Cast aluminum alloy 356 T51 hardened impeller 15" Diameter wheel 1-1/8" Diameter bore 1/4" Keyway One piece integral cast, no rivets Backward Incline/Airfoil Balanced statically and dynamically - 2 planes to ISO G6.3 - Same Spec that is used for aircraft turbine rotors. This allows for vibration free operation and gives longer motor life. Spark resistant non-ferrous, non-steel material as required by NFPA fire codes. Magnesium-Aluminum Alloy - Extremely strong with full material handling strength. Unbreakable, even in material handling applications. Stronger than steel by weight. Cast to perfect airfoil shape for high performance and low noise. Lower weight allows motor to drive air, not overcome high inertial loads.