Magnesium casting set for change.

AFS Technical Department, Schaumburg, Illinois 

Cast magnesium components provide design engineers unique advantages over other available lightweight materials, and U.S. magnesium casting shipments indicate that the material is widely becoming a metal of choice for engineers looking to design components with minimum weight while retaining strength and property needs.

Magnesium's combination of strength and weight is especially useful in automobile components where it can reduce vehicle weight and improve performance. Magnesium sheet and wrought technologies hold long-term potential for vehicle application, but the quickest path to increased magnesium content in automobiles is through advancing the processes used to cast it.

Casting Process Development

Most magnesium castings currently are made using two processes: sand casting and diecasting-related technologies. The aerospace industry has used magnesium sand cast components for decades; the process knows almost no limitations on component size or geometry, and the limited production capabilities make it ideal for high-integrity castings requiring special processing. However, this industry is seeing limited growth today.

The automotive industry is leading the way in cast magnesium applications as shipments for vehicles is forecast to rise from about 50,000 tons in 2003 to an estimated 150,000 tons in 2009. Most of this growth has been by components produced by high pressure diecasting and related technologies, such as vacuum diecasting and semi-solid casting technologies. However, diecast geometries are limited by the steel die sections and moving cores in the tooling, and some high integrity and structural components cannot be produced with acceptable levels of porosity via high pressure diecasting.

In response to those limitations, recent research has been targeted at developing and optimizing manufacturing technologies that will fill the gap between the low production sand casting process and the high production diecast processes. Such developments would enable high quantity, low cost production of cast magnesium components with geometries and properties not currently possible.

Lost Foam

The lost foam process is capable of making complicated parts by consolidating several fabricated components into a single integral cast part--benefits that could be combined with the weight advantage of magnesium to create a market for new magnesium applications.

Until recent years, two primary technical issues limited the development of magnesium lost foam casting. First, magnesium's low thermal heat content might cause incomplete fill (i.e. the heat required to evaporate the foam pattern could draw enough heat from the magnesium to cause premature solidification). Second, there was potential chemical reactivity between magnesium and the coated foam pattern. Low density foam materials can correct the first concern, as fewer decomposition products present during metal filling can help maintain heat in the metal as it advances through the foam pattern during casting. The second problem can be mitigated by the use of inhibitors in the sand and aided by the use of a high-porosity pattern coating.

Initial testing of magnesium lost foam casting has been successful. Castings have filled with few problems and, when poured with filters typically used in sand casting, produced castings with grade B x-ray quality.

Low Pressure Permanent Mold

Although low pressure permanent mold casting is an established technology for casting aluminum components, combining high solidification rates with high production rates, this technology has not been transferred effectively to magnesium alloys until recently. A major technical barrier had been the lack of an available mold coating designed for magnesium.

Coatings manufactured for aluminum casting are not suitable due to a reaction between molten magnesium and moisture released from the binder in the coating, which forms magnesium oxide panicles. When the solidified component is removed from the coated surface, these panicles pull coating from the mold substrate, leading to rapid deterioration of the coating. Recent research has developed a non-reactive permanent mold coating for magnesium casting that inhibits this reaction by adding a fluorinated compound. The compound induces the production of magnesium fluoride, which mixes with magnesium oxide to produce a solid solution that takes the form of a thin, flexible layer on the magnesium metal, preventing further reaction between the magnesium and the coating.

Alloy development will be critical to the future development of magnesium permanent mold casting. Existing alloys were designed for sand or die casting and will have to be characterized for permanent molding, or new alloys will have to be developed. A second key to further development in the process will be a better understanding of the metallurgical issues of permanent molding.

Squeeze Casting

Squeeze casting can produce castings with improved ductility and higher tensile properties than conventional high pressure diecasting due to reduced porosity. Squeeze cast components can be heat treated, while many die castings cannot. Unfortunately, squeeze casting of magnesium alloys is still in its infancy. Recent studies of the properties and microstructures of squeeze cast magnesium alloys AZ91D, AM50B and AE44, has demonstrated technical feasibility for magnesium and substantial mechanical casting improvements over diecasting. However, this work involves the production of test castings. Continuing research will focus on applying these findings to the production of actual components. Technical challenges include the lack of tested lubricants, oxide reduction and metal handling problems during casting and the potential for microstructure segregation.