Quality Criteria

Role of materials, porosity, structure and shape at the in manufacturing.

Quality systems for PM must take into account some unique technical aspects connected to the fact that the raw material is in powder form and the powders intrinsic residual porosity.

The following are some of the technical aspects that should be addressed:

1. Role of materials

Raw materials are offered in standard grades.

The supplier, in the specification certifies the range and the capability of the material in the customer's process.

In spite of that, some of the ranges may prove to be insufficiently tight, especially at the pressing stage, and may compel the supplier to manage the variability accordingly.

2. Role of porosity and structure

In the case of rigid die forming, the density distribution within the components, especially those having an intricate shape, represents an important source of variability for the local properties and the dimensional tolerances of the parts.

In order to achieve the homogeneity in the local properties and dimensional tolerances of the parts the following aspects must be considered:

  • Uniformity of powder distribution in the die cavity during filling
  • The way in which the press drives and controls the movements of the press tool set
  • The mechanical efficiency of both the press and the tooling
  • Residual porosity, which can be managed by the aspects outlined above, may have several implications:
    • Penetration of agents is strongly promoted by the interconnection of the pores
    • Decrease to the precision level - this can be compensated by using softer powders
    • Control of the evolution of the structure during thermal treatments; but only in relation to its kinetics.
  • The use of secondary operations

3. Role of shapes

The dimensional tolerances of the product will depend largely
on the following aspects:

  • The structural requirements of the tool sets
  • The elastic deformation of the press and the punches

Intricate and asymmetric shapes are possible with PM, however it is important to note that they involve an increased variability during sintering as well as during subsequent thermal treatment, if specified.

Role of materials, structure and shape at the design stage.

Quality systems for PM must take into account some unique technical aspects connected to the fact that the raw material is in powder form and the powders intrinsic residual porosity.

The following are some of the technical aspects that should be addressed:

1. Role of materials

In relation to the physical-mechanical properties of the end product, the composition represents a fundamental item.

Owing to the complex mechanisms in play during the sintering operation, the choice of the alloy influences the dimensional stability of the product; therefore the correct choice of composition is vital.

There are numerous formulations that, other characteristics being equal, can offer precise components.

The most common compositions and their relevant properties are listed in the ISO Standards, in particular ISO 5755.

2. Role of porosity and structure

In exactly the same way as with parts produced by traditional processes, the formulation of the alloy and thermo-mechanical operations determine the structure obtainable in sintered parts.

However in relation to porosity and structure two important factors have to be considered:

Powder Metallurgy enables the porosity level remaining in the component at the end of the cycle to be controlled.

The distribution of alloying elements in the structure, and the way in which they could concentrate in the vicinity of the pores, thus improving the local strength and compensating in some degree for the weakening effect of the voids.

3. Role of shapes

Designers should take into account design rules, and the experience of the supplier, and discuss with them the producibility of parts with boundary shapes and the dimensional tolerances compatible with a given shape-material.

Shape designs depend on the forming method that has been chosen, as with other processes, this may have some restrictions in terms of dimensions, weight, and profile freedom.

This section will discuss the main areas of PM in relation to the fundamental and optional tests necessary to ensure the quality of PM parts and their corresponding ISO standards.

The additional tests available are also listed, however these are used predominately in order to confirm and correlate the results of other tests; and to investigate problems.

Mechanical Components

Many of the tests used to check the Mechanical composition of the products are the same as those used for other production methods.

Fundamental Tests

  • Density ISO 2738
  • Hardness
  • Specific Mechanical Tests
  • Dimensional Tolerances

Hardness is an extremely useful test for the following reasons:

The amount of indirect information supplied (correlation with the degree of sintering, with mechanical strength characteristics, with the wear resistance of heat treated parts, etc.)

Its simplicity and rapidity, without the need to destroy the part.

Optional Tests

  • Impregnating oil content ISO 2737
  • Integrity
  • Fatigue Strength*
  • Wear Resistance*
  • Corrosion Resistance**
  • Roughness

*Fatigue and wear tests are undertaken only in exceptional cases, e.g. when changes of material or of the production process are being considered.

**Corrosion resistance may be included in the functional characteristics of the part and as it is influenced by the sintering and subsequent secondary operations, it may be one of the tests to put in the specification.

Additional Tests

  • Metallographic Structure
  • Chemical Composition

 

Bearings

The performance of a bearing is assessed by a series of tests of different technical importance. Before making these tests the costs, the availability of instrumentation and the manpower necessary for a complete inspection procedure, must be considered.

Fundamental Tests

  • Density ISO 2738
  • Oil content ISO 2737
  • Radial crushing strength - ISO 2739
  • Dimensional tolerances
  • Nature of impregnating oil
  • Chemical analyse
  • Alloying elements

Although chemical analysis is one of the fundamental tests it is not often carried out; both for logistical reasons e.g. chemical laboratory services are not commonly available, and because of the cost.

* Alloying elements - Elements to check:
Bronze-based materials Tin and possibly graphitic carbon.
Iron-based materials Copper and graphitic carbon (when expected)
Bronze-iron-based material Copper, tin, graphitic carbon and combined carbon

Optional Tests

  • Microstructure
  • Fluid permeability
  • Surface roughness
  • Hardness
  • Additional Tests
  • Fluid permeability ISO 4003/4022
  • Hardness ISO 4498/1

 

Filters

The fundamental tests to verify the functional performance of filters are: Geometric characteristics .

Apart from the warning that compressed air instruments must not to be used (the porosity will give a wrong reading), dimensional checks on filter are carried out by traditional methods.

In general the test are limited to cross section and height.

Fluid permeability

The fluid permeability is one of the fundamental parameters of filters operation.

The determination is carried out according to the ISO standard 4022, and with simple instruments

Filtration threshold (maximum pore size)

There is a direct correlation between fluid permeability and the particle size range of the original powders.

The filtration threshold is measured by the bubble test method, following the procedures of ISO standard 4003.

Fundamental Tests

  • Geometric characteristics
  • Fluid permeability - ISO 4022
  • Filtration threshold - ISO 4003
  • Optional Test
  • Microstructure
  • Fluid permeability
  • Surface roughness
  • Hardness

Additional Tests

  • Chemical composition
    Determination of chemical composition is an additional test to be used only if the onset of corrosion during operation gives rise to doubt.
    The elements to check are:
    • Type of filter Element to check
    • Bronze Filters TIN
    • Nickel Silver Filters Nickel and Zinc
    • Stainless Steel Filters Chromium, Nickel, Molybdenum, and if present Carbon
    Controlled porosity is also employed in the manufacture of metal filters and diaphragms .They have the advantage over their ceramic counterparts of higher strength and resistance to mechanical and thermal shock.
  • The close control over the pore size and permeability is achieved by the use of powders having a narrow range of particle size.
  • Perhaps the most commonly used filter elements are made of bronze (89/11 Cu/Sn), and spherical powder are used.
  • The filter profile is formed by a loose packing of the powder in the mould and the inherently poor compressibility of spheres is no disadvantage. Where products are required to have limited or localised porosity, conventional pressing is necessary and irregularly shaped particles are more suitable.
  • Metal filters are available in a wide range of materials including copper,nickel,bronze,stainless steel and 'Monel',
  • and are widely used for the filtration of fuel oils, chemical solutions and emulsions.
  • They are also efficient in separating liquids of varying surface tension, and have been successfully applied to jet engine fuels, where water is removed at the same time as the fuel is filtered.
  • Similar devices are widely used for sound damping on air compressors and the likes.

 

Components for Magnetic Applications

The shape of the piece influences the testing of the magnetic properties. There are no universally valid standards and therefore methods and devices must be agreed from time to time by the supplier and customer.

Testing of components produced with soft magnetic alloys

Tests on sample devices :

  • The testing of sintered parts for magnetic uses may be carried out on devices specifically designed for that purpose.
  • These devices reproduce the service conditions considered most important for the parts under examinations and allow evaluation relative to the results.

Test on the sample device and contemporaneously, on test pieces:

To limit the number of tests required in a laboratory investigation it is possible to sinter a certain number of test pieces along with the production parts.

  • These should be of the same original powder, have a constant and known density, generally equal to the average density of the parts being examined.
  • Inspection of the magnetic characteristics of the test pieces may be carried out in parallel with the testing of the parts in the sample device and its possible to separate the influence of the material and the sintering cycle from that of pressing in the analysis of the results.

Testing of components produced with hard magnetic alloys

  • Usually it is possible to test the magnets directly when the shape, direction of magnetisation and dimensions are compatible with the apparatus and measuring instruments.
  • The magnets are compared with standards magnets.
  • This type of test supplies a comprehensive verdict on the acceptability or not of magnets, but does not permit the discovery of the causes of any defects.
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