Nodular cast iron, as a kind of high strength cast iron material with properties close to steel, has the advantages of low manufacturing cost, good ductility, excellent fatigue strength and wear resistance, and excellent mechanical properties It is widely used in machine bed, valve, crankshaft, piston, cylinder and other parts of automobile engine. Lost mold casting technology is a kind of technology without parting surface, The near net forming method of complex precision casting without sand is widely used in the production of cast steel, cast iron, aluminum alloy and magnesium alloy parts. Lost mode ductile iron, made of Due to its high carbon content, graphitization expansion and other characteristics during solidification, casting surface wrinkling, shrinkage cavity and porosity, carbon black and other defects are often caused by poor coating performance. As a result, Improving coating properties of epc is an important link to ensure casting yield.
In epc production, the quality of coating is very important to the casting quality. Epc coating should have good permeability, strength, sintering and peeling properties Such as performance. At present, research is generally carried out on the working properties of coating, the interaction mechanism between coating and EPS decomposition products, the effect of coating on casting quality and surface composite coating Research, in the coating composition improvement, additive ratio of coating performance, coating preparation process. From the production and use of epc coating, the production of coating should be standardized Preparation process, high casting yield; The lost mold casting coating market is chaotic, the coating formula is complex and expensive, the preparation process is cumbersome, and even the formula composition is wrong. It not only affects the yield of epc casting, but also hinders the development of EPC technology and coating industry.
1 Coating requirements of ductile iron in epc casting
The casting temperature of nodular cast iron is usually 1380 ~ 1480℃, slightly lower than that of steel castings. The density of nodular cast iron is 7.3g/cm3, much higher than that of magnesium and aluminum alloy, so the nodular cast iron liquid The effect of heat and force on the coating during filling is more significant than that of magnesium and aluminum alloy. In the ductile iron process of epc, the coating works because of the vacuum negative pressure process Under the state, on the one hand, the inner side of the coating needs to resist the dynamic pressure and static pressure of the high temperature ductile iron liquid, and the inner and outer pressure difference of the coating is large, and the coating is easy when the high temperature strength is insufficient Cause casting surface sag or even casting deformation. Nodular cast iron is characterized by high casting temperature and rapid decomposition of EPS. Gaseous products account for the majority of decomposition products, including a small amount of liquid State products and solids. Decomposition products are generated in large quantities and fill the whole coating cavity, in order to avoid the failure of decomposition products to discharge the coating, resulting in ductile iron pores and wrinkles Coating should have good air permeability for the occurrence of defects such as skin and carbon deposition. The coating performance index should be controlled within a reasonable range in affecting the strength and permeability of the coating. The refractoriness, sinter property, aggregate shape and particle size have direct influence on the strength and permeability of epc coating. In the process of coating preparation, fire-resistant bone Material selection is particularly important.
2 Formulation and process of ductile iron coating for epc
Refractory aggregate is the main component of coating. The performance of coating is closely related to the physical and chemical properties of refractory aggregate. The use of suspension prevents refractory aggregate in the paint Sedimentation, so that the coating has good thixotropy. During the pouring process, the coating needs to undergo a strong thermal effect, and the compound use of different binders ensures the coating preparation The coating has the ability to maintain strength over a range of temperatures. Coating from raw materials to the formation of coating to prepare, coating and drying three main processes. High quality coating Material in addition to good performance, its technological performance is also very important, coating preparation, coating process processes need to be simple, convenient operation, coating surface smooth, no pinhole, crack and so on.
3 Coating properties of ductile iron epc casting
3.1 Coating strength
As a refractory coating coating on the surface of the shape, the vanishing mode coating with high strength can not only improve the strength and stiffness of the shape, but also can be used as a metal liquid and mold The effective barrier between the sand ensures that the coating can withstand the dynamic and static pressure and external adsorption pressure brought by the high temperature metal liquid filling process, and reduces the mechanical sand sticking of the casting. So coating strength is an important index to measure the performance of coating.
3.2 Study on porosity and permeability of coatings
Molten metal pouring, EPS appearance under the action of high temperature rapidly gasification decomposition, with the metal liquid front forward, decomposition products from the cavity through the coating discharge, metal liquid The discharge of mold filling and decomposition products is in dynamic equilibrium. If the air permeability of the coating is too low, the decomposition products cannot be discharged from the cavity in time, which will lead to the pores and carbon deposition defects under the skin of the casting And so on. If the air permeability of the coating is too high, the mold filling speed is fast, easy to cause mechanical sand sticking. With the increase of coating thickness, the influence of coating density on air permeability will decrease gradually. High permeability The coating has larger average particle diameter and wider particle size distribution.
4 Influence of coating on defects of nodular cast iron
4.1 Influence of coating on casting surface wrinkling defects
In lost mode casting, when EPS appearance meets the decomposition process of high-temperature metal liquid, the liquid decomposition products formed float on the surface of metal liquid or stick to the coating, which is difficult to be completely decomposed. When the metal When the liquid is cooled and solidified, the surface tension of the decomposed residue is different from that of the liquid metal. As the casting shrinks, wavy or trickle folds are formed on the casting skin.
4.2 Effect of coating on casting carbon deposition defect
The carbon deposition defect is caused by the large amount of generated carbon adsorbed on the surface of the coating, which cannot be discharged from the cavity and adhered to the surface of the casting. It is usually reflected in the casting as the carbon film with bright surface. The concave of casting is filled with carbon black, etc. In epc production, casting defects are closely related to appearance material, casting composition, coating and casting process.
5 Research direction of coatings for ductile iron epc casting
Nodular cast iron lost mold casting, because of its high pouring temperature, foam appearance large gas generation, negative compaction casting. The coating of nodular cast iron is subjected to molten metal during mold filling Strong scour effect and internal and external pressure difference, the coating must have good strength at high temperature, in order to prevent the metal liquid flow process caused erosion of the coating and affect the casting table Surface quality, so the composition and properties of refractory aggregate and the influence of additives on the strength of coating become the key to improve the performance of coating.