Round 1

Reviewer 1 Report

The paper studied the production of hot deformed Nd-Fe-B magnets. The novelty of this work can be assessed as relatively low, because many researchers have produced hot deformed Nd-Fe-B magnets. The authors study more literature on Nd-Fe-B magnets and revise the introduction and the references extensively. For example,

(Hot deformation)

 (1) M. Leonowicz and H. A. Davies, “Effect of Nd content on induced anisotropy in hot deformed Fe-Nd-B magnets”, Mater. Lett. 19 (1994) 275-279. (2) T. Saito, M. Fujita, T. Kuji, K. Fukuoka, and Y. Syono, “The development of high performance Nd-Fe-Co-Ga-B die upset magnets”, J. Appl. Phys. 83 (1988) 6390-6392. (3) R. K. Mishra, T. Y. Chu, and L. K. Rabenberg, “The development of the microstructure of die-upset Nd-Fe-B magnets”, J. Magn. Magn. Mater., 84 (1990) 88-94.

(SPS-Hot deformation)

(1) J. Song, M. Yue, J. Zuo, Z. Zhang, W. Liu, D. Zhang, J. Zhang, Z. Guo, W. Li, “Staructure and magnetic properties of bulk nanocrystalline Nd-Fe-B permanent magnets prepared by hot pressing and hot deformation”, J. Rare Earths 31 (2013) 674-678. (2) Y. H. Hou, Y. L. Huang, Z. W. Liu, D. C. Zeng, S. C. Ma, Z. C. Zhong, “Hot deformed anistropic nanocrystalline NdFeB based magnets prepared from spark plasma sintered melt-spun powders”, Mater. Sci. Eng. B 178 (2013)990-997. (3) T. Saito, S. Nozaki, Y. Sajima, D. Nishio-Hamane, “Coercivity of Nd-Fe-B hot-deformed magnets produced by the spark plasma sintering method”, AIP Adv. 7 (2017) 056205.

The authors have already published similar papers: (1) A. Ikram, M. F. Mehmood, M. Podlogar, A. Eldosouky, R. S. Sheridan, M. Awais, A. Walton, M. M. Krzmanc, T. Tomse, S. Kobe, S. Sturm, K. Z. Rozman, “The sintering mechanism of fully dense and highly coercive Nd-Fe-B magnet from the recycled HDDR powders reprocessed by spark plasma sintering”, J. Alloys Compd. 774 (2019) 1195-1206. (2) A. Ikram, F. Mehmood, R. S. Sheridan, M. Awais, A. Walton, A. Eldosouky, S. Sturm, S. Kobe, K. Z. Rozman, “Particle size dependent sinterability and magnetic properties of recycled HDDR Nd-Fe-B powders consolidated with spark plasma sintering”, J. Rare Earths 18 (2020) 90-99. Although the authors added the results of the hot-deformation study, the reported magnetic properties are not so interesting.

The paper does not meet with the minimum standard of Metals, thus shall be rejected.

Author Response

Reviewer #1:

The paper studied the production of hot deformed Nd-Fe-B magnets. The novelty of this work can be assessed as relatively low, because many researchers have produced hot deformed Nd-Fe-B magnets. The authors study more literature on Nd-Fe-B magnets and revise the introduction and the references extensively.

We thank the reviewer for the comment, and we are glad to reintroduce the context of this manuscript.

Primarily the research was focused on this kind of work because of its exceptional industrial precedence. The inception of European Training Network for the Design and Recycling of Rare-Earth Permanent Magnet Motors and Generators in Hybrid and Full Electric Vehicles named DEMETER http://etn-demeter.eu/ advocates the dire necessity at present to evaluate the “direct re-usage”, “direct recycling” and “indirect recycling” methods for the rare-earth permanent magnets recovery and finally their potential applications in novel electric motors. Sure, things are not novel when we read the literature for established Nd-Fe-B commercial alloys, but this manuscript projects the findings of contemporary technologies to the recycled materials. Without scientifically proving the merits of recycled materials, they cannot be transferred to industrial practices for consumer applications and this is the main objective behind DEMETER and the presented manuscript. It is important to understand that these rare-earth based systems are highly prone to oxidation, which limit their processing approach. Usually, the recycled powder contains 4-times as much oxygen content compared to the commercially developed material. This is the drawback of recycling because the oxygen content tends to rise with the number of steps involved. Firstly, we evaluated the technology i.e. Spark Plasma Sintering [1] and then it was important to establish the link between particle size, oxygen content and sinterability; as it was an industrial requirement [2].

The hydrogen-based cost effective and efficient methods have been circulated since 2015 to highly impact the circular economy of rare-earth based permanent magnets in the near future [3-11]. Therefore, metal and magnet recycling have monumental consequences for the future of green technologies.

Now on to the reviewer’s remark that many researchers have produced hot deformed Nd-Fe-B magnets. This is only true for the commercial and fresh counterparts as the recycled material has not been subjected to hot deformation yet. Therefore, the resultant outcome is important to organize for industrial practices. Secondly, the respected reviewer suggested us to review more literature, so we organized concurrent publications in our manuscript so relate that the recycled material has not progressed to the same level as the commercial/fresh Nd-Fe-B based magnets.

The reviewer exemplified:

For example,

(Hot deformation)

(1) M. Leonowicz and H. A. Davies, “Effect of Nd content on induced anisotropy in hot deformed Fe-Nd-B magnets”, Mater. Lett. 19 (1994) 275-279. (2) T. Saito, M. Fujita, T. Kuji, K. Fukuoka, and Y. Syono, “The development of high performance Nd-Fe-Co-Ga-B die upset magnets”, J. Appl. Phys. 83 (1988) 6390-6392. (3) R. K. Mishra, T. Y. Chu, and L. K. Rabenberg, “The development of the microstructure of die-upset Nd-Fe-B magnets”, J. Magn. Magn. Mater., 84 (1990) 88-94.

(SPS-Hot deformation)

(1) J. Song, M. Yue, J. Zuo, Z. Zhang, W. Liu, D. Zhang, J. Zhang, Z. Guo, W. Li, “Staructure and magnetic properties of bulk nanocrystalline Nd-Fe-B permanent magnets prepared by hot pressing and hot deformation”, J. Rare Earths 31 (2013) 674-678. (2) Y. H. Hou, Y. L. Huang, Z. W. Liu, D. C. Zeng, S. C. Ma, Z. C. Zhong, “Hot deformed anistropic nanocrystalline NdFeB based magnets prepared from spark plasma sintered melt-spun powders”, Mater. Sci. Eng. B 178 (2013)990-997. (3) T. Saito, S. Nozaki, Y. Sajima, D. Nishio-Hamane, “Coercivity of Nd-Fe-B hot-deformed magnets produced by the spark plasma sintering method”, AIP Adv. 7 (2017) 056205.

True that the literature is related to Nd-Fe-B but all the publications reported the work done on melt-spun materials which were subjected to hot deformation. However, in this manuscript, we were dealing only with the recycled HDDR material and not the melt-spun ribbons. These both nanocrystalline Nd-Fe-B systems differ significantly and cannot be intermixed to devise results or a technological strategy. The progress of recycling with hydrogen-based technologies, like hydrogen decrepitation (HD) or Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) processes, has drawn significant attention for the end-of-life scrap. The recycled melt-spun ribbons, on the other hand, are at the moment missing the mark due to the separate reprocessing scheme as the HD treated materials can be directly sintered. Similarly, the HDDR method is a widely applicable method for developing anisotropic and high coercivity bonded magnets with nanostructured Nd₂Fe₁₄B grains (≤ 400 nm). The melt-spun ribbons as produced are never anisotropic and their grain size is less than 100 nm on average, such that hot-pressing results in exceptional good magnetic properties. On the contrary, the remanence and maximum energy products are limited due to the isotropic nature of ribbons or pressed magnets. This makes a 3-step process to develop anisotropy in melt-spun ribbons. After the treatment, the HDDR Nd-Fe-B powder contains individual anisotropic grains with c-axis (easy) direction of magnetization (meaning the HDDR powder is readily anisotropic). The mechanism of sintering, grain growth kinetics, grain morphology, grain boundary structure and derivative magnetic properties thus are very different for the melt-spun ribbons and the HDDR Nd-Fe-B.

Therefore, the literature cited by the reviewer may not be adequate to draw a comparison, the next point to consider is that whether these well-established methods (hot deformation & SPS) are applicable for the recycled HDDR Nd-Fe-B. Previously, there is no literature available to cite the recycled material, thus all the work performed in this regard for industrial potential is credibly new.

The reviewer did cite us as following:

The authors have already published similar papers: (1) A. Ikram, M. F. Mehmood, M. Podlogar, A. Eldosouky, R. S. Sheridan, M. Awais, A. Walton, M. M. Krzmanc, T. Tomse, S. Kobe, S. Sturm, K. Z. Rozman, “The sintering mechanism of fully dense and highly coercive Nd-Fe-B magnet from the recycled HDDR powders reprocessed by spark plasma sintering”, J. Alloys Compd. 774 (2019) 1195-1206. (2) A. Ikram, F. Mehmood, R. S. Sheridan, M. Awais, A. Walton, A. Eldosouky, S. Sturm, S. Kobe, K. Z. Rozman, “Particle size dependent sinterability and magnetic properties of recycled HDDR Nd-Fe-B powders consolidated with spark plasma sintering”, J. Rare Earths 18 (2020) 90-99. Although the authors added the results of the hot-deformation study, the reported magnetic properties are not so interesting.

The paper does not meet with the minimum standard of Metals, thus shall be rejected.

The work done by us on the recycled HDDR Nd-Fe-B system is pioneering. The recycled material had an application in the low-performance plastic bonded magnets only. However, with the aid of spark plasma sintering, we established that the bulk magnets can also be made in a variety of sizes. We also proposed the scheme to control the processing route for achieving optimal microstructure and magnetic properties. Eventually, the as SPS-ed magnets returned M_r/M_S values ~0.5 which suggested isotropic nature, just like the as-prepared or hot-pressed melt-spun ribbons. In order to improve the remanence, the hot deformation work was applied and presented in this study which is clearly new for the recycled HDDR Nd-Fe-B material. The proof-of-concept is important in our study to validate the hot deformation mechanism for the HDDR material, which has not been adequately proposed in the literature and it is clearly explained in the manuscript.

Additionally, the impact of thermal treatments on the microstructure and resultant magnetic properties has never been studied after the hot deformation of either commercial or recycled HDDR Nd-Fe-B system. This study validated these principles to devise industrial recycling and reprocessing strategy of rare-earth permanent magnets. Most importantly, we proposed a model to define the mechanism behind the improvement in the magnetic properties (simultaneously) after the thermal treatment. This has been widely missing for the HDDR Nd-Fe-B system and in general, can also be applied to melt-spun ribbons. The work done on reprocessed melt-spun ribbons can therefore also be considered novel because of the lack of literature, and the same is applicable for the HDDR Nd-Fe-B system because it gives more freedom to the industrial producer to develop isotropic or anisotropic plastic bonded magnets or bulk sintered/hot deformed magnets.

Lastly, the mechanism of coercivity is not clearly understood for the HDDR Nd-Fe-B system, whereas it is pinning dominant in nanocrystalline melt-spun ribbons and nucleation controlled in micron-sized single crystal Nd-Fe-B sintered magnets. The level of understanding for this system requires extensive study of fresh as well as the recycled counterparts, and we are making stepwise efforts in this arrear to answer the pertinent questions asked by scientists as well as to cater the ever-growing demands of industrial partners. The dependent magnetic properties can only be refined to an extent in the recycled material because of limitation in oxygen constricted processing. Henceforth the presented results importantly also suggested the retention of coercivity (resistance to demagnetization) as the texture is imparted in the system to enhance the remanence. The derivative magnetic properties are credibly better than the starting recycled HDDR powder, whereby in previous literature on HDDR Nd-Fe-B system the coercivity declined by 50 – 90 % as the texture was increased (remanence improvement) [12]. This to an extent rendered hot deformed magnets with tiny coercivities useless for the commercial applications, but now with our proposed method; it is not only useful for the recycled material but also suitable for the fresh HDDR Nd-Fe-B powder. We strongly believe that this paper to check all the points in terms of scientific explanation, definition of experiments, proof of hypothesis and application viability.

References (in this letter of revision)

Ikram, A.; Mehmood, M.F.; Podlogar, M.; Eldosouky, A.; Sheridan, R.S.; Awais, M.; Walton, A.; Maček Kržmanc, M.; Tomse, T.; Kobe, S., S.; Šturm, S.; Zužek Rožman, K., The sintering mechanism of fully dense and highly coercive Nd-Fe-B magnets from the recycled HDDR powders reprocessed by spark plasma sintering. Journal of Alloys and Compounds 2019, 774, 1195-1206. Ikram, A.; Mehmood, F.; Sheridan, R.S.; Awais, M.; Walton, A.; Eldosouky, A.; Šturm, S.; Kobe, S.; Rožman, K.Z. Particle size dependent sinterability and magnetic properties of recycled HDDR Nd-Fe-B powders consolidated with spark plasma sintering. Journal of Rare Earths 2019. Binnemans, K.; Jones, P.T. Rare earths and the balance problem. Journal of Sustainable Metallurgy 2015, 1, 29-38. Li, X.; Yue, M.; Zakotnik, M.; Liu, W.; Zhang, D.; Zuo, T. Regeneration of waste sintered Nd-Fe-B magnets to fabricate anisotropic bonded magnets. Journal of Rare Earths 2015, 33, 736-739. Goodenough, K.M.; Wall, F.; Merriman, D. The rare earth elements: Demand, global resources, and challenges for resourcing future generations. Natural Resources Research 2017, 27, 201-216. Binnemans, K.; Jones, P.T.; Müller, T.; Yurramendi, L. Rare earths and the balance problem: How to deal with changing markets? Journal of Sustainable Metallurgy 2018, 4, 126-146. Diehl, O.; Schönfeldt, M.; Brouwer, E.; Dirks, A.; Rachut, K.; Gassmann, J.; Güth, K.; Buckow, A.; Gauß, R.; Stauber, R.,. Towards an alloy recycling of Nd-Fe-B permanent magnets in a circular economy. Journal of Sustainable Metallurgy 2018, 4, 163-175. Nakamura, H. The current and future status of rare earth permanent magnets. Scripta Materialia 2018, 154, 273-276. Prosperi, D.; Bevan, A.I.; Ugalde, G.; Tudor, C.O.; Furlan, G.; Dove, S.; Lucia, P.; Zakotnik, M. Performance comparison of motors fitted with magnet-to-magnet recycled or conventionally manufactured sintered NdFeB. Journal of Magnetism and Magnetic Materials 2018, 460, 448-453. Reimer, M.; Schenk-Mathes, H.; Hoffmann, M.; Elwert, T. Recycling decisions in 2020, 2030, and 2040—when can substantial NdFeB extraction be expected in the EU? Metals 2018, 8. Skokov, K.P.; Gutfleisch, O. Heavy rare earth free, free rare earth and rare earth free magnets - vision and reality. Scripta Materialia 2018, 154, 289-294. Li, X.-q.; Li, L.; Hu, K.; Chen, Z.-c.; Qu, S.-g.; Yang, C. Microstructure and magnetic properties of anisotropic Nd-Fe-B magnets prepared by spark plasma sintering and hot deformation. Transactions of Nonferrous Metals Society of China 2014, 24, 3142-3151. Liesert, S.; Kirchner, A.; Grünberger, W.; Handstein, A.; De Rango, P.; Fruchart, D.; Schultz, L.; Müller, K.H. Preparation of anisotropic NdFeB magnets with different and contents by hot deformation (die-upsetting) using hot-pressed HDDR powders. Journal of Alloys and Compounds 1998, 266, 260-265.

Reviewer 2 Report

referee report
metals-689946
Spark Plasma Sintering as an Effective Hot Deformation Tool for Reprocessing the Recycled HDDR Nd-Fe-B Powder
Awais Ikram, Muhammad Awais, Richard Sheridan, Allan Walton, Spomenka Kobe, Franci Pušavec,
Kristina Žužek Rožman

This manuscript discusses developments of the recycling of Nd-Fe-B magnets using spark-plasma sintering, which is an interesting
topic, fitting well to the scope of Metals. Overall, the manuscript is well organized and quite well prepared.
There are, however, several points in the present manuscript, which require clarification, and there are numerous problems
concerning the text formatting and style.

First remark concerns the abstract: When reading it once, it is difficult to understand the goal of the present research from
the first sentences. It would be useful to rephrase it.

The detailed points are the following:
(1) When citing a researchers work in the main text, the author's name is sufficient -- no prenames required.
(2) abstract, line 14: Please define the HDDR.
(3) line 69: 'rare-earth-lean systems': The text discusses Nd-Fe-B, and a lean system would imply a similar material to a rare-earth
magnet -- so what?
(4) line 102: define.
(5) line 112: define.
(6) line 122: too much 'applied'.
(7) line 135: What is this for a sign? In the mathematics, one should use a sympol like \times.
(8) the abbreviation for hour and minutes is h and min.
(9) line 270: 'remanent' or 'remnant'.
(10) line 279: Similarly.
(11) line 343: Kronmüller.

The writing style and the formatting of the reference list is not acceptable. Please use proper journal abbreviations. For all the paper titles, it is not acceptable just to copy them from other sources and write them in fully minor style. There are chemical formulae or abbreviations which are fully senseless in the present way. Please also check the correct spelling of all the authors cited, e.g., Fähnle, Kronmüller, etc.

Furthermore, some careful check of the English is required -- did the English coauthors read the manuscript?

Overall, the manuscript contains interesting material. After performing the requested changes, the manuscript may be suitable for publication in Metals.

Author Response

Please find the comments attached in the word file with title Reviewer 2 to evaluate the answers against the queries put forward.

Thank you and kind regards

Author Response File: Author Response.pdf

Reviewer 3 Report

In the paper studied the low-pressure hot deformation (HD) methodology to reprocess the HDDR Nd-Fe-B powders from the end-of-life (EOL) waste. The paper and results is novel and interesting for scientific community to warrant publication and adhere to the journal's standards.
The comments on this study are as follows:
1. In the introduction describes in detail the methods for producing permanent magnets and the effect on the properties and influence of alloying elements for ex. Cu. But the influence of alloying elements of the investigated alloy Nd13.4Dy0.67Fe78.6B6.19Nb0.43Al0.72 not discussed.
2. On page 6, in line 216 the remanent magnetization marked as Br and in the rest of the text by Ir.
3. Figures 3 and 4 show of the magnetic properties, however, the dependence of the properties is not linear and extremal (both coercive force and remanent magnetization) and saturation magnetization is not shown. It is necessary to give a more detailed discussion of the results obtained and the reasons for the extreme (increase and decrease) behavior of the properties.

Author Response

Please find answers to the your comments and questions in the attached file under Reviewer # 3 section, which have also been accomodated in the revised manuscript.
Thank you and kind regards

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

There is nothing new in this paper, other than the claim for the production of hot-deformed magnets from the recycled HDDR powder. The authors have already published their results for the recycled HDDR powder (#1-3). #1 A. Ikram, M. F. Mehmood, M. Podlogar, A. Eldosouky, R. S. Sheridan, M. Awais, A. Walton, M. M. Krzmanc, T. Tomse, S. Kobe, S. Sturm, K. Z. Rozman, “The sintering mechanism of fully dense and highly coercive Nd-Fe-B magnet from the recycled HDDR powders reprocessed by spark plasma sintering”, J. Alloys Compd. 774 (2019) 1195-1206.  #2 A. Ikram, F. Mehmood, R. S. Sheridan, M. Awais, A. Walton, A. Eldosouky, S. Sturm, S. Kobe, K. Z. Rozman, “Particle size dependent sinterability and magnetic properties of recycled HDDR Nd-Fe-B powders consolidated with spark plasma sintering”, J. Rare Earths 18 (2020) 90-99.  #3 A. Ikram, M. F. Mehmood, Z. Samardzija, R. S. Sheridan, M. Awais, A. Walton, S. Sturm, S. Kobe, K. Z. Rozman, “Coercivity increase of the recycled HDDR Nd-Fe-B powders doped with DyFe₃ and processed via spark plasma sintering & the effect of thermal treatment”, Materials 712 (2019) 1498.

In this paper, they added the new results of the hot-deformed magnets. However, the energy products of the hot-deformed magnets, (BH) max = 110 kJ/m³ as prepared and (BH) max = 144 kJ/m³ after annealing, were inferior to those of the reported values in the old literature, for example, “The production and characterization of bonded, hot-pressed and die-upset HDDR magnets”, by P. J. McGuiness, C. Short, A. F. Wilson and I. R. Harris, J. Alloys Compd. 184 (1992) 243-255.

As far as I know, the first paper for the application of SPS method to the Nd-Fe-B magnets is “Spark plasma sintering of Nd-Fe-B magnetic alloy”, by Z. G. Liu, M. Umemoto, S. Hirosawa, H. Kanekiyo, J. Mater. Res. 14 (1999) 2540-2547 and that for the application of die-upsetting of the SPS Nd-Fe-B magnets is “Microstructure and magnetic properties of anisotropic Nd−Fe−B magnets prepared by spark plasma sintering and hot deformation, by X. Li, Li Li, Z. C. Chen, S. Q. Qu, C. Yang, Trans. Nonferrous Met. Soc. China 24 (2014) 3142−3151”. The authors should have studied literature on SPS and die-upsetting of Nd-Fe-B magnets.

In addition, the recycled HDDR powder has already been studied; “A. Lizandru, I. Poenaru, K. Guth, R. Gaus, and O. Gutfleisch “A systematic study of HDDR processing conditions for the recycling of end-of-life Nd-Fe-B magnets”, J. Alloys Compd. 724 (2017) 51-61.”

I will look forward to seeing their results turned into new scientific and engineering insight that will advance the field of permanent magnets.

Author Response

Please find attached pdf file for better formatting and interpretation of answers.

Reviewer #1:

There is nothing new in this paper, other than the claim for the production of hot-deformed magnets from the recycled HDDR powder. The authors have already published their results for the recycled HDDR powder (#1-3). #1 A. Ikram, M. F. Mehmood, M. Podlogar, A. Eldosouky, R. S. Sheridan, M. Awais, A. Walton, M. M. Krzmanc, T. Tomse, S. Kobe, S. Sturm, K. Z. Rozman, “The sintering mechanism of fully dense and highly coercive Nd-Fe-B magnet from the recycled HDDR powders reprocessed by spark plasma sintering”, J. Alloys Compd. 774 (2019) 1195-1206.  #2 A. Ikram, F. Mehmood, R. S. Sheridan, M. Awais, A. Walton, A. Eldosouky, S. Sturm, S. Kobe, K. Z. Rozman, “Particle size dependent sinterability and magnetic properties of recycled HDDR Nd-Fe-B powders consolidated with spark plasma sintering”, J. Rare Earths 18 (2020) 90-99.  #3 A. Ikram, M. F. Mehmood, Z. Samardzija, R. S. Sheridan, M. Awais, A. Walton, S. Sturm, S. Kobe, K. Z. Rozman, “Coercivity increase of the recycled HDDR Nd-Fe-B powders doped with DyFe₃ and processed via spark plasma sintering & the effect of thermal treatment”, Materials 712 (2019) 1498.

Thank you for the comment. We believe any study on the recycled material which has not been previously reported should be considered as potentially new work. To date and to the best of our knowledge, no scientific report suggests any results related to the recycled HDDR Nd-Fe-B system treated by hot deformation, which validates the requirement to pursue and develop results.

In a stepwise sequence, the investigations were performed on single type of recycled HDDR Nd-Fe-B system which hold high industrial merits. Initially the spark plasma sintering approach was validated by sintering kinetics and microstructure development in relation to the magnetic properties, which goes by A. Ikram, M. F. Mehmood, M. Podlogar, A. Eldosouky, R. S. Sheridan, M. Awais, A. Walton, M. M. Krzmanc, T. Tomse, S. Kobe, S. Sturm, K. Z. Rozman, “The sintering mechanism of fully dense and highly coercive Nd-Fe-B magnet from the recycled HDDR powders reprocessed by spark plasma sintering”, J. Alloys Compd. 774 (2019) 1195-1206.  

Then the relationship of particle size was established with sinterability and oxygen content in: A. Ikram, F. Mehmood, R. S. Sheridan, M. Awais, A. Walton, A. Eldosouky, S. Sturm, S. Kobe, K. Z. Rozman, “Particle size dependent sinterability and magnetic properties of recycled HDDR Nd-Fe-B powders consolidated with spark plasma sintering”, J. Rare Earths 18 (2020) 90-99.  

Then in order to improve the coercivity of the recycled HDDR Nd-Fe-B powder, we undertook doping experiments and the resulted improvement was obtained of 70% improvement in coercivity in the third publication by Ikram et al. (2019, Materials). But importantly, the recycled HDDR Nd-Fe-B powder was produced by d-HDDR reprocessing route, which develops easy-axis anisotropy in the powder. Our publications as indicated by the reviewer developed isotropic magnets only in all these three studies with M_r/M_S values ≤ 0.5, as the focus was on retaining highest possible coercivities.

So, the hot deformation study was undertaken to enhance the texture in the system keeping in view the usability of the reprocessed magnets that do not loose their coercivities after forging. Thereby, a multitude of different forging conditions were devised to serve the recycled HDDR Nd-Fe-B system. Now this may not seem groundbreaking, but the present work definitely illustrates new experimental results, reported for possible publication. This is only true for the commercial and fresh counterparts as the recycled HDDR Nd-Fe-B had not been subjected to hot deformation yet. Therefore, the resultant outcome is considerably important to organize for the industrial practices.

Again, reciprocating the respected reviewer, that many researchers have produced hot deformed Nd-Fe-B magnets. True that the literature is related to Nd-Fe-B but all the publications reported the work done on melt-spun materials which were subjected to hot deformation. However, in this manuscript, we were dealing only with the recycled HDDR Nd-Fe-B and not the Nd-Fe-B melt-spun ribbons. These both nanocrystalline Nd-Fe-B systems differ significantly and cannot be intermixed to devise results or a technological strategy. The progress of recycling with hydrogen-based technologies, like hydrogen decrepitation (HD) or Hydrogenation-Disproportionation-Desorption-Recombination (HDDR) processes, has drawn significant attention for the end-of-life scrap. The recycled melt-spun ribbons, on the other hand, are at the moment missing the mark due to the separate reprocessing scheme as the HD treated materials can be directly sintered. On the contrary, the HDDR method is a widely applicable method for developing anisotropic and high coercivity bonded magnets with nanostructured Nd₂Fe₁₄B grains (≤ 400 nm) [1]. The melt-spun ribbons as produced are never anisotropic and their grain size is less than 100 nm on average, such that hot-pressing results in exceptional good magnetic properties. In contrast, the remanence and maximum energy products are limited due to the isotropic nature of ribbons or pressed magnets. This makes a 3-step process to develop anisotropy in melt-spun ribbons. After the treatment, the HDDR Nd-Fe-B powder contains individual anisotropic grains with c-axis (easy) direction of magnetization (meaning the HDDR powder is readily anisotropic). The mechanism of sintering, grain growth kinetics, grain morphology, grain boundary structure and derivative magnetic properties thus are very different for the melt-spun Nd-Fe-B ribbons and the HDDR Nd-Fe-B system. The flakes of melt-spun ribbon usually range from 1 to several microns in size and exhibit shape of rectangular blocks with 100 nm sized regular faceted grains inside. While the ~400 nm sized HDDR Nd-Fe-B grains are faceted in form of tetrakaidekahedrons rather than simple cubes and the average particles size ranges from 50 – 1000 um of very random shape after hydrogen decrepitation and pulverization. Therefore, the hot deformation conditions and the resultant microstructure will altogether be different for the melt-spun ribbons and HDDR Nd-Fe-B powders/magnets.

To draw a generic comparison, the commercial MQU-F grade melt-spun ribbons (isotropic flakes) have H_Ci ~1560 kA/m (nearly µ_oH_C > 1.9 T) and B_r = 0.6 T. When hot deformed, these melt-spun Nd-Fe-B ribbons have yielded B_r ~ 1.5 T [2] and even after grain boundary diffusion process with Nd₆₀Tb₂₀Cu₂₀, the achieved B_r  = 1.38 T and H_Ci ≥ 2000 kA/m. Whereas the commercial HDDR MF-15P powder has H_Ci ~ 1100 kA/m and B_r = 1.35 T (anisotropic powder). Consequently, the hot deformed HDDR Nd-Fe-B system has returned B_r = 1.22 T but the H_Ci was significantly lower to only 181 kA/m and BH_max of 121 kJ/m³ with 69% deformation ratio [3]. Clearly, in no way a comparison can be drawn between the achieved magnetic properties of as-is or hot-deformed melt-spun ribbons [4-15] and the HDDR Nd-Fe-B powders [16-20]. Currently there is no investigative evidence on the hot deformation of MF-15P type HDDR Nd-Fe-B powders to compare the process, microstructure and properties after the hot deformation.

Therefore, the literature cited by the respectable reviewer may not be adequate to field a comparison. The next point to consider was whether these well-established methods (hot deformation & SPS) will be applicable for the recycled HDDR Nd-Fe-B? Previously, since there is no literature available to cite the recycled material, thus all the work performed with regards to the industrial potential is credibly new.  

In this paper, they added the new results of the hot-deformed magnets. However, the energy products of the hot-deformed magnets, (BH) max = 110 kJ/m³ as prepared and (BH) max = 144 kJ/m³ after annealing, were inferior to those of the reported values in the old literature, for example, “The production and characterization of bonded, hot-pressed and die-upset HDDR magnets”, by P. J. McGuiness, C. Short, A. F. Wilson and I. R. Harris, J. Alloys Compd. 184 (1992) 243-255.

Since the reviewer agrees that the new results are presented, here we only reported the values for the recycled HDDR Nd-Fe-B system. As previously mentioned, all our previous 3 publications on the recycled HDDR Nd-Fe-B and isotropic SPS treated magnets can only be related, but the results presented here were never part of those papers, or as cited by the reviewer.

We duly suggest the correction to the reviewer’s comment that our optimally hot-deformed magnets from the recycled HDDR Nd-Fe-B powder yielded H_Ci = 960 kA/m, J_r = 1.01 T and BH_max = 180 kJ/m³. These values are significantly better than Li et el. has reported [3] and also better than the results suggested by reviewer above i.e. McGuiness et al. [21] on the die-upset HDDR Nd-Fe-B magnets at H_Ci = 700 kA/m, J_r = 1.05 T and BH_max = 150 kJ/m³. When better results are outcome of a recycled feedstock and not cited before, it should be considered pragmatically as a new research report. The proof-of-concept is important in the concurrent study to validate the hot deformation mechanism for the HDDR material, which has not been adequately proposed in the literature and in-turn clearly explained in the manuscript.

Importantly, the impact of thermal treatments on the microstructure and the resultant magnetic properties has never been analyzed after the hot deformation for either the commercial or the recycled HDDR Nd-Fe-B system. This report justified these principles to devise industrial recycling and reprocessing strategy of the rare-earth permanent magnets (REPMs). Most importantly, we proposed a model to define the mechanism behind the improvement in the magnetic properties of HDDR Nd-Fe-B forged magnets (simultaneously) after the thermal treatment. This has been broadly lacking in the HDDR Nd-Fe-B system and can also be applied to the Nd-Fe-B melt-spun ribbons. Our work gives more freedom to the industrial producer to develop isotropic or anisotropic plastic bonded magnets or bulk sintered/hot deformed magnets from the recycled HDDR powder with superior remanence and BH_max to the former type REPMs.

To reiterate the crucial factor again, the mechanism of coercivity is not clearly understood for the HDDR Nd-Fe-B system, whereas it is pinning dominant in nanocrystalline melt-spun ribbons and nucleation controlled in micron-sized single crystalline Nd-Fe-B sintered magnets. The level of understanding for this system requires extensive study of the fresh as well as the recycled counterparts, and we made stepwise efforts in this arrear with the previous publications (isotropic magnets). The dependent magnetic properties can only be refined to an extent in the recycled material because of the limitation in oxygen constricted processing. Therefore, the presented results essentially also suggested the retention of coercivity (resistance to demagnetization) as the texture is imparted in the system to enhance the remanence. The derivative magnetic properties are credibly better than the starting recycled HDDR powder, whereby as per previous reports in the HDDR Nd-Fe-B system [3] the coercivity declined by 50 – 90 % as the texture was increased. This definitely renders the hot deformed magnets with miniscule coercivities useless for the commercial applications, but now with our proposed method i.e. low pressure-controlled forging; texture can be developed and energy products raised not only in the recycled material but also suitably in the fresh HDDR Nd-Fe-B powder.

As far as I know, the first paper for the application of SPS method to the Nd-Fe-B magnets is “Spark plasma sintering of Nd-Fe-B magnetic alloy”, by Z. G. Liu, M. Umemoto, S. Hirosawa, H. Kanekiyo, J. Mater. Res. 14 (1999) 2540-2547 and that for the application of die-upsetting of the SPS Nd-Fe-B magnets is “Microstructure and magnetic properties of anisotropic Nd−Fe−B magnets prepared by spark plasma sintering and hot deformation, by X. Li, Li Li, Z. C. Chen, S. Q. Qu, C. Yang, Trans. Nonferrous Met. Soc. China 24 (2014) 3142−3151”. The authors should have studied literature on SPS and die-upsetting of Nd-Fe-B magnets.

Thank you for citing the literature. The work done by Liu et al. [22] refers to Nd-Fe-B melt-spun ribbons processed by Spark Plasma Sintering (SPS). It may not be a fair comparison of this report on melt-spun ribbons with our results of hot deformation HDDR Nd-Fe-B magnets, but sequentially our controlled SPS experiments on the recycled HDDR powder developed significantly better magnetic properties [23, 24] than reported by Liu and co-workers back in 1999 [22]. So, the proof-of-concept is valid with SPS for both melt-spun and HDDR Nd-Fe-B nanocrystalline materials, but these both systems cannot be compared in respect of their microstructure and derivative magnetic properties as previously elucidated.

The proper hot deformation study by Li et al. [3] utilized the SPS system for the HDDR Nd-Fe-B magnets has been cited in our manuscript. The hot deformation was performed on the HDDR Nd-Fe-B pressed magnets of comparable magnetic properties to the recycled HDDR powder. Clearly a sizeable decline in the coercivity (~70 %) was reported for 69% deformation ratio, i.e. B_r = 1.22 T and BH_max = 121 kJ/m³ only measured parallel to the pressing direction. Even with 50 – 70% deformation ratio in range of temperatures from 750 – 900 ^OC, the most optimal B_r = 1.19T and BH_max = 143 kJ/m³, still yielded H_Ci 495 kA/m which is 50% lower than the values we obtained from the recycled material. We do anticipate, the magnets from the fresh HDDR Nd-Fe-B powder like MF-15P having intrinsic H_Ci = 1120 kA/m and B_r = 1.35 T can reach better values than this recycled material. Therefore, considering literature findings, our results have the potential to become the state of the art for the hot deformed recycled HDDR Nd-Fe-B type REPMs.

In addition, the recycled HDDR powder has already been studied; “A. Lizandru, I. Poenaru, K. Guth, R. Gaus, and O. Gutfleisch “A systematic study of HDDR processing conditions for the recycling of end-of-life Nd-Fe-B magnets”, J. Alloys Compd. 724 (2017) 51-61.”

We agree in the recent past; several researchers like Lixandru et al. [16], Kimiabeigi et al. [25], Yang et al. [26] and Li et al. [27] have worked on the different recycling methods with the HDDR Nd-Fe-B system. But all these publications either reported the effectiveness of the HDDR reprocessing or the resultant properties were tethered to either the magnetic powder or for the polymer bonded applications only. For the first time, our consortium brought this concept of utilizing the SPS for developing the bulk magnets out of the recycled HDDR Nd-Fe-B powder, and in all these investigations the focus was on retaining the high coercivity and its augmentation (by doping) in the isotropic magnets [23, 24, 28]. Only the validity of different processing modality in SPS was tested in this current manuscript and forging has proven very effective in the recycled HDDR Nd-Fe-B magnets. The description of mathematical model to relate the heat treatment effect in the improvement of the magnetic properties in our manuscript comprehensively defines the processing conditions and connecting them to the microstructure. The next logical steps should be grain boundary modification of the hot-forged HDDR Nd-Fe-B system and/or disclose the relationship of interfaces with high-resolution analytical electron microscopy, but these tasks are out of scope for this manuscript.

I will look forward to seeing their results turned into new scientific and engineering insight that will advance the field of permanent magnets.

Following detailed explanation to the remarks and queries posted by the respected reviewer, we strongly believe that this paper to testifies scientific explanation, definition of the experiments, proof of hypothesis and industrial application viability of hot-forging the recycled HDDR Nd-Fe-B powder. With ~23% improvement in the remanent magnetization over the high coercivity but isotropic HDDR Nd-Fe-B based-magnets as reported previously and consequently better BH_max and H_Ci in combination to the articles cited by the referee. The obtained values of BH_max and H_Ci are relatively higher for the HDDR Nd-Fe-B system and only so far in the recycled material which advocates the possibility to publish results. Clearly for the recycled HDDR Nd-Fe-B system our reported values are the state of the art and in comparison to the previous literature, relatively all the three engineering parameters (H_Ci, B_r and BH_max) were retained better than the starting feedstock.

With meticulous control of SPS forging system, may be the commercial grade HDDR Nd-Fe-B alloys becomes comparable to the nanocrystalline Nd-Fe-B melt-spun ribbons in terms of their excellent magnetic properties after the hot-deformation. But due to certain limitations in the recycling process, there may be constraints to yield very high remanent magnetization and BH_max values without compromising the coercivity. Simultaneously, the research is still absent on developing the melt-spun ribbons from recycled feedstock or end-of-life Nd-Fe-B magnets and reprocessing them with omni functional SPS.

References (in this letter to editor)

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Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.