The results of this study also suggest that the T6 heat treat- ment standard shown in Table 1 for the A356.2 alloy must be revisited to include optimization of the incubation and artificial aging.
Conclusions
The salient conclusions and contributions from this study are: · A sequence of precipitation reactions during the aging heat treatment of A356.2 alloy cast in a per- manent mold has been proposed describing the critical stages during incubation at room tempera- ture and artificial aging at 155C (311F).
· The precipitation reaction during aging is strongly influenced by the rate of quenching subsequent to solutionizing. Higher quenching rates result in higher vacancy concentrations in the primary Al phase matrix and higher kinetics of the precipita- tion reactions.
·
Incubation at room temperature subsequent to solu- tionizing and quenching has a significant influence on the precipitation reaction sequence and kinetics during artificial aging at 155C (311F). Specifically, the location of Mg atoms in the primary Al matrix (whether trapped in individual or co-clusters, or in the super saturated solid solution of the Al phase at the end of incubation) is critical to the rate of pre- cipitation and number density of precipitates dur- ing artificial aging. More Mg atoms in the SSSS of Al at the end of incubation will cause a higher rate of precipitation during artificial aging; and vice- versa. Higher rates of precipitation during artificial aging will result in higher yield strengths and lower ductility and vice-versa.
·
A356.2 alloy could be tailored to have high strength (~235 MPa) and nominal elongation (~2%) as well as a nominal strength (~195 MPa) and high elonga- tion (>8 %) by solely varying the extent of incuba- tion at room temperature after solution heat treat- ment and quenching.
·
Commercial T6 heat treatment specification, ASTM Standard B917/B917M 2008, needs to be refined and elaborated to include the specific ef- fects of incubation coupled with artificial aging on the mechanical properties of the cast component.
Acknowledgments
The authors would express their deepest gratitude to Mr. Tom Prucha (American Foundry Society, Inc., USA), Dr. Diran Apelian (Metal Processing Institute (MPI), Worcester Poly- technic Institute (WPI), Worcester, MA, USA) and Mr. John Jorstad (JLJ Technologies Inc., USA) for their laborious and extensive review of this article and providing us with very information and thought provoking comments, revisions and discussions. We were indeed fortunate to have the three lead- ing experts in this field of science review this work.
International Journal of Metalcasting/Fall 2011
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