CORROSION RESISTANCE OF THE CAST Mg ALLOYS BY NOVEL MICROSTRUCTURAL PHASE MODIFICATIONS

Abstract

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.