| The study presented in this thesis was performed on Al-Si-Cu-Mg 319 type alloys to investigate the role of solution heat treatment on the dissolution of copper-containing phases (CuAl2 and Al5Mg8Cu 2Si6) in 319 type alloys containing 0, 0.3 and 0.6 wt% Mg levels, to determine the optimum solution heat treatment with respect to the occurrence of incipient melting, in relation to the alloy properties. Two series of alloys were investigated: a series of experimental Al-7 wt% Si-3.5 wt% Cu alloys containing 0, 0.3, and 0.6 wt% Mg levels. This series was prepared in the laboratory using pure elements. The second series was based on industrial B319 alloy (containing 0.3 wt% Mg), where the Mg level was increased to 0.6 wt% by adding pure Mg metal to the melt. In order to study the effect of modification, strontium was added in the amount of 150 ppm to both the experimental and the industrial alloys, to provide a set of Sr-modified alloys. Thus a total of ten alloys were investigated in the present case.;The results show that in the as-cast condition, Cu segregation occurs at grain boundaries, and the presence of Sr or Mg can worsen the segregation. When Mg and Sr are added at the same time, however, the segregation is weakened to some extent compared to when they are added individually. After heat treatment, especially in the Sr-modified alloys, the copper begins to distribute evenly across the dendrites as well as in the matrix, with increasing solution time and temperature; the amount of undissolved Al2Cu decreases and Cu in the matrix increases, reaching a maximum after a solution heat-treatment of 490°C/8h. Addition of Mg to 319 alloys (experimental or industrial) leads to the formation of the low-melting-point, insoluble complex phase Al 5Mg8Cu2Si6. Increasing the Mg addition to 0.6 wt% increases the volume fraction of this phase and both pre-eutectic and post-eutectic Al5Mg8Cu 2Si6 precipitation is observed. In solution heat treatments where the last solution temperature used exceeds the melting point of the Al5Mg8Cu2Si6 phase, incipient melting of this phase occurs, causing severe deterioration of the alloy mechanical properties.;The presence of Sr results in the modification of the eutectic Si particle morphology from a coarse, flake-like form in the non-modified alloys to a fine, fibrous form in the Sr-modified alloys. A corresponding depression of the Al-Si eutectic temperature is also observed. However, strontium also leads to the segregation of the copper phase in areas away from the eutectic silicon regions, so that the Al2Cu phase has a tendency to precipitate in the more massive block-like form rather than its finer eutectic-like form. This change in the Cu-phase morphology slows its dissolution rate during solution heat treatment so that when (a) the solution time of the first-step treatment is not sufficiently long to dissolve the Al2Cu particles, and (b) the temperature of the second-step treatment is higher than the melting point of Al2Cu, incipient melting will take place, and as a result of volume shrinkage during quenching, porosity formation will be observed. The addition of Sr is also found to lead to increases in area percent porosity and pore length, particularly at the 520°C solution treatment temperature.;The tensile properties, i.e. ultimate tensile strength (UTS), yield strength (YS), percentage elongation (%El), and quality index (Q) values obtained show that the addition of Mg to the experimental 319 alloys leads to an increase in the YS and UTS, but a degradation in the %El. In the non-modified alloys, the loss of elongation is overcome by the increase in strength, thus the Q values are increased. In the modified experimental alloys, the degradation of elongation is not balanced by the increase in strength, thus the Q values are decreased. Magnesium increases the yield strength more than the ultimate tensile strength. The optimum combination of Mg and Sr is that of 0.3wt% Mg with 150 ppm Sr. The corresponding tensile properties in the as-cast condition are 260 MPa (YS), 326 MPa (UTS), l.50% (El%), 352 MPa (Q), showing an increase of 79% and 40% for YS and UTS, respectively, a decrease in elongation of 38%, and an increase in the quality index of 21% compared with the base alloy. Further increasing the Mg content leads to degradation of the tensile properties.;For the alloys studied, in the absence of Mg, the recommended solution heat treatment is 450°C/4h + 500°C/4h + 520°C/4h, for which the corresponding tensile properties and quality index are 385 MPa (UTS), 240 MPa (YS), 5.25% (%El), and 493 MPa (Q). In the Mg-containing alloys, the optimum solution heat treatment is 490°C/8h + 520°C/4h; the corresponding tensile properties are 445 MPa (UTS), 334 MPa (YS), 4.24% (%El), and 539 MPa (Q), respectively. (Abstract shortened by UMI.)... |
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