Research progress of coatings for ductile iron epc casting

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 wear resistance, excellent mechanical properties  It is widely used in machine bed, valve, crankshaft, piston, cylinder 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 magnesium alloy parts.  Lost mode ductile iron, made of  Due to its high carbon content, graphitization expansion other characteristics during solidification, casting surface wrinkling, shrinkage cavity porosity, carbon black 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 peeling properties  Such as performance.  At present, research is generally carried out on the working properties of coating, the interaction mechanism between coating EPS decomposition products, the effect of coating on casting quality surface composite coating  Research, in the coating composition improvement, additive ratio of coating performance, coating preparation process.   the production 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 expensive, the preparation process is cumbersome, even the formula composition is wrong.  It only affects the yield of epc casting, but also hinders the development of EPC technology 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 aluminum alloy, so the nodular cast iron liquid  The effect of heat force on the coating during filling is more significant than that of magnesium 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 static pressure of the high temperature ductile iron liquid, the inner outer pressure difference of the coating is large, the coating is easy when the high temperature strength is insufficient  Cause casting surface sag even casting deformation.  Nodular cast iron is characterized by high casting temperature rapid decomposition of EPS. Gaseous products account for the majority of decomposition products, including a small amount of liquid  State products solids.  Decomposition products are generated in large quantities fill the whole coating cavity, in order to avoid the failure of decomposition products to discharge the coating, resulting in ductile iron pores wrinkles  Coating should have good air permeability for the occurrence of defects such as skin carbon deposition.  The coating performance index should be controlled within a reasonable range in affecting the strength permeability of the coating.  The refractoriness, sinter property, aggregate shape particle size have direct influence on the strength permeability of epc coating.  In the process of coating preparation, fire-resistant bone  Material selection is particularly important.  

2 Formulation 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 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, the compound use of different binders ensures the coating preparation  The coating has the ability to maintain strength over a range of temperatures.  Coating raw materials to the formation of coating to prepare, coating 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 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 only improve the strength stiffness of the shape, but also can be used as a metal liquid mold  The effective barrier between the sand ensures that the coating can withstand the dynamic static pressure external adsorption pressure brought by the high temperature metal liquid filling process, 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 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 the cavity through the coating discharge, metal liquid  The discharge of mold filling decomposition products is in dynamic equilibrium.  If the air permeability of the coating is too low, the decomposition products cannot be discharged the cavity in time, which will lead to the pores carbon deposition defects under the skin of the casting   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 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 stick to the coating, which is difficult to be completely decomposed.  When the metal  When the liquid is cooled solidified, the surface tension of the decomposed residue is different that of the liquid metal. As the casting shrinks, wavy 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 the cavity 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 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 internal 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 affect the casting table  Surface quality, so the composition properties of refractory aggregate the influence of additives on the strength of coating become the key to improve the performance of coating.