The manufacturing of powder metal products and the science of powder metallurgy represents cutting edge technology and utilizes innovative materials and processes. With lower production costs and greater geometrical and material flexibility, more and more products that are typically associated with standard machining are now being manufactured with powder metal technology. Although it is thought of as a 20th-century technology, its roots go back over 5000 years to the Egyptians who used a rudimentary form of iron powder metallurgy as early as 3000 BC for weapons and ornaments. In the early 20th century, at the dawn of the industrial age, powder metal manufacturing reached a turning point. A tungsten filament, which is a powder metal product, was adopted for use in the newly invented light bulb, beginning the first mass production of the process. Sintered tungsten filaments are still used in light bulbs today.
The advantages of powder metals are as relevant now as they were to the ancients; because parts are sintered rather than cast, the amount of energy and effort required is greatly reduced. But the BTUs are just the beginning; because parts are formed in a die and sintered, they are at or very close to their final dimensions. This greatly reduces costly secondary machining operations and scrap and is further enhanced by the processes’ inherently good surface finish. Because the raw material is in a powder form, it also allows for the use of a wide range of specialty alloys. These advantages have spurred exponential growth in powder metallurgy materials technology; in fact, the term “powder metal” is no longer an adequate description due to the addition of so many non-metal substances. Modern powder metal products are rarely made from metal alone, incorporating ceramic fibers and intermetallic compounds. Some contain no metal at all, such as whisker-reinforced ceramic matrix composites or oxide dispersion-strengthened intermetallic compounds. The number of materials used in the powder metal industry has become so large that the term “particulate materials” has come to be used to describe this material category.
The advantages of powder metals are as relevant now as they were to the ancients; because parts are sintered rather than cast, the amount of energy and effort required is greatly reduced. But the BTUs are just the beginning; because parts are formed in a die and sintered, they are at or very close to their final dimensions. This greatly reduces costly secondary machining operations and scrap and is further enhanced by the processes’ inherently good surface finish. Because the raw material is in a powder form, it also allows for the use of a wide range of specialty alloys. These advantages have spurred exponential growth in powder metallurgy materials technology; in fact, the term “powder metal” is no longer an adequate description due to the addition of so many non-metal substances. Modern powder metal products are rarely made from metal alone, incorporating ceramic fibers and intermetallic compounds. Some contain no metal at all, such as whisker-reinforced ceramic matrix composites or oxide dispersion-strengthened intermetallic compounds. The number of materials used in the powder metal industry has become so large that the term “particulate materials” has come to be used to describe this material category.
Process BasicsThe powder metal manufacturing process holds the material in three basic states: raw, formed and sintered. Raw materials are prepared through various methods such as solid-state reduction, electrolysis, atomization, centrifugal atomization, mechanical comminution, thermal decomposition, and mechanical alloying, just to name a few. The processes of raw material manufacturing warrants volumes of text to adequately credit the innovation that is going into developing these new materials (see future blogs for more info). Once a raw material is selected, it is blended with lubricants, which help to decrease the porosity and create greater pore-free density characteristics of the compacted parts.The compaction process can be performed either hot or cold. Cold compaction, or cold isostatic pressing, is suited for materials that will require extensive secondary operations such as hot extrusion, hot rolling, and forging. Cold compaction is typically used to produce semi-fabricated products including bars, billets, sheets, and roughly shaped components. Hot compaction, or hot isostatic pressing, combines compaction, sintering, and vacuum into one operation and produces parts of much higher density than cold compaction. It requires typical process temperatures of 1120°C and pressures of 100 MPa. This equates to a much greater capital investment of equipment, but the end result is a part that requires much less finishing. For parts formed with cold compaction, sintering is a separate process, which is performed in a two-zone furnace. The first zone burns off the lubricants, while the second zone, which runs at a higher temperature, bonds the particles. Specific atmospheres including vacuum may also be required during the sintering process, depending on specifications. Once sintered, the parts are ready for secondary operations, which can include anything from heat treating to forging.
The versatility of this process and the materials used have allowed it to find its way into a growing number of applications. This trend is certain to grow.
As I researched this subject, it became clear that a single blog on powdered metallurgy would not suffice. Look for future blogs covering various process-related subjects.
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