Powder metallurgy is one of the important course mechanical engineers must know. Products made of this material are common around us most especially auto parts. Powder metallurgy comprises a family of production technologies, used to manufacture components of various types. Besides, powder metallurgy which is abbreviated as PM is a term covering a wide range of ways in which materials or components are made from metal powders.
Today you’ll get to know the definition, importance, types, process, and application as well as the advantages and disadvantages of powder metallurgy in our modern life.
What is powder metallurgy?
Powder metallurgy is a manufacturing process that is used for producing finished or semi-finished objects. This is done by compressing the metal powder into suitable dies. This metallurgical process is one of the cheapest that offers high quality, and strength. It is also used to get complex shapes with a high degree of accuracy. Because of this, the process is suitable for mass production. Powder metallurgy mainly involves four basic steps which include powder preparation, mixing and blending, compacting, and sintering. All these will be further explained in this article.
Powder metallurgy is a process that has existed for more than 100 years, which is over the past quarter-century. It has been a superior method of producing high-quality parts for different important applications. The process was successful because it offers greater benefits over other metal forming processes such as forging and metal casting, etc. These benefits further include material utilization, shape complexity, and near-net-shape dimensional control, among others. Because of this, powder metallurgy was recognized as green technology.
Furthermore, powder metallurgy is used to make unique components that are impossible to get from melting or forming. A very similar important product is tungsten carbide (WC). It is used to cut and form other metals and is made from WC particles bonded with cobalt. This process is mostly used in industry for tolls of various types and globally ~50,000 tonnes/year (t/y) is made by PM.
Applications of powder metallurgy
Below are the applications of powder metallurgy components in various fields.
The Automotive applications
About 80% of powder metallurgy parts are for automotive applications. Around 75% of these components are for transmissions both automatic and manual and engine parts. These transmission applications include synchronizer system parts, clutch hubs, gear shift components, planetary gear carriers, turbine hubs, clutch, and pocket plates. The engine parts that are made with powder metallurgy include pulleys, sprockets, and hubs, particularly the ones associated with the engine timing belt system, valve guides, valve seat inserts, PM lobes for assembled camshafts, balancer gears, camshaft bearing caps, and engine management sensor rings.
Some other automotive system uses powder metallurgy in some of its parts which include:
- Oil pumps, especially gears.
- Shock absorbers such as piston rod guides, piston valves, and end valves.
- Anti-lock braking systems (ABS), its sensor rings.
- Flanges, oxygen sensor bosses of exhaust systems.
- Variable valve timing systems.
- Exhaust gas recirculation (EGR) systems
- Chassis components
- Continuously variable transmissions.
Other applications of powder metallurgy include:
- Aerospace applications
- Oil and gas industry.
- Health care sector, etc.
In a quick grip applications of powder metallurgy are:
- Cutting tools like cemented carbide tools, ceramic tools, etc. are Powder metallurgy products.
- Electric bushes made by mixing Cu and Ag with graphite is P/M product.
- Nozzles for rockets and missiles.
- Small parts in automotive and appliance applications where the ability to produce a nearly final shape requiring minimum machining, provides a strong economic advantage.
- Bearing, Bushes, etc.
- Magnetic soft metals like Fe, Fe-3Si, etc. are easily formed into a final shape by P/M.
Read more: Understanding the Soldering Process
Powder metallurgy process
Just as earlier stated, power metallurgy has basic four processes which include:
Before an object can be produced, the material must be converted into power. The various processes of producing such powder include atomization, grinding, chemical reaction, electrolysis process, etc.
Mixing and blending:
This powder metallurgical process involves mixing two or more material power to produce alloy material of high strength. Well, depending on the product requirement. This step ensures even distribution of the powder with additives, binders, etc. to improve the flow characteristic of the powder, lubricants are sometimes added in the blending process.
This process is to compress the prepared powder mixture into pre-defined dies. Compacting ensures voids are reduced and increases the density of the product. The powder is compacted into a mold using pressure to form a product which is called green compact. This means the product is formed by compacting. The pressure used ranges from 80 to 1600 MPa. Although the pressure depends on the properties of metal powder and binders. i.e., for soft powder compacting pressure is about 100 – 350 MPa, and for steel, iron, etc. The pressure is between 400 – 700 MPa.
Because the green compact produced by compressing is not that strong and cannot be used as a final product, sintering is performed. Sintering means heating the green compact at an elevated temperature so that a permanent strong bond can be obtained. The powder metallurgy process provides strength to the green compact and converts it into a final product. Generally, the sintering temperature is about 70 to 90 percent of the melting temperature of metal powder.
Read more: The Three basic types of soldering
Since the sintered object is more porous compared to fully dense material. The density of the product depends on the press capacity, sintering temperature, compressing pressure, etc. Sometimes, products do not need high density, making the sintered products to be used directly as final products. Although, a highly dense product is sometimes required (e.g., production of bearing). The secondary operation is required to make the product high density and high dimensional accuracy. The commonly performed secondary operation include sizing, coining, infiltration, hot forging, impregnation, etc.
watch the video below to learn the about powder metallurgy process:
Types of powder metallurgy process
Powder metallurgy processes can be performed in various ways, depending on the products to be produced. Below are the various types of powder metallurgy processes found in metal powder industries.
Conventional powder metallurgy process:
In the below diagram, the conventional types of powder metallurgy processes are explained. It involves mixing elemental or alloy powders, compacting the mixture in a die, and then sintering or heating. Just as explained above, the resultant shapes in an atmosphere-controlled furnace metallurgically bond the particles.
Just as earlier mentioned, most powder metallurgy parts weigh less than 5lb. (2.27 kg), although some parts that weigh 35 lb. (15.89 kg) can be fabricated in conventional PM equipment. Parts like bushings and bearings have simple shapes. Well, there is a sophisticated PM process today that can produce components with complex contours and multiple levels. These machines are quite economical.
Metal Injection Molding (MIM)
Metal injection molding has the manufacturing capability of producing complex shapes in large quantities. Fine metal powders typically less than 20 microns are used in this process. These metal powders are custom-formulated with a binder (different thermoplastics, waxes, and other materials) into a feedstock. The feedstock is fed into a cavity (multiple cavities) of a conventional injection-molding machine. When the “green” component is removed, almost all the binder is extracted by thermal or solvent processing. The rest of the binder is removed during the sintering process, which is performed in a controlled atmosphere furnace.
These types of powder metallurgy processes are very similar to plastic injection molding and high-pressure die casting. They can also produce many of the same shapes and configuration features. However, they have limited to relatively small (typically less than 250 grams), highly complex parts that would otherwise require extensive finish machining. The benefits of this metallurgical process are due to its ability to produce mechanical properties close to wrought materials. It is a net-shape process technology with good dimensional tolerance control. Finally, metal injection molding parts offer nearly unlimited shape and geometric-feature capability. It also has the capability of high production rates through the use of multi-cavity tooling.
Isostatic pressing is a popular type of powder metallurgy forming process. It applies equal pressure in all directions on a powder compact. Because of this, the achievement of maximum uniformity of density and microstructure without the geometrical limitations of uniaxial pressing.
Isostatic pressing can either be performed cold or hot. The diagram below describes the cold isostatic pressing. Cold isostatic pressing (CIP) is used to compact green parts at ambient temperatures. However, the hot isostatic pressing (HIP) fully consolidates parts at elevated temperatures through solid-state diffusion. This hot-pressing process can also be used to remove residual porosity from a sintered PM part.
Metal Additive Manufacturing
Metal additive manufacturing abbreviated as AM and can also be called 3D printing. This process has the potential to profoundly change the production, time-to-market, and simplicity of components and assemblies. The process does not work like the conventional or subtractive manufacturing processes (e.g., lathe machining or drilling). These processes create parts by removing material, which is not so in additive manufacturing. Additive manufacturing builds parts using a layer-by-layer process directly from a digital model.
In this process, molds or dies are not used which is why there is no wastage of much material which led to expense to the manufacturing process. Additive manufacturing has been used as a design and prototyping tool for a long period. However, its focusses is now changing to the direct production of components which include aircraft engine parts, medical implants, and jewelry.
This manufacturing process is not a single type of technology or process. However, while all additive manufacturing systems employ a common layer-by-layer approach, they still use a wide variety of materials, technologies, and processes.
Additive Manufacturing technologies that use metal powders include:
- Laser sintering (LM/SLS/SLFS)
- Selective inkjet binding (SIB)
- Electron beam melting (EBM)
- Laser powder forming (LPF)
- Powder bed process
- Fused deposition modeling (FDM)/Extrusion
Advantages and Disadvantages of the powder metallurgy process
Below are the benefits of the powder metallurgy process:
- Cost-effectiveness for mass production since there is no further machining cost, labor cost, etc.
- A highly skilled operator is not required.
- Some alloys can only be produced by PM technology.
- Bimetallic and laminated products are easily produced with this method.
- High production rate. Up to 500 to 1000 pieces of parts can be produced in an hour.
- The complex shape can be easily produced.
Despite the great advantages of the powder metallurgy process some limitations still occur. below are the disadvantages of the powder metallurgy process:
- The cost of equipment is high.
- it is expensive for a single production.
- Intricate designs can be difficult to produce because of the less flowability of metal powder.
- Complete uniform dense products cannot be produced.
- Product sizes are restricted due to pressing capacity.
- The metal powder which can produce an explosion cannot be used.
- It is difficult to cast low melting point metals by the PM process.
- Final products can experience low impact and fatigue properties.
Powder metallurgy is a process that has made mass production parts easier. It has been available for a while now but still advancing as technology keeps increasing. Today we’ve learned the definition, types, process, and applications of powder metallurgy. We’ve also examined its advantages and disadvantages.
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