Investigation On The Microstructure,Mechanical Properties And Corrosion Behavior Of Low Pressure Cold Sprayed Aviation Aluminum Alloy Coating

Aluminum alloys have been widely employed in the aviation field because of its low density,high specific strength and good machinability.However,in service,aluminum-based components may suffer damages such as erosion,scratches and cracks due to impact with foreign matters/fragments or repeated polishing.Therefore,it is necessary to repair the damaged area to restore its original finish and corrosion resistance without changing or affecting the underlying base metal.Due to the low working temperature,the sprayed material will not melt during spraying and the structure and properties of the powder are basically retained in the coating material.Thus,cold spraying technology has a great advantage in the restoration of aluminum alloy components.At present,few studies have been conducted on how to use low pressure cold spraying technology to deposit Al-based coatings with high density,good mechanical properties and corrosion resistance,thus no useful guidance can be provided for the further industrial application of coating.In this dissertation,2024-Al2O3 composite coatings,powder pre-annealed 2024 coatings and different Al-based coatings were deposited on AA 2024-T3 substrate by low pressure cold spraying equipment respectively.The effects of alumina weight content,proeprties of raw material powder and alloying elements on deposition efficiency,mechanical and corrosion properties of aluminum-based coating and corrosion failure mechanism of coating were studied.The main results are as follows:In the aspect of optimizing the weight ratio of aluminum alloy to alumina of the composite coating,it was found that the 2024-20 wt.%Al2O3 composite coating showed a compact structure,with porosity of 0.55%,and a bonding strength higher than 50 MPa,At the same time,after immersion in 3.5 wt.%NaCl for 720 hours,the coating was corroded on the surface only,indicating that the coating can effectively isolate the corrosive solution and protect the matrix.However,the composite coating with higher alumina content(40 wt.%,60 wt.%)showed a degradation in density and protective effect.The reason can be summed up as follow:the passivation film on the surface of dense 2024-20%Al2O3 coating is more complete,and thus provides better protection effect.The powder annealing treatment results showed that annealing reduced the dislocation density and hardness of the powder.When 2024 powder was annealed at 300? for 2 hours,the hardness decreased from 1.598 GPa(for the as-received powder)to 1.087 GPa,and the deposition efficiency increased from 1.2%(for the as-received powder)to 5.5%.After annealed at 400?,the hardness of the powder further decreased to 0.915 GPa.However,due to powder oxidation,additional energy was needed to break the oxide layer,resulting in an increase in the critical speed.Thus,the deposition rate of the powder decreased to 2.0%.The electrochemical tests showed that the open circuit potential and self-corrosion potential shifted negatively and the corrosion activity increased with the increase of annealing temperature.But at the same time,the coating deposited by the annealed powders showed better re-passivation ability.The immersion test showed that the corrosion rate of the four coatings in 3.5 wt.%NaCl solution was close,and there was no significant difference in corrosion resistance.With regard to the performance of Al-based coatings,the results revealed that low-pressure cold spraying could deposit high-density coatings.The porosity of 5083 coating is 1.50%,the bonding strength is around 51 MPa,the hardness is more than 130 HV0.1,which were significantly better than CP-Al coating.The immersion test in 3.5 wt.%NaCl solution showed that in the initial stage,the corrosion rate of pure Al coating was the lowest because of its high purity;when the immersion time was more than 480 hours,the corrosion rate of 5083 coating was the lowest because of the accumulation of dense corrosion products on the coating surface.In addition,during the immersion,pitting corrosion occurred firstly on coatings surfaces,and with the extension of immersion time,the corrosion turned into uniform corrosion.