0 Introduction
Automotive engine cylinder heads have a complex structure and are generally cast from cast iron or aluminum alloy materials, as shown in Figure 1. The structure of the cylinder cavity is complex, and the local thickness is easy to use Shrink. The cylinder head is mounted on top of the cylinder block and seals the cylinder from above and forms the combustion chamber. It is often in contact with high-temperature and high-pressure gases and is therefore subjected to high thermal and mechanical loads. The main waste leakage and sand hole, the manufacturing process is complex and the parameters are many and the process is changeable. There are many casting production processes, and each process has a wide variety of paper process data record tables, which are scattered in different stations, and the query, statistics and analysis of data are time-consuming, and the annual output of two types of castings exceeds 200,000 pieces, and the failure rate is more than 5%, and the quality fluctuates greatly. At present, the production of domestic automobile cylinder blocks and cylinder heads mainly adopts green sand, resin self-hardening sand and triethylamine cold core forming process. In the later stage of solidification of castings, the gas solute in the liquid phase is highly concentrated, and when the shrinkage occurs at the hot joint, vacuum holes will be generated, and the existence of holes provides a strong channel for the precipitation of gas, which then hinders the replenishment of metal liquid and aggravates the formation of shrinkage defects. Common defects in cylinder heads are shown in Figure 2. The use of computer numerical simulation technology to simulate the investment precision casting process can understand the casting solidification process in advance and predict the defects generated in the casting process, and can determine the ideal process scheme before the actual production, avoid technical risks, and improve product quality. Huazhu CAE/InteCAST software system (hereinafter referred to as Huazhu CAE system) of Huazhong University of Science and Technology is one of the most powerful and widely used casting process analysis software in China, and the prediction of shrinkage porosity and shrinkage porosity defects has reached a quantitative level. Using CAE software, taking the process of cylinder head casting of an automobile engine as an example, this paper introduces the influence of the key process parameters of sand casting, namely pouring temperature and mold temperature, on shrinkage defects by using CAE simulation software.
Figure 1 Three-dimensional view of cylinder castings, gating systems, etc
Figure 2 Defect topography of a cylinder head casting
1 Test materials and methods
1.1 Test Materials
The casting material is compacted graphite cast iron, the model is RU450, and the molding sand is for green sand casting Sand, reused sand + bentonite + pulverized coal and other additives, the sand core is mainly cold box process, silica sand or recycled sand + cold core resin. As shown in Table 1, the main process parameters are pouring temperature, mold temperature, filling time, etc., and the pouring temperature and mold temperature are selected as the main influencing factors for simulation experiments. The simulation protocol is shown in Table 2.
Table 1 Process parameters
| 工艺参数 | 浇注温度/℃ | 铸型温度/℃ |
| 范围 | 1360~1400 | 20~40 |
Table 2 simulates the experimental protocol
| 编号 | 浇注温度/℃ | 铸型温度/℃ |
| 1 | 1360 | 20 |
| 2 | 1370 | 20 |
| 3 | 1380 | 20 |
| 4 | 1390 | 20 |
| 5 | 1400 | 20 |
| 6 | 1360 | 30 |
| 7 | 1370 | 30 |
| 8 | 1380 | 30 |
| 9 | 1390 | 30 |
| 10 | 1400 | 30 |
| 11 | 1360 | 40 |
| 12 | 1370 | 40 |
| 13 | 1380 | 40 |
| 14 | 1390 | 40 |
Table 3 Meshing Scheme
| 网格 类型 | 网格 类型 | 铸件网格 数/个 | 最大边长 /mm | 最小边长 /mm | 浇注重量 /kg | 铸件毛重 /kg | 工艺出品率 (%) |
| 均匀 网格 | 13296465 | 960627 | 3.5 | 3.5 | 408 | 283 | 69.33 |
1.2 Numerical simulation software and methods
The numerical simulation software used is Huazhu CAE system, which mainly uses its gravity compensation function to predict the formation of shrinkage porosity and shrinkage porosity during solidification. In the pre-treatment stage, the 3D model of the casting, as well as the pouring system, is meshed, and the meshing scheme is shown in Table 3.
2 Test results and analysis
2.1 Simulation results of color temperature for casting solidification
The simulation results are shown in Figure 3, due to the use of the flat group flat assembly vertical pouring process and the bottom injection top rise system, the solidification of the cylinder head is carried out from the bottom to the top in order.
Figure 3 Solidification color temperature of castings
2.2 Simulation results of porosity defect evolution
In the temperature range of the experiment (pouring temperature 1 360~1 400 °C, mold temperature 20~40 °C), it can be seen from Figure 4 that there are different amounts of shrinkage porosity in the hot joint position inside the cylinder head.
Figure 4 Simulation results of crater defects
2.3 The influence of pouring temperature and mold temperature on the number of shrinkage defects
Figure 5 Number of shrinkage holes in the cylinder head at different pouring temperatures
The number of crater defects at different pouring temperatures is shown in Figure 5, when the mold temperature of the cylinder head is 20 °C, the number of shrinkage holes decreases first and then increases with the increase of pouring temperature, and the number of shrinkage holes is at least 22 when the pouring temperature is 1 370 °C, and when the casting temperature is 30 °C and 40 °C, the number of shrinkage holes is at least 22The number of shrinkage porosity increased with the increase of injection temperature, and the number of shrinkage porosity was at least 24 and 22 at the pouring temperature of 1 360 °C, respectively.
Fig. 6 Number of crater holes in the cylinder head at different mould temperatures
The number of crater defects at different mold temperatures is shown in Figure 6, and the number of shrinkage porosity is not sensitive to the mold temperature in the range of 20~40 °C when the pouring temperature is between 1 370~1 400 °C.
2.4 Sensitivity analysis of shrinkage porosity defects to pouring temperature and mold temperature
From Figures 5 and 6, it is easy to see that the number of shrinkage holes has an approximate linear relationship with the pouring temperature and the mold temperature, and the linear correlation coefficient r between the number of shrinkage holes and the pouring temperature and the mold temperature can be obtained. by (2-1).
When the mold temperature is 20 °C, the correlation between the number of shrinkage porosity and the pouring temperature is r1=0.725 1, the correlation coefficient between the number of shrinkage porosity and the pouring temperature is r2= 0.919 9 when the mold temperature is 30 °C, and the correlation coefficient between the number of shrinkage porosity and the pouring temperature r3= 0.910 5 when the mold temperature is 40 °C;The correlation coefficient between the number of shrinkage porosity and the mold temperature r4= -0.993 4 when the injection temperature is 1 360 °C, and the variance of the number of shrinkage porosity is 0 when the pouring temperature is 1 370 °C, which is considered irrelevant. When the pouring temperature is 1 380 °C, the variance of the number of shrinkage pores is 0, which is considered irrelevant. When the pouring temperature is 1390 °C, the correlation coefficient between the number of shrinkage porosity and the mold temperature r5= 0 is considered to be irrelevant. At a pouring temperature of 1 400 °C, the correlation coefficient between the number of shrinkage porosity and the mold temperature r6= -0.866. In summary, it can be seen that when the casting temperature is 20~40 °C and the pouring temperature is 1 360~1 400 °C, the influence of pouring temperature on the number of shrinkage porosity is more obvious than that of the casting temperature, that is, the number of shrinkage porosity is more sensitive to the pouring temperature.
3 Conclusion
(1) The cylinder head is made of compacted graphite cast iron RU450, which has a complex internal cavity structure, which is locally thick and easy to shrink, and is prone to shrinkage porosity defects at the internal thick wall.
(2) When the casting temperature of the cylinder head is 20 °C, with the increase of the pouring temperature, the number of shrinkage holes decreases first and then increases, when the pouring temperature is 1 370 °C, the number of shrinkage holes is at least 22, when the casting temperature is 30 °C and 40 °C, the number of shrinkage holes increases with the increase of pouring temperature, and when the pouring temperature is 1 360 °C, the number of shrinkage holes is at least 24 and 22, respectively.
(3) When the pouring temperature is 1 370~1 400 °C, the number of shrinkage pores is not sensitive to the casting temperature at 20~40 °C.
(4) When the casting temperature is 20~40 °C and the pouring temperature is 1 360~1 400 °C, the influence of pouring temperature on the number of shrinkage porosity is more obvious than that of the casting temperature, that is, the number of shrinkage porosity is more sensitive to the pouring temperature.