1. Introduction
Cylinder heads are crucial components in engines, whether for automotive, locomotive, or industrial machinery. Their complex structures, high – performance requirements, and the need for precise manufacturing make the casting process a key area of focus. The quality of cylinder head castings directly affects the engine’s performance, reliability, and durability. This article aims to comprehensively analyze the casting process design and optimization of cylinder heads, covering aspects such as material selection, process planning, defect prevention, and process improvement through case studies.
2. Cylinder Head Structures and Their Impact on Casting
2.1 Complex Internal Cavities
Cylinder heads often have intricate internal structures, including multiple channels for gas flow (intake and exhaust ports) and coolant circulation (water jackets). For example, in the 12V190 cylinder head, the internal water cavity has numerous curved surfaces, and in the EN31 aluminum alloy cylinder head, the 气道水套 (air passages and water jackets) are cross – stacked. These complex cavities pose challenges in terms of sand core design and placement during casting. Table 1 summarizes the characteristics of different cylinder head internal structures.
| Cylinder Head Model | Internal Structure Feature | Impact on Casting |
|---|---|---|
| 12V190 | Many curved surfaces in the water cavity | Difficult to ensure sand core accuracy and stability during casting, may lead to uneven wall thickness |
| EN31 | Cross – stacked air passages and water jackets | Requires precise sand core alignment to avoid defects in the overlapping areas |
2.2 Varied Wall Thickness
Some cylinder heads have areas with significant differences in wall thickness. The 12V190 cylinder head has locally thin walls, which can cause issues like shrinkage porosity and uneven cooling during solidification. Table 2 shows the wall thickness distribution and its potential problems in different cylinder heads.
| Cylinder Head Model | Wall Thickness Range | Potential Problems |
|---|---|---|
| 12V190 | Local areas as thin as 3 – 5 mm (before improvement) | Shrinkage porosity, cold shuts due to rapid cooling |
| EN31 | Basic wall thickness 5 mm | Difficulty in achieving uniform solidification, risk of internal defects |
3. Material Selection for Cylinder Heads
3.1 蠕墨铸铁 (Compacted Graphite Iron) for 12V190 Cylinder Head
The 12V190 cylinder head is made of compacted graphite iron. This material offers high – temperature resistance and excellent fatigue resistance, which are essential for its application in gas engines. The chemical composition of the compacted graphite iron is carefully controlled. Table 3 shows the technical requirements and actual test results of the chemical composition for the 12V190 cylinder head.
| Element | Technical Requirements (%) | Test Results (%) |
|---|---|---|
| C | 3.65 – 3.85 | 3.77 |
| Si | 2.2 – 3.0 | 2.63 |
| Mn | 0.5 – 0.8 | 0.73 |
| P | ≤0.040 | 0.031 |
| S | ≤0.030 | 0.010 |
| Cu | 0.5 – 0.7 | 0.66 |
3.2 铝合金 (Aluminum Alloy) for EN31 Cylinder Head
The EN31 cylinder head is made of ZL101A aluminum alloy. This alloy is chosen for its low density, good thermal conductivity, and relatively high strength – to – weight ratio, which are beneficial for automotive engine applications. The chemical composition of ZL101A is also strictly regulated. Table 4 shows the chemical composition of ZL101A alloy.
| Element | Content (%) |
|---|---|
| Si | 6.5 – 7.5 |
| Mg | 0.25 – 0.45 |
| Ti | ≤0.20 |
| Fe | ≤0.20 |
| Cu | ≤0.10 |
| Mn | ≤0.10 |
| Zn | ≤0.10 |
| Al | Balance |
4. Casting Process Design
4.1 General Process Plan
4.1.1 12V190 Cylinder Head
The 12V190 cylinder head casting process is designed with considerations of existing 工装 (tooling) and similar casting experiences. It adopts a one – mold – two – piece design, with a sand – to – iron ratio of 1.66:1 and a casting linear shrinkage rate of 1%. The machining allowance for critical surfaces such as the combustion surface, intake surface, and exhaust surface is set to 5 mm. A combination of machine molding, machine core – making, and manual core – making is used. The casting mold is coated with a water – based graphite coating by pouring, and the sand core is coated with an alcohol – based graphite coating through brushing and dipping. The mold assembly and core – setting process follows a layered core – assembly technique.
4.1.2 EN31 Cylinder Head
For the EN31 cylinder head, due to its complex structure, a more elaborate process plan is required. The casting process may involve techniques such as tilt – pouring to ensure better filling and solidification. The mold design needs to take into account the accurate positioning of sand cores for the complex internal cavities.
4.2 Core – Making Materials and Processes
4.2.1 12V190 Cylinder Head
- Furan Resin Sand and 覆膜砂 (Coated Sand): The upper and lower molds and the peripheral contour core of the 12V190 cylinder head are made of furan resin sand. The internal cavity cores for the water chamber and air passages are made of coated sand. Coated sand is chosen for its good dimensional accuracy and excellent collapsibility, which is beneficial for cleaning the internal cavity cores after casting.
- Electrode Graphite for Bolt Hole Cores: The 12V190 cylinder head has six evenly – distributed bolt – connecting holes cast directly. The bolt – hole cores are made of electrode graphite. This material not only ensures the strength of the core to prevent breakage during mold assembly but also has a chilling effect to prevent local shrinkage porosity in the casting.
4.2.2 EN31 Cylinder Head
The EN31 cylinder head, with its complex internal structure, may use special – formulated sand core materials to ensure dimensional stability and good collapsibility. For example, some high – performance resin – bonded sands may be used to meet the requirements of complex cavity formation.
4.3 Mold Venting Design
4.3.1 12V190 Cylinder Head
During the casting process of the 12V190 cylinder head, furan resin sand and coated sand will release a large amount of gas under the action of high – temperature molten iron. To prevent gas – related defects such as porosity, an effective venting system is designed. The upper mold is equipped with 5 flat risers (30 mm×10 mm×300 mm), and the top of the triangular core, spark plug core, and auxiliary water – chamber core is designed with Ø10 mm×300 mm vent holes. The lower mold sidewall has venting grooves, and additional vent holes can be drilled manually if necessary.
4.3.2 EN31 Cylinder Head
In the casting of the EN31 cylinder head, especially when using tilt – pouring, the venting design needs to consider the changing position of the molten metal during the pouring process. Venting channels are designed to ensure that gas can be effectively discharged from the mold cavity to avoid the formation of gas – related defects.
4.4 Pouring System Design
4.4.1 12V190 Cylinder Head
Based on the balanced solidification theory, the pouring system of the 12V190 cylinder head is designed to ensure smooth molten iron filling, good slag – skimming effect, and uniform mold temperature distribution. It consists of a sprue with molten iron filtering function, a pouring basin, and slow – flow horizontal runners distributed in the upper and lower molds. The horizontal runners allow the molten iron to flow through the “lap – joint” form of the upper and lower molds. A “step – type” internal runner is designed to make the molten iron enter the casting in a semi – enclosed manner. The ratio of the cross – sectional areas of the internal runner, mold runner, and sprue is 内模在. This pouring system can make the molten iron filling stable, with good slag – skimming performance, and promote uniform solidification of the casting, reducing the occurrence of casting defects such as slag inclusions, cold shuts, and porosity.
4.4.2 EN31 Cylinder Head
For the EN31 cylinder head, the pouring system is designed according to its structure and the requirements of tilt – pouring. The design focuses on ensuring that the molten aluminum alloy can evenly fill the complex cavity, and the slag and gas can be effectively separated. The shape and size of the runners and gates are carefully calculated to control the flow rate and pressure of the molten metal.
4.5 Melting and Pouring Process
4.5.1 12V190 Cylinder Head
- Melting Chemical Composition Control: The melting process of the 12V190 cylinder head follows the principle of “high carbon equivalent, low phosphorus and sulfur content, and a certain amount of manganese content”. Considering the thin – walled areas of the cylinder head, the carbon content is controlled at the upper limit.
- 蠕化及孕育处理 (Creeping and Inoculation Treatment): The tapping temperature is controlled at 1510 – 1550 °C. Creeping treatment is carried out by in – stream addition in the ladle, and after that, an inoculant is added to the molten iron surface for inoculation treatment. After the inoculation, 0.1% – 0.15% of cryolite powder (by weight of the total molten iron) is covered on the molten iron surface, and the slag is skimmed at least 4 – 5 times using perlite until the slag and impurities in the ladle are completely removed. The pouring temperature is controlled at 1360 – 1390 °C, the pouring speed is 20 – 25 s/ mold, and the pouring mass is 200 kg/ mold. During the pouring process, attention is paid to gas – guiding and slag – blocking operations to ensure a smooth and orderly pouring process. After pouring, the mold is cooled for 12 h before opening and cleaning.
4.5.2 EN31 Cylinder Head
The melting and pouring process of the EN31 cylinder head also requires strict control. The melting temperature of the aluminum alloy is precisely controlled to ensure the quality of the molten metal. During pouring, factors such as pouring speed and tilt – angle (in tilt – pouring) are carefully adjusted to ensure proper filling and solidification of the casting.
5. Process Testing and Verification
5.1 12V190 Cylinder Head
After the casting process design of the 12V190 cylinder head is completed, metal molds and other tooling are manufactured. Process tests are then carried out. During the tests, operations such as molding, core – making, and coating are carried out in accordance with the process design requirements. In the mold – assembly process, the internal cavity resin – sand core and coated – sand core are assembled in four layers following the principle of “from outside to inside, from bottom to top”. After the tests, the samples are subjected to various inspections. Table 5 shows the inspection results of the 12V190 cylinder head samples.
| Inspection Item | Requirements | Test Results |
|---|---|---|
| Chemical Composition | As shown in Table 3 | Met the requirements |
| Mechanical Properties | Tensile strength ≥ 300 MPa, Yield strength ≥ 240 MPa, Elongation ≥ 1.5%, Hardness 170 – 217 HB | Tensile strength 402 MPa, Yield strength 330 MPa, Elongation 4.0%, Hardness 187 HB |
| Metallographic Structure | F + P matrix, Creep rate 60% | F + 45%P matrix, Creep rate 85% |
| Dimensional Accuracy | Contour dimensions meet design requirements, machining allowance is reasonable | Met the requirements |
| Internal Quality | No porosity and other defects in the internal structure | Some areas with thin walls were found before improvement |
However, during the internal density inspection of the first – batch test cylinder heads, it was found that the local wall thickness between the spark – plug hole and the air passage in the lower water chamber was only 3 mm, which was lower than the required 8 mm. The reason was that the bottom of the spark – plug hole core had no sand – core positioning, and the round – steel core support only played a supporting role, resulting in the deviation of the sand – core position during pouring. To solve this problem, the structure of the spark – plug hole sand core was improved by designing a process core – head and canceling the round – steel core support. After the improvement, the problem of thin walls was effectively solved.
5.2 EN31 Cylinder Head
For the EN31 cylinder head, after the process design is completed, trial production is carried out. The trial – produced castings are inspected by X – ray 探伤 (radiography) and mechanical property tests after T6 heat treatment. The results show that the casting quality meets the technical requirements, which verifies the effectiveness of the designed casting process. Table 6 shows the inspection results of the EN31 cylinder head samples.
| Inspection Item | Requirements | Test Results |
|---|---|---|
| Hardness | 90 – 120 HBW, single – piece hardness difference ≤ 10 HBW | Met the requirements |
| Yield Strength | ≥ 180 MPa | Met the requirements |
| Tensile Strength | ≥ 295 MPa | Met the requirements |
| Elongation | ≥ 4% | Met the requirements |
| Pinhole Degree | ≤ 2 levels according to JB/T7946.3 – 2017 | Met the requirements |
| Internal Quality | No cracks, cold shuts, shrinkage porosity, etc. | Met the requirements |
6. Optimization and Improvement of the Casting Process
6.1 Optimization of Sand Core Design
Based on the problems found in the process tests, the sand core design of cylinder heads can be optimized. For example, in the 12V190 cylinder head, the improved spark – plug hole sand core structure ensures accurate positioning during the core – setting process and enhances the stability of the sand core during pouring. In general, for cylinder heads with complex internal structures, the use of advanced sand – core manufacturing techniques, such as 3D printing of sand cores, can improve the accuracy of sand – core shape and position, reducing the risk of casting defects.
6.2 Adjustment of Pouring Parameters
The pouring parameters, such as pouring temperature, pouring speed, and pouring volume, can be adjusted according to the actual situation. In the 12V190 cylinder head casting, if the pouring temperature is too high, it may cause excessive gas evolution from the sand core and increase the risk of porosity. By precisely controlling these parameters, the quality of the casting can be improved. For the EN31 cylinder head in tilt – pouring, the tilt – angle and the change of pouring speed during the tilt – pouring process also need to be optimized to ensure uniform filling and solidification of the casting.
6.3 Improvement of Molding Materials and Processes
The selection of molding materials can be further optimized. For example, new types of resin – bonded sands with better performance can be explored to improve the strength and collapsibility of the sand mold and sand core. In addition, the molding process can be improved. For instance, the use of vacuum – assisted molding techniques can enhance the compactness of the sand mold, reducing the occurrence of defects such as sand – inclusion.
7. Conclusion
The casting process design and optimization of cylinder heads are complex and systematic projects. Through in – depth analysis of cylinder head structures, rational selection of materials, scientific design of casting processes, strict process testing and verification, and continuous optimization and improvement, high – quality cylinder head castings can be produced. The case studies of the 12V190 and EN31 cylinder heads illustrate the importance of each link in the casting process. By continuously improving the casting technology, the production cost can be reduced, the product quality can be enhanced, and the requirements of modern engine manufacturing for high – performance cylinder heads can be better met. Future research can focus on the application of new materials, new casting technologies (such as additive manufacturing in casting), and more precise process control methods to further improve the quality and performance of cylinder head castings.
