One possible contribution to the changes in relative quali- ty for the T6 temper is derived from the earlier discussion related to the microstructural characteristics of the solu- tion treated condition. One feature identified which con- tributed to the equivalent defect fraction, and which was present on all fracture surfaces, is the hard Si phase. Im- portantly, the strength of the Si present in Al-Si castings may only be 200-330 MPa.18
This is less than the 0.2%
proof stress of the T6 treated HPDC material (351 MPa), but is between the 0.2% proof stress and tensile strength of the T4 treated material (217 MPa and 386 MPa, re- spectively). Therefore, it may be suggested that as the strength of the Al grains was increased by heat treatment, such that the strength of the bulk material approached or exceeded that of the Si particles, the Si phase may actu- ally have had less relative influence on the fracture pro- cess. This follows because fracture and de-bonding of the hard Si phase occurs on the assumption that the matrix Al surrounding the Si particles undergoes deformation in the crack tip plastic zone, leading to the accumulation of dislocations at the Si particles and their subsequent frac- ture. As may be appreciated, the plastic zone in the crack tip of the T6 material is expected to be smaller than that present in the more ductile T4 or solution treated mate- rial, influencing crack propagation.20
Another reason for
both the low ranges of equivalent defect fractions and high Weibull modulus values for the T6 condition follows on from the observation that no large oxide flake defects were observed on the fracture surfaces of these samples, discussed in the next section.
Defects on fracture surfaces
Samples produced in the T4 temper were examined in greater detail. The T4 temper typically displays higher levels of ductility than the as-cast condition, usually also with increases to 0.2% proof stress and tensile strength. One hundred samples were tested meaning a thorough statistical analysis of the defects most likely to cause early fail- ure in the heat treated conditions was able to be conducted. Previously, it has been shown that for the as-cast condition, ei- ther a foam-like shrinkage defect or oxide films / flakes present on the fracture sur- face contributed most significantly to early failure.1
Increasing the melt velocity at the
gate had the greatest effect on reducing the proportion of the foam-like shrinkage po- rosity defects. Oxide films or flakes present in the microstructure were not significantly influenced by velocity at the gate, but they appeared able to be reduced by melt degas- sing with argon, leading to a reduction in flaw size distribution (i.e. an increase in Weibull modulus).1
In the current experi- International Journal of Metalcasting/Fall 2011
Figure 11. A summary of the area fraction equivalent defects on the fracture surface (average is circles (Ο), with high-low ranges shown), derived from Figure 10. The respective values of Weibull modulus, m, for tensile strength (♦) from Table 5 are shown on the second y-axis and provide a comparative measure of the flaw size distribution in the material (i.e. the lower the Weibull modulus, the greater the flaw size distribution).
57
ments, the lowest 10% of samples were placed aside for further analysis. These corresponded to all samples dis- playing 4% engineering strain or less (See also Figure 3d), which also corresponds to samples displaying values of q<0.2 (Figure 10).
All ten samples were examined with a stereo microscope, and Table 6 summarises the outcomes. Each sample may be identified uniquely by its respective elongation at failure and tensile strength value. Table 6 shows that each of the lowest five data points displayed naked-eye visible planar oxide flake defects present on the fracture surface. Of the remaining samples, four did not display naked eye visible defects present on the fracture surface. Closer inspection by stereomicroscopy however revealed the four samples having 3.7 or 3.8% elongation did have planar oxide flakes present on the fracture surface, but these were “undercut” features. One example, displaying both types of defect to- gether on the fracture surface is shown in Figure 12. The feature can be described as a “twisted ribbon” in the form shown schematically in Figure 12b, such that half of the defect appears visible and half of the defect appears un- dercut. One additional feature is the angle to the tensile direction in which these oxides were often observed; many were present at an angle roughly between 30 and 60° to the tensile axis (e.g. Figure 12c and d). It is important to consider that in a random distribution, 1/3 of these planar oxide defects present in the gage length of the casting would be present at between 30 and 60° to the tensile
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