Study Of Chemical Conversion–Micro Arc Oxidation Composite Process Of ZL104 Aluminum Alloy

Owning outstanding physical and mechanical properties, as well as the abundant storage on earth, cast Al-Si alloys are widely used in the industry fields of electronic communication, aerospace technology, automobile manufacturing and so on. Nevertheless, due to the problem of poor corrosion resistance and low hardness, further application of cast Al-Si alloys are limited. Therefore, it is essential for cast Al-Si alloys to improve their surface properties by surface treatment.Micro arc oxidation(MAO) is a surface treating method derived from anodizing. It combines multiple mechanisms of chemistry, electrochemistry, physical chemistry and plasma chemistry, aiming at forming a metallurgical bonding ceramic coating on the in situ surface of the valve metal substrate. Compared with other treating methods, micro arc oxidation has some outstanding advantages, such as the eco-friendly electrolyte, simple process, easily operated equipments and so on. Concurrently, the coatings prepared by this method can exhibit some advantages like high hardness as well as excellent corrosion resistance. But for high silicon cast Al-Si alloys, it is very difficult to forming a passive coating at the beginning of the micro-arc oxidation due to the higher electrical resistivity of silicon with respect to the aluminum matrix. This prolongs the arcing time,reduces the film growth rate and improves the energy consumption.In this paper,a prefab coating on ZL104 aluminum alloys by chemical conversion treatment is made in advance to cover the silicon on the surface of the substrate, thus suppressing the influence of silicon in the early stage of micro-arc oxidation, which reduced thearcing time from 168 seconds to 52 secondsand energy consumptiondecreased from 7.7869kw·h/(m2·μm) to 4.5448kw·h/(m2·μm).SEM, EDS, XRD and other means of detection were utilized to characterize the micro morphology, cross section, composition and structure of MAO coatings. Concurrently, electrochemical workstation, 3D profiler, Brinell hardness tester were used to detect the corrosion performance, roughness, hardness of the coatings. The main phase composition of the coating was γ-Al2O3, and a small amount of α-Al2O3 and Si O2 existed, too. Compared to the MAO coating without prefab coating,the layer thickness increased from 5.3μm to 10.5μm,thefilm growth rate increased from 0.367μm/min to 0.7μm/min, the corrosion current density decreased from 0.7031μA.cm-2 to 0.3140μA.cm-2,thesurface microhardness increased from 517.7 HV to 617.8HV, the surface roughness increases from 0.63μm to 0.91μm.