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Powder Metallurgy
Alessandro Anzalone, Ph.D. Hillsborough Community Community College, Brandon Campus
Agenda 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.. 10
Introduction How Ho w P/ P/M Par Partts Are Are Mad adee Metal Po Powders Powd Po wdeer Co Comp mpac acttio ion n Sintering Seco Se cond ndar ary y Op Oper erat atio ions ns P/M P/ M Pro Produ duct ctss and and Thei Theirr Uses Uses Fact Fa ctor orss in De Desi sign gn of P/M P/M Pro Produ duct ctss Examples Refe Re fere renc nces es
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Introduction Powder metallurgy (P/M) is one of the four major methods of shaping metals (machining, hot and cold plastic deformation, casting, and P/M). The P/M process is essentially the compression of finely divided metal powder into a metallurgical bond between the particles. Although the P/M manufacturing method dates to the nineteenth century, it was not until recent decades that this field gained wide acceptance and use, and technological advances in P/M continue to grow very rapidly. Products that are difficult if not impossible to produce by other means are being manufactured with P/M at high production rates at very competitive cost. Parts manufactured by the P/M process have found a widespread use in a variety of applications. P/M products are used in the transportation industry (automobiles and trucks), in farm and garden equipment, and in household appliances. Many new applications will be found in the future for this unique method of forming metals.
How P/M Parts Are Made The basic conventional process of making PIM parts consists of two basic steps—compacting (molding) and sintering. In addition to these two basic manufacturing steps involved in the P/M process, secondary operations are commonl erformed to im art final desired ro erties to the P M product (e.g., coining, sizing, repressing). In the first step (compacting), loose powder (or a ble nd of different powders) is placed in a die and is then compacted between punches. This operation is commonly performed at room temperature. The compacted part, called a briquette or green compact, is now a solid shape; however, green compact can easily be broken or chipped and requires careful handling. In the second step (sintering) the briquette is heated in an appropriate atmosphere to a temperature high enough to cause the powder particles to bond together by solid-state diffusion and to homogenize any alloy constituents in the powder. Melting does not normally occur. The P/M part is now ready for use unless other finishing operations are needed.
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How P/M Parts Are Made Secondary operations may include sizing, machining, heat treating, tumble finishing, plating, or impregnating with oil, plastics, or liquids. Secondary operations can significantly increase the cost of the finished part, , possible, complete the product in the first two basic steps. However, the sintering process tends to deform and shrink the shaped briquette slightly, so some parts (e.g., precision gears) require a finishing operation to obtain the desired tolerances.
How P/M Parts Are Made
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How P/M Parts Are Made
http://www.youtube.com/watch?v=1Mjsi2F2MrY
Metal Powders A number of different metals and their alloys are used in P/M (e.g., iron, alloy steel, stainless steel, copper, tin, lead). The three most important methods of producing metal powders are (1) atomization, (2) chemical methods and electrol tic rocesses. Atomization is a process in which a stream of molten metal is transformed into a spray of droplets that solidify into powder. Molten metal spray can be produced in several ways. The most common method is to use a stream of high-velocity gas to atomize the molten metal. This method has several variations. In one method, the gas stream is expanded through a venturi tube, which siphons the molten metal from the crucible located below the tube. The gas breaks the stream of molten metal into small droplets that then solidify as they are carried by the gas stream. Iii another variation, the crucible with bottom gate is located above the gas tubes. The metal flows under the influence of gravity and passes through the gas stream, which breaks the molten metal stream. The solidified metal droplets are then collected n a collection chamber. In addition to gas, water and synthetic oils can also be used in the atomization process.
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Metal Powders Several chemical methods can be used to make metal powders, including reduction and precipitation. Chemical reduction is a process in which metal powders are formed by chemical reaction between metal oxides and . ., . monoxide reacts with oxygen in the metal oxide, producing pure metal. The electrolytic process that is utilized to precipitate metal powders begins in the electrolytic cell where the source of desired metal is the anode. As the anode is dissolved the desired metal is deposited on the cathode. After this step is complete, the metal deposit is removed from the cathode and is washed and dried.
Metal Powders In each process, the powders may be ground further to a desired fineness, usually in a ball mill. Metal powders are screened, and larger particles are returned for further crushing or grinding. The powders are classified accordin to article size and sha e in addition to other considerations such as chemical composition, impurity, density, and metallurgical condition of the grains. Particle diameters range from about 0.002 in. to less than 0.000 1 in. Test sieves are used to determine particle size. This method of testing has been standardized throughout the industry.
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Metal Powders Powders are often blended by tumbling or mixing. Lubricants (e.g., graphite) are added to improve flowability of material during feeding and pressing cycles. Deflocculants are also added to inhibit clumping and to improve .
Powder Compaction Compacting or pressing gives powder products their shape. Pressing and sintering techniques can be separated into two types: conventional and alternative. The method most commonly used today is the conventional a roach which consists of the ressin o eration first followed b sintering. The alternative techniques can be classified into (1) alternative compaction methods, (2) combined compaction and sintering, and (3) alternative sintering methods. In the conventional compacting process, the powder is pressed unidirectionally in a single- or a double-acting press. Unlike liquids, which flow in all directions under pressure, powders tend to flow mainly in the direction of the applied pressure. Engineering properties such as tensile and compressive strength depend to a great extent on the density of compacted material. Hot pressing, in which the powder is pressed in the die at a high temperature, produces a density approaching that of rolled metal. Die compaction can be done either hot or cold.
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Powder Compaction Compaction of powders with various presses has the advantage of speed, simplicity, economy, and reproducibility. Such compaction produces a strong, dimensionally accurate, and relatively inexpensive product; , . must be relatively small. Parts with a large aspect ratio will have uneven densities, being denser nearest the punches. These parts may have nonuniform and uncertain properties and should not be made by die compaction. Grooves or undercuts or parts with thin sections cannot be made by simple die compaction, so not every part is a good candidate for powder metallurgy; however, some of these limitations are overcome by alternative forming techniques such as split-die techniques to provide undercuts, isostatic pressing, and densification methods.
Powder Compaction Advanced Processes Because conventional presses can compact powder along only one axis, such presses cannot make some shapes. including hollow hemispheres, long arts and internal threads however one method allows ressure to be applied from all directions: isostatic pressing. In cold isostatic pressing (CIP), the powder is loaded into molds made of rubber or other elastomeric material and subjected to high pressures at room temperature. Pressure is transmitted to the flexible container by water or oil. The compacted parts are removed and sintered, followed by secondary operations if needed. With hot isostatic pressing (HIP), an inert gas such as argon or helium is used in a pressure chamber to provide the squeeze. This gas is reclaimed between each batch of pressings. Hot isostatic pressing provides more density and achieves a finer microstructure than the cold process. Powders are often preformed to an oversize shape prior to being placed in the isostatic chamber. Heat is applied to the preform by induction for a short time while the gas pressure compacts the preform. Temperatures may be as high as 1600 to 2000°F (871 to 1093°C) with pressures in excess of 15,000 psi. Isostatic pressing is useful only for certain special applications.
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Powder Compaction CIP is a comparatively slow process, and HIP is even slower. Parts made by either CIP or HIP are not limited by the shape constraints of rigid tooling.
Powder Compaction Powder Forging Fully dense P/M parts equaling or surpassing the mechanical properties of wrou ht roducts are bein roduced in commercial uantities b owder forging (P/F). The green compact or preform is made in a c onventional press and then sintered. These operations are then followed by a restrike (forge) that brings the part to the final density. Mechanical properties may sometimes exceed those of wrought metals because a more uniform composition is achieved in P/M processes. Fatigue strength and impact strength are particularly high in powder forgings compared with conventional P/M parts. P/M bearing races have been shown to outlast wrought steel races by a factor of 5 to 1.
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Powder Compaction Metal Powder Injection Molding A P/M technology that borrows a plastic injection molding process shows . , variations of this process can use plastic molding machinery. In order to inject powders into molds, the particle size must be much finer than that used for conventional P/M processes. This “dust” is combined with a thermoplastic binder. The molding step is performed at injecting pressures of about 900 psi and about 325°F (163°C). The result is a green compact that is sintered in the conventional fashion after the thermoplastic binder has been removed in an oven at about 400°F (204°C). Thin walls, high densities, unsymmetrical shapes, and accurate dimensions are possible with this method.
Powder Compaction Metal Powder-to-Strip Technology Direct rolling of metal strip from a powder slurry (powder- to-strip process) is a rocess in which thin stri s are directl roduced avoidin numerous hot or cold rolling operations. In this process, an appropriate powder mix is blended with water and a cellulose binder to form a fine slurry. The slurry is deposited on a moving band as a continuous film. After dr ying, the moving strip is compacted between rolls and then sintered, first to remove the binder and then to bind the particles. It is rolled a second time and resintered to remove porosity. As in all these advanced P/M processes, metals or alloys that cannot be formed in any other way can be produced with powdered metals. Bimetal alloys can be produced in a strip, and high-strength titanium strip is being produced for the aircraft industry.
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Powder Compaction
Powder Compaction Powder Extrusion Metal powders can be hot extruded with or without presintering. Metal owders are laced inside a can that is then evacuated and sealed. The unit is then heated and extruded. Metal billets and tubing are made from powder by this process.
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Sintering In solid-phase sintering the green compact part must be heated to 60 to 80 percent of the melting point of the constituent with the lowest melting point. This usually requires from 30 minutes to 2 hours in a sintering . changes that take place during the process of solid phase sintering: 1. Diffusion This takes place on or near the surface of the particles as the temperature rises. For example, any carbon present in the voids between the particles will diffuse (penetrate) into the metal particles. 2. Densification Particle contact areas increase considerably. Voids decrease in size, therefore lowering porosity. As a result, there is an overall decrease in the size of the part during the sintering process. The green compact must be made larger to allow for this shrinkage. 3. Recrystallization and Grain Growth Because sintering is usually carried out well above the recrystallization temperature of metals, grain growth can occur within and in between particles. Methods of inhibiting excessive grain growth are often used, since large grains tend to weaken metals.
Sintering Alternatively, liquid-phase sintering is carried out above the melting point of one of the constituents. When one of the blended metal powders has a melting point below the sintering temperature, a liquid phase of that metal fills the voids between the articles that do not melt. Infiltration is a process in which the pores or voids of a sintered or unsintered compact are filled with a metal or alloy of a lower melting point. For example, a steel—copper compact is heated at a temperature lower than the melting point of the steel compact and higher than the melting temperature of copper. The molten copper is drawn into the pores of the compact and fills the voids. This process increases densities and tensile strengths considerably.
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Sintering High density and low porosity are not always desirable. Porous filters or prelubricated bearings are produced by loose sintering or by combining the powder with a combustible or volatile substance, which is later . sometimes formed with very low pressures or none at all, and then loose sintered (called pressureless sintering) followed by a cold forging operation. Sintering furnaces on production lines are typically of the continuous type. Furnace atmospheres usually consist of a hydrocarbon gas; however, with certain metals or alloys other gases may be used. Some manufacturers use a nitrogen gas atmosphere for both ferrous and nonferrous metals.
Secondary Operations For many products the slight variations in dimension that occur during sintering are acceptable; however, where close dimensional tolerances must be maintained the product must be finished after sintering. Common secondar o erations are densification sizin im re nation infiltration heat treatment, surface treatment and machining.
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P/M Products and Their Uses A wide array of small parts made by the P/M process. Manufactures often choose P/M over other manufacturing methods because of the following characteristics of P/M parts: 1. Superior engineered microstructures and properties with precise control 2. Consistent properties and quality 3. Controlled porosity for filters and self-lubrication 4. Very low scrap loss 5. Wide variety of shape designs 6. Unlimited choice of alloys and composites 7. Low-cost, high-volume production 8. Good surface finishes 9. Close dimensional tolerances 10. Little or no machining required.
P/M Products and Their Uses Surprisingly, P/M steels are almost as strong as wrought steels, and powder forging processes increase the tensile strength. The versatility of the P/M process allows parts to be made lighter than with other manufacturing rocesses. P M rocesses allow arts to be made of ver hard metals such as tungsten carbide cutting tools for machine tools. The P/M process can also be used to make friction materials (in the form of bimetal powder materials that are bonded to a steel base) and aluminum-based antifriction materials containing graphite, iron, and copper. Copper— nickel powders are often formed as a layer on steel strip and then sintered, The sintered strip is impregnated with babbitt metal and formed into bearings for automobile and aircraft engines.
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P/M Products and Their Uses Some disadvantages of P/M are found in the conventional cold die compacting and sintering process. Since P/M products are somewhat porous and present a larger internal surface to any corrosive atmosphere, . tend to have reduced plastic properties (ductility and impact strength) compared with conventionally produced metals. Probably the greatest advantage in adopting the P/M process over other methods of manufacture is that it allows for redesign that makes use of the great versatility of the P/M process. Many small mechanisms such as those found in pneumatic drills, electronic printers and sequencers, door locks, firearms, and sewing machines have a number of small parts that can often be combined into one piece by redesigning for powder metallurgy. Savings in production can be realized and the mechanism can be simplified as well. P/M is not suitable for every metal product, but its use should always be considered when designing a part to be manufactured.
Factors in Design of P/M Products When parts are made by the conventional powder metal process, several elements of design should be observed. Thin sections and feathered edges should be avoided. Generous fillet radii should always be pro vided in a die and internal holes should have rounded corners. External corners should be chamfered, and narrow deep slots should be avoided. Splines or key- seats should have rounded roots. Secondary machining processes are common practice for precision P/M parts. Holes, tapers and drafts, counter- sinks, threads, knurls, and undercuts usually must be machined after the parts are sintered; however, holes in the direction of pressing are readily produced in P/M parts. Round holes are easiest to produce, but shaped holes, keys, splines, hexagonals, squares, and any blind holes can also be made.
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Factors in Design of P/M Products
Factors in Design of P/M Products
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Examples
Examples
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Examples
References 1.
2. . 4.
R Gregg Bruce, William K. Dalton, John E Neely, and Richard R Kibbe, , Modern Materials and Manufacturing Processes, Prentice Hall, 3rd edition, 2003, ISBN: 9780130946980 http://www.ktf-split.hr/glossary/image/ball_mill.gif . . . . http://www.xhmfm.com/english/cp.htm
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Powder Metallurgy
Alessandro Anzalone, Ph.D. Hillsborough Community College, Brandon Campus
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