The hot isostatic pressing process (HIP) is to put the powder compact or the powder in the package into a high-pressure container that subjects the powder to high temperature and isostatic pressure and sintered into a dense part. This method can use metal or ceramic packages (mild steel, Ni, Mo, glass, etc.), and use nitrogen or argon as the pressurizing medium to thermally densify the material. After half a century of development and improvement, it has produced nuclear fuel rods, powder metallurgy high-temperature alloy turbine disks, tungsten nozzles, ceramics, and metal-based composites using hot isostatic pressing technology. It has a wide range of applications in the preparation of cermets, cemented carbides, refractory metal products and their compounds, powder metallurgy products, metal-based composites, functional gradient materials, etc.
Densification process of hot isostatic pressing
We can roughly divide the densification process of hot isostatic pressing into three stages.
Particle approach and rearrangement stage
Before heating and pressurization, there are a lot of gaps between loose powder particles. At the same time, due to the irregular shape of powder particles and the uneven surface, they are mostly in point contact. Therefore, the number of other particles in direct contact with one particle (particle coordination number) is very small. When applying external force to the powder, under the action of compressive stress, the powder body may undergo the following situations:
Randomly stacked powders will translate or rotate and approach each other.
Some powders are squeezed into adjacent gaps.
Some larger bridge holes will collapse, etc.
As a result of the above changes, the adjacent coordination number of particles is significantly increased. As a result, the gaps in the powder body are greatly reduced and the relative density is rapidly increased.
Plastic deformation stage
The first stage of densification greatly increases the density of the powder. The contact area between particles increases dramatically, and the particles are in conflict or wedged with each other. To continue to densify the powder, the external pressure can be increased to increase the compressive stress on the contact surface of the particles. Increase the temperature to reduce the critical shear stress that is not conducive to the plastic flow of the powder. If the increase the pressure and temperature at the same time, it will be more effective for continued densification. When the compressive stress on the powder exceeds its yield shear stress, the particles will undergo plastic deformation in the form of slip.
Diffusion creep stage
After a large amount of plastic flow of powder particles occurs, the relative density of the powder body quickly approaches the theoretical density value. At this time, the powder particles are connected as a whole. The remaining pores are no longer connected but are dispersed in the powder matrix, like bubbles suspended in a solid medium. These pores initially exist in irregular narrow shapes. But under the action of surface tension, they are spheroidized into a round shape.
The volume fraction of the remaining pores will continue to decrease during the spheroidization process. The contact area between particles increases to such an extent that the effective compressive stress on the powder body no longer exceeds its critical shear stress. At this time, the mechanism of plastic deformation caused by the sliding of a large number of atomic groups will no longer play a major role. It mainly completes the densification process by the diffusion creep of a single atom or a hole. Therefore, the densification process of the entire powder body slows down and finally approaches the maximum terminal density value.
It is worth noting that the above three stages are not completely separate. In the hot isostatic pressing process, they often work simultaneously to promote the densification of the powder body. It is just that when the powder body is in different shrinkage stages, different densification processes play a leading role.
Hot isostatic pressing device
The hot isostatic pressing device is mainly composed of pressure vessels, gas boosting equipment, heating furnaces, and control systems. The pressure vessel part mainly includes sealing rings, containers, top covers, and bottom covers. The gas boosting equipment mainly includes gas compressors, filters, check valves, vent valves, and pressure gauges. The heating furnace mainly includes heaters and thermocouples. The control system consists of power controllers, temperature controllers, and pressure controllers.
Today’s hot isostatic pressing devices are mainly large-scale, high-temperature, and use diversified atmospheres. The HIP heating furnace mainly adopts three heating methods: radiation heating, natural convection heating, and forced convection heating. Its heating element materials are mainly Ni-Cr, Fe-Cr-A1, Pt, Mo, and C.
Hot Isostatic Pressing Process
Carry out the powder filling generally in a vacuum or inert gas atmosphere. To increase the density of the filled powder, it should vibrate the package continuously. To obtain uniform shrinkage, the density of the filled powder should not be less than 68% of the theoretical density. After filling, the package should be evacuated and sealed. This is because the hot isostatic pressing process consolidates the molded powder and material through pressure difference. Once the package is not sealed tightly, the gas medium enters the package, which will affect the sintering of the powder. In addition, vacuum sealing can remove air and water, prevent oxidation reactions, and hinder the sintering process.
Four different cycle modes
Among them, the temperature increase and pressure increase, pressure maintenance, temperature decrease, and pressure reduction stages are called high temperature and high pressure cycles. According to the order of temperature increase and pressure increase. It can be divided into four different cycle modes, each with its advantages.
- Cold loading cycle.
Pressure increase precedes temperature increase, and both reach their respective peaks at the same time. This method is conducive to better control of the geometry of thin-walled metal packages. - Hot loading cycle.
Pressure increases after the temperature reaches a certain value. This method is particularly important when using glass packages. Premature pressure increase will cause brittle glass to break. - Post-heat cycle.
This method is similar to the cold loading cycle, and the pressure is increased before the temperature is increased. The difference is that the temperature is increased only after the pressure reaches the peak, and the pressure is maintained. This method promotes the recrystallization of powder particles through plastic deformation, thereby reducing the molding temperature. - The most effective cycle.
The temperature and pressure are increased at the same time, thereby shortening the hot isostatic pressing time and obtaining the highest efficiency.
Characteristics of hot isostatic pressing
- By using hot isostatic pressing sintering, the densification of ceramic materials can be completed at a much lower temperature than pressureless sintering or hot pressing sintering. It can effectively inhibit many adverse reactions or changes in materials at high temperatures. Such as abnormal grain growth and high-temperature decomposition.
- Through the hot isostatic pressing sintering process, a dense ceramic sintered body with uniform microstructure and almost no pores can be prepared under the condition of reducing or even without sintering additives, significantly improving various properties of the material.
- Through the hot isostatic pressing post-treatment process, the residual pores in the sintered body can be reduced or even eliminated, and the surface cracks can be healed. Thereby improving the density and strength of the ceramic material.
- The hot isostatic pressing process can accurately control the size and shape of the product without the use of expensive diamond cutting processing. Under ideal conditions, the product has no shape change.
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