10 influencing factors of high-energy ball milling in material preparation

Application of High Energy Ball Milling Technology in Material Preparation and Analysis of 10 Influencing Factors

The high-energy ball milling method involves mechanically mixing powders of different materials in a certain ratio. Under the repeated impact of the grinding ball medium, the powder undergoes collision, impact, shear, and compression, resulting in continuous deformation, fracture, and welding. High strength and long-term grinding make the powder fully uniform and refined, ultimately becoming a composite powder with dispersed reinforcement distribution.

The high-energy ball milling method was initially only a method of mixing composite powders to achieve sufficient uniformity. With the understanding of high-energy ball milling, the forced force during the milling process will introduce a large number of strains, defects, and nanoscale microstructures. By using high-energy ball milling, nanomaterials such as nanocrystalline pure metals, nanoscale reinforced composite powders, nanometallic intermetallic compounds, etc. can be prepared, making it difficult to achieve alloying of certain substances using traditional melting processes The synthesis of new substances such as non equilibrium and quasi steady states has become possible.

1. Preparation of oxide dispersion strengthened alloys

The current industrial application of high-energy ball milling is largely focused on the preparation of oxide dispersion strengthened alloys. This type of alloy has complex components and is difficult to produce using ordinary metallurgical methods. This type of alloy produced by high-energy ball milling has the characteristic of extremely fine oxide particles (5 ^ 50nm) uniformly dispersed (particle spacing~100nm) on a solid solution matrix formed by multiple elements, thus having high high-temperature strength and high-temperature creep resistance.

2. Preparation of nano powder

Nanomaterials are currently a hot research topic in the field of materials science. Despite the shortcomings of pollution from ball milling media and uneven powder particle size, high-energy ball milling is still a hot technology highly praised by nanomaterial researchers. It is understood that Australia's Advanced Powder Technology Joint Co., Ltd. has successfully synthesized nano ZnO, ZrO2, CeO2, ZnS, CdS, Ce2S3 and other powders with uniform and fine particle size (nanoscale) and extremely low impurity content using a ball mill (usually a stirring type) to induce solid-state displacement reaction and dissolve by-products. By controlling the composition of reactants, ball milling and heat treatment conditions, it is possible to provide nano powders of various sizes for users to choose.

3. Preparation of raw materials for surface treatment

Fe based, Co based, and Ni based alloy powders synthesized by high-energy ball milling (other alloying elements usually Cr, Al) can be used as raw materials for corrosion and wear resistant coatings prepared by plasma spraying or other methods. The dispersion of oxides and carbides formed during high-energy ball milling is very beneficial for material protection. Adding Y can form Y2O3 dispersed phase and similar composite phase during ball milling. This type of ball milling powder can be used as a raw material for preparing diffusion barrier coatings, which reduce the inverse effect of concentration gradient between the substrate and the coating.

4. Preparation of solder

Zoz from Germany and Fukuda Metal Sheet and Powder Co., Ltd. from Japan have jointly developed high-energy ball milling products for coatings and solders. The outstanding advantage of this product is that the chemical composition distribution is very uniform and there is no segregation phenomenon.

5. Preparation of hard alloys

Nanocrystalline hard alloy, as the mainstream cutting tool in modern industrial processing, has always attracted the attention of material researchers. There are two methods for preparing this alloy by ball milling. One is to synthesize nano WC powder using W and C, and the other is to grind and refine WC and Co powder mixture to achieve nanocomposition. The grain size is generally several to tens of nanometers, and the sintered hard alloy grain size is usually several to 200 nanometers.

Factors affecting high-energy ball milling

The factors that affect high-energy ball milling include ball milling equipment, ball milling speed, ball milling time, ball type and size, ball material ratio, ball milling temperature, and process control agents. These factors are not independently influenced, but rather act together. Among them, ball milling time is the most important influencing factor. Generally speaking, the optimal ball milling time is when the powder reaches the equilibrium stage of cold welding and crushing, which varies depending on the ball milling equipment, ball material ratio, ball milling temperature, and ball milling speed. When the ball milling exceeds the optimal milling time, more pollution and imperfect phases will be introduced, resulting in a decrease in performance.

1. Ball milling medium

Ball milling media are grinding objects stored in high-energy ball milling tanks for crushing and welding ball milling materials. Reasonable selection of process parameters such as material, size, and ratio of ball milling media can improve ball milling efficiency, shorten ball milling time, and have important significance in reducing energy consumption.

(1) Material of ball milling medium

The commonly used materials for ball milling media can be non-metallic materials such as stainless steel and hard alloy, as well as non-metallic materials such as ceramics. The material of ball milling media directly determines the hardness, density, and other characteristics of the ball milling media, and it needs to be reasonably selected based on factors such as the properties of the ball milling materials and the properties of the materials to be prepared. The material of the ball milling medium is usually used in conjunction with the matching ball milling tank body to ensure that the medium and tank body are processed from the same material, avoiding contamination of the ball milling material by introducing various impurities due to different materials between the medium and tank body.

(2) Size and ratio of ball milling medium

Generally speaking, the average size of ball milling media has a significant impact on the flakiness and size of ball milling materials, so the design of ball milling media size and ratio is directly related to the electromagnetic parameters of ball milling materials. It is worth noting that during the ball milling process, not only can some ball milling media of the same size be selected, but also different sizes of ball milling media can be selected. However, the motion state of ball milling media of different sizes is more complex.

In general, compared to single size ball milling media, mixed size ball milling media have a more prominent ball milling efficiency. This is because larger size ball milling media first effectively crushes the ball milling materials, and then a large number of smaller size ball milling media will high-frequency squeeze and collide with the ball milling materials, converting the gravitational potential energy of the ball milling media more efficiently into the ball milling materials.

2. Ball to material ratio

The ball to material ratio generally represents the mass ratio of ball milling media to ball milling materials, and also uses the material to ball ratio to represent the mass ratio of materials to ball milling media. The material to ball ratio and ball to material ratio are reciprocal to each other. In the early stage of ball milling, one of the methods to improve ball milling efficiency is to increase the ball material ratio. As the ball material ratio increases, the weight of the ball milling medium corresponding to the unit mass of ball milling material increases, which means that the weight potential energy of the medium corresponding to the unit mass increases, which is conducive to improving the frequency of material compression and crushing.

3. Medium filling rate

The filling rate of the medium represents the percentage of the volume of the ball milling medium to the volume of the ball milling tank. Due to the need for a certain amount of space for free collision between ball milling media and ball milling materials, the selection of loading amount is crucial. Research has shown that when the filling rate of the medium is higher than 50%, the increase in medium filling will promote the impact of the ball milling medium on the ball milling material, improve the energy conversion efficiency of the ball milling medium to the ball milling material, and effectively shorten the ball milling time. However, it is understood that the higher the medium filling rate, the better. When the medium filling rate is too high, the limited space of the ball milling medium in the ball milling tank cannot effectively crush and cold weld the ball milling material, resulting in an extension of the ball milling time and affecting the performance of the prepared material.

4. Ball milling atmosphere

In the high-energy ball milling process, the ball milling material will be fiercely impacted by the ball milling medium, generating many irregular surfaces. These new surfaces may react with the gas inside the tank during the high-temperature ball milling process. Sometimes, it is necessary to avoid chemical reactions triggered by the new surface and air. At this time, inert gas is usually filled or air is exhausted for protection, but sometimes to prevent situations such as rapid temperature changes in the ball milling tank, Special gas will be filled into the tank.

5. Process treatment agent

High energy ball milling is a competitive process between powder fracture and cold welding, and excessive cold welding is not conducive to the refinement and alloying process of the powder; Secondly, in high-energy ball milling, the powder is easily adhered to the grinding ball and the wall of the ball milling tank, reducing the powder yield. To alleviate these trends, appropriate process control agents such as stearic acid, solid paraffin, liquid alcohol, and carbon tetrachloride can be added to suppress cold welding, promote fracture, and thereby improve powder yield.

The process treatment agent can reduce the tendency of powder adhesion to the wall and ball during ball milling, because it refines the powder while in-situ surface modification is carried out. For example, stearic acid is a solid surfactant that can adsorb on the unsaturated broken bonds on the surface of powder particles and grinding balls, reducing surface energy and interface energy between powder particles and grinding bodies, thereby reducing the degree of adhesion of powder particles to grinding tools.

It is crucial to choose an appropriate and appropriate process treatment agent based on the cold welding characteristics of the powder, the chemical and thermodynamic stability of the process treatment agent itself, the amount of powder, and the ball milling equipment. Usually, the grinding of brittle materials does not require the addition of process control agents. In addition, process treatment agents sometimes undergo chemical reactions with highly active powder components or products produced by ball milling, changing the composition of the products and causing new powder pollution. From this, it can be concluded that no process control agent is universal, and there is a critical value for its dosage, above which the powder particle size tends to decrease.

6. Ball milling time

The ball milling time has a great impact on the sheet-like structure and average size of the ball milling materials. The longer the ball milling time, the deeper the crushing and squeezing effect on the ball milling materials between the ball milling medium, the ball milling tank, and the ball milling medium. The more energy is generated in the ball milling process. The impact of ball milling time on different ball milling materials varies and is closely related to factors such as the composition, size, and properties of the ball milling materials. In addition, if the high-energy ball milling time is too long, it may maintain the ball milling temperature at a high state, increase the possibility of oxidation pollution of the ball milling materials, and also cause agglomeration of the ball milling materials, greatly affecting the material performance.

7. Ball milling speed

The increase in ball milling speed and the extension of ball milling time have similar effects, both increasing ball milling energy and affecting material performance. If the ball milling speed reaches or exceeds the critical speed, it will also cause the ball milling medium to move tightly against the inner wall of the tank, without throwing the ball milling medium off, greatly weakening the collision and compression effect of the ball milling medium on the ball milling material, which is not conducive to the transformation of the material into a sheet structure.

The ball milling medium has different effects on different ball milling materials, and even has significant differences in the ball milling effect of the same ball milling material at different ball milling stages. This is mainly because at different stages of ball milling, ball milling materials have different requirements for high-energy ball milling process parameters. If the ball milling process parameters are changed according to different requirements, it is possible to make targeted improvements to the ball milling process parameters based on the performance of the required materials.

Researchers found during the preparation of carbonyl iron powder absorbing materials by ball milling that when the ball milling speed reached 200 r/min, the absorbing material began to transform from an ellipsoidal structure to a flat structure; Moreover, an increase in ball milling speed will make the thin sheet shape of the material more obvious, effectively improving the aspect ratio of the material. However, a higher ball milling speed will cause the material to fracture and form many small fragments, resulting in a decrease in the complex magnetic permeability and an increase in the complex dielectric constant of the material, which is not conducive to the impedance matching of the material.

8. Graded ball milling

When preparing barium ferrite materials by ball milling, researchers used different sizes of ball milling media for two high-energy ball milling. Firstly, agate ball milling media with a ball milling size of 6 mm was used, and then zirconia ball milling media with a ball milling size of 2 mm was used. The results showed that the material was significantly refined after two stage ball milling, and the size distribution of the material was more uniform; The residual magnetization of the prepared barium ferrite material gradually increases.

9. Ball milling temperature

The temperature of ball milling has a significant impact on the refinement process of high-energy ball milling, as excessive temperature can cause effective strain relaxation of high-energy ball milling into nanomaterials, leading to an enhanced trend of grain size growth, making it difficult to obtain nanocrystals. At present, low-temperature ball milling is gradually receiving attention because the brittleness of metal powders increases at low temperatures, which is beneficial for refinement. At the same time, without the use of process treatment agents during low-temperature ball milling, cold welding can be effectively suppressed, avoiding powder pollution caused by process treatment agents.

Research has shown that nanomaterials prepared by low-temperature ball milling also have higher thermal stability. This is mainly due to the formation of stable oxides or nitrides during low-temperature ball milling, which are dispersed on the surface and interface of nanocrystals and play a pinning role on the interface, thereby preventing the grain size from growing during high-temperature heat treatment. At present, low temperature ball milling has also become an important method to prepare nano powders.

However, low-temperature ball milling is usually carried out in liquid nitrogen or liquid argon medium. Therefore, further exploration is needed on how to control and maintain the ultra-low temperature of the ball milling tank during the experimental process, how to effectively control the loss of cooling medium, and how to reduce the pollution of the ball milling medium at low temperatures.

10. External physical energy field

Traditional high-energy ball milling usually uses a single mechanical energy to process the powder, which limits the energy input obtained by the powder. Only the powder that receives the impact and shear effects of the ball in the ball milling chamber can receive the energy input. Introducing other physical energy into the ball milling process and utilizing an external energy field to assist ball milling can accelerate the refinement of the powder structure or promote the mechanical alloying process. This is a feasible way to improve the efficiency of high-energy ball milling and also a research direction for the development of high-energy ball milling. At present, ultrasound, magnetic field, electric field, and temperature field have been successfully combined and applied in high-energy ball milling processes, and substantial progress has been made in using plasma as a special energy field with high energy and activity to assist ball milling.

Reference source:

The Influence of High Energy Ball Milling Process on the Electromagnetic Properties of Absorbing Materials, Wang Mangang, Nanjing University of Posts and Telecommunications, 2017

Preparation and Heat Treatment of High Energy Ball Milling Ti55Cu17.5Ni17.5Al10/7075 Aluminum Matrix Composite Materials, Hu Yuan, South China University of Technology, 2018

Research on the main factors promoting powder refinement in high-energy ball milling, Dai Leyang et al., Jimei University, 2008

Industrial Application of High Energy Ball Milling in the Field of Material Preparation, Ma Mingliang et al., Jiujiang University, 2006