| Aluminum alloy is widely used in aeronautic and aerospace engineering due to its development.Aluminum is the most cost-effective weight reduction material in the automotive industry.However,low hardness,poor wear resistance and other shortcomings have limited the application and development of aluminum alloy.Improving the hardness and wear resistance of aluminum alloy is an important issue.Arc-nitriding technology has the advantages of low equipment cost,simple operation,and high bonding strength between the coating and the substrate.Friction Stir Processing(FSP)can control the thickness and forming of the composite material layer by controlling the length and shape of the stirring pin.In this paper,in-situ fabrication of Al N and FSP treatment of composite coatings were studied.The morphology and component of nitrided layers was analyzed,and the formation mechanism of Al N was studied;the microstructure and morphology after FSP was analyzed.Based on the hardness and wear resistance of the coating,the evolution of the FSP and the strengthening mechanism were explained,and the corrosion resistance of the composite coating was also studied.The aluminum alloy substrate was treated with nitrogen arc and this paper analyze the microstructure and the formation mechanism of the nitrided la yer.The Al N prepared by nitrogen arc exists in dendrite structure and layered structure.The thickness of the aluminum nitride layer is more than 300 ?m.Under the effect of arc,the surface of the aluminum alloy substrate is melted to form a molten pool.Nitrogen is ionized into atoms and ions,then diffuses into the molten pool under the effect of diffusion and convection.Nitrogen reacts with aluminum to produce aluminum nitride.The nitrided surface was subjected to FSP treatment.The microstructure and properties of the composite coating were analyzed.After FSP,the thi ckness of the composite coating is about 400 ?m.The aluminum nitride is broken into fine particles and it is well embedded in the aluminum alloy matrix uniformly.The average micro-hardness of the composite coating is more than 110 HV,significantly increase the hardness.Nano-indentation tests show that the hardness and modulus of the composite coating are greatly improved.Scratch tests show that the composite coating is more resistant to scratches at the same force and sliding distance.The friction and wear experiment shows the friction coefficient of the composite coating was about 0.15 and the base material was about 0.5.The wear loss of the composite coating was also lower than that of the base meta l and the wear scar was shallow and narrow.Its wear resistance is significantly improve.The evolution of the structure during the FSP and the strengthening mechanism of the composite coating were analyzed.Under the effect of the stirring pin,the aluminum nitride will be squeezed,broken into small particl es,and flow with the aluminum alloy thermoplastic material.Al N particles will be dispersed on the surface of aluminum alloy substrate to produce aluminum nitride particle reinforced surface composite coating.The strengthening mechanism are the second phase strengthening and deformation strengthening.The corrosion behavior of the composite coating shows that the dispersion of aluminum nitride has little effect on the polarization behavior of the aluminum alloy surface.The corrosion potential of the composite layer increases,so its corrosion sensitivity reduces.The corrosion form of the composite coating is pitting corrosion.The aluminum that surrounds the aluminum nitride particles is preferentially dissolved to cause pitting corrosion.As the corrosion pits grows together,exfoliation corrosion occurs until all of the aluminum in the surface layer is dissolved,leaving the corrosion product and unreacted Al N phase,which constitutes a corrosion barrier and reduces the corrosion rate. |
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