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1. What is vacuum hot pressing sintering?
A process in which material forming and sintering are completed at the same time by applying pressure to loose powder placed in a mold of defined shape or heating a powder compact in a vacuum environment.
2.The characteristics of vacuum hot pressing sintering:
During hot pressing, the powder is in a thermoplastic state, the deformation resistance is small, and the required molding pressure is small.
Since heating and pressing are carried out at the same time, it is helpful for the contact, diffusion and flow mass transfer of powder particles, which can reduce the sintering temperature and shorten the sintering time, thereby inhibiting the growth of grains.
Hot-pressing sintering is completed under vacuum conditions, and the sintered body produced has low porosity, high densification, and fine grains, and the product has good mechanical and electrical properties.
3. Hot pressing sintering densification process:
Stage 1: Pore Connectivity
Including the initial and middle stages of sintering. The pressure causes plastic yielding in the particle contact area; power exponential creep occurs in the expanded contact area, causing material migration; volume diffusion and grain boundary diffusion occur between atoms and vacancies; dislocations also cause grain boundary sliding through climbing. The density of the sintered body increases rapidly.
Stage Two: Hole Isolation
Also known as the end of sintering. The pores are located at the grain boundaries, and intragranular micropores are not excluded. At this time, the mechanism of the first stage still exists, but creep and diffusion become the main mechanism of sintering, the material shrinks slowly, and the product has a high degree of densification.
Hot pressing sintering densification process
In the later stage of sintering, the pressure of closed pores increases, which offsets the effect of surface tension, weakens the diffusion effect of grain boundaries, and slows the volume diffusion rate, which makes it difficult to shrink in the later stage. Hot pressing provides an external driving force to compensate for the counteracted surface tension, allowing sintering to continue.
Under the condition of hot pressing, the solid powder may exhibit non-Newtonian fluid properties. When the shear stress exceeds its yield point, it will flow, resulting in an increase in the mass transfer rate, and the closed pores can be eliminated by the viscous or plastic flow of the material.
4.The application of hot pressing sintering
(1)Powder metallurgy industry: mainly used to improve the performance of powder metallurgy products. Applicable materials include metal matrix composite materials and various targets.
Comparison of properties of hot-pressed sintered and cold-pressed sintered copper-based electrical contact composites
The hot-pressing molding pressure is small, the product densification degree is high, and the material properties are far superior to cold-pressing sintering.
When the hot pressing temperature is 750℃, the time is 120min, and the pressure is 28MPa, the density of the target material reaches 95.3%.
SEM photo of Ag-B target cross section
When the hot pressing temperature is 1350~1380℃, the pressure is 25~30MPa, and the time is 1.5~2h, the relative density of the target material is over 99%.
SEM image of cross-section of W-Si alloy target
(2)Ceramic industry: for the preparation of difficult-to-form materials. Such as silicon nitride ceramics, aluminum nitride ceramics and multi-component composite ceramics.
Hot Press Sintering Laminated Composite Ceramic Materials
Using silicon nitride powder as raw material, the hot pressing temperature is 1700~1800℃, and the applied pressure is 20~30MPa.
Hot Press Sintering Silicon Nitride Ceramic Materials
5. Strict requirements of vacuum hot pressing process on equipment
The heating chamber can accommodate hot pressing components and molds.
The degree of vacuum can meet the requirements of hot pressing process.
The temperature can be precisely controlled and automatically adjusted, and the uniformity is good.
High precision of pressure control, centering of upper and lower pressure heads.
The compression molding material is resistant to high temperature, can bear pressure, and does not react with the sintered body.
The equipment is safe and reliable, and the operation is stable.
6. Vacuum hot pressing furnace
(1)Technical characteristics of ACME Vacuum Hot Pressing Furnace
The thermal field of the heating chamber is calculated by the thermal state, and the heating element and the heat insulation layer are designed in a modular manner, so that the furnace temperature uniformity is good.
Equipped with imported pressure sensor and weighing device, the output pressure control precision is high.
Equipped with imported displacement sensor, the displacement stroke control precision is high.
It is driven by servo motor to ensure the stability of the pressure holding process.
(2)Advantages of ACME Vacuum Hot Pressing Furnace
It can realize one-key automatic operation, intelligent and efficient.
Vacuum/atmosphere hot pressing optional, conventional/quick cooling optional, strong equipment applicability.
Professional sealing structure to ensure long-term use of equipment without leakage under vacuum, high temperature and high pressure conditions.
The indenter material is resistant to high temperature and pressure, and the indenter has good neutrality and flatness.