What is Sintering? Definition, How It Works, Types, and Process
Sintering is a broad family of methods that forms integral and potentially highly complex parts from a wide range of materials, by compressing and then fusing powders and binders. The part goes through two stages. The first stage makes the required shape a weak and poorly integrated, oversize component. The second stage bakes that component to expel the binder material and causes the remaining particles to fuse together and coalesce into a full-strength part. This approach is becoming widespread in metals and ceramics, as a way of producing complex net-shape parts of great mechanical properties by a low per-part cost process that is highly repeatable. This article will define sintering, how it works, the types of sintering, and the different processes.
Sintering is the process of first bonding powder shapes into integrated solids by compaction. The shape is then heated to below the melting point of one of the powders to enable thermal fusion to bond particles. This burns away any intermediate bonding agent that previously served to hold together the form and bonds the remaining “green” (i.e., unsintered) materials. This process forms solid objects from powdered metal, ceramic, or composite materials.
When the compressed-powder parts are heated, causing particles to bond, the process consolidates any voids. This results in a close-to-100% density, approximating the properties of the main material. Temperatures for processing are precisely controlled. To fuse, the contact point must barely melt in order to fully retain the pre-fusion shape while joining as one body. Figure 1 are examples of sintered parts:
Sintered parts in mass production.
Image Credit: Surasak_Photo
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:
- A primary part powder blend, with a bonding agent, is formed into a desired shape. The bonding agent sticks the powder together to make the shape of the part. This bonding agent can be water but it is more commonly a wax or a polymer.
- When the green part is fired, the bonding agent evaporates or burns away.
- The temperature then rises sufficiently for one of two processes that are essentially identical to occur. Either the primary particles heat enough to just begin to melt, causing the individual particles to fuse at their surfaces. Or an intermediate bonding agent such as bronze melts and couples between the particles, leaving the primary component power in an unaltered state.
Sintering processes are important in a variety of applications, including:
Used to produce components of great hardness, toughness, and precision.
- Used to produce intricate shapes and geometries that are hard to achieve using more normal manufacturing methods.
- Merges the properties of multiple materials, delivering the toughness of one component married to the abrasion resistance of another.
- Lower-cost tooling for manufacturing complex parts and geometries. The complexity must be produced once in a primary press or mold tool and reproduced accurately in bonded powders.
- Enable rapid mass production of components while retaining precision and repeatability.
Various approaches fall under the broad title of sintering, including:
- Solid-State Sintering: Powdered material is heated to a temperature just below the melting point. This bonds the particles together by atomic diffusion at the grain boundaries.
- Liquid Phase Sintering: Uses the addition of a small amount of a solvent liquid to the powder to induce low porosity and bonding. This liquid is then driven off, generally by heating, to create an integrated solid.
- Reactive Sintering: Uses a chemical reaction of at least one of the phases of powder particles during heating. It alters the chemistry which results in particle coupling in the chemically changed mass.
- Microwave Sintering: A novel approach applied to ceramics. Heat is induced using microwaves, and it is claimed that this results in the faster and more complete integration of the structure.
- Spark Plasma Sintering: Uses an electric current and physical compression of the powder to integrate the powder into a whole.
- Hot Isostatic Pressing: Uses high pressure and high temperature applied to a powder to form the required shape and fuse the particles.
- Cold sintering: Uses a transient solvent and pressure to consolidate polymer powders into a solid mass.
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.
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:
- Composition: Add and mix the required primary materials and primary coupling agents.
- Compression: Press the powder (slurry or dry) to the required shape.
- Heat: Heating aims to achieve the removal of the primary coupling agent and fusion of the primary material into a low-porosity whole.
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:
- Polymers: For rapid prototyping, filter and silencer manufacture, and specialist composite components.
- Metals: Most small metal components such as gears and pulleys can be made by sintering. Coarse metal powders are also sintered to make filters, silencers, and oil-loaded bearings.
- Ceramics: By some measure, most ceramics are manufactured by a form of sintering. In particular, zirconia and alumina ceramics are appearing as options in 3D printing. Small parts such as gears and bearings for high-temperature use are often sintered from ceramics.
Components that are produced through sintering are listed below:
- Automotive parts such as gears and actuators.
- Electrical components such as switchgear.
- Cutting tools of all types, for milling, drilling, and reaming.
- Aerospace components such as fuel valve components, actuators, and turbine blades.
- Biomedical implants such as prosthetic joints.
Sintering offers a variety of benefits:
- The process can produce highly repeatable and accurate parts.
- The cost of establishing production is easily amortized over large production.
- Parts can achieve great cosmetic results and require no finishing processes.
- Non-machinable geometries for mass production are easily achieved.
Some of the risks of the sintering process including:
- Powder consistency can vary if not well controlled, altering shrinkages or overall component properties.
- Initial forming process controls must be precise to achieve consistent and repeatable results.
- Post-forming “cure” processes are varied and require very tight control to precisely set shrinkage and prevent distortion.
- The cost of establishing production is high, so if the product doesn’t sell this can be wasted.
- Production process variations can result in weak and variable parts.
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:
- Wear suitable protective equipment, to protect from heat and potential airborne hazards.
- Use a well-ventilated area. Sintering can produce fumes and vapors that should be treated as harmful.
- Follow safe procedures for handling materials, as sintering powders can be hazardous.
- As with all hot processes, have a fire extinguisher on hand and know how to use it.
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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