Overview

In practice, P/M components may require closer tolerances, increased mechanical properties and features not possible by simply sintering. Most of the operations that accomplish these processes are performed on P/M components in the same manner as on cast or wrought components. However, porosity frequently imposes limitations on some secondary operations.

In this chapter various types of secondary post consolidation treatments are described. Even though such treatments increase the cost, in a very large number of cases the process can prove economical in comparison to other competing technologies.

8.1 Machining

Sintered parts do have some porosity and it is, therefore, necessary to differentiate their machining behaviour from those of fully dense wrought products. The major differences are, as follows:

1. when a porous metal is machined, the depth of work hardening is more important than for the wrought metal, as in case of former, pores create stress concentration.

2. The temperature at the tool end causes oxidation of the pore surface.

3. The surface porosity enhances tool vibration, which is submitted to fatigue.

Pleney and Eudier1 have elegantly summarized various stages of tool wear. The stages are:

• Adhesion wear of tool

• Influence of the oxidation of parts

• Tool oxidation

• Tool wear by fatigue.

Although P/M parts are often specified because machining operations can be eliminated, in some cases it is more economical to leave certain part details desired for machining, rather than to incorporate them in the pressed configuration. Parts made by P/M techniques generally require machining methods that differ from those used...