ing from the solution treatment temperature may however cause stresses to be re-introduced into the microstructure. Some evidence of quench related dislocations within the α- aluminium has been observed in TEM studies of the A380 alloy evaluated here, although it was not considered to be prolific enough to cause a substantial effect on properties.
It is therefore important that the changes to the microstruc- ture mentioned above contribute significantly to the ob- served improvement in quality facilitated by solution treat- ing the HPDC material.
Assessments Based on the ludwik-Holloman Equation
Further analysis was conducted for the T4 and T6 conditions. Figure 10 shows the flow curves and experimental data from Figure 4, now plotted on a quality chart, for the solution treated, T4 and T6 conditions. Here, because the relative quality of the different heat treat- ed conditions was expected to be the same, constant quality (iso-q) lines can be added, to show the entire scope of properties that may be derived via age hardening heat treatments. As may be appreciated, varying the time and temperature of the age hardening heat treat- ment for strengthening will move the range of properties within these lines. The relative quality was similar for each of the three tem- pers and most values fall between the iso-q values of 0.2 and 0.4. Plotting the equivalent defect fraction present on the fracture surface from equation 8 for the respective values of strain hardening exponent, n 11
, in Figure 10,
reveals additional information. For the solu- tion treated condition, the equivalent defect fraction was 0.07-0.23. For the T4 temper, the range of equivalent defect fractions were very close to the solution treated conditions, at 0.06-0.22. The similarities in these results are to be expected, because it follows that the only difference between the solution treated and T4 tempers is related to strengthening within the aluminium grains by very fine GP zones. Casting defects such as porosity, inclusions or oxides etc. must be constant. However, surprisingly, this trend does not continue for the T6 treated material. For this temper, the equivalent defect fractions pres- ent on the fracture surface now decreased, and ranged from 0.03 to 0.09. This result was particularly interesting because the T6 temper also displayed the highest value of Weibull modulus, m, and therefore the lowest flaw size distribution. A summary of the respec- tive equivalent defect fractions and Weibull
56
modulus values for all conditions examined are shown in Figure 11. Here, it is interesting to note that the equivalent fraction of defects present on the fracture surface does not necessarily follow the same trend as the Weibull modulus. For example, compared to the as-cast condition, the solution treated material displays a greater flaw size distribution (i.e. lower value of m), but a lower defect fraction present on the fracture surface. That is, the range of defect sizes was broad- er, but the total equivalent defect fraction decreased. The T4 treated material displays a slightly higher value of m and a similar equivalent defect fraction to the solution treated material. For the T6 samples, the flaw size distribution was reduced, and the equivalent defect fraction was also reduced. It would therefore appear clear that the relative quality for the T6 treated material was significantly improved.
(a)
(b)
precipitates within the aluminium grains, while b) shows coarse θ (Al2 precipitates near to a grain boundary. (a) [011]α
grains in (b) were close to the same [001]α orientation). (a) after ref. 3 , (b) [001]α
Figure 9. Examples of heterogeneous precipitation within the as-cast microstructure (TEM). a) shows inhomogeneously distributed θ’ (Al2
Cu) Cu)
. (Note that both
Figure 10. Quality chart for heat treated conditions (solution treated, T4 and T6) showing model flow curves, calculated from σ=Kεn
), Eqn 8) for the values of n shown. Iso-q lines from 0.01 to 1 are also provided on the plot.
n / e-εi εi n
, Eqn 7, and the equivalent defect fraction on the
fracture surface (dashed lines, derived from solving f=1-( e-εh εh
International Journal of Metalcasting/Fall 2011
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