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Simulation of Solidification Process


Experimental measurement provides the basis for verifying the results of numerical simulation and for specifying the boundary conditions of the calculation. Simulation was con- ducted at this stage for only the non-insulated shell moulds, using the ProCast software of the ESI Group. In the initial calculation the thermal-physical parameters and boundary conditions from the database of this software were used. Certain deviations in the temperature waveforms of both the metal and the mould were established between the measured and simulated values. Increasing the agreement was subse- quently solved by changing the boundary conditions of the calculation. In the optimization of numerical calculation, use was made in particular of: •


• •


changing the coefficient of heat transfer between the metal and the mould, αmet-mould


changing the coefficient of mould emissivity, εmould


changing the coefficient of heat transfer from the mould into the surroundings, αmould-surr


To specify the value of mould emissivity, experimental mea- surement was performed via comparing data from thermo- couples and thermocamera. In this way, the optimum value of emissivity coefficient εm


was determined in the range from 0.7 to 0.8.


to the measurement and the simulation results coming closer together, as illustrated in Fig- ure 18.


The changes in boundary conditions were aimed in particular at obtaining agreement of metal temperatures and solidification time, which are decisive from the viewpoint of crystallization. Agreement of shell mould temperatures is only a secondary criterion. It came to light that in the calculations it will be necessary to anticipate thermally variable val- ues of the mould emissivity and of the coef- ficient of heat transfer between the metal and the shell mould, αmetal-mould


. These changes led .


the heat axis of castings towards the shell mould axis could be proved (Figure 19). This shift was expected on the ba- sis of theoretical analysis of heat transfer and experimental measurement of temperatures.


Simulation of Structure


Numerical simulation of the structure in individual castings of the experimental mould was performed using the ProCast software. We regard these calculations as an introductory study with the aim of obtaining parameters for numerical prediction of the structure of real castings. The calculation was performed for the N155 (Cr-Ni-Co alloy steel) and for solidification in non-insulated moulds. The structure ob- tained by calculation was compared with the actual structure of castings in the corresponding cross-section of test cylin- ders. Initial conditions were found under which very good agreement of metallographic and simulated microstructures was obtained. An illustration of structures for dia. 50 mm (1.96 in) cylinder can be seen in Figure 20. In the simulated structure of this casting the shift of the heat centre away from the geometrical axis is clear to see. (The dark point in the left figure is a section across the thermocouple.)


Figure 16. Shell moulds with different insulation of dia. 50 mm (1.96 in.) cylinders.


For a practical application of numerical simu- lation to shell moulds it is necessary to respect the necessity of describing precisely the shape of shell mould and the thickness of its walls. Deviations in the simulated and the actual shape may lead to considerable differences in the results.


Based on optimized values of boundary con- ditions, the temperature field was calculated for castings solidifying in non-insulated shell moulds. The effect of the ingate opening into the distribution ring on the solidification process of individual castings was proved in these calculations. In particular, the shift of


78


Figure 17. Temperature curves in dia. 50 mm (1.96 in) cylinder castings with different insulations.


International Journal of Metalcasting/Spring 10


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