an effect on improving castability, but this is most important at the higher melt velocity. At the lower velocity, increased Cu level has had little effect on Weibull modulus, but higher Zn content has reduced this value and hence has reduced the relative quality because the flaw size distribution is greater.
Despite the differences due to composition which could be identified using the Weibull statistics in Tables 5 (a & b), such variations in chemistry would of course be considered to be normal during production, where the metal composi- tion is expected to vary continually within the specification. Values of K and n were again determined for the data sets using the procedures outlined in the previous section. Table 6 shows all of the strain hardening exponents (n) and values of the strength coefficient (K) recorded for each of the three alloys, at both melt velocities.
The accumulated true stress and true strain data is presented in Fig. 10, which shows all of the combined tensile test re- sults plotted together with the model flow curve representing all 150 tensile tests of the A380 alloy samples.
Figure 11 shows the corresponding quality chart describ- ing all three alloys, at both melt velocities together with the model flow curve describing the data, based on an n value of 0.259 and a K value of 883 (Table 6). Since all three alloys are within the same compositional specification and the values are similar, it is not necessary to plot lines of constant quality (iso-q lines) on the diagram. Points corresponding to values of q of between 0.01 and 1 are marked. As well as describ- ing the behavior of the three different A380 alloys, the curves of Fig. 11 also accommodate reasonably well the combined results from other specimen geometries, literature values for A380 alloy test bars, and samples machined from castings,17-23 where these are converted to true-stress and true-strain values. Therefore, it may be suggested that Fig. 11 could permit ten- sile properties from any casting made from A380 alloy to be evaluated against this common baseline. As will be appreciated, this plot is valid for A380 alloy with 0.2% proof stresses between 160 and 190 MPa, and lower values of 0.2% proof stress and tensile strength resulting from a coarser Al grain, or Si particle size, causes the strength coefficient K to be reduced and hence the curve may be offset downwards. Similarly, increased Mg content above 0.2% for other 380 type alloys (i.e., C380) causes the strength coefficient to be raised and the curve is then offset upwards. What is also interesting to consider from these derivations of n and K, is their relative magnitude, and therefore, the scope for im- provement in the alloy. In particular, it is interesting to note that, A380 alloy is reaching less than 20% of its maximum potential, and that if straining was not limited by defects (i.e., a defect free casting) neck- ing would only begin at a true stress value of 622 MPa and a true strain value (where e = n) of ~0.26. This would correspond to an engineering stress of
48
480 MPa, and engineering strain of almost 30% (assuming of course that flow instability did not arise leading to the onset of early necking).
The results and spread of data shown by the flow curve of Fig. 11 have additional meaning. By solving Eqn 14 for the value of strain hardening exponent n, a relationship between the strain outside of the defect (i.e., true plastic strain), and the strain inside the defect, may be related to the fraction of features which have an equivalent effect to defects present on the fracture surface. That is, it must be taken into account that defects spread throughout the microstructure are numerous in type and their interactions may be extraordinarily complex.
Figure 10. Experimental true stress–true strain data for samples of Alloys 1-3 produced at 26m/s and 82 m/s overlaid with the model true stress-true strain curve derived using the Ludwik-Holloman equation. Values of the strength coefficient, K, and the strain hardening exponent, n, derived to generate the model flow curve are shown within the plot.
Figure 11. Quality indices (q) plotted along the flow curve determined for the combined results for three A380 alloys at two different melt velocities. Area fraction equivalent defects are plotted on the second Y axis, determined by solving Eqn 14 for the value of strain hardening exponent, n, shown. Symbols are the same as for Fig. 10.
International Journal of Metalcasting/Summer 2011
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