This article is devoted to powder metallurgy (PM) and the peculiarities of that branch of materials science. The technological and economical advantages of PM methods compared with traditional metallurgical processes for production of metal products are discussed. The main stages of PM technology, namely the processes of powder production, shaping, and densification of the end bodies, and the effects of these operations on both human health and the environment are also discussed. In that respect, the methods of metal powder production are viewed in more detailed. There are several methods for metal powder production and features of these methods that determine the set of physical, chemical, and technological properties of the powders. That relationship is demonstrated using scanning electron microscopy (SEM). Toxicity and other harmful properties of certain metal powders with numerous practical applications are also discussed.
POWDER METALLURGY METHODOLOGY
Powder metallurgy (PM) is an industrial method for obtaining metal or metal-like powders, molding semifinished goods from powders, and manufacturing particles from them by a thermal process, called sinter-ing. The sintering temperature is below the melting temperature of the main component in the powder mixture. Because of the similarities between methods for ceramic (shaping – thermal treatment) and PM (molding – sintering) production, the end bodiesproduced by PM are also called metalloceramics. The term “powder metallurgy” soundsprovocative or at least suspicious on the pages of a publication when the authorwants to propagandize the achievements of a technology, the first step of whichis metal or ceramic powder production. Could a technology be connected with powderproduction and its processing in a way that is friendly to both environment andhumanity, especially when our every-day practice shows that metallurgy is a mainnatural pollutant? We want to show here the achievements of a technology thatis not only economically profitable but environmental favorable.
HISTORICAL PERSPECTIVE
The first historical example for industrial applications of PM is the chiseling of high-quality platinum (Pt) coins by intaglio. The method was developed by Sobolevski and Liubarski in Russia and practiced from 1826 to 1844.1The first modern powder metallurgy product was the tungsten (W) filament of electric light bulbs, developed in the early 1900s. During this time, the production of materials and products was closely related to the achievements of the necessary technological conditions, primarily the realization of high temperatures and the reliability of the corresponding equipment and materials. Although the process has existed for more than 100 years, over the past quarter century it has become widely recognized as a superior way to produce high-quality parts for a variety of important applications. This success is due to the privileges that the PM process offers over other metal-forming technologies (such as forging and metal casting), advantages in material utilization, shape complexity, and near-net shape dimensional control. These, in turn,yied benefits of lower costs and greater production versatility.
ENVIRONMENTAL IMPACT
PM is closely connected with technologies that determine its relationship to environmental protection. Obtaining and manipulating solids in powder state is an essential feature of PM. If PM is limited to the production of metal or metal-like powders, it would be just a part of metallurgy and could not be a progressive, technologically, and economically attractive method combined with metallurgy, materials science, and metalworking. Elimination (in most cases) or at least minimization of machining of the end article leads to economic advantages. As more than 97% of the starting materials reach the finished product, powder metallurgy is a process that conserves both energy and materials. Elimination of scrap losses, which directly reflects on environmental protection, is another privilege of the PM method, providing many possibilitiesto create waste-free and environmentally friendly processes.
ADVANTAGES AND APPLICATIONS OF PM
PM could use wastes obtained by other traditional metallurgical processes. The utilization of copper oxides obtained after cable production is a good example. This simple technology allows the burning of engine oil, presented as an impurity, to obtain pure fragile copper oxide flakes. After milling of copper oxide and reduction with hydrogen (H2) at 450 °C, one obtains pure copper (Cu) powder of very high quality that is suitable for use in the electrical industry for production of copper-graphite brushes. Controlling the parameters of the processes of milling and reduction (type of mill, milling conditions, time, and temperature), one could obtain Cu powder with defined chemical, physical, and technological properties. In this example, another advantage of PM is demonstrated — the creation of composite materials from physically and chemically different (as copper and graphite) components. Very often, PM is the only technology able to lead to the production of materials and articles with specific properties, such as self-lubricating bearings, hard alloy cutting tools from tungsten carbide (WC)-based alloys, magnet materials, copper-graphite brushes for electric engines, catalysts, and hydrogenstorage materials for hydrogen economics, among others.
Figure 1 presents an SEM image of boron carbide (B4C) particles. This material is directly and dramatically connected with human body protection. Dense plates from B4C powders find numerous applications in the military for anti-bullet protection. Dense B4C pellets find applications in nuclear energy as neutron-absorbing material, thus controlling peaceful nuclear reactions. PM is the only method for producing this superhard material.2 In some cases, the conversion of a cast or wrought component to powdered metal provides a cost savings of 40% or higher. To produce a part for a contemporary reactive jet, weighing 0.45 kg, by a traditional metallurgical technology requires 8.6 kg of molded metal; the PM technique requires only 2.95 kg of the metal. Machine parts produced by PM are maintained very close to dimensional tolerances. That is very important for lowering costs. PM allows strict control on technological parameters, such as pressing, sintering temperature, etc. The combination of chemical (purity, presence of oxygen layers or impurities, etc.), physical (particle size and size distribution, surface area, etc.), and technological (fluidity, pressability, and sinterability) properties of starting powders are responsible for the behavior of PM articles during the processes of their shaping and densification by pressing and sintering. This determines the importance of the route for metal powder production. There are different industrial methods for this as all are responsible for the specific set of properties that is the “calling card” of the powder. It is customary to subdivide the methods of metal powder production into physical-chemistry and mechanical. The first method is based on deep chemical and physical transformation of the composition and structure of the starting material. Reduction, electrolysis, and thermal decomposition are base methods in that group. The “snow balls” of B4C shown in Figure 1 are obtained by so called plasmo-chemical synthesis.
The second type of method transforms starting material into powder without a change in composition. Dispersion of the metal melts and milling in different apparatuses are good examples. Interaction of certain metal powders with water and hydrogen (e.g., Al powders) makes water pulverization very dangerous. Strict measures to prevent hydrogen accumulation and its explosion should be taken. Figure 2 shows an SEM image of molybdenum (Mo) powder with a mean particle size of approximately 200 µm obtained by ball milling. Morphological peculiarities reveal products with low technological properties (pressability and sinterability). Sometimes, metal powder production is a contradiction of technological and work safety viewpoints. Low temperature reduction of metal oxides, in order to obtain fine-grained metal powders that lead to high sinterabili-ty, may result in products with pyrophoric properties. These powders, in contact with air, tend to self-ignite. The burning proceeds at very high speed and very high temperatures. Iron (Fe) and cobalt (Co) powders, which find numerous applications in PM, are good representatives of these materials. Strict safety measures should be taken during manipulation and storage of fine (< 10 µm) and ultrafine (< 0.5 µm) Fe and Co powders. Organic liquids, vacuum techniques, and an inert atmosphere of argon (Ar) are used to protect such powders from self-ignition.3
NICKEL POWDERS
Figures 3 and 4 show SEM images of nickel (Ni) powders obtained by hydrogen reduction of nickel oxide at 800°C and thermal decomposition of nickel carbonyl [Ni(CO)4], respectively. Differences in morphological properties of these powders obtained by different methods are obvious. Nickel powders find numerous applications in many branches of industry: production of specialsteels, superalloys, and alloys with controlled expansion (named Kovar), shape-memory alloys, Ni-La hydrogen accumulation alloys, dental alloys, implants, etc.
Being technologically attractive for specialists and curious for readers, these particles could be harmful and dangerous for human health. Nickel is an allergic agent and may cause different dermal effects. It is estimated that as many as 10–20% of women and 1–2% of men have been sensitized to nickel. The mechanism of nickel penetration by skin is based on the dissolving of pure metal by sweat. It is advisable not to wear cheap jewelry and to always demand a test for nickel allergy from your dentist if nickel-containing alloys for metal-to-ceramic dental constructions are used. Nickel also has carcinogenic effects and may cause throat or lung cancer. That is why very strict precautions should be taken when developing Ni powder metallurgy. These measures should include human and environment protection and strict observation of the corresponding standards for nickel dust concentrationsin the air of work places.
There are many materials that can be toxic but are safe to use when handled in the right way and exposures kept at acceptable levels. Wearing protective clothes and respiratory masks as well as strict control of dust levels in the air are necessary to minimize the risk of inhaling nickel or nickel-containing powders.
CONCLUSIONS
Based on materials science knowledge, PM is a very flexible and versatile technology. Newer methods and cheaper high-quality materials appear as a result of PM R&D activities. In that respect, we can show a metal injection molded (MIM) technique whose products have shown constant progress over the past several years. Strong markets for MIM products include tools, medical parts,electronics, firearms, etc. Spark plasma sintering (SPS) is a new process for obtaining fully dense, high-quality materials in a short time frame, thus retaining a fine-grained structure in the product. Recently, new methods for obtaining nanostructured and nanosized metal powders, alloys, and compounds, such as mechanochemical synthesis, self-propagating high-temperature synthesis (SHS), and combinations of these methods with thermal synthesis, have drastically changed the technological design for metal and ceramic powder synthesis.4 After intense mechanical treatment in high-energetic mills, such as attritors and planetary mills, certain metal powders acquired pyrophoric properties.5,6 In the presence of oxygen, some mechanically induced interactions proceeded by explosive kinetics. That is why very strict safety measures, including the use of protective inert atmospheres, glove-boxes, etc., should be observed when working with mechanically activated metal powders. PM supplies the industry with low-cost production of precision, high-performance materials and products and no other metalworking process can match its competitive advantages. Could it be friendly to the environment? It all depends on us.
References:
POWDER METALLURGY METHODOLOGY
Powder metallurgy (PM) is an industrial method for obtaining metal or metal-like powders, molding semifinished goods from powders, and manufacturing particles from them by a thermal process, called sinter-ing. The sintering temperature is below the melting temperature of the main component in the powder mixture. Because of the similarities between methods for ceramic (shaping – thermal treatment) and PM (molding – sintering) production, the end bodiesproduced by PM are also called metalloceramics. The term “powder metallurgy” soundsprovocative or at least suspicious on the pages of a publication when the authorwants to propagandize the achievements of a technology, the first step of whichis metal or ceramic powder production. Could a technology be connected with powderproduction and its processing in a way that is friendly to both environment andhumanity, especially when our every-day practice shows that metallurgy is a mainnatural pollutant? We want to show here the achievements of a technology thatis not only economically profitable but environmental favorable.
HISTORICAL PERSPECTIVE
The first historical example for industrial applications of PM is the chiseling of high-quality platinum (Pt) coins by intaglio. The method was developed by Sobolevski and Liubarski in Russia and practiced from 1826 to 1844.1The first modern powder metallurgy product was the tungsten (W) filament of electric light bulbs, developed in the early 1900s. During this time, the production of materials and products was closely related to the achievements of the necessary technological conditions, primarily the realization of high temperatures and the reliability of the corresponding equipment and materials. Although the process has existed for more than 100 years, over the past quarter century it has become widely recognized as a superior way to produce high-quality parts for a variety of important applications. This success is due to the privileges that the PM process offers over other metal-forming technologies (such as forging and metal casting), advantages in material utilization, shape complexity, and near-net shape dimensional control. These, in turn,yied benefits of lower costs and greater production versatility.
ENVIRONMENTAL IMPACT
PM is closely connected with technologies that determine its relationship to environmental protection. Obtaining and manipulating solids in powder state is an essential feature of PM. If PM is limited to the production of metal or metal-like powders, it would be just a part of metallurgy and could not be a progressive, technologically, and economically attractive method combined with metallurgy, materials science, and metalworking. Elimination (in most cases) or at least minimization of machining of the end article leads to economic advantages. As more than 97% of the starting materials reach the finished product, powder metallurgy is a process that conserves both energy and materials. Elimination of scrap losses, which directly reflects on environmental protection, is another privilege of the PM method, providing many possibilitiesto create waste-free and environmentally friendly processes.
ADVANTAGES AND APPLICATIONS OF PM
PM could use wastes obtained by other traditional metallurgical processes. The utilization of copper oxides obtained after cable production is a good example. This simple technology allows the burning of engine oil, presented as an impurity, to obtain pure fragile copper oxide flakes. After milling of copper oxide and reduction with hydrogen (H2) at 450 °C, one obtains pure copper (Cu) powder of very high quality that is suitable for use in the electrical industry for production of copper-graphite brushes. Controlling the parameters of the processes of milling and reduction (type of mill, milling conditions, time, and temperature), one could obtain Cu powder with defined chemical, physical, and technological properties. In this example, another advantage of PM is demonstrated — the creation of composite materials from physically and chemically different (as copper and graphite) components. Very often, PM is the only technology able to lead to the production of materials and articles with specific properties, such as self-lubricating bearings, hard alloy cutting tools from tungsten carbide (WC)-based alloys, magnet materials, copper-graphite brushes for electric engines, catalysts, and hydrogenstorage materials for hydrogen economics, among others.
The second type of method transforms starting material into powder without a change in composition. Dispersion of the metal melts and milling in different apparatuses are good examples. Interaction of certain metal powders with water and hydrogen (e.g., Al powders) makes water pulverization very dangerous. Strict measures to prevent hydrogen accumulation and its explosion should be taken. Figure 2 shows an SEM image of molybdenum (Mo) powder with a mean particle size of approximately 200 µm obtained by ball milling. Morphological peculiarities reveal products with low technological properties (pressability and sinterability). Sometimes, metal powder production is a contradiction of technological and work safety viewpoints. Low temperature reduction of metal oxides, in order to obtain fine-grained metal powders that lead to high sinterabili-ty, may result in products with pyrophoric properties. These powders, in contact with air, tend to self-ignite. The burning proceeds at very high speed and very high temperatures. Iron (Fe) and cobalt (Co) powders, which find numerous applications in PM, are good representatives of these materials. Strict safety measures should be taken during manipulation and storage of fine (< 10 µm) and ultrafine (< 0.5 µm) Fe and Co powders. Organic liquids, vacuum techniques, and an inert atmosphere of argon (Ar) are used to protect such powders from self-ignition.3
Figures 3 and 4 show SEM images of nickel (Ni) powders obtained by hydrogen reduction of nickel oxide at 800°C and thermal decomposition of nickel carbonyl [Ni(CO)4], respectively. Differences in morphological properties of these powders obtained by different methods are obvious. Nickel powders find numerous applications in many branches of industry: production of specialsteels, superalloys, and alloys with controlled expansion (named Kovar), shape-memory alloys, Ni-La hydrogen accumulation alloys, dental alloys, implants, etc.
Being technologically attractive for specialists and curious for readers, these particles could be harmful and dangerous for human health. Nickel is an allergic agent and may cause different dermal effects. It is estimated that as many as 10–20% of women and 1–2% of men have been sensitized to nickel. The mechanism of nickel penetration by skin is based on the dissolving of pure metal by sweat. It is advisable not to wear cheap jewelry and to always demand a test for nickel allergy from your dentist if nickel-containing alloys for metal-to-ceramic dental constructions are used. Nickel also has carcinogenic effects and may cause throat or lung cancer. That is why very strict precautions should be taken when developing Ni powder metallurgy. These measures should include human and environment protection and strict observation of the corresponding standards for nickel dust concentrationsin the air of work places.
There are many materials that can be toxic but are safe to use when handled in the right way and exposures kept at acceptable levels. Wearing protective clothes and respiratory masks as well as strict control of dust levels in the air are necessary to minimize the risk of inhaling nickel or nickel-containing powders.
CONCLUSIONS
Based on materials science knowledge, PM is a very flexible and versatile technology. Newer methods and cheaper high-quality materials appear as a result of PM R&D activities. In that respect, we can show a metal injection molded (MIM) technique whose products have shown constant progress over the past several years. Strong markets for MIM products include tools, medical parts,electronics, firearms, etc. Spark plasma sintering (SPS) is a new process for obtaining fully dense, high-quality materials in a short time frame, thus retaining a fine-grained structure in the product. Recently, new methods for obtaining nanostructured and nanosized metal powders, alloys, and compounds, such as mechanochemical synthesis, self-propagating high-temperature synthesis (SHS), and combinations of these methods with thermal synthesis, have drastically changed the technological design for metal and ceramic powder synthesis.4 After intense mechanical treatment in high-energetic mills, such as attritors and planetary mills, certain metal powders acquired pyrophoric properties.5,6 In the presence of oxygen, some mechanically induced interactions proceeded by explosive kinetics. That is why very strict safety measures, including the use of protective inert atmospheres, glove-boxes, etc., should be observed when working with mechanically activated metal powders. PM supplies the industry with low-cost production of precision, high-performance materials and products and no other metalworking process can match its competitive advantages. Could it be friendly to the environment? It all depends on us.
References:
- Schatt, W. et al. Pulvermetallurgie, Sinter-und Verbundwerkstoffe.“ Veb Deutscer Verlag Fur Grundstoffindustrie, August 1977.
- Ghosh, D. et al. “Dynamic Indentation Response of Fine-Grained Boron Carbide.” Journal of the American Ceramic Society, June 2007.
- US Patent, No 4030913, 1977.
- Radev D. et al. “Mechanically Activated Self-Propagating High-Temperature Synthesis of Nanometer-Structured MgB2.” Physica C, January 2005.
- Radev D, and Klissurski D. “Properties of TiB2 Powders Obtained in a Mechanochemical Way.” Journal of Alloys and Compounds, April 1994.
- Radev D. “Pulvermetallugie des Titans.“ Metall, January 1998.
