| The application of magnesium alloy is restrained by their easy oxidizing and wearing. A coating on the surface of magnesium alloy can solve these problems,such as minute arc oxidized film,all kind of plated film.But these films have some similar problems that the film is very thin and easy wearing.Hot spray technology can form a coat on the magnesium alloy surface that improves the oxidizing and wear resistance of magnesium alloy.But the high temperature during spray process will cause oxidizing in the interface between magnesium alloy and coat,the coat is a low compactness too.Cold spray technology is a new technique that can set up a coat under low temperature and can avoid magnesium alloy oxidized.During spray process,compressive stress exerts in the cold spray coat,which can produce high strength and compactness in coat.Therefore,cold spray technology can solve the problems of magnesium alloy surface oxidizing and wearing.In this paper,we have systematically studied the technology of a coat on the magnesium alloy surface by cold spray technology,in order to set up a technique basis for its application.The paper major research works include the following contents.First,Zn-Al and Al-Si-Fe series alloy chemical compositions have been designed for cold spray,the aim is raising the corrosion resistance and the wear-resistance and improving heat-resistance of magnesium alloy surface.The alloy powders are prepared with the supersonic gas atomization equipments.The results show that alloy powder has globular or near globular shape,narrow distribution of grain size and powder amount less than 400 meshes reaching 80%,which can completely meet the requirement for cold spray.The physical and mathematical model for describing gas and gas-particle two-phases flowing in nozzle has been set up during cold spray process;gas pressure distribution and gas-powder two-phase flowing velocity in nozzle have been simulated with the software. The results show that the traditional nozzle structure has some possibility for further improvement.In order to improve the spray effect,the supersonic nozzle has been designed on the basis of fluid mechanics and aerodynamics.With the consideration of the nozzle design theory and viscosity boundary layer theory,the nozzle structure has been optimized.New nozzle structure increases the area and cone angle of pipe outlet.The calculating simulations results show that new structure nozzle can raise acceleration effec! of gas and powder,powder spraying velocity from nozzle increased.The utilization ratio of spray powder can be increased more than 10%under fixed site spray test.Through studying the cold spray process on the magnesium alloy surface with rapidly solidified Zn-Al and Al-Si-Fe alloy powder,the suitable technology parameters have been determined.The characteristic of coat microstructure and diffusion law of alloying elements has been analyzed.The appropriate parameters of cold spray Al-Si-Fe alloy powders on magnesium alloy surface are that:the pressure of spray gas to alloy powder is 2.1MPa,the temperature of spray gas is 550℃and the spray distance is 35mm.ZA20 alloy powder appropriate parameters are pressure of spray gas 2.15MPa,spray gas temperature 300℃,spray distance 40mm.ZA27alloy powder appropriate parameters are pressure of spray gas 2.1MPa,spray gas temperature 330℃,spray distance 20mm.The coat has the characteristics of rapidly solidified and compacted microstructure.The bonding way between coat and substrate alloy is mechanical joint.The diffusion treatment has been conducted on the coated samples with different heat treatment process.The results show that coat microstructure becomes more compact and homogeneous after heat treatment,accompanied with the diffusion between coating and substrate alloy.Moreover, the diffusion quantities diffused into coating from substrate alloy are more than that of diffused into substrate alloy from coating.As temperature is rising and time increasing,the diffusion degree between coat and substrate alloy improves.However,the diffusion layer varies little when the temperature rises to 300℃and the time increases to 3h.The bonding way changes from mechanical joint to metallurgical joint;the bonding strength gets raised after diffusion treatment.The corrosion behavior of the coats has been studied.The Zn-Al and Al-Si-Fe alloy coat performs better corrosion resistance than that of the substrate magnesium alloy in both salt mist corrosion and electrochemical corrosion.The corrosion weight loss of ZA20 and ZA27 alloy coat is 0.055 and 0.072 times respectively than that of AZ91D magnesium alloy.The self-corrosion potential of AZglD magnesium alloy,ZA27 and Al-Si-Fe alloy coat are about -1.7V,-0.2V and -1V respectively,moreover,polarization curves of ZA27 and Al-Si-Fe alloy coat have a clear passivation region,which can protect the substrate from being corroded.AZ91D magnesium alloy and cold spray coats have different corrosion mechanism,which the cold spray coat of ZA27 alloy takes slight homogeneous corrosion and AZ91D magnesium alloy is heavy local corrosion.The danger of homogeneous corrosion for cold spray coat is far inferior to the local corrosion for AZ91D magnesium alloy.Al-Si-Fe alloy coat takes local corrosion also,but the degree of corrosion is very light.Wear test shows that the wear-resistance of cold spray coat is far better than that of AZ91D alloy.AZ91D magnesium alloy weight loss under lubricating wear condition is 5.92 and 11 times respectively than that of ZA20 and ZA27 alloy coat.The wear thickness of AZ91D magnesium alloy is 1.88 and 2.2 times respectively than that of ZA20 and ZA27 alloy coat.Under dry wearing condition,the wear weight loss of ZA27 alloy coat is 16% than that of AZ91D magnesium alloy. |
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