| Aluminum is the active metal with good electrical conductivity, thermal conductivity and low density. However, the poor mechanical properties limit its application. Therefore increasing the surface hardness and wear resistance of aluminum and its alloys becomes the focus of the research work.AlN crystals are covalent substances with high melting point, high strength, high hardness, good thermal conductivity and so on. Domestic and abroad researchers have used many methods to prepare AlN thin films with good performance including PVD, CVD, plasma nitriding and ion implantation. But those methods require a vacuum environment and complex equipment resulting in high preparation cost.The arc nitriding process was applied in this thesis to prepare nitrided layers on aluminum and its alloys. A commonly used TIG welding machine was used with gas mixture of different N2 proportions in Ar. And pure aluminum, Al-Si and Al-Si-Cu alloys were applied as substrates to experiment the influence of arc currents and atmosphere on the microstructures and properties of the nitrided layers.The microstructures of nitrided layers on the pure Al substrate were consisted of great amount of AlN dendrites in the deeper area and of sandwich structures including AlN and Al silid solution lamellars grown alternately in the middle area. In the upper area a wider Al silid solution lamellar was found. After the local substrate was melted by the arc of high temperature and the molten pool formed, AlN was composed between N and Al elements. AlN crystal nuclei were generated on the solid surface of interface in the bottom of the nitrided layer and then they grew up to AlN dendrites. Their growth consumed a large number of dissolved nitrogen and decreased the nitrogen concentration among the AlN dendrites and around the middle and upper area. The Al melt solidified and Al solution lamellar formed at first. Because of the sharp decrease of the N solubility in the Al solution before and after solidification, N element diffusion was initiated from the Al solution lamellar to the solid-liquid interface. Then an AlN lamellar was formed by nitriding reaction in which some N elements diffused to the interface and joined the reaction from the Al melt near the interface. Thus the'Al solution lamellar'and'AlN lamellar'grew alternately and finaly the Al melt completely solidified. In the end the special sandwich structures with characteristics of composites were developed among the AlN dendrites and in the middle and upper area of nitrided layers. Sometimes a thicker Al solution lamellar appeared in the top area because the soluble N concentration was low enough in the left Al melt. This is the forming mechanism of the microsturctures in the nitrided layer on the pure Al substrate.With nitrogen proportions increasing, the arc voltage and the thickness of nitrided layers increased as well. And the content of AlN dendrites increased for the N concentration increasing in the molten pool. when the nitrogen content was 75 percent in the arc atmosphere, the AlN dendrites arrived to the highest proportion in the microsturctures of the nitrided layer. But in the total N atmosphere the AlN dendrites grew less developed because of low N concentration in the molten pool by overmelting the pure Al substrate.The microstructures of the nitrided layers on Al-Si alloy substrate were consisted of AlN dendrites and the sandwich structures as well. But existence of element Si in the Al-Si alloy led to increasing nitrogen solubility and accelerating the nitriding reaction. More AlN phases were formed and then led to denser AlN dendrites and less sandwich structures. With currents increasing, the content of AlN dendrites first increased and then decresead. Because increasing the heat input can not only accelerate nitriding reaction, but also make the substrate melted too much which would decrease the nitrogen content in the nitrided layers. When the current was 90 A, the content of AlN dendrites increased to the most. With nitrogen content in the arc atmosphere increasing, the content of AlN dendrites increased and to the highest proportion when it was 75 percent.In addition, the microstructures of the nitrided layers on Al-Si-Cu alloy substrate were consisted of AlN dendrites and the sandwich structures. With currents and nitrogen content in the arc atmosphere rising, the variation of microstructures in the nitrided layers on Al-Si-Cu alloy substrate was the same as those on Al-Si alloy substrate.The hardness of the nitrided layers prepared on pure Al, Al-Si and Al-Si-Cu alloy substrates by arc nitriding were obviously higher than that of the substrates because of the harder phase AlN. The highest hardness among all nitrided layers was over 10 times higher than that of the substrates. With currents increasing,the hardness of the nitrided layers on Al-Si and Al-Si-Cu alloy substrates first increased and then decreased .When the current was 90A, the hardness reached to the highest. With nitrogen content in the arc atmosphere increasing, the hardness of the nitrided layers on pure Al, Al-Si and Al-Si-Cu alloy substrates gradually increased. When the nitrogen content is 75 percent in the arc atmosphere, the hardness reached to its peak. Compared with substrates, the abrasion resistance of the nitrided layers prepared on pure Al, Al-Si and Al-Si-Cu alloy substrates by arc nitriding were obviously improved. With the nitrogen content increasing, the abrasive weight loss of the nitrided layers on the pure Al substrate was lower and decreased to its lowest value of 11.8mg by which the abrasion resistance of the nitrided layer was the 6 times higher than that of the substrate when the nitrogen content was 75 percent. Moreover 100 percent nitrogen content made the weight loss back-up. For the nitrided layers on Al-Si and Al-Si-Cu alloy substrates with currents increasing,the abrasive weight loss values reduced first and then rised. Under the condition of pure nitrogen atmosphere, 2.5mm/s of traveling speed and 90A of the arc current, the abrasive weight loss of the nitrided layer on Al-Si alloy substrate was reduced to 8.7mg which value is the 8 times lower than that of the substrate. At the same time the abrasive weight loss was only 6.6mg for the nitrided layer on Al-Si-Cu alloy substrate which led to the abrasive resistance 10 times higher than that of the substrate.With the currents increasing, the weight loss and friction coefficients of the nitrided layers on Al-Si and Al-Si-Cu alloy substrates first decreased and then increased. When the current was 90A, they reached to the lowest value reflected to the best friction resistance. With nitrogen contents in arc atmosphere increasing, the weight loss and friction coefficients of the nitrided layers on Al-Si and Al-Si-Cu alloy substrates decreased. when the nitrogen content was 75 percents, they reached to the lowest value led to the best friction resistance. Under the condition of pure nitrogen atmosphere, 2.5mm/s of arc traveling speed and 90A of the arc current, the weight loss of friction wear of the nitrided layer on Al-Si alloy substrate was only 2.9mg from which value the friction resistance was 5 times higher than that of the substrate. At the same time the weight loss was 3.8mg for the nitrided layer on Al-Si-Cu alloy substrate led to the same frictional resistance improvement.The nitrided layers prepared by the arc nitriding process were metallurgically bonded to the substrates, with high hardness and excellent wear resistance. They would have a potential future of practical application.
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