The ProCAST software was used to simulate the casting solidification process of the ZG30SiMnMoV steel caterpillar board. The influence of the pouring temperature on the casting defects and the influence of the pouring rate and pouring temperature on the effective stress of the casting were analyzed, and the experimental verification was carried out. The results show that when the pouring temperature is 1,610 °C, the shrinkage cavity and porosity defects inside the caterpillar board are significantly reduced; the effective stress of the casting is not significantly affected by the pouring rate, and appropriately reducing the pouring temperature can significantly reduce the effective stress of the casting, resulting in a casting of good quality.
The caterpillar board is a key component in the walking mechanism of the roadheader. It is connected by pins to form the caterpillar board chain. If one of the caterpillar boards has a problem, it will affect the entire caterpillar board chain and the normal operation of the roadheader. Therefore, the improvement and optimization of the casting technology of the caterpillar board to improve its product quality and service life are of great significance for the improvement of the comprehensive performance of the equipment. The Lost Foam Casting technology has less pollution, high dimensional accuracy of castings, small machining allowance, and high production efficiency, and is known as the “green casting technology”. In actual production, the best casting process plan for the product is usually formulated based on experience and continuous trial and error, which not only increases the development cost but also prolongs the manufacturing cycle, which is not conducive to the competition of the enterprise in the market. The development and application of numerical simulation technology in the field of casting can effectively reduce the number of tests and shorten the product manufacturing cycle, and is a very important auxiliary means in the process of formulating the casting process plan.
In this project, the ProCAST software was used to simulate the casting solidification process of the caterpillar board. Through the simulation analysis of the temperature field and stress field of the casting, the casting defects were predicted, which provided a reference for the enterprise to formulate the casting process plan in the future production.
1.Pre-processing for Numerical Simulation
1.1 Establishment of Geometric Model
During the filling process of the molten metal and the pattern, heat transfer will occur, and the convective heat transfer control equation should be considered when modeling: ρLcL∂T/∂l = ρLcL,μ + ∂T/∂x + λL∂T/∂x^2 + ρpLΔfS (1)
In the equation, ρp is the density of the foam pattern; ρL is the density of the molten metal; λL is the thermal conductivity of the molten metal; cL is the specific heat capacity of the molten metal; L is the latent heat of crystallization; ΔfS is the solid fraction of the foam pattern.
The contour dimensions of the part are 1,500 mm × 150 mm × 433 mm, the minimum wall thickness is 13 mm, the maximum wall thickness is 35 mm, the volume is about 0.04 m^3, and the mass is about 310 kg. The part has uneven wall thickness, a large internal cavity volume, and a complex structure. The required production parts have a complete structure, no defects that affect the strength on the surface, dense internal structure, and no shrinkage cavity, porosity, cracks, and other defects.
1.2 Finite Element Mesh Generation
The 3D solid file generated by Solidworks was saved in the x-t format, the ProCAST software was run, and the x-t format file was imported into the VisualMesh program. After being checked and repaired by the MeshCAST module, a 3D finite element mesh file was generated. The length of the mesh unit for the casting, pouring system is 10 mm, the length of the mesh unit for the mold shell and sand mold is 40 mm, the total number of meshes is 1,498,610, the total number of nodes is 96,830, and the 3D finite element mesh division of the casting and sand mold.
1.3 Setting of Boundary Conditions and Parameters
The casting grade is ZG30SiMnMoV steel, the refractory coating uses Zircon Sand Ⅱ, and the sand mold material uses Olivine Sand; the contact surface type between the casting and the refractory coating is COINC, and the interface heat transfer coefficient is 750 W/(m^2 · °C), the contact surface type between the refractory coating and the sand mold is CONIC, and the interface heat transfer coefficient is 300 W/(m^2 · °C); using Lost Foam Casting, the sand mold stress type is rigid body. The ProCAST material library provides the complete thermophysical parameters of ZG30SiMnMoV, as shown in Figure 4. Among them, the solidus temperature of ZG30SiMnMoV steel is 1,393 °C, and the liquidus temperature is 1,493 °C.
In this project, the Solidworks software was used to perform 3D solid modeling of the casting. According to the structural characteristics of the caterpillar board part, a Lost Foam Casting process plan was designed, using 4 open risers for feeding and pouring in the middle: two open risers with a diameter of 160 mm were placed in the area with the maximum wall thickness of 35 mm in the middle, and 1 open riser with a diameter of 120 mm was placed in each of the areas with a wall thickness of 13 mm on both sides. The diameter of the sprue is 50 mm, the height is 340 mm, the diameter of the large end face of the gate is 120 mm, the cross-sectional size of the runner is 50 mm × 40 mm, the length is 740 mm, the cross-sectional size of the ingate is 40 mm × 20 mm, and the number of ingates is 2.
During the solidification process, the thermophysical parameters of the material will change with the temperature, which is reflected in the numerical simulation equation as that each time step may contain multiple physical property values. Therefore, in the process of numerical simulation calculation, if there is no accurate thermophysical parameter, it is difficult to ensure the accuracy of the simulation results.
2. Simulation Test Results and Analysis
2.1 Influence of Pouring Temperature on Defects
During the solidification process of the molten metal, due to the wide crystallization temperature range of the alloy, when growing from the liquid state to the solid state, a basic metal skeleton is first formed. At this time, the liquid and solid phases coexist. When continuing to grow to the solid state, the molten metal needs to flow between the dendrites of the skeleton for feeding. As the solid fraction increases, the flow channels between the dendrites gradually solidify, forming a closed isolated region. After the liquid metal in the region is completely solidified, shrinkage porosity will be formed inside the casting. If the pouring temperature is too high, the liquid shrinkage of the casting will increase, and the volume of the shrinkage cavity (porosity) formed inside the casting will increase. The volume shrinkage rate εV’ of the molten metal from the pouring temperature (TP) to the liquidus temperature (TL) can be expressed as: εV’ = αV'(TP – TL) × 100%
In the equation, αV’ is the liquid volume shrinkage coefficient of the alloy.
Using the ProCAST software to calculate the influence of different pouring temperatures on the casting defects, the initial temperature of the sand mold is 25 °C (room temperature), the pouring speed is 14 kg/s, and the distribution of casting defects when the pouring temperatures are 1,610 °C and 1,650 °C is calculated respectively. The Cut-off method or the Niyama criterion can be used to confirm the location where the closed liquid isolated domain is generated, so as to determine whether shrinkage porosity and shrinkage cavity will be generated in these sensitive areas.
It can be seen that when the pouring temperature is 1,650 °C, there are large areas of defects at both ends of the caterpillar board, and the porosity of the shrinkage cavity area in the middle is 17.68%, which has a great impact on the mechanical properties of the casting and will shorten the service life of the casting, and there are certain safety hazards during use.
When the pouring temperature is 1,610 °C, the defects at both ends of the caterpillar board are significantly reduced, and only small areas of defects exist. The porosity of the shrinkage cavity area in the middle is reduced to 5.42%, which has little impact on the mechanical properties and service life of the casting.
Increasing the pouring temperature will increase the liquid shrinkage of the alloy and the total volume of the shrinkage cavity (porosity). When the pouring temperature is increased from 1,610 °C to 1,650 °C, according to Equation (2), the volume shrinkage rate can be calculated to increase by 1.5% – 1.75%; through the ProCAST simulation calculation, the total volume of the shrinkage cavity (porosity) is increased from 0.42 mL to 3.33 mL, and the porosity is increased from 5.42% to 17.68%. It can be seen that different pouring temperatures have a certain impact on the defects of the casting. When the pouring temperature is 1,610 °C, the shrinkage cavity (porosity) formed inside the caterpillar board is significantly reduced.
2.2 Stress Field Analysis
The solidification process of the molten metal goes through three stages: liquid, solid/liquid two-phase coexistence, and solid. The thermophysical and mechanical properties of the material change greatly, and the three regions may coexist at the same time. Therefore, the stress-strain constitutive relationship designed for solidification simulation is very complex. During the phase transition of the molten metal, if the maximum stress is greater than the tensile strength of the material at this time, cracks will occur. Hot cracking is a common defect in the casting process and an important factor affecting the quality and performance of the casting. Therefore, a reasonable casting process design plays an important role in reducing the casting stress and preventing hot cracking.
2.2.1 Influence of Pouring Speed on Effective Stress
When the initial temperature of the sand mold is 25 °C (room temperature) and the pouring temperature is 1,610 °C, the effective stress distribution of the casting when the pouring speeds are 14 kg/s and 12 kg/s is calculated respectively.
It can be seen that when the pouring speeds are 14 kg/s and 12 kg/s, the maximum effective stress of the casting is 40.11 MPa and 40.15 MPa respectively, indicating that the pouring speed has little obvious influence on the maximum effective stress of the casting.
2.2.2 Influence of Pouring Temperature on Effective Stress
When the initial temperature of the sand mold is 25 °C (room temperature) and the pouring speed is 14 kg/s, the effective stress distribution of the casting when the pouring temperatures are 1,610 °C and 1,650 °C is calculated respectively.
During the cooling process of the casting, the cooling speed of each part is different, resulting in different shrinkage amounts at the same time. At the same time, due to the different heat dissipation and cooling conditions, the time for each part to reach the solid-phase transition temperature will also be different, and the degree of phase transition will also be different. During free expansion, the elongation in the length and width directions is Δl = α(TP – TS)l and Δd = α(TP – TS)d, respectively, and the strain in the length and width directions is: ε1 = Δl/l = α(TP – TS) εd = Δd/d = α(TP – TS)
That is, when the temperature of the casting decreases from the pouring temperature (TP) to the solidus temperature (TS), the strain in each direction is ε = α(TP – TS)
In the equation, α is the linear expansion coefficient of the material, and its value changes with the temperature.
When the pouring temperature is increased from 1,610 °C to 1,650 °C, according to Equation (5), the strain in each direction is increased by 7.5% – 7.75%, and at the same time, the cooling time of the casting is prolonged. The time for the shrinkage process to be blocked by the sand mold, pouring system, and riser is also correspondingly increased, and the resulting casting stress is also increased. It can be seen that when the pouring temperatures are 1,610 °C and 1,650 °C, the maximum effective stress of the casting is 40.11 MPa and 43.21 MPa respectively. It can be seen that the maximum effective stress of the casting is greatly affected by the pouring temperature.
3. Production Verification
Through the simulation of the solidification process of the caterpillar board casting, the location and size of the defects were predicted. Through the optimization of process parameters and production test verification, the optimal casting process plan was determined.
It can be seen that there are no obvious defects on the surface of the caterpillar board casting, and the qualification rate is greatly improved. After the feedback from the customer’s use, the service life of the caterpillar board is increased by about 20%.
4. Conclusions
(1) The lower the pouring temperature of the Lost Foam Casting, the fewer the shrinkage cavity and porosity defects of the casting. However, if the temperature is too low, the fluidity of the molten steel will be reduced, resulting in incomplete combustion of the foam pattern in the mold, and the casting is prone to defects such as underfilling, cold shut, and slag inclusion. When the pouring temperature of the caterpillar board casting is 1,610 °C, the shrinkage cavity (porosity) defects inside are significantly reduced.
(2) The effective stress of the casting is not significantly affected by the pouring speed. Considering the poor fluidity and easy oxidation of the molten steel, the pouring speed of the caterpillar board casting is selected to be 14 kg/s.
(3) The pouring temperature has a great influence on the effective stress of the casting. Appropriately reducing the pouring temperature can significantly reduce the effective stress of the casting.