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Gating system optimization of low pressure casting A356 aluminum alloy intake manifold based on numerical simulation

* Jiang WenmingMale, born in 1982, Ph.D. Research interest: precision casting technology of non-ferrous alloy.E-mail: jwenming@163.com.

Received: 2013-06-15 Accepted: 2013-12-12

*Jiang Wenming 1, 2 and Fan Zitian 2

1. School of Mechanical & Electrical Engineering, Wuhan Institute of Technology, Wuhan 430073, China; 2. State Key Laboratory of Material Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China

Currently, aluminum alloys are widely used in the automotive industries due to their excellent

castability, corrosion resistance, as well as high strength to weight ratio [1-5]. Low pressure casting technology is regarded as a near net shape method and very suitable for producing complex and thin-walled aluminum alloy castings, such as aluminum alloy intake manifold castings because of high precision, high efficiency as well as high internal quality [6-9].

Generally, foundries usually optimize casting process depending on experience. However, it is difficult to know the metal flow and temperature distribution during mould filling and solidification, which are undoubtedly important to obtaining high quality castings. Nowadays, with the rapid development of computers, the numerical simulation technology makes the simulation of mould filling

Abstract: To eliminate the shrinkage porosity in low pressure casting of an A356 aluminum alloy intake manifold casting, numerical simulation on filling and solidification processes of the casting was carried out using the ProCAST software. The gating system of the casting is optimized according to the simulation results. Results show that when the gating system consists of only one sprue, the filling of the molten metal is not stable; and the casting does not follow the sequence solidification, and many shrinkage porosities are observed through the casting. After the gating system is improved by adding one runner and two in-gates, the filling time is prolonged from 4.0 s to 4.5 s, the filling of molten metal becomes stable, but this casting does not follow the sequence solidification either. Some shrinkage porosity is also observed in the hot spots of the casting. When the gating system was further improved by adding risers and chill to the hot spots of the casting, the shrinkage porosity defects were eliminated completely. Finally, by using the optimized gating system the A356 aluminum alloy intake manifold casting with integrated shape and smooth surface as well as dense microstructure was successfully produced.

Key words: low pressure casting; A356 aluminum alloy; numerical simulation; optimization; intake manifold

CLC numbers: TG146.21/TP391.9 Document code: A Article ID: 1672-6421(2014)02-119-06

and solidification processes a possibility [10-13]. The numerical simulation can help researchers to know the metal flow and temperature distribution during filling and solidification of the molten metal. This allows one to predict potential defects as well as to optimize the casting process.

In this study, the numerical simulation of filling and solidification processes of low pressure casting A356 aluminum alloy intake manifold castings was carried out using the ProCAST software, and the gating system of the mould was optimized based on the simulation results. Finally, the casting practice was carried out with the optimized gating system to validate the simulation results; and the A356 aluminum alloy intake manifold castings with smooth surface and good internal quality were obtained.

1 Initial and boundary conditions

Figure 1 shows the three-dimensional model of the intake manifold casting. The complicated and thin-

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Temperature Thermal conductivity Specific heat ℃ W·(mm·℃)-1 J·(g·℃)-1

Si Mg Ti Fe Sr Al

7.10 0.31 0.23 0.17 0.05 Balance

25 0.007 0.554

Density g·mm-3

Specific heat J·(g·℃)-1

Latent heat J·g-1

Thermal conductivity W·(mm·℃)-1

Solid phase temperature (℃)

Liquid phase temperature (℃)

(2.4 – 2.7)×10-3 0.963 389 0.151 555 615

Fig. 1: Three-dimensional model of intake manifold casting

Fig. 3: Pressure curve of low pressure casting process for intake manifold casting

Fig. 2: Initial design of gating system

Table 1: Chemical composition of A356 alloy (wt.%)

Table 3: Thermal and physical parameters of mould material

Table 2: Thermal and physical parameters of A356 aluminum alloy

The test material was A356 aluminum alloy, and its chemical composition is shown in Table 1. Table 2 and Table 3 show the thermal and physical parameters of the A356 aluminum alloy and mould material, respectively. The mould is a ceramic shell mould. The numerical simulation on filling and solidification processes of low pressure casting intake manifold castings was carried out using the ProCast software. Figure 2 shows the initial design of the gating system, which is very simple, with only one sprue. The step size was set at 2 mm and the total number of meshes was 912,250.

2 Experimental proceduresFirstly, the crucible was preheated to 200 to 300 ℃ in the holding furnace. Then, the aluminum ingot, preheated to 300 ℃ was placed in the crucible and was heated in a crucible furnace. When the temperature of the molten metal reached 750 ℃, the molten metal was refined using argon gas, and was then ready for pouring. The pressure curve during the low pressure casting process is shown in Fig. 3.

The gating system was improved step by step based on the simulation results, and finally the optimized one was obtained. Then the A356 aluminum alloy intake manifold casting was prepared using the optimized gating system. The metallographic sample was cut from the maximum wall thickness of the intake manifold casting, and then the sample was etched using 0.5% hydrofluoric acid solution after polishing. The microstructure of the intake manifold casting

was observed using an Me F-3 metallographic microscope, and the macrostructure at the cross section of the intake manifold casting was observed using a stereomicroscope. The average grain size was calculated according to the following equation:

(1)where A is the average area of grain, which was measured using the ImageTool software.

3 Simulation results3.1 Simulation results of casting with initial

designed gating systemFigure 4 shows the simulat ion results of f i l l ing and solidification processes of the casting with the initial designed gating system. It can be seen that the molten metal fills from the sprue [Fig. 4(a)]; the filling process of the molten metal is not stable, and the liquid level shows an obvious disturbance, as shown in Fig. 4(b). In this case, the gas is easily involved in the molten metal and results in porosity defects. When the filling time is 4 s, the casting is fully filled [Fig. 4(c)].

walled intake manifold part has a 3 mm minimum wall thickness and 3.8 mm average wall thickness; and the outline dimensions are 410 mm × 251 mm × 164 mm.

0 06.

0 05.

0 04.

0 03.

0 02.

0 01.

0 00.0 50 100 150 200

Pres

sure

(MPa

)

Time (s)

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Fig. 4: Simulation results of filling and solidification processes with initial scheme

Fig. 5: Simulated shrinkage porosity defects of casting with initial designed gating system

Fig. 6: Improved design of gating system with one runner and two in-gates

(a) 1.0 s (b) 2.1 s (c) 4.0 s

(d) 10.0 s (e) 50.0 s

Figures 4(d) and 4(e) show the solidification processes of the intake manifold casting. It can be clearly seen, according to the temperature distribution, that the casting does not follow sequential solidification, and many isolated liquid islands can be observed. When the isolated islands cannot be filled with liquid metal, shrinkage porosity defects will generate.

Figure 5 shows the simulated shrinkage porosity defects of the intake manifold casting. As can be seen, many shrinkage porosity defects can be observed through the casting, especially in some positions with heavy wall thickness. It is too difficult to obtain denser casting if shrinkage porosity defects cannot be eliminated. So the gating system of the intake manifold must be improved.

3.2 Simulation results with improved and optimized gating system

Considering the stability of the filling process of the molten metal, the initial designed gating system of the intake manifold casting was improved, as shown in Fig. 6. One runner and two in-gates were added.

Figure 7 presents the simulation results of filling and solidification processes of the intake manifold casting under the improved gating system. As can be seen in Fig. 7(a), the molten metal was filled from the sprue to runner, to the in-gates, and then to the mould. The filling state of the molten metal [Fig. 7(b)] is very stable when compared to the initial

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(a) 1.5 s (b) 3.0 s (c) 4.5 s

(d) 10.0 s (e) 50.0 s

Fig. 7: Simulation results of filling and solidification processes of casting with improved gating system

Fig. 8: Simulated shrinkage porosity defects of casting with improved gating system

Fig. 9: Gating system further improved by adding risers and chill

scheme. In this case, the gas will be not be easily involved in the molten metal so the porosity defects can be avoided. When the filling time is 4.5 s, the mould is fully filled (Fig. 7(c)). Compared to the initial design, the filling time is slightly longer for the casting with the improved gating system. It means that the filling velocity is slower than that of the initial scheme. The faster filling velocity of the molten metal will increase the gas involvement and nonstationarity and result in some porosity defects. Figures 7(d) and 7(e) show the solidification processes of the intake manifold casting under the improved scheme. The temperature distribution of the casting shows obviously that this casting does not follow the sequential solidification either, and many liquid isolated islands can again be observed. Therefore, the gating system should be further improved in order to overcome the above problems.

Figure 8 shows the simulated shrinkage porosity defects of the intake manifold casting under the improved scheme. It can be seen that the shrinkage porosity defects are fewer than that of the initial scheme. However, some shrinkage porosity defects are again observed, mainly in the hot spots of the casting.

To remove the shrinkage porosities in the hot spots of the casting, the gating system was further improved. The risers and chill are added on the hot spots of the casting in order to realize sequential solidification and improve the feeding capacity of the molten metal, as shown in Fig. 9. Figure 10 shows the simulated shrinkage porosity defects of the casting with the gating system after further improvement.

Riser

Chill

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The shrinkage porosity defects were fully transferred into the risers. The inner part of the intake manifold casting does not show any shrinkage porosity defects. Therefore, the optimized gating system with risers and chill is very useful for the

elimination of shrinkage porosity defects.

4 Casting practiceFigure 11 shows pictures of the aluminum alloy intake manifold obtained based on the optimized gating system. The A356 alloy intake manifold casting with smooth surface and clear outline was obtained, and the surface roughness of the intake manifold casting reached Ra 3.2 to 6.3 μm. Figure 12 shows the sectional drawing of the part with maximum wall thickness, which was cut along the A–A section, as shown in Fig. 11(a). Figure 13 shows the microstructure of the sample cut from the same position with maximum wall thickness. According to Fig. 12 and Fig. 13, it can be seen that the obtained A356 alloy intake manifold part has a good internal quality. No porosity defects were observed in the cross section of the part with maximum wall thickness, and the microstructure is dense. The microstructure consists of equiaxed grains, no dendrites were observed, and the average grain size is 117.6 μm.

Fig. 12: Sectional drawing of intake manifold part Fig. 13: Microstructure of intake manifold part

Fig. 11: Pictures of aluminum alloy intake manifold obtained using optimized gating system

Fig. 10: Simulated shrinkage porosity defects with gating system further improved by adding risers and chill

5 ConclusionsThe filling and solidification processes of the low pressure casting of the A356 intake manifold casting can be visualized through the numerical simulation and the shrinkage porosity defects can be predicted. The gating system of the casting was optimized according to the simulation results.

(1) When the gating system consists of only one sprue, the filling of the molten metal is not stable, and the casting does not follow sequential solidification. Many shrinkage porosities are observed throughout the casting.

(2) After the gating system is improved by adding one runner and two in-gates, the filling time is prolonged from 4.0 s to 4.5 s, and the filling of molten metal becomes stable; but the casting

A

A

(a) (b)

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The study was supported by the National Natural Science Foundation of China (No. 51204124), the China Postdoctoral Science Foundation (No. 2012M511610) and the Scientific Research Foundation of Wuhan Institute of Technology (No. 14125041).

still does not follow sequential solidification. Some shrinkage porosity is also observed in the hot spots of the casting.

(3) Finally, the risers and chill are added to the hot spots to further improve the gating system and, as a result, the shrinkage porosity defects were eliminated completely when using the optimized gating system.

(4) The practical experiment validated the simulation results. The A356 aluminum alloy intake manifold casting with integrated shape and smooth surface (Ra 3.2 to 6.3 μm) as well as dense microstructure was successfully produced using the optimized gating system. The optimization of the gating system based on the numerical simulation can supply good guidance for the production of intake manifold castings.

References[1] Ran G, Zhou J E, and Wang Q G. Precipitates and tensile

fracture mechanism in a sand cast A356 aluminum alloy. Journal of Materials Processing Technology, 2008, 207(1-3): 46-52.

[2] Jiang W M, Fan Z T, Liu D J, et al. Influence of process parameters on fil l ing ability of A356 aluminium alloy in expendable pattern shell casting with vacuum and low pressure. International Journal of Cast Metals Research, 2012, 25(1): 47-52.

[3] Jiang W M, Fan Z T, Liu D J, et al. Correlation of microstructure with mechanical properties and fracture behavior of A356-T6 aluminum alloy fabricated by expendable pattern shell casting with vacuum and low-pressure, gravity casting and lost foam casting. Materials Science and Engineering A, 2013, 560(1): 396-403.

[4] Jiang W M, Fan Z T, Liao D F, et al. A new shell casting process based on expendable pattern with vacuum and

low-pressure casting for aluminum and magnesium alloys. International Journal of Advanced Manufacturing Technology, 2010, 51(1-4): 25-34.

[5] Zhang L Y, Jiang Y H, Ma Z, et al. Effect of cooling rate on solidified microstructure and mechanical properties of aluminium-A356 alloy. Journal of Materials Processing Technology, 2008, 207(1-3): 107-111.

[6] Zhang B, Maijer D M, and Cockcroft S L. Development of a 3-D thermal model of the low-pressure die-cast (LPDC) process of A356 aluminum alloy wheels. Materials Science and Engineering A, 2007, 464(1-2): 295-305.

[7] Cleary P W. Extension of SPH to predict feeding, freezing and defect creation in low pressure die casting. Applied Mathematical Modelling, 2010, 34(11): 3189-3201.

[8] Fu P H, Luo A A, Jiang H Y, et al. Low-pressure die casting of magnesium alloy AM50: Response to process parameters. Journal of Materials Processing Technology, 2008, 205(1-3): 224-234.

[9] Murugan G, Raghukandan K, Pillai U T S, et al. Influence of transverse load on the high cycle fatigue behaviour of low pressure cast AZ91 magnesium alloy. Materials and Design, 2009, 30(10): 4211-4217.

[10] Song Nannan, Luan Yikun, Bai Yunlong, et al. Computer simulation of core filling process of cast high speed steel roll. China Foundry, 2009, 6(4): 314-318.

[11] Mi Guofa, Liu Yanlei, Zhao Hengtao, et al. Numerical simulation and optimization of Al alloy cylinder body by low pressure die casting. China Foundry, 2008, 5(2): 99-103.

[12] Jiang Junxia, Wu Zhichao, Chen Liliang. et al. Numerical simulation and analysis of mould filling process in lost foam casting. China Foundry, 2008, 5(3): 175-178.

[13] Wang Y C, Li D Y, Peng Y H, et al. Numerical simulation of low pressure die casting of magnesium wheel. International Journal of Advanced Manufacturing Technology, 2007, 32(3-4): 257-264.