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The method of refining crystal grains

1. Introduction

The performance of metal materials is closely related to their internal structure, and grain size is one of the key characteristics of the structure. Fine grains can give metal materials higher strength, hardness, plasticity, and toughness, while improving their wear resistance, corrosion resistance, and other properties. Therefore, refining grain size has important practical significance in the field of metal material processing and manufacturing, and is widely used in many industries such as aerospace, automotive manufacturing, and mechanical processing.

2. Refine grain size through metamorphic treatment

2.1 Principle

Modification treatment is the process of adding a small amount of modifier to the molten metal to alter its crystallization process and refine its grains. Modifiers can act as heterogeneous cores during metal crystallization, increasing the number of crystalline cores while hindering grain growth and refining the grains.

2.2 Common Deteriorants and Their Applications

2.2.1. Sodium modifier: Sodium is a commonly used modifier in aluminum alloys. For example, by adding an appropriate amount of sodium salt mixture (such as sodium fluoride, sodium chloride, etc.) to hypoeutectic aluminum silicon alloys, sodium atoms can adsorb on the surface of silicon crystals, changing the growth mode of silicon crystals and transforming eutectic silicon from coarse needle like to fine fibrous or coral like, while refining α - Al grains, significantly improving the mechanical properties of aluminum alloys.

2.2.2. Titanium and boron modifiers: For metals such as steel and aluminum alloys, titanium and boron can form small compounds (such as TiB ₂, TiC, etc.), which can act as heterogeneous cores, promote grain nucleation, and refine grains. Adding a small amount of titanium to steel can form particles such as TiN and TiC, preventing the growth of austenite grains during heating; Adding an intermediate alloy of titanium and boron to aluminum alloys can effectively refine the grain size of cast aluminum alloys.

2.3 Effect and Limitations

Metamorphic treatment can significantly refine grain size, improve the performance of metal materials, and the operation is relatively simple with low cost. However, the selection and amount of modifier need to be strictly controlled. Excessive addition of modifier may cause the modifier itself to aggregate, form inclusions, and affect the quality of metal materials. In addition, the effect of spoilage treatment may deteriorate over time and require repeated spoilage treatment.

3. Vibration and agitation refine grain size

3.1 Principle

In the process of metal crystallization, applying vibration (such as mechanical vibration, ultrasonic vibration) or stirring to the melt can break the growing dendrites, increase the number of crystal cores, and promote convection in the melt, making the temperature and composition more uniform, which is beneficial for grain refinement.

3.2 Specific methods and applications

3.2.1 Mechanical vibration: In the process of metal casting, vibration devices are used to vibrate the mold or melt. For example, when casting aluminum alloys, the use of vibration casting can significantly refine the grain size of the aluminum alloy and improve the mechanical properties of the castings. Mechanical vibration can also be applied in the continuous casting process to improve the quality of continuous casting billets.

3.2.2. Ultrasonic vibration: Ultrasonic vibration has higher energy and stronger penetration ability. Insert the ultrasonic probe into the metal melt, and the ultrasonic waves will generate cavitation and acoustic flow effects in the melt. When the tiny bubbles generated by cavitation rupture, they will generate high temperature and high pressure, which will impact the growing grains; The acoustic flow effect can promote the flow of the melt, making the grains finer and more uniform. Ultrasonic vibration has been widely used in the crystallization process of light metals such as magnesium alloys and aluminum alloys.

3.2.3. Stirring: Stirring can be divided into mechanical stirring and electromagnetic stirring. Mechanical stirring directly stirs the melt through a stirrer to achieve uniform temperature and composition, break dendrites, and increase crystal cores. Electromagnetic stirring is the use of an alternating magnetic field to generate induced current in a melt. The interaction between the induced current and the magnetic field generates Lorentz force, driving the flow of the melt and achieving contactless stirring. Electromagnetic stirring is commonly used in the continuous casting process, which can effectively refine the grain size of the continuous casting billet and improve the quality of the billet.

3.3 Effect and Limitations

Vibration and stirring methods can effectively refine grain size, especially suitable for the production of large castings and continuous casting billets. However, vibration and stirring equipment may increase production costs, and the parameters of vibration and stirring (such as vibration frequency, stirring speed, etc.) need to be precisely controlled, otherwise the ideal refinement effect may not be achieved, and even have adverse effects on the metal melt.

4. Rapid solidification to refine grain size

4.1 Principle

Rapid solidification refers to the solidification of a metal melt at an extremely high cooling rate (usually greater than 10 ⁴ -10 ⁶ ℃/s). Under rapid solidification conditions, the diffusion of metal atoms is greatly limited, and there is no time to form coarse grains, only small equiaxed crystals or even amorphous structures can be formed.

4.2 Implementation method and application

4.2.1. Atomization method: Metal melt is atomized into small droplets through high-pressure gas or a rotating disk, and the droplets rapidly cool and solidify during flight, forming small powder particles. The grain size of these powder particles is very small, and after subsequent processing (such as hot pressing, hot isostatic pressing, etc.), high-performance metal materials can be made. The atomization method is widely used in the preparation of metal powders such as high-speed steel and aluminum alloys  

4.2.2. Melt spinning quenching method: Metal melt is sprayed onto a high-speed rotating cooling roller, and the melt rapidly spreads and solidifies on the surface of the cooling roller, forming a thin strip. Due to the extremely fast cooling rate, the grain size in the thin strip can reach the nanometer level. The melt spinning method is commonly used to prepare amorphous alloys and nanocrystalline alloys, and has potential application value in fields such as electronics and magnetic materials.

4.3 Effect and Limitations

Rapid solidification can obtain extremely small grains or even amorphous structures, significantly improving the strength, hardness, and wear resistance of metal materials. However, the rapid solidification process equipment is complex, the cost is high, and the shape and size of the prepared materials are subject to certain limitations, usually requiring subsequent processing to obtain the desired part shape.

5. Heat treatment to refine grain size

5.1 Principle

By controlling heat treatment parameters such as heating, insulation, and cooling, metal materials can undergo recrystallization or phase transformation, thereby refining grain size. Recrystallization refers to the process in which a metal undergoes cold deformation and is heated to a certain temperature, causing the deformed grains to re nucleate and grow into new undistorted grains. Grain refinement through phase transformation is achieved by utilizing the reconstruction of metal grains during the phase transformation process.

5.2 Specific methods and applications

5.2.1. Recrystallization annealing: For metal materials that have undergone cold deformation (such as cold rolling, cold drawing, etc.), recrystallization annealing treatment is carried out. Heat the metal above the recrystallization temperature and hold for a certain period of time to induce recrystallization of deformed grains, forming small equiaxed grains. For example, after cold rolling, copper material can be significantly refined in grain size through appropriate recrystallization annealing treatment, which improves the plasticity and toughness of the copper material.

5.2.2. Normalization treatment: For steel parts, normalization is the process of heating the steel above the critical temperature, holding it for a certain period of time, and then cooling it in the air. Normalization can refine the austenite grains in steel, adjust the microstructure of steel, and improve the comprehensive mechanical properties of steel. For example, after normalizing treatment, the grain size of medium carbon steel is refined, and its strength and toughness are improved, which can be used to manufacture mechanical parts with higher requirements.

5.3 Effect and Limitations

The method of refining grain size through heat treatment is relatively simple to operate, cost-effective, and suitable for metal parts of various shapes and sizes. However, the heat treatment effect is influenced by various factors such as the composition, degree of deformation, and heat treatment process parameters of the metal material, and precise control of the heat treatment process is required to achieve the desired refinement effect.

6. Plastic deformation refines grain size

6.1 Principle

Plastic deformation can cause the internal grains of metal materials to break and deform, increasing the distortion energy of grains and providing driving force for recrystallization. During the subsequent recrystallization process, small equiaxed grains are formed.

6.2 Specific methods and applications

6.2.1. Cold deformation: Cold rolling, cold drawing, cold extrusion and other cold deformation methods can cause significant plastic deformation in metal materials, leading to grain elongation and breakage. For example, when cold rolling steel plates, as the deformation increases, the grains gradually elongate. When the deformation reaches a certain degree, recrystallization annealing treatment can be carried out to obtain a fine grain structure and improve the strength and hardness of the steel plate.

6.2.2. Equal Channel Angular Squeezing (ECAP): Equal Channel Angular Squeezing is a severe plastic deformation method. Place the metal sample into a specially designed mold with two intersecting channels. Under pressure, the sample undergoes significant shear deformation by passing through equal channel angles. After multiple equal channel angular squeezes, the grain size of metal materials can be significantly refined, even reaching the sub micron or nanometer level. Equal channel angular pressing is commonly used to prepare ultrafine grained metal materials and has potential applications in aerospace, biomedical and other fields.

6.3 Effect and Limitations

The plastic deformation refinement method can obtain a relatively uniform fine grain structure, improving the strength and hardness of metal materials. But the degree of plastic deformation needs to be strictly controlled, and excessive deformation may lead to cracking of the metal material; Meanwhile, metal materials after plastic deformation usually require recrystallization annealing treatment, which increases the complexity of the process.

7. Conclusion

Refining grain size is an effective way to improve the properties of metal materials, and various methods such as modification treatment, vibration and stirring, rapid solidification, heat treatment, and plastic deformation have their own principles, characteristics, and application ranges. In actual production, appropriate grain refinement methods or a combination of multiple methods should be selected based on factors such as the type of metal material, the shape and size of the parts, performance requirements, and production costs to achieve the best grain refinement effect, improve the performance and quality of metal materials, and meet the needs of different industries.