According to BCC research, the powder metallurgy market (including metal powders and sintered parts) exceeded $25 billion in 2022 and is expected to reach $34 billion in 2027.
Why do engineers favor powder metallurgy (PM) so much?
I’d say it’s due to its ability to achieve near-net shapes, cost-effectiveness in mass production, tight tolerances, and consistent quality from batch to batch.
This article will give you an in-depth understanding of what is powder metallurgy process.
Tables of content.
Powder metallurgy is a vast manufacturing process that covers powder production, material processing, and part fabrication. It is an excellent method for producing structural parts at fast speeds and with tight tolerances.
Who wouldn’t like that?
Powder production is the first step in PM. It’s no exaggeration to say that the characteristics of the powders determine the quality of the final components. Among the various powder production methods, the most common are gas atomization and water atomization.
Gas atomization is the process of breaking up molten metal into metal droplets by high-pressure gas. The metal droplets solidify during flight and settle in the bottom collection tank. You can utilize air, nitrogen, argon, and helium as atomizing media. Gas atomized powder is spherical and has a low oxygen content (<0.1%).
Gas atomization is suitable for powder metallurgy, metal injection molding, hot isostatic pressing, and additive manufacturing (3D printing).
GA has the capability to produce the powders of copper, aluminum and its alloys, magnesium, zinc, titanium, titanium alloys, nickel-base alloys, cobalt-base alloys, lead, and tin.
If you find gas atomized powder too costly, you can opt for water atomized powder. Water atomization, as the name implies, is the process of converting molten metal into metal powder by atomizing it with water. Although water atomized powder is relatively cheap, it has a higher oxygen content.
This powder has an irregular shape, low packing density, and poor fluidity. Nevertheless, the irregular shape is conducive to the mechanical interlocking between the powders, and could press out the green compact with a higher density. Water atomized powder is mostly applied in PM and MIM.
Chemical decomposition mainly produces carbonyl iron powder and carbonyl nickel powder. Take carbonyl iron powder as an example. First, the iron raw material reacts with carbon monoxide to produce iron carbonyl. Then it is decomposed at high temperatures to produce iron powder. The specific chemical reaction formula is as follows.
The particle diameter of carbonyl iron powder is 1~8 microns and is spherical. Carbonyl iron powder is very suitable for MIM.
This process is ideal for producing reduced iron powder. You make it by passing hydrogen or carbon monoxide into iron oxide. The resulting powder, known as sponge iron powder, has an irregular shape with many internal pores. To obtain the finished powder, additional steps such as grinding, sieving, annealing, and decarburizing are necessary.
This irregularly shaped iron powder is well-suited for manufacturing workpieces that require superior green strength. Its porous nature also makes it perfect for the production of oil-impregnated bearings.
Mixing is the process of mixing metal powders with lubricants and binders.
Powders include mixed powders and pre-alloyed powders.
Pre-alloyed powders are produced by adding alloy components to molten metal when reproducing powder particles. The pros of pre-alloyed powders are their uniform microstructure and mechanical properties, but the cons are that they have increased hardness and poor compressibility.
Mixed powder generally refers to a mixture of iron powder, copper powder, carbon powder and a binder. The disadvantage is that it is difficult to mix evenly, but the advantage is that it is easy to produce dense workpieces. The effects of adding lubricants are as follows:
Compaction is a forming process where metal powder is filled into a mold cavity with a powder box, and then pressed into shape by the powder metallurgy press. The resulting product is called “green compact‘’.
There are several main compaction methods:
Single action compaction: In this process, the upper punch applies pressure and the lower punch and die do not move. This method easily results in a product with high density at the top and low density at the bottom. Do it is only suitable for thinner components. This press is cheap.
Double action compaction: During the forming process, the top punch and the bottom punch compress the powder at the same time. This results in a uniform green density
Floating die: In this method, the upper punches press down, the lower punch does not move, and the die drops to half of the upper punch stroke.
The first two methods utilize a demolding technique where a downward punch ejects the workpiece. In the latter method, the die continues to descend, exposing the workpiece at the top of the die.
Green compact is as strong as chalk, so you need sintering process to improve its mechanical strength.
The lubricants and binders mentioned earlier assist in powder flow and forming. However, they must be removed before the high-temperature sintering process, as they hinder bonding and densification between the powders. Typically, pre-sintering is conducted at 500 to 900 degrees Celsius for 30 to 45 minutes, during which these lubricants and binders evaporate into gas. It’s important to heat gradually, as heating too quickly might cause defects such as bubbles, bursts, or cracks.
High-temperature sintering diffuses and bonds metal particles at temperatures below their melting point, usually between 80% and 90% of the melting point. It includes 3 stages.
When the green compact is heated to the sintering temperature, atoms begin to diffuse at the contact points between the particles, forming surface contacts (also known as necks).
As the sintering time increases, diffuse becomes more pronounced. The atoms diffuse through the volume and crystals. The necks become larger, and the distance between atoms decreases. As a result, the product becomes less porous and denser. This stage is significant to enhance the mechanical strength of the finished product.
In the final stage of sintering, interconnected pores become isolated pores. Gas is trapped inside the pores but could be slowly expelled. Due to the resistance of compressed gas, it is difficult to density. It also leads to rapid growth of gain structure. At this stage, you must balance the densification and gain growth to avoid compromising the mechanical properties of the sintered parts.
The shrinkage of the components during the initial and final stages of sintering is minimal, typically around 2% to 3%. The majority of sintering phenomena, including significant densification and bonding, occur during the intermediate stage.
Cooling is the gradual reduction of the workpiece from the high sintering temperature to room temperature, which takes about 2 hours.
Cooling not only helps to eliminate thermal stresses but also prevents defects caused by cooling too quickly.
The main functions of the sintering atmosphere are as follows:
Common sintering atmospheres include:
Here are the typical sintering temperatures and atmospheres used for various metal powders.
| Materials | Temperature(℃) | Atmosphere |
| 17-4 PH | 1200–1360 | Hydrogen |
| 316L | 1250–1380 | Hydrogen |
| 410 | 1250-1375 | Hydrogen |
| 420 | 1200–1340 | Nitrogen |
| 440C | 1200–1280 | Nitrogen |
| 304 | 1250–1375 | Hydrogen |
| Ti-6Al-4V | 1140–1250 | Argon/ Vacuum |
| Inconel 718 | 1200–1280 | Vacuum |
If you want to achieve more complex shapes or improve the dimensional accuracy and surface finish of your workpieces, secondary operations are necessary.
Although PM can create complex shapes, it is challenging to manufacture parts with transverse holes and grooves. So that’s why CNC machining comes into play.
Sizing restores the inner diameter, outer diameter, flatness, and other dimensions of sintered part to the correct size. During sizing, the sintered parts are placed in a die, and pressure is applied to correct any dimensional deviations that occur during sintering, such as shrinkage or warping.
For example, in a product with a diameter of 25 mm, sizing enables improve the tolerance from IT8-IT9 to IT6-IT7. Sizing helps maintain strict dimensional tolerances and enhances the surface finish of the product. Besides, sizing can seal the surface pores of oil-impregnated bearings.
PM products contain many pores, which is not ideal for high-pressure environments. Although copper infiltration could fill these pores, it is more expensive. Resin impregnation is a more cost-effective alternative. Typically, resin is impregnated into the pores of parts using high pressure or vacuum.
When a bearing is in operation, lubricating oil is usually added to reduce friction. Yet, for some bearings that are difficult to access, it is inconvenient to add lubricating oil. This is where oil-impregnated bearings are needed. The oil impregnation process is similar to resin impregnation, so I won’t go into details here.
Copper infiltration is a process used to enhance the mechanical properties of sintered structural parts, typically iron-based. During this process, copper is melted and drawn into the pores of the porous metal parts by capillary action, improving strength and density.
Copper infiltration improves the density, strength, hardness and wear resistance of sintered components.
Heat treatment is mainly to improve product strength and toughness. Common ones include carburizing and nitriding.
During compacting, due to the gaps between the upper punch, lower punch, core rod, and die, burrs are easily formed after powder filling.
You can employ tumbling and sandblasting to remove these burrs. Tumbling knocks the workpiece against the ceramic grinding piece, and the friction by a vibration grinding machine. This reduces burrs and improve surface finish.
Sandblasting is the process of impacting the surface of a product with an abrasive media such as sand, aluminum oxide, or silicon carbide. This process is carried out in a closed environment. Sometimes, sandblasting is performed first, followed by tumbling, to achieve a burr-free surface finish.
Electroplating is a surface finish process that uses an electric current to deposit a thin metal layer from an electrolyte onto a sintered component. This process enhances the surface properties of PM components, boosting corrosion resistance, wear resistance, and electrical conductivity.
After machining, sandblasting, heat treatment, and other processes, sintered products may have grease, fine sand, chemical residues, and other contaminants adhered to them. These must be removed, and we often use ultrasonic cleaning machines to clean them.
Iron
Iron has good strength and magnetism and is inexpensive. Most structural parts are made from iron-based materials.
Stainless Steels
Stainless steel has excellent corrosion resistance due to its chromium content of more than 10%. It also offers reliable strength, hardness, and magnetic properties. Common stainless steels include the 300 series, 400 series, and 17-4PH.
Copper
Copper, including bronze and brass, has favorable electrical and thermal conductivity, making it ideal for electronic components. Copper is suitable as a substrate for self-lubricating bearings and can also improve the density and strength of iron-based parts through the copper infiltration process.
Aluminum Alloys:
Aluminum alloys are suitable for making lightweight structural parts.
Titanium Alloys
Titanium alloys, such as Ti-6Al-4V and Ti-6Al-5Nb, offer elevated strength, excellent corrosion resistance, and good biocompatibility. They have a range of applications including aerospace, medical, and automotive.
Nickel-Based Superalloys
Nickel-based superalloys, like Inconel® 718, are known for their excellent oxidation resistance, high-temperature strength, corrosion resistance, along with strong creep resistance. They are primarily used in the aerospace, automotive, and chemical industries.
Soft Magnetic Materials
Iron-based powders mixed with other elements create magnetic materials used in motors, transformers, and magnetic sensors.
Common lubricants include zinc stearate, lithium stearate, and Acrawax (ethylene bis-stearamide, EBS).
In terms of lubrication, zinc stearate is best, lithium stearate is second, the Acrawax is worst. Most iron-base structural parts employ zinc stearate. Since EBS can be easily removed by heating without residue, it is suitable for products with strict control of pollutants, like stainless steel parts.
Polyvinyl alcohol (PVA) is a common binder.
Punches are responsible for pressing metal powder. The number of punches depends on the shape of the product. Punches are mostly made of tool steel, such as M2 and A2 (AISI standard) or SKD11 (JIS standard).
The die is where the metal powder is formed and needs to withstand the axial pressure and the lateral pressure exerted by the powder. Therefore, the die is mostly made of CPM10V, ASP-60, or C11.
The core rod serves to create the internal features of the product.
Because the core rod is long and easily breaks during demolding, it is made of elevated tensile strength tool steel.
The function of powder mixer is to mix metal powder and lubricant evenly to meet the pressing requirements.
There are several types of powder mixing equipment: V-shaped mixer, double cone mixer, and drum mixer.
The tonnage of powder compaction presses typically ranges from 35 to 1,000 tons. Powder metallurgy presses are mainly divided into mechanical presses, hydraulic presses, and electric presses (servo-motorized presses). Mechanical presses are fast, while hydraulic presses provide more uniform pressure. Electric presses are the newest type and have the capability to manufacture products with complex shapes and tight tolerances.
Sintering furnaces include conveyor belt furnaces, pusher furnaces, vacuum furnaces, and others.
Conveyor belt furnaces and pusher furnaces are continuous furnaces with high production efficiency, capable of sintering 2-3 tons of components per day. Vacuum furnaces, on the other hand, are mostly batch furnaces, typically sintering about 200 kg of products at a time.
Automobile manufacturing is an important market for the powder metallurgy industry. There are more than 1,000 powder metal components in automobiles.
Engine components:
Timing gears
Camshaft sprockets
Valve guides
Oil Pump rotors
Oil Pump gears
Connecting rods
Transmission Components:
Synchronizer Hubs
Planetary Gear Carriers
Clutch Plates
Shift Forks
Others: ABS Sensor Rings and exhaust manifold flanges
Metal injection molding is a special powder metallurgy process. It has the capability to manufacture products that are small, highly accurate, and biocompatible.
Examples are metallic orthodontic brackets orthopedic implants, medical devices, and surgical tools.
Aerospace engineers favor lightweight, high temperature resistant, and mechanically strong parts. Because it not only ensures the safety of aircraft in harsh working environments but also reduces fuel consumption.
Powder metallurgy is able to manufacture these excellent performance products, such as compressor blades, turbine disks, and combustion chamber liners.
Although there are not as many powder metal parts on motorcycles as on cars, there are still many, including
You also see the PM applications in the tools. Some gears in power tools, bushing can be made by PM.
In addition, diamond tools for grinding, cutting or drilling are often made using PM. Diamond particles are embedded in a metal matrix created by PM.
Since powdered metal parts have multiple internal pores, it is an ideal process for manufacturing porous filters. Sintered filters have a wide range of applications, including but not limited to filtering water, filtering oil and gas, reducing noise, and diffusing oxygen.
Advantages
Near Net Shape
PM is a near net shape process, which means it has high material utilization and less secondary processing.
Materials
Some high melting point metals and hard metals are difficult to machine, but PM overcomes this challenge. A wide range of materials, including ferrous and non-ferrous metals, are suitable for powder metallurgy
Complex Shapes
Powder metallurgy is capable of manufacturing components with complex shapes, such as thin walls, varying step heights, along with angled features. These characteristics are difficult to achieve with forging and stamping.
Tight Tolerance
PM could achieve strict dimensional tolerances. Usually within ±0.05mm is acceptable.
Mass Production
PM processes like pressing and sintering can be highly automated, enabling the quick production of large quantities with minimal human intervention. Continuous sintering furnaces, like conveyor belt furnaces, are built for non-stop operation, boosting output and efficiency.
Batch Consistency
The variations between batches of powdered metal parts are minimal. Consistency between batches is important for the stable and reliable operation of the products.
Green Manufacturing
PM is considered a green manufacturing process for the following. Powder metallurgy generates very little waste and typically uses less energy than processes like casting or forging due to lower processing temperatures and fewer steps. Additionally, PM produces less harmful wastewater and gases compared to traditional manufacturing methods.
Disadvantages
Size& Shape
Due to the limitations of powder metallurgy presses, PM is not suitable for producing very large parts.
Density
Powder metallurgy is not capable of manufacturing full dense products, and the density usually does not exceed 95%.
Tooling& Equipment
As we all know, powder metallurgy molds are very precise, and materials like tungsten carbide used in these molds are quite expensive, leading to substantial mold costs. Furthermore, the powder metallurgy process demands more equipment, which increases the initial investment.
Surface Finish
Unlike other workpieces, powder metal parts have many pores inside. So the pores need to be filled before the electroplating process. This will increase production costs.
The scale of the Chinese PM market grew from US$1.9 billion in 2017 to US$2.35 billion in 2021. Chinese powder metallurgy technology is well-developed, and its products are consistently high in quality. These products have been serving the European and American automotive industries for many years. Moreover, the pricing is highly competitive.
If you have any questions or insights, please feel free to share them in the comments below.
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