TPT May 2008

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 T D 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

 Figure 14 : Technological scheme and drawing process parameters

 Figure 15 : Structure of the metal in a longitudinal section of tube

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. http://www.aerztezeitung.de/docs/2006/04/28/078a0401. 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, 2 nd 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. 45 th 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.

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