TPT May 2008

Manufacture of small diameter magnesium tubes with the hot extrusion and drawing processes By Alexandr Golovko, research specialist, National Metallurgical Academy of Ukraine Co-authors: Ch Krause, Th Hassel, Fr-W Bach * , Institute of Materials Science, Leibniz University Hanover, Germany

Abstract The development of biodegradable, cardiovascular implants (stents) made of an absorbable magnesium calcium alloy demands an accurately defined production process. In order to evaluate the material, magnesium alloys with a calcium content of 0.4-2 mass per cent were used in research. The microstructure, mechanical properties and corrosion behaviour were investigated. Tube extrusion and the further drawing process is a very promising concept to develop the manufacture of a semi-product for absorbable magnesium stents. In the tests undertaken, hot extruded tubes were reduced by a drawing process that was carried out both with and without the use of a mandrel. Investigation was carried out into the influence of the drawing process sequence and certain heat treatments between the drawing steps on the mechanical properties. Other areas were investigated including the temperature-deformation conditions of the extrusion and drawing processes (and their effects on tensile strength), the elongation and the microstructure of tube samples. The results show that an absorbable Mg alloy is available which can easily be produced. It was found to exclusively contain essential mineral nutrients with a non-toxic character. 1. Introduction The demand for implants manufactured from biodegradable materials has led to the development of production technologies for magnesium tubes as a basic material for stents [1, 2] . Magnesium- calcium alloys (MgCa-alloys) have, in contrast to other magnesium alloys, the advantage of a very low poisonous potential during degradation in the body. Magnesium alloys with a content of 0.6-0.8 per cent calcium have a particularly consistent corrosion behaviour. This behaviour could be an indication of a moderate degradation velocity. This is necessary in order to guarantee mechanical solidity over a certain period and ensure a low amount of gas produced during the degradation process. The production of magnesium tubes can be achieved by first producing tube pre-profiles of magnesium alloys by hot extrusion of ingots or extruded rods. Following this, hot drawing takes place (sink drawing without mandrel and subsequent drawing with a long mandrel); the process ends with cold drawing that is carried out to increase strength and improve surface quality. In order to carry out drawing after extrusion, the tubes should preferably have a low wall thickness at high plasticity. The aim of this project was to determine those factors that define the extrusion process and warm drawing of thin-walled tubes of the

alloying system Mg-Ca. It was the aim to determine the influence of technological parameters on the mechanical properties and the structure of the tubes. The experiments were carried out in three steps: 1. Experiments concerning the influence of the calcium content, deformation and temperature conditions of extrusion on the mechanical properties of the tubes 2. Influence of the billet temperature and the extrusion ratio on tubes’ mechanical properties and microstructure 3. Experiments of sink drawing and drawing on a long mandrel A detailed analysis of the extrusion of magnesium alloys was carried out in [3, 4] . It can be seen that details about ideal temperature-velocity- parameters during the extrusion of magnesium alloys on standardised industrial alloys are limited. Information about the extrusion of MgCa- alloys or uniform results about acceptable outflow velocities and ideal relation of container temperature to billet temperature are not available. It is known from literature that the use of extruded billet enables high extruding velocities [5] . The authors are not aware of any other thesis that deals with the drawing of magnesium tubes, especially of tubes made from MgCa-alloys. In the literature that does exist, extensive facts about the drawing of tubes of steel and nonferrous metal cannot be transferred to the present projects with MgCa-alloys. Investigation priorities of tubes of steel and nonferrous metal – such as energy consumption and productivity – are irrelevant for the production of magnesium tubes for biomedical applications. It is more important to develop a stable drawing process and to establish the influence of temperature and deformation parameters on the tube geometry and properties.

2. Extrusion process 2.1 Experimental methods

Experiments were carried out with a vertical hydraulic press with the nominal force of 800kN at the Institute of Materials Science of the Leibniz University, Hanover. The extrusion tool consists of a stem and dummy block in which a long mandrel is installed, together with a conical die (figure 1). The diameter of the container bore is 29mm, while the ram speed is constant and equal to 1.25mm/s.

 Figure 1 : Extrusion tool – 1) extrusion die, 2) container bore, 3) billet, 4) mandrel, 5) die

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