Powdered Metal Parts
The term "powdered metal parts" refers broadly to any solid metal part made through a process known as "powder metallurgy," which involves metal pressed into powder form.
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Powdered metal parts are used in a diverse set of industries, including automotive engineering, computer technology, electronics, hardware, and lawn and garden. Within these industries and others, powdered metal parts are valued for their properties, including magnetism and porosity, the latter which naturally dampens sound. The applications of these durable metal parts include automobile parts, bearings, bushings, magnetic assemblies, structural parts, and filtration systems.
The list of metal materials that can be converted into powder metal parts is quite long and includes steel, bronze, brass, aluminum, copper, and iron. All of these metals have different properties and different uses. Steel, including tool steel and stainless steel, has high strength and is popular for making parts related to automobile weight reduction.
Meanwhile, bronze and brass are both quite dense and mechanically useful. Bronze, however, is the denser of the two and exhibits the highest mechanical performance. Therefore, it is quite popular in the creation of self-lubricating bearings. Next, aluminum exhibits properties of lightness, high flammability, and conductivity. These qualities make it popular for use with high strength structural applications and pyrotechnics. Copper, on the other hand, exhibits quality thermal and electrical conductivity. Thus, copper parts are often used in electrical contractor and heat sink applications. Finally, iron powder, which contains a graphite additive, is useful in the production of filters, bearings, and structural parts.
Before powdered metal can be formed into a part, it must be formed. This is where the process of powder metallurgy, which takes metal from powder to a finished part, comes in. To kick things off, raw metal material is converted into powder by any of the following methods: atomization, chemical reduction, electrolytic techniques, mechanical alloying, or pulverization. Note that atomization is the most common process by which metal is turned into powder.
Regardless, once the raw metal material has been processed into powder form, it is impregnated with a lubricant that acts as a friction reducer when the powder is sent to the pressing dies. Next, the raw metal powder goes on to said pressing dies or any other tools and machines used in the forming process. Common among forming processes are forging, molding, and pressing. Once the metal has undergone forming, it is put through either sintering or metal injection molding. Most often, manufacturers choose sintering.
There are a number of reasons that sintering is more popular than metal injection molding. For one, sintered parts tend not to be shape sensitive, a fact that allows manufacturers to create more diverse parts. In addition, sintering, which uses approximately 97% of the material inputted, produces little waste. During sintering, the compacted raw materials (called green parts) are heated together in a furnace at temperatures below their melting part. In this way, the particles of the green parts are bonded, but the parts’ shapes do not change. Once sintering is complete, the powdered metal part has its final properties, and it also has gained extra strength and better control over its porosity. More porosity control means a part can self-lubricate and can be better used in liquid and gas filtration.
The alternative to sintering that some manufacturers use is metal injection molding, which offers more design freedom, reduced waste, and reduced assembly costs. It is useful in the creation of small, thin, complex, high performance, and high density parts with higher levels of corrosion resistance and magnetism. Such parts are used widely in the automotive, computer, dental, electronic, firearm, hardware, and medical industries. This process, however, is only useful in the creation of small, thin parts, and it comes with higher tooling costs than sintering. The process begins when manufacturers add wax or polymers/thermoplastics to the powdered metal via resin impregnation. Then, the metal and the additives are heated together until they become pliable. Once pliable, the combination is injected into a mold, which is then clamped shut. The heated metal is held here until it takes on the shape of the mold, cools, and solidifies.
If necessary, a powdered part can undergo secondary operations like machining, heating, deburring, or sizing. Finished powdered metal parts look solid, but technically, they are not. Rather, they are made up of many small, interconnected capillaries, which is why they are about 25% porous. If an application requires these pores be sealed, a manufacturer can put the finished part through a few treatments. These include infiltration with a metal that has a lower melting point, impregnation with oil or plastic resin, and steam treatment. Parts with very tight tolerances, such as bearings and bushings, sometimes require secondary sizing.