permanent mould casting, which is a cyclic operation and not a onetime event. Chills with no cooling will accumu- late heat with every cycle. As contact with the die surround- ing them would likely be imperfect, they would likely rise steadily in temperature until there were of no further use. In fact this is what is observed with pulls and moving inserts in permanent mould dies. Water cooling would be necessary to make these controllable in a cyclical operation.
Effect of Chill Conditions on Porosity
Porosity has a direct relation with the mechanical properties of the casting. It is known that the numbers and sizes of the pores can be greatly reduced by increasing the cooling rate. However, the air gap formation at the casting-mold interface reduces the heat transfer rate subsequently increasing the so- lidification time. The movable chill used in this experiment reduces the solidification time compared to the fixed chill. Hence, it is necessary to compare the effect on porosity in all four chill conditions.
The casting samples under different scenario were observed under the optical microscope. The pores greater than 10 μm were located within a 500 μm X 500 μm area at predetermined locations within the casting. Figure 20 shows the average number of pores measured at selected locations (2.5 mm, 7.5 mm, 15 mm, 37.5 mm and 52.5 mm) within the castings pro- duced under different conditions. It can be seen that the aver- age numbers of pores are much higher in fixed chill without water cooling than movable chill with water cooling. The av- erage number of porosity in samples cast with movable chill with water cooling is 27 % lower than cast with water cooled fixed chills. It can also be confirmed that porosity is best re- duced by moving the chill before the eutectic temperature is reached in Scenario D. In general it is found that, the porosity can be reduced by using movable chill which improves cool- ing rate by reducing the air gap at the interface.
Temperature Profile for Different Casting Scenarios
Effect of Chill Conditions on SDAS
The results show that the size of SDAS with movable chills can be reduced by up to 26 % compared to fixed chills. Fur- ther it is observed that dendrite arm spacing can be refined by effective heat transfer during the initial stage of solidifi- cation as in Scenario D. The SDAS sizes are reduced by 32 % compared to Scenario B.
Conclusions
The following conclusions can be drawn from this study: Cooling Effect
It was previously expected that a fixed chill design with cooling (Scenario B) would extract heat more effectively than without cooling. However, the presented experiments have shown that it actually becomes ineffective after a rapid growth of an initial air gap. This phenomenon is believed to be the result of sudden freezing of metal as the casting comes in contact with the water cooled chill.
Simulation of Air Gap through HTC
The effect of air gap has been simulated by comparing the heat transfer coefficients for different scenarios. Experi- mental results indicate air gap formation can be reduced us- ing a movable chill and a higher heat removal rate can be achieved, thereby reducing the solidification time and hence improving productivity of a given casting.
Solidification Time
The effective solidification time with a movable water cooled chill has decreased by over four times compared to a fixed water cooled chill which is at present being used in the industry.
Time, sec Figure 19. Temperature vs. time for different scenarios. International Journal of Metalcasting/Spring 11 77
Temperature, C
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