Research On The Sodium Silicate Matrix Binder For The Warm Box Process

Sodium silicate in sand casting has the advantages of green and pollution-free,low cost,fast hardening and good quality casting,which will have great application potential in Chinese casting industry.However,there are still some defects on the application of sodium silicate,such as poor collapsibility,strong moisture absorption and easy aging.Therefore,it is of great practical significance to improve the performance of sodium silicate bonded sand?SSBS?.In this paper,the core-making method of warm box was used.Firstly,the effect of composite modifier?including potassium hydroxide,lithium hydroxide and disodium hydrogen phosphate?on the properties of SSBS was studied.Secondly,the effect of nano magnesium oxide?Mg O?,nano silicon dioxide?Si O₂?combined with ultrasonic treatment on the properties of SSBS was further studied.In addition,the influence of breakdown agent consisting of titanium dioxide?Ti O₂?,zinc oxide?Zn O?and silicon carbide?Si C?on the properties of SSBS was studied.The influence of hardening process?core box temperature and heating time?on the properties of SSBS was discussed,and the optimal core box temperature and heating time was determined.The modification mechanism of SSBS was analyzed by means of FTIR,XRD,SEM and EDS.The results showed that when the composite modifier consisting of potassium hydroxide,lithium hydroxide and disodium hydrogen phosphate was added by 3wt.%,0.6wt.%and 3wt.%of sodium silicate respectively,the thermal strength,4h strength,24h strength and 800°C residual strength of modified samples were 1.313MPa,2.889MPa,2.616MPa and 0.751MPa.Compared with the unmodified samples,the thermal strength,4h strength and 24h strength of the modified samples was improved by 66%,37%and 56%.However,the 800°C residual strength of modified samples didn't reduce but increased by 27%,so the collapsibility was not improved.The composite modifier could change the intermolecular bonding of sodium silicate,which promoted the bonding bridges of the modified samples smoother and denser.Therefore,the strength of the samples at room temperature were significantly improved.Based on the addition of potassium hydroxide,lithium hydroxide and disodium hydrogen phosphate,when the additive amount of nano Mg O and nano Si O₂ were1.5wt.%and 1wt.%of sodium silicate respectively and the ultrasonic treatment time was 15 minutes,the thermal strength,4h strength,24h strength and 800°C residual strength of the samples were 1.461MPa,3.602 MPa,3.326 MPa and 0.541 MPa.The modification of nano powder and ultrasonic treatment could enhance the peak value of Si-O bond and P-O bond of the binder,which improved the strength of the samples at room temperature.The existence of nano particles could split the bonding film and promote the formation of long cracks on the bonded bridges in the process of heating at 800°C,which resulted in the decrease of residual strength.Based on the modification of composite inorganic modifier,nano powder and ultrasonic treatment,when the additive amount of Ti O₂,Zn O and Si C were 0.4wt.%,0.3wt.%and 1wt.%of the raw sand,the thermal strength,4h strength,24h strength and800°C residual strength of the samples were 1.921MPa,4.532 MPa,4.253 MPa and0.379MPa.After adding the breakdown agent,the gap between the sand particles reduced obviously and the number of bond bridges between the sand particles increased,so the normal temperature strength of the sample was improved.Furthermore,the coefficient of thermal expansion and cold shrinkage of each component of the breakdown agent were different,which resulted in bonded bridges damaged seriously after being heated at 800°C and facilitated further reduction of residual strength.The optimum hardening process parameters were obtained as following:the core box temperature was 150°C and the heating time was 120s.The available time of modified molding sand was 2.5h.The gas evolution of modified samples at 800°C reached the maximum value of 5.3 ml/g/s in the 22th second,and the gas-generating rate of modified samples at 800°C reached the maximum value of 1.1 ml/g/s in the 11th second,which could meet the requirements of foundry production and reduce the occurrence of blow holes and other defects in castings.