Abstract
Magnesium alloys are gaining in popularity as materials of choice for automotive and aerospace applications. Magnesium alloys have the lowest density of all structural metals, effectively making their specific properties highly attractive. Lost Foam Casting (LFC) is a novel near-net-shape manufacturing process utilizing expanded polystyrene (EPS) as a mold filler. Presence of the EPS in the casting cavity promotes formation of unique casting defects.These include misruns, folds, entrapped polystyrene pyrolysis products and potentially increased levels of gas porosity. There is very little published literature on the feasibility of casting magnesium alloys by the LFC process. This research was an attempt to evaluate the effect of selected LFC process variables on AZ91R magnesium alloy castings produced by the LFC process. In this work, the effect of melt superheat, casting section thickness, EPS foam properties and the application of vacuum during mold filling were investigated and correlated to the casting quality and molten flow behavior. Further, detailed thermal analysis was carried out to determine the solidification history of the castings. The results of the thermal analysis were used to determine the effect of the cooling rate on the development of the casting microstructure. Moreover, the morphology and the mode of second phase (Mg17Al12) precipitation were studied and quantified. The results suggest that application of vacuum during the mold filling process increased the metal flow lengths. However, the casting soundness deteriorated due to the applied vacuum. Variations in the density of the vacuum cast horizontal bars were explained through the presence of partially solidified metal. The molten metal flow was further influenced by the foam density and bead fusion. Greater flow lengths were observed in the high density 1.6 pcf foam castings. in the low density 1.3 pcf foam castings, numerous casting defects were associated with the presence of the liquid-EPS pyrolysis products. In general, the thermal analysis suggested that non-equilibrium alloy solidification promoted the formation of the lamellar non-equilibrium Mg17Al12 precipitate, and this was confirmed by optical microscopy.
Highlights
Computer aided thermal analysis was carried out for the solidification o f magnesium alloy castings produced by the Lost Foam Casting (LFC) process
Lost Foam Casting (LFC) is a full mold casting process with expandable polystyrene (EPS) foam as the m old filler.^ This novel casting process is a viable alternative to the established casting processes, such as sand casting and high-pressure die casting
The AZ91 alloy starts to solidify at the liquidus temperature o f 595 °C and solidification is complete at the solidus temperature o f 470 °C.® The M g-rich section o f the binary phase diagram shows a maximum solid solubility o f 12.7 wt% Al at the eutectic temperature o f 437 “C. U nder equilibrium conditions, the Mg-Al alloy should initially solidify as a single phase a-M g solid solution
Summary
B .l - Effect o f pouring temperature and vacuum on the casting density for the 1.6 p c f foam castings 172. B.4 - Effect o f section thickness and pouring temperature on the local solidification time (LST) for gravi ty cast 1.6 p c f foam trials viu. Table 4; M etallographic specimen polishing procedure. 18 Continuous precipitation o f M gnA hz in the M g-8wt% Al alloy; (a) Optical micrograph (xlOOO). Figure 19;Discontinuous precipitation o f MgivAl^ in the M g-8wt% Al alloy; (a) Optical micrograph. 57: Convex metal front o f the 1.5 cm thick bar Figure 59: Convex m etal front o f the 1.0 cm thick bar Figure 64: Adhering sand grains at the bottom side o f the casting
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