Influence of homogenization treatment on microstructure and mechanical properties of Al-Zn-Mg alloy extruded by porthole die

Abstract

The effects of homogenization parameters on microstructure and mechanical properties of Al-Zn-Mg profile extruded by porthole die were investigated. During homogenization treatment, the eutectic phases in as-cast billet dissolved or transformed to S(Al2CuMg) phase, while the compositions of Fe-containing phase varied slightly and only partial of them dissolved. Moreover, the fine dispersoids precipitated during air cooling. The billet hardness was decreased by increasing homogenization temperature. If the water quenching was employed, the precipitation of fine dispersoids was prevented, resulting in strong solution strengthening and relatively high hardness. The extruded profile could be divided into welding zone with fine grained structure and matrix zone with elongated and coarse grains. The quantity of coarse phases in extruded profile continuously decreased with increasing the homogenization temperature, while the fine dispersoids showed the opposite tendency. The hardness in welding zone is always lower than that in matrix zone. The profile extruded from the as-cast billet exhibited the highest tensile strength but the lowest elongation. The coarse phases exhibited stronger strengthening effect than the fine dispersoids, while they are harmful on elongation. After homogenizing the billets or increasing the homogenizing temperature, the corrosion in welding zone was reduced, while the corrosion in matrix zone became more severe.