 MPa.     

Optimization of chemical reactions between alumina/silica fibres and aluminium-magnesium alloys during composite processing

An Al-Si-;Cu-Mg alloy reinforced with alumina/silica fibres (Fiberfrax®, alumina/silica ratio=45/55) has been extensively characterized in terms of microstructure, interfacial chemical reactions and mechanical properties. The composite was fabricated by squeeze casting. The above characteristics were measured as a function of (a) calcination temperature of the fibre preform before infiltration, and (b) subsequent composite heat treatment. The main reaction that occurs during the processing of aluminium alloy matrix composites is the reduction of silica in the binder and fibres by magnesium from the matrix. When calcined below 1000°C, the fibres remain amorphous with a coating of porous silica binder. In this condition, the reinforcement reacts strongly with the matrix during heat treatment of the composite. In contrast, at high calcination temperatures (1200°C), the fibres transform partially into mullite and the silica binder densifies; these fibres are somewhat less reactive with the matrix. In both cases, the matrix/reinforcement reactions are very strong during high-temperature heat treatment, leading to a complete reduction of silica in some cases. The degradation caused by chemical reactions adversely affects the mechanical properties of these composites. Therefore, in order to optimize the mechanical properties of this composite, the fibre preform first must be calcined at high temperature, then the composite heat treatment limited to low temperature.     

Microstructure and mechanical properties of γ base titanium aluminide produced from extruded elemental powders

Abstract

An elemental powder mixture of composition Ti–35 wt-%Al was cold extruded, reactive hot isostatic pressed at 1000°C, and subsequently annealed at 1250°C. A duplex microstructure was formed having elongated lamellar regions of Ti₃Al/TiAl within a TiAl matrix. Specimens were tested in compression at temperatures between room temperature and 900°C. The material exhibits good deformability at 700°C and above. In this region, the typical stress–strain behaviour accompanying dynamic recrystallisation is observed. Matrix and lamellar regions both undergo deformation. Below 700°C, pseudoplasticity occurs, which is related to homogeneous formation of microcracks. The observed behaviour is compared with that of a cast material of similar composition.

MST/1545     

Transient liquid phase bonding of Gamma Met PX: microstructure and mechanical properties

Abstract

An investigation of microstructural development and structure–property relationships of transient liquid phase (TLP) bonded current generation γ-TiAl alloy, known as Gamma Met PX, is presented. This joining technique employed a composite interlayer consisting of a non-melting constituent (TiAl alloy) and a liquid forming constituent (copper). The microstructures of the bonds, identified using light microscopy and scanning electron microscopy, are correlated with room temperature mechanical performance. These studies suggested that joints can retain properties similar (i.e. >90%) to that of the bulk material when employing a suitable composite interlayer, bonding conditions and post-bond heat treatment. Additionally, comparisons are drawn between wide gap TLP bonding of an earlier generation γ-TiAl alloy, Ti–48Al–2Cr–2Nb (at.-%), and wide gap TLP bonding of Gamma Met PX.     

Thermal degradation of silica fibre-reinforced aluminium

At elevated temperatures, the aluminium matrix reduces the reinforcing silica fibres to silicon, severely lowering the mechanical properties. The kinetics of the degradation process have been studied and results of previous work, in which silica rods were attacked by molten aluminium, have been confirmed. Fibres corrode at a constant rate (of penetration) at a fixed temperature and the activation energy for the silica reduction/ alumina formation process was found to be 280 kJ mol^–1, in close agreement with previous work. The alumina is formed within the boundaries of the original silica fibres. An activation energy of 170 kJ mol^–1 has been determined for the commencement of the reaction and it appears that a higher activation energy is required for initiation if solid, rather than liquid aluminium is participating in the reaction. A technique is proposed for the production of alumina fibres in situ in composite materials, based on the deliberate reduction of silica and other oxides in aluminium matrices. It is suggested that the effectiveness of the method may depend upon the relative volumes of the products and the reactants. Shrinkage may cause fibre-matrix separation and porosity formation, as in the Al/SiO₂ composite.     

The interfacial microstructure of joined single crystal and polycrystalline alumina

The interfacial microstructures of 96 and 98% polycrystalline alumina joined with single crystal sapphire have been investigated in relation to the joining parameters. Joining has been evaluated based on either using a thin spin-on silica interlayer or by placing the alumina and sapphire in direct contact. The materials were joined by placing the coated or uncoated surfaces in contact and heating in the range of 1340–1475 °C with minimum external load. With the aid of a silica interlayer, sapphire and 98% polycrystalline alumina were successfully joined in 180 min at 1400 °C and above, while samples without a silica interlayer failed to join under these conditions. However, sapphire and 96% polycrystalline alumina were joined both with and without the use of silica interlayer. A variety of interfacial morphologies have been observed, including amorphous regions, fine crystalline alumina, and intimate contact between the sapphire and polycrystalline alumina.     

Influence of microstructures on press bonding of 5083 aluminium alloys

Abstract

Solid state bonding of superplastic 5083 (SP5083) and 5083–H323 aluminium alloys has been studied. Press bonding was employed to bond the aluminium alloys in air. Temperature, time, deformation, and grain size were used as process variables whose effects were assessed in terms of bond strengths. The measured shear strengths were strongly influenced by the bonding parameters and alloy microstructures. The strengths of highly deformed specimens were very sensitive to the bonding temperature, and the combined effects of bonding temperature and holding time was very dependent on the microstructural conditions. Using alloy sheets with a finer grain structure resulted in higher bond strengths.     

Gold-to-alumina solid state reaction bonding

This paper describes methods by which strong vacuum-tight bonds between gold and alumina can be fabricated in an air atmosphere. Temperature was found to be a sensitive factor in bond formation; the higher the temperature, the stronger the bond, provided that the temperature remained below the melting point of gold. Intimate contact between the bonding gold and alumina surfaces during formation of the bond was also an important consideration. This was achieved by using an optically flat and polished ceramic surface and applying a low pressure of about 1 MPa to the bonding couple. Further enhancement of the contact can be achieved by depositing an additional thin layer of gold onto the ceramic surface by evaporation or by other means, and by thorough cleaning by high temperature heat treatment prior to bonding. Good bonding occurs in the range of 1 to 100 h. Bond strengths as high as 80 MPa were achieved.     

Bond strength and microstructure investigation of Al2O3/Al/Al2O3 joints with surface modification of alumina by titanium

Joints of Al₂O₃/Al/Al₂O₃ are formed by liquid-state bonding of alumina substrates covered with thin titanium film of 800 nm thickness using an Al interlayer of 30 or 300 μm at 973 K under a vacuum of 0.2 mPa for 5 min and an applied pressure of 0.01 MPa. The bond strength of the joints is examined by a four-point bend testing at room temperature coupled with optical, scanning and transmission electron microscopy. Results show that: (i) bonding occurs due to the formation of a reactive interface on the metal side of the joint with the presence of Al₃Ti precipitates (ii) a decrease in Al layer thickness leads to stronger Al₂O₃/Al/Al₂O₃ bonds accompanied by a change of both the distribution of reaction products (Al₃Ti) in the region of the interface and the failure surface characteristics.     

Ceramic joining IV. effects of processing conditions on the properties of alumina joined via Cu/Nb/Cu interlayers

Multilayer copper/niobium/copper interlayers consisting of 3 m thick cladding layers of copper on a 125 m thick niobium core layer were used to join aluminum oxides at 1150°C or 1400°C, or both. Three microstructurally distinct aluminum oxides were joined—a 25 m grain size 99.5% pure alumina, a submicron grain size 99.9% pure alumina, and single crystal sapphire. Two-phase interlayer microstructures containing both copper-rich and niobium-rich phases developed during bonding. In some cases, the initially continuous copper film evolved via Rayleigh instabilities into an array of discrete copper-rich particles along the interlayer/alumina interface with concurrent increases in the niobium/alumina contact area. Processing conditions (temperature and applied load) and the alumina microstructure (grain size) impacted the extent of film breakup, the morphologies of the copper-rich and niobium-rich phases, the interlayer/alumina interfacial microstructure, and thereby the strength characteristics. Joints possessing a large copper/alumina interfacial area fraction were comparatively weak. Increases in bonding pressure and especially bonding temperature yielded interfaces with higher fractional niobium/alumina contact area. For joined polycrystals, such microstructures resulted in higher and more consistent room temperature fracture strengths. Joined 99.9% alumina polycrystals retained strengths >200 MPa to 1200°C. Relationships between processing conditions, interlayer and ceramic microstructure, and joint strength are discussed.     

Investigation of the mechanical behaviour of magnesium composites

Stress/strain and fracture toughness behaviour of a commercial heat-treatable magnesium alloy reinforced with up to 20 volume% short alumina fibres was studied at room and elevated temperatures. Microscopic examination of the composites, which were prepared by conventional squeeze casting, revealed damage of a small portion of the fibres during the infiltration process. Sufficient chemical reaction between the matrix alloy and alumina reinforcement tends to produce a good bond at the fibre/matrix interface. The tensile-related properties of the composites increased at room and elevated temperatures with increasing content of the reinforcement. The ductility and fracture toughness of the composites decreased at room temperature with increasing reinforcement content. While failure strains of the composites were slightly improved at higher testing temperatures, the fracture toughness decreased significantly as the testing temperature exceeded 100°C. Examination of the fracture surfaces of specimens tested at room temperature showed a mixed mode fracture appearance with predominantly brittle cleavage fracture. The fracture surfaces of specimens tested at temperatures above 100°C revealed increasing fibre/matrix interface debonding and fibre pull-out with increasing testing temperature. Micromechanism examinations of crack initiation and propagation indicated that the fracture process of the composites may be matrix controlled.     

Parameters affecting transient oxide formation on FeCrAl based foil and fibre materials

Abstract

The effects of oxidation temperature and atmosphere on the formation of alumina scales on two commercial FeCrAl foil materials have been investigated. The oxidized specimens were characterized using a range of surface analysis techniques including SEM, XRD, laser induced optical spectrometry (LIOS), AES and XPS. During oxidation at temperatures exceeding 1000°C, the protective oxide scales formed on FeCrAl-alloys consist mainly of alpha alumina. At lower temperatures, however, formation of transient alumina modifications, has been observed. Although after longer oxidation times transformation into the stable alpha alumina occurs, the high initial growth rate of the metastable oxide phases could lead to a critical depletion of the Al-reservoir in thin walled (e.g. 20 (m) components, resulting in early breakaway failure. The occurrence of metastable oxides cannot simply be correlated with alloy composition.