Study On Preparation And Properties Of Composite Enamel Layer On Aluminum Alloy Surface

In the machinery, aerospace, automotive and food industries, aluminum alloy can be used as ideal materials for automotive and motorcycle to lighten weight and for energy conservation due to the advantages of small density, light quality and good formability. However, its application scope is severely restricted by the low surface hardness and poor wear resistance. Enamel is one kind of inorganic glass glaze, which have properties of high temperature resistance, high corrosion resistance and high hardness. After surface enameled, aluminum alloy could behave characteristics of the strength and toughness, as well as the chemical stability and mechanical properties. In this way, the surface property of aluminum alloy is improved.Enamel layer with high hardness, anti-wear and anti-corrosion properties on aluminum alloy surface is preplanned. Because of its wide use in industry, 6061 aluminum alloy is chose as a matrix and treated by enamel. Different pretreatment methods to aluminum alloy samples were investigated: anodic oxidation, Ni/Al interlayer by plasma spraying and non pretreatment. The size of enamel powder was submicron through ball milling. Sintering temperature, heat preservation time, sample pretreatment were chose as variables in orthogonal experiment. The effects of different process parameters on the microstructure and properties of the enamel layer were studied. The enamel granularity after ball milling was analyzed by laser particle size analyzer. The hardness and thickness of layer were analyzed by micro-hardness tester and eddy current thickness meter. Surface morphology and interface morphology between matrix and layer were analyzed by scanning electron microscope, laser scanning confocal microscope and metallographic microscope. The properties of anti-wear, impact resistance and anti-corrosion were tested by friction and wear test, mechanical shock test and acid corrosion test.The results showed that the optimum sintering process were anodic oxidation pretreatment, sintering temperature of 540℃, holding time of 4 min under coating hardness as examination index. In the test of wear, mechanical shock and acid corrosion, enamel layer with anodic oxidation was better than the sample with Ni/Al layer and the sample without pretreatment. Comparing the surface morphology and cross-sectional morphology of different samples, the layer organization of anodic oxidation pretreatment was compact and no obvious bubbles, micro cracks and other defects. Its surface was smooth and glabrous. There were a large numbers of pores, micro-cracks and other defects in the enamel layer of sample with Ni/Al layer pretreatment and sample without pretreatment. Its surface was uneven. The combination of interface was weak and some micro seams existed.The distance of atomic diffusion is shortened and the solubility of the particles in the liquid phase is increased by the use of submicron enamel powder. At the same time, the surface energy of system and the driving force of sintering are increased, which results in the increase of sintering rate and the decrease of sintering temperature to achieve the purpose of low temperature and fast sintering. The contact area and wet ability between enamel layer and transition layer are increased because of the porous surface of anodic oxide film. Serrated mechanical anchors can be formed between anodic oxidation film and matrix after superfine molten slurry into the micro pores, which make the combination more closely. Because of the anchor-shaped combination and complex shape between enamel layer and anodized film, aluminum alloy can withstand a greater impact force, and the ceramic layer can transfer a portion of force to the aluminum alloy matrix. So the enamel layer has a good resistance to mechanical shock. In addition, the reaction between aluminum alloy matrix and alkaline glaze can be prevented by anodic oxidation film effectively. The large difference of thermal expansion coefficient is relieved. Anodic oxidation film plays a good role in the transition, eliminating air bubbles, micro-cracks and other defects. These eventually lead to the enamel layer has a higher density and smoothness.