4.1 Annealing

It is customary that the powder producer delivers the powder to the fabricator ready for mixing. The aims of annealing are:

1. to soften the powder

2. to reduce the residual amount of oxygen, carbon and/or nitrogen from the powder.

The annealing operation may be done in an atmosphere furnace or a vacuum furnace. The former may be of batch or continuous type. The furnace construction is similar to that given in Chapter 6, which deals with sintering technology. Annealing temperatures are kept as low as possible to minimize sintering.

4.2 Powder Mixing

The term 'blending' is strictly applied to a one component operation, whereas mixing involves more than one type of powder, e.g. mixing of solid lubricant with a metal powder or powders of several other metals. Sometimes the additive acts as lubricant as well as alloying addition, e.g. graphite in iron powder.

Various variables in the powder mixing process have been highlighted by Hausner.1 They are:

1. Type of mixer

2. Volume of the mixer

3. Geometry of the mixer

4. Inner surface area of the mixer

5. Constructional material and surface finish of the mixer

6. Volume of the powder in the mixer before mixing

7. Volume of the powder in the mixer after mixing

8. Volume ratio of component powders

9. Volume ratio of mixer to powder

10. Characteristics of component powders

11. Type, location and number of loading and emptying devices

12. Rotational speed of mixer

13. Mixing time

14. Mixing temperature

15. Mixing medium (gaseous or liquid)

16. Humidity, when mixing in air.

Mixing efficiency is best when the powder...