S. V. Klinkov

The features of cold spray nozzle design

This article presents the peculiarities of the supersonic nozzle design for the cold gas-dynamic spraying. The procedure to produce the high particle velocity by correct choice of the geometrical dimensions of the accelerating nozzles is described. Numerical and experimental research of wedge-shaped nozzles shows that there is a nozzle with its particular dimensions for a given type of particles that produces the maximum possible particle velocity at the moment of impact on a target surface.

On some aspects of gas dynamics of the cold spray process

This paper presents an overview of results of recent studies conducted at the Institute of Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Science in the field of gas dynamics and heat transfer of the supersonic air jet under conditions typically used in the cold spray process. These studies are related to various aspects of the problem including a flow in the nozzle and the outflow of the jet, as well as effects of the interaction of the jet with a flat obstacle. They are conducted with a supersonic nozzle with a rectangular section at the exit with a Mach number M0 between 2 and 3.5. The gas flow in the nozzle is theoretically and experimentally studied. It is shown that the boundary layer on the walls of the nozzle affects significantly the flow parameters (for example, Mach number M, pressure p, temperature T, and density ρ of the gas). A method of calculation of the gas parameters in the flow core of the nozzle is suggested, and it is shown that they depend mainly on the ratio of the nozzle width to its length. The results of the investigation of the supersonic air jets with stagnation temperature ranging from 300–600 K flowing in the atmosphere are presented. The corresponding dimensions of the jets, profiles, and axial distributions of the gas parameters are obtained. The interactions of the supersonic jet with the flat obstacle are studied. Self-similarity of the distribution of the pressure and of the Mach number on the obstacle surface is shown for the jets with various values of the Mach number and the angle of impingement. The oscillation regimen of the jet impingement, as well as a compressed layer structure is observed with the aid of a Schliren visualization technique. Some problems of heat exchange of the jets with the obstacle are considered. Distributions of stagnation temperature and heat exchange coefficient in the near-wall jet are obtained. The temperature of the obstacle for the stationary case is calculated, and it is shown that for heat conductive materials the surface temperature is lower than the stagnation temperature due to the redistribution of heat inside of the substrate.

Measurements of Cold Spray Deposition Efficiency

An analytical model of the kinetics of coating formation during cold spray is presented. The model is used to correct experimental data on deposition efficiency. The experimentally observed values are shown to be affected by experimental conditions, such as the velocity of substrate motion, the number of passes, the mass of a single portion of powder, and the exposure time of a given surface section. It is noted that experimental conditions can exert a significant effect on the consequences of the high-speed interaction of particles with a substrate. Relations are suggested that allow one to correct the results of deposition efficiency determined experimentally and to avoid mistakes in interpreting the data obtained.

Experimental study of deformation and attachment of microparticles to an obstacle upon high-rate impact

Results of an experimental study of the high-rate (400–1200 m/sec) interaction of spherical aluminum particles with a surface are given. Particle deformation was studied by means of a microscope. The mean values of the degree of particle strain were determined by statistical processing for specimens with different hardness (hardened and unhardened steel and copper) and produced in different spraying regimes (the pressure and temperature of a gas in a plenum chamber and a working gas). A relation between the degree of particle strain and the impact velocity was obtained by using the design particle velocities for the corresponding parameters.

Recently Patented Facilities and Applications in Cold Spray Engineering

Currently available facilities in Cold Spray are limited in use because of some unwanted features. The first feature is powder feeding into working gas, possibly uniform heating and acceleration up to necessary temperature and velocity. The second is connected with a dilemma service term/cost of working nozzle production. Another problem is collection of un-deposited powder. It allows cost-effective production of articles by re-using the powder and prevents the environment pollution. For success marketing there is a need for new applications including those where Cold Spray that from can be embedded (build in) easily and cost-effective. In the present paper, the most important patents published since the last 3 years and claiming the solutions of typical problems in convenient Cold Spray devices allowing increasing effectiveness and reducing the costs of both the devices units and products, prolong service term and searching new application areas are reviewed. Modernization has reached by material replacing, geometry, and embedding additional units into main design. Concerning new applications the patents cover the area connected with the production of corrosive, electrical/magnetic and other barrier coating articles.

Cold Spraying Activation Using an Abrasive Admixture

In the present study, on the basis of a two-probability model we theoretically analyze the influence of surface erosion and activation due to abrasive particles impacting on the substrate (coating) surface on the total deposition efficiency of a sprayed mixture of abrasive and metal particles. Relations for the total deposition efficiency derived with regard to the activation effect due to abrasive particles were obtained. In the experiments, the deposition efficiency was measured as a function of the abrasive (SiC) concentration in the initial mixture with copper powder, the size of the abrasive particles, and the working-gas (air) stagnation temperature. Modeling data are compared to the present and previously published experimental data, and a good agreement between the data is shown.

Current status of the cold spray process

This chapter focuses on several researches undertaken in the areas—the gas-dynamics of cold spray, interaction of a high-speed particle with the substrate and bonding mechanism, and related technologies and applications. Obtaining coatings by the cold spray method involves the use of a high-velocity gas flow for the accelerating and heating of particles. The particle velocity that can be reached in a cold spray facility is limited by the gas velocity. Owing to the use of high gas pressures, long nozzles, and fine particles, the particles move with a velocity close to the gas velocity that can be increased by using gases with low molecular weights and by gas heating. This approximation implies that the gas flow in a converging/diverging nozzle is isentropic and one dimensional. The gas is perfect and has a constant ratio of specific heats. Based on isentropic computations, it is concluded that the particle temperature is almost independent of pressure, but the particle velocity increases by 15% if the pressure is doubled.

Discovery of the cold spray phenomenon and its basic features

This chapter addresses the experiments that resulted in the discovery of the cold spray process and established the basic physical principals of the process. Two aspects of the cold spray process are important for better understanding and improving the process. First is the physics of high speed particle impact to explain bonding mechanism, and second is the gas-dynamics to optimize spray parameters and provide as high particle velocity as possible. Cold spray process offers many advantages that helps in producing and repairing a wide range of industrial parts, such as turbine blades, pistons, cylinders, valves, rings, bearing components, pump elements, sleeves, shafts, and seals for many industries. Various coatings may add strengthening, hardening, wear resistance, corrosion resistance, electro-magnetic conductivity, thermal conductivity, and other properties.

Gas-dynamics of cold spray

This chapter discusses the gas-dynamics and thermal effects associated with a supersonic jet exhausting from the nozzle and its interaction with the substrate in the cold spray method. To accelerate particles, two types of nozzles are used—nozzles with circular and rectangular sections. The chapter investigates the gas-dynamics of jets exhausting from conical nozzles with circular cross sections—symmetrical-axis flows. An analysis of the features of such jets, as applied to the cold spray method, reveal that the use of nozzles with rectangular cross sections is also significant. With the same ratio of the nozzle-exit and throat cross sections, nozzles with a rectangular section can provide a wider spray beam in the direction of the smaller size of the section and a narrower beam in the direction of the larger size of the section. Such nozzles can also decrease the effect of particle deceleration in the compressed layer in front of the substrate by decreasing the thickness of the layer itself. The issues of acceleration of finely dispersed particles in supersonic nozzles and formation of flat and comparatively thin two-phase jets, which ensure high deposition efficiency over the area, are of significant interest in the process of cold spray. The chapter concludes that the particle velocity reached immediately before the impact on the substrate plays the most important role in deposition of coatings by cold spray method.

Preliminary study of cold spraying using radial supersonic nozzle

The paper deals with the problem of coating production on internal pipe surfaces by the cold spraying. It reports on a new assembly design with a radial supersonic nozzle that greatly facilitates the application of aluminium, copper and nickel coatings onto internal pipe surfaces. Visualisation and Pitot tube measurements of the airflow produced by the nozzle assembly assist to predict the range of internal pipe diameters, in which the pipes could be successfully coated from the inside using the designed instrument. The velocity of aluminium particles has been measured at the exit from the radial nozzle and calculated the particle velocities under the actual cold spray conditions. Finally, the mass and energy consumption characteristics of the process and the deposition efficiencies achievable under typical experimental conditions have been calculated. Those calculations have allowed us to evaluate the cost of potential introduction of the cold spray process into pipe production.