RU2099442C1 - Method of gas-dynamic application of powder material coating - Google Patents

Abstract

FIELD: application of coatings. SUBSTANCE: gas-dynamic method of compacting of refractory material powder is effected in cyclic conditions by alternation of the working and preparatory processes. The working process may last for about several seconds, and the preparatory process - for about a minute. The preparatory period is used for accumulation of energy and accomplishing of relaxation processes required for preparation of equipment members to the next spraying process. EFFECT: reduced process expenditures and reduced price of products. 3 dwg

Description

 The invention relates to the field of powder metallurgy, in particular to a technology for molding products from powder materials and to a coating technology.

 Known methods of spraying coatings using gas-plasma, plasma, electric arc and detonation units [1] In gas-plasma and plasma devices, heating and acceleration of the powder is carried out using a high-temperature gas or plasma jet. In electric arc devices, the sprayed metal is melted in an electric arc between converging wires and dispersed by a stream of air. In detonation plants, the powder is heated and accelerated by gas detonation products.

 A common distinguishing feature of these methods is that the adhesion of particles to the surface and between themselves and the formation of a monolithic layer occurs due to welding of colliding molten particles. Using the above-mentioned plants, coatings are applied to restore the dimensions of parts and eliminate surface defects, as well as to protect against corrosion and impart special physical or mechanical properties to the surface. The technological process of spraying includes cleaning the surface, forming the necessary roughness on it, applying a sublayer, etc.

 Forming of finished products or workpieces of parts by thermal methods is carried out by sequential spraying of layers on the workpiece, which is then smelted or removed mechanically. For spraying, metal or composite powders with particle sizes from 20 to 150 micrometers are used. Smaller particles are destroyed at high temperature, and larger ones do not have time to warm up before melting.

 The main problems of using gas thermal methods are associated with harmful gas emissions, noise and radiation exposure to maintenance personnel, the complexity and high cost of individual components of technological equipment, and their short service life. As a rule, the spraying process is carried out in an inert atmosphere in order to reduce the influence of high temperature on the powder and the product.

The speed of the powder particles when spraying with an electric arc metallizer, a gas flame burner or a plasma torch with a subsonic nozzle does not exceed 150 m / s. In a detonation installation, it reaches 600-800 m / s, due to which a higher strength and density of the coating is ensured (up to 20-30 kgf / mm ² compared to 3-5 kgf / mm ² with plasma spraying).

 Detonation technology appeared about 20 years ago. A little later, in the early eighties, gas-plasma and plasma heads appeared with supersonic nozzles (Loval nozzles). As an example, one of such devices is a nozzle with a combustion chamber, taken from the description of the Browning patent [2]. FIG. 1 shows a general view of a Browning device with which it is possible to spray both powder and wire materials. In the Browning device, the wire is melted and dispersed or the powder is heated in a prechamber, where gaseous products of combustion are fed from the combustion chamber through the corresponding channels. The nozzle channel profile and nozzle length are selected so that the molten particles cannot come into contact with its walls. At the exit, the particles acquire a velocity of the order of several hundred meters per second. When hitting a substrate or hardened layer of an already sprayed powder, metal droplets well fill surface irregularities. Keep in mind that at too high a speed, droplet splashing and deterioration in mold quality can occur. Certain difficulties in using a supersonic nozzle arise also due to the inability to completely eliminate the sticking of the powder material inside the nozzle. Introducing the powder into the supersonic jet at the outlet is inefficient due to a sharp decrease in the acceleration section and the destruction of the jet. Therefore, a contradiction arises between the desire to introduce powder into the gas before the critical section and the need to eliminate the possibility of collision of accelerated liquid particles with the walls of the channel along its entire length.

 In the description of Browning's invention, these problems are specified, but not resolved. The way out of this situation is opened by the spraying method, in which the powder does not heat up to the molten state. The idea of the possibility of "cold welding" of small metal particles with high-speed collision with a solid surface was expressed in Shestakov’s invention back in 1967 [3] The proposal for cold welding of particles in dynamic mode was not developed at the time. Apparently, this is due to the fact that for the implementation of the cold spraying mode, new proposals were needed on the design of the nozzle assembly. Shestakov himself believed that his method was feasible with the help of known devices (sprayers, shot blasts, sandblasting machines, etc.) with "a slight modernization of them."

The phenomenon of cold gas-dynamic spraying was rediscovered independently at the Institute of Theoretical and Applied Mechanics SB RAS in the early 80s. In experiments on the study of supersonic flow around bodies of revolution in a wind tunnel, where finely dispersed aluminum powder is injected into it for visual flow, the formation of dense aluminum coatings on the frontal surfaces was observed. Later, the discovered phenomenon was studied purposefully. It turned out that a similar coating is formed when a supersonic gas-powder jet blowing out from a nozzle of large elongation is inflated onto a solid surface. The principles of calculation and design of the nozzle assembly were developed. It was found that the methods found are suitable for a wide range of powder metals, including sufficiently refractory ones, such as iron, titanium, molybdenum, etc. It turned out that most metals are sprayed in a stream of light gas (helium or hydrogen) at room temperature, and a stream of air or nitrogen at a temperature of 100-400 ^o C. For example, aluminum is well sprayed with air at a temperature of 100-150 ^o C. The main provisions of gas-dynamic spraying are reflected in [4] FIG. Figure 2 shows one of the possible schemes for the installation of gas-dynamic spraying, which can also be used for molding products. It consists of a source of compressed air (compressor), a powder feeder, a nozzle assembly, a heater, a spraying chamber and a device for carrying out the relative movement of the nozzle and the sprayed part. The actual installation should also have a device for cleaning exhaust gases and other auxiliary units.

 In this description, the cited invention [4] is used as a prototype.

 When gas-dynamic spraying, a supersonic nozzle is used, in some respects similar to the patented Browning (Fig. 1). The most significant differences from the Browning device are in the elongation (in the ratio of the transverse dimension and the length of the nozzle) and in the form of a cross section of the nozzle channel in the supersonic section. In gas-dynamic spraying, a nozzle with an elongation of more than 30 is used. The cross section at the exit has a rectangular or oval shape with a large aspect ratio. A large elongation is necessary to ensure a high particle velocity, and the specified cross-sectional shape is chosen in order to reduce the zone of inhibited gas in front of the surface where the particles reduce speed.

 In FIG. Figure 3 shows one of the possible schemes of the nozzle assembly for spraying an anti-corrosion aluminum or zinc coating. Heated air and powder are fed into the prechamber. Honeikomb is necessary for crushing large eddies of air. Cooling the nozzle reduces the likelihood of powder sticking inside it.

A nozzle for spraying aluminum with a particle size of 5-10 μm can have a critical section of about 10 mm ² and a supersonic expanding part with a length of about 120 mm with an exit section of 3 x 10 mm. The pressure and air temperature at the same time adhere to the level of 2.5 MPa and 200 ^o C.

 The problems of the gas-dynamic molding or spraying method increase with increasing particle sizes, especially at high melting points.

 In order to ensure the optimal (in this case) combination of speed and temperature parameters of the gas-powder jet, while remaining in the mode of unmelted powder particles, powerful nozzle devices and auxiliary equipment are needed.

,
где m масса частицы, F ее модель, V_p ее скорость, C_x коэффициент сопротивления, ρ и V плотность и скорость газа, C_x, r, V в данной задаче постоянные величины.We give some estimates. It can be approximately assumed that the gas flow in the supersonic section of the nozzle and behind the exit section has a constant velocity, density and temperature. In this section, the particles acquire the necessary speed due to aerodynamic drag. The equation describing the acceleration of a particle has the form

,
where m is the mass of the particle, F is its model, V _{p is} its speed, C _{x is} the drag coefficient, ρ and V are the density and velocity of the gas, C _x , r, V in this problem are constant values.

Здесь a=3/4 C_x ρ/ρ_p,ρ_p плотность материала частицы;

,
где V_рк скорость частицы в конце разгонного участка; d диаметр частицы. Для газодинамического компактирования стального порошка с размером частиц 50 мкм, при условии, что они нагреты до температуры не менее 600^oC, необходима скорость порядка 600-800 м/с. (В этом случае полная энергия каждой частицы равна ее тепловой энергии при температуре плавления). Если в качестве ускоряющего газа используется воздух или азот при числе Маха М 20, при температуре и давлении торможения 2000^oK и 4 МПа, то длина разгонного участка будет порядка 0,4-0,6 м. Следовательно, сопло будет иметь сверхзвуковой канал примерно такой же длины.Integrating the given differential equation under the condition of zero initial speed, we obtain the formula for calculating the length of the acceleration section:

Here a = 3/4 C _x ρ / ρ _p , ρ _{p is the} density of the particle material;

,
where V _{pk is} the particle velocity at the end of the acceleration section; d particle diameter. For gasdynamic compaction of steel powder with a particle size of 50 μm, provided that they are heated to a temperature of at least 600 ^o C, a speed of the order of 600-800 m / s is required. (In this case, the total energy of each particle is equal to its thermal energy at the melting temperature). If air or nitrogen is used as the accelerating gas at a Mach number of M 20, at a braking temperature and pressure of 2000 ^° K and 4 MPa, then the acceleration section will be of the order of 0.4-0.6 m. Therefore, the nozzle will have a supersonic channel of approximately same length.

In order for the gas not to brake due to friction against the walls, the critical section of the nozzle should be of the order of 100 mm ² . In this case, the flow rate of compressed air will be about 0.35 kg / s (20 m ³ / min under normal conditions).

. Однако на практике такой путь не всегда приемлем из-за очень высокого давления (10 МПа).It is possible to reduce nozzle dimensions and reduce gas consumption by increasing the pressure in the prechamber. Indeed, with an increase and a proportional decrease in all nozzle sizes, a similarity is observed in Mach and Reynolds numbers, which means the similarity of the temperature velocity fields and gas densities in all sections of the nozzle, including the outlet section. The velocity of the powder particles in the outlet section is proportional to the length of the nozzle and the density of the gas. Since the product of the characteristic length and the characteristic gas density in the case under consideration retains its value, the particle velocity at the exit also does not change. (Strictly speaking, it changes, but by a small amount, due to non-compliance with the similarity in the flow around particles, which leads to C _x ≠ const). The gas flow rate in the short nozzle turns out to be many times less than its length. This is easy to show, given that

. However, in practice, such a path is not always acceptable due to very high pressure (10 MPa).

 The way out of this situation is possible by organizing a cyclic mode of operation. It is proposed to alternate the processes of spraying and preparation for spraying. The spraying preparation mode is used to store energy, as well as to carry out certain relaxation processes necessary for preparing equipment elements for the next spraying process.

 As an example, consider a cycle consisting of a spraying process lasting 6 s and a preparation process lasting 60 s (1 min). During the preparation, compressed air accumulates in the receiver, heat is accumulated in heat exchangers (heaters), heat is removed from the heated nozzle, the workpiece is cooled, etc. Thermal and mechanical processes according to the proposed scheme are cyclical. This, the proposed scheme differs from the scheme of continuous stationary action. The units of the cyclic action installation operate in an unsteady short-periodic mode, therefore their design is simpler according to the principle of operation and execution. For example, the nozzle cooling system, which in the case of continuous operation of the installation should provide a very large specific heat removal, can be loaded with a much smaller average heat flow during cyclic operation if the mass of the nozzle is used as a buffer thermal capacity. The capacity of a compressor with a receiver in a cyclic installation is 11 times less than its required capacity for continuous operation. Similar considerations can be made with respect to heaters.

 The preparatory process from each cycle can be combined with the process of installing the nozzle assembly in a new position. For example, when spraying the necks of the crankshaft in a plant with a nozzle with a cross section of 10 by 10 mm, one neck can be processed in almost one cycle in 6 seconds of clean time. In this case, during preparation for the next spraying mode, the nozzle can be moved to a new position for spraying the next neck, etc.

 In some cases, the protected proposal opens up the only practicable way for the gas-dynamic spraying (molding) method, while retaining its main advantages, which include good molding quality, low cost of working gases, and simplicity of technological equipment. In this description, estimates are made for the case when the working gas is air or nitrogen, the sprayed material is a fine powder of alloy steel. Similar conclusions can be drawn for other refractory metals, as well as other working gases, in particular, the products of combustion of liquid or gaseous fuels and air.

 This proposal can be used in the design of plants for gasdynamic compaction and spraying of powder coatings in those cases where increased strength and density are required.

Sources of information
1. Yu. S. Borisov, Yu.A. Kharlamov, S.I. Sidorenko, E.N. Ardagovskaya. Thermal coatings of powder materials. Directory. Kiev: Naukova Dumka, 1987, p. 543.

 2. Jawes A. Brawning. United States Patent N 4416421 Nov. 22.1983.

 3. Autosvid. 198112 USSR In 23. A method of cold welding / A.I. Shestakov, publ. 06/09/1967 // Discoveries. Inventions, 1967, N 13.

 4. Auto 16187778 USSR C 23 C 4/00. A method of producing coatings / A.P. Alkhimov, V.F. Kosarev, N.I. Nesterovich, N.I. Papyrin // Discovery. Inventions, 1991, N 1.

Claims (1)

 The method of gas-dynamic coating of powder materials, including the supply of powder particles using a high-speed gas jet supplied from the nozzle assembly, and spraying them onto a surface to be coated, characterized in that the particles are supplied with a heated gas stream and the spraying is carried out in a cyclic mode with alternating spraying and preparation for spraying, and preparation for spraying is carried out by accumulating compressed gas in the receiver, thermal energy in the gas heater and heat removal from the nozzle node.

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