2) Cold Pressure Shaping Technique
Die Compaction. Compaction in metal dies is one of the most
important methods for shaping metal powders and this still accounts for
the bulk of commercial production. Both experimental and theoretical
work have been carried out by numerous workers in attempts to explain
exactly the behavior of metal powder when subjected to external pressure
in a metal die .
When the die cavity is uniformly filled with metal powder it gives
rise to certain packing density, but a certain
(a)
fig 22. Die compacting : to) Typical pressure-denaltay curve,
(b) Tools for die compaction
amount of bridges are also formed, when the powder is slightly
pressed by the application of one punch or more punches[ fig. 32 (b) ] the
first densification occurs by particle movement and rearrangement
causing improved packing density (Here all bridges are collapsed). The
degree to which this occurs is dependent on particle size distribution,
particle shape, Preliminary treatment powders, amount of lubricant, etc.
The antiparticle movement ,produces. Superficial abrasion of surface film
comprising
oxides absorbed atmospheric gases which
are usually
present or wedged between the metallic surface. These films obstruct the
occurrence of direct metal contact and, thus the formation of appreciable
antiparticle bonding, Because of the antiparticle movement the films
surrounding the particles are ruptured which are usually more brittle with
respect to the metal underneath, The initial contact areas between the
particles are so small that even a slight punch load causes very high
pressure on these areas thereby breaking tip the surface films at the areas
of contact. At this stage compacting produces only dense peeking of
powder particle, neither deformation nor adhesion between the individual
particles has at yet occurred.
If the pressure is further increased, clean particles are brought Close
enough . together and they adhere to each other with come Strength, This
metal-to- metal contact tins been described by Jones as" cold welding" or
adhesion. Further increase in pressure causes deformation of the particles
and large areas of contact and mechanical interlocking of neighboring
grains will take place as well as filling or voids by the squeezing or small
particles, , Strength and density are thus increased.
Deformation is the major mechanism of densification, with regard
to production, of high density parts. Both Types of deformation – elastic
and plastic may take place. Most of the elastic deformation will be
recovered on removal of stress from the compact. This may occur before,
during and after ejection from tie die cavity.
Following compassion, ejection of the pressed part is effected by
the movement of the die or punch., The density and strength of the
unsintered compact are called green density and green strength
respectively.
Many studies' on the effect of applied pressure on the density and
its distribution within the compact have been made in detail
by many workers particularly M,Yn Balshin, J. Wulft with his
associates R, H Uncke and others. Put forward one, which has got wide
acclaim, is that of shaping Koltholf and Konopicky:
ln (1/1 -D)= Kp-1-constant (Equation 1)
where D is the relative density, P the applied pressure and
compaction constant, If we plot density as a function of the spring
compacting pressure, the graph as shown in fig (23) obtained.
Fig. 23. Schematic illustration of basic green density distribution
for single end die compaction.
3) Hot pressures shaping technique
Hot Isotactic Pressing (HIP), the simultaneous application of heat
and high pressure, has become a standard production process in many
industries. In the HIP unit a high temperature furnace is enclosed in a
pressure vessel. Work pieces are heated and an inert gas, generally argon,
applies uniform pressure. The temperature, pressure and process time are
all
controlled
to
achieve
the
optimum
So these properties are :
A. High-melting-point sheet metal
B. High temp inert gas or vitreous fluid
C. Pressures as high as 100 MPa
D. Temperatures of 1200˚C (2200˚F)
material
properties.
E. Used for making high-quality parts
Ex. valve lifte
• Advantages:
• 100% density
• Good metallurgical bonding of the particles
• Good mechanical properties
• Compacts of uniform grain structure and density
• Disadvantages:
• Wider dimensional tolerances
• Higher equipment cost and production time
• Small production quantities