Magnesium parts with muscle

Bruce Mark
Production and New Program Manager, Magnesium and Intermediate Zinc Castings
FisherCast Global Corp. Peterborough, Ont.

Magnesium alloys increasingly replace aluminum, zinc, and other materials in structural automotive parts because they are much lighter. Magnesium's low density significantly reduces weight by volume compared to aluminum and zinc. Magnesium is 33% lighter than aluminum and just about a quarter the weight of zinc. Yet magnesium has the highest strength-to-weight ratio of all structural metals, except for titanium.

Its density is only slightly above that of plastic, but its high (72 W/m-K) thermal conductivity lets it dissipate heat more effectively. This makes magnesium a better candidate for parts that see elevated temperatures where creep is a concern. Additionally, magnesium damps out vibration and noise, resists impacts and dents, and is fully recyclable.

High-pressure, hot-chamber die-cast AZ-91D magnesium, for example, now takes the place of aluminum in small components for steering columns, valve covers and housings, shift actuators, and intake-manifold blades. Magnesium die-cast components also go into small engines, power tools, and electronic and medical devices.

Comparing total component costs
Low density gives magnesium the best economics/volume compared to aluminum components of equal geometry. Some plastics are slightly lighter than magnesium and competitive in price/volume. But the physical and mechanical properties of magnesium often let it outperform plastic where certain strength, stiffness, heat dissipation, dampening, EMI shielding or recyclability are important.

Weight reduction is a strong motivator in many applications, but magnesium's net-shape castability in the hot-chamber die-casting process is often the reason for using it. Designers should evaluate the total cost of a small die-cast component, rather than just looking at price/pound. Net-shape castings eliminate secondary machining and can significantly reduce processing time and waste material.

The alloy has excellent ductility and rapid solidification which can cut cycle times in half. Magnesium also doesn't degrade tools as readily as aluminum so dies can last three times longer.

AZ-91D magnesium alloy has high fluidity along with dimensional stability and consistent, predictable shrinkage rates. These properties make for castings with intricate, complex geometries. Component wall thicknesses on the order of 0.040 in. are typical and walls can be as thin as 0.020 in. for short distances without losing component integrity. This net-shape ability comes from a combination of the alloy's properties, tooling techniques, and high-pressure casting technology. It often outweighs magnesium's low density as the criteria for selection.

The tolerance of the die-cast tool plays a significant role in production of as-cast close tolerances. Flashing at the tool faces makes hot-chamber die casting uneconomical when deburring or secondary finishing is required. In conventional die-casting tools, molten alloy is forced into the cavity until it flashes out between adjoining tool surfaces. Hot-chamber die-casting tools assemble with tolerances of ± 0.0001 in., forming a tight seal around the cavity and eliminating flashing. Linear tolerances are typically ±[0.0008 in. + (0.001 × D)] where D = the dimension in question and Cpk = 1.33. Straight -ness is 0.001… in./in., flatness is 0.0015 in., perpendicularity is 0.001 in./in. and concentricity is within 0.002 -in. TIR. Surface finish is typically 16 to 64 µ in. and certain bores can be cast to a dimensional tolerance of ±0.0005 in.

Magnesium has a low volumetric specific heat, (1.03 J/gm-K) for efficient heat dissipation. Less heat/volume reduces solidification time. Test parts cool quickly and then can be cast up to 50% faster than comparable aluminum components.

Although hot-chamber high-pressure die casting produces net shape magnesium components, machining may be required for specialized features. Magnesium easily machines and has low resistance to cutting so deeper cuts and higher feed rates can be used.

Alloy properties
Magnesium, like most metals, is inherently conductive and shields out EMI. But its fluidity provides a bonus over other alloys. Magnesium and aluminum housings of equal weight shield equally well. As frequency in creases, however, it takes a thinner wall to get the same shielding. In the 1-GHz frequency spectrum into which most commercial applications fall, the high fluidity of AZ-91D magnesium alloy allows casting thin walls that have structural integrity. This provides weight and cost advantages over aluminum.

High-purity AZ-91D has corrosion resistance similar to mild steel and can serve in many applications without a protective coating. Galvanic corrosion, however, can be an issue when AZ-91D touches dissimilar metals to form an electrical contact near an electrolyte or continuous conducting liquid path. But eliminating one of these catalysts neutralizes galvanic corrosion.

Zinc is compatible with magnesium, as are some of the aluminum alloys with restricted copper content. You can eliminate electrolyte accumulation by using tapped blind holes with threaded studs, instead of nuts and bolts. The component can be designed so moisture will drain away or be positioned where moisture won't accumulate. The two materials can be insulated against electrical contact in the presence of electrolytes by coating the components, using nonabsorbent tapes, or fabricated insulators. Washers, spacers, and bushings between surfaces break the galvanic circle. Operating temperature and load may determine the best method to use.

Design for magnesium die casting
It's best to include the die-casting supplier in the initial planning to gain maximum benefit from magnesium alloy and the hot-chamber die-casting process. This will ensure the design incorporates the right die-casting requirements and specifications. Early die-caster involvement can also cut costs in both design and production. Discussions should cover critical properties and establishment of dimensional and datum schemes. FEA, mold flow, and thermal analysis will verify the results.

Before finalizing a design, die casters calculate process factors such as flow vectors, gate and runner design, fillets, radii, draft, metal velocity, and fill time. Even minor tweaks here can improve performance and reduce costs. For example, elements such as ribs may make the component stronger, more stable, and dense, while at the same time allowing use of thinner walls and less material. Reducing cross sections or adding recesses can also cut material needs. Adding a fillet to an inside corner or extra threads to a bolt connection are techniques used to reduce creep and ensure that loads are retained over a longer period of time.

Applications otherwise handled with an assembly of one or more parts can often be die cast in one operation. Saving come from eliminating separate components, joining operations, and inventory.

Here are a few basic guidelines for die casting:

• Wall thickness should be as uniform as possible to avoid local hot spots during solidification that could cause porosity or voids.
• Transitions from one section thickness to an other should be gradual to avoid stress concentrations.
• Edges and corners should be rounded to allow smooth filling of the die with the molten magnesium.
• Draft angles of 2 to 5° are recommended, although 1 to 3° and zero draft can be used in certain applications.
• Lettering should be raised rather than sunken into the component surfaces.

Debunking the flammability myth
Finally, the issue of flammability needs to be put to rest with no thanks to the high school chemistry teacher who few seem to have forgotten. There are flammability issues with handling and casting the powdered magnesium alloy, but this is the concern of the die casting supplier, not the end user of the die cast component. Once cast, the component is usually the last thing to ignite in a fire. Magnesium alloys for die casting have an ignition temperature of 850°F. As the metal has good conductivity, a high energy source is needed to get this ignition temperature. Tests have shown that even in a car fire, the vehicle is consumed before magnesium parts will burn.

FisherCast Global Corp. (866) 536-2278, fishercast.com

Edited by Jean M. Hoffman