What Is Powder Metallurgy?
Powder metallurgy is a term covering a wide range of ways in which materials or components are made from metal powders. This process can avoid, or greatly reduce, the need to use metal removal processes, thereby drastically reducing yield losses in manufacture and often resulting in lower costs.
Powder metallurgy is a metal forming process in which compacted metal powders are heated to just below their melting point. Although the process has been around for more than 100 years, over the past quarter-century it has been widely recognized as a superior method for making high-quality parts for a variety of important applications.
This success is due, among other things, to the advantages that the process offers over other forming technologies such as forging and metal casting, advantages in material use, the complexity of the shape, and the control of the shape in the vicinity of the network shape. These in turn contribute to sustainability and make powder metallurgy a recognized green technology.
Powder Metallurgy Process
These production technologies generally involve all or most of the following process steps:
- Production of Metal Powder
- Mixing and Blending
- Compaction and shaping of the powder
- Sintering of the compact to enhance integrity and strength
Sometimes, this process accomplished with some secondary operations like sizing, coining, infiltration, hot forging, etc.
1. Powder production
Properties of products produced using powder metallurgy are dependent on the characteristics and properties of the powder.
One of the processes used to produce powder for powder metallurgy is melt atomization. In this process, liquid metal is broken into tiny droplets that cool and solidify into minute particles.
Though atomization is the most common method for producing powder, other processes include chemical reduction, electrolytic deposition, grinding, and thermal decomposition.
Regardless of which process is used, all metals and alloys can be converted into a powder.
Prior to mixing and blending the powder, it is evaluated and tested for its appropriateness for the powder metallurgy process. The factors that are considered are flow rate, density, compressibility, and strength.
2. Mixing of powders
In the mixing and blending process, powders are combined with other powders, binders, and lubricants to ensure the final part has the necessary characteristics. Blending and mixing can be completed wet or dry depending on the type of powder metallurgy process and the requirements of the part.
The four most common blending and mixing techniques are rotating drum, rotating double cone, screw mixer on the interior of a drum, and blade mixer on the interior of a drum.
3. Forming of the mixed powder into a compact
Compacting involves pressing and compressing the powder mixture into the desired shape or die. When done properly, compacting reduces potential voids and significantly increases the density of the product. The compressed and pressured form is referred to as a green compact, an indication that the part was formed by compacting.
Compacting pressure is between 80 MPa and 1600 MPa. Each type of metal powder requires a different amount of compacting pressure depending on its properties.
In soft powder compacting, the pressure is between 100 MPa and 350 MPa. For more resilient and harder metals, such as steel and iron, the pressure is between 400 MPa and 700 MPa.
4. Sintering of the compact to enhance integrity and strength
Though the green compact has been stressed and pressed at extreme pressure, it is not strong enough to be used. In order to produce a permanent bond between the metal particles, the green compact is sintered or heated at high temperature.
In essence, sintering produces the final usable product or part. The atmosphere of the sintering is controlled such that it has a sufficient amount of carbon to produce a neutral or carburizing environment, which determines the properties of the sintered materials.
Sintering is a heat treatment wherein large numbers of parts, in compacted form, are subjected to temperatures that are sufficient to cause the loose particles to unite and bond, forming a solid piece.
The required temperature fluctuates in accordance with the type of metal but is always slightly lower than the metal’s melting temperature.
Compacting presses the particles of the green compact to form a shape. Regardless of the pressure applied during compacting, there are still minute porous spaces in the green compact.
During sintering, the material is heated at high temperatures to close the porous spaces and strengthen the part.
5. Secondary operations
The application of finishing processes to the sintered part. In powder metallurgy, such processes are often referred to as “secondary operations”.
Advantages of the powder metallurgy process
- Minimizes machining by producing parts at or near-final dimensions
- Minimizes scrap losses by typically using more than 97% of the raw material in the finished part
- Allows a variety of alloy systems
- Produces a good surface quality
- Provides materials that can be heat-treated to increase strength or wear resistance
- Provides controlled porosity for self-lubrication or filtration
- Facilitates the creation of complex or unique shapes that would be impractical or impossible with other metalworking processes
- Is suitable for the production of medium to high volume components
- Provides long-term performance reliability in critical applications