TPT May 2008

 Figure 8 : Influence of billet temperature on grain size

 Figure 9 : Influence of

extrusion ratio on grain size

size nearly corresponds to the grain size of the billet structure. The grain size at the tube end is well distinguished from the grain size in the middle of the tube. This can be explained by the two contrary thermal processes involved: heating during the plastic deformation and cooling down of the magnesium alloy at contact with the extrusion tool. An increase of extrusion ratio causes bigger grains (figure 9). During the extrusion with an extrusion ratio above approximately 80, the grain size decreases due to increased metal outflow velocity.

used for tube heating during drawing. Heating is achieved by four heating cartridges placed in a case. For the planned experiments, two drawing alternatives were used: sink drawing (without mandrel) and mandrel drawing [7, 8] . Sink drawing is appropriate for great diameter reduction. The mandrel drawing is necessary for wall thickness decrease. The sink drawing of the tubes of alloy MgCa0.8 was carried out by drawing dies with a diameter of 5.3 to 2.9mm with molybdenum disulphide as a lubricant. The drawing velocity came to 15, 45 or 75mm/s, and the drawing tool temperature was 320 or 410°C. After the heat exchange between tube and tool/equipment, the temperature was measured by means of a thermographical camera. In order to ensure a black tube surface, drawing was carried out without deformation. 3.2 Results If deformation grades lower than 0.1 are present during cold deformation, it results in grain refining and a strength increase in the first working stage (figure 10). Therefore cold drawing can be applied as a final step in order to reach the required grain size and mechanical properties. Analysing cold drawing made at an earlier stage – of wire made from technical pure magnesium (99.9 per cent) with deformation of more than 0.13-0.16 – has shown that drawing is not practicable because of crack formation and breaking out of tube tag. From the technological point of view, drawing of magnesium tubes in warm conditions is the ideal scenario. Heating of a tube was carried out in ‘continuous furnace’ conditions, which consists of copper tube installed on the heated drawing die case. The air temperature variation along the furnace length can be seen in figure 11.

3. Drawing process 3.1 Experimental methods

Drawing was carried out at the laboratory chain drawing bench at the Institute of Materials Science at the Leibniz University, Hanover. The drawing bench has an engine output of 3.0kW and maximal turning moment of 1.7kN × m. The maximum drawing force runs to 20kN, while velocities are 0.1-150mm/s. Preliminary experiments have shown that thin-walled magnesium tubes cool down very fast, so that a furnace heating before the drawing is insufficient. Therefore a heated drawing die support was

 Figure 10 : Change of structure during cold drawing

 Table 5 : Dependence of the tube heating temperature versus the drawing tool temperature T D

and drawing velocity v

( ° С )

320

410

Т

0

v (mm/s)

15

45

75

15

45

75

s (mm)

0.3 0.5 0.7 0.3 0.5 0.7 0.3 0.5 0.7 0.3 0.5 0.7 0.3 0.5 0.7 0.3 0.5 0.7 214 234 180 153 169 127 130 167 123 273 301 218 182 215 157 161 225 141

( ° С )

Т

D

124 ›

M ay 2008

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