Effect Of Heat Treatment And Composite Coatings On The Properties Of The In-mold Oxidation Layer Of Cast Aluminum Alloy

Owing to the advantages of low density,high strength,good machinability,excellent electrical conductivity and thermal conductivity,cast aluminum alloys have been widely used in automotive,ship building,aerospace and other industrial fields.However,with the emergence of environmental problems such as acid rain,the corrosion of aluminum alloy castings has attracted wide attention.Because of the limitations of the naturally formed oxide films on aluminum alloys,thickening the oxide film has become an important method to improving its corrosion resistance.In this paper,the effect of heat treatment on the microstructure and properties of in-mold oxidation layer is compared and analyzed.The composite coating treatment of the in-mold oxidation heat treated samples by phosphate coating and surface silylation is studied.Finally,the application of the in-mold oxidation treatment technology in the sand lined metal mold is explored.In-mold oxidation treatment can stably generate an oxide layer with a thickness of 2?4 ?m on the surface of the casting,which is much thicker than the oxide film generated in the natural state,and the combination of oxide layer and matrix is close.After T4 and T6 heat treatment,the oxide layer and the matrix are still closely combined,and there is no phenomenon of separation or falling.However,due to the presence of thermal stress,the oxide layer has micro cracks,and the corrosion resistance of the sample also be reduced.Comparing and analyzing the effects of the two heat treatment processes,the corrosion resistance of the oxide layer decreases more significantly after T6 heat treatment.After the heat treatment of the in-mold oxidation layer,two different processes were used to obtain a composite coating.With the assistance of the steam,the phosphate ceramic composite coating with a thickness of about 3 ?m was prepared on the surface of the oxide layer,and the combination of the coating and the oxide layer is tight.The coating can significantly improve the corrosion resistance of the oxide layer after heat treatment of T6,the polarization resistance value is increased by more than 10 times,and the stability of the coating is improved.Besides,two kinds of silane reagents were used to obtain a composite coating of heat treated in-mold oxidation layer,and the silane film which has uniform thickness was obtained on the surface of the sample,and the corrosion resistance of the sample was also improved obviously.The paper explored the application of the in-mold oxidation treatment in sand-lined metal mold,and as the carrier of the oxidation coating,a sand core with a thickness of 8 mm is attached to the cavity of the metal mold,finally an oxide layer on the surface of the casting was obtained which the thickness is about 3?4 ?m.The oxide layer has the similar section morphology with the sample which was prepared by sand mold,and the sample has excellent corrosion resistance in 3.5% Na Cl solution.