Evaluating Magnesium Casting Processes
The microstructures from four emerging magnesium casting methods were compared to find a process that produces parts with superior properties.
Liang Wang, Ratessiea Lett, Sergio Felicelli and John Berry, Mississippi State University, Mississippi State, Mississippi
ing of magnesium can be prone to porosity due to turbulent melt flow and air entrapment. Sand casting is best for limited production volumes. In order to broaden magnesium’s use, particularly for automotive ap- plications, recent research has been conducted to develop additional pro- cesses to cast the material at higher volumes than sand casting. A recent study performed at Missis-
M
sippi State University set out to evalu- ate four emerging magnesium casting processes through microstructure char- acterization, mechanical testing and scanning electron microscope (SEM) analysis of fracture surfaces. Various industrial partners provided passenger control arms produced via: • indirect squeeze casting, • low pressure permanent mold, • T-Mag, • ablation. Often a steel stamping, the con-
trol arm is a potential application for conversion to magnesium cast- ing to reduce weight in vehicles. Test samples from 12 locations
were cut from each control arm, which were heat treated prior to testing (Fig. 1). Microstructure char- acterization included grain size, area percentage of porosity, porosity size distribution and defect analysis. Of the four methods tested,
the ablation and T-Mag pro- cesses produced castings with the best mechanical properties and defect and fracture surface analysis performance.
agnesium castings are mainly produced in two ways, high pressure di- ecasting or sand casting. High pressure diecast-
Potential Processes Squeeze and low pressure per-
manent mold casting are established technologies currently being used with metals other than magnesium. T-Mag and ablation are new processes to the industry that have been used minimally in commercial production. Squeeze casting produces castings
with improved ductility and higher tensile properties than conventional high pressure diecasting, reducing porosity. In the process, molten metal is injected into a metal die cavity, and as the metal solidifies, pressure is ap- plied. Technical challenges to casting magnesium via this process include the lack of tested lubricants, oxide reduc- tion and metal handling challenges during casting, as well as the potential for microstructure segregation. Low pressure permanent mold
casting combines high solidification rates with high production rates. Low pressure is used to push molten metal into a mold through a riser tube, as the furnace is below the mold cavity. The goal of the process is to control the
molten metal flow as much as possible to ensure a tranquil fill of the mold cavity, resulting in superior mechanical properties, uniform casting shape, and excellent surface finish. The current coatings used to
cast aluminum cause a reaction be- tween magnesium and the moisture released from the coating binder, which forms magnesium oxide par- ticles. The particles pull coating from the mold substrate and significantly deteriorate the coating. A non-re- active coating has been developed. Unique permanent mold magnesium alloys also could be developed to make this process viable. T-Mag is a new permanent mold
casting process invented specifically for magnesium by the Commonwealth Scientific and Industrial Research Orga- nization (CSIRO), which is the national government body for scientific research in Australia. In the process, a metal die is filled smoothly from the bottom up, similar to low pressure permanent mold, which can minimize air entrap- ment and oxide generation for supe- rior mechanical properties. The process is in an enclosed unit that prevents cover gas (used to prevent oxidation in the melt) from entering the atmosphere. Tooling and operation costs
are economical. However, T-Mag technology is owned by CSIRO, so facilities must license it. It has yet to make it to North America for production purposes. In the ablation process, a
Fig. 1. Twelve samples were extracted from each control arm for microstructural analysis.
30 Metal Casting Design anD PurChasing
sand mold bonded with soluble binders is sprayed with water or coolant after pouring. As the water spray removes the mold aggregate, it impinges directly
July/august 2011
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