Study On The Deposition Behavior Of Hard Particles On Magnesium Alloy Surface By AC-HVAF Spraying

Activated combustion-High Velocity Air Fuel(AC-HVAF)belongs to a type of thermal spraying technology,which accelerates and heats spray particles to a certain degree through a high-temperature and high-speed flame stream emitted from a Laval nozzle,and then collides with the substrate to form a coating.This technology is widely used in industrial production such as surface protection of metal materials and parts repair.At present,the preparation of hard coatings on magnesium alloys by AC-HVAF has gradually developed to solve the problems of low hardness,low wear resistance,and poor corrosion resistance of magnesium alloys.However,the deposition behavior of hard particles on soft magnesium alloy substrates has not been systematically investigated.Therefore,based on the actual use of WC-10Co-4Cr spray powder,AZ80 magnesium alloy substrate,and spray gun operating conditions,this paper used Fluent simulation software to simulate the working process of AK07 AC-HVAF spray gun,and obtains the corresponding flame flow state and physical information of spray particles.Based on the obtained particle physics information,ABAQUS/explicit was used to solve the collision deposition process between particles and the substrate,and the effects of particle diameter,spray distance,incident angle,and substrate state on particle deposition behavior were discussed.At the same time,a discrete deposition experiment of WC-10Co-4Cr particles was designed,three basic deposition morphologies were obtained,verified the reliability of the simulation results,and analyzed the changes in the microstructure and properties of the magnesium alloy substrate during the spraying process.The research results show that:(1)The experimental results show that there are three basic deposition forms of WC-10Co-4Cr particles on the surface of magnesium alloy: embedded type,broken type,and cavity type.Embedded deposition morphology: spherical powder particles are completely or slightly deformed and embedded in the impact pit,forming effective adhesion;Crushed deposition morphology: Spherical powder particles are broken during the collision deposition process,resulting in particles partially adhering to the impact pit;Cavity type: Only bright circular pit edges caused by impact are retained on the surface of the substrate,and the surface inside the pit is smooth without powder particles.The effective deposition form(embedded type,broken type)and the bonding method of the substrate are both physical bonding or mechanical bonding.Both simulation and experimental results indicate that particle size is the main reason for different deposition states,spraying distance does not significantly affect particle deposition behavior,and small size particles(≤ 10 μm)easier to form cavity type deposits,larger particle sizes(≥ 15 μm)easier to form broken deposits,and only moderately sized particles can form more effective deposits.(2)The impact dynamics simulation of particle collision deposition shows that an increase in particle incidence angle will cause uneven strain distribution on the surface of the impact pit on the substrate,and make the substrate less prone to metal sputtering.However,the incidence angle of particles has no significant impact on the tendency of particles to form effective deposition.Increasing the preheating temperature of the substrate can enable particles to have greater penetration depth,while improving the possibility of effective deposition after particle collision deposition.During multi-particle deposition,the highest temperature point of particles occurs in the region where severe plastic deformation occurs,corresponding to the region where the advance small particles are located.(3)The hydrodynamic simulation results of the working process of the spray gun indicate that the maximum temperature in the combustion chamber can reach 2299 K,and the maximum flame flow velocity at the nozzle outlet can reach 1600 m/s.The temperature rise of particles mainly occurs in the combustion chamber,while the acceleration mainly occurs in the nozzle interior.Different oxygen in air/C3H8 and different types of nozzles can cause significant changes in the flame flow state.The lower the oxygen fuel ratio,the higher the temperature in the combustion chamber,and the faster the flame flow speed;The 4L and 5L nozzles with Laval type have lower flame temperature and higher particle velocity,which are more in line with the low-temperature and high-speed goals pursued by AC-HVAF.(4)During the actual spraying process,high-temperature flame flow and high-speed particles have a significant impact on the microstructure and properties of extruded magnesium alloy substrates.As the number of spraying passes increases,the heat affected zone continues to deepen,and there is a recrystallization refinement behavior in the matrix structure,mainly through grain boundary arcing and sub crystal transformation.At the same time,the second phase dissolves back and the twins disappear;Due to the combined effect of the second phase dissolution and recrystallization behavior,the hardness of magnesium alloy matrix exhibits a trend of decreasing first and then increasing.