CORROSION RESISTANCE OF THE CAST Mg ALLOYS BY NOVEL MICROSTRUCTURAL PHASE MODIFICATIONS
A method of improving the corrosion resistance of magnesium alloy castings containing more than about 2 per cent by weight of aluminum is described. The method comprises: first selecting a casting process suitable for developing at least on the surface of the casting a microstructure comprising aluminum-depleted magnesium grains surrounded by an aluminum-rich layer and preferably incorporating at least some of an intermetallic compound based on the composition Mg 17 Al 12 ; and second, heat treating at least the outer layer of the casting to promote additional intermetallic compound precipitation as required.
1 . A method of improving the corrosion behavior of magnesium alloy castings comprising an average composition of more than about 2 per cent by weight of aluminum, by developing a microstructure in which regions of varying aluminum content are functionally arranged to impart improved corrosion resistance, the method comprising:
cooling the casting at a rate sufficient to develop in the casting a microstructure comprising regions of less than average aluminum content generally surrounded by a layer of at least greater than average aluminum concentration; and
subjecting the casting to a heat treatment at a temperature and for a duration sufficient to promote an altered microstructure comprising an at least partially continuous network of a magnesium-containing intermetallic compound which at least partially encloses the regions of less than average aluminum concentration.
2 . The method of claim 1 wherein the intermetallic compound is based on Mg 17 Al 12 .
3 . The method of claim 1 wherein the heat treatment comprises holding at a temperature of about 232° C. for a period of about 6 hours.
4 . The method of claim 1 wherein the heat treatment comprises holding at temperature of between 200° C. and 300° C. for a period of between 2 hours and 10 hours.
5 . The method of claim 1 wherein the casting is manufactured by a casting process is selected from the group consisting of die casting, permanent mold casting, green sand casting, dry sand casting and investment casting.
6 . The method of claim 1 wherein the aluminum content of the alloy is in the range of about 3 percent by weight to about 10 percent by weight.
7 . The method of claim 1 wherein the heat treatment process is conducted using one of the group consisting of furnace heating, induction heating and radiant heating.
8 . The method of claim 1 wherein the corrosion performance of the alloy is determined by the technique of Potentiodynamic polarization.
9 . A method of imparting a predetermined level of corrosion resistance to magnesium alloy castings comprising aluminum as a major alloying element, by development of a microstructure comprising an at least partially continuous network of a magnesium-containing intermetallic compound which at least partially encloses regions of a magnesium-rich solid solution, the method comprising:
(a) selecting a casting process and alloy composition;
(b) determining, based on experience, experiment or modeling, the cast microstructure by consideration of the alloy composition, particularly its aluminum content, and the cooling rate of the casting process; then
(c) assessing, based on experience, experiment or modeling the level of corrosion protection conveyed by the microstructure, and; either
(d) terminating the method or enhancing the corrosion protection by; either
(e) heat treating the casting; or
(f) by selecting a different alloy composition or casting process and repeating the above steps beginning with step (b).
10 . The method of claim 9 wherein heat treating the casting comprises subjecting the casting to a heat treatment of suitable duration and at suitable temperature to promote an at least partially continuous distribution of a magnesium-containing intermetallic compound.
11 . The method of claim 9 wherein the heat treatment comprises holding at a temperature of about 232° C. for a period of about 6 hours.
12 . The method of claim 9 wherein the heat treatment comprises holding at a temperature from between 200° C. and 300° C. for a period of between 2 hours and 10 hours.
13 . The method of claim 9 wherein the casting is manufactured by a casting process is selected from the group consisting of die casting, permanent mold casting, green sand casting, dry sand casting and investment casting.
14 . The method of claim 9 wherein the aluminum content of the alloy is in the range of about 3 percent by weight to about 10 percent by weight.
15 . The method of claim 9 wherein the corrosion resistance is assessed using Potentiodynamic Polarization.
16 . An aluminum-containing magnesium alloy article of specified average aluminum concentration heat treated to develop a microstructure comprising an at least partially continuous network of a magnesium-containing intermetallic compound which at least partially encloses regions of a magnesium-rich solid solution and thereby impart enhanced corrosion protection to the article.
17 . The corrosion-resistant, aluminum-containing magnesium alloy article of claim 14 wherein the magnesium-containing intermetallic compound is based on the composition Mg 17 Al 12 .
18 . The corrosion-resistant, aluminum-containing magnesium alloy article of claim 15 wherein the aluminum-magnesium intermetallic compound based on the composition Mg 17 Al 12 is present in an amount ranging from 3 to 15 per cent by volume.