It is possible to produce components via powder metallurgical processes that have physical properties that approach those of analogous wrought materials. However, to achieve this requires repressing and resintering to be carried out, which in turn add to the cost of production.
Properties of powder metallurgy parts can also be influenced by post heat treatments.
Typical Properties of Powder Metallurgy Components
Typically parts produced by powder metallurgy will have tensile strengths approximately 75% of those of produced/machined from wrought stock.
Doe to the porosity of powder metallurgy parts, the hardness will often appear lower then parts produced from wrought materials. However, the hardness of the actual particles will be harder, and be almost equivalent to wrought materials.
The porosity and relatively lower sectional bonding area also lead to lower ductility’s compared to wrought materials.
Component shape is limited only by the forming process and the associated die.
For parts formed using uniaxial pressing, all parts will have a vertical symmetry. Where the parts have several wall thicknesses and/or steps, the upper and lower punches may consist of numerous individual punches.
Die design and hence shape is a fact that must be kept in mind when designing components to be manufactured by powder metallurgy.
Components with Multiple Diameters
Multiple diameters can be designed into components, however may involve multiple pressing operations to produce. Multiple pressing operations help to ensure consistent density throughout the part.
As a design rule, the number of different diameters should not exceed the number of pressing operations that the press is capable of, or the tooling dictates. Should more diameters be required, these can be machined in at a later stage.
Design Features that are not Powder Metallurgy Friendly
Some component features are not suited to powder metallurgy forming processes, in particular uniaxial pressing. These include:
• Holes at angles (where the angle is not vertical)
• Reverse tapers
• Diamond knurls
The other design attribute that is required, is that the part must be able to be ejected from the mould after pressing.
While these design features may not be possible with uniaxial pressing, they can be produced using more complex forming processes, but will obviously add to production costs.
Wall Thickness Limitations
Very small wall thicknesses can cause problems due to powder filling. A wall thickness of less than about 0.075mm is generally not recommended for this reason, but the minimum practical wall thickness may be dictated by the powder.
Another feature that should be avoided in pressed parts is abrupt changes in diameter. These cause pressure differentials in the pressed item, which in turn induce distortions during sintering.
Pressed parts also have an maximum length to diameter ratio. For hollow cylindrical parts, this ratio is approximately 2.5. For solid cylinders, or where the wall thickness is greater than about 3mm, a length to diameter ratio of 4 can be used.
The size of powder metallurgical components is largely dictated by production hardware.
The minimum size is controlled the precision of dies and the press as well as powder flow. The upper limit of size is however controlled by the amount of pressure the press can deliver.
Powder Metallurgy Defects
Where air has not been able to escape from the die, it will be entrapped within the green component. This usually is a result of trying to press components too fast and results in lamination cracking, in a direction perpendicular to the direction of pressing. It is caused by in inability of adjacent particles to mechanically interlock due to the layer of compressed air.
Blowouts occur when all the entrapped air tries to escape at a single point where the die and punch meet.
Powder metallurgy can be a viable alternative to other fabrication processes as it suited to mass production. While actual throughput rates will be dependent on the equipment capacities, it is not uncommon to be able to produce small parts at a rate of several thousand per hour.
Other factors that may influence the rate at which components can be produced include:
• Powder flow rates i.e. how fast dies can be filled
• The rate at which entrapped air can be expelled from dies