Due to its generous hydraulic basins, more than 90% of electricity in Quebec, Canada, is produced by water turbines. However, numerous sites cannot be economically, technically or environmentally developed using conventional hydropower technologies (i.e., hydroelectric dams). Novel low power water turbines are sought to suit shallow, high flow rivers; they would be particularly effective in remote areas where they could replace fuel powered generators.
Fig. 1. The 20.2-lb. (9.1 kg) aluminum A356 water turbine blade is 36.6 in. (930 mm) long, with thickness ranging from 1.7 in. (44 mm) to 0.5 in. (13 mm).
Each propeller for a 15kW water turbine uses five cast aluminum blades fixed to its axle. The Centre de Métallurgie du Québec, Trois-Rivières, Québec, Canada, was tasked with providing the technology to produce 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 castability. 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 methods for production—sand casting using traditional gating and sand casting with the direct pour method. If the metallurgical properties 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 operational parameters, such as pouring temperature and pouring time, for both methods. Metallurgical properties, including dendrite fineness (secondary dentrite arm spacing) and microporosity along with tensile properties, were measured at 13 locations.