Influence of Spraying Parameters on the Uniformity of Bolt Surface Coating
Aluminum coatings are applied to the surface of titanium alloy fasteners to prevent galvanic corrosion when these fasteners are connected to aluminum alloy structural components.However,because of the complex geometries of these bolt surfaces and influences of the spray-operation parameters,such aluminum coatings often have an uneven thickness.This inconsistency can lead to bolt connections with decreased reliability and galvanic corrosion protection.Presently,there is a lack of extensive research on the factors affecting the uniformity of these coatings.This study utilized the Euler-Lagrange method to develop a physical model of the two-phase flow in an atomizing spray field.This innovative model was designed to explore how variables like the spray distance,air intake pressure,and sector control pressure impacted the thickness of the coatings applied to bolt surfaces.To verify the theoretical insights provided by the model,a series of corresponding spray experiments were meticulously conducted,and their outcomes were compared with the simulation results.The findings showed that at shorter spray distances,there was a significant variation in the velocity of droplets as they hit different parts of the bolt,leading to a generally thicker and more uneven coating.A notable issue was the greater deposition of paint at the base of the threads,where the coating thickness sometimes exceeded 30μm.When the spray distance was increased,the droplets spread out more,complicating their interaction with the environment and making their trajectory harder to predict.This often resulted in a thinner and less uniform coating on the bolt surface.The role of the air intake pressure was found to be crucial in determining the degree of paint atomization.At lower intake pressures,the atomization process was incomplete,producing larger droplets with an uneven distribution across the surface,which led to the formation of uneven patches.At higher intake pressures,the droplets became excessively fine,spreading across a larger area but ultimately resulting in a thinner coating layer on each individual bolt.The process of adjusting the sector control pressure was instrumental in changing the trajectory of the paint droplets.This adjustment significantly affected the overall spray pattern and specific placement of droplets on the target surface.At lower sector control pressures,the liquid spray was more concentrated within a smaller angular range,resulting in a thicker coating.In contrast,higher sector pressures broadened the overall spray range but caused droplets in the central area of the spray cone to lose momentum and scatter to the sides,which led to a thinner coating.Simulations indicated that the most uniform coating thickness was achieved with a spray distance of approximately 190 mm,an air intake pressure of approximately 0.4 MPa,and a sector control pressure of approximately 0.1 MPa.To verify the accuracy of the simulation in predicting the characteristics of the bolt coating formation and thickness,corresponding parameter experiments were conducted.These experiments were carried out using an automated spraying device.After the coating application and curing,the bolts were sectioned and encapsulated in epoxy resin.The coating thickness was carefully observed and recorded using a metallurgical microscope at a magnification of 400×.These results were then compared with the simulation results.A good correlation between the experimental and simulation outcomes was observed,confirming the feasibility of using the two-phase flow physical model for studying the spray field and predicting the distribution of the coating thickness.This study adopted the Euler-Lagrange method to simulate the process of spraying a bolt surface.The distribution of the coating thickness was analyzed,and a viable method for optimizing the spray parameters was developed.
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