Misplaced Pages

#260739

105-579: In this way, powder metallurgy can be 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 . Tungsten carbide is used to cut and form other metals and is made from tungsten carbide particles bonded with cobalt. Tungsten carbide is the largest and most important use of tungsten , consuming about 50% of the world supply. Other products include sintered filters, porous oil-impregnated bearings, electrical contacts and diamond tools. Powder metallurgy techniques usually consist of

210-403: A copper alloy laced with lead. Since the beginning of metallurgy the majority of castings were simple one to two piece molds fashioned from either stone or ceramics. However, there is evidence of lost wax castings in numerous ancient civilizations. The lost wax process originated in ancient Mesopotamia . The earliest known record of lost-wax casting is a clay tablet written in cuneiform in

315-441: A cubic high-temperature form, β-WC, which has the rock salt structure . The hexagonal form can be visualized as made up of a simple hexagonal lattice of metal atoms of layers lying directly over one another (i.e. not close packed), with carbon atoms filling half the interstices giving both tungsten and carbon a regular trigonal prismatic, 6 coordination . From the unit cell dimensions the following bond lengths can be determined:

420-485: A bond length of 171 pm for W C . Sintered tungsten carbide–cobalt cutting tools are very abrasion resistant and can also withstand higher temperatures than standard high-speed steel (HSS) tools. Carbide cutting surfaces are often used for machining tough materials such as carbon steel or stainless steel , and in applications where steel tools would wear quickly, such as high-quantity and high-precision production. Because carbide tools maintain

525-494: A clay cylinder so molten metal could be poured down the center, filling and solidifying in the open spaces. This process allowed one hundred coins to be produced simultaneously. In the Middle East and West Africa the lost wax technique was used very early in their metallurgy traditions while China adopted it much later. In Western Europe lost wax techniques are considered to have been hardly used especially in comparison to that of

630-424: 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. If the temperature is above the melting point of a component in the powder metal part, the liquid of the melted particles fills the pores. This type of sintering is known as liquid-state sintering. A major challenge with sintering in general

735-579: A complex strain path the powder particles deform, generate a large amount of frictional heat and bond together to form a bulk solid. Theoretically, fully continuous operation is possible as long as the powder can be fed into the process. 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 0.2 to 1 m. Modern presses are largely automatic and operate at high speeds (on

840-409: A custom-made tungsten carbide guitar slide . The hardness, weight, and density of the slide give it superior sustain and volume compared to standard glass, steel, ceramic, or brass slides. Tungsten carbide has been investigated for its potential use as a catalyst and it has been found to resemble platinum in its catalysis of the production of water from hydrogen and oxygen at room temperature,

945-522: A die of the desired shape, and then sintering the compressed material together, under a controlled atmosphere. The metal powder is usually iron, and additives include a lubricant wax, carbon , copper , and/or nickel . This produces precise parts, normally very close to the die dimensions, but with 5–15% porosity, and thus sub-wrought steel properties. This method is still used to make around 1 Mt/y of structural components of iron-based alloys. There are several other PM processes that have been developed over

1050-421: A finely divided form. Inhalation of heavy metals can result in many health issues. Lead and cadmium are generally toxic, and cobalt can cause asthma and fibrosis in sensitive individuals. Tungsten carbide Tungsten carbide ( chemical formula : WC ) is a chemical compound (specifically, a carbide ) containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide

1155-462: A flexible mould, which is then immersed in a high-pressure gas or liquid from all directions (isostatic). After sintering, this manufacturing process produces very little scrap metal and can be used to make many different shapes. The tolerances that this process can achieve in combination with sintering 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

SECTION 10

#1733085407261

1260-474: A lower temperature fluid bed process that reacts either tungsten metal (or powder) or blue WO 3 with CO / CO 2 gas mixture and H 2 gas between 900 and 1,200 °C. WC can also be produced by heating WO 3 with graphite , either directly at 900 °C or in hydrogen at 670 °C, followed by carburization in argon at 1,000 °C. Chemical vapor deposition methods that have been investigated include: Solid tungsten carbide

1365-413: 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, one of the most critical steps in powder metallurgy processes, is the process of compacting metal powder through

1470-498: A number of characteristics are increased including the strength , ductility , toughness , and electrical and thermal conductivity of the material. If different elemental powders are compact and sintered, the material would form into alloys and intermetallic phases. 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

1575-411: A procedure used for recovery of scrap cemented carbide due to its selectivity. Tungsten carbide has a high melting point at 2,870 °C (3,140 K), a boiling point of 6,000 °C (6,270 K) when under a pressure equivalent to 1 standard atmosphere (101.325 kilopascals), a thermal conductivity of 110 W/m·K, and a coefficient of thermal expansion of 5.5 μm/m·K. Tungsten carbide

1680-845: A sharp cutting edge better than steel tools, they generally produce a better finish on parts, and their temperature resistance allows faster machining. The material is usually called cemented carbide , solid carbide, hardmetal or tungsten-carbide cobalt. It is a metal matrix composite , where tungsten carbide particles are the aggregate, and metallic cobalt serves as the matrix. It has been found wear and oxidation properties of cemented carbide can be improved by replacing cobalt with iron aluminide. Tungsten carbide cutting tools can be further enhanced with coatings such as titanium aluminium nitride or titanium chromium nitride to increase their thermal stability, and prolong tool life. Tungsten carbide, in its monolithic sintered form, or much more often in cemented tungsten carbide cobalt composite (see above),

1785-616: 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. Powder can be obtained through gas or water atomization, centrifugal atomization, chemically reducing particulate compounds, electrolytic deposition in appropriate conditions, simple pulverization and grinding, thermal decomposition of particulate hydrides or carbonyls, precipitation out of solution, and also condensation from vaporized metal. Atomization

1890-549: A thin rod of tungsten carbide is 6220 m/s. Tungsten carbide's low electrical resistivity of about 0.2  μ Ω·m is comparable with that of some metals (e.g. vanadium 0.2  μ Ω·m). WC is readily wetted by both molten nickel and cobalt . Investigation of the phase diagram of the W-C-Co system shows that WC and Co form a pseudo binary eutectic . The phase diagram also shows that there are so-called η-carbides with composition (W,Co) 6 C that can be formed and

1995-684: A tungsten carbide brick onto a plutonium sphere, known as the demon core , causing the subcritical mass to go supercritical with the reflected neutrons . He fell into a coma and died 25 days after the accident. Trekking poles , used by many hikers for balance and to reduce pressure on leg joints, generally use carbide tips in order to gain traction when placed on hard surfaces (like rock); carbide tips last much longer than other types of tip. While ski pole tips are generally not made of carbide, since they do not need to be especially hard even to break through layers of ice, rollerski tips usually are. Roller skiing emulates cross country skiing and

2100-411: Is 25 to 45% shrinkage of the powder after compacting. 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

2205-468: Is 5% to 10% higher than with other powder metallurgy processes. 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. Advantages over standard powder compaction are

SECTION 20

#1733085407261

2310-446: Is a common means of making washstands, washstand tops and shower stalls, with the skilled working of multiple colors resulting in simulated staining patterns as is often found in natural marble or travertine . Raw castings often contain irregularities caused by seams and imperfections in the molds, as well as access ports for pouring material into the molds. The process of cutting, grinding, shaving or sanding away these unwanted bits

2415-413: Is a fine gray powder, but it can be pressed and formed into shapes through sintering for use in industrial machinery , engineering facility , mold industry , cutting tools , chisels , abrasives , armor-piercing bullets and jewelry . Tungsten carbide is approximately three times as stiff as steel , with a Young's modulus of approximately 530–700 GPa, and is twice as dense as steel . It

2520-549: 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 . Simple atomization techniques are available in which liquid metal

2625-683: Is also anticipated to be a human carcinogen by the American National Toxicology Program . Casting Casting is a manufacturing process in which a liquid material is usually poured into a mold , which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various time setting materials that cure after mixing two or more components together; examples are epoxy , concrete , plaster and clay . Casting

2730-444: Is an important zone as it prevents oxidation from immediate contact with the air or a phenomenon known as rapid cooling. All of the three stages must be carried out in a controlled atmosphere containing no oxygen. Hydrogen, nitrogen, dissociated ammonia, and cracked hydrocarbons are common gases pumped into the furnace zones providing a reducing atmosphere, preventing oxide formation. Hot isostatic pressing (HIP) compresses and sinters

2835-401: Is applied from all directions, lower compaction pressures are required to produce higher densities of powder in the end product. 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

2940-445: Is attributed as one of the first civilizations to use casting methods to mass produce coins. Around the middle of the first millennium BC (1000 BC - 1 BC), coins used were made from silver but as the millennium progressed the coins shifted to a cast copper alloy. New technology was developed to mass produce the new copper coins. Introduced was a multi piece stackable coin template mold. Multiple molds were placed on top of one another into

3045-446: Is called "fettling" in UK english. In modern times robotic processes have been developed to perform some of the more repetitive parts of the fettling process, but historically fettlers carried out this arduous work manually, and often in conditions dangerous to their health. Fettling can add significantly to the cost of the resulting product, and designers of molds seek to minimize it through

3150-415: Is comparable with corundum (α- Al 2 O 3 ) in hardness , approaching that of a diamond, and can be polished and finished only with abrasives of superior hardness such as cubic boron nitride and diamond powder, wheels and compounds. Tungsten carbide tools can be operated at cutting speeds much higher than high-speed steel (a special steel blend for cutting tools). Tungsten carbide powder

3255-514: Is complex and dependent on the material and furnace (temperature and gas) conditions. There are six main stages that sintering processes can be grouped in which may overlap with one another: 1 initial bonding among particles, 2) neck growth, 3) pore channel closure, 4) pore rounding, 5) densification or pore shrinkage, and 6) pore coarsening. The main mechanisms present in these stages are evaporation , condensation , grain boundaries , volume diffusion , and plastic deformation . During this process,

Powder metallurgy - Misplaced Pages Continue

3360-413: Is currently the most commonly used method of electric pulse consolidation in general. Resistance sintering currents usually reach about 1 kA per square centimer, while electric discharge sintering voltages of up to several kilovolts also require very high currents, over 10 kA per square centimer. Resistance sintering techniques are consolidation methods based on temperature, where heating of the mold and of

3465-495: Is extremely hard, ranking about 9 to 9.5 on the Mohs scale , and with a Vickers number of around 2600. It has a Young's modulus of approximately 530–700 GPa, a bulk modulus of 379-381 GPa, and a shear modulus of 274 GPa. It has an ultimate tensile strength of 344 MPa, an ultimate compression strength of about 2.7 GPa and a Poisson's ratio of 0.31. The speed of a longitudinal wave (the speed of sound ) through

3570-485: 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,

3675-693: Is flat and lacks transparency. Often topical treatments are applied to the surface. For example, painting and etching can be used in a way that give the appearance of metal or stone. Alternatively, the material is altered in its initial casting process and may contain colored sand so as to give an appearance of stone. By casting concrete, rather than plaster, it is possible to create sculptures, fountains, or seating for outdoor use. A simulation of high-quality marble may be made using certain chemically-set plastic resins (for example epoxy or polyester which are thermosetting polymers ) with powdered stone added for coloration, often with multiple colors worked in. The latter

3780-449: Is forced through an orifice at a sufficiently high velocity to ensure turbulent flow. The usual performance index used is the Reynolds number . At low Re 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

3885-427: 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

3990-550: Is generally utilised as a button insert, mounted in a surrounding matrix of steel that forms the substance of the bit. As the tungsten carbide button is worn away the softer steel matrix containing it is also worn away, exposing yet more button insert. Tungsten carbide is also an effective neutron reflector and as such was used during early investigations into nuclear chain reactions, particularly for weapons. A criticality accident occurred at Los Alamos National Laboratory on 21 August 1945 when Harry Daghlian accidentally dropped

4095-522: 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

4200-422: Is knowing the effect of the process on the dimensions of the compact particles. This is especially difficult for tooling purposes in which specific dimensions may be needed. It is most common for the sintered part to shrink and become denser, but it can also expand or experience no net change. The main driving force for solid-state sintering is an excess of surface-free energy. The process of solid-state sintering

4305-456: Is made by single-level tooling. A more complex shape can be made by the common multiple-level tooling. 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

Powder metallurgy - Misplaced Pages Continue

4410-406: Is monitored and used to manipulate the furnace conditions or to oversize the compact materials in order to achieve the desired dimensions. Although, sintering does not deplete the compact part of porosity . In general, powder metal parts contain five to twenty-five percent porosity after sintering. Most sintering furnaces contain three zones with three different properties that help to carry out

4515-709: Is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods. Heavy equipment like machine tool beds, ships' propellers, etc. can be cast easily in the required size, rather than fabricating by joining several small pieces. Casting is a 7,000-year-old process. The oldest surviving casting is a copper frog from 3200 BC. Throughout history, metal casting has been used to make tools, weapons, and religious objects. Metal casting history and development can be traced back to Southern Asia (China, India, Pakistan, etc). Southern Asia traditions and religions relied heavily on statue and relic castings. These items were frequently made from

4620-544: Is now generally of the sabot type. SLAP, or saboted light armour penetrator , where a plastic sabot discards at the barrel muzzle, is one of the primary types of saboted small arms ammunition. Non-discarding jackets, regardless of the jacket material, are not perceived as sabots but as bullets. Both of the designs are, however, common in designated light armor-piercing small arms ammunition. Discarding sabots such as are used with M1A1 Abrams main gun are more commonplace in precision high-velocity gun ammunition. Tungsten carbide

4725-573: Is often used in armor-piercing ammunition , especially where depleted uranium is not available or is politically unacceptable. W 2 C projectiles were first used by German Luftwaffe tank-hunter squadrons in World War II . However, owing to the limited German reserves of tungsten, W 2 C material was reserved for making machine tools and small numbers of projectiles . It is an effective penetrator due to its combination of great hardness and very high density. Tungsten carbide ammunition

4830-715: Is possible to produce parts that are less than 0.1 square inches (0.65 cm) and larger than 25 square inches (160 cm). in area and from a fraction of an inch (2.54 cm) to approximately 8 inches (20 cm) in length. Small mechanical presses can generally compact about 100 pieces per minute. In die compaction, 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

4935-431: Is prepared using techniques from powder metallurgy developed in the 1920s. Powdered tungsten carbide is mixed with another powdered metal, usually cobalt (alternatives include nickel , iron and paraffin wax ) which acts as a binder . The mixture is pressed, then sintered by heating it to temperatures of 1,400 °C (2,550 °F) to 1,600 °C (2,910 °F); the binder melts, wets, and partially dissolves

5040-1720: 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 tin , 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. Many special products are possible with powder metallurgy technology. A non-exhaustive list includes Al 2 O 3 whiskers coated with very thin oxide layers for improved refraction; iron compacts with Al 2 O 3 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

5145-479: Is resistant to acids and is only attacked by hydrofluoric acid / nitric acid (HF/ HNO 3 ) mixtures above room temperature. It reacts with fluorine gas at room temperature and chlorine above 400 °C (673 K) and is unreactive to dry H 2 up to its melting point. Finely powdered WC oxidizes readily in hydrogen peroxide aqueous solutions. At high temperatures and pressures it reacts with aqueous sodium carbonate forming sodium tungstate ,

5250-541: Is roughly 10 times harder than 18k gold. In addition to its design and high polish, part of its attraction to consumers is its technical nature. Special tools, such as locking pliers, may be required if such a ring must be removed quickly (e.g. due to medical emergency following a hand injury accompanied by swelling). Tungsten carbide is widely used to make the rotating ball in the tips of ballpoint pens that disperse ink during writing. English guitarist Martin Simpson uses

5355-449: Is the most efficient type of powder compacting (the following subcategories are also from this reference). This operation is generally only applicable on small production quantities, and although the cost of a mold is much lower than that of pressing dies, it is generally not reusable and the production time is much longer. Production rates are usually very low, but parts weighing up to 100 pounds can be effectively compacted. Because pressure

SECTION 50

#1733085407261

5460-435: Is the process of binding a material together with heat without liquefying it. It is usually conducted at atmospheric pressure and under carefully controlled atmosphere composition. To obtain special properties or enhanced precision, secondary processing like coining or heat treatment often follows. One of the older such methods is the process of blending fine (<180 microns) metal powders with additives, pressing them into

5565-634: 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 . Any fusible material can be atomized. Several techniques have been developed that permit large production rates of powdered particles, often with considerable control over

5670-439: Is used by many skiers to train during warm weather months. Sharpened carbide tipped spikes (known as studs) can be inserted into the drive tracks of snowmobiles . These studs enhance traction on icy surfaces. Longer v-shaped segments fit into grooved rods called wear rods under each snowmobile ski. The relatively sharp carbide edges enhance steering on harder icy surfaces. The carbide tips and segments reduce wear encountered when

5775-399: Is used extensively in mining in top hammer rock drill bits, downhole hammers , roller-cutters , long wall plough chisels, long wall shearer picks, raiseboring reamers, and tunnel boring machines . In these applications it is also used for wear and corrosion resistant components in inlet control for well screens, sub-assemblies, seal rings and bushings common in oil and gas drilling. It

5880-404: Is usually done with a die press, but can also be done with explosive shocks or placing a flexible container in a high-pressure gas or liquid. Sintering is usually done in a dedicated furnace, but can also be done in tandem with compression (hot isostatic compression), or with the use of electric currents. Since the advent of industrial production-scale metal powder-based additive manufacturing in

5985-610: The reduction of tungsten trioxide by hydrogen in the presence of water, and the isomerisation of 2,2-dimethylpropane to 2-methylbutane. It has been proposed as a replacement for the iridium catalyst in hydrazine -powered satellite thrusters . A tungsten carbide coating has been utilized on brake discs in high performance automotive applications to improve performance, increase service intervals and reduce brake dust. The primary health risks associated with tungsten carbide relate to inhalation of dust, leading to silicosis -like pulmonary fibrosis . Cobalt-cemented tungsten carbide

6090-645: The shoes ; in the United States, borium – chips of tungsten carbide in a matrix of softer metal such as bronze or mild steel – may be welded to small areas of the underside of the shoe before fitting. Tungsten carbide is also used for making surgical instruments meant for open surgery (scissors, forceps, hemostats, blade-handles, etc.) and laparoscopic surgery (graspers, scissors/cutter, needle holder, cautery, etc.). They are much costlier than their stainless-steel counterparts and require delicate handling, but give better performance. Tungsten carbide, typically in

6195-417: The 2010s, selective laser sintering and other metal additive manufacturing processes are a new category of commercially important powder metallurgy applications. The powder metallurgy "press and sinter" process generally consists of three basic steps: powder blending (or pulverisation), die compaction, and sintering . Compaction of the powder in the die is generally performed at room temperature. Sintering

6300-511: The Indus valley civilization. There were no pieces of lost wax found in the capital of Anyang during the Shane dynasty (1600-1040 BC) while a large amount (100,000 pieces) of piece-mould fragments were found. This led to the conclusion that lost wax was not performed in the capital during this dynasty. However, the discovery of a mask made using the investment moulding dated at around 1300 BC indicated that

6405-478: 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

SECTION 60

#1733085407261

6510-856: The WC phase to high temperatures using plasma, then quenching in inert gas (plasma spheroidization). This process causes macrocrystalline WC particles to spheroidize and results in the non-stoichiometric high temperature phase WC 1-x existing in a meta-stable form at room temperature. The fine microstructure of this phase provides high hardness (2800–3500 HV) combined with good toughness when compared with other tungsten carbide compounds. The meta-stable nature of this compound results in reduced high temperature stability. At high temperatures WC decomposes to tungsten and carbon and this can occur during high-temperature thermal spray , e.g., in high velocity oxygen fuel (HVOF) and high energy plasma (HEP) methods. Oxidation of WC starts at 500–600 °C (773–873 K). It

6615-448: 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 t/in to 50 t/in (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

6720-408: The ancient city of Sparta, Babylon, which specifically records how much wax is needed to cast a key. The earliest-known castings in the global archaeological record were made in open stone molds. There are two types of lost wax methods, direct lost wax method and indirect lost wax method. The direct molding method is to make the wax material into the same wax mold as the casting by hand or other tools;

6825-410: The application of high pressures. Most powder compaction is done with mechanical presses and rigid tools, but hydraulic and pneumatic techniques can also be used, as well as methods that combine compaction with sintering, like hot isostatic compaction. Traditional metalforming processes, including rolling, forging, extrusion, and swaging, are also used. The density of the compacted powder increases with

6930-448: The bottom of the bin. In subsequent operations the powder is dried. This is called water atomization. Water atomization cools and solidifies the metal particles more rapidly than gas atomization. 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 microstructure will be. Particles produced this way will also have

7035-466: The brittleness of these phases makes control of the carbon content in WC-Co cemented carbides important. In the presence of a molten phase such as cobalt, abnormal grain growth is known to occur in the sintering of tungsten carbide, with this having significant effects on the performance of the product material. There are two forms of WC, a hexagonal form, α-WC ( hP2 , space group P 6 m2, No. 187), and

7140-484: The canister, yet filled outside. In fixed mold tooling, the mold is contained within the canister, which facilitates automation of the process. 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

7245-408: The casting process (such as the runners and risers). Plaster and other chemical curing materials such as concrete and plastic resin may be cast using single-use waste molds as noted above, multiple-use 'piece' molds, or molds made of small rigid pieces or of flexible material such as latex rubber (which is in turn supported by an exterior mold). When casting plaster or concrete, the material surface

7350-463: The casting with iron bands. In metalworking, metal is heated until it becomes liquid and is then poured into a mold. The mold is a hollow cavity that includes the desired shape, but the mold also includes runners and risers that enable the metal to fill the mold. The mold and the metal are then cooled until the metal solidifies. The solidified part (the casting) is then recovered from the mold. Subsequent operations remove excess material caused by

7455-568: 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

7560-458: The compression of a powder, and heating (sintering) it at a temperature below the melting point of the metal, to bind the particles together. Powder for the processes can be produced in a number of ways, including reducing metal compounds, electrolyzing metal-containing solutions, and mechanical crushing, as well as more complicated methods, including a variety of ways to fragment liquid metal into droplets, and condensation from metal vapor. Compaction

7665-516: The distance between the tungsten atoms in a hexagonally packed layer is 291 pm, the shortest distance between tungsten atoms in adjoining layers is 284 pm, and the tungsten carbon bond length is 220 pm. The tungsten-carbon bond length is therefore comparable to the single bond in W( CH 3 ) 6 (218 pm) in which there is strongly distorted trigonal prismatic coordination of tungsten. Molecular WC has been investigated and this gas phase species has

7770-487: The entire casting manufacturing route. Casting process simulation was initially developed at universities starting from the early ' 70s , mainly in Europe and in the U.S. , and is regarded as the most important innovation in casting technology over the last 50 years. Since the late ' 80s , commercial programs (such as PoligonSoft, AutoCAST and Magma) are available which make it possible for foundries to gain new insight into what

7875-416: The extreme pressure without deforming or bending. Tools must be made from materials that are polished and wear-resistant. Shock consolidation, or dynamic consolidation, is an experimental technique of consolidating powders using high pressure shock waves. This technique is useful for very large products, including those over 3000 tons and larger than 100 square inches. These are commonly produced by impacting

7980-454: The fluidity of molten copper, allowing them to cast more intricate designs. For example, the dancing girl of Mohenjo-daro is a copper alloy casting that most likely utilizes the lost wax technique. Lost wax casting can be dated back to 4000 BC or the Chalcolithic period. One of the oldest studied examples of this technique is a 6,000-year old amulet from Indus valley civilization . India

8085-513: The form of a cemented carbide (carbide particles brazed together by metal), has become a popular material in the bridal jewelry industry due to its extreme hardness and high resistance to scratching. Even with high-impact resistance, this extreme hardness also means that it can occasionally be shattered under certain circumstances. Some consider this useful, since an impact would shatter a tungsten ring, quickly removing it, where precious metals would bend flat and require cutting. Tungsten carbide

8190-454: The frequent necessity for protective atmospheres or simple vacuum during forming and cooling stages. HIP produces products often of higher quality than other processes. However, HIP is expensive, and generally unnattractive for high-volume production, due to the high cost of placing the powder in a flexible isolating medium that can withstand the temperatures and pressures ( canning ) and then removing it from that medium ( decanning ), as well as

8295-470: The from around ten seconds to ten minutes; and electric discharge sintering, which use capacitor banks to achieve higher currents and voltages, and take from tens of microseconds to tens of milliseconds. Resistance sintering techniques include spark plasma sintering (SPS), plasma-activated sintering (PAS), and pulse electric current sintering (PECS). electric discharge sintering techniques include capacitor discharge sintering . Currently, spark plasma sintering

8400-527: 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 cm) to 20 square inches (130 cm). in area and from 0.1 to 4 inches (0.25 to 10.16 cm) in length. However, it

8505-428: The high-temperature stage, is used to produce solid-state diffusion and particle bonding. The material is seeking to lower its surface energy and does so by moving toward the points of contact between particles. The contact points become larger and eventually a solid mass with small pores is created. The third zone, also called the cooling period, is used to cool down the parts while still in a controlled atmosphere. This

8610-513: The indirect molding method is to make the wax mold through the mold. The direct molding method requires craftsmen to have a high technical level, otherwise the quality of castings cannot be guaranteed. However, the limitation of manual direct molding is that its efficiency is too low to achieve mass production. In this regard, indirect moulding has advantages. In indirect moulding, artisans usually make moulds from stone, wood, clay or other plastic materials. Early civilizations discovered lead aided in

8715-470: The last fifty years. These include: 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. In these early manufacturing operations, iron

8820-516: The long time periods involve, which can range from 6 to 8 hours. These techniques employ electric currents to drive or enhance sintering. A combination of mechanical pressure and electrical current, passed through either the powder or the container, significantly reduces the sintering time compared to conventional solutions. There are many classifications of these techniques, but they can be divided into two main categories: resistance sintering techniques, which apply lower voltages and currents and take on

8925-471: The lost wax technique may have influenced other regions in China. Historians debate the origin of the development of the cannon but most evidence points to Turkey and Central Asia in the 18th and 19th century. The casting process of a cannon is a bit more complex with the use of a clay core, a template which has clay moulded around it and then broken out followed by an assembly in a casting pit that involves binding

9030-431: 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

9135-413: 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

9240-421: The opposite extreme, large extrusions on a tonnage basis may be feasible. 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

9345-414: The order of m/s). 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

9450-400: The part simultaneously by applying heat on the order of 2300 °F (1250 °C), in the case of iron, or 2750 °F (1500 °C) in the case of nickel alloys. 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. In most applications of powder metallurgy

9555-419: The possibility of thinner walls and larger workpieces. The height-to-diameter ratio has no limitation. No specific limitations exist in wall thickness variations, undercuts , reliefs, threads, and cross holes. No lubricants are needed 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

9660-509: 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 in the range 0.5–300 mm (0.020–11.811 in). Hard metal wires of 0.1 mm (0.0039 in) diameter have been drawn from powder stock. At

9765-533: 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. Strictly, the phrase "continuous process" should be used only to describe modes of manufacturing which could be extended indefinitely in time. Normally, however,

9870-436: The production cycle below, which offers a readily automated and high production rate process: Typically the tools are held in the vertical orientation with the punch tool forming the bottom of the cavity. 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 m) and

9975-413: The required component properties. This has benefits beyond a reduction in pre-production sampling, as the precise layout of the complete casting system also leads to energy , material, and tooling savings. The software supports the user in component design, the determination of melting practice and casting methoding through to pattern and mold making, heat treatment , and finishing. This saves costs along

10080-499: The shape of the mold, the material being cast, and sometimes by including decorative elements. Casting process simulation uses numerical methods to calculate cast component quality considering mold filling, solidification and cooling, and provides a quantitative prediction of casting mechanical properties, thermal stresses and distortion. Simulation accurately describes a cast component's quality up-front before production starts. The casting rigging can be designed with respect to

10185-426: 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. If the temperature of the compact parts is raised too quickly, the air in the pores will be at a very high internal pressure which could lead to expansion or fracture of the part. The second zone, known as

10290-593: 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 . Iron, nickel, uranium, and beryllium submicrometre powders are obtained by reducing metallic oxalates and formates . Exceedingly fine particles also have been prepared by directing

10395-432: The snowmobile must cross roads and other abrasive surfaces. Car, motorcycle and bicycle tires with tungsten carbide studs provide better traction on ice. They are generally preferred to steel studs because of their superior resistance to wear. Tungsten carbide may be used in farriery , the shoeing of horses , to improve traction on slippery surfaces such as roads or ice. Carbide-tipped hoof nails may be used to attach

10500-401: 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

10605-402: 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

10710-470: The tungsten grains, binding them together. The cobalt-tungsten composites specifically are known by a number of trade names, including Widia and Carboloy. There are two well-characterized compounds of tungsten and carbon: tungsten carbide, WC , and tungsten semicarbide , W 2 C . Both compounds may be present in coatings and the proportions can depend on the coating method. Another meta-stable compound of tungsten and carbon can be created by heating

10815-462: The workpiece with an explosively accelerated plate. Despite being researched for a long time, the technique still has some problems in controllability and uniformity. However, it offers some valuable potential advantages. As an example, consolidation occurs so rapidly that metastable microstructures may be retained. Isostatic powder compacting is an alternate method of powder compaction. In cold isostatic compaction, fine metal particles are placed into

10920-499: Was extracted by hand from a 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

11025-414: Was first synthesized by H. Moissan in 1893, and the industrial production of the cemented form started 20 to 25 years later (between 1913 and 1918). Colloquially among workers in various industries (such as machining ), tungsten carbide is often simply called carbide. Tungsten carbide powder is prepared by reaction of tungsten metal (or powder) and carbon at 1,400–2,000 °C. Other methods include

#260739