In the course of the increasing discussions about a reduction of the CO2 emissions
magnesium has gained importance since it is the lightest metal for structural applications. Currently
magnesium alloys are almost exclusively used as cast parts in the automotive industry because due
to their microstructure extruded magnesium profiles exhibit a strong asymmetry in the mechanical
properties under tensile and compressive loading (strength differential effect).
In order to improve the mechanical properties a detailed knowledge about the influence of the
different extrusion parameters on the microstructure of the extrudates is necessary. Therefore, the
parameters extrusion method, billet temperature, product speed, extrusion ratio and cooling
condition were varied for the extrusion of the magnesium alloys AZ31, AZ61 and AZ80.
Subsequently the microstructure was analyzed and the mechanical properties determined. With an
additional analysis of the deformation modes of the extruded and cold deformed products it could
be discovered that an improvement of the mechanical properties can be achieved by a modification
of the extrusion process.
Since the strength differential effect in caused by twinning which due to the texture of the
extrudates is only active under a compressive loading along the extrusion direction the modification
of the extrusion process aims at a suppression of this twinning. Because on the one hand compared
to that for dislocation glide the Hall-Petch-Constant for twinning is bigger a grain refinement of the
extruded products could be achieved by a predeformation using ECAE similar processes. On the
other hand a process has been developed where the profiles are extruded into a hydrostatic counter
pressure in order to alter the texture during the extrusion. Thereby the twinning is already activated
during the extrusion. Both modifications of the extrusion process result in an increase of the critical
resolved shear stress for twinning during the subsequent cold deformation and thus in improved
mechanical properties.
The Accounting for Materials with Strength Differential Effect in Calculating the Durability in Order to Raise the Exploitation Effectiveness
