Surface Modification Of Die-casting Molds By A Composite Technique Of Plasma Electrolytic Oxidation

The percentage of light metal application is increasingly growing, one of theforming process which is most widely used is die casting. Some kinds of failure, such assoldering and thermal fatigue, are easily happening on die casting molds after they havebeen long time work. So it is necessary to further improve the efficiency of the diecasting process in extending die life and surface quality of the castings. The process ofhot dipping pure aluminum on H13steel substrates followed by plasma electrolyticoxidation （PEO） was studied to form alumina ceramic coatings for protective purpose.H13steel bars were first dipped in pure aluminum melts then a reactive iron-aluminumintermetallic layer grew at the interface between the melt and the steel substrate. Thereactive layer was mainly composed of intermetallic Fe-Al （Fe₂Al₅）, the thickness ofaluminum layer and Fe-Al intermetallic layer were mainly influenced by dipping time（1.5¹2min）, and dipping temperature （710⁷60℃）. After PEO process, uniform Al₂O₃ceramic coatings have been deposited on the surface of aluminized steel. The followingmicrostructure observation and mechanical property tests was undertook is this work:â‘ The cross-sectional observation and elements distribution of samples afterhot-dipping process and PEO process were characterized by means of scanning electronmicroscope （SEM） with an Energy dispersive spectroscopy （EDS）. The PEO coatedsamples have the layers as ceramic layer, Al layer, intermetallic layer and substratefrom outside to inside. Besides the surface morphology PEO samples were porousobserved by SEM.â‘¡The samples after hot-dipping were characterized by means of X-raydiffraction （XRD） to determine the phase composition in intermetallic layer and surfacelayer. The intermetillics were composed by phase Fe₂Al₅and a little FeAl₃.â‘¢The thickness tendency influenced by dipping time and dipping temperature ofintermetallic layer and surface Al layer was observed and measured by opticalmicroscopy. The thickness of intermetallic layer grows quickly when increasing thedipping temperature and dipping time.â‘£The distribution of micro-hardness across the composite coating wasdemonstrated and the microhardness of the outer ceramic coatings were1700HV whichmuch higher than that of H13substrate.⑤The thermal fatigue resistance of PEO coatings and traditional anodic oxidation coatings was compared. PEO coatings showed excellent thermal fatigue resistance.