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M
ay
2008
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A Ø30
×
50mm billet extruded rod, made from MgCa-alloy, was
used. The surfaces were turned down and bored to a diameter of
0.2mm bigger than the mandrel diameter. Molybdenum disulphide
was used as a lubricant, applied on the operation surfaces of the
mandrel and die.
Extrusion experiments were carried out in two steps by varying the
following parameters:
In the second series the tool temperature corresponded to the billet
temperature. To compensate for the cooling of the billet during
transportation from furnace and charging into the container, it was
inserted for 10 minutes into the container prior to the extrusion
process.
2.2 Results
The main factors limiting the extrusion process of magnesium tubes
are the maximum extrusion force and the failure of the mandrel or
extruded metal due to hot cracks.
The use of lubricants reduces the extrusion force whereas a great
amount of lubricant deteriorates the surface quality of the extruded
tubes.
The extrusion process is nearly impossible with an extrusion ratio of
more than 80-100 and billet temperature of less than 350°C as the
metal cannot flow out of channel between mandrel and die.
The application of high billet temperatures facilitates the metal flow
and reduces extrusion force but can also cause hot cracks (figure
2). These were noticed especially at extrusion ratios higher than
70-80. The most probable zone for the formation of hot cracks is
located near the tube entrance, and corresponds to the maximum
extrusion force.
In figure 3 the behaviour of the metal structure in the deformation
zone can be seen. The basic structure of the billet material is
very inhomogeneous; grain size varies from 1 to 30µm. Grain
size decreases towards the extrusion die. Grains in the tube have
a globular form in the cross section as well as in the longitudinal
section.
Mechanical properties of the tubes are satisfied: tensile strength
runs to 160-200MPa, while tensile elongation is 10-18 per cent.
In comparison to extruded round bars used as a billet material
the tubes have 30 per cent lower strength at a significantly higher
elongation value. In figure 4 and table 1 there are listed the
dependences of the tension strength and elongation versus billet
temperature, container temperature and extrusion ratio for alloys
with varying calcium content.
Elongation A
5
decreases by increasing the calcium content and
increasing the billet temperature Т
В
as well as the extrusion ratio R
(figure 4 (bottom) and table 1). In contrast to this tensile strength R
m
increases by increasing the calcium content. Under normal operating
conditions an increase of the extrusion ratio and temperature leads
to insignificant decrease of tensile strength R
m
.
The tensile strength with Т
В
= 410°С increases in contrast to Т
В
= 350°С. The reason for this at the high extrusion ratio could be
that the process takes place with the lower temperature but with
the maximal extrusion force. There is no extrusion force drop after
the initial stage of the process and the ram speed decreases at an
amount of quadruple to quintuple.
Figure 2
:
Hot cracks in extruded MgCa-tubes
Figure 4
:
Dependencies of elongation and tensile strength on the calcium
content, extrusion ratio and container temperature: a, b: T
B
= 410°C,
T
C
= 380°C; c, d: T
B
= 350°C, R = 70a
Figure 3
:
The structure of metal in different parts of the deformation zone
Figure 5
:
Strain-stress diagrams for tubes of alloy MgCa0.8
a) dependence on billet temperature, b) dependence on extrusion ratio
(‘M’ is tube middle, ‘E’ is tube end)