The powder forging process in diagram outline:
1. Pre-alloyed powders (180–325 µm) are accurately metered into die cavity (±2g) and compacted into preform shape to 80–85% density.
2. Preforms are heated to forging temperature in a controlled atmosphere (N2/10%H2) either in muffle/mesh belt furnace or an induction heater.
3. Hot preforms are quickly transferred to mechanical forging press (closed dies) and forged/pressed to 98–99.5% theoretical solid density.
- Pre-alloyed atomised powders with low oxygen content and low non-metallic contents are preferred.
- Powder properties such as particular size range, apparent density and flow rate need to be closely controlled for consistency.
- Powder is accurately metered into the cold compaction dies (±2 g).
- Powder is cold compacted at 460–540 MPa to 80–85% density, into a close-to-form preform shape. Production rate is 100–300 components h-1, depending on size of component (0.25–9 kg).
- Preforms are heated in either mesh belt or induction heaters with a protective reducing atmosphere such as N2/10%H2 or endothermic gas, to the required forging temperature; pre-alloyed steel powder, 900–1000˚C; pre-alloyed aluminium powder, 400–450˚C.
- Hot preforms are quickly transferred, usually by mechanical transfer or robots, to a mechanical forging press and are forged (or hot pressed) to final shape. The porosity remaining after cold compaction is eliminated and final densities of 98–99.5% can be achieved.
- The components are either automatically ejected into a quench tank, or are slow cooled again in a protective atmosphere.
- Production rates are 300–600 components h-1.
- Production runs are typically 50,000–200,000 per month.
- A wide range of materials can be powder forged, depending on the final properties required in the component.
- Components used for automatic transmission parts in automobiles are produced from pre-alloyed atomised steel powder with low oxygen and non-metallic contents.
- Automobile connecting rods are manufactured in Fe-0.4 carbon steel, which can be hardened and tempered to give mechanical properties as good as, or better than, wrought alloy.
- Other steel compositions include steels suitable for carburising or through-hardening applications, e.g. Fe-1.8Ni-0.45Mo-0.2Mn-0.35C with a tensile strength of 930 MPa and elongation of 13%.
- Pre-alloyed aluminium alloy powders include the following compositions:
- A major advantage of powder forged materials is that all the mechanical properties are virtually isotropic, i.e. usually no differences in the longitudinal and transverse properties.
- Pre-alloyed powders are expensive. Alternatives include premixed powder and low-cost powders produced from machining swarf or chips.
- Correct preform design is critical to produce a sound powder forged product. Multi-level cold compaction can produce uneven densities in the preform, which might lead to surface cracking on hot repressing or forging.
- Gear teeth and splines can be forged to final shape, eliminating machining operations.
- Features such as undercuts cannot be forged.
- Typical “as-powder forged” dimensional tolerances are ±0.13 mm.
- Powder forgings have better surface finishes than those produced by conventional hot forging techniques.
- Less machining, finishing and minimal material wastage (close-die forging) lead to cost savings over conventional forging techniques, at high production volumes.
This article is a part of Manupedia, a collection of information about some of the processes used to convert materials into useful objects.