What is Powder Metallurgy Process

Powder metallurgy (PM) is an advanced fabrication technique that transforms fine metal powders into solid, high-performance components. The process relies on two primary stages: first, compacting the loose powder into a specific shape, and second, heating it, a step known as sintering, to a temperature just below its melting point. This thermal energy causes the particles to fuse and solidify without completely liquefying. Finally, secondary operations like coining or heat treatment are often utilised to achieve ultra-precise dimensions or enhanced mechanical properties.

The Basics of the Powder Metal Manufacturing Process

While the powder metal manufacturing process has many variations, its core principles remain the same. The process can be divided into four distinct stages. These are:

Powder production

Compaction

Sintering

Secondary processes

1. Powder production

Powder production is the initial step in powder metallurgy. It refers to the procurement of the metal powder used as a raw material in the manufacturing process. This step must provide high output while creating powder grains in a specific size range for the downstream processes to work.

There are various methods available to obtain metal powders. The most common ones are mechanical crushing, electrolytic deposition, atomisation, and chemical reactions.

In mechanical crushing, metal blocks are crushed into smaller pieces using equipment such as jaw crushers and stamping mills. The pieces are further ground down to the required dimensions using special machinery such as ball mills. Mechanical crushing is highly suitable for hard and brittle metals like iron alloys. Sieving may be employed as a secondary step to separate any remaining larger particles.

Electrolytic deposition is another reliable method to produce high-purity powder for metals such as copper, iron, chromium and magnesium. The process uses electrolysis to deposit metal powder on the cathode, which is then extracted. The process has high energy consumption, making it feasible only for high-value metals.

Atomisation refers to the use of a high-pressure media to create fine metal powder. This is typically done by passing a stream of hot molten metal over pressurised gas or liquid media, which breaks it into tiny fragments that promptly solidify into fine powder.

Chemical reactions such as oxide reduction, precipitation and thermal decomposition are used to create metal powder with the desired size, shape and purity. The preferred process depends on the specific metal at hand. Chemical reactions provide a very high-purity (>99.5%) metal powder.

2. Compaction

Compaction refers to the compressing of metal powders into dies using high pressure from mechanical, hydraulic, or pneumatic equipment. This is a crucial step to minimise the voids between neighbouring powder particles (densification) to improve strength and related mechanical properties. The most common compaction method is die pressing, which uses mechanical, servo-electric and hydraulic presses depending on the throughput required.

While compaction is generally carried out at room temperature, some processes, such as hot isostatic pressing (HIP), use elevated temperatures to combine this stage with sintering.

3. Sintering

In this stage, the compacted powder is heated to encourage the powder particles to merge into each other at the surface. This converts the powder into a single mass with superior mechanical properties.

Sintering provides benefits such as increased density, ductility, strength, electrical conductivity, thermal conductivity, and toughness. The process, however, is far from simple and varies from material to material.

When sintering occurs below the metal’s melting point (as is typically the case), this is known as solid-state sintering. However, in specific cases, such as in the production of cermets, electronic ceramics, cemented carbides and tungsten heavy alloys, sintering may be done above the melting point to accelerate the inter-particle bonding, and fill the pores in the compact. This process is known as liquid-state sintering.

An important concern associated with sintering is material shrinkage. The increase in density reduces the overall volume of the material. This shrinkage varies with sintering temperature and duration, particle size, initial density, compaction pressure and cooling rate. As a result, the sintering process requires close monitoring and fine-tuning to obtain the desired dimensions. This is usually done by manipulating furnace conditions and starting with a larger size to account for the shrinkage.

Sintering, however, does not completely eliminate porosity. Parts typically retain a residual porosity of 5% to 25% after sintering.

4. Secondary processes

A sintered part often goes through secondary processes to prepare it for service. Some examples of these are deburring, machining, heat treatment, and surface treatment.

These processes are used to achieve the final dimensions, improve desirable characteristics, and add functional features like self-lubrication or specialised surface textures.