Aerospace Technic and Technology

The one-dimensional gas-dynamic model for calculation of acceleration and heating of particles which takes into account space from the nozzle outlet to the substrate has been improved. One cycle of particle acceleration by a gas flow can be divided into three parts: mixing a gas flow with powder; particle movement and acceleration in the divergent part of the nozzle; the movement of gas-powder flow from nozzle outlet to substrate. It is known that cold spray coating formation depends on the normal component of particle velocity towards the surface to be sprayed. Each material obtains its value of the critical velocity when the coating formation process starts. At particle velocities above critical, they adhere to the substrate and form coating due to plastic deformation of particles, and at velocities below critical value surface erosion or spraying with low efficiency is observed. One of the features of the process of cold gas-dynamic spraying is a relatively small distance between the nozzle outlet and the deposited surface, which leads to the occurrence of the reverse flow of the gas stream (bow shock) reflected from the substrate. The reflected flow significantly inhibits the trajectory of particles of sprayed powder which need to be investigated. Impact temperature and velocity of aluminum and nickel particles with size 25 microns with a substrate for SK-20 supersonic nozzle of DIMET-405 low-pressure cold spraying machine has been calculated. Although the one-dimensional isentropic gas-dynamic model, which is usually used to calculate flow parameters, describes flow only along the axis of the nozzle, excluding heat transfer with nozzle and friction loss on the inner walls, which leads to overestimated results of calculations, its utilization allows to optimize the geometry of the nozzle channel and develop a technological process of the spraying process. Mathematical modeling of two-phase flow dynamics of the cold spraying process was performed using the MATLAB software. Comparison of simulation results with experimental data to determine the flow velocity and temperature showed that the theoretical calculations differ from the experimental ones by no more than 10 %.

cold gas-dynamic spraying; coatings; spraying parameters; particles temperature and velocity; critical velocity

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