propagating crack is expected to be important. What is of course particularly noticeable from fracture studies e.g.4 was that the crack path was always very straight and rela- tively planar in T6 treated material, but was microstruc- turally convoluted and often branched in T4 treated mate- rial. As a result, the T4 material will most likely interact with more of the microstructural flaws within the alloy on convoluted fracture surfaces, whereas the T6 material tends to interact with only those defects and other features which are directly in the fracture path, or very near to it. It then is reasonable to suggest that for the T6 material, planar defects and oxides would actually need to be large enough to initiate cracking to be observed on the fracture surfaces, and in such cases the material would most likely fail with little or no ductility.
summary and conclusions
Earlier work examining casting quality for HPDC’s has been extended to heat treated conditions in one A380 alloy. It has been found that:
1. Solution treatment has a particularly positive in- fluence on microstructural quality of HPDC’s. This improvement arises because of changes oc- curring within the microstructure, namely, a) the fragmentation and spheroidization of the Si phase; b) the dissolution of hard Cu-bearing phases, c) homogenisation of the solute elements within the aluminium grains, and d) the assumed relaxation of residual stresses. In the current samples, d) was considered to have minimal effect. Factors a), b) and c) together, decrease the contiguity of the re- sidual hard phases present in the solidified eutectic. It can be concluded that each of these features must contribute to the equivalent defect fraction on the fracture surface in the as-cast condition. In the as- cast condition, the equivalent defect fraction on the fracture surface was 0.21-0.32, and following solu- tion treatment, after the abovementioned changes had occurred, the range was reduced to 0.07-0.23.
2. Material heat treated to a T4 or T6 temper retains the same relative quality ranges as the solution treated condition, when following the method of Cáceres.9,10
In the solution treated and T4 tempers,
nearly the same equivalent defect fractions were present on the fracture surface (e.g. 0.06-0.22 for the T4 temper). For a T6 temper, the equivalent fraction of defects present on the fracture surface was reduced to 0.03-0.09. In the T6 temper, the Weibull modulus was also substantially increased, meaning the flaw size distribution was reduced. In the solution treated and T4 tempers, naked eye vis- ible film or flake defects were observed on the frac- ture surfaces of the samples with the lowest 5% of elongation values recorded. No such defects were
60
observed on the T6 fracture surfaces. It is proposed that in addition to the absence of the planar oxide film or flake defects on the fracture surfaces in the T6 temper, the reduced influence of hard Si par- ticles where yielding occurs above the Si fracture stress is important in improving the relative quality of the T6 treated material.
3.
Ductile domains were observed at the oxide-metal interface of the planar defects in T4 treated mate- rial. This suggests that the microstructure immedi- ately adjacent to the defect yielded, before becom- ing a site for stress localisation at its edges, initiat- ing failure.
4. In statistical testing of T4 treated material, the lowest 9% of the results were found to display planar defects present on the fracture surfaces, which adversely influenced properties. Two types of planar oxide film or flake defects were observed; one was easily seen by the naked-eye on the fracture surface, and the second was found to be an undercut, observable by tilting in stereo- microscopy. Both contributed to the lower values of elongation observed.
Acknowledgements
The authors would like to thank Andy Yob, Dayalan Gunas- egaram and Maya Gershenzon for assistance with casting of samples.
rEfErEncEs
1. Lumley, R.N., Deeva, N, Gershenzon, M., “An Evaluation of Quality Parameters for High Pressure Diecastings”, Int. J. Metalcasting, vol.5, issue 3, pg. 37-56 (2011).
2. Lumley, R.N., O’Donnell, R.G., Gunasegaram, D.R., Givord, M, International Patent Application, WO2006/066314.
3. Lumley, R.N., O’Donnell, R.G., Gunasegaram, D.R., Givord, M., “Heat Treatment of High Pressure Diecastings” Metall. and Mater. Trans A, vol. 38A, p.2564-2574 (2007).
4. Lumley, R.N., “A Preliminary Evaluation On The Fracture Toughness Of Heat Treated Aluminium High Pressure Diecastings”, Adv. Mater. Res., vols. 41-42, pp.99-104 (2008).
5. Lumley, R.N. “Technical Data Sheets for Heat-Treated Aluminium High-Pressure Die Castings”, Die Cast. Eng., vol. 52, no.5, p.32-36 (2008).
6. Lumley, R.N., Gershenzon, M., and Gunasegaram, D.R., “Alloy Design for Heat Treatment of High Pressure Diecastings”, Mat. Sci. Forum, vol. 618-619, p.331-339 (2009).
7. Lumley, R.N., Polmear, I.J., Curtis, P.R., “Rapid Heat International Journal of Metalcasting/Fall 2011
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