Bonding mechanism from the impact of thermally sprayed solid particles

Power particles are mainly in solid state prior to impact on substrates from high velocity oxy-fuel (HVOF) thermal spraying. The bonding between particles and substrates is critical to ensure the quality of coating. Finite element analysis (FEA) models are developed to simulate the impingement process of solid particle impact on substrates. This numerical study examines the bonding mechanism between particles and substrates and establishes the critical particle impact parameters for bonding. Considering the morphology of particles, the shear-instability–based method is applied to all the particles, and the energy-based method is employed only for spherical particles. The particles are given the properties of widely used WC-Co powder for HVOF thermally sprayed coatings. The numerical results confirm that in the HVOF process, the kinetic energy of the particle prior to impact plays the most dominant role in particle stress localization and melting of the interfacial contact region. The critical impact parameters, such as particle velocity and temperature, are shown to be affected by the shape of particles, while higher impact velocity is required for highly nonspherical powder

Estimation of cooling rates during close-coupled gas atomization using secondary dendrite arm spacing measurement

Al-4 wt pct Cu alloy has been gas atomized using a commercial close-coupled gas-atomization system. The resulting metal powders have been sieved into six size fractions, and the SDAS has been determined using electron microscopy. Cooling rates for the powders have been estimated using a range of published conversion factors for Al-Cu alloy, with reasonable agreement being found between sources. We find that cooling rates are very low relative to those often quoted for gas-atomized powders, of the order of 10 K s for sub-38 μm powders. We believe that a number of numerical studies of gas atomization have overestimated the cooling rate during solidification, probably as a consequence of overestimating the differential velocity between the gas and the particles. From the cooling rates measured in the current study, we estimate that such velocities are unlikely to exceed 20 m s

Numerical modelling of droplet break-up for gas atomisation

High-pressure gas atomisation (HPGA) technology has been widely employed as an effective method to produce fine spherical metal powders. The physics of gas atomisation is dominated by rapid momentum and heat transfer between the gas and melt phases, and further complicated by break-up and solidification. A numerical model is developed to simulate the critical droplet break-up during the atomisation. By integration of the droplet break-up model with the flow field generated high-pressure gas nozzle, this numerical model is able to provide quantitative assessment for atomisation process. To verify the model performance, the melt stream is initialized to large droplets varying from 1 to 5 mm diameters and injected into the gas flow field for further fragmentation and the break-up dynamics are described in details according to the droplet input parameters

Computational validation of an isentropic plug nozzle design for gas atomisation

During high pressure gas atomisation (HPGA), the molten metal stream is disintegrated to produce spherical powders when energy is transferred from the gas to the melt. Conventional annular-slit nozzle (ASN) in close-coupled atomisation generates an under-expanded gas jet with characteristic shock waves which consume a great deal of energy through expansion. An isentropic plug nozzle (IPN) is developed in this paper to reduce the shocks and maximize kinetic energy being transferred from the gas to instablize the melt stream. The performance of the IPN is examined using a numerical model which includes gas flow dynamics, droplet break-up mechanism and particle tracking. The numerical results demonstrate a good improvement of gas dynamics and powder yield from the IPN design in comparison with the ASN, in particular when hot gas is employed

Computational simulation of metal droplet break-up, cooling and solidification during gas atomisation

In an atomisation process for powder production, metal droplets go through undercooling, recalescence, peritectic and segregated solidification before fully solidified. The cooling process is further complicated by droplet break-up during the atomisation. This paper describes a numerical model which combines both cooling and break-up in a single computation. The dynamic history of droplets is solved as discrete phase in an Eulerian gas flow. The coupling between droplet and gas flows are two-way, in which the heat and momentum exchanges affecting the gas flow are treated as source/sink terms in the fluid equations. The droplet models were employed a gas atomisation process for metal powder production and good agreement is achieved with the results in open literature. The model results further confirm that thermal history of particles is strongly dependent on initial droplet size. Large droplets will not go through undercooling while small droplets have identifiable stages of undercooling, unclearation and recalescence. The predictions demonstrate that droplets have very similar profiles during gas atomisation and the major factor influencing the atomisation and solidification process of droplets are in-flight distance

Computational validation of an isentropic plug nozzle design for gas atomisation

During high pressure gas atomisation (HPGA), the molten metal stream is disintegrated to produce spherical powders when energy is transferred from the gas to the melt. Conventional annular-slit nozzle (ASN) in close-coupled atomisation generates an under-expanded gas jet with characteristic shock waves which consume a great deal of energy through expansion. An isentropic plug nozzle (IPN) is developed in this paper to reduce the shocks and maximize kinetic energy being transferred from the gas to instablize the melt stream. The performance of the IPN is examined using a numerical model which includes gas flow dynamics, droplet break-up mechanism and particle tracking. The numerical results demonstrate a good improvement of gas dynamics and powder yield from the IPN design in comparison with the ASN, in particular when hot gas is employed

Numerical modelling of metal droplet cooling and solidification

In an atomisation process for power production, metal droplets go through undercooling, recalescence, peritectic and segregated solidification before fully solidified. The cooling process is further complicated by droplet break-up during the atomisation. This paper describes a numerical model which combines both cooling and break-up in a single computation. The dynamic history of droplets is solved as discrete phase in an Eulerian gas flow. The coupling between droplet and gas flows are two-way, in which the heat and momentum exchanges affecting the gas flow are treated as source/sink terms in the fluid equations. The droplet model is employed to a gas atomisation process for metal powder production and good agreement is achieved with the results in open literature. The model results further confirm that thermal history of particles is strongly dependent on initial droplet size. Large droplets will not go through undercooling while small droplets have identifiable stages of undercooling, unclearation and recalescence. The predictions demonstrate that droplets have very similar profiles during gas atomization and the major factor influencing the atomization and solidification process of droplets are in-flight distance

Three-dimensional simulation of primary break-up in a close-coupled atomizer

Until now much of the modelling activity around close-coupled gas atomization has been mainly focused on gas-only flow with discrete phase interaction using Lagrangian-based models. However, this approach is unable to supply valuable information regarding the primary break-up mechanism of the melt being injected. Furthermore, much of existing numerical work is based on two-dimensional axisymmetric geometries, and therefore suffers the absence of three-dimensional flow features. In order to overcome these aspects the authors have carried out an analysis using a three-dimensional geometry by means of an Eulerian, Volume of Fluid, model to accurately present the early stages of melt stream behaviour at the atomizer’s melt inlet. The study investigates the mechanisms associated with primary break-up, and the results obtained highlight three modes under which a close-coupled atomizer may operate