Low-pressure casting technology enables castings to fill and solidify under pressure, so it is widely used to produce large-scale engine cylinder heads, etc. With continuous development of information technology, smart casting has also received more and more attention. Low-pressure casting technology is developing in direction of lightweight, digitalization and intelligence. By using numerical simulation software, influence of process factors such as temperature, pressure and cooling rate on solidification and crystallization process of castings can be intuitively analyzed, occurrence of defects such as air entrainment, hot spots and key positions during casting filling and solidification can be predicted and analyzed, corresponding process measures can be taken to improve casting density and thermodynamic properties. In view of characteristics of aluminum alloy single-cylinder cylinder head of motorcycle engines, such as compact structure, uneven wall thickness and complex sand core shape, aluminum alloy cylinder head mold structure and sand core design characteristics are analyzed and studied, MAGMA software is used to simulate and analyze influence of factors such as temperature, pressure and cooling rate of low-pressure casting of aluminum alloy cylinder head on solidification structure. Low-pressure casting is used for production, mold process is optimized and improved to provide a reference for production of similar products.

Figure 1 is a part diagram of a single-cylinder aluminum alloy cylinder head of a motorcycle engine. It can be seen that part has a compact structure, uneven wall thickness and complex internal shape. MAGMA software is used to simulate the overall structure design of cylinder head mold and filling solidification process, defect calculation and analysis function is used to predict shrinkage defects. Orthogonal test method is used to compare and verify filling temperature, mold preheating temperature, pressure and cooling time of cylinder head low-pressure casting. Weight influencing factors are found in interacting process factors for fine control. Improve organization density inside aluminum alloy cylinder head casting, especially in key position of combustion chamber.
Outline size of motorcycle engine cylinder head casting is 216mm*170mm*127mm, made of A356 aluminum alloy, with a weight of about 6.3kg and uneven wall thickness. Since casting has complex profiles in six viewing directions, has different concave and convex shapes, outer shape needs to be formed by multiple mold components, as shown in Figure 2. External structure of cylinder head mold consists of six parts, including upper mold, lower molds and four side molds. Materials of each part of mold are SKD61 heat-resistant steel. Upper shape of cylinder head is formed in upper mold, shape of side is composed of front and rear side molds, left and right side molds. Lower mold forms inner cavity of combustion chamber. At the same time, a pouring system is designed in lower mold, used to place sand core and position. Due to complex and compact overall structure of cylinder head, internal wall thickness is uneven and varies a lot, solidification shrinkage stress of molten metal in different parts of cavity is quite different. Therefore, during solidification process of local position with large wall thickness, it is easy to produce hot nodes, causing shrinkage defects, especially in last solidified part of casting.

 

Figure 1 Parts of a single-cylinder cylinder head of a motorcycle engine

 

Figure 2 Design of mold structure of a single-cylinder cylinder head of a motorcycle

 

Figure 3 Design of internal sand core of a motorcycle cylinder head
Since bottom of cylinder head is thicker, in order to ensure smooth filling and sequential solidification of molten metal during low-pressure casting process, and to reduce heat loss of pouring system, low-pressure casting gate is installed at the bottom thick wall position of casting, and try to avoid thin wall position of combustion chamber. In order to complete rapid injection of aluminum alloy liquid into cavity in a short time, pouring system is designed to be divided into 4 trapezoidal cylindrical gates to inject aluminum liquid into mold at the same time. Under action of low-pressure casting air pressure, aluminum alloy liquid enters pouring system through liquid riser and then enters cavity. Maximum size of each gate is φ20mm. Specific pouring system and riser are shown in Figure 4.
Combustion chamber area is subjected to repeated high-temperature combustion and power output during driving of motorcycle, internal structure density and dimensional accuracy of aluminum alloy in combustion chamber area are required to be high. Therefore, primary quality target of low-pressure casting production of cylinder head is to ensure density of combustion chamber area and reduce occurrence of solidification shrinkage in this area. CAE software is used to perform numerical simulation of low-pressure casting of cylinder head from two aspects of low-pressure casting filling temperature change and solidification rate, predict possible defects of casting, and provide a basis for improving solidification crystallization quality of key areas.
Combustion chamber area is subjected to repeated high-temperature combustion and power output during driving of motorcycle, internal structure density and dimensional accuracy of aluminum alloy in combustion chamber area are required to be high. Therefore, primary quality target of low-pressure casting production of cylinder head is to ensure density of combustion chamber area and reduce occurrence of solidification shrinkage in this area. CAE software is used to perform numerical simulation of cylinder head low-pressure casting from two aspects: low-pressure casting filling temperature change and solidification rate, predict possible defects of castings, and provide a basis for improving solidification crystallization quality of key areas.

 

Figure 4 Design diagram of cylinder head low-pressure casting pouring system

Table 1 Initial parameters of numerical simulation of low-pressure casting of aluminum alloy cylinder head

 

Figure 5 Temperature field simulation of cylinder head low-pressure casting filling process

 

Figure 6 Solidification cooling simulation and defect prediction of cylinder head low-pressure casting
Solidification quality of cylinder head low-pressure casting is interactively affected by many process factors such as aluminum liquid temperature, mold preheating temperature, pressure increase rate and solidification cooling control. In order to further accurately optimize process design, four factors, namely, aluminum liquid temperature, boost pressure, mold preheating temperature, and filling cooling time, were selected as experimental factors in low-pressure casting of cylinder head. Orthogonal test simulation was performed according to 4 factors and 3 levels (see Table 2), and a total of 9 parameter combination schemes were generated, as shown in Table 3. Solidification quality is evaluated and judged by shrinkage volume of casting. The larger shrinkage rate, the greater tendency of shrinkage defects.

Table 2 Orthogonal test factor level table

Table 3 Orthogonal test scheme results
Cooling system of low-pressure casting machine adopts multi-channel sequential cooling for cooling control, controlling local temperature change of mold to extend cooling time of casting, which is particularly suitable for castings with complex cooling sequences such as cylinder heads. Figure 7 shows design of cooling channel position in the front and rear side molds of cylinder head, Table 4 shows cooling timing control table of front and rear side molds of cylinder head. In order to realize solidification sequence from top to bottom in low-pressure casting of aluminum alloy single-cylinder cylinder heads, cooling circuit sequence of front and rear side molds is controlled by mold temperature controller. It is opened after aluminum liquid enters mold cavity, is opened and closed in cooling order of No. 1, No. 2, and No. 3. No. 2 and No. 5 are cooled synchronously, No. 3 and No. 6 are cooled synchronously, No. 1 and No. 4 are cooled synchronously, thereby assisting cylinder head low-pressure casting to solidify from top to bottom, realizing true mold temperature-cooling time sequence control.

 

Figure 7 Cylinder head side mold cooling circuit diagram

Table 4 Cylinder head front and rear side mold cooling sequence control table

 

Figure 8 Pressure setting curve and optimized cylinder head cooling and solidification simulation verification

(1) From simulation results of low-pressure casting aluminum alloy cylinder head filling and solidification, it can be seen that combustion chamber area is most likely to produce shrinkage and pore defects because shrinkage channel is easily cut off, becomes an isolated liquid phase.
(2) Through orthogonal experiments, various process influencing factors of cylinder head low-pressure casting are collected, and simulation experiments of different combinations are carried out to find out that the largest influencing factors of cylinder head low-pressure casting process factors are pressure control and cooling solidification control.
(3) According to results of orthogonal test process factor range sorting, holding pressure is refined segmented, cooling channel is automatically opened and closed during production of cylinder head low-pressure casting, so as to realize digital solidification quality control of cylinder head low-pressure casting.