TPT May 2008

 Figure 2 : Hot cracks in extruded MgCa-tubes

A Ø30 × 50mm billet extruded rod, made from MgCa-alloy, was used. The surfaces were turned down and bored to a diameter of 0.2mm bigger than the mandrel diameter. Molybdenum disulphide was used as a lubricant, applied on the operation surfaces of the mandrel and die. Extrusion experiments were carried out in two steps by varying the following parameters: In the second series the tool temperature corresponded to the billet temperature. To compensate for the cooling of the billet during transportation from furnace and charging into the container, it was inserted for 10 minutes into the container prior to the extrusion process. 2.2 Results The main factors limiting the extrusion process of magnesium tubes are the maximum extrusion force and the failure of the mandrel or extruded metal due to hot cracks. The use of lubricants reduces the extrusion force whereas a great amount of lubricant deteriorates the surface quality of the extruded tubes. The extrusion process is nearly impossible with an extrusion ratio of more than 80-100 and billet temperature of less than 350°C as the metal cannot flow out of channel between mandrel and die. The application of high billet temperatures facilitates the metal flow and reduces extrusion force but can also cause hot cracks (figure 2). These were noticed especially at extrusion ratios higher than 70-80. The most probable zone for the formation of hot cracks is

located near the tube entrance, and corresponds to the maximum extrusion force. In figure 3 the behaviour of the metal structure in the deformation zone can be seen. The basic structure of the billet material is very inhomogeneous; grain size varies from 1 to 30µm. Grain size decreases towards the extrusion die. Grains in the tube have a globular form in the cross section as well as in the longitudinal section. Mechanical properties of the tubes are satisfied: tensile strength runs to 160-200MPa, while tensile elongation is 10-18 per cent. In comparison to extruded round bars used as a billet material the tubes have 30 per cent lower strength at a significantly higher elongation value. In figure 4 and table 1 there are listed the dependences of the tension strength and elongation versus billet temperature, container temperature and extrusion ratio for alloys with varying calcium content. Elongation A 5 decreases by increasing the calcium content and increasing the billet temperature Т В as well as the extrusion ratio R (figure 4 (bottom) and table 1). In contrast to this tensile strength R m increases by increasing the calcium content. Under normal operating conditions an increase of the extrusion ratio and temperature leads to insignificant decrease of tensile strength R m . The tensile strength with Т В = 410°С increases in contrast to Т В = 350°С. The reason for this at the high extrusion ratio could be that the process takes place with the lower temperature but with the maximal extrusion force. There is no extrusion force drop after the initial stage of the process and the ram speed decreases at an amount of quadruple to quintuple.  Figure 4 : Dependencies of elongation and tensile strength on the calcium content, extrusion ratio and container temperature: a, b: T B = 410°C, T C = 380°C; c, d: T B = 350°C, R = 70a  Figure 5 : Strain-stress diagrams for tubes of alloy MgCa0.8 a) dependence on billet temperature, b) dependence on extrusion ratio (‘M’ is tube middle, ‘E’ is tube end)

 Figure 3 : The structure of metal in different parts of the deformation zone

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M ay 2008

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