Microstructure Effects on the Machinability of AM-Produced Superalloys

Round 1

Reviewer 1 Report

Comments and suggestions

 1. Line 100: “…layer height at 30 mm, hatch spacing at 90 mm, and beam compensation at 60 mm.”  Millimeters??

2. Why were tubular samples used?

3. Table 3: insert the full name of the terms, not only the symbols or acronyms

4. Section 2.5. Mechanical properties of the AM material

 Authors specify:

“For the AM tube a quasi-static engineering stress versus strain curve obtained for a horizontal (XY) and vertical (ZY) test piece at room temperature is displayed in Figure 2.”

 But in the Figure 2 are presented the results for the dog-bone shape samples. Furthermore, Figure 2 is the same with the Figure 12 from 10 reference: “High strain rate effect on tensile ductility and fracture of AM fabricated Inconel 718 with voided microstructures”

 5. If the aim of the paper is: “This paper investigates microstructure effects on the machinability of IN718 …” why the authors present the tensile tests?

 6. Many paragraphs or sentence are the same as in reference 10.

 For example:

Current paper: “The anisotropic tensile properties of as-built XY and ZY test pieces have been widely reported [10–17] with XY displaying higher strength and lower ductility than ZY. The horizontal build plane (XY) is orientated to the circumferential and radial axes of the tube whilst the ZY is orientated along the vertical build axis (Z).”

 Reference 10: “The anisotropic tensile properties of as-built XY and ZY test pieces have been widely reported [13,21,28,43,54] with XY displaying higher strength and lower ductility than ZY….. The horizontal build plane (XY) is orientated to the circumferential and radial axes of the tube rings”

 7. What is the depth of cut? If the tube thickness is 2 mm.

 8. Figures 5 and 6: Why the authors presented the chips shape in a simulation/digital format? They used a real machine-tool and they obtained the real chips.

 

Author Response

Dear reviewer,

The authors are extremely grateful to the reviewer for providing a very detailed summary of the improvements needed to the manuscript.

The authors accept the first version of the manuscript submitted could have been improved before it was submitted which would have made the reviewing task easier. That said the authors have now attended to all the comments, implemented the amendments suggested, and addressed each of the questions in the six criteria, which has resulted in extensive improvements throughout the manuscript that is now being resubmitted for review.

To assist the reviewers with reviewing the amended manuscript, we have included a pdf formatted version with corrections and new content highlighted in yellow. We will refer to this document and line numbers to address each of the comments below.

The details answer to reviewer comments are listed below:

• Line 100: “…layer height at 30 mm, hatch spacing at 90 mm, and beam compensation at 60 mm.”  Millimeters??

The units are in micronmeters and the typo has been corrected in line number 142 and 143

e.g. layer height at 30 mm, hatch spacing at 90 mm, and beam compensation at 60 mm. Other unit tipos were found and these have all been corrected in the revised manuscript.

• Why were tubular samples used?

The line numbers 116 to 121 have been added to the end of section 1 to explain this.

A method frequently used to conduct machinability studies of alloys is orthogonal cutting which uses a tube workpiece and a single edge of the cutting tool. The tool feed motion engages the workpiece on the length axis of the tube. The tool edge cuts across the full tube thickness normal to the long axis of the tube so the width of the chip is nominally equally to the tube thickness. The depth of cut in this arrangement is equal to the feed rate.

On a separate note, this approach can help to reduce the complexity of a 3D dimensional machining process involving multiple cutting edges such as in milling and/or, 3 force components (main cutting, feed and radial) in turning down the outside diameter of a bar or facing the end of bar, to enable a fundamental understanding of the machining science using a 2D dimensional machining configuration.

• Table 3: insert the full name of the terms, not only the symbols or acronyms.

The full names of the terms have now been added to the table 3 on the line number 193 to 194.

• Section 2.5. Mechanical properties of the AM material

 Authors specify:

“For the AM tube a quasi-static engineering stress versus strain curve obtained for a horizontal (XY) and vertical (ZY) test piece at room temperature is displayed in Figure 2.”

 But in the Figure 2 are presented the results for the dog-bone shape samples. Furthermore, Figure 2 is the same with the Figure 12 from 10 reference: “High strain rate effect on tensile ductility and fracture of AM fabricated Inconel 718 with voided microstructures”

The opening sentence has been corrected on the line number 201 and 202 under section 2.5.

“The AM material was mechanically tested using the tensile test method, and the method and results are described in reference (13).”

• If the aim of the paper is: “This paper investigates microstructure effects on the machinability of IN718 …” why the authors present the tensile tests?

The Figure 2 significantly enhances visual impact than the data in table 4 which lists the mechanical properties obtained for the AM tested material. The table 4 confirms the AM material is in accord with the powder suppliers’ specifications for the AM material in the stress relieved condition.

• Many paragraphs or sentence are the same as in reference 10.

 For example:

Current paper: “The anisotropic tensile properties of as-built XY and ZY test pieces have been widely reported [10–17] with XY displaying higher strength and lower ductility than ZY. The horizontal build plane (XY) is orientated to the circumferential and radial axes of the tube whilst the ZY is orientated along the vertical build axis (Z).”

 Reference 10: “The anisotropic tensile properties of as-built XY and ZY test pieces have been widely reported [13,21,28,43,54] with XY displaying higher strength and lower ductility than ZY….. The horizontal build plane (XY) is orientated to the circumferential and radial axes of the tube rings”

The whole of section 2.5 (line numbers 200 to 216) has been extensively amended and rephrased to ensure that the content and information provided, relates specifically to the discussion section (3.4) in this paper. Furthermore, a citation to the paper 13 has been added to the title of the Figure 2 on the line number 214. New data has been added to the table 4 that has not been previously reported or published.

 

 

• What is the depth of cut? If the tube thickness is 2 mm.

The depth of cut in this arrangement is equal to the feed – see line number 225. In this study the feed was 0.05, 0.1 and 0.15 mm/rev as shown in Table 5 on the line number 233 to 234. See also reply to comment 2 above.

• Figures 5 and 6: Why the authors presented the chips shape in a simulation/digital format? They used a real machine-tool and they obtained the real chips.

These are images of real chips obtained from the machining trials for different process conditions. Using the optical microscope described in section 2.8 (on the line number 256) a chip image was captured in 3D at x150 magnification. The topology of the chip surface enabled enhanced visualization of the serrated character of the chip forms.

Author Response File: Author Response.pdf

Reviewer 2 Report

Investigation of the machinability of superalloys obtained by additive manufacturing technology is an actual topic for modern industry. The authors of the paper " Microstructure effects on the machinability of AM produced superalloys" present an experimental investigation of the Inconel 718 behavior under different conditions of the post-treatment. It was shown that a smaller grain size leads to a change in the fracture mechanism during machining treatment. The paper is well-written and presents interesting results for the prediction of optimal production conditions. The manuscript may be accepted for publication after correction accordingly following comments:

1.                 The practical value of the paper may be increased: the authors should summarize the optimal machining conditions for AM products in the conclusion part.

2.                 The mechanical properties in Table 4 do not correspond to the stress-strain curves in Figure 2. The strength of Inconel 718 is significantly higher than 33 MPa. The value of the elongation also should be changed accordingly stress-strain curve.

3.                 What does mean “texture index” in Line 344 (Taylor factor or other characteristics)? Additional information should be added to the manuscript.

4.                 Minor corrections:

-                     The confidence interval should be added to the values of the mechanical properties.

-                     Table 5 is not informative and may be removed from the manuscript.

-                     Line 366: cooling rate during AM processing is about 10⁵ K/s but not 10^-6 K/s.

Author Response

Dear reviewer,

The authors are extremely grateful to the reviewer for providing a very detailed summary of the improvements needed to the manuscript.

The authors accept the first version of the manuscript submitted could have been improved before it was submitted which would have made the reviewing task easier. That said the authors have now attended to all the comments, implemented the amendments suggested, and addressed each of the questions in the six criteria, which has resulted in extensive improvements throughout the manuscript that is now being resubmitted for review.

To assist the reviewers with reviewing the amended manuscript, we have included a pdf formatted version with corrections and new content highlighted in yellow. We will refer to this document and line numbers to address each of the comments below.

The details answer to reviewer comments are listed below:

• The practical value of the paper may be increased: the authors should summarize the optimal machining conditions for AM products in the conclusion part.

The following sentences have been added to the conclusion.

Line number 485 to 486

At higher f = 0.15 mm/rev and for all Vc, the AM material displayed a significantly lower Ft than the WR material.

Line number 501 to 504

From a practitioner’s perspective to establish a process window for finish ma-chining the AM produced Inconel 718 material further work is needed for example both tool wear studies and workpiece surface integrity analysis for a reduced set of process parameters. Nonetheless f = 0.15 mm/rev is a good starting point for this task.

• The mechanical properties in Table 4 do not correspond to the stress-strain curves in Figure 2. The strength of Inconel 718 is significantly higher than 33 MPa. The value of the elongation also should be changed accordingly stress-strain curve.

The data provided in the table was incorrect and this has now been corrected in the revision.

• What does mean “texture index” in Line 344 (Taylor factor or other characteristics)? Additional information should be added to the manuscript.

The Texture Index used in this paper is a measurement scale used in pole figures to determine the degree in which grain structures have preferred crystallographic orientation (texture). Mechanical working of alloys leads to textured grain structures and the more severe the work the high higher the texture. To avoid misunderstanding in interpretation, since we are dealing with a comparative or relative scale, we think the use of the word index is misleading and this has therefore been replaced. For WR microstructure it is (max = 2.57; min = 0.35) and AM microstructure (max = 5.58; min = 0.02).Lines 420 to 426

• Minor corrections:
  • The confidence interval should be added to the values of the mechanical properties.

The authors have included the sample standard deviation in table 4 in the revised manuscript.

• • Table 5 is not informative and may be removed from the manuscript.

The authors would prefer to retain table 5 because it clearly describes the experimental plan for the machining trials.

• • Line 366: cooling rate during AM processing is about 10⁵ K/s but not 10^-6 K/s.

According to published literature, cooling rates in laser melting powder bed fusion have been measured at 10^-5 to 10^-6 K/s as indicated in the article below and reported in several others. Nonetheless, we have slightly amended the data in the manuscript providing an indicative range on the line number 103 and 104

https://www.nature.com/articles/s41598-022-18096-w

Wang, R., Garcia, D., Kamath, R.R. et al. In situ melt pool measurements for laser powder bed fusion using multi sensing and correlation analysis. Sci Rep 12, 13716 (2022). https://doi.org/10.1038/s41598-022-18096-w

“……The measured maximum cooling rate has a range of 6.5–56 °C/µs, highly consistent with the value (1–40 °C/µs) reported in Hooper’s56 work. The thermal gradient presented in this work has a range from 1.6 to 8.7 °C/µm, which has the same magnitude as the number (5–20 °C/µm) presented in Hooper’s work as well. Notice that the level of magnitude accuracy for microstructure estimation is usually enough.”

Author Response File: Author Response.pdf

Reviewer 3 Report

The article is devoted to the actual topic. But before accepting the article, a number of improvements need to be made.

1. Expand the abstract and rework it. Add information about the additive technology method used. Present the results obtained in the article in more detail.

2. Expand the introduction and describe in more detail the similar studies you referred to. You can also strengthen the review by adding new research on this topic to it.

https://doi.org/10.3390/ma16134529

https://doi.org/10.1007/s00170-021-07639-6

https://doi.org/10.1007/978-3-030-70566-4_3

3. In the methodology section, indicate the number of samples prepared for the study. It would also be interesting for the reader to give the total time for the manufacture of samples. Indicate that the printing was carried out in a vacuum.

4. Lines 221-230. You give formulas for calculating cutting forces. Immediately after the formulas, define all variables used, even k, f. Follow through the article to ensure that all variables used are defined.

5. When describing your results in sections 3.3 and 3.2, you often describe known things. And the results of your work in a number of provisions are consistent with the work already done. You refer to them yourself. It is clear that with an increase in feed, stepping will appear on the sample. BUT you'd better describe and highlight your new results more explicitly in these sections.

6. Figure 7 for the EX structure shows several areas (bands) with fine grains. What is the origin of these regions? It would be good to provide an explanation in the text of the article.

7. It would be good in the conclusions to give any recommendations on the processing of the alloy under study.

Author Response

Dear reviewer,

The authors are extremely grateful to the reviewer for providing a very detailed summary of the improvements needed to the manuscript.

The authors accept the first version of the manuscript submitted could have been improved before it was submitted which would have made the reviewing task easier. That said the authors have now attended to all the comments, implemented the amendments suggested, and addressed each of the questions in the six criteria, which has resulted in extensive improvements throughout the manuscript that is now being resubmitted for review.

To assist the reviewers with reviewing the amended manuscript, we have included a pdf formatted version with corrections and new content highlighted in yellow. We will refer to this document and line numbers to address each of the comments below.

The details answer to reviewer comments are listed below:

• Expand the abstract and rework it. Add information about the additive technology method used. Present the results obtained in the article in more detail.

More detail to the abstract has been added to address the comment from the reviewer from line 15 to 26. It should be noted the word count limit on the abstract of 200 is almost reached at 199.

• Expand the introduction and describe in more detail the similar studies you referred to. You can also strengthen the review by adding new research on this topic to it.

https://doi.org/10.3390/ma16134529

https://doi.org/10.1007/s00170-021-07639-6

https://doi.org/10.1007/978-3-030-70566-4_3

The authors have provided further content to the introduction section, describing in more detail the studies cited, and strengthened the review by adding new content notably citations 10, 11 and 12, in which the findings are discussed in the context of this paper. We thank the reviewer for the 3 interesting papers. They are very informative and of good quality, and we are grateful that these works have been brought to our attention for future consideration. But this paper is intentionally focused on the Nickel based superalloy Inconel 718 and the process effects on grain structure that effect machinability arising from the laser melting powder bed fusion process. We would prefer, if acceptable to the reviewer, to limit the scope of the review to papers to this alloy system. Furthermore, we are open to future research collaboration and in this regard, we invite you to correspond with the submitting author Dr JOSE DIAZ ALVAREZ.

https://doi.org/10.3390/ma16134529

Title: Production of Workpieces from Martensitic Stainless Steel Using Electron-Beam Surfacing and Investigation of Cutting Forces When Milling Workpieces.

Sentence from abstract: “The aim of this study was to investigate cutting force when milling 40 × 13 stainless steel samples obtained via electron-beam surfacing”

https://doi.org/10.1007/s00170-021-07639-6

Title: Additive manufacturing of AISI 420 stainless steel: process validation, defect analysis and mechanical characterization in different process and post-process conditions

Sentence from abstract: “The effect of LPBF process parameters and different heat treatments on density, defect characteristics and locations, roughness and mechanical properties of AISI 420 were investigated in this paper.”

https://doi.org/10.1007/978-3-030-70566-4_3

Title: Process Parameters Effect on Weld Beads Geometry Deposited by Wire and Arc Additive Manufacturing (WAAM)

Sentence from abstract: “In this research, geometry and temperature evolutions of single beads have been investigated according to process parameters modifications”

• In the methodology section, indicate the number of samples prepared for the study. It would also be interesting for the reader to give the total time for the manufacture of samples. Indicate that the printing was carried out in a vacuum.

Three tube workpieces were printed in Inconel 718 with the same process parameters using a Renishaw AM250 Selective Laser Melting machine at the University of Derby in UK was added to the line number 123.

The build processing time was approximately 23 hours was added to line number 146.

An argon purge pressure atmosphere in the build chamber maintaining an oxygen content <1000 ppm (0.1%) was added to the line number 147.

• Lines 221-230. You give formulas for calculating cutting forces. Immediately after the formulas, define all variables used, even k, f. Follow through the article to ensure that all variables used are defined.

We have followed through the article to ensure all variables used in this section have been clearly defined. We have also defined all variables under abbreviations after the abstract.

• When describing your results in sections 3.3 and 3.2, you often describe known things. And the results of your work in a number of provisions are consistent with the work already done. You refer to them yourself. It is clear that with an increase in feed, stepping will appear on the sample. BUT you'd better describe and highlight your new results more explicitly in these sections.

All phrases in the manuscript have been amended, replacing words such as ‘obvious’ and ‘not uncommon’ and ‘it is expected’, to ensure the significance of the new findings in this paper are understood by the reader.

• Figure 7 for the EX structure shows several areas (bands) with fine grains. What is the origin of these regions? It would be good to provide an explanation in the text of the article.

The following sentences have been added to line number 420 to 424. Please also note we have replaced the label ‘Ex’ with ‘WR’.

It is observed that bands of fine grain structures are visible in the WR material amongst larger neighboring grains. This feature could be attributed to more rapid recrystallization of grain structures during annealing that were in a more severely cold worked state from producing the tube leading to different rates of grain growth in the population.

• It would be good in the conclusions to give any recommendations on the processing of the alloy under study.

The following sentences have been added to line number 497 to 500.

In future machining studies of AM materials the effect of the border should be considered. For example, it may be better to fabricate material samples with and without border scan to establish this effect on machinability and if needed, to design the AM process for finish machining.

From a practitioner’s perspective to establish a process window for finish machining the AM produced Inconel 718 material further work is needed for example both tool wear studies and workpiece surface integrity analysis for a reduced set of process parameters. Nonetheless f = 0.15 mm/rev is a good starting point for this task.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

In the titles of Figures 5 and 6, please, insert the mention that these images are from optical microscope (OM) and their magnification. Usually, on the images from OM/SEM has to be indicated the scale/magnification used.

 

Author Response

Dear reviewer,

The authors are extremely grateful to reviewers for their time and help to improve the first version of the manuscript.

To assist the reviewers with reviewing the amended manuscript, we have included a pdf formatted version with corrections in red and new content highlighted in yellow. We will refer to this document and line numbers to address each of the comments below.

The details answer to reviewer comments are listed below:

1. In the titles of Figures 5 and 6, please, insert the mention that these images are from optical microscope (OM) and their magnification. Usually, on the images from OM/SEM has to be indicated the scale/magnification used.

Thank you for providing us with this indication to expand the information contained in the figures. Figures 5 and 6 were updated accordingly. Lines 360 and 379 respectively.

Best regards,

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have properly anwered most of previous comments. However, one point is still questionable:

1. Line 414: Cooling rate dulring AM of metallic materials should be 10⁶ K/s. The authors incorrectly recalculate °C/µs  to K/s. 6.5–56 °C/µs = 6 500 000 - 56 000 000 K/c = 6.5 - 56 10⁶ K/s [13]. In case of the colling rate of 10^-6 =0.0000001 K/s, the authors need to wait the end of the crystallization about 1 000 000 000 s (32 years).

 

Author Response

Dear reviewer,

The authors are extremely grateful to reviewers for their time and help to improve the first version of the manuscript.

To assist the reviewers with reviewing the amended manuscript, we have included a pdf formatted version with corrections in red and new content highlighted in yellow. We will refer to this document and line numbers to address each of the comments below.

The details answer to reviewer comments are listed below:

1. Line 414: Cooling rate dulring AM of metallic materials should be 106 K/s. The authors incorrectly recalculate °C/µs to K/s. 6.5–56 °C/µs = 6 500 000 - 56 000 000 K/c = 6.5 - 56 106 K/s [13]. In case of the colling rate of 10-6 =0.0000001 K/s, the authors need to wait the end of the crystallization about 1 000 000 000 s (32 years).

Thank you very much for bringing our mistake to our attention again. It has been corrected appropriately. The correction was made in line 104.

Best regards

Author Response File: Author Response.pdf

Reviewer 3 Report

In general, the authors have finalized the article well. The authors could write recommendations on post-processing in more detail in the conclusion.

Author Response

Dear reviewer,

The authors are extremely grateful to reviewers for their time and help to improve the first version of the manuscript.

To assist the reviewers with reviewing the amended manuscript, we have included a pdf formatted version with corrections in red and new content highlighted in yellow. We will refer to this document and line numbers to address each of the comments below.

The details answer to reviewer comments are listed below:

 

1. In general, the authors have finalized the article well. The authors could write recommendations on post-processing in more detail in the conclusion.

As we include in the previous version “From a practitioner’s perspective to establish a process window for finish machining the AM produced Inconel 718 material further work is needed for example both tool wear studies and workpiece surface integrity analysis for a reduced set of process parameters. Nonetheless f = 0.15 mm/rev is a good starting point for this task.” . However, a recommendation to perform the post- processing was included in lines  480-481: “However, the coolant may serve an important role over a longer cut duration.”

Best regards,

Author Response File: Author Response.pdf