Powder metallurgy processes products Powder Metallurgy (PM) has been used since the 1920s to produce structural components, self-lubricating bearings and cutting tools. We review the PM process and current developments in production technology and applications.

Powder Metallurgy (PM) has been used since the 1920s to produce structural components, self-lubricating bearings and cutting tools. We review the PM process and current developments in production technology and applications.

The PM process typically entails compressing a powder within a container to produce a compact with sufficient cohesion to enable it to be handled safely; then heating this in a protective atmosphere, to a temperature below the melting point of the main constituent.This sintering process welds the individual particles together, creating engineering components with sufficient strength for their intended use.

Wrought products also can be produced from powder and some scientifically exciting developments of great industrial potential have taken place.One specialised variant on this process - production of Soft Magnetic Composite (SMC) components - entails a heating step, which cures a resin binder, added to insulate individual iron particles (no sintering occurs).

In some cases, a minor constituent becomes molten at the sintering temperature, and the process is referred to as liquid phase sintering. The amount of liquid phase must be limited so that the part retains its shape.

Sometimes compaction is done at the elevated temperature; this is termed hot pressing, or pressure sintering.Often the sintered part is subjected to additional processing - repressing, plating; and in special cases, eg manufacturing filter elements from spherical bronze powder, no pressure is used - the powder is heated in a shaped mould (loose powder sintering).

Why make parts from powder?The main reasons for using PM are cost savings compared with alternative processes, or to get properties unattainable by other methods. The former is the driving force for structural/mechanical parts applications - mainly iron-based parts, but with significant quantities of copper, brass, bronze and aluminium parts - and rarer metals such as beryllium and titanium.

Historically, mechanical properties have been no better than for equivalent parts made by forging or machining from wrought bar; but parts are entirely fit for purpose, may be more dimensionally accurate than forgings - and are very attractive economically.However, it's now possible to produce sintered parts with properties equal to or better than parts made by more traditional routes.

This is down to developments over the past decade in the materials used, and in modifications to the production process at the compaction stage (Warm Compaction, High Pressure Cold Compaction, High Velocity Compaction); during Sintering (Activated Sintering, Ferrite Phase Sintering); or the post-sintering densification processes (Powder Forging, Surface Cold Rolling).PM's potential for developing special properties is manifested in several ways:* Porous Materials: ie parts having a significant, controlled porosity, which serves a useful purpose.

The chief products are filters and oil-retaining ('self-lubricating') bearings.* High melting point metals: the refractory metals - tungsten, molybdenum, tantalum - are difficult to produce by melting and casting and may be brittle in the cast state.

A sintered powder compact with relative density below 90% can be mechanically deformed at elevated temperature, gradually developing a microstructure with preferred orientation to give the now dense material useful ductility even at ambient temperatures.* Composite Materials: Ie two or more metals, which are insoluble, even in the liquid state, or mixtures of metals with non-metallic substances.

* Electrical contact materials: eg copper/tungsten, silvercadmium oxide.* Hard Metals: cemented carbides for cutting tools and wear parts.

Tungsten carbide bonded with cobalt was the first of this class of material and still has most of the market. However other carbides - plus nitrides, carbonitrides and borides - are being used in increasing quantities; and substitutes for cobalt (scarce, expensive) have been developed eg Ni, Ni-Co, Ni-Cr, nickel-based superalloys and complex steels.

* Friction materials: for brake linings and clutch facings in which abrasive and other non-metallic materials are embedded in a copper or other metallic matrix.* Diamond cutting tools: especially grinding wheels, in which small diamonds are uniformly dispersed in a metal matrix.

* Dispersion- strengthened materials: wrought products containing finely dispersed non-metallic phases have been developed and put into service. Referred to as ODS materials if the strengthening particles are oxides, strength, especially at elevated temperatures is superior to cast and wrought metals of similar basic composition.

* Soft magnetic composite (SMC) materials: consisting of iron powder particles insulated from each other by a cured resin binder.* Magnetic components: economic and technical advantages have been found in the production of components for magnetic applications.

* High-duty alloys: high speed steels and superalloys based on Ni and/or Co can be PM-processed to deliver superior properties to cast/forged products. Powder is compacted into a blank or billet which is then subjected to forging or extrusion, followed by forming in traditional ways.

The PM route delivers a higher yield of usable material; and a finer, more uniform microstructure that confers improved mechanical properties - and, for cutting tools and wear parts, longer life.PM has also allowed the development of new types of materials with micro-crystalline or even amorphous (glass-like) structures produced by cooling droplets of molten metal at high rates.

The final consolidated product has very high strength, ductility and thermal stability. Higher performance hard (or permanent) magnetic materials are also in this category; processing such materials by PM techniques brings superior magnetic properties.