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By selecting the right combination of alloy and processing method, a wide range of temperature exposed drive train parts can be made out of die cast magnesium, including engine blocks and automatic transmissions as probably the most demanding components. Successful new alloys for these purposes must fulfill a multitude of requirements to offer a viable solution, including mechanical properties, corrosion properties, die castability and recyclability. Therefore, selection of alloys must be based on the customers' requirements, at the same time as other factors are optimised. In this paper, results from the ongoing alloy development work by Hydro Magnesium are presented, focusing mainly on creep resistant alloys within the Mg-Al-RE system. High temperature tensile data, tensile creep-, stress relaxation- and bolt load retention results from a selection of AE alloys and reference alloys are presented.

As more and more magnesium metal is used and recycled, particularly for safety-sensitive components, the maintenance of metal cleanliness remains of critical importance. The term metal quality may be defined as 1) chemical composition 2) inclusions and porosity inside the metal, 3) the surface appearance and 4) consistency. This definition may apply to both ingots and cast parts. Chemical composition of ingots will influence on the cast parts, but also process operation at the die casters' will contribute to the chemical composition of the final product. The inclusion contents of a cast component is only indirectly determined from the cleanliness of the ingots from where it origins, emphasising the need for metal cleanliness assessment in the die casting shop. Beside the die casting process itself, the ingot surface appearance and the housekeeping in the die casting furnaces are important for the property of the product.

The development of creep resistant alloys for automotive drive train components has proven to be metallurgically challenging. This paper discusses the principles of high temperature alloy development, featuring metallurgical, microstructural and processing aspects of some alloys, relative to their high temperature performance. The creep resistant alloys within the Mg-Al base system obtain their creep resistance by a relatively low content of Al, and addition of elements that form stable intermetallic phases within the grain mantle. Various third elements affect the high temperature performance differently. The results demonstrate that rare earth elements (RE) show a remarkable potential as the third element(s).

The current strong interest in magnesium alloy die castings for automotive applications relies heavily on the “High Purity” concept. The basic knowledge of the detrimental effects of heavy element impurities on the general corrosion of magnesium alloys has been known for more than 60 years. However, it was not until the 1980's that the full potential of high purity alloys was recognized. The level of impurities (in particular, copper, nickel and iron) that can be tolerated in a die cast part exposed to a corrosive environment necessitates careful control throughout the complete manufacturing process, from ingot to finished product. In the present paper, some basic principles for the production of high purity alloys and the influence of subsequent melt handling practice in the die casting shop are discussed.

Hexavalent chromium-containing baths have traditionally been used as pre-treatments on magnesium alloys. Environmentally friendly alternatives to these baths are being investigated. One alternative seems to be a phosphate-permanganate treatment consisting of a mixture of sodium dihydrogenphosphate and potassium permanganate. The phosphate-permanganate treatment has shown equivalent performance to a standard chrome pickle, both as stand-alone corrosion protection and as a base for subsequent coating. Phosphate-permanganate treatments must be combined with a deoxidizing process to be successful. Another alternative is a fluorozirconate treatment. The paint base properties of fluorozirconate are quite close to what is obtained using a chrome pickle.

The effect of strain rate on tensile properties of cold chamber die cast AZ91D, AM60B and AM50A test bars is reported. The strain rate was varied from 15 s1- to 130 s-1, a range typical of deformation and crash. All tests were done at room temperature. The properties measured include fracture elongation and ultimate tensile strength values. The results are discussed in terms of the work hardening characteristics and strain rate sensitivities of the materials, and parameters in a material model suggested by Johnson-Cook have been determined. It has been found that flowstress increases and that elongation is not affected by strain rates from 15 s-1 to 130 s-1. The energy absorption during deformation increases therefore with the speed of deformation, emphasizing the positive properties of magnesium die cast alloys for safety related applications.

Permanent mould cast medallions and 3mm die cast test plates of 47 different AZ91 based alloys covering 3-129 ppm Ni, 7-2850 ppm Cu, 87-1740 ppm Si and 0-100 ppm Co were produced. Medallions and plates were subjected to 72 hours immersion in 5% NaCl solution at 25C and to the 10 day ASTM-B117 salt spray test. The results include: a) for AZ91, the corrosion rate values anticipated from salt spray testing of die cast test plates can be calculated from the results of immersion tests on permanent mould cast medallions; b) the effect of Co on corrosion of AZ91 is 35-75 times more detrimental than Cu and thus similar to that of Ni; and c) Si showed almost no effect on the corrosion rates in the alloys examined.