Powder and additive
Powder metallurgy and sintering: what goes wrong and where to inspect
Metal powder is compacted in a die and sintered in a furnace to make gears, bushings, sprockets and structural parts with little or no machining.
01How the process works
Mixed powder is pressed to a green compact, sintered below the melting point so particles bond, and optionally sized, oil impregnated, heat treated or machined. Metal injection moulding (MIM) adds a binder for complex small parts.
Density and its distribution determine strength. Cracks in green compacts, density variation and dimensional change in sintering are the main risks, verified by density measurement, sectioning and hardness.
- Powder mixing. Powder and lubricant blended.
- Compaction. Pressed in die to green part.
- Sintering. Furnace with protective atmosphere.
- Sizing and finishing. Calibration, impregnation, heat treatment.
- Inspection. Density, dimensions, hardness.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Low or uneven density | Fill, press settings | Archimedes density, sectioning |
| Green cracks | Ejection, design | Visual, sectioning |
| Dimensional change | Sinter conditions | Gauges, CMM |
| Decarburisation | Atmosphere | Hardness, metallography |
| Blisters | Lubricant burn-off | Visual |
| Low hardness | Heat treatment | Apparent hardness test |
03Where an inspector should look
Before production
- Material grade and density spec
- Tool approval
During production
- Green weight and density
- Sinter atmosphere
After production
- Density, hardness
- Dimensions
- Crack check on samples
04Standards and references
ISO 5755 (sintered materials), ISO 2738 (density), ISO 4498 (apparent hardness), MPIF Standard 35.
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.