Powder Metallurgy - An Introduction

Background

Powder metallurgy is the name given to the manufacture of shaped components or semi-finished products (e.g. bar and sheet) from metal powders. Both technical and economic advantages are gained by using this method of fabrication.

Powder metallurgy components are being used in ever increasing quantities in a wide variety of industries as the technology combines unique technical features with cost effectiveness.

Many of the methods and techniques described here are also used to produce sintered hard metals also known as ‘carbides’ or ‘tungsten carbides’, however these materials are not discussed in this article.

Advantages of Powder Metallurgy for Part Production

The technical and commercial advantages of producing parts from powder can be summarised as below:

•        production to near nett shape

•        few or no secondary operations

•        high material utilisation from low levels of ‘in process scrap’

•        homogeneous powder, and hence part, chemical composition due to absence of gross solidification segregation and uniform pre-alloyed powder particle composition

•        unique compositions and structures possible as there is no melting e.g. introduction of specific particles to give special properties such as silica and graphite in brake pads, and porosity in bearings for oil retention

•        non-equilibrium compositions possible e.g. copper-chromium alloys

•         metallurgical structures are usually fine and isotropic e.g. carbide distribution in atomised high speed steel powder parts

Disadvantages and Limitations of Powder Metallurgy for Part Production

Inevitability there are some limitations including:

•         costs of powder production

•         limitations on the shapes and features which can be generated e.g. the process cannot produce re-entrant angles by fixed die pressing or radial holes in vertically pressed cylinders

•         the size will always change on sintering. This can usually be predicted as it depends on a number of factors including ‘as-pressed’ density which can be controlled

•         potential workforce health problems from atmospheric contamination of the workplace

Design Considerations

The powder metallurgy manufacturer is often confronted by a drawing for a component designed with an alternative manufacturing process in mind. It is not sensible or desirable for the powder metallurgy component manufacturer to attempt to quote or produce to these drawings or designs as it is likely that certain features cannot be produced. It is desirable to redesign the component so that it can fulfil its design function, as well as take full advantage of the powder metallurgy process, in particular cost effective manufacture to near nett shape with close dimensional tolerances.

There are a few simple factors to consider:

•        the need to avoid feather-edged tooling. Stresses on the edge would cause it to deform under pressing loads and bind against the mating parts of the tooling. (This problem can be overcome by the use of a small flat section.)

•        the inability of the powder metallurgy process to introduce re-entrant angle and cross holes. Such features would have to be machined using a post processing step.

•         sharp corners should be avoided, being replaced by small radiuses.

•        the need to be able to eject the part from the tools after pressing

Other design rules relate to the practicalities of producing certain tooling configurations and have to be considered on a component-by-component basis.

Practicalities of Production

There are constraints on the height and wall thickness of parts. The initial height of powder in the die is about 2.5 times that of the pressed compact. Consequently components with final dimensions of height to wall thickness of greater than about 16:1 are not possible. Filling also becomes a problem with this height to wall ratio when the wall thickness itself is small. Usually wall thicknesses of less than 2 mm at this ratio are not possible.

Normally a powder metallurgy component is a single, pressed and sintered part. It is possible to produce a complex component, impossible to press in one operation, by pressing two simpler sub-components and co-sintering, or sinter-brazing them together during the sintering process.

Co-sintering relies upon the interdiffusion of the two parts during sintering with no additional joining aid, whereas sinterbrazing uses an intermediate layer of a braze material between the two parts, which joins them together whilst they are at the sintering temperature. A third option is to resistance weld the two parts together after sintering.

It is highly desirable to seek the advice of a powder metallurgy component manufacturer to ascertain the most cost effective design for the component by discussing the function of the component and all of the critical features.

Summary

The powder metallurgy process offers a unique opportunity to produce close-to-size components cost effectively as secondary operations and process scrap are greatly reduced or totally eliminated. It can also deliver unique materials

Primary author: G. Greetham

Abstracted from Materials information Service, edited by Stephen Harmer.

For further details on the Materials Information Service or this publication visit The Institute of Materials