Vapour deposited aluminium alloys

The vapour deposition process and the properties of an Al---Cr---Fe alloy produced by this process are described. Comparisons are made with some of the rapidly solidified alloys that have been reported.     

Cavitation erosion of aluminium alloys

Cast aluminium-silicon, cast aluminium-zinc and mechanically alloyed aluminium alloys were eroded in distilled water using a 20 kHz ultrasonic vibratory device. The erosion was measured by weight loss, and the damaged surface was examined using metallographic and profilometric techniques. The maximum differences in the incubation period, in the linear erosion rate and in the mass loss after a 10 h exposure for the nine materials investigated were 620%, 740% and 250%, respectively. The mechanically alloyed materials had by far the best combination of erosion properties. The cast Al-Si alloys had the poorest resistance to erosion. Age hardening was particularly beneficial with the Al-Si alloy. The main mechanism of material removal in all the alloys is by plastic deformation and ductile fracture.     

Press quenching of aluminium alloys

Abstract

The literature concerning press quenching is reviewed and the relevant precipitation kinetics are presented. Such kinetics may be presented on either continuous cooling curve or time–temperature–property diagrams. The homogenisation and subsequent reheating necessary are discussed in terms of microstructure and composition. Thermal considerations include heat transfer occurring both during the extrusion process and cooling after leaving the die. The interaction of extrudability and press quenching is discussed and it is shown that the ultimate solution must be one of compromise. The press effect is shown to be important and some results concerning the press quenching of 2000 series alloys are presented.

MST/742     

Formability of two aluminium alloys

Abstract

The suitability of two recently developed aluminium alloys (an Al–Mg–Mn alloy and an Al–Li–Cu alloy) for press forming applications has been examined. The characterisation involved the experimental determination of microstructural aspects, tensile properties, and formability parameters such as average plastic strain ratio and planar anisotropy. The forming limit diagram has been experimentally evaluated. A detailed analysis of the strain distribution profiles obtained from punch stretching experiments has been attempted. An attempt has been made to correlate the crystallographic texture with the formability parameters. The fracture surfaces of the punch stretched samples were observed using scanning electron microscopy with a view to obtaining a correlation between fracture behaviour and formability. The alloys, in particular the Al–Mg–Mn alloy, have been found to possess good stretchability but both show very limited drawability. Texture analysis indicated negligible earing during deep drawing. These alloys are suitable for stamping applications where stretching constitutes the major proportion of the deformation.     

The wetting of carbon by aluminium and aluminium alloys

The contact angle made by molten aluminium with vitreous carbon was measured by the sessile drop method in vacuum at temperatures up to 1100° C. The effect on wetting behaviour of the oxide layer on the molten metal was highlighted by using two samples of aluminium in different states of oxidation. The investigation involved the variation of certain parameters affecting the stability of the oxide film, e.g. the temperature, additions of Ti, Si, Cr, Be, Ca and Li to aluminium and the time held at a certain temperature. The state of the molten aluminium surface under various experimental conditions was determined indirectly by surface tension measurements.     

Laser-arc hardening of aluminium alloys

The characteristics of a joint laser-arc hardening of the aluminium alloys V124 and Al25 are investigated. Optimum parameters for the process are established. The laser-arc source gives a greater hardening effect in comparison with that of the arc source at treatment rates over 17 mm s. A microstructural analysis of the solidified melt zone showed that it represents α-phase dendrites surrounded by the eutectic Al-Si in which silicon has a fibre structure. With the aid of quantitative metallography, the parameters of the melt crystallization are determined for the case of fusion by the laser-arc source.     

Work softening in aluminium and some dilute aluminium alloys

In general, the results show that work softening is a feature of aluminium and some aluminium alloys. It is thought likely that it is to be found in other face centred cubic metals and further work here would be interesting.Work softening is closely associated with the formation of a cell structure and it is apparent that the stage at which work softening occurs can be influenced by the impurity content and by the rate of deformation. Both these conditions influence the cell formation and the Stage II/Stage III transition of a single crystal.Seeger, (8) has proposed that the cell structure is a result of cross slip of screw dislocations from pile-ups behind Lomer-Cottrell barriers. Hirsch, (9) proposed that the cell structure formed as a result of screw dislocations cross slipping from their original slip planes to form relatively strain free cell walls. Both these mechanism involve easy cross slip of large numbers of screw dislocations and this is just the effect proposed by Seeger to occur at stresses at which linear hardening changes to parabolic hardening, i.e. Stage III. However, observations made during work on single crystals have shown that the cell structure is well developed at the end of linear hardening and consequently, if these mechanisms for cell formation are correct, some other stress relaxation phenomena must be associated with the Stage II/Stage III transition.In the present work, the cell structure developed in the very early stages of deformation. It can be seen from the curves of hardness against rolling reduction, Fig. 2, for the four metals that these are all of the same pattern and that the stage at which work softening starts, i.e. the region of fluctuating hardness, is immediately preceded by a high rate of work hardening. This is similar to the high rate of work hardening which immediately precedes parabolic hardening in the stress/strain curve of a single crystal. It is therefore thought that the mechanism responsible for parabolic hardening and work softening could be the same.It is thought likely that work softening will occur whatever the mode of deformation provided that a cell structure is present and is maintained. It appears that the cell structure is likely to be maintained provided that the stress system does not become complex, as for example, in the necking of a tensile specimen, where the cell structure has been shown to disintegrate.     

Liquid phase sintering of aluminium alloys

The principle that alloys are designed to accommodate the manufacture of goods made from them as much as the properties required of them in service has not been widely applied to pressed and sintered P/M aluminium alloys. Most commercial alloys made from mixed elemental blends are identical to standard wrought alloys. Alternatively, alloys can be designed systematically using the phase diagram characteristics of ideal liquid phase sintering systems. This requires consideration of the solubilities of the alloying elements in aluminium, the melting points of the elements, the eutectics they form with aluminium and the nature of the liquid phase. The relative diffusivities are also important. Here we show that Al–Sn, which closely follows these ideal characteristics, has a much stronger sintering response than either Al–Cu or Al–Zn, both of which have at least one non-ideal characteristic.     

Fracture toughness of cast aluminium alloys

An investigation was carried out to evaluate the fracture toughness of cast aluminium alloys of different microstructural complexity, brought about by alloy constitution and cooling rate of castings. In all cases the three-point bend specimens, which had a thickness of 15 mm, did not provide valid plane — strain stress intensity factor values. The fracture susceptibility at a given stress level reckoned in terms of the conditional plane strain stress intensity factor (K _Q) was found to be lowest in aluminium-4.5% copper alloy castings and the susceptibility increased with increase in microstructural complexity. Casting cooling rate in these castings is likely to affect the damage potential of a given defect at yield stress to a greater extent than the fracture susceptibility at a given stress.     

Some characteristics of icosahedral aluminium alloys

Preliminary results of a study of the transformation behaviour, and those of microhardness measurements on quasi-crystalline Al-14 at.% Mn and Al-22 at.% Mn alloys are reported.     

Quench factor analysis of aluminium alloys

Abstract

A model was developed almost 20 years ago which described how precipitation during the quench affected the development of properties of aluminium alloys during subsequent aging treatment. This model was the basis for an analytical process, known as quench factor analysis, that was used to predict the effects of quench path on corrosion characteristics and strength. The purpose of this paper is to provide a theoretical basis for the model and to review how quench factor analysis has been used in solving industrial problems. Several investigators have confirmed that quench factor analysis is an effective predictive method for all quenching conditions save one. The exception is when material has been quenched below the knee of the C-curve and subsequently reheated above the knee before the quench is complete. Applications include the design of quench systems, the development of quench practices which optimize combinations of high strength and low residual stress and distortion, and predictions of the magnitude of loss in strength as a result of unsuitable quenching conditions. By combining quench factor analysis with homogeneous nucleation theory, interactions between quenching and aging conditions have been clarified, and aging treatments have been developed which minimize the low and variable strengths caused by less than ideal quenching conditions. Quench factor analysis also adequately describes the rate of loss in toughness of an AA 6000 series extrusion alloy for those cooling conditions which produced commercially significant loss in strength. The latest use of quench factor analysis is in a specification for quenchants for aluminium alloys.

MST/573     

Environmentally assisted cracking of aluminium alloys

A short review of the stress corrosion cracking (SCC) behaviour of aluminium alloys is given. Mechanisms of environmentally assisted cracking are outlined. For aluminium alloys, in which stress corrosion cracks propagate predominantly along grain boundaries, anodic dissolution and hydrogen embrittlement have been proposed. Transgranular stress corrosion cracking occurring at severe loading conditions has found particular interest concerning localised corrosion‐deformation interactions. Accelerated test methods for assessing the SCC behaviour are described, including the slow strain rate testing technique and the breaking load method. Results of recent studies on environmentally assisted cracking of aluminium alloys are summarised. Most of the work published in the last two decades has been on aluminium‐lithium based alloys and improved high strength Al‐Zn‐Mg‐Cu alloys in corrosion resistant retrogressed and re‐aged tempers.     

The weldability of lithium-containing aluminium alloys

Lithium-containing aluminium alloys have reduced density and increased elastic modulus compared with conventional aluminium alloys. Many such alloys are currently under development for aircraft applications, which usually involve mechanical fastening. Consequently, the weldability of lithium-containing aluminium alloys is currently receiving relatively little attention. The weldability of lithium-containing aluminium alloys is reviewed. The vast majority of the welding studies performed have been on the Soviet Al-5 wt% Mg-2 wt% Li alloy, 01420. Alloy 01420 and other lithium-containing aluminium alloys are indeed fusion weldable, and weldments having high joint efficiencies have been made.     

Analytical electron microscopy of aluminium alloys

X-ray microanalysis and electron energy loss spectroscopy of thin foils constitute the important techniques of high resolution chemical analysis using the electron microscope. The technique of x-ray microanalysis is discussed in this paper with particular emphasis on the study of aluminium alloys using a dedicated scanning transmission electron microscope (stem). The principle of determining chemical composition from observed x-ray peak intensities including the absorption of x-rays and beam broadening in thin foils are considered. The accuracy of peak intensity measurement and detection limits in x-ray microanalysis are illustrated with reference to Al-Mn alloys. The Cliff-Lorimer (k) factors for manganese, iron and copper with respect to aluminium were obtained from standard samples. Identification of phases in 1100 and 1200 aluminium and 3008 (Al-Mn-Zr) alloy were carried out from measured intensities of x-ray peaks. The experimental results emphasize the value of developing techniques for extracting the particles from the aluminium matrix. The transition phases formed in Al-6%Zn-3%Mg and Al-4% Cu were investigated by micro-diffraction and x-ray microanalysis.     

Microstructure and strength of Si-Ti-C-O,fibre-reinforced aluminium and aluminium alloys

Microstructures of Si-Ti-C-O fibre-reinforced aluminium and aluminium alloys were investigated by scanning electron microscopy, and both conventional and analytical transmission electron microscopy. In the latter samples, some inclusions were observed between the matrix and the fibres. From the electron diffraction, high resolution microscopy and compositional analysis by energy-dispersive X-ray, the inclusions were identified as the α-Al-Si-Fe phase. Since the longitudinal three-point bending strength decreases with the increase of iron content, it was concluded that the α-phase inclusions on the surface of the fibre contribute to the lower strength of the composites based on this alloy.     

Effect of hydrogen in aluminium and aluminium alloys: A review

Susceptibility of aluminium and its alloys towards hydrogen embrittlement has been well established. Still a lot of confusion exists on the question of transport of hydrogen and its possible role in stress corrosion cracking. This paper reviews some of the fundamental properties of hydrogen in aluminium and its alloys and its effect on mechanical properties. The importance of hydrogen embrittlement over anodic dissolution to explain the stress corrosion cracking mechanism of these alloys is also examined in considerable detail. The various experimental findings concerning the link between hydrogen embrittlement and stress corrosion cracking are also discussed.