This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. 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. Powder metallurgy is also used to make unique materials impossible to get from melting or forming in other ways. WC is used to cut and form other metals and is made from WC particles bonded with cobalt. 2010s, selective laser sintering and other metal AM processes are a new category of commercially important powder metallurgy applications. Compaction is generally performed at room temperature, and the elevated-temperature process powder filling sintering is usually conducted at atmospheric pressure and under carefully controlled atmosphere composition.
Powder forging: A «preform» made by the conventional «press and sinter» method is heated and then hot forged to full density, resulting in practically as-wrought properties. The can is vibrated, then evacuated and sealed. It is then placed in a hot isostatic press, where it is heated to a homologous temperature of around 0. Powders employed in ECAS can avoid binders thanks to the possibility of direct sintering, without the need of pre-pressing and a green compact. 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, though mass manufacturing of PM products did not begin until the mid or late 19th century. 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 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. 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. The most commonly used powders are copper-base and iron-base materials. 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. 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. 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. Atomization is accomplished by forcing a molten metal stream through an orifice at moderate pressures. The collection volume is filled with gas to promote further turbulence of the molten metal jet.
Simple atomization techniques are available in which liquid metal is forced through an orifice at a sufficiently high velocity to ensure turbulent flow. 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. 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. 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. 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. The density of the compacted powder increases with the amount of pressure applied.
1000 psi to 1,000,000 psi have been obtained. 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. Production rates of 15 to 30 parts per minute are common. 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. Better workpiece materials can be obtained by repressing and re-sintering.
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. 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. 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. Along with having walls thicker than 0. 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. However, it is possible to produce parts that are less than 0. This procedure, together with explosion-driven compressive techniques is used extensively in the production of high-temperature and high-strength parts such as turbine disks for jet engines.
Isostatic powder compacting is a mass-conserving shaping process. Fine metal particles are placed into a flexible mould and then high fluid pressure is applied to the mold, in contrast to the direct pressure applied by the die faces of a die pressing process. 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. There are many types of equipment used in isostatic powder compacting. There is the mold containing the part, which is flexible, a flexible outer pressure mold that contains and seals 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.
Typical workpiece sizes range from 0. It is possible to compact workpieces that are between 0. 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. Argon gas is the most common gas used in HIP because it is an inert gas, thus prevents chemical reactions during the operation.
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. After compaction, powdered materials are heated in a controlled atmosphere in a process known as sintering.
During this process, the surfaces of the particles are bonded and desirable properties are achieved. Sintering of powder metals is a process in which particles under pressure chemically bond to themselves in order to form a coherent shape when exposed to a high temperature. The temperature in which the particles are sintered is most commonly below the melting point of the main component in the powder. The main driving force for solid state sintering is an excess of surface free energy. Most sintering furnaces contain three zones with three different properties that help to carry out the six steps above. The first zone, commonly coined the burn-off or purge stage, is designed to combust air, burn any contaminants such as lubricant or binders, and slowly raise the temperature of the compact materials. During this process, a number of characteristics are increased including the strength, ductility, toughness, and electrical and thermal conductivity of the material.
As the pore sizes decrease, the density of the material will increase. As stated above, this shrinkage is a huge problem in making parts or tooling in which particular dimensions are required. The shrinkage of test materials is monitored and used to manipulate the furnace conditions or to oversize the compact materials in order to achieve the desired dimensions. 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. One recently developed technique for high-speed sintering involves passing high electric current through a powder to preferentially heat the asperities. 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. Powders can also be rolled to produce sheets.
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. Extrusion processes are of two general types. Extrusions with binders are used extensively in the preparation of tungsten-carbide composites. For softer, easier to form metals such as aluminium and copper alloys continuous extrusion may also be performed using processes such as conform or continuous rotary extrusion. These processes use a rotating wheel with a groove around its circumference to drive the loose powder through a forming die. Through a combination of high pressure and a complex strain path the powder particles deform, generate a large amount of frictional heat and bond together to form a bulk solid.
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 to 30 m and diameters from 0. 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. 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. Many special products are possible with powder metallurgy technology.
Extremely thin films and tiny spheres exhibit high strength. 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. You can help by adding to it. The special materials and processes used in powder metallurgy can pose hazards to life and property. Sintering of Chemically Preconditioned Tin Powder». An Overview of Powder Metallurgy and Its Big Benefits». M Parts for Soft Magnetic Applications».
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Alting, Leo1994 Manufacturing Processes Reference Guide, Industrial Press Inc. Metastable Microstructure in Dynamically Consolidated γ Titanium Aluminide». Materials Science and Engineering: R: Reports. An earlier version of this article was copied from Appendix 4C of Advanced Automation for Space Missions, a NASA report in the public domain. German, «Powder Metallurgy and Particulate Materials Processing,» Metal Powder Industries Federation, Princeton, New Jersey, 2005. Delicious and nutritious to nurture and nourish your mind and body.
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