TPT May 2008

128 / 132

May 2008 126 ›
In figure 14, an example can be seen for a scheme
of drawing tubes with an external diameter of 1.8mm
and a wall thickness of 0.2mm. The first five working
steps were carried out without a mandrel and with a
drawing tool temperature of TD of 350-380°C, while
the drawing velocity came to approximately 50mm/s.
At these steps, the diameter is decreased from 6.4 to
2.9mm and the wall thickness changed in the range
of 0.1-0.15mm.
Subsequently a mandrel drawing (mandrel with a
diameter of 1.5mm) was used. The recommended
drawing tool temperature is 280-300°C, with a
drawing velocity of approximately 30-50mm/s. The
wall thickness reduced down to 0.2mm. The mandrel
was then pulled out of the tube. As a final working
step, cold drawing is used to improve surface quality
as well as grain refining.
During the hot drawing process the grain size reduces
from 10-20µm (in extruded tube-billet with diameter
Ø6.4mm and wall thickness 0.5mm), to 3-7µm (tube
Ø1.8×0.2mm). Micrographs from the tube longitudinal
section show globular grains (figure 15).
4. Conclusions
A technological method has been established that
reveals how tube-billet of Mg-Ca-alloy is manufactured by hot extrusion
from ingot or extruded rod, drawn with and without a long mandrel, and
finished with the method of cold drawing.
It has been possible to determine the influence of calcium content
(ranging from 0.4-2.0 per cent), extrusion ratio, billet and container
temperature on the tensile strength and elongation of tube samples.
It has been established that the increase of the calcium content and
the extrusion ratio reduces the plasticity of the tubes, increases the
maximum force and normally reduces the tensile strength.
The rise of temperatures (of billet and container) implicates the
decrease of the extrusion forces, but results in deterioration of the
mechanical properties and the occurrence of hot cracks. Stable
extrusion of Mg-Ca tubes Ø6x0.4mm has been demonstrated.
The tubes have sufficiently high plasticity as compared with billet
material (elongation is 10-16 per cent) and tensile strength is more
than 160MPa.
Magnesium alloys with calcium content of 0.6-0.8 per cent offer
advantageous corrosion stability. For the alloy MgCa0.8, it is
possible to establish the influence of the extrusion ratio (35-130)
and billet temperature on mechanical properties and the metal
structure.
A description has been given of the main factors limiting the process
of capillary tubes of Mg-Ca alloy hot drawing and typical defects. A
definition has also been given of the behaviour of wall thickness (in the
range Ø3-6mm) at hot sink drawing.
It has been established that by increasing the total deformation with
hot and cold drawing of MgCa0.8-alloy, there is a reduction of the
grain size, elongation and an increase in tensile strength. Grain size
of tubes Ø1.8x0.2mm is approximately 3-7µm compared with the
grain size of 10-20µm in extruded tubes Ø6.4x0.5mm.
References
1. Ärzte Zeitung online: Sich auflösender Stent besteht Test bei Patienten
mit einer KHK.
asp, 28.04.2006.
2. Fr-W Bach, Th Hassel, A Golovko, Ch Hackenbroich,
A Meyer-Lindenberg: Resorbierbare Implantate aus Magnesium durch
Mikrolegieren mit Calcium, deren Verarbeitung und Eigenschaften:
Biomaterialien, 6. Jahrgang, Heft 3, Okt. 2005, S. 163.
3. Bach Fr-W, Golovko AN, Hassel Th, The features of tube extrusion
made of Mg-Ca alloys at vertical hydraulic press.//Metallurgical and
Mining Industry, 2005, No 1, p45-52.
4. Fr-W. Bach, Th Hassel, AN Golovko: The Influence of the chemical
composition and extrusion parameters on the mechanical properties of
thin-walled tubes made of magnesium-calcium alloys. Modern problems
of metallurgy. Vol 8. Plastic forming of metals, 2005, S 379-384.
5. Zholobov VV, Zhverev GI, Extrusion of metals, 2nd edition,
M Metallurgy, 1971.
6. Th Hassel, Fr-W Bach, A Golovko, Ch Krause. Investigation of the
mechanical properties and the corrosion behaviour of low-alloyed
magnesium-calcium alloys for use as absorbable biomaterial in the
implant technique: magnesium technology in the global age. 45th Annual
Conference of Metallurgists of CIM, Montreal, Canada. P 359-370.
7. K Lange: Umformtechnik, Band 2: Massivumformung. 2 Auflage,
Springer, Berlin 1988.
8. Shevakin Yu F, Rytikov AM, Seydaliyev FS, Manufacture of tubes of
nonferrous metals, M Metallurgizdat, 1963.
National Metallurgical Academy of Ukraine
4 Gagarin Avenue, 49600 Dnepropetrovsk, Ukraine
Fax: +380 562 39 85 59
Email:
 Figure 15: Structure of the metal in a longitudinal section of tube
 Figure 14: Technological scheme and drawing process parameters