Dip-coating of aluminium by lead

A study of the problems connected with the coating of aluminium by molten lead has been made. A theoretical approach shows that lead only wets the aluminium surface if the latter is completely free of oxide. Contact of liquid lead with a “clean” aluminium surface may be effected by using an intermediate layer of molten chlorides as a flux and this process has been studied using electrochemical methods. It has been shown that the flux (formed by a eutectic mixture of KCl and ZnCl₂) reacts with the aluminium to form a surface layer of various Al-Zn alloys as shown by electron probe microanalysis. After removal of all traces of chlorides, the resulting surface may be readily wetted by molten lead at the appropriate temperature.     

Wetting and spreading of molten aluminium against AlN surfaces

Wetting and spreading of molten aluminium against AlN substrates were investigated between 1100 and 1290°C. The contact angles decreased linearly with time under isothermal conditions between 1100 and 1200°C. The isothermal rate of spreading of molten aluminium against AlN substrates was constant between 1220 and 1290°C and the rate increased exponentially with increasing temperature. Crystals of Al4C3 nucleated and grew on the substrate surface beneath the liquid. However, the formation of Al4C3 may not be solely responsible for the changes in contact angle and spreading. It is postulated that carbon contamination from the substrate and/or experimental equipment coupled with the low oxygen partial pressure of the chamber in the presence of graphite, were primarily responsible for the observed contact angle and spreading phenomena. The activation energy for the spreading process was 448 kJ mol-1, suggesting the presence of some chemical reaction at the interface. Carbon-rich aluminium may be initiating a continuous surface reaction with the AlN substrates by reducing the native oxide layer on the substrate surface.     

Mechanism of scale growth on liquid aluminium

Abstract

Metal loss is an unavoidable consequence of the high production melting of aluminium and its alloys. Losses must be minimised, for reasons of economy and to ensure optimum quality of cast and wrought products. Aluminium losses during melting and casting result primarily from the formation of dross, a mixture of oxide and melt. Results are presented from an initial study aimed at reducing melt loss through a knowledge of the mechanism by which dross is formed. Work has centred on an understanding of the initial stages in oxide scale growth, a study of growth kinetics, and observations of the subsequent breakdown of these initial scales to form dross. It was found using TEM that a thin surface oxide of γ-alumina forms rapidly on melts of commercially pure aluminium at 750°C providing a highly effective barrier confining the molten aluminium. This thin surface oxide reduces scale growth to a low level before the onset of breakaway oxidation. Localised failure of this protective oxide film results in exudations forming on the melt surface, the size and number of which increase with exposure time. These exudations would appear to be the onset of dross formation. In parallel studies of the characteristics of wetting of aluminium to alumina it is shown that the reported non-wetting is a result of the presence of this thin alumina film on the melt surface. Once broken, wetting of the alumina takes place and it is shown that this accounts for the exudation of aluminium through the surface oxide and hence dross formation.

MST/990     

Influence of initial treatments of aluminium on the morphological features of electrochemically formed alumina membranes

The fabrication of alumina membranes by electrochemical oxidation of annealed aluminium was investigated. Porous layers were grown in 0.4 M H₃PO₄ at −1 and 5 °C at different anodising times. The morphology of the outer surface was found to be dependent on temperature and charge density, with both determining the extent of chemical dissolution of the anodic oxide. The inner-surface morphology was found to depend on the applied voltage only. The chemical dissolution rate of anodic oxide grown on annealed aluminium was found to be lower than that formed on unannealed aluminium under otherwise identical conditions. Such a difference in behaviour is explained in terms of a higher finishing degree of annealed aluminium determining a lower defect concentration in the anodic oxide.     

Coloration of molten aluminium surface in various atmospheres

Various kinds of coloration were observed on a surface of metallic aluminium heated above its melting temperature under seven different atmospheres with different po₂. Similar distributions of the coloration on a diagram of temperature (600–1400 °C) and time (1–90 min) were found among these atmospheres. Yellowish colours (beige, gold, yellow, blonde, camel and sand) appeared over a fairly wide range of the diagram. From estimated thicknesses determined by weight increases and the order of appearance of the colours, it was concluded that the colour is caused by interference by the oxidized thin film produced on the smooth surface of the molten aluminium.     

Wetting of aluminium oxide by molten aluminium and other metals

The sessile drop technique has been used to measure the contact angle of molten aluminium, aluminium-nickel and aluminium-copper alloys, copper and gold, with sapphire, ruby and recrystallised alumina. Measurements were madein vacuo, and as a function of time and temperature over the range 800 to 1500° C. Cinematography and time-lapse photography were used. At temperatures below 950° C, sessile drops of aluminium reached equilibrium only after a period of time which increased with decrease in temperature and could be in excess of one hour. A rapid increase in contact area occurred around 900° C. Above 1150° C drops of aluminium and of the aluminium alloys were observed to spread and contract repeatedly. Contractions were observed with both polycrystalline and single-crystal alumina, although they were much more pronounced with the latter, and were associated with the formation of a series of reaction rings on the plaque. Ruby and sapphire behaved similarly. The shape of the rings depended on the crystallographic orientation of the plaque: the reaction profile tended to terminate in certain low index directions. Neither contractions nor reaction was observed with copper or gold. The observations are discussed in terms of the combined effects of evaporation, chemical reactivity and interfacial tensions in the system.     

Oxide film formation on aluminium nitride substrates covered with thin aluminium layers

The growth of oxide films on aluminium nitride substrates covered by vapour-deposited aluminium films of 1.5 and 4 m thickness has been studied in air at atmospheric pressure as a function of temperature. Oxide films were grown by oxidation in air at temperatures between 800 and 1300°C. The kinetics of the growth of oxide films on such substrates was observed to be complex. In particular, there are three subsequent periods of observed oxide growth: (1) an initial period of rapid oxide growth, (2) an incubation period with very slow oxide growth, and (3) a second period of relatively fast oxide growth.     

Fabrication of gradient distribution alumina short fibre reinforced aluminium alloy

Gradient distribution alumina short fibre reinforced 6061 aluminium alloy have been fabricated by taking advantage of preform compressive deformation during squeeze casting. Pressure was applied mechanically by a punch. Velocity of the punch, pre-heat temperature of the preforms and pouring temperature were controlled during the infiltration of molten 6061 alloy into alumina short fibre preforms. The distribution of hardness along the infiltration direction in the composites was measured and the distribution of volume fraction along the infiltration direction was calculated by the hardness. Velocity of the inflow, pre-heat temperature of the preform, pouring temperature of the molten metal, binder content of the preform and volume fraction of fibres, all have a very great effect on the gradient distribution of alumina short fibres in the aluminium alloy composites.     

Anodization of evaporated aluminium on Ti-6 wt% Al-4 wt% V

Evaporated aluminium on Ti-6 wt% Al-4 wt% V alloy was anodized in phosphoric acid and other electrolytes. The anodic oxide formed was characterized by various techniques and it was found that a duplex oxide forms in which titanium has diffused through the aluminium oxide film and appears at the surface and throughout the film.     

Oxidation of aluminium nitride

The oxidation kinetics of polycrystalline aluminium nitride substrates in air at temperatures in the range 1150 to 1750°C have been studied by measuring the weight increase in the oxidized samples. At the lowest temperature, the oxide layer was not continuous on the AlN surface and the oxidation kinetics followed a linear rate law with an activation energy of 175 kJ mol^–1. At all the higher temperatures, the growth kinetics followed a parabolic rate law with an activation energy of 395 kJ mol^–1. Samples oxidized at these higher temperatures were covered with a dense oxide layer having a fine-grained microstructure.     

Amorphous and crystalline oxides on aluminium

In order to study the transition from amorphous to crystalline aluminium oxides, high purity aluminium was oxidized in dried oxygen of atmospheric pressure at 673 and 913 K. The morphology of the oxide layers produced was investigated by optical microscopy methods (phase and interference contrast) and scanning and transmission electron microscopy. The crystallinity was determined using electron (micro)diffraction methods. The existence of a hitherto undescribed transition aluminium oxide was established. Interface coarsening resulting from the formation of “tops” and “depressions” or “dimples”, as consequences of vacancy creation and injection in the substrate surface layers, was quantified by Auger depth profiling. The X-ray photoelectron spectroscopy technique revealed differences in the coordination of the aluminium cation between the amorphous and crystalline oxides.     

Production and properties of SiCp-reinforced aluminium alloy composites

A vacuum infiltration process was developed to produce aluminium alloy composites containing various volume fractions of ceramic particles. The matrix composites of aluminium with 9.42 wt%Si and 0.36 wt%Mg containing up to 55 vol% SiCp were successfully infiltrated and the effect of infiltration temperature and volume fraction of particle on infiltration behaviour was investigated. In addition to aluminium powder, magnesium was used to improve the wetting of SiC particles by the molten aluminium alloy. The infiltration rate increased with increasing infiltration time, temperature and volume fraction of particle, but full infiltration appeared at the optimum process parameters for the various volumes of fraction composite compacts. In addition, the microstructure, hardness, density, porosity and wear resistance of the composites were also examined. It is observed that the distribution of SiC particles was uniform. The hardness and density of the composite increased with increasing reinforcement volume fraction and porosity decreased with increasing particle content. Moreover, the wear rate of the composite increased with increasing load and decreased with increasing particle content.     

Preparation of cast aluminium alloy-mica particle composites

The optimum conditions for producing cast aluminium alloy-mica particle composites, by stirring mica particles (40 to 120 m) in molten aluminium alloys (above their liquidus temperatures), followed by casting in permanent moulds, are described. Addition of magnesium either as pieces along with mica particles on the surface of the melts or as a previously added alloying element was found to be necessary to disperse appreciable quantities (1.5 to 2 wt.%) of mica particles in the melts and retain them as uniform dispersions in castings under the conditions of present investigation. These castings can be remelted and degassed with nitrogen at least once with the retention of about 80% mica particles in the castings. Electron probe micro-analysis of these cast composites showed that magnesium added to the surface of the melt along with mica has a tendency to segregate around the mica particles, apparently improving the dispersability for mica particles in liquid aluminium alloys.The mechanical properties of the aluminium alloy-mica particle composite decrease with an increase in mica content, however, even at 2.2% the composite has a tensile strength of 14.22 kg mm^–2 with 1.1% elongation, a compression strength of 42.61 kg mm^–2, and an impact strength of 0.30 kgm cm^–2. The properties are adequate for certain bearing applications, and the aluminium-mica composite bearings were found to run under boundary lubrication, semi-dry and dry friction conditions whereas the matrix alloy (without mica) bearings seized or showed stick slip under the same conditions.     

Ellipsometric and X-ray specular reflection studies on naturally grown overlayers on aluminium thin films

The results of detailed investigations on the natural surface layer formed at room temperature on aluminium films exposed to air are presented. Aluminium films of high perfection, deposited onto very smooth glass substrates, have been studied over a 2 year period using ellipsometry. Soft X-ray specular reflection analysis revealed a composite surface layer structure composed by a thin (d₂ = 0.8 nm) very compact alumina layer in contact with the aluminium substrate and by a thick (d₃ = 3 nm) hydrated oxide layer. A new computer procedure was applied for this composite layer system, which evaluated 72 ellipsometric experimental data and achieved a best fit of the measured and calculated Ψ and Δ values. The resultant optical constants of the aluminium substrate were n = 1.09, 0.95, 0.535 and 0.370 for λ = 579 nm, 546 nm, 436 nm and 365 nm respectively, whereas k = 6.72, 6.40, 4.96 and 4.23 respectively for the same mercury lines. Among widely scattered data from the literature, these are in good agreement with results of Hass on the assumption of a similar surface layer structure, using n₂ = 1.77 (alumina) and n₃ = 1.58 (hydrated oxide).

Our optical constants for aluminium were applied for evaluating ellipsometric experimental data obtained during the 2 year period. A slight systematic change in the d₂ and d₃ values of the samples was found, owing to hydration.     

Spreading kinetics of liquid metals on mild steel

Abstract

The kinetics of the spread of molten copper, Cu–Mn alloy, and aluminium on solid mild steel were investigated at temperatures from 800 to 1100°C under an anhydrous borate flux. The molten copper and Cu–Mn alloy seem to spread largely by surface diffusion and the spread of molten aluminium follows linear kinetics. The spread coefficient, interfacial tension, and work of adhesion are evaluated from the relationship between an observed rate constant and Yin's theoretical formulism. The spread of molten copper on mild steel indicates complete wetting, while the spread of molten aluminium shows the least wetting. In the aluminium–mild steel spread system, the spread tends to increase with temperature: with increasing temperature, the spread coefficient increases and the interfacial tension decreases, while the work of adhesion remains much the same.

MST/743     

Reaction chemistry at joined interfaces between silicon nitride and aluminium

Joined interfaces of HIPed additive-free silicon nitride ceramics/aluminium braze bonded at a low temperature of 1073 K for 18 ks or at a high temperature of 1473 K for 1.8 ks in vacuum of 1.3 mPa and of β silicon nitride powders/aluminium powders bonded at the low temperature for 1.8 ks or 18 ks in the same vacuum are identified by analytical transmission electron microscopy and X-ray diffraction method. Mullite, some small crystals and β′-sialon are detected at the interface of the ceramics/aluminium braze bonded at the low temperature and 15R AIN-polytype sialon, β′-sialon, aluminium nitride, mullite and silica-alumina noncrystalline are detected at that bonded at the high temperature. At the interface of the two kinds of powders, aluminium nitride and silicon are also detected besides β′-sialon and silica-alumina noncrystalline even though the bonding was conducted at the low temperature. The interfacial reactions of the joints are influenced not only by bonding temperature but also by the oxide formed at the interface before bonded.     

Modelling of degradation of nickel-pigmented aluminium oxide photothermal collector coatings

Thermal degradation phenomena occurring in nickel-pigmented aluminium oxide coatings were investigated by combining detailed microstructural analysis with modelling of the optical properties. Scanning electron microscopy and Auger electron spectroscopy sputter profiling showed that the initial film consisted of a nickel-pigmented aluminium oxide layer close to the substrate. This layer supported a porous aluminium oxide layer that had a rough outer surface. While the solar absorptance degraded substantially (from 0.94 to 0.71) after heat treatment at 350°C for tens of hours, the aluminium oxide film morphology and thickness remained virtually unchanged and there was apparently no redistribution of nickel within the coating. Instead, the optical quality of the film degraded through oxidation of the nickel particles. These observations were supported by an optical model of the coating which produced the spectral reflectance properties measured both before and after the thermal ageing.     

Influence of oxide film defects generated in filling on mechanical strength of aluminium alloy castings

Abstract

The influence of oxide film defects generated from the filling process on the mechanical strength of aluminium alloy castings has been investigated. Using numerical simulation and experimental validation, the investigation aims to reveal the relationships among the liquid aluminium flow behaviour in the filling, the likely oxide film defect distribution caused by surface turbulence and the final mechanical strength of the castings. CFD modelling was used to investigate the liquid metal flow behaviour and the likely oxide film defect distribution in the filling at different ingate velocities. In particular, a numerical algorithm - Oxide Film Entrainment Tracking (OFET, 2-D) has been proposed and developed for predicting such oxide film defect distribution in the liquid aluminium throughout the filling. Also, light microscopic and SEM techniques were used to identify the microstructures of oxide film casting defects. The Weibull statistics method was employed to quantify the effects of oxide film distributions on the mechanical strength and reliability of the acquired aluminium alloy castings for different runner systems. It was found from the numerical simulations that the ingate velocities acquired using different runner systems have significant influence on the distribution of oxide film defects generated by surface turbulence in the filling process, which results in the disparities of the final mechanical strength of the castings. The results of the mechanical property test and the SEM micro-structural analysis of the castings are consistent with the numerical simulations.     

Heating sequence and hydrogen evolution in alloyed aluminium powders

The results reported here, showing the effect of a non-continuous degassing sequence on the Al-20Si-3Cu-1 Mg powder, are a complement of previous work concerning the continuous degassing of the same powder. The degassing experiments were carried out, under high vacuum, in the temperature range 20 to 550 °C in a horizontal furnace heated at a uniform heating rate of 2.5 °C min^–1. The partial pressures of the released gases were monitored and analysed during the heating phase by a computerized Edwards EQ80F residual gas analyser (RGA). RGA measurements indicate that water and hydrogen are the main degassing products. A complete degassing can only be achieved if the sample is heated up to a temperature where the chemical reactions are finished in the applied time. Thermodynamical equations alone are not enough to explain the kinetics of degassing of aluminium powders. The diffusion of aluminium through its surface oxide layer (Al₂O₃), described by the self-diffusion of aluminium, can explain to a large extent the kinetics of degassing aluminium powders.     

Synthesis of boride and nitride ceramics in molten aluminium by reactive infiltration

The infiltration of solid powder mixtures with molten aluminium has been investigated as a potential route for the synthesis of ceramic/metal composites. Either titanium or tantalum powder was mixed with boron nitride flakes for the reaction powder mixture. The infiltration occurred spontaneously at 1473K for both [Ti+BN] and [Ta+BN] powder mixtures. Owing to reactions between the starting materials, both boride and nitride ceramics were produced in molten aluminium. TiB2 and AlN were produced from the [Ti+BN] powder mixture, and TaB2 and AlN were produced from the [Ta+BN] powder mixture. When the [Ti+BN] powder mixture was used, a reaction producing Al3Ti took place immediately after the infiltration of the molten aluminium, and a subsequent reaction producing TiB2 and AlN proceeded gradually. The time required to convert BN flakes to TiB2 and AlN particles at 1473K was in the range of 1800–3600 s. On the other hand, when the [Ta+BN] powder mixture was used, there was an initial incubation period to allow the tantalum and molten aluminium to react with each other. The reaction between tantalum, BN and aluminium took place after this incubation period. This revised version was published online in November 2006 with corrections to the Cover Date.