The sintering process of diamond segments

1. Overview of sintering

Sintering is one of the most basic processes in the powder metallurgy production process, and it is also the last major process. It plays a decisive role in the performance of the final product. Sintering is the "gateway" of diamond blade production.

 

The sintering process is a series of complex physical and chemical processes such as diffusion, melting, flow, shrinkage and recrystallization between the compacted metal matrix powders under hot pressing, which produces a strong bond between the matrix particles, making the product strong and having a certain hardness and strength.

 

The sintering process mainly refers to the sintering system, and the curve is drawn according to the relationship between temperature and time. Therefore, the sintering temperature must be determined first according to the composition and ratio of the saw blade matrix, the size and specifications of the saw blade, etc., and then the heating method and the insulation time after reaching the sintering temperature must be determined.

 

• 1) Determination of sintering temperature

For a certain saw blade product, since its matrix composition has been determined, its sintering temperature is a fixed temperature range. This range is generally specified at the optimal temperature sintering point plus or minus 10°C, and the sintering temperature is generally about 2/3 of the melting point of the main component, that is, Tsintering>2/3Tmelting. The actual sintering temperature is determined by trial sintering test blocks. The first group of test blocks is determined according to the above temperature. After the trial sintering, the color, surface condition, crystallization, etc. of the test blocks are observed to determine whether they are over-burned or under-burned. Then the sintering temperature of the second group of test blocks is given, and the third and fourth groups of test blocks are tested in the same way until the appropriate sintering temperature is obtained.

 

• 2) Sintering time

Sintering time and sintering temperature are a pair of related parameters, because when the sintering temperature is appropriately increased, the insulation time of the sintering point can be relatively shortened, but if the control is improper, the product will be deformed, the grains will grow, and even segregation will occur, affecting the product quality. When the sintering temperature is low, the insulation time of sintering must be extended, otherwise the product will be under-burned. The sintering temperature difference cannot be too large, generally around 20oC.

 

2. Theoretical analysis of the sintering process

After the matrix powder is sintered, the strength of the sintered body increases. First, the bonding strength between the powder particles increases. During sintering, due to the high temperature, the movement of atoms in the powder body intensifies, allowing more atoms to enter the contact surface between the particles to form a bonding surface. Moreover, as the bonding surface expands, the strength of the sintered body also increases. The bonding surface expands to form a sintering neck, so that the original particle interface forms a grain interface, and as the sintering continues, the grain boundary can move into the interior of the particle, resulting in the growth of the grain.

 

The increase in the strength of the sintered body is also reflected in the reduction of the pore volume and the total amount of pores, as well as the change in the shape of the pores. As the sintering neck grows, the pores that were originally interconnected between the particles gradually shrink into closed pores and then become rounded. The size and number of pores are also changing, that is, the number of pores decreases, while the average pore size increases.

 

The formation of the particle bonding surface usually does not lead to the shrinkage of the sintered body. Therefore, densification does not mark the beginning of the sintering process, and only the increase in the strength of the sintered body is an obvious sign of sintering. With the growth of the sintering neck, the reduction of the total pore volume, and the shortening of the distance between particles, the densification process of the sintered body really begins. As mentioned above, in addition to the growth of the sintering neck during the sintering process, the compact can be densified and shrunk; the surface area will decrease; the strength can be increased, and the conductivity will increase. These parameters provide the possibility of describing the sintering process. In most cases, the sintering process is accompanied by a decrease in the size of the sintered body.

 

Its densification parameter Φ can be expressed as:

Φ=(ρs–ρg)/(ρt-ρg)

where ρs--sintered body density;

ρt--theoretical density;

ρg--compacted density.

 

The figure below shows the changes in some parameters when isothermal sintering is carried out at two temperatures (T2›T1), reflecting the main effects of time and temperature on the sintering process.

 

 

The isothermal sintering process of powder can be roughly divided into three stages with unclear boundaries (schematic diagram as follows). 

 

(1) Initial stage - the initial stage of sintering, or the bonding stage. The original contact points or contact surfaces between particles are transformed into grain bonding, that is, a sintering neck is formed through atomic migration processes such as nucleation and growth. This stage mainly involves metal recovery, volatilization of adsorbed gas and water, and decomposition and removal of the forming agent in the compact.

 

(2) Intermediate stage - sintering neck growth stage. The large-scale migration of atoms to the particle bonding surface causes the sintering neck to expand, the distance between particles to decrease, and a continuous void network to form. At the same time, due to grain growth, the grain boundary moves across the pores, and the pores disappear in large numbers where the grain boundary sweeps. Increased density and strength are the main characteristics of this stage.

 

(3) Final stage - closed pore spheroidization and shrinkage stage. Most pores are completely separated, the number of closed pores increases greatly, and the pore shape tends to be spherical and continues to shrink. In this stage, the entire sintered body can still shrink slowly, but this is achieved by the disappearance of small pores and the reduction of the number of pores. However, there are still a small number of residual isolated small pores that cannot be eliminated.

 

Quanzhou Huice Tools Company has many years of experience in producing diamond segments. If you want to obtain more knowledge and information, please contact us.