Magnesium die casting

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In magnesium die casting , preheated ingots , also called ingots, are charged in the melting and holding furnaces, which are equipped with steel crucibles on every die casting machine. Usually electric or gas fired furnaces are used. For safety reasons, the bars must be preheated before being immersed in the molten metal . Because of the strong tendency of molten magnesium to react with the oxygen in the air, it is imperative that a protective gas atmosphere covers the molten metal. For this purpose a mixture of 0.2% sulfur hexafluoride (SF 6 ) in dry air or nitrogen or a mixture of 0.5-2% sulfur dioxide in dry air is used. The protective gas must be supplied in such a way that burning and oxidation are prevented when melting. Too high a concentration of SF 6 causes the steel parts to be attacked.

The metal surface must be smooth and clean.

Sulfur dioxide is preferred over SF 6 because SF 6 is an extremely powerful greenhouse gas. Alternatively, R134a is used.

Because liquid magnesium alloys can be transported in heated, nickel-free steel pipes, a variety of metering devices are in use. Clean magnesium parts can also be melted in hot chamber die casting machines. Liquid magnesium has a low solubility against iron, for this reason different types of pumps can be fed through heated steel pipes with molten magnesium for dosing into the shot cylinder. The casting temperature varies from one alloy to another and has an impact on the casting process. The casting temperature for AZ91 (MgAl9Zn1) is typically in the range of 650–680 ° C for cold chamber die casting machines and 620–640 ° C for hot chamber die casting machines. Alloys with lower aluminum contents are cast at slightly higher temperatures around 660–690 ° C.

In general, cold chamber die casting machines that are used for aluminum can also be used for magnesium. Important differences between aluminum and magnesium are the density, which is only 2/3 that of aluminum. The lower density of magnesium compared to aluminum means that the inertia is lower and for the same amount of metal a higher flow rate is achieved under pressure.

So it takes less time to fill a mold with magnesium than with aluminum. The filling time for magnesium will also be shorter for cast parts with thin walls and large flow paths. For this reason, some magnesium die casters work with shot piston speeds that exceed 10 m / s. The spec. Casting pressures are generally in the range of 30-70 MPa. Cold chamber die casting machines with clamping forces up to over 4000 tons are commercially available.

Mainly smaller parts with shot weights of up to 2 - 3 kg are produced on hot chamber die casting machines. The casting pressure is lower than with cold chamber machines, typically in the range of 20 to 30 MPa.

Example components

  • Hot chamber die casting: PC cases, carriers, cases for mobile phones.

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