Die Materials for Zinc and Magnesium Die Casting

Why the two alloys are read together

Zinc alloys are cast at relatively low temperatures and molten zinc alloys do not rapidly attack ferrous metals, so the material requirements for the dies and for the machine parts in contact with the melt are less rigorous than for aluminum or copper. Magnesium is poured hotter than zinc and does attack the die far less than aluminum does, and the published comparison puts a magnesium die at two to three times the life of an aluminum die.

Zinc die casting

In the die casting of zinc alloys the die temperature is relatively low, usually 160 to 245 °C (325 to 475 °F). Hot work tool steels are therefore not generally required for the dies. For extremely long runs and for high dimensional accuracy, however, a hot work tool steel such as H13 will provide optimum die life.

Die hardness in the casting of zinc alloys is less critical than for alloys of higher casting temperature, and steels prehardened by the manufacturer to any maximum hardness consistent with reasonable machinability can generally be used. The typical hardness is 29 to 34 HRC (280 to 320 HB).

Cores and slides follow the same logic. Hardenable stainless steels such as type 440B are often used for cores, and a hot work tool steel such as H13 can be used for both cores and slides. Because these tool steels respond readily to nitriding they can be selectively hardened, so the main portion of a core or slide can be nitrided for wear resistance while the end portions are masked to resist nitriding and keep their resistance to heat checking and spalling. Lubricating the slides and cores with molybdenum disulfide or colloidal graphite in oil keeps the action smooth and minimizes wear.

Ejector pins of nitrided H11 tool steel or 7140 alloy steel are available as stock items for insertion in the dies. On the machine side, the pots, goosenecks and other components that contact the molten metal are usually gray or ductile cast iron, and can be cast steel. Sleeves and nozzle seats receive high wear and have been made from nitrided alloy steel, hot work tool steel such as H13, high speed tool steel and stainless steel, and they are frequently made replaceable to permit inexpensive repair.

Die life for casting zinc alloys is generally much longer than for casting aluminum, magnesium or copper alloys, and it is not unusual for dies to last for 1,000,000 shots. The service life of a die is directly related to the temperature of the metal being cast, the thermal gradients within the die and the frequency of exposure to high temperature. Maximum die life depends largely on well-designed dies, trained operators and a rigidly enforced program of machine and die maintenance.

Magnesium die casting

The materials used for magnesium dies are similar to those used for aluminum. Dies and cores are typically hot work tool steels such as H11 and H13, heat treated to 44 to 48 HRC, and nitriding the die surfaces is often done. Die life can be appreciably lengthened if the die surface is finished to 0.38 µm (15 µin.). Stress relieving and repolishing the die surface after approximately 25,000 shots is sometimes carried out to retain life.

Magnesium runs hotter than zinc at the die. The optimum die temperature range is 260 ± 14 °C (500 ± 25 °F), and after the dies are preheated into that range the heat supplied by repeated casting is normally enough to hold it. There is usually a need to cool the die to stay inside the range, so the dies carry a means for water cooling, and control of die temperature is necessary to maintain casting quality. If the die temperature is too low, misruns are likely.

The advantage magnesium holds over aluminum is in the way the melt behaves at the die surface. Molten magnesium does not weld onto the die in the same way as aluminum does, so the expensive dies last much longer, and the published figure is two to three times the life of an aluminum die.

The two alloys side by side

The table collects the die material, hardness, core and slide practice, surface finish and typical die life for both alloys in one place.

Die materials and die practice for zinc and magnesium die casting, reproduced from the Zinc and Zinc Alloys article and the Magnesium and Magnesium Alloys article in ASM Handbook, Volume 15, Casting. The two alloys sit at the low end of the die casting temperature range and are treated together on that basis.

Grades and stock

The hot work grades named for these two alloys are covered on the H11 tool steel page and on the H13 tool steel page. Selective nitriding of cores and slides is set out on the tool steel nitriding guide.

The stainless core grade named for zinc is covered on the 440B stainless steel page, which gives the hardening and tempering practice for that grade.

The pressure die casting component materials, including the shot sleeve and the other parts that see the melt, are listed on the die casting die component materials page. The grade by grade selection for the same process is set out on the tool steel for die casting dies and inserts page.

Source: ASM Handbook, Volume 15, Casting (9th Edition Metals Handbook), ASM International, 1988.