for both melt velocities were used to form portions of the average true stress–true strain curve for the alloy. Values of n were then determined from the slope of the log-log plot of the line of best fit for each data set; that is, a) 26 m/s, b) 82 m/s, and c) both velocities combined. At 26 m/s, the strain hardening exponent, n, was 0.274, and the strength coefficient K was 906. At 82 m/s, n was 0.261, and K was 872. When both data sets were combined n was 0.267 and K was 888. All analyses showed reasonable agreement. To validate the technique, a comparison was made between the model true stress–true strain flow curve generated from the combined average values of n (0.267) and K (888) shown in Fig. 4, with the same curve determined experimentally for a single specimen diecast at a velocity at the gate of 82 m/s. All true stress and true strain data for both velocities are also plotted. The results therefore confirm that this method pro- vides a valid means of determining useful values for n and K, for the alloy A380. An added advantage is the ability of the model to produce values consistent with those resulting from a statistically large batch of tensile tests, in this case a total of 50 tests.
(a)
From these values of n and K, a quality chart based around the model flow curve representative of Alloy 1 was derived, and is shown in Fig. 5. In Fig. 5 it can be seen that for the samples produced at 26 m/s, the comparative iso-q values as defined by Cáceres9,10
are between 0.05 and 0.1, and for the
samples produced at 82 m/s, the iso-q values are between ~0.1 and 0.17. Here it is important to note that the lowest values for the 82 m/s data set were still greater than the high- est value for the 26 m/s data set.
Effects of Alloy chemistry
Two additional A380 alloys were tested (Alloys 2 and 3 from Table 1). Alloy 2 had a higher level of Cu compared to Alloy 1, and Alloy 3 had a higher level of Zn. These alloys were tested to determine if there were any quantifiable differences in quality which could be observed because of the composi- tional changes. Raising Cu level within an A380 alloy may produce higher levels of strength, but also greater quantities of hard intermetallic domains in the solidified eutectic that adversely influence ductility. The susceptibility to hot tear-
(b)
(c)
(d)
elongation levels for the two conditions tested. International Journal of Metalcasting/Summer 2011
Figure 3. Weibull distributions of Alloy 1 produced at 26 or 82 m/s. Views (a) and (c) show the distribution and values of Weibull modulus, m for the UTS and elongation at failure, Ef
; (b) and (d) show the probability of failure at stress or 43
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