COST 507 - Repositório Aberto da Universidade do Porto

Thermodynamic calculations have also shown that the driving force AG is not
affected by Ag or Sm. For supersaturated metastable solutions, the finer
precipitate size scales with the initial nucleus size which in turn depends on the
magnitude of AG. The higher the driving force for precipitation, the finer the
dispersion and the higher the strength. For this purpose the ageing
temperature was selected to be 155° C, in contrast to 170° C conventionally
used in the A357 alloy.
The time of artificial ageing was determined from hardness measurements as
a function of time at 155° C. Peak hardness is obtained at 25h for A357+Cu
and A357+Cu+Ag, and at 20h for A357+Cu+Sm. Artificial ageing was then
performed at 155° C for the times discussed just above.
3.4 Mechanical Properties
The results of the tensile testing of the heat-treated alloys are summarized in
Fig. 7. The elongation values are lower than expected, however they are
comparable since identical casting conditions were used in all alloys. The three
alloys with Cu, Ag and Sm additions possess better combinations of strength
and ductility than the reference conventional A357 alloy. Since all alloys have
been casted in the same way it can be deduced that the improvement in
mechanical properties is due to the alloying elements and the artificial ageing
followed. In comparison to the reference A357 alloy, the alloys containing Cu
posses higher strength and ductility due to the formation of fine dispersion of Θ-
phase. In addition the lower temperature ageing treatment (higher AG)
produces also a finer dispersion of Mg 2 Si precipitate.
Silver on the other hand enters the (Al) solid solution providing solid solution
strengthening, while Sm forms fine particles of the rhombohedral phase.
Due to the fact that the strengthening dispersions are very fine, both strength
and ductility in these alloys increase.
A357 A357+CU A3S7+Cu+Ag A357+Cu+Sm
Fig. 7: Yield strength, ultimate tensile strength and elongation for alloys A357
(reference), A357+Cu, A357+Cu+Ag and A357+Cu+Sm.
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