Powder Metallurgy Process: Definition, Application, Advantages, Types [PDF]

Powder Metallurgy Definition

Powder metallurgy is a cost-effective manufacturing process for producing finished or semi-finished objects by compressing metal powder into dies. It offers high quality, strength, and the ability to create complex shapes accurately, making it suitable for mass production. The process involves four steps: powder preparation, mixing, compacting, and sintering. It is recognized as a green technology due to its benefits over other metal-forming processes. Powder metallurgy is also employed to create unique components. Tungsten carbide is created by it.

Powder Metallurgy Process

These factors make this process most suitable for mass production. It mainly involves four basic steps.

1. Powder Preparation
2. Mixing and Blending
3. Compacting
4. Sintering

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

1. Powder Preparation

• This is a first and basic step for producing an object by powder metallurgy process. Any material can convert into powder.
• There are various processes of producing powder such as atomization, grinding, chemical reaction, electrolysis process, etc.

2. 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 process ensures even distribution of powder with additives, binders, etc.
• Sometimes lubricants are also added in the blending process to improve flow characteristic of powder.

3. Compacting

• Compacting means compressing the prepared powder mixture into pre-defined dies.
• This step ensures to reduce the voids and increase the density of the product. The powder is molded by the application of pressure to form a product which is called green compact.
• It involves pressure range from 80MPa 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.

4. Sintering

• The green compact, produced by compressing, is not very strong and can’t be used as a final product.
• This step involves heating of green compact at an elevated temperature which ensures a permanent strong bond between adjacent particles.
• 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.

5. 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.
• 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 is required to obtain high density and high dimensional accuracy.
• The most common secondary operation used is sizing, hot forging, coining, infiltration, impregnation, etc.

Types of powder metallurgy process

Conventional powder metallurgy process

The diagram below explains conventional powder metallurgy processes involving mixing powders, compacting them in a die, and then sintering. The resulting shapes bond the particles in an atmosphere-controlled furnace. Most powder metallurgy parts are lightweight, but some can be made using conventional equipment. However, there is a cost-effective advanced PM process available today that can create complex components with intricate contours and multiple levels.

Metal Injection Molding (MIM)

Metal injection molding (MIM) can mass-produce complex shapes using fine metal powders (<20 microns). The powders are mixed with a binder to create a feedstock. This is then injected into a cavity using an injection-molding machine. After removing the “green” component, most of the binder is extracted through thermal or solvent processing. The remaining binder is eliminated during sintering in a controlled atmosphere furnace.

MIM processes resemble plastic injection molding and high-pressure die casting, allowing for the production of similar shapes and configurations. However, MIM is limited to relatively small and highly complex parts (<250 grams) that would otherwise require extensive machining. This metallurgical process offers the benefits of closely matching the mechanical properties of wrought materials, net-shape manufacturing, precise dimensional tolerance control, and the ability to create parts with virtually any shape or geometric feature. Additionally, MIM enables high production rates through the use of multi-cavity tooling.

Isostatic Pressing

Isostatic pressing is a popular powder metallurgy forming process that applies equal pressure in all directions to a powder compact. This ensures maximum uniformity of density and microstructure without the limitations of uniaxial pressing.

Isostatic pressing can be performed either cold or hot. Cold isostatic pressing (CIP) is used to compact green parts at room temperature. On the other hand, hot isostatic pressing (HIP) fully consolidates parts at elevated temperatures through solid-state diffusion. HIP can also be employed to eliminate residual porosity from a sintered PM part.

Metal Additive Manufacturing

This is also known as 3D printing. It has the ability to revolutionize production, time-to-market, and component simplicity. Unlike traditional subtractive methods, AM builds parts layer by layer from a digital model, rather than removing material. Additive manufacturing does not require molds or dies, resulting in minimal material wastage and reduced manufacturing expenses. Additive manufacturing is now increasingly utilized for the direct production of components. It is used for aircraft engine parts, medical implants, and jewelry.

Powder Metallurgy Advantages

• 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 increase in 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.

Powder Metallurgy Disadvantages

• The high initial cost of metal powder.
• The size of the parts produced is limited due to large presses and need to get required compressing pressure.
• The equipment used for the operation is costly.
• It is impossible to have a completely dense product.
• Pressure up to 100 tonnes capacity is used even for a small product.
• The metal powder is expensive and in some case 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

Automotive Applications

Approximately 80% of powder metallurgy parts serve automotive purposes, with about 75% specifically designed for transmissions (both automatic and manual) and engine components. These transmission applications encompass various parts such as synchronizer system components, clutch hubs, gear shift elements, planetary gear carriers, turbine hubs, clutch plates, and pocket plates.

Engine parts manufactured using powder metallurgy include pulleys, sprockets, hubs (particularly those 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.

Powder metallurgy is also utilized in various other automotive systems for specific parts, including:

• Oil pumps, particularly gears.
• Shock absorbers, including piston rod guides, piston valves, and end valves.
• Anti-lock braking systems (ABS), specifically sensor rings.
• Flanges and oxygen sensor bosses in exhaust systems.
• Variable valve timing systems.
• Turbochargers.
• Exhaust gas recirculation (EGR) systems.
• Chassis components.
• Continuously variable transmissions.

Other Applications of Powder Metallurgy

• To produce a porous product
• 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 of tungsten wires for filaments in the lamp industry.
• Diamond impregnated tools are produced by a mixture of iron powder and diamond dust.
• To produce electrical contract material such as 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 is extremely necessary in the following cases:

1. The difference in the melting temperature of the two elements.
2. Melting and solidification cause poor quality.
3. Melting causes loss of identity of the constituents.
4. Some metals do not form a liquid solution.

Powder Metallurgy Characteristic

• 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 a 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

Video on Powder Metallurgy Process

Conclusion

Powder metallurgy simplifies mass production of parts and continues to advance with evolving technology. In this post, we have explored the definition, types, process, applications, advantages, and disadvantages of powder metallurgy. If you found this information valuable, please share it with fellow technical students. Thank you for reading, and see you next time!