For different casting defects (such as sand sticking, porosity, sintering, veining, shrinkage porosity at hot spot, etc.), the foundry should analyze the defect characteristics and causes together with the coating suppliers, and reasonably select casting coatings with different functions.
1. Abnormal blowhole of gen3 cylinder block
During the test of ZHY casting anti veining coating, abnormal pores appeared at the position of cross beam and marking surface of gen3 cylinder block after processing (as shown in Figure 1), and the coating for normal production has no such defects. According to the existing technical conditions of our company, the process parameters of two kinds of coatings are compared. The gas evolution of asico coatings is about 3ml / g higher than that of current coatings. Due to the limitation of sand core structure conditions, the gas emission capacity of pores can not be increased. Through negotiation with suppliers, the gas evolution of coatings is reduced to be close to that of current coatings, but the effect is not obvious.
The results show that the shape of sand core is complex and heated seriously, and there is no exhaust channel. The analysis shows that: when the permeability of the coating is too high, it is difficult to block the gas decomposed by the resin, and it is easy to produce porosity defects, so the permeability of the coating should be reduced to achieve the purpose of blocking the gas escape from the sand core . The air permeability of the coating was reduced from 38 to 33 by means of refining and grading the size of refractory aggregate and improving the permeability and covering ability of the coating.
The gas evolution and permeability of the coating are related to the formulation of the coating. Different castings have different sensitivity to the gas evolution and permeability of the coating. Under the condition that the exhaust capacity of the casting can not be increased, the gas evolution and permeability of the coating should be adjusted according to the specific situation to achieve the purpose of releasing or plugging the gas in the sand core. For gray cast iron cylinder, it is generally recommended that the gas evolution of the coating should be controlled within 40 ml / g and the air permeability should be controlled between 30 ~ 38. It should be pointed out that the one-sided pursuit of too low gas generation, too high or too low air permeability will affect the other properties of the coating and break the balance of various indicators of the coating.
2. Sintering at process socket of gen3 cylinder block
The process nest of gen3 cylinder block is “island shaped sand core” (as shown in Fig. 2). During the pouring process, it is surrounded by molten iron. It is difficult to resist the thermal effect of high temperature molten iron on the sand core by using ordinary water-based coating, and it is easy to produce sintering defects (as shown in Fig. 3). A special process is needed for cleaning. Nearly 1000 pieces of products to be treated have been overstocked, which seriously affects the cleaning efficiency of castings. According to the characteristics of the sintering defects, the coating with good heat insulation effect should be selected to reduce the erosion of high temperature molten iron to the sand core and avoid the penetration of molten iron into the sand core to form the defects of iron coated sand. Through the test, the following conclusions are drawn: the heat insulation coating and anti veining coating have the above functions, which can effectively solve the sintering defects at the process socket of the cylinder block.
3. EA113 cylinder block local sintering and water jacket veining
The EA113 cylinder block developed by our company in the early stage had local sintering (broken core) defects at the bottom center of both ends of the water jacket (as shown in Fig. 4). The pneumatic milling cutter and other cleaning tools were too long to reach, which seriously affected the cooling water circulation in the inner cavity of the engine cylinder block and had to be scrapped. Sometimes, the single scrap rate was as high as 10%, Seriously affect the company’s economic efficiency and quality reputation. Through the analysis of the defect parts, it is found that the weak part of the whole water jacket sand core is the part where local sintering (core breaking) occurs. The wall thickness is only about 3.5mm, and it is located at the far end of the nozzle, which is easy to produce loose defects. When the temperature of quartz sand reaches a certain temperature, phase transformation occurs β Quartz sand to α When the resultant force between quartz and the buoyancy of molten iron is greater than the high temperature cohesive force of the sand core in this part, cracks will occur, veinlet defects will occur in slight cases, and partial sintering (core breaking) will occur in severe cases. According to the causes of the defects, the coating should be selected to delay the phase transformation time of silica sand or increase the concession of sand core. The anti veining coating meets the above requirements. The test results show that the effect is good, and the veining, sintering and core breaking defects of the cylinder block water jacket are effectively controlled. In view of Veining defect, casting coating should be selected reasonably. The author has used tellurium powder coating and graphite coating to solve the veining defect of cylinder water jacket, and the result shows that the veining defect is larger. The analysis shows that tellurium is a highly active element, which can strongly reduce the surface tension of molten iron, strengthen the penetration of molten iron, and promote the generation of Veining defects, while graphite coating seriously aggravates the veining defects of water jacket due to its good thermal conductivity.
4. Leakage of cylinder head
The leakage defect occurred in the cylinder heads of platinum four and six cylinders produced by our company. After analysis, the leakage was caused by shrinkage at the hot spot of the casting. In order to solve the problem, a layer of chilling tellurium powder coating was applied at the leakage, and then the conventional coating was applied normally. It is worth noting that the amount of te must be strictly controlled in the preparation of chilling tellurium powder coating. On the premise of ensuring the compact hot spot structure and no shrinkage leakage, the less the amount of TE, the better. Generally, it can be controlled at about 10%. Due to the high density of metal powder, the suspending agent and thickener with good suspension performance should be used, and they should be stirred evenly before use. In order to prevent metal powder precipitation, it is better to dip another layer of coating on the chilling coating to ensure that the tellurium powder coating has good high temperature iron erosion resistance (to avoid te spalling with low melting point), resulting in hard spots in other parts of the casting, It affects the mechanical properties of castings.
As mentioned above, our company used tellurium powder coating and graphite coating to solve the cylinder water jacket veining defects, and the results showed that the veining defects were more serious. The analysis shows that te is a highly active element, which can strongly reduce the surface tension of molten iron, strengthen the penetration of molten iron, and promote the generation of Veining defects, while graphite coating has good thermal conductivity, which seriously aggravates the veining defects of water jacket.