Numerical Simulation of Liquid-Solid Conversion Affecting Flow Behavior During Casting Filling Process
Liu CAO, Fei SUN(), Tao CHEN, Zihao TENG, Yulong TANG, Dunming LIAO
State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, China
Cite this article:
Liu CAO, Fei SUN, Tao CHEN, Zihao TENG, Yulong TANG, Dunming LIAO. Numerical Simulation of Liquid-Solid Conversion Affecting Flow Behavior During Casting Filling Process. Acta Metall Sin, 2017, 53(11): 1521-1531.
Misrun and cold shut are common defects in casting productions, which could make surface accuracy of castings poorer, even leading to cracking and casting scraps in them. The formation process of misrun and cold shut is hard to be observed directly only by experiment measures, since casting filling process is in a state of high temperature flow inside mold. The key to predict the defects accurately is the way to handle the effect of liquid-solid conversion on flow behavior. On the basis of existing methods for treating liquid-solid conversion, a calculation model of mushy region flow behavior through measurement of solid-fraction is developed, which can effectively investigate the flow behavior of mushy region in different stages. Generally, the critical solid-fraction method is adopted for mushy region with high solid-fraction, in consideration of that only the speed of high solid-fraction region is supposed to be zero during casting filling process. The variable viscosity method is applied for mushy region with low solid-fraction, due to casting filling process being unlikely to form toothpaste-like flow. However, the porous medium drag-based model is used for mushy region with middle solid-fraction, because only the middle solid-fraction region can be equivalent to porous medium. Combining the above three methods, a flow-field calculation program considering the effect of liquid-solid conversion on flow behavior during casting filling process is developed, in which finite volume method (FVM) is included for discretization equations; the pressure implicit with splitting of operator (PISO) algorithm is added for coupling pressure and velocity; the volume of fluid (VOF) algorithm is also combined for interface tracking. An numerical simulation of water-filled S-shaped channel is performed in the case of taking no account of liquid-solid conversion, and the simulated results coincide better with the experimental results, which certifies for its accuracy as an adopted model. Since the bottom filling casting craft is commonly used in single-shape casting, a comparison between the calculated results obtained using other single models and those using this model at different control parameters, is needed. The better agreement between them indicates that this new model is appropriate for calculating the flow behavior in mushy region.
Fund: Supported by New Century Excellent Talents in University (No.NCET-13-0229) and National Science & Technology Key Projects of Numerical Control (No.2012ZX04010-031)
Fig.1 Schematics of the evolution process of dendrite morphology in fluid liquid (a) starting stage of dendrite growth (b) middle stage of dendrite growth (c) final stage of dendrite growth
Fig.2 Calculation flow chart used for the pressure implicit with splitting of operator (PISO) algorithm
Parameter
Value
Unit
Water density
1000
kgm-3
Air density
1
kgm-3
Water dynamic viscosity
1×10-3
Pas
Air dynamic viscosity
1×10-5
Pas
Water-air surface tension coefficient
0.07275
Nm-1
Acceleration of gravity
ms-2
Inlet velocity
ms-1
Outlet pressure
0
Pa
Table 1 Parameters used in simulation of S-shaped channel
Fig.3 Comparisons between experimental results (a~c) and simulated results of volume fraction of water (d~f) and velocity of liquid-gas phase (g~i) in S-shaped channel filled with water at 7.15 ms (a, d, g), 25.03 ms (b, e, h) and 39.34 ms (c, f, i)
Fig.4 Geometric and mesh model of bottom filling casting craft with simple shape
Parameter
Value
Unit
Aluminum alloy density
2385
kgm-3
Air density
1
kgm-3
Aluminum alloy dynamic viscosity
0.003
Pas
Air dynamic viscosity
1×10-5
Pas
Aluminum alloy-air surface tension coefficient
0.871
Nm-1
Acceleration of gravity
ms-2
Inlet velocity
ms-1
Outlet pressure
0
Pa
Liquidus temperature of aluminum alloy
660
℃
Solidus temperature of aluminum alloy
560
℃
Latent heat of aluminum alloy
300
Jg-1
Inlet temperature
670
℃
Mold temperature
30
℃
Heat transfer coefficient between lower surface and mold
6000
Wm-2K-1
Heat transfer coefficient between other surfaces and mold
100
Wm-2K-1
Table 2 Parameters used in simulation of bottom filling casting craft
Fig.5 Comparisons of simulated results of solid-fraction of aluminum phase (a~c), velocity of liquid-gas phase (d~f) and volume fraction of aluminum phase (g~i) under critical solid-fractions of 0.5 (a, d, g), 0.75 (b, e, h) and 0.95 (c, f, i) at 5 s Color online
Fig.6 Comparisons of simulated results of solid-fraction of aluminum phase (a~c), velocity of liquid-gas phase (d~f) and volume fraction of aluminum phase (g~i) with incremental multiples as 1 (a, d, g), 10 (b, e, h) and 100 (c, f, i) at 5 s (The incremental multiple means the viscosity difference between solid and liquid phase) Color online
Fig.7 Comparisons of simulated results of solid-fraction of aluminum phase (a~c), velocity of liquid-gas phase (d~f) and volume fraction of aluminum phase (g~i) with porous medium drag coefficients of 1 (a, d, g), 10 (b, e, h) and 100 (c, f, i) at 5 s
Fig.8 Simulated results of solid-fraction of aluminum phase (a~c), velocity of liquid-gas phase (d~f) and volume fraction of aluminum phase (g~i) at different times of 1.5 s (a, d, g), 3 s (b, e, h) and 5 s (c, f, i) by using the calculation model for mushy region flow behavior through measurement of solid-fraction
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