This net-shaped prototype magnesium valve body was Thixommolded from a single mold, permitting dimesional variation and inclusin of bosses and ribs.
The Thixomolding process produces lightweight net- or near-net-shape engineered components, such as this magnesium throttle body.
A prototype four-cylinder engine magnesium valve cover from Ford.
|
Magnesium, the lightest of all the commonly used metals, is increasing in automotive applications, but cost and other technical challenges limit more widespread use. While Sir Humphrey Davy could identify but not isolate aluminum, in 1808 he had better luck isolating magnesium. Its chief advantage is that it has only one-quarter the mass of steel and two-thirds the mass of aluminum. According to the International Magnesium Association, magnesium can be machined faster with less wear and tear on casting tools and has the best strength-to-weight ratio of any of the commonly used structural metals.
Magnesium's low heat content per unit volume permits die casting rates that are frequently one and a half times faster than aluminum, and its low reactivity with iron usually results in twice the die life of a similar aluminum part. Magnesium is dimensionally stable because of its consistent shrink rate, and close casting and machining tolerances can be held without stress relief.
Magnesium is thixotropic, with thixotropy refering to the property of semi-solid materials to become more fluid when shear energy is applied. In conventional die casting, it is necessary to superheat the metal material to provide proper flow and mold filling. In Thixomat Inc.'s Thixomolding process, magnesium is heated only to a semi-solid form by simultaneous application of shear and temperature to control viscosity. Temperatures are 100°C (212°F) cooler than in die casting.
At room temperature, pellet-sized chips of magnesium alloy are fed into the hopper and into the barrel of a Thixomolding machine under a blanket of cool argon gas. The material is progressively heated in the barrel from a solid state to a uniform, semi-solid state under tightly controlled conditions. After the material has achieved a predetermined shot size, it is injected at very high pressure and speed into the vacuumized mold cavity. Molded parts are automatically removed. According to the company, the process permits the elimination of virtually all secondary machining operations and produces parts with significantly reduced porosity, better surface finish, and tighter dimensional tolerances. Many molds built for plastic-injection molding can be used for Thixomolding.
Production automotive magnesium components are predominantly used in interior applications, such as steering column brackets, instrument panels, seat frames, steering wheels, ashtray doors, sunroof track assemblies, and mounting structures for compact disc/cassette players (AEI June 2000). In 1990, a typical vehicle consisted of 1.4 kg (3 lb) of magnesium. Ten years later, a typical vehicle has about 3.6 kg (8 lb) of magnesium. Future applications may expand to high-temperature areas under the hood, such as automatic transmission cases and engine components. Noranda Inc. recently developed three die-cast (cold chamber) magnesium alloys, and further evaluations for high-temperature strength and creep resistance continue.
In 1996, Ford invested in the Australian Magnesium Corp. (AMC) to ensure an affordable and reliable source of magnesium. Starting in 2002 when AMC begins production, Ford is guaranteed 50% of its output at a pre-arranged price. It is little wonder then that Ford projects its use of magnesium will grow from 2.3 kg (5 lb) per vehicle today to equaling current aluminum composition levels of about 113 kg (250 lb) per vehicle over the next two decades. Ford's projected use of 40,000 to 50,000 t (44,100 to 55,115 ton) of magnesium in the 2004/05 timeframe includes body structures as well as instrument panel structures. The 2001 Ford Explorer will have a magnesium instrument panel structure, which will eventually reach across to the entire F-Series line.
