Sequential aging of aluminum silicon casting alloys
View Patent ↗Aluminum castings having increased elongation and tensile strength are obtained by sequential aging a solutionized casting followed by rapid heating to nucleation temperature followed by rapid cooling, then reheating to precipitate growth temperature.
1. A method of using a multiple step artificial aging process for an automotive engine component made from an aluminum silicon alloy casting comprising relatively thick regions and relatively thin regions, the method comprising:
a) solution heat treating the casting to dissolve alloying elements, following by cooling;
b) rapidly heating the casting to a nucleation temperature in a range of 400° F. to 500° F. at a rate of at least 1.5° F./min such that a heating rate differential between the relatively thick regions and relatively thin regions is no greater than 7° F./min, and holding at a temperature at least equal to the nucleation temperature for a time sufficient to induce nucleation throughout the casting;
c) cooling the casting to a temperature about 100° F. or more lower than the nucleation temperature,
d) heating the casting to a temperature lower than the nucleation temperature to facilitate growing precipitates as distinct phase in the casting, and
e) cooling the casting to ambient temperature.
2. The process of claim 1 , wherein the Brinell hardness of the casting decreases and the tensile strength and elongation both increase relative to a single-step aging process.
3. The process of claim 1 , wherein a liquid heat treating medium or a fluidized bed furnace is used to achieve the rapid heating.
4. The process of claim 1 , wherein the differential time to temperature is lowered by contacting heavier sections of the casting with an increased volume of hot fluid.
5. The process of claim 1 , wherein the heating rate of the casting in step b) is minimally 1° F./s averaged over the heating time to the nucleation temperature.
6. The process of claim 1 , wherein the heating rate of the casting in step d) is minimally 1.5° F./s averaged over the heating time to the precipitate growth temperature.
7. The process of claim 1 , wherein the temperature in step c) is sufficiently low such that precipitate growth does not occur.
8. The process of claim 7 , wherein the cooling rate is a cooling rate more rapid than that obtained in a forced air furnace.
9. The process of claim 8 , wherein cooling is accomplished in a liquid, in a fluidized bed, by impingement of a gas jet, or a combination thereof.
10. The process of claim 1 , wherein in step c) the casting is cooled to a temperature lower than that required to grow precipitates, and the casting is reheated in step d) to a temperature sufficient for growth of precipitates.
11. The process of claim 1 , wherein following nucleation in step b) precipitate growth in the casting is rapidly quenching to a temperature at which growth of precipitates is interrupted, followed by precipitate growth at a temperature lower than the nucleation temperature.
12. The process of claim 1 , wherein a slowest heating section of the casting reaches the nucleation temperature in 100 minutes or less.
13. The process of claim 1 , wherein a slowest heating section of the casting reaches the nucleation temperature in 60 minutes or less.
14. The process of claim 1 , wherein a slowest heating section of the casting reaches the nucleation temperature in 30 minutes or less.
15. The process of claim 1 , wherein the average heating rate to nucleation temperature in step b) is ≧5° F./minute.
16. The process of claim 1 , wherein the average heating rate to nucleation temperature in step b) is ≧3° F./minute.
17. The process of claim 1 , wherein the heating rate differential in step b) is less than 5° F./minute.
18. The process of claim 1 , wherein the differential between the heating rates of the thin and thick sections in step d) is less than 7° F./minute.
19. The process of claim 1 , wherein the multiple step ageing process results in both a higher tensile strength than a T6 aged casting and a higher tensile elongation than a T7 aged casting.