Effects of Zr additions on structure and tensile properties of an Al-4.5Cu-0.3Mg-0.05Ti (wt.%) alloy

Abstract

The effects of different Zr additions (0.05wt.%-0.5wt.%) on the structure and tensile properties of an Al-4.5Cu-0.3Mg-0.05Ti (wt.%) alloy solidified under a high cooling rate (18 °C·s-1), in as-cast and T6 heat-treated conditions were studied. The as-cast structure of the alloy consists of equiaxed grains of α-Al with an average size of 64 μm which is unaffected by the Zr additions, indicating the ineffectiveness of Zr in the grain refinement of the alloy. Scanning electron microscopy, along with X-ray diffraction analysis revealed the presence of elongated θ-Al2Cu at the grain boundaries; in addition, coarse Al3Zr particles exist in the intergranular regions of the 0.5wt.% Zr-containing alloy. After the T6 heat treatment, the elongated θ particles were fragmented; however, the coarse Al3Zr particles remained unchanged in the microstructure. Also, the formation of fine β′-Al3Zr and θ″-Al3Cu/θ′-Al2Cu phases during T6 heat treatment was revealed by transmission electron microscopy. The results of the tensile tests showed that the Zr additions increase the strength of the alloy in both as-cast and T6 heat-treated conditions, but reduce its elongation, especially with 0.5wt.% Zr addition. The 0.3wt.% Zr-added alloy in the T6 heat-treated condition has the highest quality index value (249 MPa). Fractography of the fracture surfaces of the alloys revealed ductile fracture mode including dimples and cracked intermetallic phases in both conditions.