Grain Size Distribution and Interfacial Heat Transfer Coefficient during Solidification of Magnesium Alloys Using High Pressure Die Casting Process

doi:10.1016/j.jmst.2016.09.004

Abstract

Abstract:

The objective of this study is to predict grain size and heat transfer coefficient at the metal-die interface during high pressure die casting process and solidification of the magnesium alloy AM60. Multiple runs of the commercial casting simulation package, ProCAST?, were used to model the mold filling and solidification events employing a range of interfacial heat transfer coefficient values. The simulation results were used to estimate the centerline cooling curve at various locations through the casting. The centerline cooling curves, together with the die temperature and the thermodynamic properties of the alloy, were then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting. Finally, the local cooling rate was used to calculate the resulting grain size via previously established relationships. The effects of die temperature, filling time and heat transfer coefficient on the grain structure in skin region and core region were quantitatively characterized. It was observed that the grain size of skin region strongly depends on above three factors whereas the grain size of core region shows dependence on the interfacial heat transfer coefficient and thickness of the samples. The grain size distribution from surface to center was estimated from the relationship between grain size and the predicted cooling rate. The prediction of grain size matches well with experimental results. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent interfacial heat transfer coefficient for different locations.

Key words: High pressure die casting, Grain size, Interfacial heat transfer coefficient, Solidification of magnesium alloys, Process parameters

P. Sharifi, J. Jamali, K. Sadayappan, J.T. Wood. Grain Size Distribution and Interfacial Heat Transfer Coefficient during Solidification of Magnesium Alloys Using High Pressure Die Casting Process[J]. J. Mater. Sci. Technol., 2018, 34(2): 324-334.

Figures/Tables 18

Fig. 1. Schematic view for the solidification through the thickness of casting based on the modified heat transfer approach.

Table 1 Values of thermo-physical variables were used

Thermo-physical variables	T_L (°C)	T_S (°C)	α (m² s^-1)	k (W/(m °C))	ρ (kg m^-3)	c (kJ/(kg °C))	L_f (kg J^-1)
Values	615	565	0.044	Variable from solid to liquid phase 101.8-704	Variable from solid to liquid phase 1,810-1,620	Variable from solid to liquid phase 1-1.17	37,000

Fig. 2. Typical microstructure of the skin and core regions of the die cast magnesium alloy AM60.

Table 2 Chemical composition of AM60 alloy

Alloy	Mg	Al	Zn	Mn	Si	Cu	Fe	Ni	Be
AM60	93.6	6.1	—	0.32	0.005	0.001	0.001	0.001	0.001

Fig. 3. Shock tower casting with gates and overflow. Selected locations for the characterization are marked from Locations 1 to 6.

Table 3 Reference process parameters were used in this study. In order to implement the simulation, fast shot of piston and die temperature were changed

Process parameter	Slow shot speed of piston (m s^-1)	Fast shot speed of piston (m s^-1)	Intensification pressure (bar)	Die temperature (°C)	Melt temperature (°C)	Vacuum (mbar)
Value	0.6	5	450	160	700	<30

Fig. 4. Filling sequence and solidification conditions predicted in pro-cast.

Fig. 5. Die filling time as affected by fast shot velocity.

Fig. 6. Predicted die surface temperatures in selected locations at IHTC of 15,000 W m-2 K-1, die temperature of 200 °C and fast stage velocity of 4 m s-1. This curve was directly obtained from simulation result.

Fig. 7. Temperature distribution along the die wall and casting with time for Location 1.

Fig. 8. Relation between cooling rate and IHTC at different locations for (a) skin and (b) core regions.

Fig. 9. Dependency of average cooling rate on the variation of die temperature at (a) skin and (b) core regions.

Fig. 10. Dependence of average cooling rate on the variation of filling time at (a) skin and (b) core regions.

Fig. 11. Microstructure of six locations.

Fig. 12. Comparison of the experimental results and predicted values using the modified model in terms of grain size from surface to center.

Table 4 Characteristic of six locations in the shock tower which have been produced under the reference process parameters

Location	Thickness (mm)	Average grain size in the skin region (μm)	Average grain size in the core region (μm)	Area fraction of ESG (%)
1	3	9.8	22.3	26.4
2	3.2	8.5	17.8	19.4
3	3.8	8.1	15	5.7
4	6.3	7.3	20	6.2
5	3.1	7.7	13.7	1.3
6	3.2	7.5	13.1	<1

Fig. 13. Comparison of the measured grain size and predicted values using the modified model for different IHTC value from surface to center.

Table 5 IHTC value for casting process in the literature that the pressure is applied

Reference	Process casting	Alloy	Features	IHTC (W m^-2 K^-1)
^[13]	HPDC	AM50	Effect of the thickness, pressure, fast stage velocity of piston are examined	10,820-14,780
^[14]	HPDC	ADC12	Effect of intensification pressure, fast stage velocity of piston and initial die temperature	16,450-20,760
^[24]	HPDC	AZ91	Effect of pressure cavity, slow and fast stage velocity pressure are investigated	8,500-9,000
^[25]	Squeeze casting	AM60	Pressure range: 30-90 MPa. Effect of thickness is examined	7,195-10,649
^[26]	Squeeze casting	A443	Pressure range: 30-90 MPa. Effect of thickness is examined	7,441-11,249

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