Page 462 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

2 Chapter 17 Powder-Metal Processing and Equipment

strengthening of the particles or can impart special electrical or magnetic proper-
ties to the powder.

Miscellaneous Methods. Other, less commonly used, methods for making pow-
ders are as follows:

° Precipitation from a chemical solution
° Production of fine metal chips by machining
° Vapor condensation.
More recent developments include techniques based on loiglv-temperature
extractii/e metallurgical processess-in turn based on the reaction of volatile halides
(a compound of halogen and an electropositive element) with liquid metals and the
controlled reduction and reduction/carburization of solid oxides.

Nanopowders. More recent developments include the production of nanopow-
ders of copper, aluminum, iron, titanium, and various other metals (see also
nanomaterials in Section 8.8). Because these powders are pyrophoric (ignite spon-
taneously) or are contaminated readily when exposed to air, they are shipped as
thick slurries under hexane gas (which itself is highly volatile and combustible).
When the material is subjected to large plastic deformation by compression and
shear at stress levels of 5500 MPa during processing of the powders, the particle
size is reduced and the material becomes pore free and possesses enhanced proper-
ties.

Microencapsulated Powders. These metal powders are coated completely with a
binder. For electrical applications (such as magnetic components of ignition coils
and other pulsed AC and DC applications), the binder acts like an insulator, prevent-
ing electricity from flowing between particles and thus reducing eddy-current losses.
The powders are compacted by warm pressing, and they are used with the binder
still in place. (See also pou/der-injection molding, Section 17.3.3.)

l7.2.2 Particle Size, Shape, and Distribution

Particle size usually is controlled by screening-that is, by passing the metal pow-
der through screens (siez/es) of various mesh sizes. Screen analysis is achieved by
using a vertical stack of screens, with the mesh size becoming finer as the powder
flows downward through the screens. The larger the mesh size, the smaller is the
opening in the screen. For example, a mesh size of 30 has an opening of 600 ,u.m,
size 100 has 150 /sim, and size 400 has 38 /im. (This method is similar to the num-
bering of abrasive grains. The larger the number, the smaller is the size of the abra-
sive particle.)
In addition to screening, several other methods are available for particle-size
analysis:

I. Sedimentation, which involves measuring the rate at which particles settle in a
fluid.
2. Microscopic analysis, which may include the use of transmission and scanning-
electron microscopy.
3. Light scattering from a laser that illuminates a sample consisting of particles
suspended in a liquid medium. The particles cause the light to be scattered,
and a detector then digitizes the signals and computes the particle-size
distribution.