Engine

SINTERED PART PRODUCTION

Introduction
The general sequence of operations involved in the powder metallurgy process is shown schematically in the process.

The component powders are mixed, together with lubricant, until a homogeneous mix is obtained. The mix is then loaded into a die and compacted under pressure, after which the compact is sintered.

An exception is the process for making filter elements from spherical bronze powder where no pressure is used; the powder being simply placed in a suitably shaped mould in which it is sintered. This process is known as loose powder sintering

Mixing -
The object of mixing is to provide a homogeneous mixture and to incorporate the lubricant. Popular lubricants are stearic acid, stearin, metallic stearates, especially zinc stearate, and increasingly, other organic compounds of a waxy nature.
The main function of the lubricant is to reduce the friction between the powder mass and the surfaces of the tools - die walls, core rods, etc. - along which the powder must slide during compaction, thus assisting the achievement of the desired uniformity of density from top to bottom of the compact.

Of equal importance is the fact that the reduction of friction also makes it easier to eject the compact and so minimises the tendency to form cracks.

It has been suggested that an additional function of the lubricant is to help the particles to slide over each other, but it seems doubtful whether this factor is of much significance: - good compacts can be obtained without any admixed lubricant, e.g. using die wall lubrication or isostatic pressing.

Care in the selection of lubricant is necessary, since it may adversely affect both green and sintered strengths especially if any residue is left after the organic part has decomposed.

Over-mixing should be avoided, since this increases the apparent density of the mix. Additionally, over-mixing usually further reduces the green strength of the subsequent compacts probably by completely coating the whole surface of the particles, thereby reducing the area of metal to metal contact on which the green strength depends.

The flow properties also are impaired and good flow is essential for the next step i.e. loading the powder into the die. In the special case of cemented carbides, the mixing process is carried out in a ball mill, one of the objects being to coat the individual particles with the binder metal e.g. cobalt, but as the very fine powders involved do not flow, the mixture is subsequently granulated to form agglomerates.

Pressing
- The mixed powders are pressed to shape in a rigid steel or carbide die under pressures of 150-900 MPa. At this stage, the compacts maintain their shape by virtue of cold-welding of the powder grains within the mass. The compacts must be sufficiently strong to withstand ejection from the die and subsequent handling before sintering.

Compacting
A critical operation in the process, since the final shape and mechanical properties are essentially determined by the level and uniformity of the as-pressed density. Powders under pressure do not behave as liquids, the pressure is not uniformly transmitted and very little lateral flow takes place within the die.

The attainment of satisfactory densities therefore depends to a large degree on press tool design. Densities

Sintering
Sintering is the means whereby the powder particles are welded together and a strong finished part produced

The sintering of mechanical parts is usually done in a continuous belt furnace - in special cases a vacuum furnace is used.