Magnesium castings have been produced by a range of casting processes, in a number of foundries around the world for many years. Processes used have included sand casting, low and high pressure die casting, and of recent times using thixocasting. A large range of automotive, consumer goods including computer frames and video camera housings, sporting goods and general castings have been made. Major automotive, aeronautical and transport industry castings are an important current and future growth area for magnesium alloy castings.
Structural Casting Applications
Cast magnesium alloys are being specified at an increasing rate for structural applications, particularly in the area of automotive engineering. The cause of this dramatic growth is a drive by automotive companies to reduce component mass, and to take advantage of specific physical and/or inherent mechanical properties of magnesium alloys. It has been realised by a number of original equipment manufacturers and suppliers to this industry that superior product performance can be obtained by the use of cast magnesium alloy. A total engineering approach including appropriate material grade selection, component design, casting manufacture, machining and assembly has been adopted by this group. Some manufacturers and first tier suppliers have adopted a magnesium strategy as part of their business operations, where substitution of metal components with magnesium castings is a key part of their future platform planning, involving purchasing and advanced engineering groups. Magnesium casting substitution offers the maximum benefit in the replacement of multicomponent fabricated assemblies by a fully integrated die casting. This has been achieved for a number of magnesium seat designs around the world.
AZ91D Magnesium Alloy
When die casting magnesium, design measures and optimal control of processing conditions permit a large number of present applications to be produced from AZ91D grade magnesium alloy. This grade exhibits excellent die castability, with good strength and moderate ductility. It has been considered for decades as the grade of first choice if not discounted by specific property requirements. Typical applications include manual transmission housings, pedal brackets, intake manifolds, steering column and lock housings and mirror brackets to name a few.
AM, AS and AE Series Magnesium Alloys
The AM series of alloys have been developed recently to enhance fracture toughness and hence generate energy absorbing properties. There has been a significant growth in applications for this series as design engineers further exploit the structural casting area.
Typical applications include: steering wheels, seat frames/pans/backs, instrument panels, brackets and firewall beams and door frames/intrusion beams. The AM series are characterised by slightly reduced strengths with higher ductility and impact strength compared to AZ91D grade. For applications involving tong term exposure of stressed components at temperatures exceeding 120°C, the creep properties need careful consideration and evaluation. As a first measure, design modifications to lower the level of applied stress should be considered. More recent developments, the AS and AE series of alloys, with enhanced creep performance, are available and they should be considered for such applications. These series are based on the addition of either silicon or rare earth elements respectively to promote the formation of finely dispersed particles at the grain boundaries.
Die Casting Production Considerations
A significant improvement in die life compared to die casting of aluminium can be expected. This is effected with magnesium because of the heat transfer characteristics and the reduced affinity with iron, resulting in negligible soldering and reduced erosion.
Because of the lower heat content of magnesium compared with aluminium, the metal solidifies at a faster rate, generating shorter cycle times, typically by 15-25%. Exceptional dimensional stability of the as-cast product is a particular characteristic of cast magnesium alloys. Frequently, annealing or stress relieving treatments are not required with magnesium, contrary to experiences with some cast aluminium components where some growth continues as natural ageing effects occur over extended times at moderate to elevated temperatures. Machinability is excellent, exhibiting the best characteristics of all the structural materials viz reduced machining time, lower power requirements, longer tool life, excellent surface finish frequently with a single cut and minimal tool build-up with lower overall machining costs.
During the die casting cycle each part of the casting will develop a microstructure governed by the local solidification rate and pattern. Correct design of the casting and its feeding system are essential to ensure a uniform and directional solidification pattern. Where this is not achieved, it is to be expected that a certain fraction of microporosity will form due to volume contraction during solidification. This will inhibit the achievement of the excellent properties attainable in die cast magnesium. Ductility is a significant process-sensitive parameter with the control of inhomogeneities, defects and process of paramount importance in realising the potential for structural applications.