Machine Design

Faster, more predictable die casting

Alloys of zinc and magnesium sport a wide range of physical and mechanical properties. The result is enormous flexibility in selecting die-casting materials for small, complex components.

Designers have a broad latitude of materials to choose from for small die-cast components. Each of five zinc alloys, plus a magnesium option has strengths and weaknesses for particular applications. But as die-casting mediums, they have an advantage over materials such as aluminum. Their dimensional stability and predictable shrinkage rates ensure part-to-part consistency while low thermal mass permits high cycle rates. And designers often see a tenfold increase in die life thanks to the low abrasive wear rates of these alloys when compared to their aluminum counterparts.

Special die-casting techniques let designers easily modify designs to boost structural properties. To improve creep resistance in a component with external or internal threads, for example, designers can increase thread count. Special hot-camber die-casting processes produce 360* external threads cast up to 50 threads/in. (20 threads/cm). Die casting lets the parts meet a thread tolerance class of 2A (6g) with no secondary deburring or chasing.

Similarly, uniform wall thicknesses in a design distribute stresses more evenly. And precision tooling used in hot-chamber die casting helps eliminate secondary finishing or machining -- net-shape parts exit the mold "flash-free." Net-shape manufacturing is one of the main advantages of this die-casting method as it reduces material waste and part cost.

Zinc alloys
The most common zinc alloys for hot-chamber die casting are Zamak 2, Zamak 3, Zamak 5, ZA-8, and ACuZinc. Overall, their mechanical properties compare favorably with brass and screw-machined steel.

Zamak 3 is the most widely used zinc alloy. It offers the best combination of mechanical properties and economics.

Zamak 2 and 5, with higher copper content, better resist wear. In addition, Zamak 5 offers higher creep resistance, while Zamak 2 has the highest shear strength.

ZA alloys have high aluminum content which gives them greater strength, superior wear and creep resistance, and lower densities. ZA-8, with a nominal 8% aluminum content, is well suited for high-speed, hot-chamber die-casting.

ACuZinc, with a copper content of 5 to 6%, has the highest structural strength. It also has greater hardness and good creep, wear, and corrosion resistance, as well as better lubricity. Zamak 2, with its high tensile strength and lubricity, comes a close second to some of ACuZinc's properties and may be a suitable alternative in some applications.

Ideal candidates for die-cast zinc alloys include many components that perform mechanical functions. Some typical parts are gears, shafts, cams, ratchets, levers, and pinions, as well as end bells, plates, housings, spacers, and seats.

Magnesium alloy
Magnesium is the lightest of all structural metals, but it also has the highest strength-to-weight ratio. Magnesium is two-thirds the weight of aluminum and about one-quarter that of steel and zinc. Incorporating thin walls into a magnesium die-cast component further reduces part weight, while adding ribs maintains load strength.

AZ91D magnesium alloy has excellent thermal and electrical conductivity. And the alloy's inherent EMI/RFI shielding attributes makes it economical for electronic components. The alloy's elastic energy absorption and Young's modulus make for good impact and dent resistance. And its ability to absorb energy elastically dampens sound and vibration.

These properties can make magnesium alloy the material of choice for components in automobiles, hand tools, appliances, and electronic communication devices.

A typical 1.18-in. (30-mm) component with a Cpk of 1.33 (99.73% of the samples measured will lie within the design specifications) has linear tolerances, flatness, and concentricity of about 0.002 in. (0.05 mm). Die-casting alloys easily flow into complex, intricate details and form wall sections as thin as 0.02 in. (0.5 mm).

Gears are cast to AGMA 6 while external, internal, face, helical, spur, and worm gears are cast with up to a 20* helix angle and can incorporate shafts, ratchets, and cams.

Physical properties
ACuZinc has extended zinc alloy's use in structural applications because of its creep resistance -- 49 kpsi (338 MPa) yield strength. Magnesium alloy's yield strength is somewhat less at 23 kpsi (159 MPa). But creative design can compensate by taking advantage of the alloy's strength-to-weight ratio. For example, increasing wall sections in critical areas improves mechanical performance without significantly boosting component weight. Magnesium alloy's creep-strength underload at service temperatures up to 250*F (120*C), makes it suitable for automotive applications where zinc alloys would deform under stress.

Zinc alloys, in particular ZA-8 and ACuZinc, have significantly greater hardnesses than other die-casting materials -- up to 103 and 118 Bhn (Brinell), respectively. ACuZinc's low coefficient of friction makes it a shoe-in for bearing applications. But Zamak 2, with a hardness of 100 Bhn and high lubricity, can also be considered for bearings. Magnesium alloy is at the low end of the spectrum with a hardness up to 63 Bhn.

The natural thermal conductivity of both magnesium and zinc alloys lets them remove heat better than some powdered metals or mild steel. Zinc alloys also perform better than aluminum. And both alloys are frequently low-cost material choices for heat sinks.

Magnesium and zinc alloys are excellent conductors of electricity and inherently shield EMI and RFI as well as prevent ESD. This inherent shielding, combined with light weight earns a role for magnesium alloy die-cast components in electronic devices such as camera and cell phone housings, digital board frames, and load cell trays.

Properties of zinc and magnesium alloys
Zamak 2 Zamak 3Zamak5ZA-8ACuZinc5Az-91D (Mg)
Physical properties
Thermal conductivity, Btu/ft hr°F 60.5 65.3 62.9 66.3 61.2 41.8
Electrical conductivity, %IACS 25 27 26 27.7 26.9 12.2
Mechanical properties
Tensile strength, kpsi 52 41 47.6 54.2 59 34
Yield strength (2% offset), kpsi 41 32 33 42 49 23
Shear strength, kpsi 46 31 38 40 40.6 20
Hardness, Bhn up to 100up to 82 up to 91 up to 103 up to 118 up to 63
The properties of zinc and magnesium alloys give enormous flexibility in selecting a die-casting material. Each must be examined on its own merit. While similar in many respects, they exhibit sufficient diversity to meet a wide range of application specifications for small component manufacturing. In addition, their castability opens opportunities for component cost reduction.
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