(T7) wedges, there is a difference of 15-19 µHVH25g/15s between these two conditions across the length of the wedge while the difference in the PAF for the Cu based phases is smaller near the chilled end and more pronounced at a dis- tance of 228 mm from the chill.
Figure 11a shows the plot of PAF porosity, PAF Cu based phases and λ2.
Figure 11b shows the plot for the PAF Cu based phases and dendrite microhardness for the as-cast and heat treated wedges (T7 conditions), for the W319 alloy. It is noticeable that the vertical PAF Cu scale in Figures 11a and 11b is significantly larger than in Figures 10a and 10b due to the higher Cu level and consequently the higher PAF Cu based phases present. Figures 11b and 10b show that there is a much larger difference between the as-cast and T7 conditions for both the PAF Cu based phases and for the microhardness of the metal matrix (~24-26 µHV25g/15s). This observation is presumably due to the combination of elevated Cu concentrations and the much lower start temper- ature (see Table 2) for Cu multi phase eutectic formation.
from the chill). This thermal signa- ture (latent heat event on the cooling curve) will contain other phases such as Mg2 Al15
observed in microscopy analysis in multi phase eutectic reactions. In ad- dition, the start temperature for the 4 wt% alloys is close to 25C (77F) lower than for the 1 wt% Cu alloys, and is very close to the 505C (941F) Solution Treatment temperature used. There- fore, the results shows the fact that the Cu phases in 4 wt% Cu containing al- loys may have dissolved more readily, leading to large changes in α-Al den- drite microhardness and to the drop in
(Fe,Cr,Mn)3 Si, Al5
FeSi, and small script Si2
all of which were International Journal of Metalcasting/Fall 10
Figures 12a, 12b, 13a and 13b show the plots of the same characteristics as noted above for the U328 and W328 alloys. While similar trends are ob- served for the 7 wt% Si and for the 9 wt% Si alloys the microhardness in the as-cast and heat treated conditions is considerably higher. There is a dif- ference of 20-25 µHV25g/15s between the as-cast state and the T7 condition across the length of the wedge for the U328 alloy, and this difference is 40- 45 µHV25g/15s for the W328 alloy. From Table 2 we can see that for the 4 wt% Cu alloys the volume fraction of Cu based phases measured are ap- proximately three times higher than for the 1 wt% Cu alloys (λ2
or the distance
the PAF Cu based phases measured. Neglecting to point out this fact may lead the reader to base this observation only due to the concentration level of the Cu in the alloys.
Figures 14 and 15 plot the value of largest pore diameter and the PAF porosity, respectively, that was measured for the four alloy compositions at a distance of 28, 68 and 108 mm from the chill end of the wedge. The U328 alloy appears to have both the lowest largest pore diameter and the PAF porosity. It is interesting to note, that the W319 alloy, used for Cosworth casting production, has the highest largest pore diameter and the PAF porosity.
The U328 alloy is presumed to be a preferred option to pro- vide improved fatigue performance over the U319, W319 and the W328 alloys. The authors are making this statement based on the porosity data comparison seen in Figures 14 and 15, which show the U328 alloy having the lowest porosity, and the widely held observation within the literature that porosity
Figure 11a. Value of λ2, PAF-Cu phases and PAF porosity along the wedge casting for the W319 (Al6Si4Cu).
Figure 11b. The effect of T7 treatment on the area fraction of Cu phase (%) and the Vickers microhardness (µHV25g/15s) of the W319 (Al6Si4Cu) alloy.
41
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