March 3, 2024

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INTRODUCTION TO POWDER METALLURGY

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Powder metallurgy (PM) is a term that encompasses many methods of manufacturing materials or parts from metal powders. The PM process eliminates or greatly reduces the need to use metal extraction processes, greatly reduces lost production revenue, and often reduces costs. Iron is often used for sintering. Powder metallurgy is also used to melt or shape unique materials in other ways to make them impossible. A very important product of this type is tungsten carbide (WC). WC is used for cutting and shaping other metals and is made from WC grains combined with cobalt. PM is widely used in industry for all types of tools, with ~50,000 tonnes (t/a) of PM produced worldwide. Other products include sintered filters, porous grease impregnated bearings, electrical contacts and diamond tools. Since the advent of industrial-scale powder metal additive manufacturing (AM) in the 2010s, selective laser sintering and other AM metal processes represent a new category of commercially important powder metallurgy applications. Compaction and sintering processes in powder metallurgy typically consist of three main steps: powder mixing (grinding), compaction and agglomeration. Pressing is usually carried out at room temperature, while the high temperature sintering process is usually carried out at atmospheric pressure with a carefully controlled atmospheric composition. Additional additional operations such as coining or heat treatment are often performed for special properties or improved accuracy.

One of the older methods, and one that is still used to make about 1 million tons of structural parts from iron-based alloys, is a process in which fine (micron) metal powder (normal iron) is mixed with additives such as lubricants. . It uses wax, carbon, copper and/or nickel to press it into a mold of the desired shape, then heats the pressed material (the “green part”) in a controlled atmosphere to bond the materials together by sintering. It is usually very close to the dimensions of the mold, but produces accurate parts with 5-15% porosity, so it has the properties of a mild forged steel.

There are several other PM processes that have been developed over the past 50 years. These include:

Powder Forging: A “shape” made by the conventional “press and sinter” method is heated and then hot-forged to maximum density to achieve near-forged properties.

Hot Isostatic Pressing (HIP): Here the powder (usually an atomized, spherical gas) is poured into a mold consisting of a metal “tire” of suitable shape. The can is vibrated, then emptied and sealed. It is then placed in a hot isostatic press, heated to about 0.7 homologous temperature, and subjected to an external gas pressure of ~100 MPa (1000 bar, 15,000 psi) for several hours. The result is a full density machined or cast part with better properties. HIPs were invented in the 1950s and 1960s and entered mass production in the 1970s and 1980s. [citation needed] In 2015, it was used to produce ~25,000 tons per year of stainless steel and tool steel , as well as basic superalloy components for jet engines.

Metal Injection Molding (MIM): Here a very fine spherical powder (a plastic binder or wax to near maximum solids loading (typically around 65 vol%) and cast by injection to form a “green”. “.” form layer. Complex geometric pieces. This part is then heated or otherwise treated to remove the binder (bond), resulting in a “brown” part. The part is then sintered and reduced by ~18% to produce a complex, 95-99% dense finished part (~3 micron surface roughness). Invented in the 1970s, production has increased since 2000, with global production estimated at 12,000 tonnes in 2014, worth €1.265 billion.

Electrical Current Assisted Sintering (ECAS) technology relies on electricity for small powders, which has the advantage of significantly reducing production time (from 15 minutes to a few microseconds for the slowest ECAS), eliminating the need for long heating times in ovens. It is close to the theoretical density, but has the disadvantage of a simple shape. The powders used in ECAS avoid binders due to their direct agglomeration potential without prepressing and green compaction. The mold is designed for the final shape of the part as the powder is compressed as it fills the cavity under pressure, avoiding the problem of shape deviation due to anisotropic aggregation and gravity-induced deformation at high temperature. The most common of these techniques is hot pressing, used to produce diamond tools used in the construction industry. Spark plasma sintering and electrosintering are two modern commercial industrial ECAS technologies.

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