Producing high-integrity magnesium castings using permanent mold.

Magnesium is increasingly used in the automotive industry to lighten vehicles, improve fuel economy and reduce greenhouse gas emissions. Currently, most magnesium castings are made by high pressure diecasting, which has limitations of internal shape, soundness, low casting yield and difficulty in applying heat treatment, making the process incompatible for some classes of automotive components.

A new permanent mold casting process called T-Mag, which is being developed by Australia's Commonwealth Scientific and Industrial Research Organization (CSIRO) under its Light Metals Flagship Program, addresses the need for magnesium castings with high-yield, high-integrity and a wider range of geometry.

The patented T-Mag process combines melting and casting in a single compact machine with low equipment and tooling cost. The steel melting crucible is connected to the bottom of the mold through a heated transfer pipe (Fig. 1). The mold is filled smoothly from the bottom up when the machine assembly rotates forward at a given angle around a fixed horizontal axis.

[FIGURE 1 OMITTED]

After solidification has progressed from the casting extremities to just beyond the gate, the machine rotates back to a horizontal position, causing the molten magnesium in the transfer pipe to separate from the small solidifying sprue formed in the gate area. The machine remains in this position as the casting develops sufficient rigidity to be ejected from the mold.

The bottom mold connection for filling and feeding, which is similar to low-pressure diecasting, results in a high casting yield (typically 95%), which reduces recycling costs and maximizes productivity. The furnace is essentially sealed during operation, which leads to efficient use of the cover gas needed to prevent the furnace charge from burning. Low cover gas consumption leads to cost savings and increased environmental sustainability. Tool life is expected to be better r than the aluminum permanent mold process due to the lower latent heat of fusion of magnesium alloys and the very low solubility of iron in magnesium.

Tooling and operating costs are low to moderate, which is comparable to gravity or low-pressure permanent mold casting of aluminum. T-Mag thus is ideal for high-integrity castings that can be heat-treated and welded and are of internal complexity that require sand cores and cannot be made by high pressure diecasting.

The new process capability has been demonstrated successfully with a number of prototype castings produced in continuous runs, including magnesium automotive wheel rim castings of 10 x 4.5 in. nominal size (295 mm actual diameter) (Fig. 2). This relatively small wheel was appropriate for the moderate size of the T-Mag development machine and was cast using a purpose-designed and built four-part tool. The small stub or sprue in the center of the wheel casting was all that needed to be removed during finishing, resulting in a casting yield of 95%.

[FIGURE 2 OMITTED]

Another recent demonstration casting is a motorcycle engine block for 750cc water-cooled engines (Fig. 3), which requires four sand cores with a six-part die. The block has a cycle time of 3.3 min. and a yield of 95%.

[FIGURE 3 OMITTED]

Further research and development is planned to extend the application into automotive power-train and engine and suspension components, as well as the refinement of the machine design and construction for the production of larger, high-volume components.

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