E-IJPM: Vol. 44/4 - MPIF

stearate was used as a die-wall lubricant, but the
aluminum powder per se was not lubricated.
Powder Extrusion
Extrusion tests were carried out on the 15.97
mm dia. precompacted aluminum powder billets
using a die with an orifice dia. of 11.07 mm, corresponding
to an extrusion ratio of 2.1. Prior to
extrusion the powder billets were compacted in a
die, which later was used as a container during
the extrusion process. This low extrusion ratio
was chosen to permit analysis of the densification
of the powder billets during extrusion as a function
of ram displacement and is much lower than
the recommended values of 9 or higher for complete
densification of spherical powders.8
Extrusion was carried out at room temperature
(25°C) at an extrusion (ram) speed of 30 mm/min.
The die at the end of the extrusion container was
replaced with a flat plate assembly which allowed
the extrusion container to be used as a compaction
die.
The interparticle friction within the powder billet
depends on particle size, particle shape, and
surface texture. Therefore, it is important to
determine at which point billet densification is
complete for the different powder morphologies.
Since the precompacted billets were uniaxially
compacted in the extrusion container, they exhibited
a region of high density (low porosity) at the
ram end (at which the pressure was applied),
while the other end of the billet exhibited a region
of lower density (higher porosity). As a result, the
end of the billet with lower density (higher porosity)
was towards the extrusion die. The extrusion
Figure 2. Extent of extrusion with corresponding % ram displacement of initial
billet length
Volume 44, Issue 4, 2008
International Journal of Powder Metallurgy
CONSOLIDATION OF ALUMINUM POWDER DURING EXTRUSION
tests were preformed with flat-face dies (halfincluded
angle of 90°) with a round die orifice and
a 3 mm-long bearing length.
Extrusion tests were performed on a 3 MN vertical
hydraulic press. The extrusion process was
arrested at ram positions of 12%, 23%, 33%, 44%,
58%, 70%, 83%, and 99% of the initial billet
length from the end surface of the precompacted
billets (40 mm). The extent of extrusion, with
respect to ram displacement, is shown schematically
in Figure 2. Extrusion ram pressure and
ram displacement data were collected directly
from the load cell and displacement sensors. The
Figure 3. Extrusion pressure vs. ram displacement curves as a function of ram
displacement. Aluminum powder grade AM 603
Figure 4. Extrusion pressure vs. ram displacement curves as a function of ram
displacement. Aluminum powder grade AM 605
29