Among all the components of water-based lost foam casting coating for cast iron, refractory aggregate is the main part. The most commonly used refractory aggregate is quartz powder, followed by bauxite, kaolin, talc, graphite powder, corundum powder, wollastonite and other refractory aggregates. When a single refractory aggregate is used, although the fire resistance of lost foam casting coating is high, the comprehensive properties such as suspension, sintering and conditional viscosity are poor. Therefore, in actual production, two or more refractory aggregates are often used to improve the comprehensive properties of lost foam casting coating. In order to explore the influence of different refractory aggregates on the performance of water-based lost foam casting coating for cast iron, four different types of refractory aggregates such as bauxite, kaolin, wollastonite and talc powder were selected in this study, in which bauxite was mainly added with refractory aggregate, kaolin, wollastonite and talc powder were supplemented with refractory aggregate, and the addition amount of other components.
According to the preparation method of lost foam casting coatings, 9 kinds of lost foam casting coatings in the orthogonal test scheme were developed into paste, added with an appropriate amount of water, stirred evenly, adjusted the density of lost foam casting coatings to 1.5 g / cm3, and then measured the 24-h suspension rate, conditional viscosity, gas evolution, coating strength, coating hanging ability and other performance indexes of lost foam casting coatings.
Among all the performance indexes of lost foam casting coatings, the coating performance of lost foam casting coatings is the most important. Through the analysis of orthogonal test results, it is found that the coating performance of the above nine lost foam casting coatings is grade II or above, and the coating performance is good, which can meet the actual production needs of lost foam casting coatings, In order to further explore the effects of various components in refractory aggregate on 24-h suspension rate, conditional viscosity, gas evolution and coating strength of lost foam casting coating.
Among the above three auxiliary refractory aggregates, the most significant factor affecting the 24h suspension rate of lost foam casting coating is kaolin, followed by wollastonite, and finally talc powder. The main reason is that kaolin is similar to bentonite, soft and easy to disperse and suspend in water, which can significantly improve the conditional viscosity and suspension of lost foam casting coating. The most significant factor affecting the coating strength is wollastonite. The order of influencing factors on the coating strength is wollastonite > kaolin > talc powder.
The most significant factor affecting the gas evolution of lost foam casting coating is talc powder, followed by kaolin. The order of influencing factors on the gas evolution of lost foam casting coating is talc powder > kaolin > wollastonite. The main reason is that the molecular structures of talc powder (layered magnesium silicate hydrate) and kaolin (layered silicate containing crystal water) contain crystal water, During pouring, they lose crystal water rapidly in the temperature range of 800 ~ 900 ℃ and 600 ~ 900 ℃ respectively, resulting in a large amount of water vapor, which increases the gas evolution of the coating. The most significant factor affecting the conditional viscosity of lost foam casting coating is kaolin. The order of influencing factors on the conditional viscosity of lost foam casting coating is kaolin > talc powder > wollastonite. The main reason is that kaolin is a kind of clay and clay rock dominated by kaolinite family clay minerals, which has good plasticity, When used in water-based lost foam casting coating, flocculent colloidal aqueous solution is formed, which increases the conditional viscosity of lost foam casting coating. In order to find the best level of three kinds of auxiliary refractory aggregates, the performance indexes of lost foam casting coatings are analyzed by single factor analysis method.
In the actual use of lost foam casting coating, in order to prevent the rapid precipitation of lost foam casting coating and the convenience of lost foam casting coating, lost foam casting coating is usually required to have high suspension rate and low conditional viscosity, so the suspension rate should be the maximum level, i.e. a3b3c1; The conditional viscosity takes the lowest level, i.e. a1b3c1. In addition, in order to prevent the excessive gas evolution of the coating from aggravating the porosity and carbon black defects of the casting, and prevent the defects such as burr, sand flushing, pattern deformation and pattern damage caused by insufficient coating strength, the gas evolution of lost foam casting coating should be at the minimum level, that is, a2b1c1; The coating strength takes the maximum level, i.e. a3b2c3. Among the 24-h suspension rate, conditional viscosity, gas evolution, coating strength, coating hanging ability and other performance indexes of lost foam casting coating, on the premise that the 24-h suspension rate and coating hanging ability meet the application requirements of lost foam casting coating, the first one should meet the coating strength index, followed by the gas evolution index and conditional viscosity index of coating, Combined with the primary and secondary relationship between the properties, the optimal combination is a3b2c1, that is, the best formula of lost foam casting coating.
In order to verify the performance of the lost foam casting coating with the formula, the field pouring test was carried out. After the lost foam casting coating was prepared according to the formula, the lost foam casting coating was coated by dipping and drying for three times. The pattern after lost foam casting coating and the pattern after drying are shown in Figure 1. It can be seen from the figure that the surface coating of the pattern after lost foam casting coating is smooth and uniform, and the coating is good. After it is completely dried in a constant temperature drying oven, the pouring test is carried out under vacuum and negative pressure.
The peeling of the formed lost foam casting casting and the coating after pouring is shown in Figure 2. The results show that the surface of the formed casting is smooth without casting defects such as sand sticking, porosity, carbon deposition and wrinkle. During the cleaning of lost foam castings, most of the surface coatings of lost foam castings can collapse freely. Only a small part of the coatings of hot junction parts need to be gently knocked with a hammer to peel off the whole piece, and the overall collapsibility of the coating is good. This is mainly because the casting temperature of lost foam casting of iron castings is higher, which is 40 ~ 80 ℃ higher than that of other casting methods, generally 1 400 ~ 1 500 ℃. When aggregates with different fire resistance are used together, the sintering of the coating is not only easier, but also the strength of the coating is higher. During casting, the organic binder in the coating is rapidly ablated under the thermal action of high-temperature molten metal, A large number of pores are formed to form a loose lamellar structure. The high temperature permeability of the coating is good because the pyrolysis products of the foam pattern are mainly gaseous products and less liquid products. When the vacuum pressure is poured, the gaseous and liquid products are rapidly drawn out of the cavity, so that the filling of the high-temperature metal liquid is easier, so there is no sand, porosity and carbon deposition. Wrinkle and other casting defects can well meet the actual production needs and have excellent performance.