Powder metallurgy (PM) is a growing and rappidly evolving branch of metallurgy based on the production of materials in the form of metal powders and the manufacturing of parts from these materials. Powder metallurgy processes 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. The primary market for metal powder is for complex parts manufactured by various PM technologies.
Powder metallurgy is also used to make unique materials impossible to get from melting or forming in other ways. For example, tungsten carbide (WC), which is used extensively in mining in top hammer rock drill bits, downhole hammers, and many more applications, is made by powder metallurgy.
Powders of metals and alloys also are fabricated into parts for several reasons. Most importantly, parts of complex shapes, close tolerances, controlled density, and controlled (and often unusual) properties can be produced by PM methods. The high precision forming capability of PM generates components with near net shape, intricate features and good dimensional precision pieces are often finished without the need of machining. The most common method of PM part production is the two-step process of:
- Powder compaction (by pressing). The powder is mixed at desired elemental ratios for the resulting alloy, and a binder is added to aid in the flowability of the powder in the forming process. The formulated powder is compacted to bring the powder particles in close proximity to encourage bonding. Compaction of the metal powders is performed in a rigid die under high pressure (typically around 135 to 680 Mpa). The compacted mass of powders is referred to as a green (unsintered) compact.
- Sintering. After compaction of a green compact, powdered materials are heated in a controlled atmosphere in a process known as sintering. During sintering, compacted metal powders are bonded or sintered by heating in a furnace to a temperature that is usually below the melting point of the major constituent. Sintering of powder metals is a process in which particles under pressure chemically bond to themselves in order to form a coherent shape when exposed to a high temperature. This process is known as solid-state sintering. If the temperature is above the melting point of a component in the powder metal part, the liquid of the melted particles fills the pores. This type of sintering is known as liquid-state sintering. Sintering time and temperature are the most significant factors from a practical perspective, with temperature being the most important variable. During this process, a number of characteristics are increased including the strength, ductility, toughness, and electrical and thermal conductivity of the material. If different elemental powders are compact and sintered, the material would form into alloys and intermetallic phases.
There are also other methods of powder compaction such as metal injection molding and high-temperature compaction techniques (such as hot isostatic pressing and powder forging) that consolidate metal powders to higher densities approaching or equaling that of wrought products.
Fabrication of parts by PM methods has several advantages that include:
- Producing parts with more uniform microstructure and distribution of alloying elements. Products are more predictable in their behaviour across a wide range of applications.
- The high precision forming capability of allows for production of parts with netshape or near-net-shape dimensions (thus reducing the need for machining)
- Flexibility in component design
- Some spacial materials (e.g. very hard materials) can be produced only by powder metallurgy
Main disadvantages of powder metallurgy lies in high cost of metal powders compared to the cost of raw material used for casting or forging a component. Also large or complex shaped parts are sometimes difficult to produce by PM process and parts have lower ductility and strength than those produced by forging.
Materials for Powder Metallurgy
Materials for powder metallurgy covers a very extensive range of applications. Examples of materials, that are processed via powder metallurgy, are:
- Iron/steel. Low alloy ferrous powder metallurgy materials are predominant in the Press/Sinter structural parts sector. In the automotive sector, which consumes about 80% of structural PM part production, the reason for choosing PM is, in the majority of cases, an economic one. Ultra-high-carbon steel has approximately 1.25–2.0% carbon content. Steels that can be tempered to great hardness. This grade of steel could be used for hard steel products, such as truck springs, metal cutting tools and other special purposes like (non-industrial-purpose) knives, axles or punches. Most steels with more than 2.5% carbon content are made using powder metallurgy.
- Stainless steels. Stainless steels can be also processed via powder metallurgy. A range of AISI 300 and 400 series stainless steels are available in powder form. Also, many types of blade steels are produced by powder metallurgy. The precipitation hardening stainless steel grade, AISI 17-4 PH is also frequently used in MIM (metal injection moulding) products. Of all of the available stainless grades, 17-4 PH steel generally offer the greatest combination of high strength coupled with excellent toughness and corrosion resistance. They are as corrosion resistant as austenitic grades. Common uses are in the aerospace and some other high-technology industries.
- Copper alloys. Copper alloys can be processed as PM structural parts. These can use either fully pre-alloyed powders or elemental mixes. Bronze powders can be processed into self-lubricating bearings.
- Aluminium Alloys. Mechanical properties of aluminium alloys highly depend on their phase composition and microstructure. High strength can be achieved among others by introduction of a high volume fraction of fine, homogeneously distributed second phase particles and by a refinement of the grain size. Powder metallurgy allows to prepare fine grained materials with increased solid solubility which are favourable precursors for further precipitation strengthening. Gas atomization was used for the preparation of powders. A range of aluminium alloy powders are available for powder metallurgy processing by press/sinter powder metallurgy or MIM. he applications for aluminum in powder metallurgy are typically driven by aerospace applications with an emphasis on full density composites as structural members. The powder metal alloy is typically based on the 2000 and 6000 series aluminum alloys and contains copper, magnesium and/or silicon. Structural automotive components manufactured using PM techniques have seen a large uptake over past decades due to the cost efficiency, high volume capabilities and limited post-processing needed for PM parts. Many engine components are fabricated using PM such as connecting rods, cam caps, drive pulleys and timing devices.
- Molybdenum alloys. Most common molybdenum-based alloy is is the Titanium-Zirconium-Molybdenum alloy TZM, composed of 0.5% titanium and 0.08% of zirconium (with molybdenum being the rest). It is typically manufactured by powder metallurgy or arc-casting processes. The alloy exhibits a higher creep resistance and strength at high temperatures, making service temperatures of above 1060 °C possible for the material.
- Titanium alloys. The use of titanium alloys in powder metallurgy has been steadily increasing due to the viability and cost reduction of producing near net shape parts with limited post processing. This has led them to be a focus of worldwide research and development. Titanium and titanium alloy powders are available in a number of forms. The limited use of press/sinter titanium powder metallurgy has generally used HDH (hydride-dehydride) titanium powder. The mechanical properties of titanium Ti-6Al-4V as with other PM alloys depends on the porosity, microstructure and oxygen content within the post sintered and pre-sintered alloy. Grade 5 – Ti-6Al-4V is significantly stronger than commercially pure titanium (grades 1-4) due to its possibility to be heat treated. This grade is an excellent combination of strength, corrosion resistance, weld and fabricability It is the prime choice for many fields of applications