Effect of forced-air cooling on thixo-extrusion of mechanicall stirred Al-10%Mg and Al-5.5%Zn-2.5%Mg-1.5%Cu alloy billets

In order to improve the poor tensile properties of Al-10%Mg and AA7075 alloy round bars thixo-extruded at rather higher speeds from their mechanically stirred billets, forced-air cooling treatments were made during thixo-extrusions of small specimens at the constant extrusion ratio of 36. When the constant quantity of forced-air Q_fa = 1.33 × 10^-3m ³/s) was blown on to the Al-10%Mg alloy bars thixo-extruded at various positions from the die-outlet (L_ac) under the condition of extrusion temperature (T_e) of 823K and press ram speed (V _R) of 10mm/s, the most improved tensile property comparable with the conventional hot-extruded bar could be obtained at L_ac=70mm. If the same treatments using Q_fa = 1.33 × 10^-3m ³/s were employed for thixo-extrusion of AA7075 alloy at V _R =10mm/s and T_e =833K, the remarkably improved highest elongation was achieved at L_ac= 140mm. Whether cooling treatments were carried out or not at the same extrusion conditions, in both alloys, there exist no differences in microstructures of the extruded bars and also in extrusion forces. Fractographs of the both alloy bars showed that the fracture mode changed from intergranular to transgranular ones with increasing ductility. This may be attributable to suppression of both the progress of aging and the generation of defects, such as micro-cavities, along the grain boundaries due to cooling effect by forced-air blowing.