Study Of The Growing Characteristics And Properties Of Plasma Electrolytic Oxide Coatings Processed On Micro Concave-convex Surface Of Aluminum Alloy

The ceramic oxide coating process on aluminum alloy by Plasma Electrolytic Oxide(PEO)has a broad prospect,due to its excellent physical and chemical properties,such as high wear resistance,corrosion resistance,thermal shock resistance and electrical insulating,etc.However,the high current energy consumption and low growth rate of ceramic oxide coating limit the development of PEO.It is generally known that there is a strong edge effect on the angle of the substrate surface,edge and grooves during the PEO process.Under the effect of micro discharge,the oxide coating growing perpendicular to the surface of the substrate and keeping to the end of the oxidation process.A lot of works were carried out to effectively make use of the current concentration effect and the growth characteristics,artificially control the growth process of the oxide coating,improve the forming efficiency and performance of oxide coating in this thesis.The mechanical micro grooving pretreatment was applied to build the micron level of concave and convex surface and prepared the zigzag ceramic oxide coating by PEO process.The growth rate,the structure and performance of the ceramic oxide coating were characterized by SEM,EDS.XRD,Micro Hardness Tester and Falex Multi Specimen Test Machine;The growth mechanism of PEO coating on the micro concave and convex surface and the influencing factors of welding defects were analyzed;Preliminary explored the advantages and disadvantages of zigzag oxide coating which was used in aluminium alloy engine cylinder body.The results show that there is obvious current concentration effect on the sharp corner of micro grooves during the PEO process and the coating is much thicker.The growth regularity of PEO coating in the micro grooves is different from the flat surface.In the early stage,the oxide coating growing perpendicular to the surface.With prolonging the PEO time,the oxide coatings from the two adjacent wall surfaces and the bottom surface gradually encountered and were welded together by micro discharge.The appropriate opening angle of micro grooves is beneficial to weld them together without weld defects.Meanwhile,the micro groove can be completely filled with oxide under the appropriate process parameter and continuous smooth with the coating out of the grooves.The growth rate of oxide coating was improved.The theoretical probability of that the weld defects were weld well by micro discharge on the oxide coating formed in the micro grooves was demonstrated by calculating the temperature field around the weld defects.And which has been proved in the experiment.With the increase of PEO time,the weld defects will be alleviated and further healed.While.the insufficient fill of molten oxide,thermal expansion coefficient mismatch between oxide coating materials,the volume shrinkage caused by the crystal structure transformation of alumina and the poor gas exhaust conditions in micro grooves,etc.,limit the completely healed of weld defects with the fixed dimension of micro grooves.The poor heat-transfer conditions of coatings in the micro grooves is beneficial to the crystal structure transformation of ?-Al2O3.Thus,the hardness and wear resistance of zigzag oxide coating is obviously improved.The thermal analysis results show that there is a stress concentration region around the coating-substrate interface of the zigzag oxide coating when heated and it can be alleviated by changing the structure of micro grooves.Nevertheless,the stress concentration has little influence to thermal shock failure of the coating,due to the metallurgical combination.