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.

Size dependent enhancement of helium ion irradiation tolerance in sputtered Cu/V nanolaminates

We have investigated the evolution of radiation damage and changes in hardness of sputter-deposited Cu/V nanolaminates upon room temperature helium ion irradiation. As the individual layer thickness decreases from 200 to 5 nm, helium bubble density and radiation hardening both decrease. The magnitude of radiation hardening becomes negligible for individual layer thickness of 2.5 nm or less. These observations indicate that nearly immiscible Cu/V interface can effectively absorb radiation-induced point defects and reduce their concentrations.

Aggregation and caking processes of granular materials:continuum model and simulation with application to sugar

Aggregation and caking of particles are common severe problems in many operations and processing of granular materials, where granulated sugar is an important example. Prevention of aggregation and caking of granular materials requires a good understanding of moisture migration and caking mechanisms. In this paper, the modeling of solid bridge formation between particles is introduced, based on moisture migration of atmospheric moisture into containers packed with granular materials through vapor evaporation and condensation. A model for the caking process is then developed, based on the growth of liquid bridges (during condensation), and their hardening and subsequent creation of solid bridges (during evaporation). The predicted caking strengths agree well with some available experimental data on granulated sugar under storage conditions.

Tensile properties of as fabricated, aged, and stretched SiC whisker and particle reinforced 2009 aluminium alloy

Tensile tests were carried out using specimens of 2009 aluminium alloy reinforced by either SiC whiskers or particles. The size distributions of the whiskers and particles in the matrix were obtained by image analysis. It was found that failure was a result of uniform void nucleation and coalescence in the as fabricated composites, or a result of fast crack propagation initiated by a flaw developed at clusters of SiC in the aged or stretched and aged composites. The strengths of the as fabricated composites were estimated based on the results of image analysis using continuum mechanics and dislocation theories. The estimation indicated that the tensile strengths are largely contributed to by composite strengthening, supplemented by residual dislocation strengthening and work hardening. Owing to the flaw controlled failure, the tensile strengths of the aged or stretched and aged composites were independent of aging time, aging temperature, and the amount of stretching. The elastic moduli of the composites were estimated using the Halpin-Tsai model and a good correlation was found between the measured and estimated moduli. © 1996 The Institute of Materials.

The effect of microstructure on the fracture toughness of a metal-matrix composite

The fracture behaviour and plane strain fracture toughness, KIC, of four 8090-based metal-matrix composites containing 20 weight % SiC particles, 3, 6 and 23 μm in diameter, has been evaluated as a function of matrix ageing condition. Toughness values are found to be almost independent of reinforcement size. Ageing at 170°C results in a monotonic decrease in toughness with increasing strength up to the peak condition, with no subsequent recovery in toughness on overageing. However, unlike reinforced 8090, the composites are not found to be susceptible to intergranular embrittlement on overageing. The observed trends are found to be independent of reinforcement size. These findings are explained in terms of the strength, work hardening behaviour and nature and distribution of void-nucleating particles in the matrix. © 1993.

The correlation of indentation size effect experiments with pyramidal and spherical indenters

Experiments were conducted in annealed iridium using pyramidal and spherical indenters over a wide range of load. For a Berkovich pyramidal indenter, the hardness increased with decreasing depth of penetration. However, for spherical indenters, hardness increased with decreasing sphere radius. Based on the number of geometrically necessary dislocations generated during indentation, a theory that takes into account the work hardening differences between pyramidal and spherical indenters is developed to correlate the indentation size effects measured with the two indenters. The experimental results verify the theoretical correlation.

The correlation of the indentation size effect measured with indenters of various shapes

Experimental results are presented which show that the indentation size effect for pyramidal and spherical indenters can be correlated. For a pyramidal indenter, the hardness measured in crystalline materials usually increases with decreasing depth of penetration, which is known as the indentation size effect. Spherical indentation also shows an indentation size effect. However, for a spherical indenter, hardness is not affected by depth, but increases with decreasing sphere radius. The correlation for pyramidal and spherical indenter shapes is based on geometrically necessary dislocations and work-hardening. The Nix and Gao indentation size effect model (J. Mech. Phys. Solids 46 (1998) 411) for conical indenters is extended to indenters of various shapes and compared to the experimental results. © 2002 Elsevier Science Ltd. All rights reserved.

Characterization of viscoplastic yielding of asphalt concrete

A temperature and strain rate dependent yield surface model was proposed to characterize the viscoplastic yielding of asphalt concrete. Laboratory tests were conducted on specimens that have two binders, two air void contents, and three aging periods. Strain decomposition was performed to obtain viscoplastic strain and stress-pseudostrain curves were constructed to determine the model parameters accurately and efficiently. Results indicate that a stiffer asphalt concrete has greater cohesion and strain hardening amplitude, both of which decline as temperature increases or strain rate decreases. The temperature and strain rate factors of the yield surface can be accurately determined solely by the peak stress of the strength tests. © 2013 Elsevier Ltd. All rights reserved.