Page 463 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 17.2 Productnon of Metal Powders

4. Optical methods (such as particles blocking a beam of light), in which the
particle is sensed by a photocell.
5. Suspending particles in a liquid and then detecting particle size and distribu-
tion by electrical sensors.

Particle Shape. A major influence on processing characteristics, particle shape
usually is described in terms of aspect ratio or shape factor. Aspect ratio is the ratio
of the largest dimension to the smallest dimension of the particle. This ratio ranges
from unity for a spherical particle to about 10 for flake-like or needle-like particles.
Shape Factor. Also called the shape index, shape factor (SF) is a measure of the
ratio of the surface area of the particle to its volume-normalized by reference to a
spherical particle of equivalent volume. Thus, the shape factor for a flake is higher
than that for a sphere.

Size Distribution. The size distribution of particles is an important consideration,
because it affects the processing characteristics of the powder. The distribution of
particle size is given in terms of a frequency-distribution plot (see Section 36.7 for
details). The maximum is called the mode size.
Other properites of metal powders that have an effect on their behavior in pro-
cessing are (a) flow properties when the powders are filled into dies, (b) compressibility
when they are being compacted, lc) density, as defined in various terms such as theo-
retical density, apparent density, and the density when the powder is shaken or tapped
in the die cavity.

l7.2.3 Blending Metal Powders

Blending (mixing) powders is the next step in powder-metallurgy processing. It is
carried out for the following purposes:

° Powders of different metals and other materials can be mixed in order to im-
part special physical and mechanical properties and characteristics to the PM
product. Note that mixtures of metals can be produced by alloying the metal
before producing a powder, or else blends can be produced. Proper mixing is
essential to ensure the uniformity of mechanical properties throughout the
part.
° Even when a single metal is used, the powders may vary significantly in size and
shape; hence, they must be blended to obtain uniformity from part to part. The
ideal mix is one in which all of the particles of each material (and of each size
and morphology) are distributed uniformly.
° Lubricants can be mixed with the powders to improve their flow characteristics.
They reduce friction between the metal particles, improve flow of the powder
metals into the dies, and improve die life. Lubricants typically are stearic acid or
zinc stearate in a proportion of from 0.25 to 5% by weight.
° Other additives, such as binders (as in sand molds), are used to develop sufficient
green strength (see Section 17.3), and additives also can be used to facilitate
sintering.

Powder mixing must be carried out under controlled conditions in order
to avoid contamination or deterioration. Deterioration is caused by excessive
mixing, which may alter the shape of the particles and cause work hardening,
making subsequent compaction more difficult. Powders can be mixed in air, in
inert atmospheres (to avoid oxidation), or in liquids (which act as lubricants and