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ALUMINIUM EXTRUSION INDUSTRY
file cross-section is 56 × 5 mm 2 and
the press ratio is 1 : 60. All profiles
were extruded at almost homogenous
temperature conditions and a constant
profile speed of v e = 3.6 m/min.
Threading of continuously
reinforced aluminium profiles
The application and extend of use of
new materials is strongly connected to
their machinability. Special mechanical
or physical properties, beneficial
for the application of materials, can
be disadvantageous for machining
operations. The combination of different
materials within a composite allows
a combination of different properties;
but if both materials have to be
cut or formed in one process with the
same tool, it usually evokes a compromise
in machining. Since the gains
from using composite materials like
continuously reinforced aluminium
profiles are high, the challenge faced
by research is to determine suitable
machining operations. Therefore, experimental
investigations concerning
flow-drilling and threading of steelwire
reinforced flat profiles have been
conducted.
Flow-drilling is a chipless method
of producing bushings in thin-walled
structures like sheet metals, tubes or
extrusions by using a polygon-shaped
pin with conical head made of cemented
carbide. The pin can be used
on drilling machines or machining
centres where it is accelerated up to a
defined rotational speed and fed into
the direction of the workpiece. The
pin is pressed against the workpiece
material either with a defined force,
or by using a defined feed. Friction
between the tool and the workpiece
Fig. 3: Extrusion forces double-T-profile
generates heat that lowers the yield
strength of the workpiece material.
Forming of the workpiece material
becomes easier. By pushing the pin
further into the workpiece, material,
displaced by the tool, yields in opposite
direction of the feed first and in
direction of the feed later. The material
yielding towards the spindle can
either be cut off by countersink cutting
edges of the pin, or can be formed
to a defined flange with a shoulder
of the pin. The material displaced in
the direction of the feed is formed
to a stable collar which can be used
for threading, in addition to the wall
thickness of the profile.
Threading, as a subsequent machining
operation after flow-drilling,
can be carried out via different machining
operations. Threads can either
be cut by tapping or thread milling,
or formed by thread forming. In
contrast to the conventional method
of tapping, the axial forces acting on
the workpiece are usually smaller
when using thread milling, which can
be advantageous for machining thinwalled
profiles. Additionally, cutting
parameters can be varied in a wider
range. The main advantage of the
forming process compared to cutting
operations is the resulting grain flow
within the workpiece material [6].
The workpiece material is strengthened
by work hardening. Since no
chips are created, chip packing cannot
occur and so process reliability is
increased. The field of application is
limited to certain materials. A tensile
strength of R m≤1200 N/mm 2 should
not be exceeded and the elongation
after fracture should be more than
A ≥ 5 - 8% to make cold forming feasible
[7]. While the aluminium matrix
material of the
composite extrusions
meets
the requirements,
the tensile
strength of
the reinforcement
exceeds
the limit. Experimentalinvestigations
on the
composite profiles
were conducted
to reveal
the effect of the reinforcement on the
profiles characteristics during thread
forming.
Experimental setup
The investigations have been carried
out using flow-drilling tools with countersink
cutting edges to create a flat
surface on the entry side. The forming
part of the tool has a polygonal shape
with four facets. Threading was done
with different tools. The tools used for
tapping and thread forming are made
from high speed steel coated with
TiN in case of thread forming, and
TiCN in case of tapping. The thread
milling tool is made from cemented
carbide and coated with TiAlN. The
different substrates were chosen to
cope with the special characteristics
of the processes. The tapping and the
thread forming tools are subjected
to tensile and torsional loads due to
synchronisation errors of the machine
tool. Therefore, the substrate of these
tools must be ductile. Thread milling
tools are mainly stressed by radial
loads, making a rigid substrate necessary.
Moreover, cemented carbide
is favourable for high cutting speeds.
The different titanium-based coatings
give the tools a hard, wear resistant
surface. Flat profiles with seven solid
steel wires, arranged in a regular pattern,
were machined using different
three-axis machining centres. The position
of the hole relative to the positions
of the reinforcing elements was
varied and a non-reinforced aluminium
profile was machined as reference.
The positions and the direction
of the sectional view for a qualitative
analysis of the lateral area are shown
in Fig. 4.
Flow-drilling investigations
The investigations concerning the
flow-drilling process take the feed as
an additional influencing factor into
account. A feed of f = 0.025 mm up to f
= 0.1 mm was analysed with a peripheral
speed of v c = 30 m/min. The profiles
are split into parts after machining.
The corresponding sections are
shown in Fig. 5. The outer form of the
collar and the material distribution in
the collar is comparable in all cases.
38 ALUMINIUM · 4/2010