In powder metallurgy, the sintering method depends on the product type and required properties.
1. Classification by Raw Material Composition
- Single-component sintering
- Conducted below the melting point of pure metals (e.g., refractory metals, pure iron for soft magnetic materials) or compounds (Al₂O₃, B₄C, BeO, MoSi₂, etc.)
- Performed as solid-phase sintering
- Multi-component solid-phase sintering
- Involves two or more constituents
- Sintered below the melting point of the lowest-melting component
- Many powder sintered alloys belong to this category, e.g., Cu-Ni, Fe-Ni, Cu-Au, W-Mo, Ag-Au, Fe-Cu, W-Ni, Fe-C, Cu-C, Cu-W, Ag-W
- Multi-component liquid-phase sintering
- Conducted above the melting point of the lowest-melting component
- Examples: W-Cu-Ni, W-C powder metallurgy, WC-Co, TiC-Ni, Fe-Cu (Cu>10%), Fe-Ni-Al, Cu-Pb, Cu-Sn, Fe-Cu (Cu<10%)
2. Classification by Feeding Method
- Continuous sintering
- The furnace has sections for debinding, preheating, sintering, and cooling
- Material passes continuously or in stages through each section
- High production efficiency, suitable for mass production
- Common feeding methods: push-rod, roller-bed, mesh-belt
- Batch (intermittent) sintering
- Parts remain stationary in the furnace
- Temperature control devices perform preheating, heating, and cooling cycles
- Allows customization of sintering schedules based on material performance
- Lower efficiency, suitable for small batches or single pieces
- Common furnaces: bell-type, box-type
3. Other Classification Methods
- By sintering temperature and liquid phase presence: solid-phase vs. liquid-phase sintering
- By sintering temperature: medium-temperature (1100–1700°C) vs. high-temperature sintering
- By sintering atmosphere:
- Air sintering
- Hydrogen-protected sintering (e.g., molybdenum wire furnace, stainless steel tubes, hydrogen furnace)
- Vacuum sintering
- Advanced techniques: ultra-high-pressure sintering, activated hot-press sintering
Factors Affecting Sintered Product Quality
The performance of sintered parts depends on powder characteristics, forming conditions, and sintering conditions.
- Sintering temperature and time
- Temperature and duration affect porosity, density, strength, and hardness
- Too high a temperature or too long a time can over-sinter and reduce performance
- Too low a temperature or too short a time can cause under-sintering, lowering material properties
- Sintering atmosphere
- Common atmospheres: reducing gas, vacuum, hydrogen
- Atmosphere affects sintered part properties:
- Reducing atmosphere prevents oxidation and reduces surface oxides
- E.g., iron- and copper-based products often use furnace gas or decomposed ammonia
- Hard alloys and stainless steel often use pure hydrogen
- Active metals, refractory metals (Be, Ti, Zr, Ta), TiC-containing hard alloys, and stainless steel can use vacuum sintering
- Vacuum avoids harmful gas components (H₂O, O₂, H₂)
- Can reduce sintering temperature by 100–150°C
- Reducing atmosphere prevents oxidation and reduces surface oxides