Electrodeposition on aluminium and its alloys

Aluminium alloys S1C, NS4, HE9, LM25 and the 'difficult' zinc containing U.S. specification alloy used for automobile bumpers (X-7046), have been successfully electroplated using pretreatments which utilized either conventional immersion, elevated temperature or electrolytic modified alloy zincate (M.A.Z.) deposits. Satisfactory adhesion in excess of 7•5 KN m -I was only achieved on X-7046 using an electrolytic M.A.Z. pretreatment. The limitations of simple zincate solutions were demonstrated. Growth of deposits ~as monitored using a weight loss technique and the morphology of the various deposits studied using scanning electron microscopy. The characteristics of a specific alloy and processing sequence selected had a significant influence on the growth and morphology of the N.A.Z. deposi t. These all affected subsequent adhesion of electrodeposited nickel. The advantages of double-dip sequences were confirmed. Superior adhesion was associated with a uniform, thin, fine grained M.A.Z. deposit which exhibited rapid and complete surface coverage of the aluminium alloy. The presence of this preferred type deposit did not guarantee adhesion because a certain degree of etching was essential. For a satisfactory combination of alloy and M.A.Z. pretreatment, there was a specific optimum film weight per unit area which resulted in maximum adhesion. An ideal film weight of 0•06 :!: 0•01 mg cm-2was determined for S1C. Different film weights were required for the other alloys due to variations in surface topography caused by pretreatment. S1C was the easiest alloy on which to achieve high bond strength. Peel adhesion was not directly related to tensile strength of the alloy. The highest adhesion value was obtained on S1C which had the lowest strength of the alloys studied. The characteristics of the failure surfaces after peeling depended on alloy type, adhesion level and pretreatment employed. Plated aluminium alloys exhibited excellent corrosion resistance when appropriately pretreated. The M.A.Z. layer was not preferentially attacked. There was a threshold value of adhesion below which corrosion performance ~a8 poor. Alloy type, pretreatment and coating system influenced corrosion performance. Microporous chromium gave better corrosion protection than decorative chromium.

Powder painting of aluminium

The mechanisms involved in the production of chromate-phosphate conversion coatings on aluminium have been investigated. A sequence of coating nucleation and growth has been outlined and the principle roles of the constituent ingredients of the chromate-phosphate solution have been shown. The effect of dissolved aluminium has been studied and its role in producing sound conversion coatings has been shown. Metallic contamination has been found to have a dramatic influence on chromate-phosphate coatings when particular levels have been exceeded. Coating formation was seen to be affected in proportion to the level of contaminaton; no evidence of sudden failure was noted. The influence of substrate and the effect of an acidic cleaner prior to conversion coating have been studied and explained. It was found that the cleaner ages rapidly and that this must .be allowed for when attempting to reproduce industrial conditions in the laboratory. A study was carried out on the flowing characteristics of polyester powders of various size distributions as they melt using the hot-stage microscopy techniques developed at Aston. It was found that the condition of the substrate (ie extent of pretreatment), had a significant effect on particle flow. This was explained by considering the topography of the substrate surface. A number of 'low-bake' polyester powders were developed and tested for mechanical, physical and chemical resistance. The best formulation had overall properties which were as good as the standard polyester in many respects. However chemical resistance was found to be slightly lower. The charging characteristics of powder paints during application by means of electrostatic spraying was studied by measuring the charge per unit mass and relating this to the surface area. A high degree of correlation was found between charge carried and surface area, and the charge retained was related to the powder's formulation.

Formability of high strength aluminium sheet

An initial review of the subject emphasises the need for improved fuel efficiency in vehicles and the possible role of aluminium in reducing weight. The problems of formability generally in manufacture and of aluminium in particular are discussed in the light of published data. A range of thirteen commercially available sheet aluminium alloys have been compared with respect to mechanical properties as these affect forming processes and behaviour in service. Four alloys were selected for detailed comparison. The formability and strength of these were investigated in terms of underlying mechanisms of deformation as well as the microstructural characteristics of the alloys including texture, particle dispersion, grain size and composition. In overall terms, good combinations of strength and ductility are achievable with alloys of the 2xxx and 6xxx series. Some specific alloys are notably better than others. The strength of formed components is affected by paint baking in the final stages of manufacture. Generally, alloys of the 6xxx family are strengthened while 2xxx and 5xxx become weaker. Some anomalous behaviour exists, however. Work hardening of these alloys appears to show rather abrupt decreases over certain strain ranges which is probably responsible for the relatively low strains at which both diffuse and local necking occur. Using data obtained from extended range tensile tests, the strain distribution in more complex shapes can be successfully modelled using finite element methods.Sheet failure during forming occurs by abrupt shear fracture in many instances. This condition is favoured by states of biaxial tension, surface defects in the form of fine scratches and certain types of crystallographic texture. The measured limit strains of the materials can be understood on the basis of attainment of a critical shear stress for fracture.

Control of soundness and mechanical properties in aluminium-silicon-magnesium alloys

Previously, specifications for mechanical properties of casting alloys were based on separately cast test bars. This practice provided consistently reproducible results; thus, any change in conditions was reflected in changes in the mechanical properties of the test coupons. These test specimens, however, did not necessarily reflect the actual mechanical properties of the castings they were supposed to represent'. Factors such as section thickness and casting configuration affect the solidification rate and soundness of the casting thereby raising or lowering its mechanical properties in comparison with separately cast test specimens. In the work now reported, casting shapes were developed to investigate the variations of section thickness, chemical analysis and heat treatment on the mechanical properties of a high strength Aluminium alloy under varying chilling conditions. In addition, an insight was sought into the behaviour of chills under more practical conditions. Finally, it was demonstrated that additional information could be derived from the radiographs which form an essential part of the quality control of premium quality castings. As a result of the work, it is now possible to select analysis and chilling conditions to optimize the as cast and the heat treated mechanical properties of Aluminum 7% Silicon 0.3% Magnesium alloy.

Wear of high aluminium zinc-based alloys in plain bearing applications

Two zinc-based alloys of high aluminium content, Super Cosmal alloy containing 60% Al, 6% Si, 1% Cu, 0.3% Mn and HAZCA alloy containing 60% Al, 8% Si, 2% Cu, 0.06% Mg were produced by sand casting. Foundry characteristics in particular, fluidity, mode of solidification and feeding ability were examined. Metallographic analysis of structures was carried out using optical and scanning electron microscopy and their mechanical properties were determined using standard techniques. Dry wear characteristics were determined using a pin-on-disc test, and boundary-lubricated wear was studied using full bearing tests. Results from casting experiments were evaluated and compared with the behaviour of a standard ZA-27 alloy and those from tribological tests with both ZA-27 alloy and a leaded tin-bronze (SAE660) under the same testing conditions. The presence of silicon was beneficial, reducing the temperature range of solidification, improving feeding efficiency and reducing gravity segregation of phases. Use of chills and melt degassing was found necessary to achieve soundness and enhanced mechanical properties. Dry wear tests were performed against a steel counterface for sliding speeds of 0.25, 0.5, 1.0 and 2 m/s and for a range of loads up to 15 kgf. The high aluminium alloys showed wear rates as low as those of ZA-27 at speeds of 0.25 and 0.5 m/s for the whole range of applied loads. ZA-27 performed better at higher speeds. The build up of a surface film on the wearing surface of the test pins was found to be responsible for the mild type of wear of the zinc based alloys. The constitution of the surface film was determined as a complex mixture of aluminium, zinc and iron oxides and metallic elements derived from both sliding materials. For full bearing tests, bushes were machined from sand cast bars and were tested against a steel shaft in the presence of a light spindle oil as the lubricant. Results showed that all zinc based alloys run-in more rapidly than bronze, and that wear in Super Cosmal and HAZCA alloys after prolonged running were similar to those in ZA-27 bearings and significantly smaller than those of the bronze.

Embedded fibre Bragg grating array sensors in aluminium alloy matrix by ultrasonic consolidation

Fibre Bragg Grating (FBG) array sensors have been successfully embedded in aluminium alloy matrix by ultrasonic consolidation (UC) technique. The temperature and loading responses of the embedded FBG arrays have been systematically characterised. The embedded grating sensors exhibit an average temperature sensitivity of ~36pm/°C, which is three times higher than that of normal FBGs, and a loading responsivity of ~0.1nm/kg within the dynamic range from 0kg to 3kg. This initial experiment clearly demonstrates that FBG array sensors can be embedded in metal matrix together with other passive and active fibres to fabricate smart materials to monitor the operation and health of engineering structures.

Studies in aluminium alkoxide chemistry

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Aluminium hydroxide stabilised MnFe2O4 and Fe3O4 nanoparticles as dual-modality contrasts agent for MRI and PET imaging

Magnetic nanoparticles (NPs) MnFe2O4 and Fe3O4 were stabilised by depositing an Al(OH)3 layer via a hydrolysis process. The particles displayed excellent colloidal stability in water and a high affinity to [18F]-fluoride and bisphosphonate groups. A high radiolabeling efficiency, 97% for 18F-fluoride and 100% for 64Cu-bisphosphonate conjugate, was achieved by simply incubating NPs with radioactivity solution at room temperature for 5min. The properties of particles were strongly dependant on the thickness and hardness of the Al(OH)3 layer which could in turn be controlled by the hydrolysis method. The application of these Al(OH)3 coated magnetic NPs in molecular imaging has been further explored. The results demonstrated that these NPs are potential candidates as dual modal probes for MR and PET. In vivo PET imaging showed a slow release of 18F from NPs, but no sign of efflux of 64Cu. © 2014 The Authors.