Study On Microarc Oxidation Process And Microstructure And Properties Of Hot-dip Aluminized Coating On Cast Iron

An oxidation ceramic layer was prepared on the surface of cast iron by combination of hot-dip aluminum plating and micro-arc oxidation to improve its wear resistance.The influence of hot-dip coating time and temperature on the thickness of hot-dip aluminum coating was analyzed.Effects of micro-arc oxidation voltage and duty cycle on the microstructure and properties of micro-arc oxidation ceramic coatings were determined.By comparing with micro-arc oxidation coating on pure aluminum substrate,the influence of the aluminized cast iron substrate on the micro-arc oxidation layer was studied.The results showed that the optimized process parameters for hot dip aluminizing were at 730? for 3 min.Under this process the thickness of the aluminized layer was 135?m,of which thickness of the alloy layer was 46?m.A smooth surface with less defects and needle-like FeAl₃ phases of the aluminized layers were obtained.The step-boost mode was used for the preparation of micro-arc oxidation layers.With the increase of voltage,the surface roughness of micro-arc oxidation coatings increased,and the main growth mode changed from ingrowth to outgrowth.The maximum thickness of the oxide layer was 58?m when the oxidation voltage was 550 V.The wear resistance of oxide layers was improved,of which the amount was only 2.65×10^-4mm³/?N·m?.With the increase of duty cycle,the film thickness increased,and the growth mode of the coatings changed from ingrowth to outgrowth.The friction coefficient increased gradually.The phase of the oxidation layers changed from?-Al₂O₃ to?-Al₂O₃.The hardness of the oxide layer reached a maximum at 40%?approximately 950 HV?.At this time,the amount of wear was only 1.399×10^-4mm³/?Nm?,which was about 2/5 of the iron matrix.Compared with pure aluminum,the surface of the micro-arc oxidation coatings prepared from the aluminized cast iron had less volcanic morphology,and the discharge pattern was mainly B-type discharge.There was a dense and smooth oxide layer around the“crater”.In the process of micro-arc oxidation,the aluminizing cast iron had lower arc starting voltage and less roughness and the wear resistance was better.The friction coefficient was only about 0.5,and the wear amount is only 75%of pure aluminum.However,due to the limitation of the thickness of the aluminized layer,the oxide layer prepared from the aluminized cast iron material had weaker bonding ability between the oxide layer and the iron-aluminum alloy layer when the aluminized layer diffused into the matrix,resulting in poor thermal shock resistance.