Technical Review and Discussion:
Microstructure and Mechanical Properties of Magnesium-Aluminum-Manganese Cast Alloys A. Luo and A. Sachdev, General Motors Research and Development Center, Warren, MI, USA
Reviewer: The paper says that porosity levels in the SVDC casting are very low. In micrographs (Fig. 7) there is quite a bit of shrinkage porosity present. Did you quantify your porosity levels?
Authors: Fig. 7(c) is a local area chosen to show shrinkage porosity, which is considerably more than other areas of the castings. The porosity measurement is very dependent on the areas selected under the microscope, and local area measure- ment is less accurate compared to the global density measure- ment. The revised paper states, “While porosity measurement was not done in this study, a previous publication 14
, using
density measurement, has shown that the use of vacuum can significantly reduce the gas pores in die casting, but it is not effective in reducing shrinkage-related porosity.”
Reviewer: What would be the challenges in switching to AM80 from AZ91?
Authors: There would be no obvious challenges in switch- ing to AM80 from AZ91, since the castability of AM80 al- loy is similar to that of AM60, which is routinely die cast in the industry.
Reviewer: The authors suggest that AM80 alloys provide the best balance of properties, but do not specify exactly why. The selection of the best alloy is less obvious to the average reader than might appear, and it would benefit the readers to know why the authors choose AM80, i.e. what specific criteria they employ.
Authors: As stated in the “Introduction” section, “AZ91 (Mg-9%Al-1%Zn) alloy has high strength but low ductil- ity…the strength of AM50/60 alloy is too low for critical structural applications.” The basis for AM80 alloy se- lection was to select an alloy with higher strength than AM60 and higher ductility than AZ91. The following is now in the revised text. “The above results suggests that the alloy AM80 (Mg-8%Al-0.3%Mn) provides a more balanced combination of tensile properties (higher strength than AM60 and higher ductility than AZ91), in addition to a favorable response to heat treatment in the GPMC samples.”
International Journal of Metalcasting/Fall 10
59
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