S
Yield Strength MPa (ksi) % Elongation
imilar to ablation of ice glaciers, where the snow and ice accumulated in the winter months is worn away in the summer by melting, erosion, evaporation and sublima- tion, ablation casting erodes away the sand and binder of a sand mold. In this emerging casting method, the mold is sprayed with a liquid and/or gas solvent directly
after pouring. As the solvent dissolves (or ablates) the sand mold, it facilitates rapid heat removal to solidify the molten alloy quickly (Figs. 1a & 1b). T e rapid solidifi cation achieved through ablation enhances the mechanical
properties of the casting alloys. While only a few facilities have ablation cast- ing capabilities, the emerging method has been successfully used for conventional aluminum alloys in commercial production and stood up well against other emerg- ing magnesium casting methods in a comparison study. Due to its ability to achieve fi ne microstructure and low tendency for hot tearing, the new process has opened up possibilities for casting typically diffi cult-to-cast alloys. As part of the exploration of ablation’s potential benefi t to casting technology,
researchers have studied its ability to produce wrought castings as well as alumi- num metal matrix composites (MMCs).
Boosting Metal Matrix Composites Manufacturing aluminum silicon carbide brake rotors for high volume ap-
plications via conventional sand casting can be diffi cult due to the settling of the
Table 1. Mechanical Properties of A356 Aluminum Cast in Various Methods Property
Sand
Ultimate Tensile Strength MPa (ksi)
228 (33) 262 (38) 4
179 (26) 207 (30) 3.5
243 (35) 11.0
Permanent Mold Squeeze Cast Ablation 312 (45)
silicon carbide particles. T e particles are heavier than molten aluminum and diffi cult to machine due to their high hardness. A hybrid MMC incorporating both
silicon carbide and graphite particles in the matrix of aluminum alloys is being developed to produce a compos- ite melt with silicon carbide particles, which are more dense than aluminum, along with graphite particles, which are less dense than aluminum. T e two particle types hinder the other’s move- ment to produce neutral buoyancy. T e presence of graphite also makes the aluminum silicon carbide alloy more machinable and wear resistant, with a lower density. During the solidifi cation of a
325 (47)
261 (38) 12.5
discontinuously reinforced metal matrix composite, the solidifying alloy interacts with the particulate rein- forcement to either push the particles into the last freezing, interdendritic regions or engulf them in the solidifi ed dendrites. T e phenomenon of particle pushing is a hindrance to achieving a uniform dispersion of reinforcements in cast MMCs. In order to improve the dispersion of micro-size particles, the solidifi cation rate must be high enough to promote engulfment of the particles or refi ne the dendritic microstructure. Ablation has the potential to
increase the solidifi cation rate and provide the required dispersion of the particulate reinforcement in the fi nished casting. In the process, a core package is made using a propri- etary binder. Shortly after the alloy is poured, a liquid or liquid/gas mixture is directed to the core surface. T e ab- lation media dissociates the binder and erodes the silica sand cores, enabling the ablation media solvent to come
@
Fig. 1. A solidifi ed steering knuckle is shown after the sand cores have been removed by the ablation solvent.
Jan/Feb 2012 | METAL CASTING DESIGN & PURCHASING | 37
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