The different dependencies of the mechanical properties and microstructures on hot extrusion and artificial aging processing in case of the alloys Al 7108 and Al 7175

Abstract

In view of optimizing the strength and the ductility of the almost copper-free alloy 7108 and the copper-containing alloy 7175 the influences of extrusion processes with different parameters and subsequent artificial peak aging procedures have been studied. The work was divided into three steps. In the first step the extrusion ratios, the product speeds and the billet temperatures have been varied and their influence on the extrudability of the alloys 7108 and 7175 were examined. In the second step the resulting strands were analysed in the water quenched condition to investigate the correlation of the morphology and the texture to the mechanical properties. In the last step the strands were subjected to peak aging, to analyse the influence of the extrusion parameters on the precipitation behaviour and the mechanical properties. For the analyses of the morphology and the texture a combination of optical microscopy, X-ray diffraction and electron backscatter diffraction has been used. The particles and the precipitates were analysed by scanning and transmission electron microscopy. In addition the mechanical properties were measured by compression and tensile tests. The investigations show that the almost Cu-free alloy 7108 is extrudable within a wide parameter field and that the mechanical properties of this alloy can be varied and optimized by the parameters of artificial aging processes. So the combination of the extrusion process and the artificial aging results in an elongation at fracture up to 17.4±0.2% and in strength values up to 426±1MPa. In case of the Cu-containing alloy 7175 the extrusion process is limited to lower product speeds, because the temperature increment, which is introduced through the plastic deformation, is higher in case of this alloy. Therefore, eutectic phases form and cause hot cracking of the profiles. The investigation shows that the mechanical properties of the Cu-containing alloy 7175 are mainly influenced by the extrusion process. It was shown that the extrusion process using high billet temperatures leads to a ductility up to 14.5±0.5% and to strength values up to 636±4MPa.