Powder Metallurgy

Powder Metallurgy is a technology which involves spending considerable time and effort in converting the starting material to the required powder form and then even further time and effort in “sticking” the material back together again to produce a more or less solid object.

Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape or form (compacting), and then heating the compressed material in a controlled atmosphere to bond the material (sintering). The powder metallurgy process generally consists of four basic steps: powder manufacture, powder blending, compacting, and sintering. Compacting is generally performed at room temperature, and the elevated-temperature process of sintering is usually conducted at atmospheric pressure. Optional secondary processing often follows to obtain special properties or enhanced precision.

A much wider range of products can be obtained from powder processes than from direct alloying of fused materials. In melting operations the “phase rule” applies to all pure and combined elements and strictly dictates the distribution of liquid and solid phases which can exist for specific compositions. In addition, whole body melting of starting materials is required for alloying, thus imposing unwelcome chemical, thermal, and containment constraints on manufacturing. Unfortunately, the handling of aluminium/iron powders poses major problems. Other substances that are especially reactive with atmospheric oxygen, such as tin, are sinterable in special atmospheres or with temporary coatings.

In powder metallurgy or ceramics it is possible to fabricate components which otherwise would decompose or disintegrate. All considerations of solid-liquid phase changes can be ignored, so powder processes are more flexible than casting, extrusion, or forging techniques. Controllable characteristics of products prepared using various powder technologies include mechanical, magnetic, and other unconventional properties of such materials as porous solids, aggregates, and intermetallic compounds.

There are, in fact, many good reasons why Powder Metallurgy might be chosen as the preferred route for the manufacture of a product. In broad terms, these reasons separate into two categories:

• Product cost effectiveness 

Powder Metallurgy is the most cost effective of a number of possible options for making the part. Product cost effectiveness is by far the predominant reason for choosing Powder Metallurgy and is the main driver of the structural (or mechanical) parts sector. Powder Metallurgy wins the cost competition on the basis of its lower energy consumption, higher material utilization and reduced numbers of process steps, in comparison with other production technologies. All of these factors, in turn, are dependent on Powder Metallurgy’s ability to reduce, or even possibly eliminate entirely, the machining operations that would be applied in conventional manufacture.

Powder Metallurgy’s cost effectiveness generally also requires that the particular product be made in large production quantities. If production quantity requirements are too low, there would be no opportunity to amortise the costs of the (long-lasting) forming tooling over a sufficient numbers of parts or to avoid the loss of significant fractions of potential production time in tool changeover/setting operations.

• Product uniqueness 

Some characteristic of the product (e.g. combination of chemical constituents, control over microstructure, control over porosity etc.) can be created by starting from a powder feedstock, which would be very difficult or sometimes impossible in conventional processing. Powder Metallurgy allows the processing, in an intimate mixed form, of combinations of materials that would be conventionally regarded as immiscible. Friction materials for brake linings and clutch facings in which a range of non-metallic materials, to impart wear resistance or to control friction levels, are embedded in a copper-based or iron-based matrix.

Hardmetals or cemented carbides, used for cutting tools, forming tools or wear parts. These comprise a hard phase bonded with a metallic phase, a microstructure that can only be generated through liquid phase sintering at a temperature above the melting point of the binder. Tungsten carbide bonded with cobalt is the predominant example of such a material, but other hardmetals are available that include a range of other carbides, nitrides, carbonitrides or oxides and metals other than cobalt can be used as the binder (Ni, Ni-Cr, Ni-Co etc.)

Diamond cutting tool materials, in which fine diamond grit is uniformly dispersed in a metallic matrix. Again, liquid phase sintering is employed in the processing of these materials. Powder Metallurgy enables the processing of materials with very high melting points, including refractory metals such as tungsten, molybdenum and tantalum. Such metals are very difficult to produce by melting and casting and are often very brittle in the cast state.

The production of tungsten billet, for subsequent drawing to wire for incandescent lamps, was one of Powder Metallurgy’s very early application areas. Powder Metallurgy enables the manufacture of products with controlled levels of porosity in their structure. Sintered filter elements are examples of such an application. The other prime example is the oil-retaining or self-lubricating bearing, one of Powder Metallurgy’s longest established applications, in which the interconnected porosity in the sintered structure is used to hold a reservoir of oil.