Powder metallurgy (PM) is a term covering a wide range of ways in which materials or components are made from metal powders. PM processes can avoid, or greatly reduce, the need to use metal removal processes, thereby drastically reducing yield losses in manufacture and often resulting in lower costs.
- History and capabilities
- Powder production techniques
- Sponge iron process
- Centrifugal disintegration
- Other techniques
- Powder compaction
- Die pressing
- Design considerations
- Isostatic pressing
- Isostatic powder compacting
- Geometrical possibilities
- Tool style
- Hot isostatic pressing
- Cold isostatic pressing
- Continuous powder processing
- Shock (dynamic) consolidation
- Electric current assisted sintering
- Special products
Powder metallurgy is also used to make unique materials impossible to get from melting or forming in other ways. A very important product of this type is tungsten carbide (WC). WC is used to cut and form other metals and is made from WC particles bonded with cobalt. It is very widely used in industry for tools of many types and globally ~50,000t/yr is made by PM. Other products include sintered filters, porous oil-impregnated bearings, electrical contacts and diamond tools.
The PM press and sinter process generally consists of three basic steps: powder blending (pulverisation), die compaction, and sintering. Compaction is generally performed at room temperature, and the elevated-temperature process of sintering is usually conducted at atmospheric pressure and under carefully controlled atmosphere composition. Optional secondary processing such as coining or heat treatment often follows to obtain special properties or enhanced precision.
One of the older such methods, and still one used to make around 1Mt/yr of structural components of iron-based alloys, is the process of blending fine (<180 microns) metal (normally iron) powders with additives such as a lubricant wax, carbon, copper, and/or nickel, pressing them into a die of the desired shape, and then heating the compressed material ("green part") in a controlled atmosphere to bond the material by sintering. This produces precise parts, normally very close to the die dimensions, but with 5-15% porosity, and thus sub-wrought steel properties. There are several other PM processes which have been developed over the last fifty years. These include:
History and capabilities
The history of powder metallurgy and the art of metal and ceramic sintering are intimately related to each other. Sintering involves the production of a hard solid metal or ceramic piece from a starting powder. The ancient Incas made jewelry and other artifacts from precious metal powders, mass manufacturing of PM products did not begin until the mid- or late- 19th century". In these early manufacturing operations, iron was extracted by hand from metal sponge following reduction and was then reintroduced as a powder for final melting or sintering.
A much wider range of products can be obtained from powder processes than from direct alloying of fused materials. In melting operations the "phase rule" applies to all pure and combined elements and strictly dictates the distribution of liquid and solid phases which can exist for specific compositions. In addition, whole body melting of starting materials is required for alloying, thus imposing unwelcome chemical, thermal, and containment constraints on manufacturing. Unfortunately, the handling of aluminium/iron powders poses major problems. Other substances that are especially reactive with atmospheric oxygen, such as titanium, are sinterable in special atmospheres or with temporary coatings.
In powder metallurgy or ceramics it is possible to fabricate components which otherwise would decompose or disintegrate. All considerations of solid-liquid phase changes can be ignored, so powder processes are more flexible than casting, extrusion, or forging techniques. Controllable characteristics of products prepared using various powder technologies include mechanical, magnetic, and other unconventional properties of such materials as porous solids, aggregates, and intermetallic compounds. Competitive characteristics of manufacturing processing (e.g. tool wear, complexity, or vendor options) also may be closely controlled.
Powder production techniques
Any fusible material can be atomized. Several techniques have been developed which permit large production rates of powdered particles, often with considerable control over the size ranges of the final grain population. Powders may be prepared by crushing, grinding, chemical reactions, or electrolytic deposition.
Powders of the elements titanium, vanadium, thorium, niobium, tantalum, calcium, and uranium have been produced by high-temperature reduction of the corresponding nitrides and carbides. Iron, nickel, uranium, and beryllium submicrometre powders are obtained by reducing metallic oxalates and formates. Exceedingly fine particles also have been prepared by directing a stream of molten metal through a high-temperature plasma jet or flame, atomizing the material. Various chemical and flame associated powdering processes are adopted in part to prevent serious degradation of particle surfaces by atmospheric oxygen.
In tonnage terms, the production of iron powders for PM structural part production dwarfs the production of all of the non-ferrous metal powders combined. Virtually all iron powders are produced by one of two processes: the sponge iron process or water atomization.
Sponge iron process
The longest established of these processes is the sponge iron process, the leading example of a family of processes involving solid state reduction of an oxide. In the process, selected magnetite (Fe3O4) ore is mixed with coke and lime and placed in a silicon carbide retort. The filled retort is then heated in a kiln, where the reduction process leaves an iron “cake” and a slag. In subsequent steps, the retort is emptied, the reduced iron sponge is separated from the slag and is crushed and annealed.
The resultant powder is highly irregular in particle shape, therefore ensuring good “green strength” so that die-pressed compacts can be readily handled prior to sintering, and each particle contains internal pores (hence the term “sponge”) so that the good green strength is available at low compacted density levels.
Sponge iron provides the feedstock for all iron-based self-lubricating bearings, and still accounts for around 30% of iron powder usage in PM structural parts.
Atomization is accomplished by forcing a molten metal stream through an orifice at moderate pressures. A gas is introduced into the metal stream just before it leaves the nozzle, serving to create turbulence as the entrained gas expands (due to heating) and exits into a large collection volume exterior to the orifice. The collection volume is filled with gas to promote further turbulence of the molten metal jet. Air and powder streams are segregated using gravity or cyclonic separation. Most atomized powders are annealed, which helps reduce the oxide and carbon content. The water atomized particles are smaller, cleaner, and nonporous and have a greater breadth of size, which allows better compacting. The particles produced through this method are normally of spherical or pear shape. Usually, they also carry a layer of oxide over them.
There are three types of atomization:
Simple atomization techniques are available in which liquid metal is forced through an orifice at a sufficiently high velocity to ensure turbulent flow. The usual performance index used is the Reynolds number R = fvd/n, where f = fluid density, v = velocity of the exit stream, d = diameter of the opening, and n = absolute viscosity. At low R the liquid jet oscillates, but at higher velocities the stream becomes turbulent and breaks into droplets. Pumping energy is applied to droplet formation with very low efficiency (on the order of 1%) and control over the size distribution of the metal particles produced is rather poor. Other techniques such as nozzle vibration, nozzle asymmetry, multiple impinging streams, or molten-metal injection into ambient gas are all available to increase atomization efficiency, produce finer grains, and to narrow the particle size distribution. Unfortunately, it is difficult to eject metals through orifices smaller than a few millimeters in diameter, which in practice limits the minimum size of powder grains to approximately 10 μm. Atomization also produces a wide spectrum of particle sizes, necessitating downstream classification by screening and remelting a significant fraction of the grain boundary.
Centrifugal disintegration of molten particles offers one way around these problems. Extensive experience is available with iron, steel, and aluminium. Metal to be powdered is formed into a rod which is introduced into a chamber through a rapidly rotating spindle. Opposite the spindle tip is an electrode from which an arc is established which heats the metal rod. As the tip material fuses, the rapid rod rotation throws off tiny melt droplets which solidify before hitting the chamber walls. A circulating gas sweeps particles from the chamber. Similar techniques could be employed in space or on the Moon. The chamber wall could be rotated to force new powders into remote collection vessels, and the electrode could be replaced by a solar mirror focused at the end of the rod.
An alternative approach capable of producing a very narrow distribution of grain sizes but with low throughput consists of a rapidly spinning bowl heated to well above the melting point of the material to be powdered. Liquid metal, introduced onto the surface of the basin near the center at flow rates adjusted to permit a thin metal film to skim evenly up the walls and over the edge, breaks into droplets, each approximately the thickness of the film.
Another powder-production technique involves a thin jet of liquid metal intersected by high-speed streams of atomized water which break the jet into drops and cool the powder before it reaches the bottom of the bin. In subsequent operations the powder is dried. This is called water atomization. The advantage of water atomization is that metal solidifies faster than by gas atomization since the heat capacity of water is some magnitudes higher than gases. Since the solidification rate is inversely proportional to the particle size, smaller particles can be made using water atomization. The smaller the particles, the more homogeneous the micro structure will be. Notice that particles will have a more irregular shape and the particle size distribution will be wider. In addition, some surface contamination can occur by oxidation skin formation. Powder can be reduced by some kind of pre-consolidation treatment, such as annealing used for the manufacture of ceramic tools.
Powder compaction is the process of compacting metal powder in a die through the application of high pressures. Typically the tools are held in the vertical orientation with the punch tool forming the bottom of the cavity. The powder is then compacted into a shape and then ejected from the die cavity. In a number of these applications the parts may require very little additional work for their intended use; making for very cost efficient manufacturing.
The density of the compacted powder increases with the amount of pressure applied. Typical pressures range from 80 psi to 1000 psi (0.5 MPa to 7 MPa), pressures from 1000 psi to 1,000,000 psi have been obtained. Pressure of 10 tons/in² to 50 tons/in² (150 MPa to 700 MPa) are commonly used for metal powder compaction. To attain the same compression ratio across a component with more than one level or height, it is necessary to work with multiple lower punches. A cylindrical workpiece is made by single-level tooling. A more complex shape can be made by the common multiple-level tooling.
Production rates of 15 to 30 parts per minute are common.
There are four major classes of tool styles: single-action compaction, used for thin, flat components; opposed double-action with two punch motions, which accommodates thicker components; double-action with floating die; and double action withdrawal die. Double action classes give much better density distribution than single action. Tooling must be designed so that it will withstand the extreme pressure without deforming or bending. Tools must be made from materials that are polished and wear-resistant.
Better workpiece materials can be obtained by repressing and re-sintering. Here is a table of some of the obtainable properties.
The dominant technology for the forming of products from powder materials, in terms of both tonnage quantities and numbers of parts produced, is die pressing. There are mechanical, servo-electrical and hydraulic presses available in the market, whereby the biggest powder throughput is processed by hydraulic presses. This forming technology involves a production cycle comprising:
- Filling a die cavity with a known volume of the powder feedstock, delivered from a fill shoe.
- Compaction of the powder within the die with punches to form the compact. Generally, compaction pressure is applied through punches from both ends of the toolset in order to reduce the level of density gradient within the compact.
- Ejection of the compact from the die, using the lower punch(es) withdrawal from the die.
- Removal of the compact from the upper face of the die using the fill shoe in the fill stage of the next cycle, or an automation system or robot.
This cycle offers a readily automated and high production rate process.
Probably the most basic consideration is being able to remove the part from the die after it is pressed, along with avoiding sharp corners in the design. Keeping the maximum surface area below 20 square inches (0.013 m2) and the height-to-diameter ratio below 7-to-1 is recommended. Along with having walls thicker than 0.08 inches (2.0 mm) and keeping the adjacent wall thickness ratios below 2.5-to-1.
One of the major advantages of this process is its ability to produce complex geometries. Parts with undercuts and threads require a secondary machining operation. Typical part sizes range from 0.1 square inches (0.65 cm2) to 20 square inches (130 cm2). in area and from 0.1 to 4 inches (0.25 to 10.16 cm) in length. However, it is possible to produce parts that are less than 0.1 square inches (0.65 cm2) and larger than 25 square inches (160 cm2). in area and from a fraction of an inch (2.54 cm) to approximately 8 inches (20 cm) in length.
In some pressing operations, such as hot isostatic pressing (HIP) compact formation and sintering occur simultaneously. This procedure, together with explosion-driven compressive techniques, is used extensively in the production of high-temperature and high-strength parts such as turbine blades for jet engines. In most applications of powder metallurgy the compact is hot-pressed, heated to a temperature above which the materials cannot remain work-hardened. Hot pressing lowers the pressures required to reduce porosity and speeds welding and grain deformation processes. It also permits better dimensional control of the product, lessens sensitivity to physical characteristics of starting materials, and allows powder to be compressed to higher densities than with cold pressing, resulting in higher strength. Negative aspects of hot pressing include shorter die life, slower throughput because of powder heating, and the frequent necessity for protective atmospheres during forming and cooling stages.
Isostatic powder compacting
Isostatic powder compacting is a mass-conserving shaping process. Fine metal particles are placed into a flexible mould and then high gas or fluid pressure is applied to the mould. The resulting article is then sintered in a furnace which increases the strength of the part by bonding the metal particles. This manufacturing process produces very little scrap metal and can be used to make many different shapes. The tolerances that this process can achieve are very precise, ranging from +/- 0.008 inches (0.2 mm) for axial dimensions and +/- 0.020 inches (0.5 mm) for radial dimensions. This is the most efficient type of powder compacting (the following subcategories are also from this reference). This operation is generally applicable on small production quantities, as it is more costly to run due to its slow operating speed and the need for expendable tooling.
Compacting pressures range from 15,000 psi (100,000 kPa) to 40,000 psi (280,000 kPa) for most metals and approximately 2,000 psi (14,000 kPa) to 10,000 psi (69,000 kPa) for non-metals. The density of isostatic compacted parts is 5% to 10% higher than with other powder metallurgy processes.
There are many types of equipment used in isostatic powder compacting. There is the mold, which is flexible, a pressure mold that contains the mold, and the machine delivering the pressure. There are also devices to control the amount of pressure and how long the pressure is held. The machines need to apply pressures from 15,000 to 40,000 pounds per square inch (100 to 280 MPa) for metals.
Typical workpiece sizes range from 0.25 in (6.35 mm) to 0.75 in (19.05 mm) thick and 0.5 in (12.70 mm) to 10 in (254 mm) long. It is possible to compact workpieces that are between 0.0625 in (1.59 mm) and 5 in (127 mm) thick and 0.0625 in (1.59 mm) to 40 in (1,016 mm) long.
Isostatic tools are available in three styles, free mold (wet-bag), coarse mold (damp-bag) and fixed mold (dry-bag). The free mold style is the traditional style of isostatic compaction and is not generally used for high production work. In free mold tooling the mold is removed and filled outside the canister. Damp bag is where the mold is located in the canister, yet filled outside. In fixed mold tooling, the mold is contained within the canister, which facilitates automation of the process.
Hot isostatic pressing
Hot isostatic pressing (HIP) compresses and sinters the part simultaneously by applying heat ranging from 900 °F (480 °C) to 2250 °F (1230 °C). Argon gas is the most common gas used in HIP because it is an inert gas, thus prevents chemical reactions during the operation.
Cold isostatic pressing
Cold isostatic pressing (CIP) uses fluid as a means of applying pressure to the mold at room temperature. After removal the part still needs to be sintered. It is helpful in distributing pressure uniformly over the compaction material contained in a rubber bag.
Advantages over standard powder compaction are the possibility of thinner walls and larger workpieces. Height to diameter ratio has no limitation. No specific limitations exist in wall thickness variations, undercuts, reliefs, threads, and cross holes. No lubricants are need for isostatic powder compaction. The minimum wall thickness is 0.05 inches (1.27 mm) and the product can have a weight between 40 and 300 pounds (18 and 136 kg). There is 25 to 45% shrinkage of the powder after compacting.
Solid state sintering is the process of taking metal in the form of a powder and placing it into a mold or die. Once compacted into the mold the material is placed under a high heat for a long period of time. Under heat, bonding takes place between the porous aggregate particles and once cooled the powder has bonded to form a solid piece.
Sintering can be considered to proceed in three stages. During the first, neck growth proceeds rapidly but powder particles remain discrete. During the second, most densification occurs, the structure recrystallizes and particles diffuse into each other. During the third, isolated pores tend to become spheroidal and densification continues at a much lower rate. The words "solid state" in solid state sintering simply refer to the state the material is in when it bonds, solid meaning the material was not turned molten to bond together as alloys are formed.
One recently developed technique for high-speed sintering involves passing high electric current through a powder to preferentially heat the asperities. Most of the energy serves to melt that portion of the compact where migration is desirable for densification; comparatively little energy is absorbed by the bulk materials and forming machinery. Naturally, this technique is not applicable to electrically insulating powders.
To allow efficient stacking of product in the furnace during sintering and prevent parts sticking together, many manufacturers separate ware using ceramic powder separator sheets. These sheets are available in various materials such as alumina, zirconia, and magnesia. They are also available in fine, medium, and coarse particle sizes. By matching the material and particle size to the wares being sintered, surface damage and contamination can be reduced, while maximizing furnace loading per batch.
Continuous powder processing
The phrase "continuous process" should be used only to describe modes of manufacturing which could be extended indefinitely in time. Normally, however, the term refers to processes whose products are much longer in one physical dimension than in the other two. Compression, rolling, and extrusion are the most common examples.
In a simple compression process, powder flows from a bin onto a two-walled channel and is repeatedly compressed vertically by a horizontally stationary punch. After stripping the compress from the conveyor, the compacted mass is introduced into a sintering furnace. An even easier approach is to spray powder onto a moving belt and sinter it without compression. However, good methods for stripping cold-pressed materials from moving belts are hard to find. One alternative that avoids the belt-stripping difficulty altogether is the manufacture of metal sheets using opposed hydraulic rams, although weakness lines across the sheet may arise during successive press operations.
Powders can also be rolled to produce sheets. The powdered metal is fed into a two-high rolling mill, and is compacted into strip form at up to 100 feet per minute (0.5 m/s). The strip is then sintered and subjected to another rolling and further sintering. Rolling is commonly used to produce sheet metal for electrical and electronic components, as well as coins. Considerable work also has been done on rolling multiple layers of different materials simultaneously into sheets.
Extrusion processes are of two general types. In one type, the powder is mixed with a binder or plasticizer at room temperature; in the other, the powder is extruded at elevated temperatures without fortification. Extrusions with binders are used extensively in the preparation of tungsten-carbide composites. Tubes, complex sections, and spiral drill shapes are manufactured in extended lengths and diameters varying from 0.5–300 millimetres (0.020–11.811 in). Hard metal wires of 0.1 millimetres (0.0039 in) diameter have been drawn from powder stock. At the opposite extreme, large extrusions on a tonnage basis may be feasible.
There appears to be no limitation to the variety of metals and alloys that can be extruded, provided the temperatures and pressures involved are within the capabilities of die materials. Extrusion lengths may range from 3–30 m and diameters from 0.2–1 m. Modern presses are largely automatic and operate at high speeds (on the order of m/s).
Shock (dynamic) consolidation
Shock consolidation, or dynamic consolidation, is an experimental technique of consolidating powders using high pressure shock waves. These are commonly produced by impacting the workpiece with an explosively accelerated plate. Despite being researched for a long time, the technique still has some problems in controlability and uniformity. However, it offers some valuable potential advantages. As an example, consolidation occurs so rapidly that metastable microstructures may be retained.
Electric current assisted sintering
These techniques employ electric currents to drive or enhance sintering. Through a combination of electric currents and mechanical pressure powders sinter more rapidly thereby reducing the sintering time compared to conventional thermal solutions. The techniques can be divided into two main categories: resistance sintering, which incorporates spark plasma sintering and hot pressing; and electric discharge sintering, such as capacitor discharge sintering or its derivative, electro sinter forging. Resistance sintering techniques are consolidation methods based on temperature, where heating of the mold and of the powders is accomplished through electric currents, usually with a characteristic processing time of 15 to 30 minutes. On the other hand, electric discharge sintering methods rely on high-density currents (from 0.1 to 1 kA/mm^2) to directly sinter electrically conductive powders, with a characteristic time between tens of microseconds to hundreds of milliseconds.
Many special products are possible with powder metallurgy technology. A nonexhaustive list includes Al2O3 whiskers coated with very thin oxide layers for improved refraction; iron compacts with Al2O3 coatings for improved high-temperature creep strength; light bulb filaments made with powder technology; linings for friction brakes; metal glasses for high-strength films and ribbons; heat shields for spacecraft reentry into Earth's atmosphere; electrical contacts for handling large current flows; magnets; microwave ferrites; filters for gases; and bearings which can be infiltrated with lubricants.
Extremely thin films and tiny spheres exhibit high strength. One application of this observation is to coat brittle materials in whisker form with a submicrometre film of much softer metal (e.g. cobalt-coated tungsten). The surface strain of the thin layer places the harder metal under compression, so that when the entire composite is sintered the rupture strength increases markedly. With this method, strengths on the order of 2.8 GPa versus 550 MPa have been observed for, respectively, coated (25% cobalt) and uncoated tungsten carbides.
The special materials and processes used in powder metallurgy can pose hazards to life and property. The high surface-area-to-volume ratio of the powders can increase their chemical reactivity in biological exposures (for example, inhalation or ingestion), and increases the risk of dust explosions. Materials considered relatively benign in bulk can pose special toxicological risks when in a finely divided form.