The Powder Metallurgy Process Step by Step

Powder Metallurgy Definition

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 also used to make unique materials impossible to get from melting or forming in other ways. A very important product of this type is tungsten carbide. Tungsten carbide is used to cut and form other metals and is made from tungsten carbide particles bonded with cobalt. It is very widely used in industry for tools of many types and globally 50,000 tones/year is made by powder metallurgy. Other products include sintered filters, porous oil-impregnated bearings, electrical contacts and diamond tools.

Since the advent of industrial production scale metal powder based additive manufacturing in the 2010s, selective laser sintering and other metal additive manufacturing processes are a new category of commercially important powder metallurgy applications.

Powder Metallurgy Process

Powder Metallurgy comprises a family of production technologies, which process a feedstock in powder form to manufacture components of various types. These production technologies generally involve all or most of the following process steps:

  1. Production of Metal Powder
  2. Mixing and Blending
  3. Compaction and shaping of the powder
  4. Sintering of the compact to enhance integrity and strength
Powder Metallurgy Process

Sometimes, this process accomplished with some secondary operations like sizing, coining, infiltration, hot forging, etc.

Production of Metal Powder

This is a first and basic step for producing an object by powder metallurgy process. Any material can convert into powder.

Virtually all iron powders for PM structural part production are manufactured using either the sponge iron process or water atomization. Nonferrous metal powders used for other PM applications can be produced via a number of methods.

There are various processes of producing powder such as atomization, grinding, chemical reaction, electrolysis process, etc.

Mixing and Blending:

As the name implies, this step involves the mixing of two or more material powder to produce a high strength alloy material according to the product requirement.

This can often involve the introduction of alloying additions in elemental powder form or the incorporation of a pressing lubricant.

This process ensures even distribution of powder with additives, binders, etc.

Sometimes lubricants also added in the blending process to improve flow characteristic of powder.

Forming of the mixed powder into a compact

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.

This step ensures to reduce voids and increase the density of the product. The powder is compacted into the mold by the application of pressure to form a product which is called green compact (the product gets by compacting).

  • It involves pressure range from 80 to 1600 MPa.
  • This pressure depends on the properties of metal powder and binders.
  • For soft powder compacting pressure is about 100 – 350 MPa.
  • For steel, iron, etc. the pressure is between 400 – 700 MPa.

Sintering of the compact to enhance integrity and strength

The green compact, produced by compressing, is not very strong and can’t be used as a final product.

This process step involves heating of 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. The mechanisms involved in solid phase and liquid phase sintering are discussed briefly in a later section.

This process provides strength to green compact and converts it into a final product.

The sintering temperature is generally about 70 to 90 percent of the melting temperature of metal powder.

Secondary Operation

The sintered object is more porous compared to fully dense material. The density of the product depends upon press capacity, sintering temperature, compressing pressure, etc.

The application of finishing processes to the sintered part. In the Powder Metallurgy industry, such processes are often referred to as “secondary operations”.

Sometimes, the product does not require high density and the sintered product is directly used as a final product. But sometimes, a highly dense product is required (for example manufacturing bearing, etc.)

Where a sintered product cannot be used as a finished product. That’s why a secondary operation required to obtain high density and high dimensional accuracy.

The most common secondary operation used is sizing, hot forging, coining, infiltration, impregnation, etc.

Advantages of Powder Metallurgy

The powder metallurgy process offers a number of advantages over competing metalworking technologies. All of this results in part-to-part uniformity for improved product quality, form and material flexibility, versatility of use and cost efficiency.

  • The parts can be produced clean, bright, and ready for use.
  • The composition of the product can be controlled effectively.
  • Articles of any intricate shape can be manufactured.
  • Close dimensional tolerance can be achieved.
  • The machining operation is almost eliminated.
  • Parts have excellent finish and high dimensional accuracy.
  • There is the overall economy as material wastage is negligible.
  • Metals and non-metals can be mixed in any proportion.
  • A wide range of properties such as porosity, density, etc.
  • can be achieved effectively.
  • A high production rate can be achieved.
  • Reduced production time.
  • Highly skilled labor is not required.
  • Saving in the material through reduced wastage.
  • Composition structure and properties can be controlled easily.
  • A wide range of parts with special electrical and magnetic properties can be produced.

Disadvantages of Powder Metallurgy

  • The high initial cost of metal powder.
  • The size of the parts produced is limited due to large presses and needed to get the required compressing pressure.
  • The equipment used for the operation is costly.
  • The impossibility of having a completely dense product.
  • Pressure up to 100 tones capacity is used even for a small product.
  • The metal powder is expensive and, in some cases, difficult to store.
  • Some power may present explosion hazards.
  • Dies used must be of high accuracy and capable of withstanding high pressure and temperature.
  • Parts produced have poor ductility.
  • High tooling cost.
  • The difficulty of sintering low melting powder.
  • Poor plastic properties.
  • The necessity of protective atmospheres.

Powder Metallurgy Application:

  • To produce a porous product and
  • Babbitt bearing for automobiles.
  • To produce oil pump gears for automobiles.
  • Used for production of cutting tools, wire drawing dies and deep drawing dies.
  • To produce refractory metal composites, eg: tungsten, molybdenum, tantalum For manufacturing the tungsten wires for filaments in the lamp industry.
  • Diamond impregnated tools are produced by a mixture of iron powder and diamond dust.
  • To produced electrical contract material, eg: circuit breakers, relays, and resistance welding electrodes.
  • Parts of cars, aircraft, gas turbine, electric clocks, etc.
  • Parts of vacuum cleaners, refrigerators parts of guns, sewing machines.

Powder Metallurgy Necessity or Need

Power metallurgy becomes very much in the following cases:

  • The difference in the melting temperature of the two elements.
  • Melting and solidification cause poor quality.
  • Melting causes a loss of identifying the constituents.
  • Some metals do not form a liquid solution.

Characteristic of Powder Metallurgy

  • Powder metallurgy should be heat resistant.
  • The size of the powder particles is to pass the powder through the screen (sieves) having a definite number of meshes.
  • The powder should have good plasticity.
  • It should have the ability to be cold-pressed.
  • The powder should have an excellent parking factor.
  • It should have good flowability.
  • The powder should be free from oxides and should have a clean surface.
  • The ratio of the density of the compact to the apparent density of the powder should vary between 2:1 to 3:1

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