Powder Metallurgy
Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape or form (compacting), and then heating the compressed material in a controlled atmosphere to bond the material (sintering). The powder metallurgy process generally consists of four basic steps: (1) powder manufacture, (2) powder blending,(3) compacting, (4) sintering. Compacting is generally performed at room temperature, and the elevated-temperature process of sintering is usually conducted at atmospheric pressure. Optional secondary processing often follows to obtain special properties or enhanced precision. 1] Two main techniques used to form and consolidate the powder are sintering and metal injection molding. Recent developments have made it possible to use rapid manufacturing techniques which use the metal powder for the products. Because with this technique the powder is melted and not sintered, better mechanical strength can be accomplished. History and capabilities The history of powder metallurgy (PM) and the art of metals and ceramics sintering are intimately related to each other. Sintering involves the production of a hard solid metal or ceramic piece from a starting powder. While a crude form of iron powder metallurgy existed in Egypt as early as 3000 B. C, and the ancient Incas made jewelry and other artifacts from precious metal powders, mass manufacturing of P/M products did not begin until the mid-or late- 19th century". [2] 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. [3] Other substances that are especially reactive with atmospheric oxygen, such as tin, are sinterable in special atmospheres or with temporary coatings. 4] 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,[5] 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 regulated.
Powder Metallurgy products are today used in a wide range of industries, from automotive and aerospace applications to power tools and household appliances. Each year the international PM awards highlight the developing capabilities of the technology. [6] 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. This increases the strength of the part by bonding the metal particles.
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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. )[7] 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. oda[8] 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. Equipment There are many types of equipment used in Powder Compacting. There is the mold, which is flexible, a pressure mold that the mold is in, and the machine delivering the pressure. There are also controlling devices to control the amount of pressure and how long the pressure is held for.
The machines need to apply anywhere from 15,000 psi to 40,000 psi for metals. Geometrical Possibilities 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. Tool style 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. Design Considerations 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. 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 comminution, 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, simultaneously atomizing and comminuting the material. On Earth various chemical- and flame-associated powdering processes are adopted in part to prevent serious degradation of particle surfaces by atmospheric oxygen. Atomization 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. On Earth, air and powder streams are segregated using gravity or cyclonic separation. Most atomised 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.
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 ? m. 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 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,[9] 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. [10] Other techniques 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 atomisation.
The advantage is that metal solidifies faster than by gas atomization since the heat capacity of water is some magnitudes higher, mainly a result of higher density. Since the solidification rate is inversely proportional to the particle size smaller particles can be made using water atomisation. 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 as annealing. sed for ceramic tool Powder compaction [pic] [pic] Rhodium metal: powder, pressed pellet (3*105 psi), remelted 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. [7] 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 is directly proportional to the amount of pressure applied. Typical pressures range from 80 psi to 1000 psi, pressures from 1000 psi to 1,000,000 psi have been obtained. Pressure of 10 tons/in? to 50 tons/in? 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 minutes 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. Introduction | |[pic] | | | |Powder metallurgy uses sintering process for making various parts out of metal powder. The metal powder is compacted by placing in a closed| |metal cavity (the die) under pressure. This compacted material is placed in an oven and sintered in a controlled atmosphere at high | |temperatures and the metal powders coalesce and form a solid.
A second pressing operation, repressing, can be done prior to sintering to | |improve the compaction and the material properties. | |[pic] | |The properties of this solid are similar to cast or wrought materials of similar composition. Porosity can be adjusted by the amount of | |compaction. Usually single pressed products have high tensile strength but low elongation. These properties can be improved by repressing | |as in the following table. |Material | |Tensile | |MPa | |(psi) | |Tensile | |as Percent of Wrought Iron Tensile | |Elongation | |in 50 mm | |(2 in) | |Elongation | |as Percent of Wrought Iron Elongation | | | |Wrought Iron, Hot Rolled | |331 | |(48,000) |100 % | |30 % | |100 % | | | |Powder Metal, 84 % density | |214 | |(31,000) | |65 % | |2 % | |6% | | | |Powder Metal, repressed, 95 % density | |283 | |(41,000) | |85 % | |25 % | |83 % | | | |Powder metallurgy is useful in making parts that have irregular curves, or recesses that are hard to machine. It is suitable for high | |volume production with very little wastage of material. Secondary machining is virtually eliminated. |Typical parts that can be made with this process include cams, ratchets, sprockets, pawls, sintered bronze and iron bearings (impregnated | |with oil) and carbide tool tips. | | | |Design Considerations | |[pic] | | | |• | |Part must be so designed to allow for easy ejection from the die. Sidewalls should be perpendicular; hole axes should be parallel to the | |direction of opening and closing of the die. | | |• | |Holes, even complicated profiles, are permissible in the direction of compressing. The minimum hole diameter is 1. 5 mm (0. 060 in). | | | |• | |The wall thickness should be compatible with the process typically 1. 5 mm (0. 060 in) minimum. Length to thickness ratio can be upto 18 | |maximum-this is to ensure that tooling is robust.
However, wall thicknesses do not have to be uniform, unlike other processes, which offers| |the designer a great amount of flexibility in designing the parts. | | | |• | |Undercuts are not acceptable, so designs have to be modified to work around this limitation. Threads for screws cannot be made and have to | |be machined later. | | | |• | |Drafts are usually not desirable except for recesses formed by a punch making a blind hole.
In such a case a 2-degree draft is recommended. | |Note that the requirement of no draft is more relaxed compared to other forming processes such as casting, molding etc. | | | |• | |Tolerances are 0. 3 % on dimensions. If repressing is done, the tolerances can be as good as 0. 1 %. Repressing, however, increases the cost | |of the product. | | | Powder Metallurgy - Processing | | | |Topics Covered | |Materials | | |Powder Consolidation | | |Cold Uniaxial Pressing | | |Cold Isostatic Pressing | | |Sintering | | |Hot Isostatic Pressing | | |Hot Forging (Powder Forging) | | |Metal Injection Moulding (MIM) | | |Materials | |The majority of the structural components produced by fixed die pressing are iron based.
The powders are elemental, pre-alloyed, or partially | |alloyed. Elemental powders, such as iron and copper, are easy to compress to relatively high densities, produce pressed compacts with adequate| |strength for handling during sintering, but do not produce very high strength sintered parts. | |Pre-alloyed powders are harder, less compressible and hence require higher pressing loads to produce high density compacts. However, they are | |capable of producing high strength sintered materials. Pre-alloying is also used when the production of a homogeneous material from elemental | |powders requires very high temperatures and long sintering times.
The best examples are the stainless steels, whose chromium and nickel | |contents have to be pre-alloyed to allow economic production by powder metallurgy. | |Partially alloyed powders are a compromise approach. Elemental powders, e. g. Iron with 2 wt. % Copper, are mixed to produce an homogeneous | |blend which is then partially sintered to attach the copper particles to the iron particles without producing a fully diffused powder but | |retaining the powder form. In this way the compressibilities of the separate powders in the blend are maintained and the blend will not | |segregate during transportation and use. | |A similar technique is to ‘glue’ the small percentage of alloying element onto the iron powder.
This ‘glueing’ technique is successfully used | |to introduce carbon into the blends, a technique which prevents carbon segregation and dusting, producing so-called ‘clean’ powders. | |Powder Consolidation | |Components or articles are produced by forming a mass of powder into a shape, then consolidating to form inter-particle metallurgical bonds. | |An elevated temperature diffusion process referred to as sintering, sometimes assisted by external pressure, accomplishes this. The material | |is never fully molten, although there might be a small volume fraction of liquid present during the sintering process. Sintering can be | |regarded as welding the particles present in the initial useful shape. |As a general rule both mechanical and physical properties improve with increasing density. Therefore the method selected for the fabrication | |of a component by powder metallurgy will depend on the level of performance required from the part. Many components are adequate when produced| |at 85-90% of theoretical full density (T. D. ) whilst others require full density for satisfactory performance. | |Some components, in particular bush type bearings often made from copper and its alloys, are produced with significant and controlled levels | |of porosity, the porosity being subsequently filled with a lubricant. | |Fortunately there is a wide choice of consolidation techniques available. |Cold Uniaxial Pressing | |Elemental metal, or an atomised prealloyed, powder is mixed with a lubricant, typically lithium stearate (0. 75 wt. %), and pressed at pressures| |of say, 600 MPa (87,000 lb/in2) in metal dies. Cold compaction ensures that the as-compacted, or ‘green’, component is dimensionally very | |accurate, as it is moulded precisely to the size and shape of the die. | |Irregularly shaped particles are required to ensure that the as-pressed component has a high green strength from the interlocking and plastic | |deformation of individual particles with their neighbours. |One disadvantage of this technique is the differences in pressed density that can occur in different parts of the component due to | |particle/particle and die wall/particle frictional effects. Typical as-pressed densities for soft iron components would be 7. 0 g/cc, i. e. | |about 90% of theoretical density. Compaction pressure rises significantly if higher as-pressed densities are required, and this practice | |becomes uneconomic due to higher costs for the larger presses and stronger tools to withstand the higher pressures. | |Cold Isostatic Pressing | |Metal powders are contained in an enclosure e. g. a rubber membrane or a metallic can that is subjected to isostatic, that is uniform in all | |directions, external pressure.
As the pressure is isostatic the as-pressed component is of uniform density. Irregularly shaped powder | |particles must be used to provide adequate green strength in the as-pressed component. This will then be sintered in a suitable atmosphere to | |yield the required product. | |Normally this technique is only used for semi-fabricated products such as bars, billets, sheet, and roughly shaped components, all of which | |require considerable secondary operations to produce the final, accurately dimensioned component. Again, at economical working pressures, | |products are not fully dense and usually need additional working such as hot extrusion, hot rolling or forging to fully density the material. |Sintering | |Sintering is the process whereby powder compacts are heated so that adjacent particles fuse together, thus resulting in a solid article with | |improved mechanical strength compared to the powder compact. This “fusing” of particles results in an increase in the density of the part and | |hence the process is sometimes called densification. There are some processes such as hot isostatic pressing which combine the compaction and | |sintering processes into a single step. | |After compaction the components pass through a sintering furnace. This typically has two heating zones, the first removes the lubricant, and | |the second higher temperature zone allows diffusion and bonding between powder particles. A range of atmospheres, including vacuum, are used | |to sinter different materials depending on their chemical compositions.
As an example, precise atmosphere control allows iron/carbon materials| |to be produced with specific carbon compositions and mechanical properties. | |The density of the component can also change during sintering, depending on the materials and the sintering temperature. These dimensional | |changes can be controlled by an understanding and control of the pressing and sintering parameters, and components can be produced with | |dimensions that need little or no rectification to meet the dimensional tolerances. Note that in many cases all of the powder used is present | |in the finished product, scrap losses will only occur when secondary machining operations are necessary. |Hot Isostatic Pressing | |Powders are usually encapsulated in a metallic container but sometimes in glass. The container is evacuated, the powder out-gassed to avoid | |contamination of the materials by any residual gas during the consolidation stage and sealed-off. It is then heated and subjected to isostatic| |pressure sufficient to plastically deform both the container and the powder. | |The rate of densification of the powder depends upon the yield strength of the powder at the temperatures and pressures chosen. At moderate | |temperature the yield strength of the powder can still be high and require high pressure to produce densification in an economic time.
Typical| |values might be 1120°C and 100 MPa for ferrous alloys. By pressing at very much higher temperatures lower pressures are required as the yield | |strength of the material is lower. Using a glass enclosure atmospheric pressure (15 psi) is used to consolidate bars and larger billets. | |The technique requires considerable financial investment as the pressure vessel has to withstand the internal gas pressure and allow the | |powder to be heated to high temperatures. | |As with cold isostatic pressing only semifinished products are produced, either for subsequent working to smaller sizes, or for machining to | |finished dimensions. |Hot Forging (Powder Forging) | |Cold pressed and sintered components have the great advantage of being close to final shape (near-nett shape), but are not fully dense. Where | |densification is essential to provide adequate mechanical properties, the technique of hot forging, or powder forging, can be used. | |In powder forging an as-pressed component is usually heated to a forging temperature significantly below the usual sintering temperature of | |the material and then forged in a closed die. This produces a fully dense component with the shape of the forging die and appropriate | |mechanical properties. |Powder forged parts generally are not as close to final size or shape as cold pressed and sintered parts. This results from the allowances | |made for thermal expansion effects and the need for draft angles on the forging tools. Further, minimal, machining is required but when all | |things are considered this route is often very cost effective. | |Metal Injection Moulding (MIM) | |Injection moulding is very widely used to produce precisely shaped plastic components in complex dies. As injection pressures are low it is | |possible to manufacture complex components, even some with internal screw threads, by the use of side cores and split tools. |By mixing fine, typically less than 20 ? m diameter, spherical metal powders with thermoplastic binders, metal filled plastic components can be| |produced with many of the features available in injection moulded plastics. After injection moulding, the plastic binder material is removed | |to leave a metal skeleton which is then sintered at high temperature. | |Dimensional control can be exercised on the as-sintered component as the injected density is sensibly uniform so shrinkage on sintering is | |also uniform. | |Shrinkage can be large, due to both the fine particle size of the powders and the substantial proportion of polymer binder used. |
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