Fatigue properties of Al–Si casting alloy with cold sprayed Al/SiC coating

Abstract

The fatigue properties of Al–Si alloy cold sprayed Al and Al–SiC composite coatings have been studied. The specimens coated with composites reinforced with a large volume (25%) of fine SiC particles exhibited improved adhesion strength at the interface due to crater formation, and cyclic fatigue lives at room temperature more than three times those of uncoated specimens. In high temperature low cycle fatigue tests at 250°C, the pure Al coatings showed longer fatigue lives than the Al–SiC composite coatings, which is attributed to an increment in ductility at the surface retarding fatigue crack initiation.     

Performance and selectivity of the new column flotoextraction method

The main difference between the new Column Flotoextraction (CFE) method and the conventional SX and SPRAY methods is the substitution of organic phase bubble dispersion, instead of its drop dispersion into the aqueous phase. This substitution is intended to increase the contact area of phases and to enhance the buoyancy force of the organic phase. The main aims of this study are the introduction of the CFE method and the comparison of its performance and selectivity with SX and SPRAY methods. The results indicated ∼10% and ∼20% increase of cobalt (Co) extraction from dilute (100 mg Co/l) synthetic solution in A/O = 40 and ∼1% and ∼6% increase of Co extraction from concentrated (1000 mg Co/l) synthetic solution in A/O = 30 for the CFE method compared to SX and SPRAY methods, respectively, due to improved contact surface of the phases. In the same extraction time (1 min), the increase of Co extraction from dilute and concentrated solutions in the CFE method compared to the SX method were ∼16% and ∼4%, which demonstrate improved kinetics of the extraction process in CFE method. Improvement of separation factors in the CFE method relative to SX and SPRAY methods warrants the better performance of this innovative method.     

Hardness and microstructural developments of spray formed aluminium alloys with high Mg2Si content during hot extrusion and heat treatment

Abstract

Spray forming offers the possibility of producing alloys with very fine, homogeneous microstructures. Even materials with high contents of intermetallic precipitates, which cannot be produced by casting because of the high solidification rates required, can be distributed homogeneously. Alloying aluminium with high contents of Mg and Si (>20 wt-%Mg₂Si) gives an increase in stiffness plus a significant reduction in density, but a very fine distribution of the Mg₂Si particles in the aluminium matrix is required. Therefore, such alloys are commonly produced by spray forming. Post-spraying processes such as forming and heat treatment are generally carried out to optimise properties. To examine the microstructure and hardness as a result of subsequent processing, aluminium alloys with high Mg₂Si content (22–30 wt-%) have been produced under a variety of spray forming conditions. The duration and temperature of heating before extrusion were varied. In addition, some specimens were preheated without extrusion. The influence of subsequent heat treatment was investigated by varying the age hardening parameters. Hardness measurements were conducted and the distribution and size of the precipitates were evaluated by light microscopy. Image analysis was used to study the coarsening behaviour of primary Mg₂Si. The results indicate that the subsequent processing conditions have a strong influence on the microstructure and hardness of the material. Further, a significant dependence of coarsening rate during subsequent processing on the initial state of the material after spray forming was observed. Knowledge of correlations between process parameters and microstructural development offers the possibility of optimising the hot extrusion and heat treatment parameters for high Mg₂Si containing aluminium alloys.     

Spray Deposition Methods for Improving Interfacial Strength of Copper–Epoxy Systems

A simple spray method using a plain orifice atomizer has been developed for depositing γ-aminopropyltriethoxysilane (APS) from solutions in water and in methanol onto copper surfaces. The evaporative patterns of the sprayed droplets were studied to determine the distribution of deposited APS and the percent coverage of the surface. The peel strengths between copper foil and epoxy resin were measured with and without APS deposition. It was shown that the application of APS resulted in a considerable increase in interfacial adhesion. APS applied from a 1 wt% solution in methanol resulted in a higher peel strength than when applied from a 1 wt% aqueous solution; the opposite was true with 0.2 wt% APS solutions, indicating a trade-off between deposited APS film thickness and surface coverage. In all cases, a higher concentration of APS gives a higher peel strength. APS was very effective when chemisorption occurred at the surface but much less effective when only physisorption took place. A study of the fracture surfaces showed cohesive failure inside the epoxy layer, and that the deposited APS on the copper surfaces had a long-range effect which was seen deep into the epoxy layer, well away from the copper surface.     

Evolution and analysis of γ′ rafting during creep of single crystal nickel base superalloy

Abstract

By means of TEM observation and finite element analysis, an investigation has been made into the directional coarsening of the γ′ phase for a single crystal nickel base superalloy with [001] orientation during creep at 1040°C. The results show that the strain energy change related to the elastic strain is to be the driving force for γ′ rafting. The extruded strain of the lattice in the cuboidal γ′ interfaces results in a supersaturation of the elements Ta and Al of larger atomic radius. The extrusion or expansion strain in the lattice of the cuboidal γ′ planes may repel or trap these atoms to promote the directional growth of the γ′ phase into a needle-like raft structure along the direction parallel to the stress axis under an applied compression stress, or into a meshlike raft structure along the direction perpendicular to stress axis under applied tensile stress. The normal direction of the expanding lattice is supposed to be the one in which the γ′ rafts grow. The rate of γ′ rafting is enhanced by increasing viscoplastic flow in the γ matrix and elastic strain in the γ′ phase. Therefore, there is a smaller rate of growth under compressive than under tensile stress as a result of the smaller expansion strain and viscoplastic flow occurring in the former.     

Analysis of the influence of atmospheric plasma spray (APS) parameters on adhesion properties of alumina-titania coatings

Alumina-13 wt% titania wear resistant coatings were deposited using the Atmospheric Plasma Spray (APS) process under several processing conditions. Coating adhesion was then measured locally on cross sections by the indentation test and results were correlated with process variables. In order to identify the most influential factors on adhesion, artificial intelligence was used. The analysis was based on an Artificial Neural Network (ANN) taking into account training and test procedures to predict the dependences of measured property on experimental conditions. This study pointed out primarily that adhesion was largely sensitive to parameters that modified the in-flight particle characteristics (i.e. velocity and temperature). These effects were quantitatively demonstrated and predicted with an optimized neural network structure.     

Engineering nanostructured thermal spray coatings: process–property–performance relationships of ceramic based materials

Abstract

Nanostructured powders were deposited using thermal spraying to produce coatings having internal features of nanosized dimensions. Several ceramic based materials were studied, including WC–12 wt-%Co, TiO₂, hydroxyapatite, Al₂O₃–13 wt-%TiO₂ and yttria stabilised zirconia. The effect of the thermal spray conditions on the microstructure, phase composition, properties and performance was investigated. Key nanostructural features of the coatings were identified and their potential benefit in contributing to enhanced behaviour explored. Issues relating to design strategies and process control for engineering these types of coatings with performance characteristics tailored for targeted applications are discussed.     

Aspects of porosity formation in spray deposited thin aluminium strip

Abstract

A characteristic feature of the spray deposited metal strip is that it contains porosity. The present paper has reported and explained the effect of various process parameters such as atomising pressure, deposition distance, melt superheat and melt delivery tube diameter on porosity of the spray deposited thin aluminium strip. Porosity of the spray deposited aluminium strip greatly depends not only on the liquid fraction present on the substrate, but also on the thermal condition through the substrate. It has been shown that there are several types of porosity present in the strip, namely interstitial, mechanically entrapped gas, intersplat boundary, solidification shrinkage and porosity due to hydrogen evolution. The mechanism of their formation has been discussed. An increase in the initial temperature of the substrate lowers the porosity content in the spray deposited aluminium strip.     

Comparison of the Fracture Toughness and Stress Corrosion Behaviour of Four Aluminium Alloys

Abstract

The fracture toughness and stress corrosion properties of forgings in four aluminium alloys (AA2014–T6, AA7075–T73, L83 WP and DTD 5084WP) have been compared, using precracked specimens to define K_Iscc (the critical stress intensity for stress corrosion cracking) for each alloy in the short transverse direction when tested in 3% sodium chloride solution. In the T73 condition of heat-treatment, alloy AA7075 was resistant to stress corrosion whereas the other three alloys all showed susceptibility to stress corrosion, the ratio K_Iscc to K_Ic being about 0·5 for the L83 WP and about 0·7 for the other two alloys.     

Microstructure and wear characteristics of spray formed and hot extruded Al–Si alloys

Abstract

The microstructural and wear properties of spray formed Al–6.5Si, Al–18Si and Al–18Si–5Fe–1.5Cu (wt-%) alloys have been investigated. The microstructure of the Al–6.5Si alloy exhibits the equiaxed grain morphology of the primary α-Al phase with eutectic Si at the grain boundaries. The size of the primary Si particulates in the Al–18Si alloy varied from 3 to 8 μm embedded in the eutectic matrix. Complex intermetallic phases such as β-Al₅ SiFe and δAl₄ Si₂ Fe are observed to co-exist with primary Si in the spray formed Al–18Si–5Fe–1.5Cu alloy system. The periphery of the preforms invariably showed pre-solidified particles with a large amount of interstitial pores. An extrusion ratio of 6 : 1 for these alloys led to drastic porosity reduction and extensive breaking of second phase particles. These microstructural features showed distinct variation in the wear behaviour and the coefficient of friction of the alloys. The Al–18Si–5Fe–1.5Cu alloy shows better wear resistance compared with the other two alloys, particularly at higher loads. The coefficient of friction shows a dependence upon the applied load. However, this becomes steady at higher loads. The wear behaviour of these alloys is discussed in light of the morphology of debris particles as well as that of the worn surfaces.