| A study was undertaken to investigate the effects of chemical additives, mainly zirconium (Zr) and scandium (Sc), on the microstructure, tensile properties, and hot-tearing susceptibility of A1-2wt%Cu-based alloys, Zr and Sc additions being known to improve the high temperature performance of aluminum alloys.;In the second alloy category, seventeen alloy compositions were prepared, using different additions of Zr, Sc, Ti-B, Sr, Ag, and Si. These alloys were further divided into four groups, namely, the Zr-Ti, Zr-Sr, Zr-Sc, and Si alloy groups. Tensile test bars (cooling rate 7°C/s) prepared from these alloys were solutionized for 8h at 490°C then age-hardened at temperatures of 180°C and 220°C for aging times of 2, 4, 6, 10, 16, 24, and 48h, and 0.5, 1, 1.5, 2, 4, 6, 10, 16, 24, and 48h, respectively. It was observed that combined additions of Zr-Ti, Zr-Sr, or Zr-Sc refined the grain size of the base alloy considerably, from 219 microm to 104-46 microm, since these elements form primary trialuminide intermetallics including Al3(Sc 1-xZrx), Al3(Sc1-x-yZrxTi y), and Al3Zr which act as nucleation sites for alpha-Al grains, thereby producing fine non-dendritic structures. The refined non-dendritic morphology produced by the combined additions of Zr-Ti, Zr-Sr, or Zr-Sc caused a reduction of about 65% in the amount of the Al2Cu phase in the base alloy, and a reduction in porosity of about 50%. The addition of 2 wt% Si also produced the same reduction levels in the amount of Al2Cu and porosity, brought about by the increase in the Al-Si eutectic content. Age-hardening peaks were observed after aging for 10h and 24h at 180°C, and after aging for lh and 2h at 220°C. The Zr-Sr and Zr-Ti alloy groups displayed the highest level of improvement in the tensile properties of the Al-2wt%Cu base alloy followed by the Zr-Sc and Si alloy groups, in that order. The 0.02wt%Sr-0.7wt%Zr containing alloy produced the highest ultimate tensile and yield strength values of 383 MPa and 326 MPa, respectively, after 4h of aging at 180°C.;The third alloy category comprised six alloys selected from the second category in order to investigate the effects of chemical composition and mold variables on the hot-tearing susceptibility (HTS) of the new A1-2wt%Cu alloy. The HTS results were then compared with 206 alloys containing the same chemical additions. Generally, the Al-2wt%Cu-based alloys exhibited higher resistance to hot-tearing than 206-based alloys did. It was found that an elevated mold temperature is beneficial in reducing the hot-tearing susceptibility of the Al-2wt%Cu and 206 alloys in that the HTS value decreased from 21 for the two alloys to 3 and 9, respectively, as the mold temperature was increased from 250°C to 450°C. The refinement of the grain structure obtained with the Zr-Ti or Ti additions decreased the hot-tearing severity as result of an increase in the number of intergranular liquid films per unit volume and a delay in reaching the coherency point. Increasing the Si content reduced the hot-tearing susceptibility of the A1-2wt%Cu alloy considerably; this reduction is attributed to an increase in the volume fraction of eutectic in the structure, and a decrease in the freezing range of the alloy. The addition of Sr caused a deterioration in the hot-tearing resistance of the base alloy as a result of the formation of Sr-oxides and an extension of the freezing range of the alloy. It was also observed that cs-Fe particles may obstruct the propagation of hot-tearing cracks. The 1 %wtSi-containing Al-2wt%Cu alloy was judged to be the best composition in view of its low hot-tearing susceptibility. (Abstract shortened by UMI.);In the first alloy category, ten alloys were cast using Zr and Sc as the main additives in combination with Ti-B, Sr, and Ag under slow cooling rate conditions of ∼0.3°C/s and at relatively high concentrations of 0.5 wt% each of Zr and Sc. A number of primary Zr-, Sc-, and Zr-Sc-containing intermetallics were detected in the alloys investigated, namely, the star-like Al3(Sc1-x Zrx) phase, the Al3Sc phase, the V-AlSc2Si2 phase, the Al3Zr phase, and two other Zr-intermetallic compounds. It was observed that the Al 3Zr crystals act as nuclei for the star-like phase which grows through the precipitation of layers of Al3Sc on these nuclei, with the successive substitution of Sc by Zr atoms. It was also reported that the starlike phase continues to grow in the solid state by absorbing Sc to form the Al 3Sc phase, observed in the form of a rim along the edges of the particle. Ternary AlZrSi and quaternary AIZrTiSi intermetallic compounds were also detected in this study. |
