Page 464 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 17 Powder-Metal Processing and Equipment

f-. -

(H) (b)

(C) (Ol) (G)

FIGURE l7.8 (a) through (d) Some common bowl geometries for mixing or blending powders.
(e) A mixer suitable for blending metal powders. Since metal powders are abrasive, mixers rely
on the rotation or tumbling of enclosed geometries, as opposed to using aggressive agitators.
Source: Courtesy of Kemutec Group, Inc.

make the mix more uniform). Several types of blending equipment are available
(Fig. 178).

Hazards. Because of their high surface area-to-volume ratio, metal powders can
be explosive, particularly aluminum, magnesium, titanium, zirconium, and thorium.
Great care must be exercised both during blending and in storage and handling.
Precautions include (a) grounding equipment, (b) preventing sparks (by using non-
sparking tools) and avoiding friction as a source of heat, and (c) avoiding dust
clouds, open flames, and chemical reactions.

l7.3 Compaction of Metal Powders

Compaction is the step in which the blended powders are pressed into various
shapes in dies, as shown in sequence in Fig. 17.9. The purposes of compaction are
to obtain the required shape, density, and particle-to-particle contact and to make
the part sufficiently strong for further processing. The powder (feedstock) is fed
into the die by a feed shoe, and the upper punch descends into the die. The press-
es used are actuated either hydraulically or mechanically, and the process gener-
ally is carried out at room temperature, although it can be done at elevated
temperatures.
The pressed powder is known as green compact, since it has a low strength,
just as is seen in green parts in slip casting (Section 18.2.1). The green parts are very
fragile (similar to chalk) and can crumble or become damaged very easily; this situ-
ation is exacerbated by poor pressing practices. To obtain higher green strengths,
the powder must be fed properly into the die cavity and proper pressures must be
developed throughout the part.