2.5 Magnesium Die Castings

Although a small amount of magnesium has been used in automobiles for many years (the first 1000 Minis had magnesium sumps and VW used die cast crankcases and transmission housings for the 'Beetle') its low density (about 1800 kg/m³ compared to about 2700 kg/m³ for aluminium) and the constant search for weight savings are encouraging designers to evaluate more potential applications. The ease with which die castings can be produced make this is the favoured manufacturing route for most applications. Current applications include seat frames, transmission system casings, air bag housings and lock bodies. The table below (page 2 of ref. M1) summarises the benefits of using magnesium die castings for seat frames:

Process Choice - Why Die Casting

- reduce total part count (feature integration)

- reduce seat assembly operations

- improve dimensional accuracy and repeatability

- reduce squeeks and rattles

Material Choice - Why Magnesium

- lower specific weight than other options

- better elongation than the other die casting metals

- 20% - 30% shorter cycle times than aluminium die casting

- longer die life (about double) than aluminium die casting

- ability to produce thinner walls than aluminium die casting

Economic Choice

- magnesium price stability

- investment cost reduction in comparison to current seats

Key issues that need attention are:

• i) Stress relaxation in bolted joins
• ii) the potential for creep at only moderately elevated temperatures
• iii) corrosion resistance
• iv) effects of recycled metal on properties

i and ii) One magnesium alloy, AZ91D (9% Al, 0.7% Zn, >0.15% Mn) has been shown to creep at ambient temperature under an initial applied stress of only 39% of its yield stress (ref. M2). The commonly used die casting alloy, AZ91, starts to creep at temperatures above 100^o and has a maximum operating temperature of 125^o. Where alloys are used in applications that do not suffer elevated temperatures, this need not be a problem, however power train and engine casings operate at temperatures of 100^o or more and are held together with threaded fasteners and so creep becomes an issue for these applications. Experimental work has shown that AE42 retains a greater percentage of initial compressive load than does AZ91D, and AE series alloys are generally superior in this respect to AZ91D.
Extrapolation of test results for AS21 alloys suggest that total loss of ambient temperature clamping force may occur after 20,000 hours at 150^oC (ref M1). This is probably beyond the expected life of most automotive components.

iii) Surface coatings produced on magnesium die castings by hexavalent chromium baths have been used to provide stand alone protection and as pre-treatment for painting. These baths are not considered to be environmentally friendly (effluent treatment and operator protection costs are high) so possible alternatives have being investigated. Some phosphate coatings have worked well, eg iron phosphate based 'Bonderite 1000' from Parker Amchem. Zinc phosphates are not compatible with magnesium. A 'phosphate - permanganate' treatment, using a mixture of sodium dihydrogenphosphate and potassium permanganate has performed well. Fluorozirconate treatments have also shown promise (this is used on aluminium drinks cans).

iv) Because of the increasing need to recycle materials, the effects that recycling (possibly) inferior magnesium have been investigated. Because magnesium has a higher oxygen potential than most metals, the oxydation of molten magnesium does not stop, as occurs with molten aluminium, but accelerates, hence melting must be carried out under a controlled atmosphere. Unless appropriate melt treatment and refining are carried out, magnesium may contain more inclusions than aluminium alloys. As impurities in moltem magnesium do not naturally tend to float to the surface, procedures are needed to remove these impurities. Methods tested at specialist foundries include sparging the molten metal with argon which brings dross to the surface which can then be skimmed off. The molten metal is pumped to ingot moulds through a 410 grade stainless steel filter which removes larger particles.

References
M1: Characteristics and Applications of Magnesium in Automotive Design, SP-1250, SAE, Feb 1997.
M2: W. T. Miller, Metall. Trans., 22A (1991) 873.