Cylinder Liner Requirements: As a core component of the engine, the cylinder liner is required to have high strength, high temperature resistance, and friction resistance. With the development of engine technology, higher requirements are placed on the performance of the cylinder liner. Nodular cast iron is widely used in various host factories due to its excellent mechanical properties and cavitation resistance1.
Casting Defect Problem: In the production of nodular cast iron cylinder liners by the water-cooled metal mold horizontal centrifugal casting process, a white structure appears at the thick wall of the casting, which is called inverse chill. This defect reduces the mechanical properties of the casting, increases the machining difficulty, and reduces the tool life, directly affecting the casting quality and economic benefits. The formation of inverse chill is mainly related to the chemical composition segregation of nodular cast iron, the inoculation effect of the molten iron, and the cooling conditions of the casting1.
Simulation Technology Advantage: The development of computer numerical simulation technology provides a new way for centrifugal casting process designers to explore the filling and solidification laws of centrifugal casting. Although the current casting simulation software is not mature enough for the simulation of horizontal centrifugal casting and there is a certain gap from the actual production process, it has the advantages of short development cycle, low research cost, and convenient adjustment of process parameters, and plays an increasingly important role in guiding actual production and process improvement2.
2. Initial Casting Process Simulation Analysis
2.1 Mathematical Model Establishment
Continuity Equation:
Navier – Stokes Equation:
Heat Balance Equation:
2.2 Physical Model
Component
Description
Casting
Maximum diameter Φ140mm, length 298mm, maximum wall thickness 19mm
Mold
–
Baffle
–
Insulation Coating
–
2.3 Grid Division
Object
Grid Type
Grid Density
Face Grid Number
Body Grid Number
Mold, Baffle
Non – structure tetrahedron
Reduced
–
–
Casting, Insulation Coating
Non – structure tetrahedron
Increased
99812
719843
2.4 Initial Process Conditions and Physical Parameters
2.4.1 Centrifugal Rotation Speed Calculation
The centrifugal rotation speed is calculated according to the Konstantinov empirical formula:
The calculated pouring rotation speed range of this casting is 840-1380r/min, and the actual pouring rotation speed is taken as 1200r/min according to production experience.
2.4.2 Boundary Conditions and Physical Parameters
Parameter
Value
Casting Material Chemical Composition (mass fraction, %)
Casting, Mold and Coating Heat Transfer Coefficient (W•m^-2•K^-1)
500
Mold and Cooling Water Heat Transfer Coefficient (W•m^-2•K^-1)
5000
Casting Inner Surface and Air Heat Transfer Coefficient (W•m^-2•K^-1)
20 – 60
2.5 Simulation Result Analysis
2.5.1 Solidification Process Temperature Field Analysis
In the centrifugal casting solidification process of the casting, the outer surface of the casting contacts with the insulation coating, and the insulation coating contacts with the mold. Due to the large temperature difference between the liquid metal and the mold in the initial stage of pouring, the heat of the casting is easily transferred to the lower temperature mold, so the outer surface of the casting solidifies first. The rotating inner surface of the casting contacts with the air, and the inner wall of the casting transfers heat through the air, resulting in the temperature of the inner surface of the casting being lower than that of the middle layer of the casting, so the inner surface of the casting is not easy to produce defects. The two ends of the casting contact with the baffle and the mold, resulting in faster heat transfer at the two ends of the casting than in the middle section. In addition, due to the wall thickness difference of the casting, the thick wall end has more heat. Therefore, during the air cooling process, the mold at the thick wall end of the casting heats up faster, and finally the temperature at the thick wall of the casting is higher at the end of air cooling. Although the mold temperature tends to be uniform during the water cooling process, it is difficult to make the casting cool evenly from the outside to the inside due to the existence of a hot spot at the wall thickness of the casting.
Cooling Time (s)
Temperature Field Distribution
–
As shown in Figure 4, the temperature field of the casting shows a sandwich state distribution due to the heat transfer between the inner wall and the mold of the casting
150
Inner layer temperature: 1160°C, outer layer temperature: 1120°C, middle layer temperature: 1180°C, and the two ends of the casting have lower temperatures
2.5.2 Solidification Process Solid – Liquid Phase Distribution Analysis
The metal liquid transfers heat to the surroundings through thermal radiation. The metal liquid near the mold and baffle solidifies first due to the faster cooling rate. Due to the two – way heat dissipation of the inner surface and the mold of the casting, the inner surface and the outer layer of the casting with lower temperatures solidify first, and the middle layer at the wall thickness of the casting with higher temperature solidifies slower and finally solidifies. The “A region” in Figure 6 is the last solidification region, which is also the position where the inverse chill casting defect is most likely to occur. The distance from the center position of the last solidification region to the inner wall of the casting is measured as . The actual casting defect position is distributed at about from the inner wall of the casting, which is almost the same as the last solidification region predicted by the simulation, proving that the simulation result can predict the position of the casting defect and provide a basis for process optimization.
Cooling Time (s)
Solid – Liquid Phase Distribution
150
The solidification sequence of the casting is shown, and the “A region” is the last solidification region
3. Casting Process Optimization
3.1 Optimization Measures
Increase the cooling water flow at the wall thickness position.
Reduce the thickness of the insulation coating at the wall thickness position.
Improve the cooling speed at the wall thickness part.
Improve the consistency of the casting cooling sequence.
Shorten the solidification time of the last solidification part.
Eliminate the influence of inoculation decline.
3.2 Optimization Effect
The temperature field simulation result after process optimization shows that the outer wall of the casting solidifies first and the inner wall solidifies last, realizing the uniform cooling of the casting from the outer wall to the inner wall.
The solid – liquid phase distribution at t=120s after optimization shows that the solidification sequence of the casting is improved, and the distance from the center position of the last solidification region to the inner wall of the casting is reduced to about 3.5mm.
After the actual production verification, the casting inspection qualification rate is above 99.6%, and the finished product qualification rate is 100%, which is consistent with the simulation result.
4. Conclusion
The casting simulation software is used to analyze the temperature field and solid – liquid phase distribution law of the original casting process, determine the position of the last solidification region of the casting, and the simulation result is consistent with the actual production defect position, providing data support for the optimization and improvement of the casting process.
By adjusting the cooling water flow and insulation coating thickness at the wall thickness position of the casting, the solidification time of the last solidification part is shortened, the influence of inoculation decline is eliminated, the depth of the last solidification region from the inner wall of the casting is reduced, the inverse chill defect of the casting is eliminated, and the casting quality and machining performance are improved. The optimization scheme is verified by actual production.
Through casting process simulation, the solidification law of centrifugal casting is understood, providing a theoretical basis for reducing the machining allowance of the casting and improving the material utilization rate in the future.
The method introduced in this article can provide a guiding basis for the design and development of similar products, effectively reduce the process development cycle and improvement cost, and improve economic benefits.
