What Is Sintering?
Sintering is an object that becomes dense when powdered metals are hardened and then joined together atoms at a temperature lower than their melting point.
In the initial stages of sintering, the powder particles form a neck that is bonded to each other. Diffusion of ions, atoms, and molecules on the surface of the neck causes the neck to grow larger and the surface area of the sintering object to decrease. At this point, the diameter of the neck is approximately 30% of the particle diameter.
The pores inside the sintering will change their shape to a tube-like structure and become connected to each other. Pores in this state are called open pores. Finally, the relative density exceeds 95% and the pores are dispersed within the sintered body, resulting in a closed pore state.
Uses of Sintering
Sintering is used to form materials that cannot be processed by plastic deformation, such as rolling and forging. For example, they are used in powder metallurgy and in the manufacture of ceramics. Compared to casting, where defects may or may not occur depending on the conditions of the mold and hot water, sintering offers more flexibility in shape and is therefore used for parts with complex shapes, such as automobile engine parts, drive train parts, small gears, and bearings.
Sintering can be used with almost any metal that can be made into powder. Therefore, sintering is also applied to parts made of tungsten and molybdenum, which have high melting points, and titanium, which easily reacts with other elements at high temperatures.
Other Information on Sintering
Characteristics of Sintering
The main sintering methods are atmospheric pressure sintering and pressure sintering, but research and development of electromagnetic field-assisted sintering as a new method is progressing, and some practical applications have begun.
1. Atmospheric Pressure Sintering
Atmospheric pressure sintering is a sintering method in which the atmosphere is at atmospheric pressure. There are two types of sintering methods: liquid phase sintering, in which a liquid phase is generated at high temperatures using additives to produce densification; and solid phase sintering, in which densification occurs through mass transfer between solid phases without generating a liquid phase.
2. Pressure Sintering
Pressure sintering is a method of densification by applying external pressure and includes hot press sintering and hot isostatic sintering. In hot press sintering, uniaxial pressure is applied to the powder, while in hot isostatic sintering, the powder is encapsulated in glass or metal and pressurized with gas to solidify.
3. Electromagnetic Field Assisted Sintering
Pulsed energized pressure sintering, microwave/millimeter wave sintering, and electric field assisted sintering are available. Pulsed current pressure sintering is a method of sintering by passing a pulse current of several thousand A through a conductive mold while applying pressure. Microwave and millimeter wave sintering are self-heating methods from the inside of the derivative, and heat treatment can be performed in a short time with high thermal efficiency. By raising the ambient temperature while energizing the powder compact directly, sintering can be performed at lower temperatures than with atmospheric pressure sintering.