Automotive Uses of Magnesium Alloys: Part Two

Abstrakt:

The best opportunity for wrought magnesium in the body is the use of extrusions on primary structures, such as space frames. In power train, magnesium castings are replacing some of the iron and even aluminum castings in some housings and covers. Wheels were one of the first applications for magnesium, and development of a competitive production process enabled wide use.

The body is the single largest group, accounting for over 40% of total mass. Metals (mostly steel) are the first choice for structural components. Mechanisms are made primarily from metals (with use of some plastic parts increasing), and most lightly stressed housings are molded from plastics.

The best opportunity for wrought magnesium (Mg) in the body is the use of extrusions on primary structures, such as space frames. Even if Mg space frames do not become economic in the mass market, penetration in the specialty automotive market (50-100,000 units/year) would significantly increase Mg usage. There could also be opportunities for seat frames, where Mg castings are already being used, and a combination of castings, extrusions, and perhaps sheet could be competitive. The use of Mg sheet in body panels would require development of an economical, high-volume hot-forming process.

The components in the power train are markedly different from those in the body. The engine and transmission, the main mechanical groupings in the vehicle, are characterized by complex assemblies of many individual components. A great diversity of materials is used. Magnesium castings are replacing some of the iron and even aluminum castings in some housings and covers.

However, Mg does not have the creep resistance of aluminum, and therefore it is unlikely to be used for the two most massive and critical housings in the engine: the block and the head. But in the transmission, where operating temperatures are much lower, Mg could eventually replace Al in the rather massive main housing. In fact, Mg housings are already used in transfer cases, a similar application. There are probably few opportunities for wrought Mg in power train components, where even wrought Al has gained little hold.

The chassis components are highly diverse, with characteristics between those of the other groups; the mechanisms are simpler, and many components also have structural functions. The materials are diverse, but iron and steel still play a major role. Components with significant structural function (e.g., the suspension and sub-frames) are dominated by steel, while those with mainly mechanical functions (e.g., the steering and brake systems) include more diverse materials. Al is gaining share among the chassis housings and other complex castings, and Mg could substitute for many of these, especially in unstrung components, where low mass is key.

Wheels were one of the first applications for Mg, and development of a competitive production process based on welded extruded and/or stamped components could enable wide use.

After WWII magnesium was further used in some automotive applications. Some racing cars (Mercedes 300SL, Porsche 962) had the frame and skin made from magnesium. The VW beetle was the commercial car with the largest single application of magnesium alloys (crank case and transmission housing with a total weight of 17kg) and with the production stop of the air cooled Mg engine of the beetle a tradition of 50 years of magnesium automotive applications came almost to an end.

Since then, there was a steady decrease of magnesium applications until early nineties when the interest and use especially in North America increased again due to new regulations of petrol consumption (CAFE). Though the consumption of magnesium was decreased, various automotive companies continued with magnesium parts in various vehicles (VW Polo, Passat and Golf, Porsche 911 and 928, Daimler Benz, Renault 8 Turbo, Chrysler Jeep and light truck vehicles, Ford light trucks). Sand casted magnesium wheels were used by Porsche in the sixties for their racing cars 907, 908, 910 and 917 and in 1970 for the first time in standard production for the 914/916 models.

This first generation of Mg wheels, although served life-times of more than 150,000 km, were taken out of operation due to lack of understanding of die cast technology, insufficient corrosion properties and the general recession in the seventies. Porsche started the development of the next generation of Mg wheels, produced by low pressure die casting using PAZ9110. Today most magnesium wheels are forged in 11983 to achieve better strength and fatigue properties. VW/Audi started with the B80 gearbox housing in 1996 a new era of drive train applications and the available range extended very much since then.

Magnesium alloys have two major disadvantages for the use in automotive applications; they exhibit low high temperature strength and a relatively poor corrosion resistance. The major step for improving the corrosion resistance of magnesium alloys was the introduction of high purity alloys. Alloying can further improve the general corrosion behavior, but it does not change galvanic corrosion problems if magnesium is in contact with another metal and an electrolyte. The galvanic corrosion problem can only be solved by proper coating systems.

Beside the galvanic corrosion problems related with magnesium the low temperature strength is another serious problem, limiting the use of magnesium especially for power train applications. The use for transmission cases and engine blocks requires temperature stability up to 175°C.

Additionally the widespread use of magnesium and its alloys in transportation industry is limited by some poor property profiles. Low creep resistance, high coefficient of thermal expansion (CTE), low Young’s modulus, insufficient ductility and crash energy consumption in car body structures, low fatigue stability, low corrosion and wear resistance are deficits which have to overcome by further alloy and/or process developments.

Electroplating of magnesium is also used in the automotive industry for decorative applications, especially for parts inside the car. For higher corrosion resistance electroplating is deposited as a top layer on a conversion coating/special e-coat layer system. Such a system provides with a chromium top layer a corrosion resistance of 500 h salt spray 32. In many cases it is sufficient to simply coat the counterpart and leave the magnesium uncoated (if it is no view part), as a defect in a coating on magnesium would result in an enhanced localized corrosion attack of the magnesium component. No contact corrosion of magnesium is caused by anodized AlMg3 alloy.

Conventionally galvanized steel bolts can be attached to magnesium by using a silicate sealing. The silicate sealing of galvanized steel bolts was successfully applied by Audi and VW at the B80 magnesium gear housing.

A good protection can also be obtained by multi-layer coatings on the critical counterpart, e.g. a zinc coating in combination with a cathodic dip-coating. Another possibility is the use of Sn/Zn-coatings instead of conventional zinc galvanizing.

Additionally new electrolytic deposited Al-Mg coatings for steel bolts are under development and testing. Already used are also insulating polymer coatings (nylon) or plastic caps for screw heads. In addition often washers made of aluminum (6XXX series, sometimes anodized) or polymers are used with steel bolts to minimize the contact corrosion on magnesium.

Important issue of using magnesium in auto industry is recyclability. In general, metals are more recyclable because they can be melted and reused. In particular, magnesium has a lower specific heat and lower melting point then other metals. This gives the advantage of using less energy in recycling, with recycled Mg requiring as little as about 4% of the energy required to manufacture new material.

At present, however, recycling procedures are still not fully developed and work is underway on the technologies required for recycling wastes from relatively clean factories. In the future, it may be necessary to review the material flow in order to ensure that recycled materials will account for about ten percent of total Mg alloy production.

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