1.Effect of the amount of suspending agent on the properties of spray casting coating
Based on the basic research and development experiments, the effects of different addition amounts of Attapulgite on the properties of casting coatings were investigated.
The addition amount of attapulgite directly affects the suspension stability, shear dilution index and viscosity of sprayed casting coating. The suspension and viscosity of casting coating increase with the addition amount of attapulgite. Considering the suspension property, shear dilution index and viscosity index, the appropriate amount of attapulgite suspension agent is 2.5% ~ 3.0% of the total mass of casting coating.
2.Effect of binder and addition amount on properties of sprayed casting coating
The addition of binder is too low and the coating strength is low; The addition amount is too high and the brushing performance is poor (the brush cannot be opened and the brush mark is large). Based on the previous experiments, the effect of binder on the properties of casting coating was investigated by selecting different binders (PVA and CMC) and their addition amount. The results are shown in Figure 1 (the addition amount of fixed attapulgite suspension is 2.8%).
With the increase of the amount of binder, the wear resistance and shear dilution index of the coating can be significantly improved, but the gas evolution of the casting coating will be increased, which will have adverse effects on the casting, such as gas producing holes and other defects. Through many comparative experiments and integrating various factors, the casting coating prepared with the addition amount of binder PVA at 1.0% (accounting for the mass fraction of refractory) and CMC at 0.2% (accounting for the mass fraction of refractory) has the advantages of low binder content, small air generation and good spraying effect.
3.Effect of modified additives on the properties of spray casting coatings
The water-based spraying casting coating for large castings belongs to high solid content casting coating, and because the brushing, spraying and other operations are carried out under the condition of high Baume degree, especially spraying, the Baume degree is 90 ~ 100 ° B é. If the casting coating is not modified, the spraying operation is more difficult. Either it is easy to block the high-pressure nozzle, or the nozzle is damaged faster and replaced frequently, which has an adverse impact on the spraying operation.
Finally, an end group modified silicone efka 3288 was selected to improve the spraying performance of casting coatings. Effect of modified additives on properties of casting spray coating.
Adding an appropriate amount of efka 3288 can significantly improve the shear dilution index of casting coatings, improve the rheological properties of casting coatings, and improve the spraying properties of casting coatings accordingly. However, the addition amount shall not be too much, otherwise the solid content of casting coating will be excessively reduced, which will lead to excessive carrier liquid dispersion loss in the spraying process, the coating is not easy to form, and the baking and drying time will be prolonged. According to the experimental results, it is finally determined that the amount of efka 3288 should be 0.4% ~ 0.6%.
4.Effect of dispersion time of casting coating on the properties of sprayed casting coating
The powder must be fully stirred and dispersed to fully wet the refractory particles, so as to ensure the excellent rheology of the casting coating and improve the stability of the casting coating system. Based on the previous experiments, the effect of dispersion time of casting coating in dispersion kettle on the properties of casting coating was investigated.
By prolonging the dispersion time of casting coating to a certain extent, the shear dilution index and suspension of casting coating were improved, while the conditional viscosity decreased slightly; With the continuous extension of dispersion time, the properties of casting coatings tend to be stable. According to the experimental results, the dispersion time of casting coating was finally determined as 30 min.
5.Measures to improve the sintering performance of casting coating and reduce the burning point
High purity white corundum is selected as high temperature refractory aggregate for the casting coating. Because the sintering point of alumina refractories is generally high, the burning point of white corundum used in this experiment is 1882 ℃ measured by the burning point tester, while the casting temperature of molten iron in steelis about 1630 ~ 1650 ℃. Under this condition, because the burning point of refractory is higher than the casting temperature of molten iron, the casting coating layer cannot be sintered to form a dense structure after molten . On the one hand, molten iron will penetrate into the sand mold through refractory particles, resulting in mechanical sand sticking such as acupuncture. On the other hand, it will lead to poor peeling performance of the coating, increase the cleaning workload of castings, increase the scrap rate of castings Costs will also rise.
In order to solve the problems of mechanical sand sticking and poor peeling of alumina casting coatings, sintering additives must be added to reduce the burning point of refractories and lower than the casting temperature of molten iron. Considering the heat conduction after casting and other factors, the actual sintering temperature should be more than 20 ℃ lower than the hottemperature. In this experiment, zinc oxide, cerium oxide and sintering agents mixed with zinc oxide and cerium oxide in different mass ratios were selected. The addition ratio of sintering agent was controlled at 0.8% (accounting for the total mass of refractory). The influence of sintering additives on the sintering properties of casting coatings was measured by measuring the sintering point of casting coatings.
It can be seen from the experimental results that whether zinc oxide or cerium oxide, adding a single component sintering agent can not significantly reduce the burning point of casting coating; The composite sintering agent mixed with zinc oxide and cerium oxide has obvious effect. In this experiment, four different compound ratios were set up, and their mass ratios were 1 ∶ 1, 2 ∶ 1, 4 ∶ 1 and 6 ∶ 1 respectively. The results show that when the ratio of zinc oxide to cerium oxide is 4 ∶ 1, the sintering point is the lowest, 257 ℃ lower than that without sintering additives, and 25 ℃ lower than the lowest molten iron casting temperature. Under this condition, the casting coating can be sintered, which can not only avoid mechanical sand sticking, but also improve the peeling property of the casting coating.