Wear of ZhS6U Nickel Superalloy Tool in Friction Stir Processing on Commercially Pure Titanium

Round 1

Reviewer 1 Report

This paper presents a study on the tool durability in FSW of pure Ti. The work presents some minor issues that I present in the attached file

Comments for author File: Comments.pdf

Author Response

Point 1: 1105 mm, 2335 mm

Response 1: Corrected signature of Figure 3 - added units of dimension.

Point 2: is this after traversing 2755 mm ?

Response 2: EDX maps are given for tools after 2755 mm processing. It is indicated in the caption to Figure 5.

Point 3: This is not clear. The axial displacement of the tool is not made at the same z penetration? The penetration of the tool in the parent material is not always the same along the welding line? Is the z axis controled in force? If this is the case, there will be a problem, because the load changes also due to the high temperature of the parent metal that changes along the welding

Response 3: In our work, it is not the plunge depth but the axial force that is controlled. During processing, it is constant. This explanation has been added in section 2.1. Materials and experimental set-up. Therefore, when we wear out the shoulders, the pin plunges deeper. If, however, the plunge depth is controlled by coordinates and does not change, then the shoulders will not touch the surface at all after wear.

Point 4: How the Author know the parent material strength? Sometimes we have surprises when we believe in the supplier. Nothing better than to make our tensile test to parent material

Response 4: The strength of the initial material obtained experimentally is given in this paper. Explanations are added in section 2.2. Investigative techniques.

Point 5: How were the tensile samples prepared in its thickness? Are the subjected to some surface preparation like grinding? How is the strength obtained? The section area is not always the same along the weld line

Response 5: The surfaces of the test specimens were not processed in any way. Quasi-static tensile tests of the weld material were conducted on a UTC 110M-100 testing machine at room temperature. Tests were carried out along the material to obtain directly the strength of the processed material. In fact, these passes are not fixed joints.

Point 6: the load is a result, not an input parameter, right? If we change the speed, the load changes, ...

Response 6: The axial load in this paper is an input parameter, which was reflected in section 2.1. Materials and experimental set-up.

Point 7: internal cooling!?

Response 7: The idea of internal cooling came to mind after the work was done. At the moment we are carrying out experiments, which will be reflected in the following works.

Reviewer 2 Report

The present paper aims at studying the durability of a tool in ZhS6U Ni-based superalloy used for performing friction stir processing operations of a commercially pure titanium and the influence of the tool wear on the weld quality.

The paper is interesting but it is not suitable for publication in the present form: additional results have to be shown and many aspects have to be improved.

• English has to be improved; sometimes the sentences are a little difficult to understand;
• The reviewer suggests changing the title of the manuscript, to highlight especially the study of tool wear rather than the friction stir processing;
• In the list of authors, the word "and*" is a typo;
• The introduction section is weak and needs to be improved. The reviewer suggests to better explain the aim of the work and what the innovative aspect of the research is, as compared to the scientific literature;
• The experimental methods in section 2 have to be described through sub-paragraphs;
• Provide more details about FSP operations, such as orientation of rolling direction versus processing direction, tool plunging, plunging speed and dwelling time;
• In section 2, the reference [33], cited to justify the choice of parameters at which the highest weld strength was achieved, seems inappropriate and not correct. Explain better;
• Was the FSP process conducted using force control? It would be very interesting to show the trend of the vertical force developed during FSP;
• Specify the international standard followed for tensile tests. How was the strain measured during tensile tests? Did you use an extensometer?
• The reviewer suggests plotting the behaviour of average surface roughness of the pin tip and shoulder as a function of weld length during FSP;
• Did the authors evaluate the influence of transverse speed and rotational speed on tool wear? It would be very interesting to have information on such results;
• Add the stress-strain curves of friction stir processed welds at different weld lengths and compare the mechanical properties (such as ultimate tensile strength and ultimate elongation) of the base material with the results given by welds;
• In figure 6, the reviewer suggests adding further images of axial cross-sectional view of the tool after other values of weld length;
• Why in figure 12, for a weld length equal to 0 mm, that is FSP not yet conducted, the tensile strength is 25% greater than that of the base metal? Shouldn't the value be the same as that of the unprocessed metal? Or does the dwelling time affect the tensile strength? Please, explain such result;
• Check the manuscript title of ref [4] because it should be incorrect;
• The discussion of the results has to be more supported by literature. The reviewer suggests to read the following papers, useful to support authors' statements:
  • Bruni C et al. Friction stir welding of magnesium alloys under different process parameters, Materials Science Forum, (2010), 638-642, 2010, pp. 3954-3959.
  • Gibson BT et al. Friction stir welding: process, automation, and control. J Manuf Process 2014; 16: 56–73.
  • Sahlot P  et al. Quantitative wear analysis of H13 steel tool during friction stir welding of Cu-0.8%Cr-0.1%Zr alloy. Wear 2017; 378–379: 82–89.
  • Prater T et al. A comparative evaluation of the wear resistance of various tool materials in friction stir welding of metal matrix composites. J Mater Eng Perform 2013; 22: 1807–1813.
  • Bevilacqua et al., Comparison among the environmental impact of solid state and fusion welding processes in joining an aluminium alloy, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture (2020), 234(1-2), pp. 140-156

For the aforementioned reasons, the reviewer suggests a major revision.

Author Response

Point 1: English has to be improved; sometimes the sentences are a little difficult to understand;

Response 1: English was improved.

Point 2: The reviewer suggests changing the title of the manuscript, to highlight especially the study of tool wear rather than the friction stir processing;

Response 2: The name has been changed to «Wear features of ZhS6U nickel superalloy tool at friction stir processing of commercially pure titanium».

Point 3: In the list of authors, the word "and*" is a typo;

Response 3: The source file of the manuscript, downloaded in docx format, does not contain an "and". It must be a service error.

Point 4: The introduction section is weak and needs to be improved. The reviewer suggests to better explain the aim of the work and what the innovative aspect of the research is, as compared to the scientific literature;

Response 4: The introduction identifies a scientific and practical problem: despite the large number of studies, the available materials for the manufacture of tools are either not durable enough, or do not ensure reliable weld quality, or are technologically unfeasible due to complicated processing and high cost. Various solutions to this problem by other researchers are described. The most relevant sources have been used. Welding and processing of titanium itself are quite rare compared to other materials. The variant of the problem solution is offered. It is shown that nobody has used nickel superalloys for welding of titanium before. It provides originality and novelty. No one has ever investigated the wear of nickel superalloys with FSP titanium before, which is also indicated. The stated goal is to investigate the durability of a ZhS6U Ni-based superalloy tool and the effect of its wear on a friction stir weld in titanium alloy.

Point 5: The experimental methods in section 2 have to be described through sub-paragraphs;

Response 5: Section 2 was divided into 2 sub-paragraphs: 2.1. Materials and experimental set-up and 2.2. Investigative techniques.

Point 6: Provide more details about FSP operations, such as orientation of rolling direction versus processing direction, tool plunging, plunging speed and dwelling time;

Response 6: The FSP was carried out along the rolling direction. . The FSP is carried out at constant axial load, which is a preset parameter to maintain metal flow both at the tool plunging stage and during processing. The tool travel speed is not a parameter to control. The FSP tool travel starts immediately after the shoulder contacts the metal surface. So that dwelling time is zero. The tool inclination angle is 1.5 °. Explanations have been added to the section 2.1. Materials and experimental set-up.

Point 7: In section 2, the reference [33], cited to justify the choice of parameters at which the highest weld strength was achieved, seems inappropriate and not correct. Explain better;

Response 7: In fact, the reference is to the source [34]. The error has been corrected.

Point 8: Was the FSP process conducted using force control? It would be very interesting to show the trend of the vertical force developed during FSP;

Response 8: The axial force of the processing was constant, this is specified in the paragraph 2.1. Materials and experimental set-up.

Point 9: Specify the international standard followed for tensile tests. How was the strain measured during tensile tests? Did you use an extensometer?

Response 9: The tests were conducted according to ISO 6892-1-2016. Specified in the list of literature.. Strain was measured by UTC 110M-100 testing machine. We didn't use an external extensometer.

Point 10: The reviewer suggests plotting the behaviour of average surface roughness of the pin tip and shoulder as a function of weld length during FSP;

Response 10: This is not possible at the moment as the tool has already machined 2755 mm of titanium.

Point 11: Did the authors evaluate the influence of transverse speed and rotational speed on tool wear? It would be very interesting to have information on such results;

Response 11: At this stage of the research it was important to establish the nature of the wear, which no one did exactly at constant parameters. This was done in order to assess the impact of other technological solutions on the wear in the future with exactly the same parameters. In the future, of course, the impact of speed and other parameters will be assessed.

Point 12: Add the stress-strain curves of friction stir processed welds at different weld lengths and compare the mechanical properties (such as ultimate tensile strength and ultimate elongation) of the base material with the results given by welds;

Response 12: Curves are added in Figure 12 b. In the discussion, a comparison of total elongation has been added to the figure.

Point 13: In figure 6, the reviewer suggests adding further images of axial cross-sectional view of the tool after other values of weld length;

Response 13: The tool's already cut, we can't use it after cutting. The tool was the only one to meet the minimum differences in conditions. It is known that such materials in bulk can have heterogeneity. Wear at other processing lengths can be seen in the photo (Fig. 3).

Point 14: Why in figure 12, for a weld length equal to 0 mm, that is FSP not yet conducted, the tensile strength is 25% greater than that of the base metal? Shouldn't the value be the same as that of the unprocessed metal? Or does the dwelling time affect the tensile strength? Please, explain such result;

Response 14: In fact, the first point on the graph corresponds to the strength of the processed material after 20 mm pass. You may have been confused by the large scale of values on the OX axis.

Point 15: Check the manuscript title of ref [4] because it should be incorrect;

Response 15: Corrected.

Point 16: The discussion of the results has to be more supported by literature. The reviewer suggests to read the following papers, useful to support authors' statements:

Bruni C et al. Friction stir welding of magnesium alloys under different process parameters, Materials Science Forum, (2010), 638-642, 2010, pp. 3954-3959.

Gibson BT et al. Friction stir welding: process, automation, and control. J Manuf Process 2014; 16: 56–73.

Sahlot P  et al. Quantitative wear analysis of H13 steel tool during friction stir welding of Cu-0.8%Cr-0.1%Zr alloy. Wear 2017; 378–379: 82–89.

Prater T et al. A comparative evaluation of the wear resistance of various tool materials in friction stir welding of metal matrix composites. J Mater Eng Perform 2013; 22: 1807–1813.

Bevilacqua et al., Comparison among the environmental impact of solid state and fusion welding processes in joining an aluminium alloy, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture (2020), 234(1-2), pp. 140-156

Response 16: Most of these references are irrelevant to this work, as they study completely different materials and effects. However, the works of Sahlot and Bevilacqua proved to be useful for comparison. In paragraph 3.1. Tool wear adds references and discussions.

Reviewer 3 Report

The submitted manuscript shows results of friction stir processing of commercially pure titanium by the new tool made form ZhS6U. The paper should be corrected in order to improve the paper's quality. Please find below some points that ought to be corrected:

1. Authors should insert some information about potential, specific applications of FSW methods - what's will more clearly show readers an importance of the manuscript.

2. Experimental procedure - authors have used unit of forces [kg] - it is the unit of mass. If it is a mistake (i.e. should be [kG]) - the unit [kG] it isn't SI unit. Authors have to change it.

3. Authors examine the samples taken only from the weld zone (Fig.2) - why?
In general, the main reason why the joining process uses is a joint two pieces of material. Therefore the samples for the static tensile test should be taken from the complete joint area i.e. base materials and weld area. From the practical point of view, it is very important to know what is the mechanical properties of such joint (including base materials and weld zone) then only weld zone as a separate area.
This point of view is not realistic. Authors should show the good motivation of the approach presented in the manuscript.

4. How efficient was the cooling process of the tool after each pass?
It has led to the same temperature as in the before experiments?

Author Response

Point 1: Authors should insert some information about potential, specific applications of FSW methods - what's will more clearly show readers an importance of the manuscript.

Response 1: Information about the prospects of FSW application has been added to section 1. introduction.

Point 2:  Experimental procedure - authors have used unit of forces [kg] - it is the unit of mass. If it is a mistake (i.e. should be [kG]) - the unit [kG] it isn't SI unit. Authors have to change it.

Response 2: Changed to newtons.

Point 3: Authors examine the samples taken only from the weld zone (Fig.2) - why?
In general, the main reason why the joining process uses is a joint two pieces of material. Therefore the samples for the static tensile test should be taken from the complete joint area i.e. base materials and weld area. From the practical point of view, it is very important to know what is the mechanical properties of such joint (including base materials and weld zone) then only weld zone as a separate area.
This point of view is not realistic. Authors should show the good motivation of the approach presented in the manuscript.

Response 3: Yes, but we did not get the joint, namely processed the material, so the strength of the processed material was of interest. These tests are standardised. The motivation has been added to the section 2.2. Investigative techniques.

Point 4: How efficient was the cooling process of the tool after each pass? 
It has led to the same temperature as in the before experiments?

Response 4: After each pass, the instrument was cooled to room air temperature as described in section 2.1. Materials and experimental set-up. Forced cooling was not used.

Reviewer 4 Report

This paper deals with the effect of pure titanium material over the wear of ZhS6U Ni-based superalloy tool in FSP(friction stir processing). As following authors's claim, the effect of wear of processing tool in FSP was not sufficiently studied up to now. The main contents may be fall within the scope of this journal and some modification will be helpful to enhance the quality of current version.

• The experintal process was only focused on the traveling length. Other condition may be considered such as, friction velocity, magnitude of loading.
• The shape of pin in this paper was only considered for one, conical shape, and you stated that the reduced diameter at the base of the pin was caused by heat production. May the experimental result dependent on the shape of pin.
• The effect of strength in FSP sample over the strength in base material was not sufficiently adressed in current version.

Author Response

Point 1: The experintal process was only focused on the traveling length. Other condition may be considered such as, friction velocity, magnitude of loading.

Response 1: The purpose of this paper was precisely to establish the nature of the wear and the suitability of the material for use as a tool as specified. There is no doubt that future publications will take into account changes in other conditions. In addition, in future publications we will repeat the experiment entirely in the same mode, but with other alloys and the cooling system of the tool, on which we are already working.

Point 2: The shape of pin in this paper was only considered for one, conical shape, and you stated that the reduced diameter at the base of the pin was caused by heat production. May the experimental result dependent on the shape of pin.

Response 2: Of course, it was the shape change of the tool that led to the degradation of the material quality. The entire section 3.3 is devoted to the discussion of this effect.

Point 3: The effect of strength in FSP sample over the strength in base material was not sufficiently adressed in current version.

Response 3: Commercially pure titanium is a single-phase material. The main parameters that affect the strength of the material are defect, geometry and grain structure. All these aspects have been measured and discussed in detail in the section 3.3. Quality of friction stir processing.

Reviewer 5 Report

The manuscript studies friction stir processing of commercially pure
titanium by a ZhS6U Nickel based superalloy tool. The manuscript can be accepted for publishing after addressing the following comments.

1. Some of the friction stir processing parameters are missing. What was the dwell time utilized? what was the tool tilt angle utilized? Mention the axial force in SI units of N. Was the processing performed under load control or displacement control?
2. Was the axial force suitable, because observing figure 2, it is shown that the tool did not fully plunge into the workpiece, that can cause vibrations and increase tool pin wear. Comment on this.
3. Between the successive passes after 100 mm for FSP was the tool scrubbed to remove excess material?
4. Comment on the backing plate utilized. During FSP/W of titanium alloys it has been commonly observed the sheet to be joined/create hook defects with the backing plate. So usually similar backing plate as the workpiece has been utilized. Was something similar observed, how difficult was it to remove the processed plate after FSP? 
5. Microhardness on the processed region could ideally show the localized mechanical behavior. It is highly recommended to present microhardness data for FSP region at different processed lengths.
6. Comparison of the tensile data of FSP titanium and tool wear with literature values for similar tool geometry could be made and would help in comparison of tool effectiveness. 

Author Response

Point 1: Some of the friction stir processing parameters are missing. What was the dwell time utilized? what was the tool tilt angle utilized? Mention the axial force in SI units of N. Was the processing performed under load control or displacement control?

Response 1: The FSP is carried out at constant axial load, which is a preset parameter to maintain metal flow both at the tool plunging stage and during processing. The tool travel speed is not a parameter to control. The FSP tool travel starts immediately after the shoulder contacts the metal surface. So that dwelling time is zero. The tool inclination angle is 1.5 °. Explanations have been added to the section 2.1. Materials and experimental set-up.

Point 2: Was the axial force suitable, because observing figure 2, it is shown that the tool did not fully plunge into the workpiece, that can cause vibrations and increase tool pin wear. Comment on this. 

Response 2: Indeed, in some areas there was an incomplete plunge. Apparently, it is due to the heterogeneity of the sheet itself or to an unstable regime. It certainly affects the wear.

Point 3: Between the successive passes after 100 mm for FSP was the tool scrubbed to remove excess material?

Response 3: The instrument was not cleaned in any way between the aisles to preserve the natural conditions. Explanations have been added to the section 2.1. Materials and experimental set-up.

Point 4: Comment on the backing plate utilized. During FSP/W of titanium alloys it has been commonly observed the sheet to be joined/create hook defects with the backing plate. So usually similar backing plate as the workpiece has been utilized. Was something similar observed, how difficult was it to remove the processed plate after FSP?

Response 4: A plate made of AISI 304 steel was used as a substrate. To avoid welding the workpiece to the substrate, the length of the pin was 0.5 mm less than the thickness of the workpiece. This was specified in the section 2.1. Materials and experimental set-up. There was no touching of the substrate.

Point 5: Microhardness on the processed region could ideally show the localized mechanical behavior. It is highly recommended to present microhardness data for FSP region at different processed lengths.

Response 5: Microhardness measurements were additionally performed. The results are given in the section 3.3. Quality of friction stir processing. «The same was true for the stir zone microhardness numbers, which were 1.5±0.1 GPa, 1.4±0.1 GPa and 1.6±0.1 GPa for 20 mm, 500 mm and 2700 mm processed length. The base metal microhardness was 1.3±0.1 GPa. Such a moderate hardening was the result of severe plastic deformation during FSP».

Point 6: Comparison of the tensile data of FSP titanium and tool wear with literature values for similar tool geometry could be made and would help in comparison of tool effectiveness.

Response 6: Unfortunately, no mechanical properties data could be found along the processed material. About the wear: FSW on Ti–6Al–4V carried out using similar configuration tools made of other materials also demonstrated different type of wear [16]. In particular, a W-La tool experienced severe plastic deformation while those made of WC–Co demonstrated brittle fracture.. Discussion added to the section 3.1. Tool wear.

Round 2

Reviewer 2 Report

• The introduction was not significantly improved as compared to the original version of the paper, as required by the reviewer. Only the sentence highlighted in blue has been added. The purpose of the work was not described more clearly (lines 72 and 73 only): it is not accurate to write that "The object of this study is to investigate the durability of a ZhS6U Ni-based superalloy tool and the effect of its wear on a friction stir WELD in titanium alloy", since friction stir process was performed, and not friction stir welding. The methodology used in the work has to be introduced: for example, the reviewer suggests moving the text of lines 76-78 into the introduction to better explain the approach used by the authors.

• In line 38, the authors wrote that the problem of rapid tool wear is typical in FSW processes; the reviewer suggests including FSP processes as well.

• English has not been significantly improved as compared to the previous version of the manuscript. It is suggested a further editing.

The reviewer proposes a minor revision.

Author Response

Point 1: The introduction was not significantly improved as compared to the original version of the paper, as required by the reviewer. Only the sentence highlighted in blue has been added. The purpose of the work was not described more clearly (lines 72 and 73 only): it is not accurate to write that "The object of this study is to investigate the durability of a ZhS6U Ni-based superalloy tool and the effect of its wear on a friction stir WELD in titanium alloy", since friction stir process was performed, and not friction stir welding. The methodology used in the work has to be introduced: for example, the reviewer suggests moving the text of lines 76-78 into the introduction to better explain the approach used by the authors.

Response 1: Yes, you're right. The aim has been adjusted according to the actual methodology. The motivation for the method has been added.

Point 2: In line 38, the authors wrote that the problem of rapid tool wear is typical in FSW processes; the reviewer suggests including FSP processes as well.

Response 2: FSP was included.

Point 3: English has not been significantly improved as compared to the previous version of the manuscript. It is suggested a further editing.

Response 3: English was improved.

Reviewer 3 Report

In the present form, the paper "Friction stir processing of commercially pure titanium by a ZhS6U superalloy tool" can be published.

Author Response

Thank you for your suggestions and comments.

Reviewer 4 Report

Referee’s raised issues were moderately solved in the revised form. Before condering publication, English editing may be required to enhance the quality of this form. My recommendation is “accept”.

Author Response

Thank you for your suggestions and comments.