An Experimental Study on Bushing Formation during Friction Drilling of Titanium Grade 2 for Medical Applications^†

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article discusses bushing formation during friction drilling of titanium grade 2 for medical applications. Although the subject of the article is interesting and important both from the scientific point of view and from the point of view of doctors and patients, and is worth publishing, the manuscript requires some corrections and clarifications, namely:

-          in the introduction, more attention should be paid to discussing the latest literature (i.e. from the last 5 years), nearly 60% of the literature cited in the manuscript comes from before 2018. Some of the references seem to be loosely related to the issues discussed in the article, e.g. reference [11]

-          moreover, in the introduction, it should be clearly stated what distinguishes this work from previous research and what new elements the work brings to research on friction drilling of titanium grade 2; Do the authors of the work know the results of other authors’ research on the use of friction drilling in the process of producing stabilizing plates?

-          according to the text, the tests were carried out with the force of 361 and 722 N, therefore please check the correctness and, if necessary, correct the force value given in Table 3 for room temperature.

-          in Figure 8a and 8b, the unit on the vertical axis representing the average bushing thickness is missing

-           in Figure 8b, please correct the sign of the rotational speed: from "w" to "ω"

-          due to the high temperature and tendency of titanium to absorb gases from the environment, is there a high risk of structural changes in the material that would affect the further operation of the implant? Would it be justified to carry out the process in a protective atmosphere?

-          in the conclusions, it is worth providing the optimal/specific set of parameters (F, ω, T) of the friction drilling process for the tested titanium sheets with a thickness of 1 mm (the authors only indicate the benefits resulting from preheating the material)

Author Response

Dear Reviewer,

Thank you for your valuable feedback. We took your comments into account when revising the paper and formulated answers to your concerns below. We hope the answers are to your satisfaction.

- in the introduction, more attention should be paid to discussing the latest literature (i.e. from the last 5 years), nearly 60% of the literature cited in the manuscript comes frombefore 2018. Some of the references seem to be loosely related to the issues discussed in the article, e.g. reference [11]

Thank you for your comment. 10 more recent research have added to strengthen the introduction chapter.

Reference [11] (Reference 21 in the updated version) is one of the important articles that define the structural zone of friction drilling by examining their microstructure and distance from the drilled zone, namely SZ (Stir Zone), TMAZ (Thermomechanically Affected Zone), and HAZ (Heat Affected Zone). We believe referring to this previous study is important while defining the structural zones in our samples through microstructural analysis. By doing so, we are demonstrating the reliability of our approach in accordance with the existing literature.

 

- moreover, in the introduction, it should be clearly stated what distinguishes this work from previous research and what new elements the work brings to research on friction drilling of titanium grade 2; Do the authors of the work know the results of other authors’ research on the use of friction drilling in the process of producing stabilizing plates?

We have incorporated additional recent literature sources. We believe that the last three paragraphs of the introduction chapter of the new version, make the research aim clearer by highlighting the existing gaps in the literature.

 

- according to the text, the tests were carried out with the force of 361 and 722 N, therefore please check the correctness and, if necessary, correct the force value given in Table 3 for room temperature.

There was a switch in cells when converting table 3 to the paper. At room temperature the optimal thrust force was 722N, with Pre-Heating the optimal Temperature was 1017N. Pre-heating allows a higher thrust force than the friction drilling performed at room temperature. hence the optimal thrust force in the pre-heating case lays above the upper limit chosen for the factorial analysis.

 

- in Figure 8a and 8b, the unit on the vertical axis representing the average bushing thickness is missing

Resolved

 

- in Figure 8b, please correct the sign of the rotational speed: from "w" to "ω"

Resolved

 

- Due to the high temperature and tendency of titanium to absorb gases from the environment, is there a high risk of structural changes in the material that would affect the further operation of the implant? Would it be justified to carry out the process in a protective atmosphere?

This question is indeed relevant and could be crucial for implant applications. In our study, we used EDS analysis to assess the oxygen content both right below the bushing surface and also on the Ti sheet before heating. The results indicated that there was no significant increase in the oxygen content, and the elemental distribution remained consistent between the two surfaces.

While our primary focus in this study was not on using protective atmosphere during friction drilling, it is important to consider the option. As part of our ongoing research, we will further evaluate in vitro biocompatibility and bioabsorbability studies. Future studies will delve into these aspects to comprehensively understand the implications for medical applications. Considering these in vitro testing results, incorporating a protective atmosphere during friction drilling could be explored as a process optimization strategy.

Moreover, we are currently working on mechanical testing to evaluate fixation strength of the friction drilled holes, such as pull through and angular stability testing of the fixation screws in the implants. By comparing friction drilled fixations with some other state of art processes, this research will reach a conclusion of mechanical stability.

Reviewer 2 Report

Comments and Suggestions for Authors

The results seem to be interesting and they are reported in a quite clear way. However, there is a need for certain clarifications and additions:

1. What were the dimensions of the pure titanium samples (apart from thickness) used for testing?

2. Provide more information about Berulit 935 lubricant. How much lubricant was added to the tools?

3. How many times tests and measurements were repeated?

4. Do the input parameters in tests (temperature, thrust force, rotational speed) correspond to real systems?

5. What do you mean by “optimal drilling parameters” (page 5, line 148)? On what basis were they determined?

6. It would be also worth clarifying where the results could be applied. Do the results have only cognitive or also applicable character?

7. References section should be checked and formatted to the required format.

Author Response

Dear Reviewer,

Thank you for your valuable feedback. We took your comments into account when revising the paper and formulated answers to your concerns below. We hope the answers are to your satisfaction.

1. What were the dimensions of the pure titanium samples (apart from thickness) used for testing?

We added both the sample size of 20 x 60 mm and size of the clamping fixture 120 x 50 x 6mm

 

2. Provide more information about Berulit 935 lubricant. How much lubricant was added to the tools?

New text: Berulit 935, is selected as lubricant for its high temperature stability (up to 950°C), in order to reduce tool wear. The tool is dipped into the Berulit 935 immediately prior to forming of each hole

 

3. How many times tests and measurements were repeated?

All tests were performed 5 times. This was stated once at the results of the factorial analysis but we repeated the statement at the different sections of the text.

 

4. Do the input parameters in tests (temperature, thrust force, rotational speed) correspond to real systems?

We don't really understand the question? The temperature, thrust force and rotational speed were applied on our platform and will be used in a CNC automated drilling station to create bushings for thin shelled medical implants

 

5. What do you mean by “optimal drilling parameters” (page 5, line 148)? On what basis were they determined?

A regression plane with method of steepest ascent was used to find the optimal parameters for the specific case of friction drilling with the described conditions (tool, sheet, ...). This was added to the text.

 

6. It would be also worth clarifying where the results could be applied. Do the results have only cognitive or also applicable character?

The results will certainly be used in creating bushings for fixation of medical implants. Moreover, we are in the process doing pre-clinical testing (biocompatibility, sterilisation, strength..) to start clinical trials

 

7. References section should be checked and formatted to the required format.

The reference section is strongly reworked

Reviewer 3 Report

Comments and Suggestions for Authors

It is an interesting work on bushing formation during friction 2 drilling.  This manuscript may be accepted in this journal after a minor revision. Please see the comments below.

1. It can be seen that there is a heating platform and a heating cartridges for sample preheating. My question is how to control or get a uniform temperature field for the sample. 

2. How to obtain the optimal drilling parameters in Table 3? Please give a detailed description. Do you consider to use some optimization method?

3. During the description of Figure 10, the author states that the HAZ to the BM are defined by looking into the grain size  distribution, with the mean grain size enhanced slightly in the room-temperature drilled sheet. Is there any reference for this determination?

 

Author Response

Dear Reviewer,

Thank you for your valuable feedback. We took your comments into account when revising the paper and formulated answers to your concerns below. We hope the answers are to your satisfaction.

1. It can be seen that there is a heating platform and a heating cartridges for sample preheating. My question is how to control or get a uniform temperature field for the sample.

The heating platform remains at the same temperature through the use of a PID controller. A small clamping fixture is mounted to this stable heated block and allowed enough time (15 minutes) to reach a steady temperature. The temperature of the Ti sheet prior to forming is verified through both a K-type thermocouple and Infra Red imaging. The text now states: In addition to using a thermocouple to control the heating block temperature, a FLIR Thermovision A20 IR camera is set up to monitor the temperature of the backside of the sheet. A secondary thermocouple at in the middle of the drilling location at the front of the titanium sheet is used to verify the correct pre-heating temperature. Once steady state is confirmed (15 minutes) the thermocouple is removed and drilling starts.

2. How to obtain the optimal drilling parameters in Table 3? Please give a detailed description. Do you consider to use some optimization method?

A regression plane with method of steepest ascent was used to find the optimal parameters for the specific case of friction drilling with the described conditions (tool, sheet, ...). This was added to the text.

3. During the description of Figure 10, the author states that the HAZ to the BM are defined by looking into the grain size distribution, with the mean grain size enhanced slightly in the room-temperature drilled sheet. Is there any reference for this determination?

While mechanically affected zones are easily observed by examining microstructure, the situation differs between the heat-affected zone (HAZ) and base metal (BM). There are distinct observations between HAZ and BM in friction welding applications in literature, as the significant heat difference makes them easily distinguishable. However, there is a lack of literature on this distinction in friction drilling.

Our observations have been conducted through scanning electron microscopy (SEM) and optical micrographs. Using Image J software, we have calculated the grain size distribution. However, we are planning to conduct an in-depth Electron Backscatter Diffraction (EBSD) analysis on these structures, providing specific coordinates for the structural zones and defining HAZ and BM.

Actually, we were also not sure whether to share our findings or to keep them after making an EBSD observation, as it will be the first observation in the literature for friction drilling of titanium and its alloys. Therefore, as part of this study, we are excluding our previous observations of HAZ and BM to ensure a more thorough investigation and gather additional data.

Article revised accordingly.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you very much for your comprehensive answers and additions. I find them satisfying