Magnesium: The Other Light Metal
By Colleen A. DeJong
Pure magnesium is about one-third lighter than aluminum, and two-thirds lighter than steel. Lighter weight translates into greater fuel efficiency, making magnesium alloy parts very attractive to the auto industry. And these lighter parts come with good ductility and elongation properties, giving the materials good dent and impact resistance, as well as fatigue resistance. The alloys also display good high-speed machinability and good thermal and electrical conductivity.
The Die Is Cast
Although magnesium alloys can be easily machined into various parts, they really stand out when die cast. They can be formed into complex shapes in one casting, often reducing cost by eliminating several steel stampings and the associated assembly.
If you were to look at a cross section of a die-cast part, you would see a very thin skin (that's coatable, by the way) covering a fine interior microstructure. Once decried as magnesium's greatest weakness, this microstructure is now recognized as one of magnesium alloy's greatest strengths. It allows the material to be cast with very thin walls, optimizing design and decreasing the component's weight. The microstructure also gives the alloys good sound and vibration dampening qualities. In fact, many luxury cars use magnesium alloys for valve covers and other under-the-hood parts, keeping the ride nice and quiet.
Engineers like die casting with magnesium alloys because they can design to specific yield strength, fatigue, and creep criteria. There is a note of caution here, however. There is relatively no creep in magnesium alloys at room temperature, but if higher temperatures are anticipated in the application, the design will need to accommodate the resulting creep factors.
Steering the Way at Ford
A few years ago, a decision was made at Ford to meet 1997 automotive safety guidelines beginning with '95 model year cars. This meant including airbags on all vehicles. And, in turn, this meant a significant amount of weight was being added to the top of the column. Adding the weight resulted in a couple of different things. First, with certain designs, the steering wheel vibrated under normal driving conditions. Using magnesium alloys not only lighten the steering column, but it also maintains rigidity better than other metals and alloys considered for the job.
How the steering column components change shape under force, and how it would absorb the energy of a collision were also important considerations for Contech Division of SPx Corp. (Kalamazoo, MI), the company producing steering column components for the Mercury Mystique and Ford Contour. During a frontal collision, the column is designed to collapse when struck by the driver. Therefore, it must maintain a balance between being rigid enough to hold up in regular driving conditions, but flexible enough to fall apart during a collision. For the original column design developed in the late '80s, Ford specified magnesium alloy AZ91D for the lower mounting bracket and mating bearing retainer. The magnesium alloy proved to have the right balance between crash energy absorption and structural integrity when tested. Contech also made a second mounting bracket for the assembly. This, too, was originally specified in AZ91D.
Soon after this, new safety guidelines came out that required protection for unbelted passengers, changing impact deformation parameters considerably. AZ91D did not hold up to the new guidelines. Rather than go through a costly redesign, Contech first went to Dow Magnesium to see if there was another material that might perform better.
With Dow's aid, Contech began hot-chamber die casting magnesium alloy AM50A, a departure from the more traditional cold-chamber die casting technique. Modifying the alloy used and the casting method gave the steering column components more desirable deformation and fracture qualities. This saved both Ford and Contech the expense of redesigning the components.
Chrysler is also using magnesium alloys for weight reduction, but not in the steering column. Instead, it's used for the mounting bracket for the anti-lock braking system (ABS) on the minivan. The supplier, ITT-Automotive (Auburn Hills, MI), produced the initial design for the bracket with 1018 hot-rolled steel. However, during final testing, the bracket failed. Reinforcement darts and a higher strength steel could have solved the problem, but it brought the total bracket weight to 2.5 lb., too heavy according to Chrysler requirements.
At this point, ITT decided to pursue using a magnesium alloy to produce the bracket. The diecaster, Diemakers Inc. (Palmyra, MO), made a few design change recommendations to the ITT engineering crew, and cast a few parts from AZ91D for testing. The prototype parts passed the test, and work with Dow Magnesium and Diemakers, corrosion concerns were eliminated. The final part weighed just 0.8 lb.
Future uses of magnesium alloys promise to go beyond steering wheel components and valve covers. Several users are making forays into using extrusions for parts with symmetrical cross sections like seating tracks. Others are looking into consolidating several components into one through advanced die casting designs. And to accommodate anticipated growth, several magnesium alloy producers are increasing capacity for both new alloy smelting and recycling.
The Trouble with Magnesium
Magnesium alloys are predicted to continue to grow in popularity (about 15% per year in the automotive industry alone), but the world's supply of magnesium, like every other natural resource, will be forever shrinking. The solution is, logically, to recycle. Especially since anywhere between 30% to 50% of the metal handled by die casters ends up as scrap.
In the past, concerns about the purity and usability of recycled magnesium have kept the automotive industry from extensively using recycled magnesium alloys. Non-metallic inclusions (NMIs) and other impurities can corrode or otherwise alter the metal. But advances in testing technologies—such as detailed image analysis, fast neutron activation analysis, and light reflectivity analysis—can tell users fairly quickly whether or not an ingot cast from recycled material is useable or not. Better testing, combined with more sophisticated smelting techniques, have pushed the issue of recycled material quality into the realm of nonissue.
As a matter of fact, according to Robert VanFleteren of Dow Chemical Co. (Midland, MI), a "significant amount of magnesium used in production is recycled." Industry experts say about 10,000 tons of material used is recycled. But that may be a conservative estimate. "The actual amount is probably surprisingly large to industry forecasters," concludes VanFleteren.
Plastics: The Inside Report
While there have been a lot of innovative uses of plastics for automotive exteriors, the latest buzz seems to be coming from the polymers and composites made for interiors.
From M.A. Hanna Engineered Materials and M.A. Hanna Color (both of Cleveland, OH) comes the Controlled Color Natural line of natural homopolymer, copolymer, and impact-modified copolymer polypropylene compounds. The uncolored line is engineered to allow processors to meet automakers' color match standards for interior trim and other molded plastic parts. The compounds can also reduce the need to store up supplies of precolored materials.
An important aspect of the Color Natural product is that the resin pellets have a guaranteed consistent color. No matter what lot the pellets come from, as long as the color concentrates are added correctly, the resulting color will be consistent. This has not always been the case with natural color resin pellets, which has kept some plastic molders from using them. Instead, they use pre-colored materials, which they order in bulk and store up in inventory.
This works fine until you consider that automakers can—and do—change color specifications for interior plastic molded parts frequently. Then what about the overstock of precolored stuff sitting in the warehouse? "More often than not, molders end up throwing it away and eating the cost," says Mike McCormack, Automotive Industry Manager at Hanna Engineered Materials.
"But with the Color Natural line, all you would have to do is adjust the mix of color ratios in the concentrate," adds McCormack.
Foam for Trim
Recently introduced by Toray Plastics America, Inc. (Front Royal, VA) is a line of foamed olefin sheet stock that can be used in low pressure molding (LPM) vacuum forming, TPO skin lamination, structural reaction injection molding (SRIM), and compression molding processing operations.
According to engineers at Toray, the material offers users greater design and processing flexibility, and increased thermal performance. From a practical standpoint, the olefin line is light weight and durable, yet softer to the touch than similar materials. These characteristics have led to the specification of sheetstock from the AP61 series of the line for door trim panels, seat backs, interior running boards, and other automotive interior parts.
Compared with PVC, the AP61 foams offer greater tear resistance, and contain no halogens or plasticizers that can migrate to the surface and affect surrounding materials. They are also compatible with low-pressure molded PP polypropylene substrate, so no synthetic backing fabric or adhesive is required for the laminate. This eliminates processing steps and lowers system costs.
From a "Green" standpoint, the foams are good candidates for both in-plant scrap reuse, and post-consumer recovery efforts.
The Nanos Have It
Nanomaterials have grain sizes on the order of a billionth of a meter. Nanocrystalline materials are comprised of grains that are each a single crystal consisting of atoms arranged in an orderly pattern. Interestingly, when you start arranging these materials on almost an atomic level like this, they begin to behave differently than their conventional counterparts. For example, nanocrystalline coppers have been produced that are up to five times harder than conventional micron-sized copper. And some nanocrystalline ceramics display a machinability and hardness rivaling some metals, but at a much lighter weight.
Their quirkiness aside, there are some real-world applications for these materials. In the automotive world, research has advanced to the point where prototypes of a spark plug that use nanomaterials is being tested. Dubbed the "railplug," this spark plug theoretically burns gasoline completely, getting the most out of the gas, and reducing emissions considerably. The composites may someday be used under the hood as high-strength springs, ball bearings, and valve lifters.
Not To Be Left Out...
...are plastics. The National Institute of Standards and Technology (NIST) is testing nylon-6 clay nanocomposite, a nanocomposite plastic compound in which particles of motmorillonite clay about one nanometer in size are dispersed throughout a specific polymer. The resulting compound (nylon-6 clay nanocomposite) is extremely flame retardant, flexible, and resistant to breakage, giving it potential in several different areas, especially as a synthetic fabric.
Polymer Woven Into Tough Fabric
Victrex Peek 381G polymer from Victrex USA, Inc. (West Chester, PA) is a material with several application possibilities. In fact, at Tetko (Depew, NY), the polymer is being woven into fabric with pore sizes starting at 10 microns, and thickness as low as 60 microns. Applications include custom engineered belts; cut, sewn, and welded panels and sleeves; stamped and heat cut parts; and gasketted and laminated assemblies.