lurgie du Québec, Trois-Rivières, Québec, Canada, was tasked with providing the technology to pro- duce cast blades, meeting stringent conditions on as-cast surface finish (≤250 rms) and geometry (±2 mm [0.08 in.]) of the theoretical envelope (Fig. 1). Plus, 140MPa minimum yield strength and 2% elongation were required in the highest stressed parts of the blade. Aluminum alloy A356
(AlSi7Mg04) was selected for its availability and excellent castabil- ity. It also exhibits good corrosion resistance when immersed in fresh river water. As for the casting process, the Centre de Métallurgie du Québec examined two meth- ods for production—sand cast- ing using traditional gating and sand casting with the direct pour method. If the metallurgical prop- erties were found to be similar for the two filling procedures, direct pour technology would make the molding much easier in a smaller flask while increasing the yield and eliminating finishing costs. In their study, the researchers
compared the optimal opera- tional parameters, such as pouring temperature and pouring time, for both methods. Metallurgi- cal properties, including dendrite fineness (secondary dentrite arm spacing) and microporosity along
Fig. 2. A conventional sprue-runner-gate system is shown.
is a perfect geometry for cast- ing, as solidification smoothly progresses from the thin end of the part to the thick extremity, providing self-feeding as long as a riser is placed at the hot end of the blade. Te traditional rigging is shown in Fig. 2. It included a sprue, sprue well with a filter at the parting line, two runners and six gates corresponding to an unpressurized gating ratio of 1:2:4 with a resulting filling time of 25 seconds. Te yield was 70%. Te cup of a direct pour
Fig. 3. In direct pour filling, the metal is poured through a ceramic filter directly into the mold cavity via pouring cups (shown in inset).
with tensile properties, were measured at 13 locations.
Traditional Gating vs. Direct Pour Mold Filling
Te wedge-shaped aluminum blade
system combines a ceramic foam filter held inside an insulating ceramic sleeve (see inset in Fig. 3). Te assembly acts as a pour- ing cup, filter and riser, allowing the metal to be poured directly into the mold cavity, eliminating the traditional sprue-runner- gate system. It considerably simplifies the molding process and promotes directional solidi- fication since the hottest metal is poured into the riser. Direct pour is particularly suited to the blade because the liquid metal drop under the filter is moderate (2 in. [50 mm]).
Comparisons Te pouring temperature
was adjusted so the predicted temperature of the liquid metal
Fig. 4. Modeled filling with the conventional gating is illustrated.
Fig. 5. Modeled filling using the direct pour cup is illustrated.
August 2014 MODERN CASTING | 41
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