Cast metal products with high grain circularity
Systems and methods for making aluminum alloy products are described including those that decrease the tendency for hot tearing or shrinkage porosity to occur during casting by introducing forced convection during the casting process. The forced convection may result in formation of high circularity grains during the solidification process, thereby increasing the permeability of the liquid aluminum alloy and decreasing the tendency for hot tearing or shrinkage porosity to occur.
1. A method comprising
feeding an aluminum alloy in a molten state into a casting cavity to form an intermediate product, wherein the intermediate product comprises:
a first zone having a first temperature below a liquidus temperature of the aluminum alloy and above a coherency temperature of the aluminum alloy;
a second zone adjacent to the first zone, the second zone having a second temperature below the coherency temperature of the aluminum alloy and above a solidus temperature of the aluminum alloy; and
a third zone adjacent the second zone, the third zone having a third temperature below the solidus temperature; and
forcing convection in at least the first zone to limit a temperature variation across the first zone;
wherein:
grains of the third zone of the intermediate product have an average circularity from 0.5 to 1.
2. The method of claim 1 , wherein the average circularity of the grains of the third zone of the intermediate product ranges from 0.6 to 1.
3. The method of claim 1 , wherein the third zone contains no defects due to hot cracking.
4. The method of claim 1 , wherein the temperature variation across the first zone is less than 2% of the liquidus temperature of the aluminum alloy.
5. The method of claim 1 , wherein the first temperature ranges from 10° C. below the liquidus temperature of the aluminum alloy to 1° C. below the liquidus temperature of the aluminum alloy.
6. The method of claim 1 , wherein the first temperature is uniform within the first zone of the intermediate product.
7. The method of claim 1 , further comprising adjusting a rate of the forced convection in at least the first zone to achieve a target material property in the third zone, wherein the target material property is one or more of an average grain size in the third zone or the average circularity of grains in the third zone.
8. The method of claim 1 , wherein the first zone of the intermediate product comprises seed grains of the aluminum alloy having a first average size ranging from 10 μm to 50 μm.
9. The method of claim 1 , wherein the first zone of the intermediate product comprises seed grains of the aluminum alloy having a first average size, wherein the second zone of the intermediate product comprises grains of the aluminum alloy having a second average size, wherein the third zone of the intermediate product comprises grains of the aluminum alloy having a third average size, and wherein at least one of the second average size or the third average size is greater than the first average size.
10. The method of claim 1 , wherein the third zone of the intermediate product comprises grains of the aluminum alloy having an average size from 20 μm to 120 μm.
11. The method of claim 1 , further comprising feeding the molten aluminum alloy into the second zone of the intermediate product to fill interstitial spaces among grains of the aluminum alloy in the second zone of the intermediate product.
12. The method of claim 1 , wherein the third zone of the intermediate product is separated from the first zone of the intermediate product by the second zone of the intermediate product.
13. The method of claim 1 , wherein the second zone of the intermediate product is disposed vertically between the first zone of the intermediate product the third zone.
14. The method of claim 1 , wherein the second zone of the intermediate product is disposed horizontally between the first zone of the intermediate product the third zone.
15. The method of claim 1 , wherein forcing convection comprises stirring the first zone of the intermediate product.
16. The method of claim 15 , wherein the first zone of the intermediate product is stirred by at least one of an ultrasonic stirrer, a mechanical stirrer, or a propeller.
17. The method of claim 16 , wherein the propeller comprises at least one of aluminum oxide, aluminum nitride, or graphite.
18. The method of claim 1 , wherein a cooling speed ranging from 1° C./second to 10° C./second is maintained.
19. The method of claim 1 , wherein a cooling speed ranging from 10° C./second to 100° C./second is maintained.
20. The method of claim 1 , wherein a cooling speed ranging from 100° C./second to 800° C./second is maintained.