| The present study was undertaken to investigate the effects of Mg content, cooling rate and aging conditions on the impact toughness of both non-modified and Sr-modified experimental and industrial 319-type alloys. The results showed that Mg content, aging conditions, and cooling rate have a significant effect on the microstructure of both experimental and industrial alloys, and consequently, on the mechanical properties. Such effects may be observed clearly in the impact toughness properties of the alloys investigated under different conditions.;The addition of Mg to the 319 alloys, especially at 0.6wt%, modified the morphology of the Si particles, and lowered the Al-Al2Cu eutectic temperature, this temperature depression increased with further Mg additions. Also, Mg caused a segregation of the copper phases which then led to the precipitation of the block-like Al2Cu phase. An increase in Mg content in the Sr-containing alloys resulted in an increase in Si particle size (area, length, and aspect ratio) and a decrease in the roundness ratio, thereby diminishing the modifying influence of Sr. The addition of Mg to the Fe-containing 319 alloys resulted in the precipitation of the Mg2Si, Q-Al5Mg 8Cu2Si6 and pi-Al8Mg3FeSi 6 phases, the Q- and pi-phases appeared in a script-like form rather than as irregularly-shaped particles. Magnesium was observed to refine the Si phase slightly, and it also had a negative effect on Sr modification, deduced from the fact that a change occurred in the microstructure which went from being a well modified one to a partially modified one.;The eutectic Al2Cu phase particles were almost completely dissolved in the aluminum matrix after solution heat treatment for all the alloys studied. Ultrafine Si particles were to be observed after the dissolution of the eutectic Al2Cu particles. These small Si particles tended to precipitate at the end of the solidification process. The block-like Al 2Cu, Q-Al5Mg8Cu2Si6, and alpha-FeAl 15Fe3Si2 intermetallics were insoluble, the plate-like beta-FeAl 5FeSi and pi-Al8Mg3FeSi6 phases dissolved partially, and the Mg2Si phase dissolved completely during solution heat treatment at 495°C/8hrs. For the range of cooling rates applied in the present study, corresponding to dendrite arm spacings of 24 mum and 50 mum, the higher cooling rate of 24 mum DAS was the dominant parameter in controlling the size and distribution of the Si particles, intermetallic phases, and porosity in both the non-modified and Sr-modified alloys.;The addition of magnesium and copper improved the hardness values of the alloy samples tested, especially in the T6 heat-treated condition. The higher cooling rate also produced an increase in hardness, especially for the non-modified Mg-containing alloys. The addition of Sr, however, decreased the hardness of both the Mg-free and Mg-containing alloys. This decrease most likely arose from a delay in Mg2Si precipitation during the aging process of the Mg-containing alloys. The T7 heat-treated alloys displayed lower values for hardness and, consequently, higher values for impact toughness than those which were T6 heat-treated. In both T6 and T7 heat-treated conditions and for both the non-modified and Sr-modified alloys, the experimental alloys demonstrated higher values for hardness and impact energy than the industrial alloys.;The porosity parameters of area percent porosity and pore length were greater in the industrial alloys than in the experimental alloys, in both the non-modified and Sr-modified conditions. These parameters increased with Sr modification and the application of a low cooling rate in all the alloys studied. When up to 0.6wt% Mg was added to the 319 alloys, both parameter values increased noticeably. Strontium modification increased the eutectic Si particle count per unit area in the as-cast condition, indicating a refinement of the microstructure; both the size and shape parameters of the eutectic Si particles were also affected. Furthermore, the addition of Sr led to the segregation of the copper phases in areas away from the modified eutectic Si as well as at the dendrite boundaries, leading to the precipitation of the Al2Cu phase in a block-like form, which, in turn, was more difficult to dissolve than the finer eutectic-like form of the phase.;Magnesium and copper decreased the impact toughness of the alloy samples tested, especially under T6 heat treatment conditions. The impact properties of 319 alloys are influenced by the microstructure which depends strongly on solidification conditions and alloy composition. (Abstract shortened by UMI.)... |
