Single crystal blades are cast blades with only one crystal grain. Directional crystallization blades eliminate transverse grain boundaries sensitive to cavities and cracks, and make all grain boundaries parallel to the stress axis direction, thus improving the service performance of the alloy.
Nowadays, single crystal hollow blades containing rhenium are the first choice and trend of turbine engines. The blade material of steam turbine must be nickel-iron alloy. That is, during normal casting, an electromagnet is used to generate a strong directional magnetic field, and the unconsolidated superalloy is solidified in the same direction under the action of the directional magnetic field, and finally a single crystal with the same atomic arrangement is formed, instead of polycrystals such as steel.
Only in this way can the neatly arranged materials resist high temperature. This is the secret of single crystal superalloy manufacturing.
Seed method
First, the seed crystal with the same composition as the single crystal to be cast is placed at the bottom of the mold shell, and then overheated molten metal is poured on the seed crystal to partially melt the seed crystal.
By properly controlling the temperature gradient in the liquid at the front of the solid-liquid interface and the growth rate of the crystal, the molten metal will grow from the unmelted seed crystal into the molten metal, and finally form a single crystal with the same crystal orientation as the seed crystal.