TPT May 2008

The increase of the container temperature in contrast to the billet temperature (figure 4) causes an increase of the metal temperature in the container and therefore a deterioration of the mechanical properties. For the application as implant material alloys with calcium content of 0.6-0.8 per cent can be recommended [6] . Therefore the alloy MgCa0.8 was investigated more intensively concerning the influence of the billet temp- erature and ram speed on the mechanical properties and the microstructure. In these experiments wall thickness was changed to influence the extrusion ratio and metal outflow velocity at the constant tube diameter (table 2).

T

= 350 ° C

T

= 410 ° C

Alloy

Extrusion ratio

B

B

[MPa]

A

[%]

R

[MPa]

A

[%]

R

m

5

m

5

55-57 70-74

–

–

177 179 199 204 198 252 195 197 266

15,7 14,4 10,3 14,2 11,4

MgCa0,4

177 160

18,5 12,1

120-130

55-57 70-74

–

–

MgCa1,2

191 167

14,0

120-130

6,2

3,6

55-57 70-74

–

–

12,2 10,1

MgCa2,0

193 184

9,2 4,8

120-130

1,2

 Table 1 : Mechanical properties of tubes of MgCa-alloys* * Container temperature is 380°C Extrusion ratio 35

51

71

89

128 4.85 5.91 0.28

Logarithmic deformation

3.56 6.54 1.05

3.93 6.57 0.72

4.26 6.34 0.50

4.49 6.15 0.43

Outside diameter of tube (mm)

Wall thickness (mm) Outflow velocity (m/min)

2.6

3.8

5.3

6.7

9.6

 Table 2 : Process parameters during the extrusion moulding with a different extrusion ratio

In figure 5 characteristic stress- strain-diagrams for tubes of alloy MgCa0.8 are shown. The ratio of yield stress to tensile strength is dependent on the extrusion conditions of 30-60 per cent, and in parts greater. Greater ratios can be seen at samples with a lower yield stress. With an increase of billet temperature, the tensile strength and yield stress drop (figure 6). Elongation firstly increases and then remains nearly constant. The mechanical properties correspond to the measured extrusion force and grain sizes. An increase of extrusion ratio leads to reduction of elongation, tensile strength and yield stress (figure 7). The grate values of tensile strength at the high extrusion ratios can be explained by the fact that the extrusion process runs at a maximum extrusion force but with much reduced ram speed. Middle grain size [µm] End grain size [µm] Extrusion ratio Grain size [µm]

Part of tube* Billet temperature [°C]

340 360 380 400 420 Billet grain size

5-10 10-20 10-30 10-35 10-40 10-20 10-25 10-35 10-25 10-40

5-35

 Table 3 : Influence of the billet temperature on the grain size * ‘Middle’ – the sample was extracted in the range of 35-40 per cent of the tube length at the tube beginning; ‘End’ in 5-10 per cent before the tube end

35

51

71

89

128 Billet grain size

8-15 (up to 20)

10-20 10-30 7-25 4-10

5-35

 Table 4 : Influence of the extrusion ratio on the grain size* * The samples were extracted in the range of 35-40 per cent of tube length at the tube beginning

In figure 8 and figure 9 the structure of billet and tube can be seen. Tables 3 and 4 contain the corresponding grain sizes. The structures of extruded billets are comparatively homogeneous. With regard to figure 8 and table 3, an increase of the billet temperature causes bigger grains. At lower billet temperatures of approximately 340-370°C, the tube grain size decreases in contrast to the initial structure. At higher billet temperatures the tube grain

 Figure 6 : Influence of billet temperature on the mechanical properties of tubes

 Figure 7 : Influence of extrusion ratio on the mechanical properties of tubes

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