Experimental Continuous Casting of Nitinol
Round 1
Reviewer 1 Report
It is an interesting research topic, and casting of NI-Ti alloy is very difficult. However, huge amount of content of the paper stay on general comment. As a scientific paper, quantative analysis is very important. In your Figure 1, it looks the surface quality is not good, how to improve the surface quality? In the trial 1, the strand compsoiton shown in Table 1 tells that Ti content decrease and Ni increase, compared to the original composition. Why Ti is lost so much? The cast product is good if they have good surface quality and uniform compsoition distribution. Information about such distribution in cross section or Axial section should be very interesting.
Author Response
Reviewer 1
It is an interesting research topic, and casting of NI-Ti alloy is very difficult.
Reviewers comment 1)
However, huge amount of content of the paper stay on general comment. As a scientific paper, quantative analysis is very important.
Response:
No doubt quantitative analysis is important. However, it is not clear what exactly was meant by this comment.
- Casting parameters: All values of the casting parameters which could be controlled or measured with the existing casting device are already in the manuscript. To obtain any additional quantitative values, additional measuring systems would be required on the casting device.
- The strands: The depth of the witness marks was not measured because the rule “the shallower the better” always applies.
- Chemical compositions: The actual chemical compositions were listed in Table 1.
- The ideal chemical composition was not defined for two reasons: a) The optimum chemical composition depends on the desired transformation temperatures, and b) Small deviations in the composition of nitinol have no significant influence on castability.
Action taken:
The following explanation of the dependence of transformation temperatures on chemical composition has been included in
lines 132-136 of the revised manuscript:
“Nickel contents below 50 at. % are usually selected if the goal is to exploit the shape memory effect. Less nickel results in higher transformation temperatures. If the the superelasticity should be exploited, the martensite start temperature must be below the service temperature range, and usually nickel contents above 50.5 at. % are selected.”
Reviewers comment 2)
In your Figure 1, it looks the surface quality is not good, how to improve the surface quality?
Response:
This comment certainly applies to the surface in Fig. 1d, which is unacceptable, especially due to many cracks at the witness marks, but it does not necessarily apply to all other surfaces, shown in Figs. 1a, b and c.
Actions taken:
Changes have been made in lines 199-202:
Before revision:
The surface quality of the strands was excellent, Figure 1.
After revision:
“The primary witness marks (regular circumferential grooves on the strands) were very shallow and narrow, and the surface between them is very smooth. Taking into account that witness marks are unavoidable in this casting process, the surface of strand 1, Fig 1a, is excellent.”
In lines 278-280 of the revised manuscript was added:
“Such surface is not desirable, but it may still be acceptable, as the surface layer of continuously cast strands is often removed (peeled off) prior to further processing in order to obtain a flawless surface without defects.”
Below Fig. 5, lines 315-324 in the revised manuscript, the following discussion was added:
“For a smoother surface with shallow witness marks the temperature in the mould must be high enough. Sufficiently high temperature assures that the solidification front cannot advance into the nozzle (consequently, no, or very shallow primary witness marks occur). It also assures complete remelting of the edges of both static and dynamic skin (consequently, very shallow or almost invisible secondary witness marks occur). But the temperature must not be to high either. It must be low enough to prevent sticking to the mould. Sticking damages the mould and shortens its service life. On the other hand, sticking increases pulling forces and, therefore, a thicker solid skin must form to withstand the pulling force at the start of the next stroke. To obtain a thicker solid skin, the cooling rate must be increased. Finding the optimum combination of casting parameters and materials for the crucible, nozzle, and mould, to fulfil all these conditions at the same time, will require further efforts.”
Reviewers comment 3)
In the trial 1, the strand compsoiton shown in Table 1 tells that Ti content decrease and Ni increase, compared to the original composition. Why Ti is lost so much? The cast product is good if they have good surface quality and uniform compsoition distribution. Information about such distribution in cross section or Axial section should be very interesting.
Response:
The inadequate chemical composition of strand 1 was not caused by the loss of titanium. Namely, the loss of chemical elements during smelting in vacuum (by evaporation) or protective atmosphere (burn-off due to residual oxygen) cannot cause such significant deviations from the targeted compositions.
As was already explained (lines 171-177 in the first version), the main reason was inadequate mixing of the melt in the graphite crucible, combined with the differences in melting temperatures and densities of Ni and Ti. Consequently, nickel settled at the bottom while the majority of the titanium remained on the top. This means that the amount of titanium missing at the bottom (the front end of the strand) was not lost, but was present on the top of the crucible.
However, we failed to mention other, minor reasons: Errors in weighing of the batch, and the fact that scrap was used in the first two trials, which means that the weighed mass included not only pure metals but also some impurities, mostly oxides, the majority of which formed the slag.
Action taken:
To address this comment of reviewer 1, the following text has been included into the manuscript at lines 231-236 of the revised manuscript:
“…while the Ti-concentration was too high closer to the surface of the melt in the crucible. Other possible reasons for inadequate composition, which, however, cannot cause such significant deviations, are: Evaporation of liquid metals in a vacuum, burn-off due to residual oxygen in the protective atmosphere, errors in weighing of the batch and the use of scrap in the first two trials, which means that the weighed mass included not only pure metals, but also some impurities, mostly oxides, the majority of which formed the slag.”
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Congratulations to the authors for their good work. The following comments will help you to improve the quality of your paper prior to publishing.
- the references are old. you can add different discussions in the introduction part talking about different methods of making NiTi parts (although not continuous) and their properties. then you can talk about why these methods are or are not proper for making NiTi parts. in addition, you can talk about the different properties of NiTi alloys and their applications. the following references are strongly recommended to be in this paragraph:
https://link.springer.com/article/10.1007/s10853-019-03375-1 (biomedical applications)
https://www.sciencedirect.com/science/article/pii/S0921509320300113 (additive manufacturing of NiTi)
- please add an image of the setup you used for continuous casting or at least add a schematic image of the setup.
- the images could be in a higher quality. Do you have higher quality images?
- in figure 2, the kikuchi patterns are for EBSD of XRF not for SEM imaging. although the machines are the same. please correct the information there. same for other images.
- in general, the quality of the images, especially the microstructures are not high. please replace them with higher resolution ones.
Author Response
Reviewer 2
Congratulations to the authors for their good work. The following comments will help you to improve the quality of your paper prior to publishing.
Reviewers comment 1)
the references are old.
Response:
Yes, some references are old, but some are not. There is even one reference from 2020. One reason why not all references are from the recent years is that we did not want to present certain problems (not yet solved) and facts addressed in the manuscript as something new when, in fact, they have been known for quite a long time. The other reason is that, in many cases, no newer references could be found. However, the comments of reviewer 2 encouraged us to add some references from the recent years.
Action taken:
The following references have been added (numbers correspond to the reference list in the revised manuscript):
[15-25], [29], [30].
Reviewers comments 2) and 3)
Reviewers comment 2):
you can add different discussions in the introduction part talking about different methods of making NiTi parts (although not continuous) and their properties.
Reviewers comment 3) :
then you can talk about why these methods are or are not proper for making NiTi parts.
Response:
Our objective was to research the feasibility of continuous casting of NiTi, because this alloy is one of not many alloys that are not yet cast continuously into strands in the industrial production.
In general, the production methods cannot be divided onto proper and improper ones. Which method is a proper one depends on the part that is to be produced.
We mentioned some disadvantages of the conventional production, that consist of ingot casting, several stages of rolling and intermediate recrystallisation, forming and/or machining.
Indeed, we failed to discuss alternative routes for the production of finished parts and to explain when continuous casting makes sense and when not. The main reason why alternative methods of final part production and post-production treatments were not addressed more comprehensively is that continuous casting is not a rival process to forming, additive manufacturing, machining…, or final production steps like heat treatments or surface treatments, which are usually necessary to obtain the desired materials/surface properties regardless of the previous production route.
Action taken:
To address comments 2), 3) and 5) of reviewer 2, the following text has been included into the Introduction:
At lines 31-57 of the revised manuscript:
The alloy belongs to a group of materials called Shape Memory Alloys (SMAs). SMAs exhibit an unconventional correlation between strain, stress and temperature, based on crystallographically reversible thermoelastic martensitic transformation. The low-temperature and the high-temperature phases are, analogous to steel technology, named martensite and austenite. Austenite has a cubic (B2) structure [21, p. 49], martensite has a monoclinic B19´ structure [21, p. 53]. High cooling rates are necessary to prevent decomposition of austenite into equilibrium low-temperature phases and to inforce martensitic transformation instead. However, since the martensitic transformation of SMAs is crystallographically reversible, high cooling rates are necessary only at the first time. Once martensite has formed, further martensitic transformations are possible at any temperature-change rate, as well upon cooling as upon heating.
Depending on temperature, stress- and deformation state, several different phenomena can occur, the most interesting being Pseudoplastic deformation, One-way shape memory effect, Two-way shape memory effect and Superelasticity. In the martensitic state, apparently plastic deformation based on coalescence of martensitic variants, practically without dislocation glide, is possible - pseudoplastic deformation. Upon heating above the austenite start temperature, a pseudoplastically deformed element starts to recover its original shape - one-way shape memory effect. With special thermomechanical treatments called training, it can be achieved that a part made of SMA changes its shape, not only upon heating, but also upon cooling - two-way shape memory effect. In an austenitic state, SMAs are capable of unusually large elastic deformation, which is enabled by stress-triggered transformation austenite à martensite – suprelastic deformation. Upon unloading, the transformation takes a reverse course and the material recovers its original shape. In different products, components are needed that are capable of sensing and responding, movement (stroke), mechanical work/force, energy absorption, vibration damping... Using SMAs, many of such components can be built without electronic controllers, switches, motors, gears, moving parts, bearings, lubrication, mechanical wear... Consequently, the devices are simpler, less material and less different kinds of materials are needed to build certain devices, less or no energy is needed for operation, operation is more reliable,and, after the end of their life time, recycling is easier and more efficient.
At lines 72-100 of the revised manuscript:
Further operations, like machining and post-processing (e.g. heat treatments or surface treatments), are necessary to create finished components of the desired shape and properties, and to incorporate those components into tools and devices [11, 12 (pp. 13-23)]. Difficult machining and poor workability due to its high strength, ductility, and high work hardening, are limiting a wider use of NiTi [Farhang-2020]. Machining of NiTi is characterised by severe tool wear, high specific cutting energy, and high strain hardening. Furthermore, functional properties are very sensitive to machining parameters [Kaya-2018].
To shorten the production efforts and costs, alternative processes can be an advantage. Relatively old alternatives to the classic route are casting and classic powder metallurgy (compressing powders into green products and sintering). However, the possibilities of these processes are quite limited with respect to the shape of the products. The compression of powders into green parts is possible only for relatively simple shapes, and not always pore-free products can be obtained [Elahinia-2012]. Today, the most versatile alternative is Additive Manufacturing (AM), including Selective Laser Melting, Electron Beam Melting and Wire-arc Additive Manufacturing. Additive Manufacturing has significant advantages in the production of very complex-shaped construction parts, prototypes, and very small series of parts. In general, the static mechanical properties of AM metallic materials are comparable to conventionally fabricated metallic components [Frazer-2014]. Nearly fully dense products can be fabricated with optimum combinations of process parameters [Pal-2018]. Even the high standards ofthe Aerospace industry can be met [Gisario-2019]. Newertheless, it is still a challenge to produce a flawless product with the intended characteristics [Dillip-2017], and the building orientation has an impact on the mechanical properties [Pal-2019]. Besides, in spite of the high quality of AM parts, post-processing involving hot working, cold working [Elahinia-2012], conventional machining [Pidge-2020], sometimes even also, heat treatments, enhance the shape memory properties [Khoo-2019] or heat treatments improve corrosion resistance and biocompatibility. By appropriate surface treatments, corrosion resistance can be improved significantly [dehghanghadikolaei-2019].
In the production of wires, rods, tubes and thin sheets the above described alternative processes cannot be applied. Such products are still produced by casting, rolling and drawing (wires). If the production starts with thick ingots, numerous stages of rolling and intermediate annealing are necessary to obtain the desired semi-finished product, which is time consuming and expensive. In such a case, continuous casting can be the preferable alternative to ingot casting.
At lines 108-112 of the revised manuscript:
Suzuki et al. [Suzuki-1999] studied ductility and other properties if Nitinol and some other Ti-based alloys at high temperatures, and suggested that titanium alloys can be produced successfully by continuous casting. However, only twin roll casting of wide bands was realised [Goryczka-2005], while successful continuous casting of Nitinol into strands has not yet been reported.
Reviewers comment 4)
in addition, you can talk about the different properties of NiTi alloys and their applications.
Response:
The most important fields of application are already listed in the Introduction. The properties of NiTi are today quite well known, and were not addressed comprehensively in order to keep the manuscript as short as possible.
However, encouraged by this comment, the Introduction has been extended.
Action taken:
Please find the inserted text at the reviewers` comments 2) and 3).
Reviewers comment 5)
the following references are strongly recommended to be in this paragraph:
https://link.springer.com/article/10.1007/s10853-019-03375-1 (biomedical applications)
https://www.sciencedirect.com/science/article/pii/S0921509320300113 (additive manufacturing of NiTi)
Response:
None of these articles is directly related to our work. The first one deals with surface coatings of selective laser melted NiTi samples. The second one deals with the microstructure and properties of selective laser melted NiTi samples.
However, we are aware of the importance of the Additive Manufacturing techniques and post-production treatments of parts. Therefore, we have included both fields (and both references) into the Introduction. The reason why they were not mentioned in the first version of the manuscript is that Additive Canufacturing and continuous casting are not rival processes.
Action taken:
The introduction was extended so that it now includes Additive Manufacturing and surface treatments. Please find the inserted text at the reviewers` comments 2) and 3).
Reviewers comment 6)
please add an image of the setup you used for continuous casting or at least add a schematic image of the setup.
Response:
A schematic image was already included in the submitted supplementary file.
Action taken: the following changes have been made in the supplementary file:
- Approximate dimensions have been added to the schematic drawing
- A photograph of the device with the operator has been added.
Reviewers comment 7)
the images could be in a higher quality. Do you have higher quality images?
Response:
The images in the submitted Word file are indeed of lower resolution. Images in higher resolution exist and will be uploaded together with the revised manuscript.
Action taken:
The images in higher resolution were uploaded as a separate zip-file
Reviewers comment 8)
in figure 2, the kikuchi patterns are for EBSD of XRF not for SEM imaging. although the machines are the same. please correct the information there. same for other images.
Response:
- The reviewer is right: EBSD and XRF systems are not used for SEM imaging. The Kikuchi patterns given in the images were recorded by the NordlysII EBSD system (producer: HKL), an additional detector in a scanning electron microscope (JEOL JSM 6500-F). XRF was used for chemical analysis, and no images or patterns can be recorded using the XRF method.
- However, the explanation of which equipment was used, was provided in Chapter 2.4 Examination of strands. In lines 132-134 it was also explained that EBSD is an additional system/detector built into the electron microscope.
- We recognise that the Figure captions of Figures 6 and 7 (not Fig. 2) imply that kikuchi patterns were obtained with SEM because the explanation that they were obtained by an EBSD detector is missing.
Action taken:
Adequate explanations have been added to the Figure captions of Figs 6 and 7.
- Fig. 6, before revision: “Kikuchi patterns of these three phases are presented on the top”;
- After revision: “Kikuchi patterns of these three phases, obtained by the EBSD system, are presented on the top”.
- Fig. 7: The following explanation has been added: “Kikuchi patterns of NiTi and NiTi2, obtained by the EBSD system, are presented on the top.”
Reviewers comment 9)
in general, the quality of the images, especially the microstructures are not high. please replace them with higher resolution ones.
Response:
The images in the submitted Word file are indeed of lower resolution. All images in higher resolution will be uploaded together with the revised manuscript.
Action taken:
- The micrograph in the Fig. 6 has been replaced,
- The quality of the micrograph in the Figure 7 has been enhanced.
- All images in higher resolution according to “instructions for authors” have been uploaded as a separate zip-file.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Congratulation for your good work!
Reviewer 2 Report
Good job with responding to the comments.
Good luck with your research.
