In the first generation of cast aluminium axial fan blades, wholly cast aluminium was used. A good aerodynamic shape of the blades could be obtained in the casting process. But aluminium has some weaknesses. The tensile and yield strength values are much lower than for steels. The strength of aluminium decreases dramatically at slightly elevated temperatures and the aluminium begins to creep. Also, during the casting process voids, cracks and other casting faults often occur. Some may be found by X-ray screening, others may pass undetected.
Even using forged aluminium does not change the fact that aluminium as a material has a low melting point and gets soft at much lower temperatures than steels.
Therefore the next generations of cast aluminium has learnt from Mother Nature. Primitive designs, such as jellyfish, worms and first generation cast aluminium axial fan blades have a core that is not stronger than the rest of the shape. More developed designs, such as mammals and later generations of fan blades, have a spine of a stronger material.
The outer cast aluminium is used to give a good aerodynamic shape. The inner steel skeleton is used to assure the strength, even at elevated temperatures, such as during emergency smoke extraction. Thus a temperature of e.g. 400°C is not very detrimental to steels, while aluminium at this temperature hardly has any strength at all.
Over time the inner skeleton of the fan blades has been developed into different shapes, each optimised for different duties. Most skeleton shapes designed to grip and retain the aluminium, which is soft at higher temperatures, resemble Christmas trees, thus the popular name for the steel skeleton in the axial fan blades from witt&sohn AG
There are the following 2 major designs for aluminum impellers:
For the type B impellers there exist several methods to fix the blades:
The more complicated a mold is, the higher is the risk of a casting fault (i.e. complete impellers cast as one part). Or the thinner a part becomes the higher the risk of faults is (i.e. thin long blades). In both cases extensive checks & examinations (i.e. X-raying) are a must to avoid later breakage of impellers because of unseen cast imperfections.
The last years the requirements for axial impellers have increased more and more concerning
If you compare primary aluminum and mild steel, primary aluminum is much more corrosion resistant than mild steel. But concerning the material strength, the yield strength is only a third of the one from mild steel (≈ 70 MPa at 20° instead of ≈ 235 MPa of mild steel). Additionally this yield strength decreases enormously at higher temperatures: Above 300°C aluminum has no relevant yield strength while a steel skeleton has lost nearly no yield strength.
Final conclusion: Impellers type B (aluminum blades with cast in skeleton) can be stressed most, but they are the most expensive ones.
