In the application dealt with in the other sections, the powder metallurgy process is used to produce materials having special characteristics that either cannot be achieved in any other way or that can be achieved only with great difficulty.
In the case of structural parts the justification for using PM is, in many cases, quite different. No special technical merit is claimed for the product in comparison with similar parts made by alternative processes-casting, forging, stamping etc. - in fact the mechanical properties are normally inferior.
The justification is economic, i.e. there is a cost saving. At first sight this may seem difficult to understand. The bulk of structural parts is based on iron, and iron powders are significantly more expensive than iron in the solid state.
The cost savings that enable this initial disadvantage to be overcome are basically twofold:
- (a) sintered parts can be produced directly to the specified dimensions, markedly reducing the amount of machining required or eliminating it completely;
- (b) in consequence of (a) material usage is very much better, scrap being almost negligible.
The saving in machining costs as a proportion of the total cost is likely to be greater the smaller the part, and until recently the bulk of parts produced from powder were small, less than 1lb in weight.
Another factor is that the size and, therefore, the cost of presses increases with the size of the part being made.
For this reason the compressibility of the powder mix is of major importance, and has militated against the use of prealloyed steel powders which, inevitably, require greatly increased pressures to make compacts of the required density.
When strengths greater than those obtainable with 'pure' iron powder are required, it is customary to add powder of alloying elements to the mix.
The choice is restricted to elements that do not oxidise in commercial protective atmospheres, and in practice copper is the most widely used in amounts up to 10%.
- Copper has the advantage of melting at a temperature below the sintering temperature used for iron (1120°C) and, therefore, alloying is rapid.
- Nickel and molybdenum can also be used, but higher sintering temperatures are necessary, involving more expensive furnaces and higher operating costs.
- The cheapest strengthening element for iron is, of course, carbon, but its use in sintered parts depends on the ability to control the composition, and since carbon reacts not only with oxygen but also with hydrogen, special atmospheres having a carbon potential in equilibrium with the steel are necessary.
Copper and copper plus carbon remain the most widely used additions.
Infiltration also is used to increase strength, the most common infiltrant being copper with a small percentage each of iron and manganese to avoid erosion. It is not necessary to infiltrate the whole part; quite often local infiltration of highly stressed areas is sufficient .
A description of some of the main PM materials below will provide some general guidelines as to alloy types.
Carbon Steels : Carbon steels with up to 0.8% carbon contents are produced and the microstructure comprises ferrite and pearlite.
- These steels may be used for lightly stressed parts at low densities and for moderately stressed parts not requiring high levels of toughness when sintered densities reach 6.9 - 7.3 g/cm3.
- They may be hardened or case hardened and also steam treated to increase strength and hardness, but with some loss of toughness.
Copper Steels : Whereas copper has a detrimental effect in wrought steels, it has a great strengthening effect in sintered steels and is usually used from 1 to 4% with a carbon content up to 1%.
- They have properties similar to carbon steels but with higher levels of strength and hardness.
- Similar heat treatment processes may also be used to improve strength, hardness and fatigue properties.
Phosphorus Steels : Small additions of phosphorus to iron acts as a sintering activator and allows the production of higher density parts with good ductility.
- Carbon may be present not usually greater than 0.6% to improve strength and dimensional accuracy.
- Phosphorus alloyed steels may be used as alternatives to copper steels when toughness combined with moderate strength is required, or copper may be added (up to 4%) to further improve properties.
Nickel Steels :
- As in wrought steels nickel is effective in increasing the toughness of sintered parts when used in the range of 2 to 6% and with carbon content up to 1%.
- Mechanical properties may be substantially improved by heat treatment.
Molybdenum Steels :
- Molybdenum when dissolved in iron does not impair the compressibility of steel powders, and makes it possible to compact fully prealloyed powders;
- hardening or case hardening (including plasma nitriding) may be used to give excellent properties and good dimensional control in sintering.
Copper-nickel, molybdenum-nickel, and copper-nickel-molybdenum Steels : Copper and especially molybdenum in association with nickel make dimensional control easier during sintering; Mo ensures a good response to hardening of parts with comparatively thick sections.
- These PM steels are usually produced to a minimum of 6.8 g/cm3 sintered density and carbon content is 0.5-0.6%.
- After heat treatment properties such as tensile strength put sintered parts made from these steels on the top of the range and they are used in applications demanding severe service conditions.
Stainless Steels : Whilst the majority of the PM steels mentioned above are made from mixtures of elemental powders or diffusion alloyed powders, PM stainless steel parts are normally made from prealloyed powders in order to guarantee the homogeneity of the microstructure - an essential requirement for adequate corrosion resistance.
- All of the standard AISI grades (304L, 316L, 410L, 430L ) are available as PM alloys and mechanical properties are satisfactory for most applications.
- Austenitic PM steels are characterised by good ductility and toughness.
(ISO and national standards provide comprehensive data on the mechanical and physical properties of PM steels, and additional data on properties is also available from powder and component producers.)
Non Ferrous Parts
The production of structural parts in non-ferrous materials is on a much smaller scale but significant quantities of copper, brass, nickel silver, and bronze parts are made, and the production of aluminium from powder is now developing .
BRONZE. In the case of bronze there is an important technical advantage.
- Because of the wide freezing range of copper/tin alloys it is difficult to avoid serious inter-dendritic porosity in bronze castings, and pressure/tight pump bodies and other hydraulic fittings are difficult to cast without a significant percentage of rejects.
By using the powder route this problem is overcome, since although there is normally a percentage of porosity in the sintered part, this is in the form of separate small holes rather than the interconnected porosity found in castings. Of course, we are talking of densities much higher than those of self-lubricating bearings.
TITANIUM ALLOYS. Made by PM are also increasing in importance with cold and hot isostatic pressing being the preferred method of consolidation.
Ti-6Al-4V alloy powders produced by blending of elemental powders or prealloyed powders produced by the plasma rotating electrode process (PREP) are used for valves, valve balls, and fittings for the chemical industry; surgical implants; fasteners for the aerospace industry; airframe components; missile casings and fins; axial impellers; compressor blades; and prototype connecting rods for the automotive industry.
A recent important development is a family of PM titanium alloy matrix composites incorporating TiC ceramic particles to improve high temperature strength, increase hardness and improve modulus of elasticity.
ALLUMINIUM. PM components made from aluminium alloy powders offer a combination of light weight (a third of that of steel), corrosion resistance, good mechanical and fatigue properties, high thermal and electrical conductivity, good machinability and the ability to be finished by a variety of processes.
- PM aluminium alloys have the same compositions as their wrought counterparts
- The powder exhibits excellent compressibility reaching 90% of theoretical density at a relatively low compaction pressure of 12 tsi and 95% at 25 tsi.
- Sintering temperatures are significantly lower than for ferrous PM parts.
- Depending on the composition of the alloy, sintering temperature can vary between 580 - 625C using low dew point nitrogen-based atmosphere.
- Very close temperature control (± 5C) is required to minimise dimensional scatter. Sizing or repressing after sintering is practically unavoidable.
- Surface finishing can include etching to achieve textures, electroplating, anodizing and painting.
Aluminium PM parts offer mechanical properties ranging from 150 to 300 MPa ultimate tensile strength which about 50% of the value of wrought aluminium alloys of the same composition.
However, further secondary processing such as hot or cold forming to attain full density can bring UTS and fatigue limit values up to wrought levels. Examples of applications include: bearing caps, gears, stators and rotors in water pumps, belt pulleys, etc.
Powder forging produces fully dense PM steel parts , such as the automotive connecting rod used in BMW V8 engines.
The production of traditional PM parts has been expanding at a significantly faster rate than the general growth of engineering production and when it was originally developed in the 1970s powder forging or sinter forging was expected to alter fundamentally the scale of the PM industry.
PROCESS In this process, a powder blank is pressed to a simple shape halfway between that of a forging billet and the required finished part.
This compact, referred to as a preform, is sintered and then hot forged to finished size and shape in a closed die.
The amount of deformation involved is sufficient to give a final density very closely approaching that of the solid metal , and consequently, the mechanical properties are comparable with those of material forged from wrought bar.
ADVANTAGES Indeed they may be superior in some respects because of the freedom of the sinter forged part from directionality, the greater homogeneity as regards composition, and a finer microstructure, as well as the absence of internal discontinuities resulting from ingot defects that are possible in forgings made from cast metal.
An additional advantage is the dimensional consistency achievable in consequence of the accurate metering of the quantity of powder used.
LIMITATIONS There are limitations to the steel compositions that can be successfully produced on a commercial scale.
- Steels containing readily oxidisable elements such as chromium and manganese - which happens to be also the cheaper strengthening elements - cannot easily be used, but special compositions , generally containing as alloying elements, nickel and molybdenum, the oxides of which are reduced in sintering atmospheres, have been developed.
- Powder forged steel parts can be heat treated in the same manner as wrought steels.
Production costs in powder forging are generally higher than in conventional casting or forging due mainly to the high price of the starting material and tooling. However, the high precision achieved in powder forging results in considerable savings on machining costs and hence savings on investments in machining operations.
This has particularly proved to be the case for powder forged connecting rods which are gaining in popularity all over the world due to their improved dimensional accuracy, higher dynamic properties, smoother running in the engine, and significant cost savings.
Many companies in North America, Japan and Europe now have large powder forging installations mainly to produce parts for the automotive industry . Such parts can have inside and outside spline forms, cam forms, and other forms that require extensive machining. In addition to the well known connecting rod other applications include bearing races, torque convertor hubs, and gears.
A new application for aluminium powders in structural shapes involves the production of lightweight foam panels or components .
The process involves mixing aluminium or aluminium alloy powders with a powder foaming agent which is a gas releasing substance.
This mixture may be compacted by various powder consolidation processes such as extrusion, hot pressing, or hot isostatic pressing, to produce a semi-product having a gas tight metallic matrix.
The semi-product can then be roll clad between conventional aluminium sheet to make sandwich panels with a foamable core layer, and due to the metallic bonding between the individual layers the product can be shaped, for example, by stamping.
- The key to the forming of the metallic foam is the heating of the metallic matrix to its melting point or above, but below that of the cladding material.
- As the core metal or alloy melts the foaming agent decomposes and releases a quantity of gas which produces large voids in the material.
- These voids remain in the metal after cooling to produce the closed porosity foam.
- This new manufacturing process is being scaled up to manufacture automotive chassis panels giving lighter weight and greater stiffness than steel sheet.
Returning to ferrous parts, the limitations as to geometry can, in some cases, be overcome by making two parts and joining them, e.g. by copper brazing or by projection welding.
- In this way, undercuts and transverse projections can be incorporated.
Another device for achieving similar results is the use of 'split-die' i.e. a die which is in two pieces with the junction at right angles to the pressing direction.
- The compact is removed by separating the two halves and taking the compact out in the middle.