The basic procedure in the manufacture of PM parts is:
1. Mix the metal powder or powders with a suitable lubricant.
2. Load the mixture into a die or mould and apply pressure.
This gives what is called a compact which requires only to have sufficient cohesion to enable it to be handled safely and transferred to the next stage. Such compacts are referred to as green, meaning unsintered: hence the terms green density and green strength.
3. Heat the compact , usually in a protective atmosphere, at a temperature below the melting point of the main constituent so that the powder particles weld together and confer sufficient strength to the object for the intended use.
This process is called sintering hence the term sintered parts. In certain cases a minor constituent becomes molten at the sintering temperature in which case the process is referred to as liquid phase sintering.
The amount of liquid phase must be limited so that the part retains its shape. In certain special cases stages 2 and 3 are combined i.e. compaction is done at an elevated temperature such that sintering occurs during the process.
This is termed hot pressing, or pressure sintering. In many cases the sintered part is subjected to additional processing - repressing, plating etc. and these will be dealt with in the appropriate sections below. In certain special cases, e.g. in the manufacture of filter elements from spherical bronze powder, no pressure is used, the powder being placed in a suitably shaped mould in which it is sintered.
This process is known as loose powder sintering.
The object of mixing is to provide a homogeneous mixture and to incorporate the lubricant. Popular lubricants are stearic acid, stearin, metallic stearates, especially zinc stearate, and increasingly, other organic compounds of a waxy nature.
The main function of the lubricant is to reduce the friction between the powder mass and the surfaces of the tools - die walls, core rods, etc. - along which the powder must slide during compaction, thus assisting the achievement of the desired uniformity of density from top to bottom of the compact.
Of equal importance is the fact that the reduction of friction also makes it easier to eject the compact and so minimises the tendency to form cracks.
It has been suggested that an additional function of the lubricant is to help the particles to slide over each other, but it seems doubtful whether this factor is of much significance: - good compacts can be obtained without any admixed lubricant, e.g. using die wall lubrication or isostatic pressing.
Care in the selection of lubricant is necessary, since it may adversely affect both green and sintered strengths especially if any residue is left after the organic part has decomposed.
Over-mixing should be avoided, since this increases the apparent density of the mix. Additionally, over-mixing usually further reduces the green strength of the subsequent compacts probably by completely coating the whole surface of the particles, thereby reducing the area of metal to metal contact on which the green strength depends.
The flow properties also are impaired and good flow is essential for the next step i.e. loading the powder into the die. In the special case of cemented carbides, the mixing process is carried out in a ball mill, one of the objects being to coat the individual particles with the binder metal e.g. cobalt, but as the very fine powders involved do not flow, the mixture is subsequently granulated to form agglomerates.
The mixed powders are pressed to shape in a rigid steel or carbide die under pressures of 150-900 MPa. At this stage, the compacts maintain their shape by virtue of cold-welding of the powder grains within the mass. The compacts must be sufficiently strong to withstand ejection from the die and subsequent handling before sintering.
A critical operation in the process, since the final shape and mechanical properties are essentially determined by the level and uniformity of the as-pressed density. Powders under pressure do not behave as liquids, the pressure is not uniformly transmitted and very little lateral flow takes place within the die.
The attainment of satisfactory densities therefore depends to a large degree on press tool design
Sintering is the means whereby the powder particles are welded together and a strong finished part produced
The sintering of mechanical parts is usually done in a continuous belt furnace - in special cases a vacuum furnace is used.
Even with the best control that is feasible in practice, there will inevitably be some variation in the dimensions of parts produced from a given material in a given die set.
Typically, it is possible for parts 'as-sintered' to be accurate to a tolerance of
-0.0508mm per mm, in the direction at right angles to the pressing -direction, and 0.1016mm per mm parallel to the pressing direction.
Dimensional accuracy can be greatly improved by re-pressing the part after sintering. This operation is called sizing.
Sizing may be done in the die that was used for compacting the powder in those cases where the dimensional change on sintering is controlled at or very near to zero, but commonly separate sizing tools are used.
Re-pressing is used also to imprint or emboss the face(s) of the component in contact with the punch(es), in which case the process is referred to, for obvious reasons, as coining.
During re-pressing the density of the part is generally increased, especially if the as-sintered density is low.
- In certain cases where strength and other mechanical properties are required to be at maximum, re-pressing is used principally to achieve such densification.
- Further improvement is achieved by re-sintering.
Hot Repressing will give even greater densification, with consequent greater improvement in the mechanical properties, but less accurate control of the final dimensions is to be expected.
Hot Isostatic Pressing
HIP is used as a post-sintering operation to eliminate flaws and microporosity in cemented carbides.
It is predicted that as HIP vessels increase in size the economics of the process will be sufficiently attractive to allow its use even on low-alloy steel structural PM parts in order to achieve full-density.
Forging is a comparatively recent technique in which a blank is hot re-pressed in a closed die which significantly changes the shape of the part, and at the same time can give almost complete density and hence mechanical properties approaching or even surpassing those of traditional wrought parts.
Sinter forging is dealt with in more detail in a later section.
An alternative method of improving the strength of inherently porous sintered parts is to fill the surface connected pores with a liquid metal having a lower melting point.
Pressure is not required:
- capillary action is sufficient, provided that the infiltrant wets the metal concerned. It is desirable, however, that the infiltrant have a limited capacity to dissolve the metal being infiltrated otherwise the surface of the part may be eroded.
The process is used quite extensively with ferrous parts using copper as infiltrant but to avoid erosion, an alloy of copper containing iron and manganese, is often used.
Clearly if the molten copper is already saturated with iron its ability to erode the surface is lost.
Other proprietary infiltrant compositions are also on the market.
However, as we saw in the section on dimensional change during sintering, the diffusion of copper into iron can lead to growth.
Infiltration is also used to make composite electrical contact material such as tungsten/copper and molybdenum/silver; the lower melting point metal being melted in contact with an already sintered skeleton of W or Mo.
This term is used for a process analogous to infiltration except that the pores are filled with an organic as opposed to a metallic material.
- An outstanding example is oil-impregnated bearing materials which are dealt with in detail later; but, increasingly, impregnation with thermo-setting or other plastic materials is being done.
- The benefits to be obtained include some increase in mechanical properties, sealing of the pores which may provide pressure-tightness and will also prevent the entry of potentially corrosive electrolyte during a subsequent plating operation.
- Additionally the machining of sintered parts is improved, a feature that is referred to in more detail later.
Although many, perhaps the bulk of sintered structural parts are used in the as-sintered or sintered and sized condition, large quantities of iron-based parts, correctly steels, are supplied in the hardened and tempered conditions.
Conventional hardening processes are used, but because of the porosity inherent in sintered parts, they should not be immersed in corrosive liquids - salt baths, water, or brine - since it is difficult to remove such materials from the pores .
Heating should be in a gas atmosphere followed by oil-quenching.
These restrictions may not apply to very high density parts 7.2 g/cc nor to parts that have been infiltrated.
Carburizing and carbonitriding of PM parts is extensively used, and again gaseous media are indicated.
Because of the porosity of the 'case' is generally deeper and less sharply defined than with fully dense steels, but this is generally an advantage rather than the revers.
A process peculiar to PM parts is steam-treatment which involves exposing the part at a temperature around 500°C to high pressure steam.
This leads to the formation of a layer of magnetite (iron oxide) on all accessible surfaces and a number of desirable property changes result.
- Firstly the corrosion resistance is increased by the filling of some of the porosity,
- and secondly, this reduction in porosity of the surface layer leads also to improved compressive strength.
- Thirdly, the oxide layer significantly increases the surface hardness and more importantly the wear resistance.
Steam-treatment is often followed by dipping in oil which enhances the blue/black appearance and still further increases the corrosion resistance.
The treatment is not generally applicable to hardened and tempered parts because the exposure to the high temperature would undo the hardening.
Heating in air at a lower temperature (200-250°C) can also be used to provide a thin magnetite layer that gives some increase in corrosion resistance, but it is much less effective than steam treatment.
Sintered parts may be plated in much the same way as wrought or cast metals, and copper, nickel, cadmium, zinc, and chromium plating are all used.
- However, it is important to note that low density parts should be sealed - e.g. by resin impregnation, before plating, to prevent the electrolyte from entering the pores and causing corrosion subsequently.
- Parts that have been oil-quenched cannot be plated satisfactorily unless the oil is removed before resin impregnation.
Recent work has shown that it may be possible successfully to plate unimpregnated porous parts with nickel by electroless plating, which process will plate also the surfaces of any of the pores into which the solution penetrates.
A large percentage of hardmetal cutting tool inserts are now coated using chemical vapour deposition (CVD) or physical vapour deposition (PVD).
The lower temperature PVD process also allows steels to be given a wear resistant layer of TiC, TiN, Al2O3 or a combination of these materials and sintered high speed steel tools are also now being coated.
Although a major attraction of PM parts is that they can be produced accurately to the required dimensions, there are limitations to the geometry that can be pressed in rigid dies, and subsequent machining, for example of transverse holes or re-entrants at an angle to the pressing direction is not uncommon.
The existence of porosity alters the machining characteristics and in general tool wear is greater than with the same composition in the fully dense form.
Carbide tools are recommended, and lower cutting speeds may be necessary.
The machinability can be improved by incorporating certain additions in the powder mix - e.g. lead, copper, graphite, sulphur or a metal sulphide such as manganese sulphide, and, as already indicated, by infiltration or resin impregnation.
If these points are borne in mind all the traditional machining processes - turning, milling, drilling, tapping, grinding, etc. - can be done quite readily.
De-burring is done with sintered parts, and is used to remove any 'rag' on edges, resulting from the compacting operation or a machining step.
Tumbling, sometimes in a liquid medium with an abrasive powder, is normally employed. At the same time, brushing and some hardening of the surface layers may occur.
Another class of wrought sintered material that is beginning to make an impact is based on particulate material - powder or chopped ribbon - that has been solidified and cooled at a very high rate such that metastable non-equilibrium microstructures result. They may be microcrystalline or amorphous.
The process is applicable only to certain alloys, and one important feature is that the matrix metal can retain in solid solution a much higher than the equilibrium percentage of the alloying element.
Providing that the densification and mechanical working is carried out at a temperature low enough to avoid destroying the non-equilibrium structure, remarkably enhanced mechanical properties can be achieved.
A major development programme is underway with alloys of aluminium, titanium, and magnesium, the hope being that their use in aircraft structures will significantly reduce the weight and increase the payload.
The high precision forming capability of PM generates components with near net shape , intricate features and close dimensional precision pieces finished without the need of machine work.
By producing parts with a homogeneous structure the PM process to enables manufacturers to make products that are more consistent and predictable in their behaviour across a wide range of applications
Attention must be given to the following design factors in the light of limited lateral flow and also of the necessity of ejecting the green part in the direction of pressing:
- Length-to-Width Ratio. The applied pressure and, therefore, the density decreases over the length of the compact. Double-ended compaction assists in equalising pressure distribution but still leaves a lower density region at the middle section of the part. Ratios of length to width in excess of 3:1 are not recommended.
- Re-entrant Grooves, Reverse Tapers and Lateral Holes cannot be moulded into the compact because of the impossibility of ejection and must, therefore be subsequently machined, although elaborate, sometimes flexible die assemblies have been designed and patented to overcome this limitation.
- Bevels require feather-edged tools, which are fragile and easily fractured; so, if design permits, the bevelled edge of the component should end in a small flat.
- Abrupt changes in section should be avoided since they introduce stress raisers which may lead to crack formation as a result of the stresses induced by the elastic expansion - spring back - that takes place as the compact is ejected from the die.
- To a first approximation the size of part that can be made is a direct function of the capacity of the press available, but the complexity of the part and number of punch motions required also influence the equation.
- These same factors are relevant also to production rates: the simpler the part the easier it is to press at high speed. With such parts rates as high as 1 part per second have been achieved using mechanical presses.
Hydraulic presses enable greater pressures to be used - up to 5,000 tonnes - but speeds are necessarily much lower, 10 parts per minute being a fairly representative high speed for parts of comparatively simple geometry .
Shapes for Correct Design
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.
Some examples of these are outlined below:
Guideline for shapes
Neither the diameter of the holes nor their distance from the edge should be less than 1.5mm. When the upper punch is withdrawn there is no longer any balance of forces acting on the compact: both elastic spring back of the lower punch that forms the hub and internal stress in the compact (still in the die) try to bend the piece: give it an adequate thickness. Avoid specifying narrow and deep splines, requiring the construction of dies with reduced and therefore weak sections. Long and narrow teeth make flow of the powder mix difficult during filling of the die cavity and the die becomes fragile. A fillet radius favours filling of the die cavity and increases the robustness of the part. Rounded corners allow better filling and increase die life. Thin walls having a thickness of less than 0.8mm limit the flow of the powder and should be avoided. Completely conical parts may cause the upper punch (diameter "d") to jam in the die during pressing. Heavy radiused corners would require very narrow, feather edges, punches. A small flat will improve both functioning and the life of the die. Specification of chamfers with angles over than 45° should be avoided. Instead a small flat area should be designed eliminating the feather edge on the punch which will give it longer life. When the chamfers serve to contain the pressing burr on "h" depth of 0.2mm is sufficient.
The flat area may be dispensed with if the parts have low density and the angle is
The depth of the bosses produced with the upper punch must not exceed:
1) For the shape A: p<0.3H
2) For the shape B: p<0.2H
Compact sections indicated by diameters "d" are produced by means of projections or recesses in the upper punch.
When compaction is complete, during the withdrawal of the upper punch the friction between the punch and the compact on the walls of diameter "d" tends to make those sections crumble: tapers are required.
Helical gears with large helix angles create excessive pressure on the teeth of the die both in the pressing and in the ejection phases.
To obtain sufficient uniformity of compact density the s/L ration should not be lower than 1:4. From the point of view of the behaviour in service, in certain cases a 1:6 ration may be allowable. Other less favourable ratios should be considered case by case. In structural parts subject to high stresses the H/D ratio should not be greater than 5, for the same reason.
Teeth with modulus smaller than 0.3 may not guarantee sufficient mechanical strength; additionally the flow of powder during filling is difficult.