Powder metallurgy is a manufacturing route in which metal powders are produced, formed into a required shape and consolidated to produce an engineering component. Sintering is the thermal stage of the process, during which bonding develops between the powder particles.
The process is widely used for components that can be produced efficiently from powders, for materials that are difficult to manufacture by conventional melting and forming routes, and where controlled porosity or particular material properties are required.
A conventional powder metallurgy manufacturing route can be divided into three principal stages:
Additional operations may include powder blending, lubricant addition, sizing, repressing, machining, heat treatment, impregnation, infiltration and surface finishing depending on the component and its required properties.
Sintering is a thermal process in which a compacted or otherwise formed powder body is heated so that bonding develops between adjacent particles. Atomic diffusion and other mass-transport mechanisms progressively reduce particle interfaces and strengthen the component.
In conventional solid-state sintering, the principal material remains below its melting temperature. Some powder metallurgy processes use liquid-phase sintering, in which a liquid phase is intentionally formed during the sintering cycle.
The component before sintering is commonly referred to as a green compact. It must possess sufficient green strength to survive handling and transfer to the sintering equipment without damage.
Powder metallurgy also introduces manufacturing constraints which must be considered during component design.
The characteristics of the starting powder strongly influence powder handling, compaction and the properties of the finished component. Important characteristics include particle size, particle-size distribution, particle shape, surface condition and chemical composition.
Metal powders can be produced by a number of mechanical, physical and chemical processes. Methods include:
Atomisation is an important industrial method of producing metal powder. A stream of molten metal is broken into droplets, commonly using high velocity gas or liquid jets. The droplets solidify to form powder particles which are subsequently collected and classified.
The atomising medium and process conditions influence particle shape, size distribution, surface condition and oxidation.
Some metal powders can be produced by chemically reducing metal oxides or other compounds. Depending on the material and process, reducing agents can include gases such as hydrogen or carbon-based reducing systems.
The resulting product may have a porous or sponge-like structure which is subsequently crushed, milled and classified to produce the required powder.
Certain metals can be processed through volatile metal compounds and subsequently decomposed to produce fine powders. Carbonyl processes are important examples and have historically been used for metals including iron and nickel.
Chemical precipitation and related processes can also be used where the required material and powder characteristics make them appropriate.
Electrolytic methods deposit metal from a suitable electrolyte. The deposited material is removed and subsequently processed into powder.
The method can provide high-purity material and distinctive particle structures, although the economics depend strongly on the metal and required powder specification.
Before forming, powders may be blended to obtain the required composition and a consistent distribution of constituents. Different particle sizes or alloying additions can be combined, and lubricants may be introduced to assist compaction and removal of the compact from the tooling.
Powder flow is important because the die cavity must fill consistently. Variation in filling can lead to variation in mass, density and ultimately the dimensions and properties of the finished components.
A number of methods can be used to form powders before or during sintering. The appropriate method depends on the material, component geometry, required density and production quantity.
Important methods include:
Where a highly porous structure is required, powder can be placed into a suitable mould and sintered without conventional high-pressure compaction. The resulting porosity can be useful in applications such as filters and other permeable components.
The mould and component geometry must permit the sintered part to be removed, and dimensional changes during the thermal cycle must be taken into account.
Other powder-based forming routes can include slurry and slip-forming techniques, particularly for ceramic and related materials.
Cold pressing is one of the principal methods used to manufacture conventional powder metallurgy components. A measured quantity of powder is introduced into a rigid die and compressed between punches.
Compaction brings the particles into close contact, rearranges and deforms them, and develops sufficient green strength for the compact to be ejected from the die and handled before sintering.
The required compaction pressure depends strongly on the powder material, particle characteristics, lubricant, component geometry and required green density.
Isostatic pressing applies pressure to the powder from multiple directions rather than principally along the axis of a rigid die. This can improve density distribution and allows some geometries which are difficult to produce using conventional uniaxial die pressing.
Cold isostatic pressing and hot isostatic pressing are distinct processes and should not be treated as interchangeable. Hot isostatic pressing combines elevated temperature with isostatic gas pressure and can be used for powder consolidation and the reduction of internal porosity in suitable components.
Hot pressing combines applied pressure and elevated temperature during consolidation. Heating assists densification while the applied load promotes intimate contact and deformation of the powder particles.
The tooling material, atmosphere, pressure and temperature must be selected for the powder being processed. Vacuum or controlled atmospheres may be required where oxidation or contamination would adversely affect the material.
After compaction, the green components are heated through a controlled thermal cycle. Where lubricants, binders or other volatile constituents have been used, an initial heating stage may be required to remove them before the principal sintering stage.
The compact is then heated to the required sintering temperature and held under controlled conditions to develop bonding between the particles. The temperature and holding time depend on the material system, component size, required properties and the particular sintering process.
The furnace atmosphere is also important. Vacuum, inert, reducing or other controlled atmospheres may be selected according to the material and the chemical reactions which must be prevented or encouraged.
Sintering develops bonds between neighbouring particles and can substantially increase the strength of the green compact. Depending on the material and process, it can also change:
Dimensional change is particularly important for production tooling. Shrinkage or growth must be characterised for the particular powder and process rather than assumed from nominal component dimensions.
Porosity is not necessarily a defect in a powder metallurgy component. Its significance depends on the intended function of the part.
For structural components, reducing porosity will often improve mechanical properties. In other applications, a controlled interconnected pore structure is deliberately retained. Porous metallic filters and oil-impregnated bearings are important examples.
A sintered component may be used directly or may undergo additional manufacturing operations to achieve the required dimensions, properties or surface condition.
Typical secondary operations can include:
Components intended for conventional powder compaction should be designed with the forming and ejection process in mind. A shape that is simple to machine is not necessarily simple to compact from powder.
Powder metallurgy competes with casting, forging, machining and other forming processes rather than universally replacing them.
It can be particularly attractive where material utilisation, high production quantity, controlled porosity or near-net-shape manufacture provide an advantage. Conventional machining or forging may be preferable for low quantities, very large components, geometries unsuitable for compaction, or applications requiring properties associated with fully dense wrought material.
The following external resources were included on the original RoyMech page and provide additional information relating to metal powders, powder metallurgy and manufacturing processes.