Powder Metallurgy is a continually and rapidly evolving technology embracing most metallic and alloy materials, and a wide variety of shapes. PM is a highly developed method of manufacturing reliable ferrous and non ferrous parts. The European Market alone has an annual turnover of over 12 Billion Euros, including ferrous, non ferrous, magnets, and hardmetals, with annual worldwide metal powder production exceeding 700’000 tonnes.

Created by mixing elemental or alloy powders and compacting the mixture in a die, the resultant shapes are then heated or “sintered” in a controlled atmosphere furnace to bond the particles metallurgically. The high precision forming capability of PM generates components with near net shape, intricate features and good dimensional precision pieces are often finished without the need of machining.Caratula

By producing parts with a homogeneous structure the PM process enables manufacturers to make products that are more consistent and predictable in their behaviour across a wide range of applications. In addition the PM Process has a high degree of flexibility allowing the tailoring of the physical characteristics of a product to suit your specific property and performance requirements. These include:

  • Structural pieces with complex shapes
  • Controlled Porosity
  • Controlled performance
  • Good performance in stress and absorbing of  vibrations
  • Special properties such as hardness and wear resistance
  • Great precision and good surface finish
  • Large series of pieces with narrow tolerances

The unique flexibility of the PM process enables products to be made from materials that are tailored to your specific needs. By using specially selected materials this capability enables refinements to be engineered into the mechanical properties of the part.

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.

The growth of the P/M industry during the past few decades is largely attributable to the cost savings associated with net (or near-net) shape processing compared to other metalworking methods, such as casting or forging. In some cases, the conversion of a cast or wrought component to powder metal provides a cost savings of 40% or higher.
PM typically uses more than 97% of the starting raw material in the finished part and is specially suited to high volume components production requirements.

There are two principal reasons for using a powder metallurgy product:

  1. cost savings compared with alternative processes, and
  2. unique properties attainable only by the PM route

In the automotive sector, which consumes about 80% of structural PM part production, the reason for choosing PM is, in the majority of cases, an economic one.
PM process enables products to be made that are capable of absorbing up to 35% of selected fluids.

Why then is PM more cost effective?

Better material utilisation with close dimensional tolerances. Conventional metal forming or shaping processes, against which PM competes, generally involve significant machining operations from bar stock or from forged or cast blanks.

These machining operations can be costly and are wasteful of material and energy.This is illustrated in the figure below which shows that material utilisation in excess of 95% can be achieved with close dimensional tolerances.

Raw material utilisation and energy requirements of various manufacturing processes.

This is a comparison between various manufacturing processes (Casting, Cold or Warm Extrusion, Hot Drop Forging, and Machining Processes) and PM sintering for a production of notch segments for truck transmission.

The PM process has:
  • the highest raw material utilisation (over 95%)
  • and the lowest energy requirement per Kg of finished part
  • comparing with the other manufacturing processes

economic table1

Energy Savings

The energy savings alone contribute significantly to the economic advantage offered by PM..

An example is given below for a notch segment used in a truck transmission, where PM consumes only around 43% of the energy compared with forging and machining and the number of process steps has been greatly reduced. .

Comparison of the PM Process and Forging and Machining (energy requirements and number of process steps)

This is an example for a notch segment used in a truck transmission, where:

  • PM consumes only around 43% of the energy compared with forging and machining, and
  • the number of process steps has been greatly reduced
PM process:

economic table2

Forging and machining :

economic table22

Cost Comparison between PM and Forging:

economic table3

PM components are designed to meet structural criteria in many applications. It is possible to produce sintered structural or mechanical parts with properties equal to and even superior to those of parts made by more traditional routes. The structural performance of a PM component usually depends on several mechanical properties either alone or in combination.

Unnotched Charpy impact strength figures for many PM materials range from 5 to around 35J.

  • Higher values are found in the case of high relative densities (>93-95%).
  • The ability of a material to resist crack initiation and propagation can be quantified only by adopting fracture mechanics concepts.

Fracture toughness values of PM materials are lower than that of wrought materials; in the case of PM steels with densities in the range 6.5 to 7.0 g/cm³, fracture toughness increases and crack propagation decreases.

  • However, the most important and experimentally verified aspect shows that in cyclic loading near the fatigue limit PM steels with a density of about 7.1 – 7.5 g/cm³ cracks propagate more slowly in comparison with nodular cast iron and wrought heat treatable steels.

This effect is due to the presence of pores which compensate for the weakening of the structure.

When components are designed with low tensile strength in critical features, such as gear teeth, the designer can use conventional design formulas.

  • Where strength of a feature is critical, it is essential to work with the PM manufacturer to optimise the design for manufacturing, determine the strength that can be reasonably specified, and establish the test and inspection procedures for maintaining performance.
  • Improvements in the strength could be achieved by filling the surface connected pores with a liquid metal that has a lower melting point (infiltration)
  • Elimination of flaws- using hot isostatic pressing.

Because tensile strength (Rm) is the oldest property used to characterise the mechanical behaviour of PM materials, and since there is a lot of experience behind the interpretation of its figures, designers have had good reasons for specifying it on their drawings.

  • They are, of course, right if the application actually requires Rm as a mandatory design value, to be directly used in calculations;
  • however, strength and fatigue concepts should not be mixed up: if parts are subject to dynamic loading, the conclusion about the performance of the component in service may only be approximate or even wrong when Rm (alone or with other properties, like elongation and impact strength) is used to establish the suitability of a material for an application where fatigue is present.

In mechanical design yield strength usually is more important than tensile strength: designers must take into account that dimensional stability is guaranteed only when stresses due to service loading are below this threshold value.

Creep is very slow plastic deformation occurring at stress levels below the yield point.

  • PM components may offer economical alternatives to injection-moulded plastics, composites and die-castings designed for temperatures to 400° F (205° C).

For example, a redesign for PM may require fewer or smaller fasteners, require less thread depth in tapped holes, allow threaded fasteners in tapped holes rather than through bolts and nuts, and eliminate the need for steel inserts to distribute concentrated loads.

When hardness is measured on a porous material, the size of the identation is the combination of two factors:

  1. the resistance of the base material to the penetration of the indenter
  2. and the resistance to plastic deformation and fracture of the welds which characterise the sintered structure, stressed beyond its yield point.

Thus compared with dense materials, hardness values of PM materials are always more scattered and scatter increases with decreasing test load.

  • For example, in the case of PM steels because there is a decrease in the number of pores whose effect is averaged during the penetration of the indenter;
  • ISO standards prescribe that the average value be indicated on drawings.

Hardness can also for an indirect and rough evaluation of Rm of PM parts, provided they are as-sintered and homogeneous as regards structure and porosity distribution; it cannot be used in the case of heat treated PM parts.

Elongation values give a good indication of the material’s ductility, ie. its ability to absorb substantial amounts of energy in case of breakage.

  • For PM designers are limited to elongation values ranging between 1 and about 10%.
  • With the same alloy composition a decrease of density also decreases elongation values; the decrease is steeper at a relative density higher than about 90% (transition from closed to open porosity).

MOE is directly affected by material density.

  • Tabulated values foe MOE indicate lower values for PM alloys than for equivalent wrought and cast alloys, higher values than for die cast alloys, and much higher values than plastics and most composites.
  • Higher MOE may allow opportunities to reduce wall thickness and eliminate reinforcing features, such as gussets and ribs, when redesigning plastics, composites and die-castings for PM.

The relation between Young’s modulus (E) and density is approximately linear for relative densities higher than 80%:

  • E value is halved (E ~ 210 Gpa in case of iron) by decreasing the density in the indicated range;
  • the decrease becomes steeper with porosity greater than 20%.

For Poisson’s ration (v) an increase of porosity up to 15-20% for ferrous PM materials involves a gradual decrease of the value of v from ~ 0.29 to ~ 0.26.

Comparison of parts manufacturing processes in terms of shaping capabilities
Property Investment casting MIM
Min. bore diameter 2mm 0.4mm
Max. depth of a 2mm dia blind hole 2mm 20mm
Min. wall thickness 2mm <1mm
Max. wall thickness unlimited 5mm
Tolerance at 14mm dimension +/- 0.2mm +-0.06mm
Surface roughness Ra 5µm 4µm

(Schlieper, EPMA MIM Short Course, Delft, September 1992)

Dent resistance is the ability of a component to withstand impact without permanent deformation.PM materials with dent resistance are not readily handled by standards methods.

This specific property can be increase leading to the reduction in porosity of the surface layer, increasing the surface hardness and the wear resistance.

Could be achieved by the filling of some of the porosity. PM parts working in mechanical devices usually require no protection for corrosion because the presence of lubricants guarantees safe operation. However, where PM parts are exposed to high humidity or corrosive media they are more prone to corrosion than their wrought counterparts. Surface pores increase the surface area exposed to the environment and they can also act as pockets where corrosive media have an enhanced effect.

  • Corrosion resistance can be assured by the use of stainless steels, or by zinc plating followed by chromating of PM steel components.
  • Steam treatment and resin impregnation also make PM parts less prone to corrosion.

The temperature ranges in which most PM components operate have little effect on copper, iron, steel and stainless steel alloys. In those applications where anticipated temperatures are high or low enough,alloy formulation can be modified to improve the properties. The stability of mechanical properties at elevated temperatures often makes PM an economical alternative to injection moulded plastics, composites and die castings, which can experience loss of strength and reduced MOE at approximately the same temperatures that induce creep. When redesigning for PM, design economies discussed previously for MOE, strength and creep may be applicable.

PM alloys exhibit an endurance limit, as do cast and wrought alloys of similar composition. The endurance limit increases with increasing component density.

  • Fatigue performance of a PM component relative to tensile strength is reasonably consistent.
  • An increase in mechanical properties is achieved when pores are filled with an organic rather than metallic material.
  • The operation prevents the entry of potentially corrosive electrolyte during subsequent plating operations.

Considering what has been said before on fracture toughness, fatigue endurance limits increase with increasing density.

  • Fatigue strength of unnotched PM steels is lower than that of wrought steels;
  • however, in the presence of notches (Kt > 2.0) the difference practically disappears, provided that density is 7.1 g/cm³ or more.
  • Moreover, porous materials are less sensitive to an increase of Kt than their dense counterparts.
  • Of course there is an increase of notch sensitivity with increase of hardness.

Fatigue strength is greater under bending than axial loading, due to the difference in the influence of pores, which is less severe in the case of bending. An improvement in strength can be obtained by local densification of the already sintered part, but the highest improvements are obtainable by means of post sintering heat treatments. Carburizing or carbonitriding provide the maximum gains as residual compressive stresses can be introduced.

The mechanical properties of structural PM components are influenced mainly by the residual porosity.

  • Porosity reduces the amount of metal actually present in a given section of the part and when it is loaded the pores themselves can act as stress raisers.

In wrought materials tensile strength can be controlled by the composition of the alloy and the structural changes brought about through heat treatment. Similar strength levels can be achieved in PM components by raising the density level but not adding alloying elements, or by maintaining an acceptable level of porosity but adding alloying elements. Yield strength follows the same pattern. However, these are extremely intricate inter-relations, and it is therefore important to discuss with the PM producer the choice of both material and production cycle. Porosity affects the magnetic properties of PM materials.

  • It must therefore be minimised when high induction (Bmax) is needed;
  • remanence (Br) is affected in the same way.

Pores also limit the mobility of domain walls, i.e. an increase in density gives higher values of permeability (µ) and lowers coercive force (Hc).

  • Other aspects, like grain size, presence of work hardening (annealing may be required), presence of impurities in the lattice (C, N), have the same importance as in the case of dense materials.

Recently, new magnetic powders based on iron where the single particles are coated with an insulating resin have been developed for AC applications (low eddy current losses).

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