Effect of deposition conditions on the properties and annealing behavior of cold-sprayed copper

The deposition of copper by cold gas dynamic spraying has attracted much interest in recent years due to the capability to deposit low-porosity oxide-free coatings. However, it is generally found that as-deposited copper has a signicantly greater hardness, and potentially lower ductility, than bulk material. In this article, copper was deposited by cold spraying using helium as the driving gas at both 298 and 523 K. Evidence is presented indicating that the material sprayed at the lower temperature exhibits a lower dislocation density throughout the grain structure than the material sprayed at the higher temperature. The low stacking fault energy of copper restricts recovery during annealing, and thus microstructural changes during annealing only proceed once recrystallization begins. The material sprayed at low temperature (with the low dislocation density) exhibited recrystallization at annealing temperatures as low as 373 K with a corresponding reduction in hardness. However, the copper sprayed with helium at 523 K was resistant to annealing at temperatures up to 473 K where the dislocations in the structure prevented recrystallization. However, at higher temperatures, recrystallization did proceed (with corresponding reductions in hardness). The fracture behavior of the copper that was cold sprayed with helium at 523 K, both in the as-sprayed condition and following annealing, was measured and explained in terms of the annealing mechanisms proposed. The original version of this paper was published in the CD ROM Thermal Spray Connects: Explore Its Surfacing Potential, International Thermal Spray Conference, sponsored by DVS, ASM International, and HW International Institute of Welding, Basel, Switzerland, May 2–4, 2005, DVS-Verlag GmbH, Düsseldorf, Germany.     

A model to predict the evolution of pitting corrosion and the pit-to-crack transition incorporating statistically distributed input parameters

A model based on deterministic equations with statistically distributed input parameters has been developed for simulating the evolution of the pit depth distribution at different exposure times and the percentage of pits that transform to stress corrosion cracks. The model has been applied to the specific case of steam turbine disc steel exposed to a range of environments under applied stress. With preliminary fitting at one exposure time, the simulation not only reflects the trends in the experimental measurement but also the model, uniquely, reproduces the statistical variability or “noise” associated with the measurements.     

Effect of cold spray deposition of a titanium coating on fatigue behavior of a titanium alloy

The deposition of titanium on a titanium alloy substrate is being examined for potential use as a surface treatment for medical prostheses. A Ti6Al4V alloy was coated with pure titanium by cold gas dynamic spraying. Coatings were deposited onto samples with two different surface preparation methods (as-received and grit-blasted). The fatigue life of the as-received and grit-blasted materials, both before and after coating, was measured with a rotating-bend fatigue rig. A 15% reduction in fatigue endurance limit was observed after application of the coating to the as-received substrate, but no significant reduction was observed on its application to the grit-blasted substrate. The reduction in fatigue endurance limit has been related to the substrate-coating interface properties, the elastic modulus, and the residual stress states. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference, (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, 2006.     

Twelve years of postgraduate education in desalination

The Master of Engineering course in Desalination Technology has now 130011 running at the University of Glasgow for Twelve years. It has had a total throughput of 82 students from all over the world. This paper describes -the development of the course over this period, and discusses some of the lessons learnt and the experience gained. Various components of the desalination. education 'market' are identified and their differing, and often conflicting, requirements are discussed. In conclusion some thoughts on future trends and training requirements are presented.     

Intermetallic phase formation in directionally solidified Al−Si−Fe alloy

The present study has been undertaken to better understand the solidification behavior of Al−Si−Fe alloys containing 7wt.% Si and 0.9wt.% Fe, with particular regard to the formation of phase during controlled solidification and influence of growth rates on intermetallic phase selection. The alloys studied were Al-7Si-0.9Fe alloys, which were produced by a modified Bridgman solidification arrangement. These alloys were solidified unidirectionally with different growth rates (1–30mm/min). The solidified microstructure of these alloys consists of the growth of primary aluminum and multiple second phase reactions.     

Numerical modeling of in-flight characteristics of inconel 625 particles during high-velocity oxy-fuel thermal spraying

A computational fluid dynamics (CFD) model is developed to predict particle dynamic behavior in a high-velocity oxyfuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a combustion chamber linked to a long, parallel-sided nozzle. The particle transport equations are solved in a Lagrangian manner and coupled with the two-dimensional, axisymmetric, steady state, chemically reacting, turbulent gas flow. Within the particle transport model, the total flow of the particle phase is modeled by tracking a small number of particles through the continuum gas flow, and each of these individual particles is tracked independently through the continuous phase. Three different combustion chamber designs were modeled, and the in-flight particle characteristics of Inconel were 625 studied. Results are presented to show the effect of process parameters, such as particle injection speed and location, total gas flow rate, fuel-to-oxygen gas ratio, and particle size on the particle dynamic behavior for a parallel-sided, 12 mm long combustion chamber. The results indicate that the momentum and heat transfer to particles are primarily influenced by total gas flow. The 12 mm long chamber can achieve an optimum performance for Inconel 625 powder particles ranging in diameter from 20 to 40 μm. At a particular spraying distance, an optimal size of particles is observed with respect to particle temperature. The effect of different combustion chamber dimensions on particle dynamics was also investigated. The results obtained for both a 22 mm long chamber and also one with a conical, converging design are compared with the baseline data for the 12 mm chamber.     

High-velocity oxyfuel reactive spraying of mechanically alloyed Ni-Ti-C powders

Recently, there has been considerable interest in producing cermet coatings with nanoscale carbide grains in the size range 50 to 500 nm. In this article, the production of nanoscale TiC grains in a Ni-based alloy matrix by reactive high-velocity oxyfuel (HVOF) spraying of metastable Ni-Ti-C powder is reported. Mechanical alloying of a Ni(Cr) prealloyed powder and Ti and C elemental powders was performed in a planar-type ball mill, and materials were characterized in detail using x-ray diffraction (XRD) and scanning electron micros-copy (SEM). Phase changes were correlated with milling time and other processing conditions. Results show that, by the selection of appropriate conditions, a metastable Ni-Ti-C powder could be obtained with the nominal composition 50wt.%Ni-40wt.%Ti-10wt.%C. Following sieving and classification, powder was produced with a particle size range of −38 to 8 μm, which is suitable for HVOF spraying. Coatings, approximately 250 μm thick, were deposited by HVOF spraying onto mild steel substrates, and the microstructures formed were investigated. XRD showed that a self-propagating high-temperature synthesis (SHS) reaction had occurred in the powder particles during spraying and that the principal phases present in the coating were TiC and a Ni-rich solid solution; small quantities of NiTi, TiO₂, and NiTiO₃ were also present. SEM revealed that the coatings had a characteristic, splatlike morphology and that TiC formed as a nanoscale dispersion, with a size range of ∼50 to 200 nm, within solidified splats. The microstructures of these reactively sprayed Ni-TiC coatings are briefly compared with those observed in HVOF-sprayed coatings deposited using prereacted SHS powder. The original version of this paper was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

A theoretical explanation of the delaying effects of overloads on fatigue crack propagation

A simple theoretical model valid for generalized plane stress conditions is described which is able to predict the effects of overloads on crack tip deformation and fatigue crack propagation rates. The model is based upon a method of superimposing crack problems in an attempt to represent the effect of residual stresses in the plastic zones. An expression for the crack opening displacement is derived which is valid whilst the crack tip is propagating through the overload plastic zone. Crack tip closure is predicted during stress cycling and, following tradition, this is regarded as reducing the effect of the stress range intensity factor on crack propagation rate thus causing retardation. The model also predicts that, whenever a simple condition involving applied stress ratios is satisfied, crack arrest will take place. In keeping with experimental observations the model predicts that during its propagation through the overload plastic zone the crack growth rate first declines to a minimum and then increases to the appropriate value which would result if the overload had not been applied. In certain circumstances the model also predicts some acceleration effects and delayed retardation.It is shown how to construct delay maps characterizing the effect of applied stress ratios on growth rate, independent of specimen geometry whenever small scale yielding conditions prevail. Such delay maps, when used in conjunction with uniform amplitude fatigue crack growth data, offer a very convenient method of predicting delay for practical situations.

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Résumé On décrit un modèle théorique simple valable pour les conditions généralisées d'état plan de tension et à même de prédire les effets de surcharge sur la déformation d'une extrémité d'une fissure ainsi que sur les vitesses de propagation d'une fissure de fatigue. Le modèle est basé sur une méthode de superposition de problèmes de fissure en vue de représenter l'effet de contrainte résiduelle dans les zônes plastiques. Une expression du COD est dérivée, valable au cours de la propagation de l'extrémité de la fissure au travers de la zône de déformation plastique correspondant à la surcharge. La fermeture de l'extrémité de la fissure est prédite au cours d'un cyclage de la contrainte et, conformément à la tradition, ceci est considéré comme un effet réducteur du facteur d'intensité de contrainte sur la vitesse de propagation de la fissure, causant ainsi un retard de propagation. Le modèle prédit également que, pour autant qu'une condition simple impliquant les rapports des contraintes soit satisfaite, il se produira un arrêt de la fissure. En considérant les observations expérimentales, le modèle prédit qu'au cours de la propagation au travers de la zône de déformation plastique correspondant à la surcharge, la vitesse de croissance de la fissure passe d'abord par un minimum et s'accroit ensuite jusqu'à une valeur adéquate qui correspondrait aux conditions où ne se produirait pas de surcharge. Sous certaines circonstances, le modèle prédit également quelques effets d'accélération ainsi que des ralentissements différés. On montre la manière de construire des cartes de retard caractérisant l'effet des rapports des contraintes appliquées sur la vitesse de croissance et ce indépendamment de la géométrie de l'éprouvette, pour autant que prévallent des conditions d'écoulement plastique à faible échelle. De telles cartes de retard, lorsqu'elles sont utilisées en complément de données sur la croissance de fissures de fatigue sous amplitude uniforme, constituent une méthode très commode pour prédire les délais qui peuvent se produire dans des situations pratiques.