Technical Review & Discussion
Effect of Incubation Coupled with Artificial Aging in T6 Heat Treatment of A356.2 Aluminum Casting Alloy J. Manickaraj, G. Liu, S. Shankar, Light Metal Casting Research Centre (LMCRC), McMaster University, Hamilton, ON, Canada
Reviewer: The paper indicates that there is evidence for the breakdown of the Mg self-clusters. To justify this, some thermal analysis and/or resistivity studies should have been performed.
Authors: Evidence for the breakdown of the Mg self-clusters was taken from reference [6] as discussed in the literature. In this reference, emphatic evidences have been shown to support the breakdown of the Self-Clusters.
Reviewer: What technique did you use to examine the se- quence of precipitation?
Authors: Micro hardness measurements and tensile tests coupled with information gathered from the literature were used to determine the sequence of precipitation.
Reviewer: Please comment on your ideas for the precipi- tation sequence described in the various figures. Can you provide a reference?
Authors: The sequence of precipitation shown as schemat- ics in Figure 5 and Figure 9 to 13 were created in this study from the results of micro hardness measurements, tensile tests and the literature.
Reviewer: In figures 12-16: How did you arrive at the Mg1.8Si, Mg2Si and Mg5Si6 compositions? Where is the evidence for such intermetallic compounds? Did you do any advanced microscopy to verify?
Authors: The composition of the precipitates Mg1.8Si, Mg2Si and Mg5Si6 were proposed from the literature, specifically in references [6, 10, 29,30,31,32,33,34] which have provided emphatic evidences to support our hypothesis. In-Situ high temperature HRTEM studies are currently being explored to confirm some of these precipitates in this alloy as well.
Reviewer: The primary focus appears to be on 6xxx alloys, not casting alloys. Previous literature suggests that precipi- tation reactions in A356 will be very different from those occurring in 6xxx alloys.
Authors: The mechanism of formation of strengthening pre- cipitates in both 6xxx and A356 alloys are determined by the amount of Si atoms, Mg atoms and vacancy concentra- tion after solution heat treatment in the primary aluminium phase. The vacancy concentration is determined by the ma- trix phase which is identical in both the 6xxx and 356 alloys. Table A below shows the average concentration of Si and Mg atoms in the primary Al phase matrix. The excess Si in the Al matrix for A356 alloy would cause a greater tendency for the formation of Si self-clusters during the precipitation reaction. However, our mechanism shows that the critical parameter controlling the mechanism of precipitation is the mobility of the Mg atoms through the Al matrix. The se- quence of precipitation would be similar for both the 6xxx alloy and A356 since this is controlled by the amount of Mg in the matrix, however, there would be a marked difference in the kinetics of the precipitation reactions only due to the excess Si content in the matrix for identical solution treat- ment and quenching rates for both the 6xxx and A356 alloys.
Our study merely used the literature information on the sequence of the precipitation reaction from the 6xxx alloy family and quantified the kinetics of the precipitation reac- tion in the A356 alloy, specifically during the natural ageing process. Publications related to this precipitation reaction in the Al matrix during heat treatment of A356 alloy have not alluded to any sequence of precipitation but merely identi- fied the terminal phases in the precipitation reaction as be- ing Mg2
Si. The two publications (cited by the reviewer) by
Apelian et al. and Gruzleski et al. present extensive mechan- ical properties as a function of natural ageing, however, nei- ther comment on the sequence of the precipitation reaction, which is critical for an in-depth understanding and control of the transient process such as natural ageing. Further, many findings of these papers could now be explained by the understanding of the kinetics of the precipitation sequence proposed in this study. A commentary on these publications has also been added to the revised manuscript.
Reviewer: The authors present detailed, but rather specula- tive discussions about ageing mechanisms, yet little direct ev- idence (e.g., via TEM or x-ray studies) about the details of the precipitates or other reactions that occur during natural aging.
Authors: The aim of this study was to propose (hypothesize) regarding the transient and dynamic sequence of precipita- tion as a function of time and explain the effect of natural ageing process on the tensile properties. It is nearly impossi- ble to carry out any visual in-situ studies to corroborate the sequences via TEM and/or X-ray studies due to the transient
Table A. Concentration of Si and Mg atoms in the primary aluminium matrix during equilibrium solidification of 6xxx and A356 alloys.
International Journal of Metalcasting/Fall 2011
35
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