Research On Sepiolite Coating For Metal Molds For Aluminum Alloy Casting

Nowadays, the development trend of automobile manufacturing technology is lightweighting, the important measures of automobile lightweighting is to replace iron casting parts by large number of aluminum alloy castings in making them. The surfaces of metal molds used in production of aluminum alloy castings must be sprayed by coating before putting them into service, its purpose is to protect metal molds and improve quality of aluminum alloy castings. In metal mold casting, the quality of spraying coating is directly related to production efficiency and quality of aluminum alloy castings.The water-based sepiolite spraying coating used for metal molds for making of aluminum alloy castings was investigated. In this investigation the sepiolite was used as main refractory aggregate, kaolin, graphite and talcum powder as auxiliary refractory aggregates, sodium silicate, polyvinyl alcohol(PVA) and silica sol as binders, sodium bentonite, sodium carboxymethyl cellulose(CMC) as suspension agents in coating formula. The different components of the sepiolite coating on the influence of suspension rate and strength of the coating were studied and metal mold spraying coating and melt aluminum pouring tests were preformed, getting results as follows:The study found that the interaction between sodium silicate and PVA was a weakening trend on strength of and suspension rate of the coating, the interaction between PVA and silica sol was an enhancing trend on the strength of the coating. When the density of coating was controlled at 1.1±0.02g/cm3, the sodium silicate was selected as only binder and the adding amount of sodium silicate reached up to 200% of sepiolite mass traction, the strength of coating could reach gradeⅠof finger wiping method. By analyzing SEM photos, it was found that the greater sepiolite was packaged by sodium silicate, the lower the porosity, the higher strength of the coating. When the adding amount of sodium silicate reached to 250% of sepiolite mass fraction, the coating layer would not occur to stripping off after pouring 15 times continuously.When the adding amount of sodium silicate was 250% of mass fraction of sepiolite, sodium bentonite and CMC were 40% and 3% of sepiolite mass fraction respectively, the 3h suspension rate of the coating could reach to 91%, the strength to gradeⅠof finger wiping method. The coating could meet application requirements of aluminum alloy metal molds.While the adding amount of kaolin was 20~60% of sepiolite mass fraction. With increase of the adding amount of kaolin, the suspension rate and strength of the coating were increased, when graphite, talcum powder amounts were 10~30% and 20~60% respectively, with their adding amount increasing, the suspension rate and strength were decreased. While graphite powder was less 5%, the strength and suspension rate of the coating would not be affected. With increase of graphite powder the porosity of coating layer and the flake structure of sepiolite would decrease. Pouring experiment found that the amounts of talcum powder and kaolin were 40% and 200% of sepiolite mass fraction respectively, the surface roughness of aluminum castings could reach Ra12.5μm.