| Cold gas dynamic spraying (CGDS, often referred to as simply ‘cold spraying’) isa relatively new technique used to deposit materials onto the surface of a substrate. Incold spraying, for a given spray material, there is a critical particle velocity; coatingformation by the particle impact velocity above its critical velocity Thus,comprehensively and systematically analyzing the influence factors of particleacceleration in the process of cold spraying and optimizing those factors can improvethe quality of coating and the deposition efficiency effectively, and provide a crucialguide for the rapid development of the cold spraying equipment as well as the wideapplication of the cold spraying process.There are many factors that affect the particle velocity, such as: Laval nozzlestructure, gas inlet parameters, particle physical property parameter, morphology andsize, standoff distance and so on. Beyond that there are still many factors that not beentaken into account. The effect of powder carrier gas on the particle deposition wasstudied in this paper, which has not been taken into account enough. While, in theexisting high pressure cold spraying system, powder is carried by a certain flow ratepowder carrier gas, and inject into the prechamber before the nozzle in radial direction.After enters into the nozzle the low temperature powder carrier gas will lead to thedecreasing of the effective temperature in the nozzle, which would case the decreasingof the coatings quality and deposition efficiency. In this paper, the effect of the powdercarrier gas on the particle deposition was studied thoroughly through theoreticalanalysis, numerical simulation and experimental analysis.1. Numerical investigation on the cold spraying supersonic flow field with apowder carrier gas is carried out using a computational fluid dynamics (CFD) program,FLUENT. The effects of the Pressure differential between powder carrier gas and mainpropulsion gas (P), the ratio of the nozzle throat diameter on the powder injectordiameter (Rd) and the length of the prechamber (Lp) on the gas flow field, the velocityand temperature of particle are studied in this study. The gas flow characteristics insidethe nozzle are quite different under different parameters (P, Rd, Lp). In the condition ofthe Lp=0, the proportion of the powder carrier gas in the nozzle increase significantlywith increasing of the pressure differential, and the gas backflow phenomenon appearbefore the nozzle throat. The gas effective temperature of gas in the nozzle drop significantly because of the gas backflow and the large proportion of the powder carriergas. So that the particle impact velocity and temperature decrease remarkably withincreasing of the pressure differential. The proportion of the powder carrier gas in thenozzle decrease when a nozzle with large throat size is used under a certain pressuredifferential. While the mixing of the two gas flow streams in the converging sectionbecomes weak as the diameter ratio Rdincreases. As a result, the injected cold carriergas along the centerline will cause a lower temperature and lower velocity of the gasflow around the centerline in the divergent section of the nozzle. Thus, increasing thediameter ratio cannot significantly increase the velocity of the gas flow and the impactvelocities of the particles on the centerline when there is no prechamber in front of thenozzle. The prechamber in front of the nozzle not only preheat the particle, but alsoimprove the heat transfer between the two gas flow streams. Appropriately increasingthe nozzle throat diameter and elongating the prechamber to30~50mm can effectivelyweaken the adverse effect of the powder carrier gas on the particle velocity andtemperature.2. A new powder feeding method is put forward, a powder feeder based on themechanical energy and gravity is designed. Moreover, a high pressure cold sprayingsystem is established, in which the pressure differential between powder carrier gas andmain propulsion gas can be measure and controlled accurately. Unlike the traditionalhigh pressure cold spraying system, the pressure between the powder feeder and thepowder feed pipe is always balance because of the designing of the pressure balance inthe developed cold spraying system. In the powder feed process, the powder outflowfrom the powder feeder depending on the mechanical energy and the powder gravityrather than the powder carrier gas. The powder carrier gas is introduced into the powderfeed pipe according the requirement of the cold spraying tests after the powder fallinginto the powder feed pipe. Furthermore, the powder carrier gas is unavoidable in thetraditional high pressure cold spraying powder feed process, which cause a negativeinfluence of powder deposition in a certain degree; while, the developed high pressurecold spraying system provide a solution to solve this problem completely, because ofthat the powder carrier gas is not necessary in the current powder feed process.3. Copper coatings on aluminum substrate are fabricated experimentally by thedeveloped high pressure cold spraying system; the effects of the parameters associatedwith powder carrier gas on the coating thickness and deposition efficiency are discussedin this paper. The experimental results show that the effect of powder carrier gas on the coating thickness and deposition efficiency is significant. Under the specific sprayingparameters in the test, with increasing of the pressure differential from0.05MPa to0.2MPa, the decline range of the coating thickness and deposition efficiency are more than50%; The degree of promotion of the coating thickness and powder depositionefficiency with increasing of the temperature of main propulsion gas is different whenthe pressure differential is different. By increasing the nozzle throat diameter andprechamber length can effectively weaken the negative influence of powder carrier gason powder deposition, and improve the powder deposition efficiency sharply in the caseof a certain pressure differential. |
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