Die cast magnesium components

Abstract

Die casting of some magnesium alloys may yield castings that are susceptible to corrosion when exposed to salt water or other aggressive oxidizing environments. Such corrosion may result from the existence of different microstructures in a cross-section of the die casting (e.g. between the surface of the part and the center) that produce galvanic couples that are susceptible to such corrosive attack. However, a die temperature may be determined for casting of the part such that a more uniform cross-sectional microstructure is produced in which minimal or negligible galvanic potentials are produced.

Description

TECHNICAL FIELD[0001]This disclosure pertains to casting of magnesium alloys in metal molds having heating or cooling capability, or other casting processes that involve thermally managed dies or molds. More specifically this disclosure pertains to temperature control of a die casting process for a magnesium alloy article so as to improve the corrosion resistance of the die cast part.BACKGROUND OF THE INVENTION[0002]There is a need to reduce the weight of automotive vehicles in order to conserve fuel. With this goal in mind low cost, creep-resistant magnesium alloys are being evaluated for cast components such as engine cylinder blocks, bed plates, oil pans, front engine covers, and other powertrain and vehicle components. Many such components may be produced by a high pressure die casting process in which structurally complex vehicle parts may be produced at a suitable production rate. Several magnesium-based alloys are available for consideration for the casting of such automotive...

AI Technical Summary

Benefits of technology

[0007]Now, it is found that the die temperature may have a profound effect on the tendency of certain magnesium alloy parts to corrode, especially in a salt water environment. This corrosion has been attributed to a rate of solidification in the dies that produces significantly different microstructures between the surface (skin) and interior (core) of the die cast part. For example, an excessively high rate of solidification of a cross-section of a die cast part can result in a preponderance of one metallurgical phase being formed at the surface of a die cast part and a different phase of different composition formed internally. The proportions of different phases with different overall composition may form a galvanic couple between surface and core regions causing them to have higher susceptibility to accelerated corrosion in a salt water environment.

[0008]In accordance with an embodiment of this invention, casting trials at varying die temperatures may be made with a specific part and magnesium alloy composition to determine a die temperature (or temperature range) that yields a cast part that does not form the aforementioned “skin-core” two-phase microstructure in a cross-section that may render the part susceptible to unacceptable corrosion in, for example, a salt water corrosion test. Depending upon the size and shape of a die cast part and die temperature, solidification of the part may yield varying distributions of alloying elements across a cross-section of the magnesium-based part. Such variation may produce an electrochemical potential gradient between regions of the cross-section that may result in galvanic corrosion, especially in a wet salty environment.

Problems solved by technology

Now, it is found that the die temperature may have a profound effect on the tendency of certain magnesium alloy parts to corrode, especially in a salt water environment.

For example, an excessively high rate of solidification of a cross-section of a die cast part can result in a preponderance of one metallurgical phase being formed at the surface of a die cast part and a different phase of different composition formed internally.