ing may also change. Similarly, increased levels of Zn may increase porosity in the casting due to its high vapour pres- sure above 420C (788F).15
is well known to cause embrittlement within cast aluminium alloys. Alternately, either element may enhance castability as well as increasing strength, so that relative quality may instead improve.
bilize needles or platelets of the Beta phase Al5
A further 25 tensile tests were conducted for samples pro- duced at 26 m/s and 82 m/s for each of Alloys 2 and 3. Val- ues of the mean (µ), one standard deviation (σ), and µ-3σ for the tensile data of each of the two conditions are shown in Tables 3 and 4. Examinations were also made of the fracture surfaces of specimens of Alloy 2 that had been produced at a velocity of 26 m/s, for three samples which had similar tensile properties. Images are shown in Figs. 6-8 and some large, characteristic defects, with different origins are ob- served. It was also noted that, although the defects ranged in
Zn has also been reported to sta- FeSi,16
which
size, shape and type, the tensile properties of the three sam- ples were virtually identical. In some cases, the defects were more homogeneously distributed than others (i.e., compare Figs. 7 and 8). However, their actual effects on tensile prop- erties were nevertheless similar.
Weibull distributions were then used to examine the accumu- lated data in greater detail. As before, the samples produced at 26 m/s were inferior to those produced at 82 m/s. The Weibull modulus and σo
values as they relate to elongation at failure for Alloys 1-3
values as they relate to tensile strength for Al-
loys 1-3 are presented in Table 5(a). The Weibull modulus and Efo
are presented in Table 5(b). Respective distributions are pre- sented in Figure 9 (for tensile strength only). For specimens produced with a melt velocity at the gate of 26 m/s, it is clear from Table 5(a) and Fig. 9(a) that Alloy 1 is inferior to Alloy 2 whereas Alloy 3 falls between the other two. This primar- ily reflects differences in one or both of position parameter and Weibull modulus. Table 5(a) shows this to also be the case for
Table 3. Data for Alloy 2 Samples Produced at 26m/s or 82 m/s
Figure 4. Experimental data for samples produced at 26m/s and 82 m/s overlaid with one experimental true-stress-true strain curve, as well as the model true stress-true strain curve derived using the Ludwik- Holloman equation. Values of the strain hardening exponent, n, and strength coefficient, K, derived are shown within the plot.
44
Figure 5. Quality chart for Alloy 1 at two different melt velocities, describing the experimental data using the flow curve derived from the Ludwik-Holloman equation, after the method developed by Cáceres.9,10
The data for
the 26 m/s condition all falls between the q = 0.05 to q = 0.1 iso-quality points, whereas that for the 82 m/s condition nearly all falls between the q = 0.1 and q = 0.2 iso-quality points.
International Journal of Metalcasting/Summer 2011
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