Effect of grain size on properties of metal materials
The finer the grain, the higher the strength and hardness. Because the finer the metal grain, the larger the total area of grain boundary, the more dislocation barriers and the more grains with different orientations need to be coordinated, the higher the resistance of metal plastic deformation.
The finer the grain, the higher the plasticity and toughness. Because the finer the grains, the more the grains in the unit volume, the more the grains involved in the deformation, and the more uniform the deformation, the larger the plastic deformation occurs before fracture. When the strength and plasticity increase at the same time, the work consumed by the metal before fracture is larger, so the toughness of the metal is better.
The method of refining grain to improve the strength, hardness, plasticity and toughness of metal is called fine grain strengthening.
Mechanical treatment method
Mechanical treatment methods include mechanical mixing and mechanical vibration.
Mechanical stirring can cause relative movement between liquid phase and solid phase, that is, convection motion of liquid metal, which causes the breaking, breaking and proliferation of dendrite arm, and achieving the purpose of refining the grain. But there are two shortcomings in this method. One is that when the melt is stirred, it is easy to get involved in gas, and it can not be supplemented in time, and it is easy to form porosity and shrinkage defects; Second, when stirring the metal liquid with high melting point, the mixer is seriously damaged, which causes pollution to the metal melt and creates new quality problems.
The mechanical vibration method is also used to break dendrites and cause nucleation proliferation by convection movement of metal melt to achieve the purpose of refining solidification structure. But in operation, when the mechanical vibration frequency increases, the refinement effect of metal solidification structure will be reduced, which causes the carbide segregation and looseness of ingot seriously.
External physical field method
The external physical field treatment technology is to apply the physical field to the metal melt before or during the solidification process, and use the interaction between the metal and the physical field to improve the solidification structure. This technology has the advantages of friendly environment and easy operation. At present, the research hotspots in this field mainly focus on the following three aspects
(1) Let the current pass through the metal melt, namely current treatment;
(2) Let the metal melt solidify in the magnetic field, namely magnetic field treatment;
(3) The solidification structure of molten metal was refined by ultrasonic treatment.