| As an inorganic environmentally friendly binder for mold and core sands,sodium silicate is the main trend of green casting materials.With the improvement of the core-making method,the application of sodium silicate sand is more and more widely used.Heat-curing as a hardening method with high efficiency still has disadvantages such as poor moisture absorption resistance and high residual strength.Therefore,modifying the heat-cured sodium silicate sand is of great significance.In this thesis,the effects of process parameters（heating temperature,heating time and the sodium silicate mass）and hydroxides（sodium hydroxide,potassium hydroxide and lithium hydroxide）on the properties of sand core were first studied.Then,the effects of curing additives（aluminum hydroxide,basic zinc carbonate and silica fume）and breakdown additives（magnesium oxide,aluminum nitride and chromite sand）on the properties of sand core were studied.The modification mechanism of sand core was analyzed through Materials Studio 2019,FTIR,XRD,SEM and EDS,etc.The results of experiment indicated that the optimum process parameters were as follows:the heating temperature was 150°C,the heating time was 120s,and the amount of sodium silicate mass accounted for 2.5%of the original sand.The instant strength,4h strength,24h strength and 800°C residual strength were 1.528MPa,3.753MPa,2.813MPa and 2.919MPa,respectively.In the modification experiment of hydroxides,the modification effect of lithium hydroxide was the best.When the addition of lithium hydroxide accounted for 0.2%of sodium silicate mass,the instant strength,24h strength and 800°C residual strength were 3.005MPa,3.486MPa and 1.749MPa.The adsorption energy between sodium silicate and water molecules was van der Waals energy,and sodium silicate could spontaneously adsorb water molecules,resulting in the reduction of strength of sodium silicate sand after placement.Li OH was easy to adsorb on the surface of sodium silicate to react and the sum of Li+coordinated silicon hydroxyl and water molecules was small.The hygroscopicity of the sand sample was reduced and the binder bonded well to the surface of sand,which improved the strength of the sand samples.Lithium hydroxide changed the remelting of sodium silicate at800°C,which improved the 800°C residual strength of the sand sample.The fracture mode of the residual strength sample was mixed fracture.Based on lithium hydroxide modification and using the optimum process parameters,when the addition of aluminum hydroxide,basic zinc carbonate and silica fume was 0.06%,0.05%and 0.9%of the original sand mass,the instant strength,4h strength,24h strength and 800°C residual strength were 3.679MPa,4.421MPa,3.934MPa and 0.818MPa.Sodium silicate（Na2Si O3）molecules could spontaneously adsorb on the surface of silica fume（Si O2）（1-1 0）and form chemical bonds,which promoted the tight bond between the binder and the sand particles,and improved the strength of the sand sample.Aluminum hydroxide formed aluminum silicate and basic zinc carbonate was heated to decompose zinc oxide and carbon dioxide,which improved the collapsibility of sand core.On the basis of sand core modified by lithium hydroxide and curing additives,and using the optimum process parameters,when the amount of breakdown additives magnesium oxide,aluminum nitride and chromite sand were 0.15%,0.3%and 0.2%of the original sand mass,the instant strength,4h strength,24h strength and 800°C residual strength were 3.921MPa,4.892MPa,4.252MPa and 0.427MPa,respectively.During heating and cooling process,water vapor shock pressure inside the bonding bridge and the different linear expansion coefficients of magnesium oxide,aluminum nitride,chromite sand and sodium silicate sand systems resulted in the increase of stress in the bonding layer,which reduced the residual strength of the sand sample.The best process parameters and the optimal compositions of modifiers were as follows:the heating temperature was 150°C,the heating time was 120s,the amount of sodium silicate mass accounted for 2.5%of the original sand,and the addition of lithium hydroxide accounted for 0.2%of sodium silicate addition.The quantity of aluminum hydroxide,basic zinc carbonate,silica fume,magnesium oxide,aluminum nitride and chromite sand were 0.06%,0.05%,0.9%,0.15%,0.3%and 0.2%of the original sand mass,respectively. |
