Page 461 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

chamber (Fig. 17.5d). The cen- Milled Production of Metal Powders 44l

malerlal

Section 17.2

rotated rapidly (about 15 O00
Incoming
’.
rev/min)
helium-filled powder
a
in

trifugal force breaks up the . , ;. ncomlng malerla
Hammers
l
~ - »@;,?,g»~@p ,_,g|l
molten tip of the electrode into g,:;,g9‘.3% _,S
t I t' I . ' ' 33'@?°3 ».L. _ ., °`
égsgéf BHIIS “ *»--`L' Crushed powder
me a Par lc CS lgdllividliid
Reduction. The reduction of (a) (D) (C)
metal oxides (i.e., removal of
oxygen) uses gases, such as FIGURE I1.6 Methods of mechanical comminution to obtain fine particles: (a) roll
hydrogen and carbon monox- crushing, (bl ball mill, and (c) hammer milling.
ide, as reducing agents. By this
means, very fine metallic oxides are reduced to the metallic state. The powders pro-
duced are spongy and porous and have uniformly sized spherical or angular shapes.
Electrolytic Deposition. Electrolytic deposition utilizes either aqueous solutions
or fused salts. The powders produced are among the purest available.
Carbonyls. Metal carbon)/ls, such as iron carbonyl [Fe(CO)5] and nickel carbonyl
[Ni(CO)4] are formed by letting iron or nickel react with carbon monoxide. The
reaction products are then decomposed to iron and nickel, and they turn into small,
dense, uniformly spherical particles of high purity.
Comminution. Mechanical coniininution (puli/erization) involves crushing
(Fig. 17.6 ), milling in a ball mill, or grinding of brittle or less ductile metals into small
particles. A ball inill (Fig. 17.6b) is a machine with a rotating hollow cylinder partly
filled with steel or white cast-iron balls. The powder or particles placed into a ball
mill are impacted by the balls as the cylinder is rotated or its contents are agitated.
This action has two effects: (al the particles periodically fracture, resulting in smaller
particles, and (b) the morphology of the particles is affected. With brittle materials,
the powder particles produced have angular shapes; with ductile metals, they are
flaky and are not particularly suitable for powder-metallurgy applications.
Mechanical Alloying. In inec/aanical alloying, powders of two or more pure
metals are mixed in a ball mill, as illustrated in Fig. 17.7. Under the impact of the
hard balls, the powders fracture and bond together by diffusion, entrapping the
second phase and forming alloy powders. The dispersed phase can result in

Dispersed Stainless
particles steel ball

I
Milling liquid Welded Ni flakes
1. 2. 3.

FIGURE l1.7 Mechanical alloying of nickel particles with dispersed smaller particles. As
nickel particles are flattened between two balls, the second, smaller phase is impressed into
the nickel surface and eventually is dispersed throughout the particle due to successive
flattening, fracture, and Welding events.