Experimental Investigation of the Influence of Wire Arc Additive Manufacturing on the Machinability of Titanium Parts

Round 1

Reviewer 1 Report

This paper studied the machinability of titanium parts that made by Wire Arc Additive Manufacturing (WAAM) through a series of drilling tests. It provides solid experimental evidence to show that WAAM leads to parts with a higher hardness and better overall mechanical properties. The research is novelty with practical guidance to both the academic community and industry thus I believe the draft should be a good paper for the journal ‘metals’.

 

My two cents: if you can discuss on the scientific reasons/theories behind your observation, it will better make your paper looks more persuasive. The readers may ask a question like this: why the WAAM can make those difference?

Author Response

This paper studied the machinability of titanium parts that made by Wire Arc Additive Manufacturing (WAAM) through a series of drilling tests. It provides solid experimental evidence to show that WAAM leads to parts with a higher hardness and better overall mechanical properties. The research is novelty with practical guidance to both the academic community and industry thus I believe the draft should be a good paper for the journal ‘metals’.

Response: We would like to thank to the reviewer for their comments and suggestions for the manuscript.

 

Point 1: My two cents: if you can discuss on the scientific reasons/theories behind your observation, it will better make your paper looks more persuasive. The readers may ask a question like this: why the WAAM can make those difference?

 

Response 1:

Discussions have been introduced to clarify the differences observed in the microstructure and in the mechanical properties that affect the drilling process:

 

“Results show lower burr values in cases of drilling of material manufactured by WAAM technology due to its higher hardness and mechanical resistance typical of the manufacturing process of the wall in which we find a heating-cooling cycle that increases the local hardness”

“Due to the influences of thermal gradient and solidification rate, the acicular α interwoven with a basket-weave structure and martensite α’ the β matrix were generated.”

Author Response File: Author Response.pdf

Reviewer 2 Report

the machining study is good;

the microstructure-properties gradient along the wall height could be investigated better;

please sight some minor comments in the manuscript file

Comments for author File: Comments.pdf

Author Response

Point 1: the machining study is good;

Response 1:

We would like to thank to the reviewer for their comments and suggestions for the manuscript.

 

Point 2: the microstructure-properties gradient along the wall height could be investigated better;

Response 2:

A discussion has been added about the differences in microstructure at different heights of the wall manufactured by WAAM technology:

 

”The microstructure of the wall in the middle zone is shown, the grain border and the dendritic structure derived from phases in which the temperature undergoes an abrupt decrease from the melting temperature necessary for the different welds production. Due to the influences of thermal gradient and solidification rate, the acicular α interwoven with a basket-weave structure and martensite α’ the β matrix were generated.”

Point 3: please sight some minor comments in the manuscript file

Response 3:

A text revision has been made. Authors hope will meet the expectations of the reviewers

Author Response File: Author Response.pdf

Reviewer 3 Report

In this work, the authors investigate the machinability of the titanium parts made with the wire arc AM process. While not a new topic, the authors provide some interesting new data and conclusions in this paper. It is generally well written, clear, and easy to follow. The results and material behavior make sense to me and are similar to what I have seen with previous work on SLM/DMLS of titanium alloys (consistency of behavior across process types would be an interesting discussion in a future paper if the authors want to tackle it). The testing and analysis procedures are well-planned and well-designed for this kind of work. The only major thing I would have liked to see included with a study like this is discussion/examination of the variance and repeatability with a variety of samples; however, I understand that this is not the focus of the study and that the authors presented quite a few other topics. 

With these general comments, I do not have any major concerns about the paper or the work is describes, but I do have some minor suggestions for improvement. I recommend acceptance after a minor revision to address the points below: 

1. The machining and other hybrid processing of AM materials is not a new topic, so the authors need to more clearly specify what is novel about the presented results

2. The quality and clarity of the figures needs improvement - please provide higher resolution figures (especially focus on improving Figures 1 and 6, as some of the panels and the scale bars are difficult to read)

3. There are quite a few acronyms and technical terms that are not obvious to a casual reader, so the authors should provide a nomenclature section at the beginning of the paper

4. It is not clear if the samples were checked for cracking or voids after machining - was this done? If so, please present some results and discussion on this.

5. Please provide some guidance for the community on how to continue this work, especially in setting up formal designed experiments to test repeatability and reliability of results (as previously specified).  

 

Author Response

In this work, the authors investigate the machinability of the titanium parts made with the wire arc AM process. While not a new topic, the authors provide some interesting new data and conclusions in this paper. It is generally well written, clear, and easy to follow. The results and material behavior make sense to me and are similar to what I have seen with previous work on SLM/DMLS of titanium alloys (consistency of behavior across process types would be an interesting discussion in a future paper if the authors want to tackle it). The testing and analysis procedures are well-planned and well-designed for this kind of work. The only major thing I would have liked to see included with a study like this is discussion/examination of the variance and repeatability with a variety of samples; however, I understand that this is not the focus of the study and that the authors presented quite a few other topics. 

Response: Appreciate the review and suggestions to continue the research work. They will be taken into consideration and it is hoped they can be covered in future works by these authors.

With these general comments, I do not have any major concerns about the paper or the work is describes, but I do have some minor suggestions for improvement. I recommend acceptance after a minor revision to address the points below: 

Point 1: The machining and other hybrid processing of AM materials is not a new topic, so the authors need to more clearly specify what is novel about the presented results

Response 1: A sentence summarizing the observed results and the novelty has been included in the paper

 

 

Results show lower burr values in cases of drilling of material manufactured by WAAM technology due to its higher hardness and mechanical resistance typical of the manufacturing process of the wall in which we find a heating-cooling cycle that increases the local hardness.

 

The novelty of this paper lies in the study of WAAM additive technology for the production of Ti6Al4V aeronautical titanium alloy parts and in experimentally addressing the drilling operation since it is one of the most recurring machining operations in this type of applications.

Point 2: The quality and clarity of the figures needs improvement - please provide higher resolution figures (especially focus on improving Figures 1 and 6, as some of the panels and the scale bars are difficult to read)

Response 2: Both figures have been modified

 

Point 3: There are quite a few acronyms and technical terms that are not obvious to a casual reader, so the authors should provide a nomenclature section at the beginning of the paper

Response 3: A list of acronyms has been added to the paper

Abbreviation	Definition
AM	Additive Manufacturing
WAAM	Wire Arc Additive Manufacturing
EASA	European Union Aviation Safety Agency
LMD	Laser Melting Deposition
SLM	Selective Laser Melting
EBM	Electron Beam Melting
PAW	Plasma Arc Welding
AMS	Aerospace Material Specification
IR	Infrared Spectroscopy
CNC	Computer Numerical Control
MQL	Minimum Quantity Lubrication
UTS	Ultimate Tensile Stress
YS	Yield Strength
Vc	Cutting Speed

 

 

Point 4: It is not clear if the samples were checked for cracking or voids after machining - was this done? If so, please present some results and discussion on this.

 

Response 4: No specific analysis of cracks or voids has been made on the wall specifically tested, but the deposition parameters have been verified. At the time of making the holes to the plate no presence of pores was found inside the material.

Point 5: Please provide some guidance for the community on how to continue this work, especially in setting up formal designed experiments to test repeatability and reliability of results (as previously specified).  

Response 5: This text is included in the further works:

“Other lines of research to consider cover the study of different drilling parameters considering the variance and repeatability with a large size samples experiment analyzing the wear of the drill. Also of interest is the comparative study of the machinability of the material manufactured by this additive technology versus other material deposition technology such as Selective Laser Melting (SLM) and Laser Metal Deposition (LMD).”