Localized Defects in Cold Die-Compacted Metal Powders
Round 1
Reviewer 1 Report
Interesting work focused on deformation at contact points between Cu particles is compacts produced by uniaxial pressing at different pressure levels.
There are some points that should be clarified:
1- Include composition and granulometry of Cu powders (level of purity, etc). How are results affected by differences in particle size?
2- Which is the grain size of Cu particles? EBSD could help in this characterization. Plastic deformation at contact points will be affected by grain size.
3- The selection of contact areas between particles is unclear. Perpendicular and parallel to the pressing direction is not precise enough. The authors should provide which are the criteria followed to make such difference. (90 and 0 degrees but with a tolerance +/-?)
4- All stress components need to be taken into account in order to analyze the corresponding strain. There are shear and normal stresses that lead to different modes of deformation. Which is the dominant mode in these experiments, if any?
5- Have the authors considered the measurement of coordination number as a function of the applied pressure?
Author Response
The authors are very grateful for the Reviewer’s valuable comments.
The enclosed manuscript “Localized Defects in Cold Die-Compacted Metal Powders” has been revised in accordance with all the Reviewers’ and Editor’s suggestions as follows:
“There are some points that should be clarified:”
Thank you for your thoughtful and helpful review. We have made the following changes per your guidance:
- Include composition and granulometry of Cu powders (level of purity, etc). How are results affected by differences in particle size?
Thank you for bringing this up to our attention. This information was added to the Materials and Methods section.
- Which is the grain size of Cu particles? EBSD could help in this characterization. Plastic deformation at contact points will be affected by grain size.
Thank you for this question. Sadly, we don’t have EBSD data; but also, hardness can be considered data that provide information regarding the grain size since they are proportionally inverse (Hall–Petch relationship). The data we collated shows that the grain size in the undeformed spheres is homogeneous in the spheres’ volume.
- The selection of contact areas between particles is unclear. Perpendicular and parallel to the pressing direction is not precise enough. The authors should provide which are the criteria followed to make such difference. (90 and 0 degrees but with a tolerance +/-?)
Thank you, we can see how this was confusing. The spheres subjected to the uniaxial-compaction show lenticular shape since, along the direction of compaction, they result in more deformed than in the plane perpendicular to the load direction. Therefore, the contact areas present on top and bottom (90° and 270°) are those parallel to the compaction direction; meanwhile, those present in the compaction plane (0° and 180°) are the contact in the transversal direction.
In the manuscript, to clarify this point, we elaborated as follows: “In the present work, the measurements were carried out in the contact areas aligned along the compaction (blue arrows) and transversal (plane perpendicular to the compaction direction) directions, and avoiding as much as possible ambiguously oriented ones, in order to observe the state of deformation in these two distinct regions”.
- All stress components need to be taken into account in order to analyze the corresponding strain. There are shear and normal stresses that lead to different modes of deformation. Which is the dominant mode in these experiments, if any?
Thank you for this question. The stresses can be different based on the observed area. The spheres close to the die and punches surfaces will be affected by the shear stress due to the presence of friction; meanwhile, those in the core of the cylindrical specimens will be mainly subjected to the normal stress due to the contact with the neighbor spheres. In this work, we focused primarily on the second type to avoid possible disturbance derived from die-spheres and punches-spheres contact.
5- Have the authors considered the measurement of coordination number as a function of the applied pressure?
Thank you for bringing this up. We thought we could derive an estimation of the coordination number by observing the contact areas for the cross-section of the selected spheres for the two loads. But since in the present work, we analyzed only two loads will not be sufficient to provide a precise relationship between them. In future works where we will explore more loads and with the additional data points, we plan to investigate the correlation between coordination number and load.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper seems interesting. However, there are a few concerns which need to be clarified.
1, The test method needs to describe more in details. How are the copper spheres used loaded in the die? Is it a single layer or multiple layers? There is not die dimensions. It would be better to add a sketch of the die and particles with the dimension of the die.
2, Page 3, line 114: It will be better to give the definitions of the terms used in the study such as dislocation density. How is it related to the packing density? It shows how the dislocation density is measured, but it does not show any correlation to the compaction.
3, Page 8, line 240 and line 246, the format of the numbers needs to be corrected. 'increase' is not a proper symbol for the formula.
4, In discussion, the work has been compared to the others. How does it represent the compaction? If the dislocation density is in use, what does it mean?
Author Response
The authors are very grateful for the Reviewer's and Editor’s valuable comments.
The enclosed manuscript “Localized Defects in Cold Die-Compacted Metal Powders” has been revised in accordance with all the Reviewers’ and Editor’s suggestions as follows:
Thank you for your thoughtful and helpful review. We have made the following changes per your guidance:
1, The test method needs to describe more in details. How are the copper spheres used loaded in the die? Is it a single layer or multiple layers? There is not die dimensions. It would be better to add a sketch of the die and particles with the dimension of the die.
This is a great addition. This information was added to the Materials and Methods section.
2, Page 3, line 114: It will be better to give the definitions of the terms used in the study such as dislocation density. How is it related to the packing density? It shows how the dislocation density is measured, but it does not show any correlation to the compaction.
Thank you for your comment. We elaborated as follows: “Due to compaction in the contact areas within the particles, plastic deformation and, consequently, increasing defectiveness are introduced in the material's structure, among which there are the dislocations. Therefore, it is expected that the etching will produce a greater quantity of pits since there the density of dislocations will be greater.”
3, Page 8, line 240 and line 246, the format of the numbers needs to be corrected. 'increase' is not a proper symbol for the formula.
Thank you, this was fixed!
4, In discussion, the work has been compared to the others. How does it represent the compaction? If the dislocation density is in use, what does it mean?
Thank you for this question. With compaction and, consequently, the plastic deformation of the material structure defects, among which there are the dislocations, are generated. Their presence is correlated with the amount of plastic deformation and, therefore, with the increasing compaction's load. The comparison with other work is to show how our results obtained with our experimental method are in agreement with literature data for the same material undeformed (e.g., annealed) and subjected to different degrees of deformation.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
All comments have been addressed properly. There is no further concern.
