What is Powdered Metal Manufacturing?

Most PM parts are created via three basic steps:

1. Blending / preparing the metal powder
2. Die compaction
3. Sintering

Additional heat treatment is often required after sintering parts in order to improve the density, dimensions and surface finish.

Here we will look at these processes in more detail before investigating specific methods for creating metal powder parts.

• Step One – Preparing the Powders: The first step involves preparing the powders for use in PM. Powders can be created through the use of a variety of methods, including atomisation, chemical reactions, crushing, electrolytic deposition, and grinding. The size and shape of the powders varies and so pulverisation and sieving can be used to further prepare the powders for use. 
• Step Two – Die Compaction: Once the powders are prepared they are subjected to a pre-determined level of pressure to compact them. This is applied at room temperature before sintering can begin.
• Step Three – Sintering: This takes place at elevated temperatures and under strictly controlled atmospheric conditions. The sintering process applies enough heat to bond the particles without reaching their melting point. This improves the part properties but the heating process can cause the component size or lead to some distortion. Sizing can be used following sintering as a ‘re-pressing’ operation to achieve net shaping of the part so that the shape and size are controlled.
• Step Four – Heat Treatment: Parts may require secondary heat treatments to enhance their mechanical properties and dimensional precision.

While the basic steps for manufacturing with powdered metals remain the same, there are a range of different techniques used to achieve this.

1. Traditional ‘Press and Sinter’ Forming:

One of the oldest methods involves blending fine iron metal (typically <180 microns) with additives like carbon, copper, and/or nickel and a wax lubricant. The wax lubricant is used to help press the metal powders into a die of the required shape for the part. This mixture is then heated in a metallurgical furnace under a controlled atmosphere to make the powdered metal bond via the sintering process. Following sintering, the part will still contain around 5-15% porosity, meaning that it is typically weaker than a fully finished product and so may require further heat treatment.

Since the 1940s, several other PM processes have been developed, including powder metallurgy processes such as powder forging, hot isostatic pressing (HIP), metal injection moulding, and electric current assisted sintering:

2. Powder Forging:

Powder forging takes a pre-form created using the traditional ‘press and sinter’ method (see above). This pre-form is then heated and hot forged to create a full-density part with as-wrought properties.

This process uses gas atomised, spherical powders that are placed into metal mould in the shape of the finished product. The mould is then sealed and vibrated before a vacuum pump removes the air from inside. The mould is then heated in a hot isostatic press as the internal pressure is increased through the use of an external gas pressure. This heats and joins the powders to create a finished part in the correct shape and with full density. Developed in the 1950s and 60s, HIP creates parts with as-wrought or better mechanical properties. This process grew in use of the ensuing decades and has led to the manufacture of super alloys used in jet engines and other aerospace applications.

4. Metal Injection Moulding (MIM):

This process used spherical metal powders of less than 25 microns in size along with either wax or plastic to act as a binding agent. The near-solid part (65% volume) is then injection moulded to create a ‘green’ part. This green part is then heated under controlled conditions to remove the binder (de-bindering). At this point, the part becomes a ‘brown’ part, which is sintered under atmospheric controls, reducing the volume by around 18%. This results in a finished part with a high density of around 97-99%. This technique can be used to create complex geometries.

5. Electric Current Assisted Sintering (ECAS):

This powder metal manufacturing process uses electric currents and does not need the use of binders. Rather than sintering, the electric currents increase the density of the powder, reducing the thermal cycle required to maintain the final part’s strength and density. This can all be achieved in mere microseconds, but is best suited for simple designs. Because the powders achieve final density under the electric current heat and pressure without distortion or shape variation, the moulds that are used are designed in line with the final part shape.

Also known as metal 3D printing, this is a newer PM process, dating back to the 1980s. This process can use metal powders as well as other materials like ceramics and polymers. Parts are formed by melting or laser sintering the material to build up a structure layer-by-layer. The layer-by-layer, ‘additive’ approach is what makes this form of PM different from the others and allows parts to be built in micrometer-thick single layers to a high level of precision and complexity. The structures are based on 3D digital models that inform the computer-aided manufacturing process. AM can create complex parts that are highly customisable in metals, composites, polymers, nanomaterials, biological and pharmaceutical materials, and ceramics.

Post-production heat treatment is used for post PM parts to control mechanical properties such as strength and surface hardness.  These heat treatments typically use precise temperature control and an inert atmosphere to prevent oxygen molecules contaminating the sintered part. Contamination will harm the mechanical and chemical properties by introducing porosity. The use of nitrogen, argon or hydrogen as inert atmospheres prevents this.

Heat treatment processes include:

• Sintering: Most PM parts are exposed to sintering, typically with a furnace atmosphere of nitrogen with 10% hydrogen, although a small quantity of methane gas ca be added to prevent decarburisation.
• Sinter Hardening: This commonly-used heat treatment increases the surface hardness of PM parts. The less severe quench means that there is less part distortion than with other traditional hardening methods. Sinter hardening uses highly controlled temperatures and an atmosphere typically of ammonia/nitrogen or hydrogen/nitrogen with a small amount of hydrocarbon gases, although other hardening and quenching treatments may use nitrogen and methonal.
• Tempering: This common heat treatment process increases the part’s toughness by modifying the microstructure. Types of tempering include martempering and austenising.
• HIP Treatments: Some AM parts require additional heat treatment following stress relieving. This takes place in a pressure vessel full of argon gas.

Powdered metals are fine metal powders that are compressed and sintered to achieve a final shape. This differs from cast parts that use liquefied metals and machined or forged parts that are created from stock metal.

The manufacturing processes used with powdered metals allow for parts with complex geometries to be created. While it is possible to cast or machine complex parts, this is not as cost effective as the manufacturing costs increase alongside the level of complexity.

An additional benefit of powdered metals is that they allow for a near net manufacturing process with very little waste material generated. This adds to the efficiency of powdered metal manufacturing as well as being more environmentally friendly than subtractive methods like machining.

The use of computer-aided manufacturing can further optimise weight and properties such as strength, hardness and stiffness in PM parts.

A variety of powdered metals can be used to create PM parts, including iron, steel, stainless steel, copper, aluminium, titanium, tin, tungsten and tungsten carbide, molybdenum, and a range of different precious metals.

Iron and steel are most commonly used for industrial purposes, although they may be combined with other materials and alloying elements to customise or improve the material properties of the finished parts.

Powder metal manufacturing provides a number of benefits for the creation of parts with a long lifecycle and high levels of performance even under high temperatures or corrosive environments. Not only is the process cost effective for manufacturing times and material usage, but also offers strong, robust, and wear-resistant parts.

Additive manufacturing also provides the opportunity to make parts with minimal weight and heightened durability as well as being able to make parts that cannot be created using convention manufacturing techniques.

The benefits of powder metal manufacturing mean that it has found a place in a wide range of industries, from aerospace and automotive to biomedical and marine.

PM is used to manufacture components for engines, lightbulb filaments, linings for friction brakes, lubricated bearings, electrical contacts, gas filters, medical devices, heat shields for spacecraft, industrial tools, and more.

The applications for PM parts continue to grow as technologies develop and improve, allowing for the cost effective creation of prototypes or fully-functional, finished parts.

Powdered metal manufacturing covers a range of techniques to create materials and components from metal powders. PM processes can save material costs compared to subtractive manufacturing processes and, particularly with metal additive manufacturing, can create complex items that cannot be made using other methods.

Powder manufacturing can also be used to create materials, such as tungsten carbide bonded with cobalt to create tools to cut and form other metals.

However, powdered metals have some limitations with regards to strength and resistance when compared to forged alloys. These characteristics can be improved by heat treating, which increases strength, durability and hardness. Heat treatments are typically used for items that required heightened toughness and wear-resistance, such as gears, sprockets and turbine hubs.