TPT May 2008

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 13 : Influence of the drawing parameters on the change of wall thickness

 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

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M ay 2008

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