Introduction to the Basic Principle of Sintering

(1) Basic principle of sintering
Sintering is one of the most basic processes in the powder metallurgy production process. Sintering plays a decisive role in the performance of the final product, because the waste caused by sintering cannot be saved by subsequent processes; on the contrary, certain defects in the process before sintering can be corrected within a certain range by adjusting the sintering process, such as appropriately changing the temperature, adjusting the heating and cooling time and speed, etc.

Sintering is the process of heating powder or powder compact to a temperature lower than the melting point of the basic components, and then cooling to room temperature in a certain way and speed. The result of sintering is that the powder particles are bonded together and the strength of the sintered body is increased. A series of physical and chemical changes occur during the sintering process, turning the aggregate of powder particles into an aggregate of grains, thereby obtaining a product or material with the required physical and mechanical properties. During sintering, in addition to the connection of powder particles, densification, alloying, heat treatment, and connection may also occur. People generally classify the metal powder sintering process into:
1. Single-phase powder (pure metal, ancient melt or metal compound) sintering
2. Multiphase powder (metal-metal or metal-nonmetal) solid phase sintering
3. Multiphase powder liquid phase sintering:
4. Melt infiltration
Usually, the first three types of sintering are what PORITE micro bearings are currently exposed to and need to understand. Usually, during the sintering process, the powder particles often undergo the following stages of changes
1. The particles begin to connect
2. The bonding neck between particles grows
3. The pore channel is closed
4. Pore spheroidization
5. Pore shrinkage
6. Pore coarsening
The various changes in the above sintering process are closely related to the movement and migration of substances. Theoretically, the mechanism is:
1. Evaporation and condensation
2. Volume diffusion
3. Surface diffusion
4. Inter-product diffusion
5. Viscous flow
6. Plastic flow

(2) Sintering process

2-1. Sintering process
The sintering process of powder metallurgy can be roughly divided into four temperature stages:
Heating
Sintering and heat preservation
Slow cooling
Quick cooling
1. Low-temperature pre-sintering stage. In this stage, the recovery of metal and the volatilization of adsorbed gas and water, the decomposition and removal of the forming agent in the pressed green sheet, etc. mainly occur. In PORITE micro copper and iron bearings, R, B, 0 (Rapid Burning 0ff) is used to replace the low-temperature pre-sintering stage. After R, B, 0, copper and iron products will oxidize, but can be reduced in the body, and can also promote sintering.
2. Medium-temperature sintering stage. In this stage, re-agglomeration begins to appear. First, in the particles, the deformed particles are restored and reorganized into new particles. At the same time, the oxides on the surface of the particles are completely reduced, and sintering necks are formed at the particle interface.
3. The sintering stage is completed by high temperature insulation. This stage is the main process of sintering. If diffusion and flow are fully carried out and nearly completed, a large number of closed pores are formed and continue to shrink, so that the pore size and the total number of pores are reduced, and the density of the sintered body is significantly increased.
4. Cooling stage: The actual sintering process is continuous sintering, so the process from slow cooling from the sintering temperature for a period of time and then fast cooling to the process of the furnace reaching room temperature is also the stage of austenite decomposition and the gradual formation of the final structure.
The temperature usually refers to the highest sintering temperature, that is, the insulation temperature, which is generally 1/2~4/5 of the absolute melting point temperature. The temperature index a=0.67~0.80, its lower limit is slightly higher than the re-formed product temperature, and its upper limit is mainly considered from economic and technical perspectives, and is selected at the same time as the sintering time. 2-2. Factors affecting the sintering process:
1. The properties of the material, including various interface energies and free energies: diffusion coefficient: viscosity coefficient: critical shear stress, vapor pressure and evaporation rate, lattice type and product morphology; heteromorphic transformation ecology, etc. 2. Powder properties: including particle size: particle shape and morphology: particle structure: particle chemical composition.
3. Physical properties of green compacts: including pressing density, pressing residual stress, deformation or destruction of oxide film on particle surface, and gas in green compact pores.
4. Sintering process parameters: including holding time, heating and cooling speed, sintering gas, etc. 2-3. Changes in size and density of green compacts during sintering
In production, the requirements for product size and shape accuracy are very high. Therefore, it is an extremely important issue to control the density and size changes of green compacts during sintering. Factors that affect the density and size changes of parts are:
1. Shrinkage and removal of pores: sintering will cause shrinkage and removal of pores, that is, reduce the volume of sintered bodies.
2. Enclosed gas: During pressing and forming, many closed isolated pores may be formed in the green compact. When heated and crushed, the air in these isolated pores will expand.
3. Chemical reaction: Certain chemical elements in the green sheet and in the sintering atmosphere react with a certain amount of oxygen in the green sheet raw material to generate gas that either evaporates or remains in the green sheet, causing the green sheet to shrink or expand.
4. Alloying: When two or more element powders are alloyed, when one element is dissolved in another element to form a solid solution, the basic lattice may expand or shrink.
5. Lubricant: When a certain amount of lubricant is mixed in the metal powder and pressed into a green sheet, at a certain temperature, the mixed lubricant is burned off to cause the green sheet to shrink. However, if the gaseous substances produced by the decomposition cannot reach the surface of the sintered body, it may cause the green sheet to expand.
6. Pressing direction: During sintering, the size change of the pressed piece is not equal in the vertical or parallel direction to the pressing direction. Generally speaking, the size change rate in the vertical direction (radial direction) is larger, and the size change rate in the parallel direction (axial direction) is smaller.
2-4. Preparation before sintering
Check whether the sintered products are suitable for the sintering temperature and the speed of the mesh belt, check the products to be sintered, and remove the unqualified pressed green sheets. Generally, the inspection is carried out according to the requirements of the process drawings. Usually, the geometric dimensions and deviations of the single weight of the product, that is, the density of the pressed green sheets and whether the pressed green sheets have broken edges and corners, delamination cracks, and severe roughening are checked.
Determine the sintering method (such as standing, lying, and discharging) according to the shape and size of the pressed green sheets. Then use a pneumatic nozzle to blow out the dust remaining on the surface of the product. In special cases, high aluminum plates must be sintered. 2-5. Finishing work after sintering
After sintering, the products must be inspected first, and the unqualified parts must be separated. Then, they must be oiled and stacked neatly according to the product classification. In special cases, the products must be placed in a common machine (drum) to remove burrs and separate the parts that stick together.
2-6. Analysis of waste products in sintering furnace
Sintering waste products include waste products that cannot be saved in the process and "reburned products" that can be transformed into qualified products through reprocessing.
1. Change and warping
2. Blistering and cracking
3. Pits
4. Dimensional deviation
5. Over-burning and under-burning
6. Oxidation and demolding
7. Metallographic defects

(3) Basic structure of sintering furnace and introduction to mesh belt sintering furnace

In order to mass produce high-quality and low-cost powder metallurgy products, the heating rate, sintering temperature and time, cooling rate and time, cooling rate and furnace atmosphere and other factors must be strictly controlled during sintering. Therefore, choosing a suitable sintering furnace is an important part of powder metallurgy production.
3-1. Classification of sintering furnaces
According to the heating method: it can be divided into fuel heating type and electric heating type:
According to the production method: it can be divided into intermittent type and continuous type:
According to the conveying method of sintered products, continuous sintering furnaces are divided into mesh belt type, bed type, push boat type and stepping type

The powder metallurgy process has the following requirements for the structure of the sintering furnace:
1. There is a sealed furnace shell or muffle sleeve to maintain the reducing atmosphere in the furnace and prevent air from entering
2. There is a stable and reliable material conveying mechanism
3. There is a preheating zone to remove the lubricant and adsorbed gas in the pressed green sheet body
4. A high-temperature sintering belt with sufficient power to ensure that the product has a sufficient sintering and heat preservation process
5. A water jacket cooling belt to prevent oxidation and form the final metallographic structure
6. A device to adjust and control the heating speed, sintering temperature, heat preservation time, cooling speed, etc.
7. A strict temperature control system
8. The temperature distribution of the furnace chamber cross section is uniform
9. The heating element must meet the requirements of the sintering temperature conditions
10. When the electric furnace is opened and closed, and when feeding and discharging, air does not flow back into the furnace, and there should be no water vapor attached to the furnace.

The powder metallurgy sintering electric furnace is generally composed of three parts: preheating belt, sintering belt, and cooling belt. The entire furnace body structure is usually connected vertically with a muffle sleeve to form a whole, and the furnace tube is passed with protective gas (as shown in the figure). The functions of the three belts in the sintering furnace are as follows:
1. Preheating belt: preheating powder compacts and burning off lubricants:
2. Sintering belt: to keep the compacts at the specified temperature for a long enough time to obtain the physical and mechanical properties required for sintered parts;
3. Cooling belt: including precooling belt and water jacket cooling belt. To slowly cool the compacts from high temperature to the recrystallization temperature, and then quickly cool to obtain the final organizational structure of the product. 3-4. Introduction to mesh belt sintering furnace
The mesh belt sintering furnace is the most commonly used sintering furnace for sintering iron-based and copper-based products. The mesh belt is made of heat-resistant alloy. Generally, the maximum sintering temperature is <1150℃. The width of the mesh belt and the size of the furnace are selected according to the size and quantity of the product. The mesh belt is driven by a transmission device to make the ring-shaped mesh belt perform continuous circulation in the furnace to achieve the purpose of material transmission. The product can be loaded in the iron mesh or directly placed on the mesh belt, and move with the mesh belt to make the compacts preheated, sintered, cooled, and finally discharged from the outlet. The specific operation process is as shown in the "3005 Sintering Furnace Operation Standard". The recently used R.B.0. device is a method of accelerating dewaxing by direct combustion of coal gas or liquefied petroleum gas. The use of the RB0 method can shorten the length of the preheating zone, save the equipment floor space, and is conducive to the discharge of lubricant vapor, saving a large amount of protective gas, while greatly increasing the output of the furnace. The specific operation process is as shown in the "R.B.0 Operation Procedure Standard".