Technical Review and Discussion
Controlled Diffusion Solidification of 2024, 6082 and 7075 Al Alloys via Tilt-Pour Casting Process Peyman Ashtari, Gabriel Birsan, Abbas A. Khalaf and Sumanth Shankar; Light Metal Casting Research Centre, McMaster University, Hamilton, ON Canada
Reviewer: Can the authors discuss the quality of the cast- ings. The authors admit that they are not attempting to opti- mize the process, but attempting to demonstrate reasonable soundness and properties. While the paper says it is funda- mental to the process that the mixture can have no time for oxides introduced by the mixing to separate prior to being cast, there seem to be numerous defects in the cast samples.
Authors: We fully agree with the reviewer that the cleanli- ness of the cast melt and the resultant observable defects in the casting would have to be mitigated by an extensive and critical study of the methods of mixing the alloys. As aptly put by the reviewer, the objective of this study was to dem- onstrate the validity of the mechanism of the CDS process in a commercial casting process. Specifically, to observe the castability of the resultant alloy mixture to obtain a reason- able sound casting free of major hot tears or cracks with a non-dendritic primary phase in the cast microstructure. The microstructural evidences, fractography and mechani- cal test results presented in this publication have presented a favorable scenario to further develop and optimize this technology for industrial production processes. There was not much emphasis laid on optimizing the method of mixing to reduce oxides and dissolved hydrogen. Further research is underway to optimize the mixing process wherein innova- tive methods have been developed and verified such as bot- tom mixing and tandem mixing to mention a couple; and it is worthy to mention that the preliminary results of these efforts have been quite encouraging.
Reviewer: Can the authors discuss why their theoretical isopleths and particularly the “B” points, were always higher than those shown in actual testing. If this is always the case, then some of the previous conclusions on the required differ- ences in liquidus temperatures might be need to be reviewed.
Authors: We think that the reviewer is alluding to the theo- retical isopleths from the phase diagram in comparison with the TL1
isopleth from the experimental thermal data. These two isopleths are nearly identical and this may not be ap-
parent due to the larger temperature scale tick levels in the phase diagram images. Further, the point “B” is obtained from the thermal data resulting from the highest tempera- ture recorded at the end of the mixing stage. This is unique to the experiment conditions and presently, could not be pre- dicted by any theoretical formulations. The differences in the liquidus temperatures of the two initial alloys would be that measured or predicted by the phase diagrams and the magnitude of this difference is a critical parameter that af- fects the morphology of the primary phase in the resultant casting and the castability of the alloy mixture in CDS.
Reviewer: The non-traditional and rarely produced prime alloys were used for mixing to get the wrought iron compo- sition. On a production basis, the availability of these alloys would seem to be a much bigger issue than concerns about the use of remelt.
Answer: Our assumption of the term “wrought iron com- position” in the reviewer’s comment refers to the low iron content in wrought Al alloys. With this assumption, we fully agree with the reviewer in that using alloys with iron com- positions defined by wrought Al alloy chemistries would be challenging in a commercial production environment. This study was merely to identify a few favorable scenarios presented by the results of this study to strengthen our at- tempts to develop a commercial processing route for the CDS process. One of the objectives of our continued re- search in this area is to develop alloys with higher and more affordable iron levels (about 0.12 wt% Fe) in them for the commercial CDS process. We understand that this may lead us to stray a bit away from many conventional wrought Al alloy chemistries but we are striving to strike an optimal balance between properties and economics for commercial production.
Reviewer: The authors should explain why 2024 did not re- spond to heat treatment (Table 7) – even a wrong or poor heat treatment would have resulted in T-6 properties higher than F or T-4.
Authors: We have added Table 6 and few words discussing this table in the revised manuscript. The alloy 2024T par- ticularly showed a poor response to heat treatment as com- pared to 6082T and 7075T alloys, the Cu content of 2024T alloy was 5.7 wt% which is higher than the nominal shown in Table 1, and this could be a reason for the poor response to heat treatment by this alloy as shown in Table 8.
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International Journal of Metalcasting/Spring 11
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