Powder sintering is a manufacturing process that has been in use since the Industrial Revolution. Powder metallurgy is closely related to the ceramic sintering process, as both utilize a base powder component and apply heat to produce a hard, solid part. In this article we discuss the sintering process as it is used in powder metallurgy, including the various materials and techniques that produce the metal components used in everyday manufacturing.
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Sintering, by definition, is a process used in manufacturing to compact solid materials. The resulting product is a harder, stronger, more durable mass due to the high heat and pressure applied forcing the atoms of the material into tighter bonds with each other. Most manufacturing processes use a sintering furnace that can provide the necessary temperatures quickly and accurately. At its most effective, sintering materials reduces porousness while enhancing strength. Powder metallurgy is the specific study of the sintering process using powdered metals.
The manufacturing of pottery is the most common use for the sintering process, used the world over. It has been utilized for thousands of years in one form or another to strengthen clay materials. Most people, however, are surprised to learn that powder metallurgy has an equally storied history dating back to the ancient Egyptians over 5,000 years ago.
In contemporary manufacturing, metal powders have become more refined. As sintering technologies have improved, the actual sintering materials have also improved. The sintering powders used in the metal manufacturing process — stainless steel, aluminum, nickel, copper and titanium alloy — make up the vast majority of powder metals used in additive manufacturing today.
Powder metallurgy covers a broad range of manufacturing, but what each process shares is the use of sintering powder for metal part fabrication. The benefits of utilizing a furnace or oven — strengthening metal parts through heat and compaction — are core components of additive manufacturing. The sintering process allows us to create components which would decompose otherwise. Because sintering does not require manufacturers to consider solid-liquid phase changes, powder metallurgy is more flexible than conventional manufacturing techniques such as casting, forging and extrusion.
A variety of terms are commonly used to describe processes that are essentially sintering. These include: powder metallurgy, MIM (metal injection molding), consolidation, caking, and firing. Powder metallurgy is the process of pressing or injecting metal powders into solid objects. MIM, on the other hand, injects metal powder slurry with a molten polymer into a plastic mold tool. The polymer is then burned away and the temperature is raised to fuse the particles.
Consolidation is widely used in the ceramics industry to describe the similar process of press molding ceramic powders to form solids that are then kiln cured. Caking is used to describe the forming of various powder particles which become bonded together to form a solid “cake”. Lastly, firing describes the heat integration of particle-based forms in the ceramics industry.
The origins of sintering lie in pre-history, as all fired ceramics are essentially sintered clay particles. The wet fusion of the clay particles forms the “green” shape, followed by firing, which integrates the discrete blobs of wet clay into a single, durable item. Additionally, some metal powder decoration and the glazing of pottery represent primitive sintering methods, whereby glass and metals are induced to fuse from powders to solids by the application of heat.
Modern sintering began as a scientific/commercial area with the work of William Coolidge. He achieved ductile tungsten wire in 1909 by hot extrusion/drawing the powder-formed billets to make lamp filaments that were more durable than before.
Sintering works in a three-stage process:
Sintering processes are important in a variety of applications, including:
Used to produce components of great hardness, toughness, and precision.
Various approaches fall under the broad title of sintering, including:
As a wide spectrum of techniques, sintering finds application in a huge range of materials. These are listed below:
A wide range of metals can be used in sinter processes of several types. This includes: iron, iron-copper, copper steels, nickel steels, stainless steels (300 and 400 series), high-strength low-alloy steels (HSLA), medium- and high-carbon steels, and diffusion hardenable steels, brass, and bronze, and soft iron magnetic alloys. All of these can be built as green parts by 3D printing and then sintered to high-quality, low-porosity parts of excellent properties. Metals can be sintered by pressing, molding, and injection molding. For more information, see our guide on Metalloids.
Most ceramic processes are considered either sintering or close to sintering. A selection of commonly 3D (SLS or paste deposited) printed and then sintered ceramics are: alumina, aluminum nitride, zirconia, silicon nitride, boron nitride, and silicon carbide. Ceramics are generally sintered by compression or press molding.
Sintered polymers fall into two categories: large and small particle sintering. Large particle sintering with high porosity is commonly applied as filtration and pneumatic silencer materials and as flow diffusion controllers. These include: polyethylene, polypropylene, and polytetrafluoroethylene.
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Small particle sintered polymers are used in 3D printing in processes such as selective laser sintering. This is used to produce integrated and high-strength components with near-native material properties and near-zero porosity. Examples are: polyamides, polystyrene, thermoplastic elastomers, and polyether-ether ketones.
Sintering of composites is a more complex group of processes, and various materials are processed in different ways. Tungsten carbide uses tungsten and carbon powders. Pressure-heat oxidation transforms the carbon to carbide. This couples the metal powder, which remains unaltered. Glass, carbon, and metal fibers are experimentally included in metal powder sinters, to enhance properties. In some regards, the processing of carbon fiber is a sintering process. An adhesive matrix is compressed and heat activated to bond the carbon component. Metal oxide ceramics are experimentally composited with polymers such as PEEK to manufacture forms of resistive semiconductors. Sintering of composites is highly varied and can be achieved by compression, molding, and in limited cases injection molding
Various glass materials are used in sintering processes, including: ceramic glazes, silica glass, lead glass, as well as sintered glass slabs made from fused silica glass powder. Sintering of glass is generally done by compression molding.
Sintering consists of a series of steps, each of which is simple but requires great precision in control. The steps include:
The sintering process generally takes only seconds to complete. The post-form sintering step, however, can take several hours to complete. Sintered manufacture of parts is a rapid process by most methods. Powders and primary binders are pressed, molded, or injection molded to the uncured, green state at which stage they are oversize, porous, and not fully bonded. Parts are then heat treated to induce particle bonding.
Sintering is a manufacturing process used across many materials, including:
Components that are produced through sintering are listed below:
Sintering offers a variety of benefits:
Some of the risks of the sintering process including:
It depends. There is a wide spectrum of materials and processes in the sintering family. Generally, the “green” processes are non-hazardous, although metal and ceramic nano-particles are reported as having medical consequences for the human body and must be handled with care. The fusion part of sintering is a high-temperature stage that often involves driving off or burning polymer/wax components which can be toxic and irritant. Ventilation is required as well as normal safety precautions with hot and potentially inflammable evaporative/combustion processes.
Safety precautions to keep in mind when sintering are listed below:
No, sintering is not the same as melting. Sintering specifically does not involve the general melting of the part, but applies sufficient heat to fuse the particles without liquefying them. An excess of heat, in the case of polymer or metal parts, risks damaging the structure or shape of the part.
No, sintered metal parts are not stronger than forged or machined stock parts. When well manufactured, sintered parts can achieve the same strength as the machined equivalents.
Correctly sintered metal parts generally take on most or all of the mechanical properties of the primary constituent. In the case of stainless steels, for example, MIM parts will generally achieve 80–90% of the fatigue strength of wrought or cast parts because of larger crystal grain size and trace porosity causing weakness.
No, sintering is not the same as welding. While the fusion of powder granules into a whole does often involve a form of welding at the contact points, sintering differs widely from any process that falls under the “welding” banner, as welding involves the full liquefaction of the filler and native material at the weld point.
This article presented sintering explained what it is, and discussed how it works and its advantages and disadvantages. To learn more about sintering, contact a Xometry representative.
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