Blog 17: What is Powder Metallurgy?
Introduction:
Powder metallurgy is an attractive manufacturing process for producing cost-effective high-performance components. These parts have superior mechanical behavior and corrosion resistance suited for a great range of engineering applications. While traditional forms of metal manufacturing like forging and casting have been around for hundreds of years, there are some applications where the best option is powder metallurgy manufacturing.
Powder metallurgy is a process for forming metal parts by heating compacted metal powders to just below their melting points. In other words, PM is a metal shaping process that creates near-net parts from powdered metal. PM comprises a family of production technologies, which process a feedstock in powder form to manufacture components of various types.
Although the process has existed for more than 100 years, over the past quarter century it has become widely recognized as a superior way of producing high-quality parts for a variety of important applications. This success is due to the advantages the process offers over other metal forming technologies such as forging and metal casting, advantages in material utilization, shape complexity, and near-net-shape dimensional control, among others. These, in turn, contribute to sustainability, making powder metallurgy a recognized green technology.
Powder metallurgy process:
Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape or form (compacting), and then heating the compressed material in a controlled atmosphere to bond the material (sintering).
The powder metallurgy process consists of five basic steps:
- Powder manufacture
- Powder blending
- Compacting
- Sintering
- Extra processing
Powder Manufacture:
Virtually all iron powders for PM structural part production are manufactured using either the sponge iron process, water atomization, centrifugal disintegration, etc. Nonferrous metal powders used for other PM applications can be produced via a number of methods.
- Sponge iron process: In the process, selected magnetite (Fe3O4) ore is mixed with coke and lime and placed in a silicon carbide retort. The filled retort is then heated in a kiln, where the reduction process leaves an iron “cake” and a slag. In subsequent steps, the retort is emptied, and the reduced iron sponge is separated from the slag and is crushed and annealed. The resultant powder is highly irregular in particle shape, therefore ensuring good “green strength” so that die-pressed compacts can be readily handled prior to sintering, and each particle contains internal pores (hence the term “sponge”) so that the good green strength is available at low compacted density levels.
- Atomization: Atomization is accomplished by forcing a molten metal stream through an orifice at moderate pressure. A gas is introduced into the metal stream just before it leaves the nozzle, serving to create turbulence as the entrained gas expands (due to heating) and exits into a large collection volume exterior to the orifice. The collection volume is filled with gas to promote further turbulence of the molten metal jet. Air and powder streams are segregated using gravity or cyclonic separation. Most atomized powders are annealed, which helps reduce the oxide and carbon content. The water atomized particles are smaller, cleaner, and nonporous and have a greater breadth of size, which allows better compacting. The particles produced through this method are normally spherical or pear shape. Usually, they also carry a layer of oxide over them.
There are three types of atomization:
- Liquid atomization
- Gas atomization
- Centrifugal atomization
- Centrifugal decentration: The metal to be powdered is formed into a rod which is introduced into a chamber through a rapidly rotating spindle. Opposite the spindle tip is an electrode from which an arc is established which heats the metal rod. As the tip material fuses, the rapid rod rotation throws off tiny melt droplets which solidify before hitting the chamber walls. A circulating gas sweeps particles from the chamber.
Powder Blending
This can often involve the introduction of alloying additions in elemental powder form or the incorporation of a pressing lubricant. Since the metal is in powder form the making of alloys becomes easier. Through this even the distribution of the metal can be mixed properly creating high-strength alloys
Compacting of Powder
The dominant consolidation process involves pressing in a rigid toolset, comprising a die, punches, and, possibly, mandrels or core rods. However, there are several other consolidation processes that are used in niche applications. The metal is pressed into the final shape they are to be and then pressed through die pressing or other machines. Some of the parts made from this process require some minor secondary operation to be used. While the rest of the parts are sent to the sintering process.
Sintering of compact powder
This process step involves heating the material, usually in a protective atmosphere, to a temperature that is below the melting point of the major constituent. In some cases, a minor constituent can form a liquid phase at sintering temperature; such cases are described as liquid phase sintering.
Secondary operations
The application of finishing processes to the sintered part. In the Powder Metallurgy industry, such processes are often referred to as “secondary operations”.
Application of Powder Metallurgy:
Powder metallurgy parts are used in a variety of end products such as:
- Hardware
- Automobile parts
- Washing machines
- Power tools
- Copiers and postage meters
- Hunting knives
- Hydraulic assemblies
- X-ray shielding
- Oil and gas drilling well-head components
Advantages of Powder Metallurgy:
The PM process provides a host of advantages over competing metalworking technologies, including:
- Cost-effectiveness
- Shape and material flexibility
- Application Versatility
- Part-to-part uniformity
