the component engine block according to customer specifi- cations and is described as follows: If a life goal of 107
cycles
without a failure occurs, then it is considered a run out for a given targeted alternating stress level. A subsequent test would be conducted on a new fatigue test sample at an incre- mentally higher stress level. If that subsequent test sample does not achieve the 107
0.04°C/sec, see Figure 6) exists for Cu diffusion into the in- terdendritic liquid resulting in a much larger PAF of the Cu based phases and less Cu to be retained in the dendrites.
cycle life target, it is considered a
failed test and results in the very next sample being tested at an incrementally lower stress level. The example of how this fatigue staircase plot is generated is shown in the results section of this paper. The method of calculating the mean fa- tigue stress from the plot is determined by taking the average stress of all the fatigue test samples which failed (higher value of stress) or all the fatigue test samples which passed (lower value of stress), whichever was lower in number. The total number of samples used includes the initial ramp up or ramp down samples. The logic behind taking the lower number of fatigue test samples, which either all passed or failed, is that it removes the possible contribution to the average stress and standard deviation of a poor estimate of the starting alternating stress (e.g. successive failures or run-outs at the beginning of the plot). The total number of fatigue samples tested for each alloy investigated was 36.
Results Metallographic and
Mechanical Characterization of Wedge Castings
Figure 10a shows the plots of the PAF porosity, PAF of the Cu based phases and λ2
for the U319 (Al7Si-
ing phases. At the same time, PAF porosity is very low due to the fact that the existing hydrogen is trapped in the dendrites and therefore, inter- dendritic feeding is better at higher solidification rates. The longer so- lidification time allows for hydro- gen diffusion that leads to stable pore growth along with extended or tortuous feeding distances.1,6,8,12 At a 228 mm distance from the chill a much longer time (cooling rate at
1Cu) alloy wedge casting. As the solidification rate slows, (from the chill toward the direction of the riser – right to left), the values of all three microstructural features goes up. At a distance of 28 mm from the chill most of the Cu is retained within the dendritic arms, not having time to diffuse into the interdendritic liquid and form Al2
Cu and other Cu bear- 40
Figure 10a. Value of λ2, PAF-Cu phases and PAF porosity along the wedge casting for the U319 (Al6Si1Cu) alloy.
The effect of the T7 treatment is illustrated in Figure 10b which shows a considerable drop in the PAF Cu based phas- es (especially for the larger distance from the chill) while dendrite hardness increased for the U319 alloy. During solu- tion treatment the Cu based phases partially dissolve into the α-Al dendrite matrix while during artificial aging precipita- tion occurs raising its hardness value. Due to the residual Sr concentrations (residual Sr concentrations defining as being ≤ 20 ppm), and due to the low solution temperature used, the Si and the Al5
thus its contribution to the change in α-Al dendrite hardness should be minimal. For the as-cast and heat treated
FeSi platelets seen in the microstructure
appear similar to the as-cast state. Usually elevated Sr con- centrations, higher solution temperatures and/or durations are required to promote spheroidization of both platelet type phases,1
Figure 10b. Effect of T7 treatment on the area fraction of cu phase (%) and the Vickers microhardness (µHV25g/15s) of the U319 (Al6Si1Cu) alloy.
International Journal of Metalcasting/Fall 10
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