Magnesium – Structurally Sound

Consider this lightweight material for small cast parts

—by Stuart Burke, Application Engineer, Fishercast

Design engineers considering a structural metal for small component production have a “check list” of materials that includes steel, powdered metal, aluminum and zinc, depending on the application. Until recently magnesium has rarely been on the list, yet it’s the lightest of all structural metals, and its physical and mechanical attributes meet a wide range of production specifications – including component piece price reduction.

As an example, take the cover for an oil reservoir. Originally designed for aluminum die casting, the requirements of the 54 mm by 64 mm by 10 mm-thick component could just as easily be met with magnesium alloy. AZ91D was specified and the component cast using the hot chamber die casting process. Weight was reduced by 30% and piece price by 23%, but the net shape of the cast cover and the much finer tolerances attainable improved part-to-part consistency and the appearance of the component. Plus, the life expectancy of the magnesium die cast tool is five times longer.

Why magnesium?

Light weight, combined with the highest strength-to-weight ratio, sets magnesium apart as a choice where a structural metal is required. It is two-thirds the weight of aluminum and about one-quarter that of steel and zinc. By design, weight can be reduced even further by reducing wall thickness and incorporating ribs to increase load strength. These properties are making magnesium an increasingly more attractive alloy for components in automobiles, hand tools, appliances and electronic communications devices.

Magnesium’s excellent dimensional stability, with minimal residual casting stress and consistent, predictable shrinkage rates, ensures part-to-part consistency in die cast magnesium alloy components not attainable with aluminum. Developments in hot-chamber die casting technology meet close tolerance specifications and produce a net shape component that requires no secondary finishing or machining.

Magnesium hot chamber die casting processes, coupled with precision tooling, can meet flash-free, precise tolerance specifications. For a typical 25 mm component with Cpk = 1.33, linear die cast tolerances, flatness and concentricity can be as close as 0.05 mm. The alloy’s fluidity allows for thin wall sections (a minimum 0.5 mm) for reduced cost, increased production rates, as well as complex, intricate detail. Gears can be cast to AGMA 6 and external as cast threads to 6g. Surface finish is 1.6 microns.

External, internal, face, helical, spur and worm gears are cast with up to a 20-degree angle and can incorporate shafts, ratchets and cams. 360-degree external threads are cast at up to 20 threads per cm with a thread tolerance class 6g and with no secondary cleaning or chasing. Intricate detail is possible in the component design, with holes and slots cast flash free. Design elements, such as ribs, can be incorporated to give increased strength, stability and density, while at the same time reducing the amount of material used.

Magnesium’s physical properties are making it a material of choice for many consumer product applications. Structurally, it is more rigid than plastic, and feels and looks more durable. Magnesium has excellent thermal and electrical conductivity with the alloy’s inherent EMI/RFI shielding attributes, making it a low-cost solution for components in electronic devices such as camera and cell phone housings, digital board frames, load cell trays, platforms and heat sinks.

Magnesium die cast components frequently perform well in an as-cast condition, but chemical treatments can be applied for corrosion protection. Platings such as nickel, brass, tin or copper lend wear resistance, conductivity and solderability.

AZ91D, designated by ASTM as die casting grade magnesium, has a tensile strength of 234 MPa; yield strength of 159 MPa; shear strength of 138 MPa and Brinell Hardness up to 63. Through creative design, advantage can be taken of magnesium’s strength-to-weight ratio. For example, component wall sections can be increased in critical areas to enhance mechanical performance while still reducing component weight.

Magnesium’s elastic energy absorption and Young’s modulus results in good impact and dent resistance, a favorable attribute for many hand-held devices. Used in power tools as motor and gear housings, magnesium’s ability to absorb energy elastically dampens sound and vibration. Good creep strength at service temperatures up to 120º C makes it suitable for automotive applications where zinc alloy would deform under stress. Steering column support brackets, electrical actuators, cam covers, pedal and alternator brackets, and air bag components are well-suited applications for magnesium.

Hot chamber technology positions the cost of magnesium die cast components favorably against many other die cast materials. The alloy’s low thermal mass allows cycle speeds up to five times faster than aluminum. Because of its low abrasion, tool maintenance costs are reduced as die life expectancy is much longer — generally about 500,000 units for small components. Although hot chamber die cast magnesium components are cast net shape, the cost of any additional secondary finishing requirements are reduced because magnesium is the easiest of the structural metals to machine.

Magnesium’s physical and mechanical attributes, and piece price reduction are adding value to a wide range of applications. In addition to the applications already mentioned, the alloy can be found in bicycles and roller blades, vacuum cleaners and wheelchairs, grass trimmers and lawn mowers.