NUMERICAL SIMULATION OF FILLING AND SOLIDIFICATION IN EXHAUST MANIFOLD INVESTMENT CASTING Yuan Fang, Jing Hu Key Laboratory of Advanced Metal Materials of Changzhou City, Changzhou University, Changzhou, P. R China Jixiang Zhou Changzhou Xiangfan Machinery Co., Ltd., Changzhou, P. R China Yun Yu Jiangsu University of Technology, Changzhou, P. R China Copyright © 2014 American Foundry Society
Abstract In this study, investment casting is used as the manufacturing method for an exhaust manifold of modified-HF stainless steel, and the filling and solidification process is simulated by Procast software. The simulated results of the preliminary design show that the porosity defects exist in the exhaust manifold and the casting yield is only 38.8%, which is in good agreement with the actual casting product. Based on the technical requirements, an optimal design is proposed by enlarging the size of the riser and decreasing the size of the downsprue. The
simulated results of the optimized design show that there is almost no porosity defect in the product and the casting yield is increased from 38.8% to 49.1%, which is confirmed by real production testing. Therefore an exhaust manifold with better casting quality and higher yield can be successfully produced by applying the optimized design.
Introduction
Unfortunately, shrinkage porosity and cavity defects can easily appear in exhaust manifold casting products by empirical processing design, which seriously affects the quality of the products, since the castability of the modified-HF stainless steel is poor due to the high concentration of alloying elements. To eliminate these problems, proper design of the gating system is necessary. This can be predicted and designed by means of computer simulation of the casting process. Numerical simulation is basically focused on the casting design, processing determination, flow pattern prediction and tooling design. It can help designers to understand the patterns of filling and solidification during casting, and make the casting process and product quality predictable, and thus possible to obtain casting products with good quality by avoiding the potential defects through selecting the optimum casting conditions. It is also helpful to shorten the trial period and reduce the production costs.5-7
As one of the eight important vehicle components, the exhaust manifold withstands severe environment including high temperature generated during operating conditions, cyclic thermal loads and engine vibrations.1,2 The efficiency of vehicle engines largely depends on the exhaust manifold’s structural design, manufacturing technology and especially, the combination properties, which is mainly dependent on the material used for the exhaust manifold. Cast iron is widely used for automotive exhaust manifolds. But with higher and higher exhaust temperatures, heat resistant austenitic steel can be chosen for improved hot oxidation resistance, thermal fatigue resistance and high temperature strength. Therefore we selected this heat resistance austenitic steel as the material for exhaust manifold in this research. The investment casting process is one of the most versatile casting processes, offering the possibility of making near net shape castings with high dimensional accuracy and high casting quality, thus with higher efficiency and lower producing costs than traditional sand casting.3,4 It is especially suitable for manufacturing components with complex geometry. Therefore, the investment casting process is considered to be the manufacturing method of choice for the heat resistant steel exhaust manifold in this work.
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Keywords: heat resistant steel, exhaust manifold, numerical simulation, investment casting, FEM
In this work, a commercial finite element package named ProCAST is employed to investigate the preliminary design in the investment casting, and then the casting process parameters are redesigned and optimized based on the numerical simulation, finally the simulation results are checked and confirmed by the real production testing.
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Materials Modified-HF stainless steel was selected as the exhaust manifold material, whose chemical composition is listed in Table 1. Mullite was selected as the mould material. The thermophysical properties of modified-HF stainless steel and mullite are shown in Figs. 1 and 2. Simulation Procedures ProCAST was adopted in this research and the initial conditions were chosen as follows: The thickness of the mould shell is 8mm. When using this software the interface between the casting and the mould is divided into three category types (COINC, NCOINC, and EQUIV). When the casting material and the mould are different, COINC or NCOINC are used. In this case, the interface nodes are symmetrical and COINC was used. The heat transfer coefficient of mould/casting, casting/air, mould/air was set as 800 W/(m2•K), 10 W/(m2•K), 10 W/(m2•K), respectively. Because of the high preheated mould temperature, radiation cannot be neglected. The radiation coefficient was set as 0.7 W/(m2•K).7,8
Figure 1. Thermophysical properties of modified-+F stainless steel are shown.
The initial parameters used in the simulation are: Pouring temperature of 1620˚C (2948˚F), preheated mould temperature of 1000˚C (1832˚F) and filling time of 10s. Then, 3-D geometry model of the exhaust manifold was drawn according to the casting design principle and experience, and finally the model was translated into a Finite Element Method (FEM) model. Results and Discussion
Figure 2. Thermophysical properties of mullite are shown.
Preliminary Design and Discussion The preliminary design is what is used by the industry according to the experience without simulation; 3-D geometry model in the preliminary design is shown in Fig. 3. Note that the downsprue is located at the outlet flange. In this preliminary design, the gating size is kept large to force the feeding path to remain open for longer duration due to the complex structure of the model. Three risers are placed at the rim of the inlet flange and three air vents are placed on the risers whose opposite end is linked to the middle of the downsprue. The model of the exhaust manifold is translated into the FEM model, which consisted of 182805 nodes and 870382 elements. During carving up the mesh for geometrical model, different mesh densities for the casting
Figure 3. The 3-D geometry model used in the preliminary design.
Table 1. Chemical Composition of Modi¿ed-+F Stainless Steel (wt.%)
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and gating system are used in order to reduce executing time. With this scheme, the casting yield was 38.8% calculated by UG software. Figure 4 shows the evolution of the filling state at different times. It clearly shows that the liquid alloy flows through the downsprue into the cavity of the casting, and Figure 4. The evolution of the ¿lling state at different times. then fills the outlet flange and pipe smoothly and steadily. Two of the four pipes are filled earlier than the others due to the different length of each pipe. At 430 steps, the front liquid metal has already filled the air vents, which will block liquid metal from further filling because of the gas backpressure, and meanwhile, the dirty melt and air in the casting cannot be discharged smoothly. Figure 5 shows the temperature distribution at the end of filling. It shows that the temperature difference in the casting is not large enough to promote sequential solidification, and the temperature of the pipe is lower since its thickness is much thinner than that of the other parts. Figure 5. Temperature distribution at the end of ¿lling.
Figure 6 shows the temperature field during different solidification states. It shows that the pipes cool faster than other parts. The risers, the junction of the two pipes and the inlet flange can remain in the liquid state much longer than the pipes area; therefore, the temperature in these sections is higher than that of the pipes as shown in Fig. 6(c), (areas 1, 2 and 3). The higher temperature Figure 6. Temperature ¿eld during solidi¿cation. zone is divided into two parts by the hole on the flange, which causes the feeding paths to be smaller as shown in Fig. 7, thus defects may be formed in the isolated temperature zone, because the feeding paths are cut off prematurely. Figure 8 shows the solid fraction at 890 steps. The liquid islands are found at the junction of the two pipes, the corner of the second flange and the risers. The liquid islands form the potential defects since no liquid feeding metal can be introduced into these areas during the process of solidification. Figure 9 presents the solid fraction at 6500 steps. Liquid metal still exists in the center of the riser when the casting is totally solidified, which means that the liquid metal in the riser cannot be effectively used to compensate the shrinkage.
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Figure 7. The isolated higher temperature zone on the Àange corner is shown.
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Therefore, the efficiency of the riser is lower, resulting in a lower casting yield. Figure 10 shows that porosity defects are found in the liquid islands, and Fig. 11 shows the cross section of porosity at the riser. It indicates that porosity exists in the contact area between the riser and casting, which will affect the surface quality of the casting. Therefore, the simulation results of the preliminary design show that porosity defects exist at the junction of the two pipes, two corners of the inlet flange and in the risers, as shown in Fig. 10. The porosity defects in the contact area between the riser and casting may have an effect on the surface quality of the casting, though those located in the junction of the two pipes and in the corner of the inlet flange are acceptable according to the technical specifications. The simulation results for the defects are in good agreement with the actual industrial castings, which shows that the boundary conditions for this simulation are reasonably set, and also implies that the preliminary design for the exhaust manifold should improve.
Figure 15 shows the solidification time. It can be seen that the solidification time for the risers and the gate is longer than that of the other parts of the casting, which means that the whole casting undergoes sequential solidification. Thus, it is beneficial for the liquid metal to compensate for the shrinkage.
Figure 8. Fraction of solid at 890 steps.
Optimized Design and Discussion Based on the simulation results of the preliminary design and the technical requirements, optimizing the gating system is a possible solution to improve the casting quality. The detailed optimizing technical methods include the following contents: Decreasing the gate size, increasing the riser size, changing the riser shape and moving the air vents to the top of the downsprue which will help to exhaust the air in the mould, as shown in Fig. 12. Using this design, the casting yield of the optimized design is increased from 38.8% to 49.1%. The boundary conditions and the cast parameters used for the optimized design are the same as those chosen in the preliminary design.
Figure 9. Fraction of solid at 6500 steps.
Because of the optimization of the gating system, mould filling and temperature field were modified. The air vents were filled behind the casting. It was beneficial to exhaust the air in the mould, as shown in Fig. 13, which indicates that the mould filling process in the optimized design was much better than that in the preliminary design due to the change in air vent location. Figure 14 depicts the temperature field at 1230 steps. The temperature of the casting is uniform except for that at the risers and the gate. 42
Figure 10. Porosity distribution detected by solidi¿cation software.
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The simulation results indicate that the porosity defects are moved to the junction of the two pipes and two corners of the inlet flange as shown in Fig. 16. And the porosity defects on the contact surface between the riser and casting that existed in the preliminary design are eliminated as shown in Fig. 17, which is attributed to the enlarged riser. Therefore, the simulation results for the optimized design show that the casting benefits from the sequential solidifica-
Figure 11. Cross section of porosity at the riser.
tion, since the enlarged risers in the optimized design can keep the feeding path open during the entire filling process, which can improve the efficiency of the feeding, and improve the casting quality and yield. Finally, the simulated test results were supported and confirmed by the actual production testing. Figure 18 shows the modified-HF stainless steel exhaust manifold manufactured by investment casting using the optimized design. It can be seen that the surface quality of the flange
Figure 12. The 3-D geometry model in the optimized design.
Figure 13. Flow ¿eld during ¿lling.
Figure 14. Temperature ¿eld at 1230 steps.
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is smooth and no obvious defects were found, which is in good agreement with the simulation results.
No. CC20120033 and Wujin Science and Technology Bureau under Grant No. WG2011005 and the Jiangsu province graduate student innovation fund.
Conclusions 1.
The simulation results of the preliminary design show that the porosity defects exist in the casting, since the feeding path was cut off prematurely due to the small riser size, which resulted in nonsequential solidification. And the casting yield of the preliminary design was only 38.8% due to the large downsprue size.
2.
Based on the technical requirements, an optimized design was proposed, details included enlarging the riser size, changing the riser shape and decreasing the size of the downsprue.
3.
The simulation results of the optimized design show that there were almost no porosity defects in the casting and the casting yield was increased from 38.8% to 49.1%, since the potential porosity can be effectively removed from the contact surface to the risers and the downsprue size was reasonably decreased.
4.
Figure 15. Solidi¿cation time of the casting.
The simulated results were checked and confirmed by actual production testing.
Acknowledgement This work is sponsored by Changzhou Science and Technology Bureau under Grant Figure 16. Porosity distribution predicted by ProCAST.
Figure 17. Cross section of porosity at the riser of the new design.
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Figure 18. The test exhaust manifold casting using the optimized design.
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Technical Review & Discussion 1XPHULFDO6LPXODWLRQRI)LOOLQJDQG6ROLGL¿FDWLRQ in Exhaust Manifold Investment Casting Yuan Fang, Jing Hu, Key Laboratory of Advanced Metal Materials, Changzhou University, Changzhou, P. R China; Jixiang Zhou, Changzhou Xiangfan Machinery Co., Ltd., Changzhou, P. R China; Yun Yu, Jiangsu University of Technology, Changzhou, P. R China Reviewer: What is meant by term (COINC)? Is this a software setting? Can you please give an explanation? Author: We are not sure what the term (COINC) exactly means, it is a ProCAST setting. The specification of software shows that the interface between the casting and the mould is divided into three kinds, which are “COINC”, “NCOINC” and “EQUIV”. When the materials of casting and the mould are different, “COINC” or “NCOINC”
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need to be selected, and in this case, if the interface nodes are symmetrical, “COINC” needs to be selected. Otherwise, “EQUIV” is selected. Therefore, we can say, COINC means that the material of casting and the mould are different, and the interface nodes are symmetrical. We added an explanation in the revised manuscript. Reviewer: Can you discuss the numbers that are on the figure 6(c)? Author: The figure 6(c) mainly depicts that the pipes cool faster than the other part so that the risers, junction of two pipes and the inlet flange can keep much longer in liquid state than the pipes. Reviewer: Do the actual castings verify the model prediction and exhibit porosity at the two corners of the inlet flange? Author: Yes, the actual castings verify the model prediction and exhibit porosity at the two corners of the inlet flange and show close correlation with the modeling prediction.
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