and dynamic nature of the precipitation reaction. The only in-situ study (visual) that could be carried out was the 3D atom probe technique which is currently cost prohibitive for such a study. However, extensive studies of such kind in the recent past were reviewed to create an in-depth understand- ing of the mechanism sequence and the in-situ micro-hard- ness test data and tensile property assessment were com- pared with the literature to develop the proposed hypothesis.
Reviewer: Authors say the focus has been on 6xxx alloys. The natural ageing of Al-Si-Mg alloys has been studied in some detail in previous literature.
studied for size and distribution but a conclusive correla- tion could not be attained with this precipitate alone. This work would be an apt extension of these archival articles.
Reviewer: How did you study the kinetics?
Author: The kinetics of the precipitates is studied by observ- ing the transient (time-dependent) changes in the micro-hard- ness value on the primary Al matrix during the natural and artificial ageing. The kinetics of the sequence of precipitation was derived from the micro-hardness vs. time data for vari- ous stages of natural and artificial ageing, mechanical tensile properties and the information gathered from the prior ar- ticles. Table 4 in the revised manuscript presents a hypotheti- cal overview of the kinetics of each precipitation reaction for various rate of quenching after solution treatment.
Reviewer: I would also suggest that the authors reconsider the use of electrical conductivity measurements. It is true that many of the clustering events may not be detectible by conductivity changes, but these measurements would add an additional ‘layer’ of information. Of course, one must take special precautions to insure that experimental errors are kept to a minimum. In my experience, these are caused largely by temperature variations in the sample during measurements.
Authors: We are currently formulating an experiment strat- egy to carry out the electrical conductivity studies and hope to verify certain aspects of the proposed mechanism with the results of this study.
Reviewer: Figures 9 to 13, are variations of Figure 5 with some ‘steps’ removed and thus somewhat redundant.
Authors: We felt that the figures 9 to 13 presented a sche- matic description of the mechanisms of precipitation reac- tion during the artificial ageing process after various levels of natural ageing process. Since each natural ageing pro- cess leads to a slightly modified precipitation pathway dur-
36
Authors: The advanced characterization techniques such as FIB, TEM and etc. would help to evaluate the exact reasons, but such a task was beyond the scope of this project.
Reviewer: Are the values in Figure 14 due to the fact that they were only operating in the lower end of the incubation time range (not up to 24 hours or above). Previous studies indicated the curve is not regular below or above 24 hours incubation; that is, there are ups and downs on both side. Some research- ers have noted peaks in performance with the 24 hour incuba- tion coupled with longer than usual aging times—seemingly overaged based on lower ageing times, but superior properties similar to those the authors have found.
Authors: In this study the precipitation sequence was pro- posed for A356.2 alloy used in the analysis by the combina- tion of information provided by the background literature and the results of the micro-hardness measurements carried out in this study. The tensile test is carried out to find the in- cubation effect in the artificial ageing. We believe the pres- ent study shows the importance of the incubation time in the heat treatment of A356.2 alloys.
Reviewer: Authors say at one point that electrical resistivity measurement is one of the more popular methods (used) in pre- cipitation studies and they follow that saying that in a recent study “
...the noise was comparable or higher than the signal...”
Authors: We meant to say that electrical resistivity measure- ment instead of time dependent micro-hardness measure- ments were more popular with past researchers in evaluating responses from precipitation reactions during ageing. How- ever, the technique has a high noise to signal ratio which has rendered much ambiguity in the interpretation of the results.
International Journal of Metalcasting/Fall 2011
Authors: We thank you for these citations and have in- cluded these references in the revised article. These ar- ticles show that natural ageing affects the mechanical properties after artificial ageing, but the precipitation se- quence were not discussed in these articles as presented in this work. Only the terminal Mg2
Si phase has been
ing artificial ageing, we feel that these images would help readers quickly identify these pathways without reviewing the elaborate descriptions that each figure would entail.
Reviewer: With increasing incubation time, the elongation should continue to increase when artificially aged.
Authors: In the present study it is observed that the incubation is affecting the yield strength and elongation. There was no direct relationship, such as increasing incubation time, that would result in a continuous increase in the elongation be- cause the precipitation reaction sequence during the incuba- tion would directly influence the precipitation sequence dur- ing the artificial aging treatment and, as shown in this study, the elongation would depend on the nature and distribution of the precipitates and clusters in the primary Al matrix at the arrest of the incubation process. Further, we were not able to identify any literature in the academic public domain to sup- port that increasing incubation would continuously increase final elongation of the sample in tensile loading.
Reviewer: The data would have been really reinforced with some FIB TEM work.
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