Influencing factors

Mechanical alloying is a complex process, and therefore a series of influencing parameters need to be optimally designed in order to obtain desired phase and microstructure. Some factors that have a significant impact on the results of mechanical alloying are listed below.

1). Grinding device

The grinding devices for producing mechanically alloyed powders are various, such as: planetary mills, vibratory mills, stirred mills and so on. Their grinding energy, grinding efficiency, the degree of contamination of the material, grinding media and the grinding vessel wall forces are different, so the grinding results may vary a lot. The material and shape of the grinding vessel also have an important influence on the grinding results. In the process, the impact of the grinding media on the inner wall of the grinding vessel and friction will make part of the material on the inner wall of the grinding vessel fall off and enter the grinding material to cause contamination. Commonly used grinding container materials are usually quenched steel, tool steel, stainless steel; quenched steel lined material and so on. Sometimes for special purposes, special materials, for example: grinding materials containing copper or titanium  are used, in order to reduce the contamination.

Planetary ball mill normally has single jar, double jar, four jars to meet the needs of different application. Powteq planetary ball mill BM6Pro with a single jar, BM20 with double jars. Planetary ball mill BM40 has four platforms, which can possess 2, 4, or 8 samples at the same time; The Planetary Ball Mill BM20Plus is a two-jar ball mill, which differs from the Planetary Ball Mill BM20,  it is suitable for processing a large quantity samples by supporting two jars up to 4 liter.

In addition, the shape of the grinding vessel is also very important, especially the shape of the inner wall design. For example, when grinding jar inner wall fixed slide plate and bumps, it will increase the sliding speed of the media and produce centripetal acceleration, enhance the friction between the media, and is conducive to the alloying process.

2). Grinding speed

The higher the rotational speed of the mill, the more energy will be transferred to the grinding material. However, it is not the case that the higher the rotational speed, the better. This is because, on the one hand, when the rotational speed of the mill is increased, the rotational speed of the grinding media will also be increased, and when it is high to a certain extent the grinding media will be close to the inner wall of the grinding vessel, and cannot produce any impact on the grinding material, which is not conducive to deformation and alloying process. On the other hand, too high a rotational speed can cause the grinding system to warm up too quickly and to become too hot, which is sometimes unfavorable, e.g., higher temperatures may lead to the decomposition of supersaturated solid solutions, amorphous phases, or other sub stable phases that need to be formed in the process.

3). Grinding time

Grinding time is one of the most important factors affecting the results. Under certain conditions, with the process of grinding, the degree of alloying will be higher and higher, the particle size will gradually reduce and eventually form a stable equilibrium state, that is, the cold welding and crushing of particles to reach a dynamic equilibrium, at this time the particle size no longer changes. On the other hand, the longer the grinding time, the more serious the pollution. Therefore, the optimal grinding time should be based on the desired results, through the test to determine.

4). Grinding media

Selection of grinding media should not only consider its material and shape such as ball, rod, etc., like the grinding vessel, but also consider its density and the size and distribution of the size, etc., the ball milling media should have the appropriate density and size in order to produce enough impact on the grinding material, which have a direct impact on the final product. For example, when milling the Ti-Al mixed powder, if the diameter of the grinding ball of 15 mm is used, a solid solution can be obtained, whereas if 25 diameter grinding balls are used, no Ti-Al solid solution is obtained under the same conditions even if the grinding is done for a longer period of time.

5). Ball to material ratio

The ball-to-material ratio refers to the weight ratio of the grinding media to the grinding material, usually the grinding media is spherical, so it is called ball-to-material ratio. The ball to material ratio for experimental research is in the range of 1:1 to 200:1, and in most cases, it is about 15:1. When doing small production runs or tests, this ratio can be as high as 50:1 or even 100:1.

Usually, it is recommended to choose grinding balls made of the same material as the grinding jar for experiments.

6). Filling ratio

Filling ratio of grinding media refers to the percentage of the total volume of the grinding media to the volume of the grinding vessel, and the filling ratio of the grinding material refers to the percentage of the loose volume of the grinding material to the gap between the grinding media. If the filling rate is too small, it will make the productivity low; if it is too high, there is not enough space to make the grinding media and materials to fully move, so as to produce less impact, and not conducive to the alloying process. Generally speaking, the filling rate of grinding media in vibration mill is between 60%-80%, and the filling rate of material is between 100%-130%.

7). Gas environment

Mechanical alloying is a complex solid-phase reaction process, ball milling atmosphere, ball milling intensity, ball milling time and other changes in any one of the parameters will affect the alloying process and even the final product. During the mechanical alloying process, mechanical energy is converted into thermal energy due to the impact between balls and balls, balls and jars, which makes the temperature inside the jar rise to a very high level. At the same time, the alloying process often occurs in the refinement of the particles, and the introduction of defects, free energy rises, it is easy to react with the ball mill atmosphere such as oxygen, so the general mechanical alloying process normally use inert gases, such as argon, etc. for the protective gas. Different ball milling atmosphere has significant impact in the reaction mode of alloying, the final product. Grinding gas environment is an important factor in the generation of pollution, therefore, generally in a vacuum or inert gas protection. However, it is sometimes necessary to grind in a special gas environment for special purposes, e.g., when a corresponding nitride or hydride generation is required, grinding may be carried out in a nitrogen or hydrogen atmosphere.

The grinding jars of planetary ball mills are divided into normal grinding jars and ventilated grinding jars. If the gas content of a substance is to be determined, a grinding jar with a ventilator can be used in combination with the proprietary software, as shown in the figure:

(Ventilation Grinding Gar for POWTEQ Planetary Ball Mill).

8). Process Control Agents

The presence of severe agglomeration, caking and wall sticking of powders in the MA process greatly hinders the MA process. For this reason, process control agents, such as stearic acid, solid paraffin, liquid alcohol and carbon tetrachloride, are often added to the process to reduce the agglomeration of the powder, sticking to the ball, sticking to the wall, and the abrasion of the grinding media and the inner wall of the grinding vessel, which can better control the composition of the powder and improve the powder yield.

9). Grinding temperature

Whether the final product of MA is solid solution, intermetallic compounds, nanocrystalline, or amorphous phase involves diffusion, and diffusion is affected by the milling temperature, so the temperature is also an important influence on MA. For example, the Ni-50% Zr powder system in ball milling when milled under the cooling of liquid nitrogen for 15h did not find that the amorphous phase is formed; and milled under the 200oC powder material found to be When milled at 200°C, the powder material was found to be completely amorphous; when milled at room temperature, partial amorphization was achieved.

The above factors are not independent of each other, e.g., the optimal grinding time depends on the type of grinding, media size, grinding temperature, and ball-to-material ratio.

Mechanical alloying has the following advantages when synthesizing high melting point alloys or intermetallic compounds: avoiding the high temperature melting and solidification process of common metallurgical methods, achieving alloying at room temperature, obtaining uniform alloys with a fine structure, and higher yields, thus it has become a good method for the production of alloys and new materials that are difficult to be prepared by conventional means.