Powder Metallurgy
Blending:
Compacting:
In powder metallurgy
process (PM), fine powders of metals and alloys are compacted together by
pressing the powder in a mould or die to control the shape of the finished
product. The pressure used for compaction are high so that the metal or alloy
particles get mechanically interlocked. The part also develops enough strength,
so that it may be taken out of the die or mould cavity without damage or
crumbling back to powder form. The product of this compaction process is known
as ‘‘green compact’’. Its strength is low, and its density is below that of
corresponding solid metal or alloy. It is also somewhat porous. To produce
useful strength level, the green compact is sintered at a high temperature but
below the melting point of the powder-metal in a neutral or reducing atmosphere.
An exchange of atoms between individual particles welds them together into a
slightly porous piece of metal of the approx. shape and size of the die or
mould cavity. The sintered component may be used as such or may undergo some
secondary operations before being used.
Now we
shall discuss, in some detail, the various steps involved in making PM
components:
Metal
powder production:
Metal powders can be
produced in several ways of which atomisation is the most important. In this
process, metal or alloy is heated until it melts. The molten metal is then
gravity fed through a nozzle where it is impinged with a high velocity stream
of water, air or nitrogen atomising it. Upon solidification, the atomised
particles of metal or alloys are of various shapes and sizes. These may have to
be pulverised to a fine powder of size below 100 microns.
Blending
means agitation of powder for homogenizing the particle sizes. While blending
some lubricants are also added so that during the next operation of compacting,
the die wear and the friction between the metal particles may be reduced. Usual
lubricants used are powdered graphite or lithium stearate. Blending is usually
done dry—no water is added.
After blending, the powders are placed in a die and compacted by pushing
a punch in under pressure. The dies are usually made of tungsten carbide to
reduce wear of the die. The use of lubricants is necessary to reduce wear of
dies and to reduce compacting force and in order that the density of ‘‘green
compact’’ is almost as high as the density of solid metal. Ejection of green
compact from the die also becomes easier with the use of lubricants. Compacting
requires high pressures of the order of 700 MPa to cause mechanical
interlocking in particles.
Before
sintering, however, the lubricant must be driven out by a low temperature
heating cycle.
Sintering:
This is
the next step in PM process. The green compacts are heated in a muffle type
furnace in a controlled atmosphere. For ferrous metals, a dissociated ammonia
atmosphere is used to control carburization or decarburization of the powder
compact. Temperatures are maintained between 60–80% of the melting point of
metal or alloy concerned. Sintering time may range from 20 minutes to 60
minutes. Sintering raises the ultimate strength of the product. It results in
diffusion bonding of particles.
Advantages of
powder metallurgy process:
The main advantage of PM process is that accurate control over powder
can be exercised permitting variation in physical and mechanical properties. If
so desired a part can be made with different densities in different portions of
the same part. Parts can be made in different shapes accurately so that no
subsequent machining is required. Small gears, parts with spline or irregular
shapes can be produced cheaply and accurately. PM process is metal and energy
efficient. PM parts are also relatively free from defects. Main disadvantage is
that initial tooling costs are high and it cannot produce parts with weak thin
sections.
Applications:
Powder metallurgy is used in the manufacture of parts for: Automobile
industry: motors, gear assemblies, brake pads Abrasives: polishing and grinding
wheels
Manufacturing: cutting
and drilling tools (using hard
metals)
Electric and magnetic devices: magnets, soft magnetic cores, batteries
Medical and dental: prostheses, amalgams
Aerospace: motors, heat shields, structural parts Welding: solder, electrodes
Energy: electrodes,
fuel cells
Other: porous filters,
bearings, sporting goods
etc.