Effects of rare earth metals on electrical conductivity and mechanical properties of commercial purity aluminium

Abstract

The effects of the addition of rare earth metals on the electrical conductivity and mechanical properties of commercial purity aluminium have been investigated both in the laboratory and in industrial trials. The electrical conductivity of commercial purity aluminium is increased by about 1% on the international annealed copper standard by the addition of lanthanum, and both its tensile strength and elongation are improved considerably by the addition of cerium. Investigation of the microstructure of the alloys containing rare earth metals shows that the apparent improvement in the electrical conductivity of commercial purity aluminium is caused by a decrease in the solid solubility of impurities in the aluminium.

MST/2032     

Microstructure of as-cast aluminium bronze containing iron

Abstract

The morphology, crystallography, and chemistry of phases present in an as-cast commercial aluminium bronze containing iron, of nominal composition (wt-%) Cu–10Al–2·5Fe (BS 1400: AB1), and the development of microstructure on cooling from elevated temperatures, have been studied using optical and electron microscopic techniques. The as-cast microstructure consists of α-phase, martensite, and iron-rich intermetallic precipitates. The α-phase is fcc copper-rich solid solution and exhibits a Widmanstätten morphology. The martensitic phase, which is derived from the high-temperature β-phase, has the 9R crystal structure. The intermetallic particles are based on Fe₃Al and have the Do₃ structure. These iron-rich particles are precipitated in the β-phase and cause a refinement of the microstructure by providing sites for the nucleation of the β-phase, to some extent, by impeding the growth of the α-phase.

MST/150     

Activation energy of mullite crystallisation from precursor fibres determined by differential scanning calorimetry

Abstract

Mullite fibres were prepared by the sol–gel method using aluminium lactate and tetraethylorthosilicate. The aluminium lactate was prepared by mixing aluminium nitrate and lactic acid with a molar ratio of 1∶3. Thermogravimetry–differential scanning calorimetry, X-ray diffraction and scanning electron microscopy analysis were used to characterise the properties of the gel and ceramic fibres. The gel fibres completely transformed to mullite fibres at 1200°C, with a uniform diameter and dense microstructure. The active energy of mullite crystallisation was 1121·7 kJ mol^?1 by the Kissinger equation, which was consistent with most data reported by other authors.     

Dynamic recrystallisation phenomena of commercial purity aluminium during friction welding

Abstract

The microstructures of commercial purity aluminium near the friction weld interface were observed by transmission electron microscopy. Large plastic deformation of aluminium occurred near the weld interface and the microstructure of the aluminium was oriented from the centre to the periphery of the weld nugget. The aluminium grains were refined, and there were many recrystallised grains which were almost dislocation free. The refined grains, which were of size ~ 1 νm, formed very near the weld interface. The grain boundary was estimated to be a large angle tilt boundary. The refined grains were mainly formed by dynamic recrystallisation during the upset stage of the welding cycle. A large amount of shear strain and heat were introduced during the friction stage, and dynamic recrystallisation started during the upset stage. Grain growth occurred during air cooling after the upset stage. The grain size was larger in the central region than in the periphery owing to the variation in temperature. The Vickers microhardness of aluminium near the weld interface increased owing to the microstructural refinement.     

Ultraviolet surface plasmons on aluminium and a non-contact technique for characterizing short-pitch gratings

Abstract

In this work we present grating-coupled surface plasmon characterization of aluminium in the ultraviolet. Light of wavelength 325 nm from a helium-cadmium laser was used to excite surface plasmon-polaritons both at a buried aluminium-silica interface and on oxidized aluminium. By fitting the angle-dependent reflectivity data to model grating theory, the optical permittivity of uncontaminated aluminium at 325 nm is deduced. These results are compared with those extrapolated from prism-coupled surface plasmon excitation in the visible. The further characterization of the oxidized aluminium allows optical non-contact profilometry to be performed on short-pitch gratings that are opaque or are comprised of materials with ill-defined optical properties. This profile characterization is demonstrated for a short-pitch grating manufactured in photoresist.     

Characterisation of aluminium alloys after HNO3/HF–NaOH–HNO3/HF pretreatment

Abstract

The effect of a pretreatment sequence on several aluminium alloys was examined by X-ray photoelectron spectroscopy and SEM. The pretreatment consisted of vapour degreasing followed by a three step sequence comprising 1 min immersions in (a) nitric acid–hydrofluoric acid solution, (b) sodium hydroxide solution and (c) a 1 or 5 min immersion in nitric acid–hydrofluoric acid as in step (a). Following each pretreatment step the alloy panels were rinsed in tap water for 1 min. The surfaces of aluminium alloys 2024-T3, 6061-T6, and 7075-T6 were examined at each stage in the pretreatment process. The neutral salt spray resistance of specimens deoxidised with this pretreatment and chromate conversion coated were compared to those deoxidised with a conventional chromate based deoxidiser before conversion coating.     

Oxidation behaviour and microstructural evolution of FeCrAl ODS alloys at high temperature

Abstract

The oxidation behaviour of a commercial oxide dispersion strengthened (ODS) FeCrAl alloy (MA 956) and of a dispersion-free FeCrAl alloy (APM) were studied during isothermal exposures in air between 1,100°C and 1,300°C. After short heat treatments, the oxide film developed on MA 956 was more protective than that developed on APM. Longer isothermal exposures were conducted on MA 956 in order to examine the long-term behaviour of the oxide layer and the microstructural evolutions of the alloy. Chemical analyses of the substrate for increasing treatment duration revealed a continuous decrease of the matrix aluminium content due to oxide growth. After 4 months at 1,300°C, aluminium content of the alloy decreased to a critical value below which a continuous and protective oxide film could no longer be formed. Longer exposure times were carried out in order to relate the different stages of the catastrophic breakaway oxidation and identify the parameters involved in this phenomenon. Other substrate evolutions like cavity growth and nature and morphology changes of the Y₂O₃ particles were observed and are discussed in relation with the oxidation behaviour.     

Microstructure and texture evolution in continuously cooled AA 3004 hot rolled sheet

Abstract

A quantitative study of variations in microstructure and texture evolution in the through thickness direction of industrially hot rolled AA 3004 aluminium alloy has been carried out. The microstructural features of the specimens were examined with the aid of the electron channelling contrast technique in conjunction with an image analysis system. The number of recrystallised grains and the size distributions of coarse and fine intermetallics were measured to evaluate the variation between surface and centre. Significant differences in the number of recrystallised grains and the average size of coarse intermetallics in the through thickness direction of the hotband were observed. After isothermal annealing of the hotband for various times the fine intermetallic area fraction increased and was higher at the centre than at the surface. Quantitative texture analysis was carried out on the specimens and various texture components estimated to characterise the through thickness texture evolution. The proportion of cube texture component was higher near the surface than at the centre and this difference increased after isothermal annealing. Since negligible change occurred in the cube content of the centre specimen, growth of cube grains was deduced to have taken place primarily near the surface region. These observations illustrate that mechanisms of cube texture formation, heterogeneous nucleation of precipitates causing retarded recrystallisation, and Zener drag are evidently applicable even to complex commercial alloys.

MST/1849     

Vacuum-lamination process for metallized polymer films

Abstract

A process is described whereby polymer films are laminated using vacuum-deposited aluminium as an adhesive. Only modest pressures are needed to produce a pressure weld between the freshly deposited metal coatings. The influence of various process parameters on the metal–metal bonding is explored, with particular reference to surface contamination and deposition rate. In addition to their commercial potential, the laminates are particularly useful as test specimens for measuring metal–polymer adhesion.

MST/178     

Effect of Stearic Acid Particle Size on Surface Characteristics of Film-Coated Tablets

Abstract

Raw material specifications are vital for many excipients used in the manufacture of tablets. Stearic acid powder is widely used in the pharmaceutical industry as a tablet lubricant. This study shows that the variation in particle size of stearic acid not only affects die wall lubrication properties (ejection force) but also affects surface characteristics of film-coated tablets. A coarser grade of stearic acid can dislodge from tablet surfaces during the film-coating process leaving pit marks, whereas a finer grade of stearic acid (less than 100 mesh) results in film-coated tablets having very smooth surfaces. The mechanism of pitting on the tablet surface is described. A specification for stearic acid particle size to overcome this problem is suggested.     

Coupled heat and fluid flow model and experimental verification of aluminium plate die casting

Abstract

The present work aims to forecast mould filling, void shape, location and size as well as columnar to equiaxed transition (CET) in commercial pure aluminium casting. A model coupling the momentum equations of the fluid flow and heat transfer equations is presented, in which metallostatic pressure, air gap and oxide layer are considered. Different casting parameters were investigated such as casting configuration by varying the plate thickness from 5 to 20 mm, melt superheat from 40 to 120°C, mould preheat up to 200°C and different pouring heads ranging from 0·3 to 0·6 m. Regarding the microstructure and void formation, the approach based on the Niyama criterion, was considered. The experimental verification of the model was achieved by gravity die casting in the form of a rectangular cavity. Voids inside aluminium plate were investigated by X-ray imaging. Microstructure and CET was investigated microscopically. The supposed model proves its validity for mould filling and in detecting the void features and CET.     

Preparation of mullite fibres by sol–gel processes using aluminium carboxylates and tetraethylorthosilicate

Abstract

Mullite fibres were prepared using aluminium carboxylates (ACs) and tetraethylorthosilicate by sol–gel process. ACs were synthesised from dissolution aluminium in a mixture of formic acid and glacial acetic acid using aluminium chloride hexahydrate as catalyst. The optimum condition for obtaining ACs is as follows: the molar ratio of aluminium, formic acid and acetic acid was 1∶3∶2·26 and aluminium chloride hexahydrate was 10 wt-%. All the Al and Si components were mixed at the molecular level and linear molecules were formed in the precursor sol. The dried gel fibres completely transformed to mullite fibres at 1200°C and the calcinated fibres had a smooth surface and uniform diameter.     

Influence of aqueous salt environments on fatigue response of SiCp reinforced aluminium alloys

Abstract

The low cycle fatigue response of aluminium alloy 2124 reinforced with silicon carbide particulates produced by a commercial powder metallurgy route has been evaluated. This assessment includes measurements of fatigue crack growth rates and ‘total life to failure’ experiments. The relative performance under aqueous saline environments, namely total immersion in a 3.5% salt solution and exposure to a salt fog of similar concentration, were then characterised. While the alloy essentially demonstrates an insensitivity to saline environments in terms of its crack growth characteristics, a process of surface corrosion and pitting is highlighted and deemed responsible for significant reductions in the total fatigue life. The implications for engineering designs attempting to make use of this class of metal matrix composite are discussed.     

Evaluation of crystallographic changes and plastic strain ratio in Al alloys

Over the past several decades, extensive research endeavours in the field of crystal plasticity (CP) have led to the development of various Taylor-type homogenisation models enabling simulation of texture evolution during plastic deformation. This contribution provides a brief summary on the quality of deformation texture modelling under various boundary conditions. The crystallographic evolutionary patterns are discussed on the basis of grain interaction phenomena incorporated in the CP calculations. The annealing textures are successfully simulated by considering strain heterogeneities in the particle affected zone and by employing orientation selection during both nucleation and growth stages of recrystallisation. It is shown that the Lankford value profiles can be accurately reproduced by CP calculations on condition of reliable grain statistics.

This paper is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.     

Effect of aluminium on microstructure and mechanical properties of self-shielded flux cored welds in low carbon steel plate

Abstract

In the present paper, the results are reported of an investigation dealing with the inclusions, microstructure and mechanical properties of self-shielded flux cored welds with different aluminium contents. Results show that the total number and number density of inclusions in the low aluminium weld are higher than those of the high aluminium weld. However, the average size and the contamination rate of inclusions in the low aluminium weld are 0·77016 μm and 0·022%, which are lower than those of the high aluminium weld. The weld with a low aluminium content contains finer inclusions and the weld with a high aluminium content contains mainly coarser inclusions. There are remarkable differences in microstructure between the welds with different aluminium contents. The reheated zones in the weld made using flux no. 1 are larger than those in the weld made using flux no. 2, which are occupied by skeletal δ ferrite columnar crystals. As a result, a drastic reduction in impact toughness is observed in the high aluminium weld. Fracture examination shows that the low aluminium fracture surface consists mainly of small ductile dimples and a quasi-cleavage fracture area. The fracture surface of the high aluminium weld comprises mainly larger dimples and a cleavage fracture area. Inclusions in the low aluminium weld are mainly small Al₂O₃–MgO spinel with an approximate round shape, and some AlN inclusions. In the case of the high aluminium weld, inclusions are almost all large AlN. Finally, the results of thermodynamic analysis agree with the trends observed in the experimental data.     

Assessment of surface bonding strength in Al clad steel strip using electrical resistivity and peeling tests

Abstract

Aluminium clad steel strip successfully combines the surface properties of an aluminium alloy coating with the satisfactory mechanical properties of the steel substrate. The production of Al clad steel strip by rolling, however, is a more efficient and economical approach compared with other processes. In this investigation, trilayer strips of aluminium/steel/aluminium were produced using the cold roll bonding technique. The bonding strength between the layers and the electrical resistivity of the samples were measured using a peeling test and four point probe test, respectively. The effects of reduction of thickness, the friction condition between the outer layer and rolls, and the rolling velocity on the bonding strength and electrical resistivity of the samples were assessed. Finally, it is shown that the resistivity test can be used as a non-destructive test for the evaluation of the quality of bonding between the layers of aluminium clad steel strip.     

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     

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