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125
M
ay
2008
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The most important technological parameters that determine metal
flow during the drawing process are the die case temperature, the
drawing velocity and the initial tube wall thickness. The temperature
in the deformation zone depends on the deformation work and the
heat that will be transferred from the ‘continuous furnace’, drawing
dies and die case.
In table 5 the temperatures of the tube after leaving the drawing
tool are listed (distance 15mm from the drawing die). The highest
temperature has the tube with a wall thickness
s
of 0.5mm, and
the lowest with
s
= 0.7mm. The reason for this is the low heating
in the drawing die with
s
= 0.7mm or a very fast cooling with s =
0.3. Therefore the tube of the alloy MgCa0.8 can be heated up to a
temperature
Т
D
of 120 to 300°C.
Thus the heating of tubes for hot drawing in a drawing tool can be
carried out with a tube motion involving drawing velocity. The main
limiting factors of this process are shown bellow.
During the first two steps of hot sink drawing (down to diameters
of 4.6mm), the process is stable at velocities of up to 75mm/s and
the above-mentioned temperatures. Subsequent tube reduction
demands a decrease of the drawing tool temperature to 350-360°C.
Furthermore to avoid an interruption at the tube tag at biting point,
it is advisable to reduce drawing velocity down to 45-50mm/s.
A further velocity decrease leads to tube overheating and breakage
due to tube thinning, which is one of the main defects at the hot sink
drawing. To avoid this, it is advisable to carry out a local tube cooling
by means of air or water-air-spray. Generally hot sink drawing of the
alloy MgCa0.8, with deformation by diameter
е
D
= ln(
D
0
/
D
1
) to 0.25
per working step, is steadily achievable.
The information about the wall thickness variation during the
drawing process without mandrel is necessary for the estimation
of inner diameter to determine the reduction schedule. The
experiments with sink drawing were carried out with a relation of
D
/
s
from 7 to 18 and logarithmic deformation
е
D
of 0.11-0.24. It
must be mentioned that with practically all deformation grades an
increase of
D
/
s
contributes to an increase of the wall thickness
(figure 13).
At low drawing velocities, a rise of deformation by diameter causes
wall thickness growth. At the high drawing velocities the wall
thickness almost remains unchanged or decreases with deformation
е
D
elevation. The increase of the drawing temperature intensifies the
above-mentioned dependences and leads to a considerable change
of the wall thickness in contrast to other process parameters. A
wall thickness decrease is possible at the combination of high
temperatures
T
D
, with great relations to
D
/
s
and value
e
D
.
During hot sink drawing of magnesium tubes, the external diameter
is similar to that when drawing of steel tubes lower than the die
diameter (in warm and cold conditions). This difference increases
with deformation and comes to 1-4 per cent with
е
D
= 0.12-0.24 and
D
/
s
= 10-16.
During the hot drawing process the metal strength grows and
elongation falls. For example, the tensile strength of tubes
(Ø2.9
×
0.5mm), made by sink drawing, is 284MPa compared to
195MPa for extruded tube-billet Ø6.5
×
0.45mm. In this example,
elongation reached 10 per cent for the tube compared to 16 per
cent for the billet.
For the mandrel drawing process, a high carbon steel wire was
applied as a mandrel. In the case of a billet, tubes with a diameter
of 2.8-3.2mm and wall thickness of 0.5-0.7mm were produced with
the sink drawing method. The lubricant molybdenum disulphide was
applied to external tube and the mandrel surfaces. The mandrel
was not pulled out after the drawing and the tube with mandrel was
used after tagging for the next drawing step.
Thus there is a reduction in likely tube damage during mandrel
extraction but the drawing of tubes that are longer than a
hundredfold in diameter is difficult to achieve because of worsened
sliding in tube-mandrel contact. This method can be used for short
tubes that Fe serve as a billet for stents. In contrast to magnesium
tube, sink drawing the tube surface roughness decreases at
mandrel drawing.
Figure 11
:
Variation of air temperature along the furnace length
Figure 12
:
Different defects at
the hot drawing of magnesium
tubes:
a – tube thinning;
b – tube corrugation;
c – destruction at the die
entrance
Figure 13
:
Influence of the drawing parameters on the change of wall
thickness