Mechanical Properties

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.

(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).