experimentation broad research plan
The most effective way to improve the room temperature TYS and LCF appeared to be an addition of Mg. This had to be limited in order to retain most of the room temperature ductility and the high temperature stability of the Mg-less alloy. Additions of 0.05 and 0.10% were first explored, fol- lowed by a methodical screening of the 0.07 – 0.20% domain in interaction with Cu (3.5 – 4.0%) and Fe (0.10 – 0.19%).
Lower levels of V were studied, decreasing from 0.21% to 0.05% along with a V-free alloy for comparison. It will be seen that V has practically no effect on the tensile properties but a strong one on creep. Consequently, it has been possible to research and present sequentially the effects of Mg, Cu and Fe on the tensile properties at constant V, and the effects of V, Mg and Cu on 300C (572F) creep.
The experimentation has thus been carried out in four phases as shown in Table 1.
detailed experimental procedure
Foundry practice: Phases 1 – 3 used AFNOR (Association Francaise de Normalisation) 18 mm diameter permanent mold cast test-pieces, later machined either to 13.8 mm for tensile testing or to other dimensions for fatigue and creep testing. The test-bars were cast from a 100 kg resistance furnace in which the alloy had been modified by an addi- tion of 110 ppm Sr, grain refined by 0.3% AlTi5B1 and degassed with a rotary impeller, using ultra high purity ar- gon. Phase 4 used simple cylindrical samples cast in dies calibrated to yield the intended solidification times. The melt treatment practice was similar, but a smaller 20 kg resistance furnace was used.
All samples of phases 1 and 3 as well as half of those of
a) AlSi7Cu0.5Mg0.3 T7
b) AlSi7Cu3.8MnVZrTi T7 Figure 1. Comparative LCF performances.
Figure 2. Solubilities of V and Zr as a function of temperature. 18 International Journal of Metalcasting/Summer 2011
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