used in the current work produce little, if any, change in po- rosity fraction.2,3
Since it is relatively easy to characterise
what has actually changed within the microstructure, those features which are directly influencing quality may be read- ily characterised.
The reasons for the differences between the as-cast and the solution treated conditions were sought by examining in de- tail the microstructural changes occurring during solution treatment. Solution treatment causes important changes in HPDCs that influence quality. These are:
1) fragmentation, spheroidization, and ostwald ripening of the si Phase
Examples are shown in Figure 6 for edge and centre re- gions of diecast alloy, where the images were taken from the same position of the head of tensile samples. (Note that the optical microscopy samples used for Figure 6 and 7 were produced from samples having a velocity at the gate of 26 m/s).1
As is readily seen, the changes to the
Si phase were significant. Not only are the size and shape of the Si particles changed, fragmenta- tion also meant that the contiguity of the Si phase was reduced. Si particles with different shapes and aspect ratios were observed in the microstruc- ture (Figure 6) and it would be expected that these particles would display a range of different frac- ture strengths. Fracture initiated at Si occurs by the tangling of dislocations around these particles (similarly for other dispersoids), exerting stress until the particle cracks, undergoes interfacial decohesion, or shears. Any of these mechanisms will cause a significant reduction in resistance to further shear, and stress localisation will often oc- cur as a result. Further stresses are placed on the next Si particle, dispersoid, or defect within the path of the slip band, which fails even more read- ily, until fracture is catastrophic.17
Si particles are reported to begin cracking at an applied plastic strain of ~0.01 with a stress of between 200-330 MPa during tensile testing (previously reported for an A356 alloy in the T6 temper).18
to cause failure of the Si has also been shown to be de- pendent on the sphericity and size of the Si particles, with greater sphericity (i.e. lower aspect ratio) and smaller size relating to a higher stress required to cause fracture and de-bonding.19
aspect ratios, or form a component of a composite defect cluster and intersect porosity, oxides or intermetallics may also display early cracking.
Figure 7 quantifies the changes to the Si phase, for the same material at edge and centre regions. The trends in each re- gion were the same, but the differences were most signifi- cant in the edge region.
Fragmentation of the Si caused the number of Si particles in the edge region (present in 122,063 µm2
) to increase
The level of stress required
Those particles which are large, have high
Figure 5. Quality charts presenting the data and model flow curves along with the equivalent defect fraction present on the fracture surface, for the as-cast and solution treated conditions. See text for details.
(a)
(b)
(c)
(d)
Figure 6. Optical images showing edge and centre regions either as-cast or heat treated. a) and b) are edge regions, c) and d) are centre regions. a) and c) show the as-cast condition, b) and d) show the heat treated condition. Solution treatment for b) and d) was 15 minutes immersion at 490C (914F) prior to water quenching.
International Journal of Metalcasting/Fall 2011 53
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