In-Depth Comparison of an Industrially Extruded Powder and Ingot Al Alloys

Round 1

Reviewer 1 Report

The presented work contains interesting results of comparative studies of the extrusion process of two forms of aluminum alloys. In the first case, the authors proposed to extrusion of a pre-compacted powder and then compared it with a conventional alloy deformed in a similar way.

The manuscript was prepared with great care. The part describing the experiment and research methodology is detailed and complete. The results were presented in a reliable and legible manner. The strength of the presented article is a number of references to research conducted previously by other authors and the comparison of the results obtained. The conclusions are supported by a correct analysis and discussion of these results.

The only drawback of the work may be the low level of scientific innovation. Research on plastic processing of aluminum and its alloys has been presented in the world for over 60 years. The proposed method of processing, the form of the material (powder consolidation) or the achieved parameters of the finished product are not the subject of new discoveries or process innovation from the scientific point of view.

Author Response

We are grateful for the comments, which have been addressed as follows:

We focused on an industrial extrusion and the subsequent conditions for additional hot forming, along determined apparent activation energies for deformation in different states. Processing maps were added to revised manuscript showing where the material flow is unstable.
The authors believe that the study of hot forming of an industrially extruded powdered Al has not been carried out before. The apparent activation energies in the present study were determined for annealed and extruded conditions which also by our knowledge were not published before.

Reviewer 2 Report

This manuscript deals with the extrusion process of pure Al powder and the comparison with pure Al ingot extrusion. Several extrusion conditions and subsequent treatments are considered. The manuscript can be published once several points are taken into account by the authors.

1. Line 24. Density instead of weight.
2. Line 28. Please check: do reinforcing particles always have a beneficial effect on density? Is it really important for the common amount of reinforcing particles in AL alloys?
3. Lines 31-34. I guess this last sentence is intended as a summary of refs 2-11. More details should be given on the experimental procedure of these manuscripts (range of volume fractions, ball milling conditions, etc.), and also some other ways of obtaining AMC should be commented (for instance not only ball milling is used, but on times reinforcing is just mixed with the powders, and then consolidated).
4. Lines 35-36. What about the effect of these ceramic particles on the movement of dislocations more than the formation of new dislocations? See for instance Journal of Alloys and Compounds Volume 728, 2017, Pages 640-644, where indicated that The strengthening of the compacts obtained from the NH₃ milled powder is the result of the N solid solution and the Al–N reactions during the subsequent sintering process.
5. Line 41-42. The previously indicated manuscript shows that AlN does not appear as a surface layer, but inside the powder particles. More in detail the same authors show that the formation of AlN can be extended to very high percentages in Powder Technology Volume 287, 2016, Pages 341-345.
6. Line 44. Some detail should be given in the introduction for reader not familiarized with the process on the different ways – direct and indirect – of extrusion.
7. Lines 58-59/61. Have you made a simple calculation on the thickness of Al2O3 surrounding particles of 1 micron for an oxygen content of 1.95%. Does it agree with the typical thickness of about 5 nm surrounding native metals (Evans UR, 1968, The corrosion and oxidation of metals: first supplementary volume. Edward Arnold, London)? Is the measurement of 1.95% correct? Details on the effect of the oxide layer can be found at J Mater Sci (2016) 51:822–835.
8. Line 84/208. Please clearly explain the reason for not having data of the bar temperature in indirect extrusion.
9. Line 129-130. Was the Al2O3 identified by any method, or is it just a supposition that the dark zone around particles is indeed Al2O3?
10. Lines 146/154. Fig 3a: Indirect or direct?
11. Line 171. Please explain the reason for the displacement between P and measured T in graphs (measurement in the extruded bar once obtained after a certain time after P starts).
12. Line 171. D3 has a longer period of pressure because extrusion speed is slower, but what about the T measured in D3? Why is it measured for a shorter period? What about the initial peak? How do these affect the measured value?
13. Line 177. D1 and D2?
14. Line 177. Please include images to show the different surfaces (smooth and cracked).
15. Line 188-189. What about the effect of grain size (did you just check the hall petch equation). In case of maintaining the effect of porosity, it should be better explained.
16. Line 208. Any comment on the different shape of P curves in indirect extrusion as compared to direct extrusion?
17. Lines 213-220. Did you check the effect of grain size or reinforcing particles coarsening on mechanical properties?
18. Line 236. True stress strain curves should always show a positive slope because the material continuously harden by strain hardening. Recrystallization could act to change this behavior. Comment should be added to explain the different curves in Fig 8.
19. Line 257. Repeated.
20. Line 307. Was this polygonization shown in the results section? How does this affect properties?. Relate to results.
21. Line 343. Not only 1080 and 10560 alloys.

Author Response

We are grateful for the comments, which have been addressed as follows:

Line 24. - Apologies, corrected.

Line 28. - The wording has been changed to specific strength, as some reinforcing particles have a higher density.

Lines 31-34. - Apologies, corrected. Additional paper is cited with recent review of Al AMC.

Lines 35-36. - Suggested paper does not talk about dislocations accommodating added particles. During ball milling, in the early stage of milling, dislocations are generated and accumulated in the grains. As milling continues, the increased density of dislocations tended to tangle, and gradually transform into LAGBs.

Line 41-42. - The following was added to manuscript: "In-situ formed AMC contains only a small amount of the Al2O3 phase (typically less than 3 vol. %), but this thin layer prevents grain growth during hot consolidation and is responsible for the excellent thermal stability of the mechanical properties of in-situ formed AMCs [8,17]. AMCs reinforced with AlN have also been produced by ball milling in NH3 atmosphere, where AlN can be introduced in higher contents and is present not only on the surface but also inside the grains [10]. "

Line 44. - The following has been added: "In the direct extrusion process, the green pellet billet is placed into a press container and pushed by a ram. Before the material begins to flow out of the die opening, the billet is pressed against the container wall and later through the extrusion die. Due to the considerable friction between the container and the green pellet billet, higher forces are needed compared to indirect extrusion. In indirect extrusion or backward extrusion, the extrusion die at the front of the hollow ram is pressed against the green pellet billet, which is prevented from movement in the container with back plate. As there is therefore no relative movement between the billet and the container, indirect extrusion produces less friction compared to direct extrusion."

Lines 58-59/61. - In the present study we are dealing with an industrial consolidation and optimal hot working window, not thickness analysis of Al2O3. For detailed analysis of powder see previous papers by Balog et al. which are cited in the paper and thoroughly analyse used Al powder.

Line 84/208. - Due to an industrial extrusion process specifics, in indirect extrusion as die is pushed with hollow ram toward stationary billet it was impossible to attach thermocouple on the profile immediately after die opening. Measuring temperature after few meters of profile was extruded did not seem important.

Line 129-130. - See previous papers by Balog et al., as explained before.

Lines 146/154. - Apologies, corrected.

Line 171. - Temperature is measured on the extruded profile (check Fig. 5 caption). Therefore, it is natural to have delay before profile starts to exit die opening (position of temperature measurement) and pushing billet with ram.

Line 171. - This was measured in an industrial extrusion workshop. As condition were not ideal, there was temporary loss of contact between thermocouple and extruded profile measurement exhibits initial peak. This dose not have any effect on peak temperature, which was of our interest.

Line 177. - Apologies, corrected.

Line 177. - Unfortunately, images of the surfaces are not available as this was proprietary material aiming to be patented at the time of extrusion tests. Only image we still have is from cross-section of the profile showing edge cracks, but would not help to explain surface damages in more detail.

Line 188-189. - Material flow is different when comparing powder billet and cast billet. Deformation is possible only with shear stress, which is homogenous in AA 1050 alloy and easier to achieve. Also grain size plays effect as dislocations can glide over larger distance in cast material. In powder material some of shear stress is needed for compaction of grain as shown in TEM images and due to small grains, dislocation glide causes dislocation tangles. Contrary to AA 1050 alloy, material flow in powder material also happens due to grain boundary sliding during deformation (consequence of cracking during hot compression tests at small strain rates).

Line 208. - In indirect extrusion friction is much smaller therefore pressure is more stable.

Lines 213-220. - Reinforcing particles were grown in-situ during green pellet preparation and their size was not subject of present paper.

Line 236. - Please remember, that these are hot compression test where softening mechanisms start to play role. In our case (Al) softening occurs via recovery and conditions for recrystallization are almost never meet. Higher the temperature larger is effect of softening. Tests at room temperature would exhibit positive slope in true stress-true strain curve, but at high temperatures initial strain hardening is countered by softening which are in balance when cure exhibits peak.

Line 257. - Apologies, corrected.

Line 307. - Polygonization after annealing heat treatment is well known mechanism in Al alloys. Prof for this is actually in activation energy for hot deformation calculated for various annealing times and compared to as extruded state.

Line 343. - Apologies, corrected.

Reviewer 3 Report

This is regarding the manuscript titled as In-depth comparison of an industrially extruded
powder and ingot Al alloys. The authors studied the mechanical properties of extruded parts of compated Al 1080 powder and compared with the properties of a cast Al alloy. The results are interesting and worth of publication.

The submitted manuscript is full of crossed words and sentences, please next time ensure you are not submitting a half processed text.

-Table 1- the concentration of O (not O2 right?)

- The quality of figure 1b and 3a,b are rather low, could you replace them with better ones?

- Figure 12 should be in section Materials and Methods

- Table 5 and text of lines 425-453 - you compare the results achieved in commertial purity Al also with alloys, could you remove the alloys from the table ? Also the text only repeats the data from the table. Also there is almost no discussion on the differences between the literature and present results.

Author Response

We are grateful for comments, which have been addressed as follows:

"The submitted manuscript is full of crossed words and sentences, please next time ensure you are not submitting a half processed text." -> The paper went through 2 evaluation rounds and the crossed-out sections are changes from the original submission. In the submitted word file, changes have been highlighted in color for better readability.

"-Table 1- the concentration of O (not O2 right?)" -> Apologies, corrected.

"- The quality of figure 1b and 3a,b are rather low, could you replace them with better ones?" -> The figures in pdf format have been compressed, but the figures mentioned in the submitted Word file look good.

"- Figure 12 should be in section Materials and Methods" -> Figure 12 depicts several options in the powder metallurgical route, and we believe that this should remain under discussion.

"- Table 5 and text of lines 425-453 - you compare the results achieved in commertial purity Al also with alloys, could you remove the alloys from the table ? Also the text only repeats the data from the table. Also there is almost no discussion on the differences between the literature and present results." -> Thank you for your comment, but we believe that the comparison with Al alloys of the 2xxx series would help the readers of the paper to understand the influence of the different matrix on the mechanical properties and to see where the industrial processing fits to the usual samples produced in the laboratory. Since different processing routes were used to produce the samples published in literature and collected in Table 5 and all of them were produced on a laboratory scale, the comparison also serves to estimate the differences between the alloy produced in the laboratory and the scale-up process on an industrial scale. In addition, Table 5 is a brief summary of the mechanical properties of aluminium matrix composites by other authors and shows where the limits can be pushed.

Round 2

Reviewer 2 Report

Dear Authors, thank you for your answers to my comments.

I can see that Authors have an answer to most of my comments, however I have not seen those answers included in the new version of the manuscript, and under my point of view including a small comment in the manuscript would help for a better understanding of the content. This is so, even if it is to avoid readers thinking in something wrong, as maybe has happened with me in some of my comments. I am in particular referring to my comments in my first review that were related to Lines 84/208, 171, 188-189, 208, 213-220 and 307.

On the other hand, I would like to insist on some points that authors should consider in some more detail. In my first review, I commented on lines 35-36, indicating that the strengthening mechanism in ceramic particles reinforced AMC could be not only the formation of geometrically necessary dislocations, but also the effect of these reinforcements on the movement of existing dislocations. Unless authors explain to the contrary, this is indeed not exactly the same mechanism as that commented by the authors, and I consider it should be commented in the introduction section as an alternative strengthening mechanism.

Moreover, I have ask in two different points, lines 58-59/61 and 129-130 of my first review, about Al2O3 particles. Authors refer to certain references in their answer, but I would like to have a clear answer where they explain the content of those references and clarify my doubts. Then, in case of the content in the manuscript being correct and clarified in those references, they should be added to facilitate readers to find them.

Author Response

We are grateful for the comments, which have been addressed as follows:

Point 1 in second revision:

Line 84/208: From description of the industrial process given in revision it should be self-evident that measuring temperature was
not possible. However, following was included "Since the die was pressed into the billet with a hollow ram, the temperature at
the exit of the die could not be measured.".

Line 171: Text that read as "In addition, Figure 5 also shows temperature as measured on the extruded profiles during direct
extrusion which was measured between 512 and 587 °C." was amended to "In addition, Figure 5 also shows the temperature measured
on the extruded profiles immediately after leaving the die during direct extrusion, which was measured between 512 and 587 °C.
Due to the temperature measurement at the exit from the die, there is a time delay after the peak pressure."

Line 188-189: Explanation for this was already in the manuscript; "Part of the difference in cylinder pressures can be attributed to
differences in grain size, and part to the porosities present in the compacted powder green pellet, which cause a significant
increase in friction and resistance to continuous material flow."

Line 208: In the description of the difference in direct and indirect extrusion process, describes differences in pressure.
Adding description in the manuscript would not help in clarifying anything as direct and indirect extrusion processes are different.

Line 213-220: This is connected to polygonization. The following has been added to the revision "This phenomenon can be confirmed by
changes in the calculated activation energy for hot deformation at different annealing times and in the extruded state, although the
grain size was not measured."

Line 307: Please see reply to the line 213-220.

Point 2:

The following has been added to the revised manuscript describing strengthening mechanisms in particle added AMCs:
"The strengthening mechanisms in particle reinforced AMC are based on Orowan strengthening, grain and substructure strengthening, quench hardening from the dislocations created to compensate for the difference in thermal expansion coefficient between the reinforcing particles and the matrix, and work hardening due to strain mismatch between the particles [15]. Experimental confirmation of this is available and indicates that for a fixed reinforcement particle, the yield strength increases with decreasing particle size [16–18]. The highest contribution to strength in the case of AMC reinforced with added ceramic particles is the formation of geometrically necessary dislocations enhanced by differences in elastic moduli and coefficients of thermal expansion in Al-matrix and ceramic reinforcements [6,19]."

In our case following holds and has been added to the introduction:
"Strengthening mechanism in the in-situ formed AMCs is unique and is attributed to the fine Al2O3 dispersoids along the Al grain boundaries, which is similar to strengthening mechanism in the oxide dispersion strengthen materials. A network of Al2O3 particles results in an increased slip resistance due to high stress from a dislocation pile-up at the particle network. The Al2O3 particle network and low angle grain boundaries do not contribute significantly to strengthening via the Orowan mechanism at room temperature and a significant contribution is due to the grain boundary strengthening. A further effect of the Al₂O₃ particle network is stabilization of the Al grain structure and prevention of grain growth, which was confirmed after long annealing times at high temperatures [23]."

Point 3:

The same powder was characterized with selective area diffraction in TEM by cited reference (Poletti, C.; Balog, M.; Simancik, F.; Degischer, H.P.; Acta Mater. 2010, 58, 3781–3789, doi:https://doi.org/10.1016/j.actamat.2010.03.021.

Text in the manuscript was amended as:
"For a more detailed characterization of the same Al 1080 powder as the powder used in the current study, see reference [22], where selective area diffraction was used to prove that the oxide layer on the grains is Al2O3."

Reviewer 3 Report

Thank you for your replies.

Round 3

Reviewer 2 Report

According to the changes in the manuscript, it can be now published.

Author Response

We are grateful for comments, which have been addressed as follows:

"According to the changes in the manuscript, it can be now published." -> The reviewer is satisfied with the corrections made, and no further suggestions were recommended.