The Full Wiki

Powder metallurgy: Wikis

Advertisements
  

Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.

Encyclopedia

From Wikipedia, the free encyclopedia

Powder metallurgy is a forming and fabrication technique consisting of three major processing stages. First, the primary material is physically powdered, divided into many small individual particles. Next, the powder is injected into a mold or passed through a die to produce a weakly cohesive structure (via cold welding) very near the dimensions of the object ultimately to be manufactured. Pressures of 10-50 tons per square inch are commonly used. Also, to attain the same compression ratio across more complex pieces, it is often necessary to use lower punches as well as an upper punch. Finally, the end part is formed by applying pressure, high temperature, long setting times (during which self-welding occurs), or any combination thereof.

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.

Contents

History and capabilities

The history of powder metallurgy and the art of metals and ceramics sintering are intimately related. Sintering involves the production of a hard solid metal or ceramic piece from a starting powder. There is evidence that iron powders were fused into hard objects as early as 1200 B.C. In these early manufacturing operations, iron was extracted by hand from metal sponge following reduction and was then reintroduced as a powder for final melting or sintering.

A much wider range of products can be obtained from powder processes than from direct alloying of fused materials. In melting operations the "phase rule" applies to all pure and combined elements and strictly dictates the distribution of liquid and solid phases which can exist for specific compositions. In addition, whole body melting of starting materials is required for alloying, thus imposing unwelcome chemical, thermal, and containment constraints on manufacturing. Unfortunately, the handling of aluminium/iron powders poses major problems. Other substances that are especially reactive with atmospheric oxygen, such as 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 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.[1]

Isostatic powder compacting

Isostatic powder compacting is a mass-conserving shaping process. Fine metal particles are placed into a flexible mold and then high gas or fluid pressure is applied to the mold. The resulting article is then sintered in a furnace. This increases the strength of the part by bonding the metal particles. This manufacturing process produces very little scrap metal and can be used to make many different shapes. The tolerances that this process can achieve are very precise, ranging from +/- 0.008 inches for axial dimensions and +/- 0.020 inches for radial dimensions. This is the most efficient type of powder compacting.(The following subcategories are also from this reference.)[2] 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.[3]

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.

Advertisements

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 with in 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 preventing 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 and the product can have a weight between 40 and 300 pounds. 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. Several of the melting and mechanical procedures are clearly adaptable to operations in space or on the Moon.

Powders of the elements Ti, V, Th, Nb, Ta, Ca, and U have been produced by high-temperature reduction of the corresponding nitrides and carbides. Fe, Ni, U, and Be 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 atomized powders are annealed, which helps reduce the oxide and carbon content. The water atomized particles are smaller, cleaner, and nonporous and have a greater breadth of size, which allows better compacting.

Simple atomization techniques are available in which liquid metal is forced through an orifice at a sufficiently high velocity to ensure turbulent flow. The usual performance index used is the Reynolds number R = fvd/n, where f = fluid density, v = velocity of the exit stream, d = diameter of the opening, and n = absolute viscosity. At low R the liquid jet oscillates, but at higher velocities the stream becomes turbulent and breaks into droplets. Pumping energy is applied to droplet formation with very low efficiency (on the order of 1%) and control over the size distribution of the metal particles produced is rather poor. Other techniques such as nozzle vibration, nozzle asymmetry, multiple impinging streams, or molten-metal injection into ambient gas are all available to increase atomization efficiency, produce finer grains, and to narrow the particle size distribution. Unfortunately, it is difficult to eject metals through orifices smaller than a few millimeters in diameter, which in practice limits the minimum size of powder grains to approximately 10 μm. Atomization also produces a wide spectrum of particle sizes, necessitating downstream classification by screening and remelting a significant fraction of the grain boundary.

Centrifugal disintegration

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 (DeCarmo, 1979), 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.

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 thermal conductivity of water is some magnitudes higher. The solidification rate is inversely proportional to the particle size. As a consequence, one can obtain smaller particles by 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.

Finally, mills are now available which can impart enormous rotational torques on powders, on the order of 2.0×107 rpm. Such forces cause grains to disintegrate into yet finer particles.

Powder compaction

Powder compaction, also known as powder pressing, 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 the punch tool forming the bottom of the cavity. The powder is then compacted into a shape and then ejected from the die cavity.[4] 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/in² to 50 tons/in² 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.

Typical workpiece materials
Workpiece material Density (grams/cc) Yield strength (psi) Tensile strength (psi) Hardness (HB)
Iron 5.2 to 7.0 5.1*103 to 2.3*104 7.3*103 to 2.9*104 40 to 70
Low alloy steel 6.3 to 7.4 1.5*104 to 2.9*104 2.00*104 to 4.4*104 60 to 100
Alloyed steel 6.8 to 7.4 2.6*104 to 8.4*104 2.9*104 to 9.4*104 60 and up
Stainless steel 6.3 to 7.6 3.6*104 to 7.3*104 4.4*104 to 8.7*104 60 and up
Bronze 5.5 to 7.5 1.1*104 to 2.9*104 1.5*104 to 4.4*104 50 to 70
Brass 7.0 to 7.9 1.1*104 to 2.9*104 1.6*104 to 3.5*104 60

Design considerations

  • Must be able to remove part from die.
  • Maximum surface area below 20 square inches.
  • Minimum wall thickness of 0.08 in.
  • Sharp corners should be avoided.
  • Should avoid height to diameter ratios greater than 7:1.
  • Adjacent wall thickness ratios greater than 2.5 to 1 should be avoided.

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 in² to 20 in². in area and from 0.1 in. to 4 in. in length. However, it is possible to produce parts that are less than 0.1 in². and larger than 25 in². in area and from a fraction of an inch to approximately 8 in. in length.

Isostatic pressing

In some pressing operations (such as hot isostatic pressing) compact formation and sintering occur simultaneously. This procedure, together with explosion-driven compressive techniques, is used extensively in the production of high-temperature and high-strength parts such as turbine blades for jet engines. In most applications of powder metallurgy the compact is hot-pressed, heated to a temperature above which the materials cannot remain work-hardened. Hot pressing lowers the pressures required to reduce porosity and speeds welding and grain deformation processes. Also it permits better dimensional control of the product, lessened sensitivity to physical characteristics of starting materials, and allows powder to be driven to higher densities than with cold pressing, resulting in higher strength. Negative aspects of hot pressing include shorter die life, slower throughput because of powder heating, and the frequent necessity for protective atmospheres during forming and cooling stages.

Sintering

Solid state sintering is the process of taking metal in the form of a powder and placing it into a mold or die. Once compacted into the mold the material is placed under a high heat for a long period of time. Under heat, bonding takes place between the porous aggregate particles and once cooled the powder has bonded to form a solid piece.

Sintering can be considered to proceed in three stages. During the first, neck growth proceeds rapidly but powder particles remain discrete. During the second, most densification occurs, the structure recrystallizes and particles diffuse into each other. During the third, isolated pores tend to become spheroidal and densification continues at a much lower rate. The words Solid State in Solid State Sintering simply refer to the state the material is in when it bonds, solid meaning the material was not turned molten to bond together as alloys are formed.[5]

One recently developed technique for high-speed sintering involves passing high electrical current through a powder to preferentially heat the asperities. Most of the energy serves to melt that portion of the compact where migration is desirable for densification; comparatively little energy is absorbed by the bulk materials and forming machinery. Naturally, this technique is not applicable to electrically insulating powders.

To allow efficient stacking of product in the furnace during sintering and prevent parts sticking together, many manufacturers separate ware using Ceramic Powder Separator Sheets. These sheets are available in various materials such as alumina, zirconia and magnesia. They are also available in fine medium and coarse particle sizes. By matching the material and particle size to the ware being sintered, surface damage and contamination can be reduced while maximizing furnace loading.

Continuous powder processing

The phrase "continuous process" should be used only to describe modes of manufacturing which could be extended indefinitely in time. Normally, however, the term refers to processes whose products are much longer in one physical dimension than in the other two. Compression, rolling, and extrusion are the most common examples.

In a simple compression process, powder flows from a bin onto a two-walled channel and is repeatedly compressed vertically by a horizontally stationary punch. After stripping the compress from the conveyor the compact is introduced into a sintering furnace. An even easier approach is to spray powder onto a moving belt and sinter it without compression. Good methods for stripping cold-pressed materials from moving belts are hard to find. One alternative that avoids the belt-stripping difficulty altogether is the manufacture of metal sheets using opposed hydraulic rams, although weakness lines across the sheet may arise during successive press operations.

Powders can also be rolled to produce sheets. The powdered metal is fed into a two-high rolling mill and is compacted into strip at up to 100 feet per minute. [6] The strip is then sintered and subjected to another rolling and sintering.[7] Rolling is commonly used to produce sheet metal for electrical and electronic components as well as coins. [8]Considerable work also has been done on rolling multiple layers of different materials simultaneously into sheets.

Extrusion processes are of two general types. In one type, the powder is mixed with a binder or plasticizer at room temperature; in the other, the powder is extruded at elevated temperatures without fortification. Extrusions with binders are used extensively in the preparation of tungsten-carbide composites. Tubes, complex sections, and spiral drill shapes are manufactured in extended lengths and diameters varying from 0.5-300 mm. Hard metal wires of 0.1 mm diameter have been drawn from powder stock. At the opposite extreme, large extrusions on a tonnage basis may be feasible.

There appears to be no limitation to the variety of metals and alloys that can be extruded, provided the temperatures and pressures involved are within the capabilities of die materials. Extrusion lengths may range from 3-30 m and diameters from 0.2–1 m. Modern presses are largely automatic and operate at high speeds (on the order of m/s).

Extrusion Temperatures Of Common Metals And Alloys
Metals and alloys Temperature of extrusion, K °C
Aluminium and alloys 673-773 400-500
Magnesium and alloys 573-673 300-400
Copper 1073-1153 800-880
Brasses 923-1123 650-850
Nickel brasses 1023-1173 750-900
Cupro-nickel 1173-1273 900-1000
Nickel 1383-1433 1110-1160
Monel 1373-1403 1100-1130
Inconel 1443-1473 1170-1200
Steels 1323-1523 1050-1250

Special products

Many special products are possible with powder metallurgy technology. A nonexhaustive list includes Al2O3 whiskers coated with very thin oxide layers for improved refractories; iron compacts with Al2O3 coatings for improved high-temperature creep strength; light bulb filaments made with powder technology; linings for friction brakes; metal glasses for high-strength films and ribbons; heat shields for spacecraft reentry into Earth's atmosphere; electrical contacts for handling large current flows; magnets; microwave ferrites; filters for gases; and bearings which can be infiltrated with lubricants.

Extremely thin films and tiny spheres exhibit high strength. One application of this observation is to coat brittle materials in whisker form with a submicrometre film of much softer metal (e.g., cobalt-coated tungsten). The surface strain of the thin layer places the harder metal under compression, so that when the entire composite is sintered the rupture strength increases markedly. With this method, strengths on the order of 2.8 GPa versus 550 MPa have been observed for, respectively, coated (25% Co) and uncoated tungsten carbides. It is interesting to consider whether similarly strong materials could be manufactured from aluminium films stretched thin over glass fibers (materials relatively abundant in space).

See also

Notes

  1. ^ International Powder Metallurgy Directory - PMawards
  2. ^ Todd, Robert H., Allen, Dell K., Alting, Leo, "Manufacturing Processes Reference Guide", 1st Edition, Industrial Press Inc., New York 1994, ISBN 0-8311-3049-0
  3. ^ http://www.pickpm.com/designcenter/isostatic.asp?locarr=2|1|2|2
  4. ^ Todd, Robert H., Allen, Dell K., Alting, Leo, "Manufacturing Processes Reference Guide", 1st Edition, Industrial Press Inc., New York 1994, ISBN 0-8311-3049-0
  5. ^ F. Thummler and W. Thomma, "The Sintering Process," Metallurgical Reviews No. 115, June (1967).
  6. ^ Manufacturing Engineering and Technology fifth edition
  7. ^ freepatentsonline.com
  8. ^ Manufacturing Engineering and Technology fifth edition

References

  • An earlier version of this article was copied from Appendix 4C of Advanced Automation for Space Missions, a NASA report in the public domain.

External links

  • [1]- Rapid manufacturing technique developed at the KU Leuven, Belgium

Advertisements






Got something to say? Make a comment.
Your name
Your email address
Message