Mechanical Properties of High Strength Aluminum Alloy EN AW-7075 Additively Manufactured by Directed Energy Deposition

Round 1

Reviewer 1 Report

In this work,  a high strength aluminium alloy has been processed via LMD technology. The work seems to be interesting, but some further efforts are required to improve the quality of the paper. Therefore, the following issues should be considered and addressed:

1. According to ASTM, DED is Directed Energy Deposition, not Direct…… Please modify the word on page 1 line29 and entire text.
2. For the AM processes, please also inset their abbreviations such as AM, DED, PBF,…. Then use the abbreviations in the text.
3. DED processes are normally divided into Powder-feed processes and Wire-feed processes. Therefore, please modify the sentence on page 1, line 30.
4. The terminology which has been used in the paper is not according to the ASTM standard, and it is highly recommended to change them and use the standard one. i.e. instead of Laser beam melting, Laser Powder Bed Fusion (LPBF) should be used.
5. Different names of DED process such as LMD or DMD have been used in the article, which is not acceptable. Since ASTM introduced these processes as DED, it would be better to use this term in the entire paper instead of LMD, DMD, and so on. For this reason, the following works can be used and cited:

• An overview of additive manufacturing of titanium components by directed energy deposition: microstructure and mechanical properties, Appl. Sci. 2017, 7(9), 883; https://doi.org/10.3390/app7090883

6. The state of art of this work is too week, and so some recent works has have been carried out in this direction should be included in the introduction. The application of DED process to repair the high-value components is also critical and should be included in the text. For this reason, the following papers can be used and cited:

• Application of Directed Energy Deposition-Based Additive Manufacturing in Repair, Appl. Sci. 2019, 9(16), 3316; https://doi.org/10.3390/app9163316.

7. In table 1, the chemical composition is in which unit? Wt.% or atom.%? the unit can be mentioned in the heading of the table and then the ‘’in%’’ can be removed from the first row of the table.
8. Which laser focus has been used for the production of single tracks?
9. In Figure 4, in the X and Y-axis there are two errors that should be corrected. The units should be mentioned in line with axis title like Porosity (%). In addition, the first alphabet of the axis titles should be uppercase.
10. In Figure 4, there is a preferred formation of the porosities, why? It seems they have been preferentially formed in the melt pool borders. The large porosities seem to be gas-induced porosity that can be formed as a consequence of alloy element evaporation or residual gas which was entrapped within the starting powder. Has this aspect been considered?
11. How much the findings of the mechanical properties which have been reported in Fig. 5, are comparable with the standard and also previous works?
12. How this kind of protection affects the microstructure of the samples? What is the expectation about the cooling rate with and without using this extra shielding gas system?
13.  

Author Response

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Reviewer 2 Report

The article discusses about research conducted on additive manufacturing of low-defect specimens of Al alloys using laser metal deposition technique by adapting shield gas coverage. The study investigated the influence of the laser power on the porosity of the test samples and their tensile strength.

The reviewer finds the article interesting and it is clear to read and understand, though some obvious lack of use of commas is observed throughout the manuscript.

Specific issues:

1. The title seems to general and fall short of directly reflecting the content of the manuscript.
2. The abstract is too general and the use of expressions "pores can build" and "pore volume can be achieved" do not indicate the direct observation of the researchers.
3. The keywords such as "aluminium" and "tensile testing" are too general.
4. The introduction section lacks in depth analysis and coverage of relevant literature.
5. Designation of vertical and horizontal axes of Figure 3 (left):  "Porosity (%)" and "Laser power (kW)" respectively.
6. Designation of vertical axis of Figure 5:  "Stress (MPa)"
7. (Optional). Consider to merge Section 3 and 4 under a section "Discussion of Results" and present the discussion of the results about porosity issues, hardness tests and tensile tests under separate subsections.
8. The conclusion section is too brief and lacks content. 
9. Line 201 in the conclusion: "... for a slight increase in hardness ..." Can you be more specific? How much did the hardness increase?
10. Line 203 - 205: "When comparing the measured ultimate tensile strength with that of other additively manufactured EN AW-7075 specimens, good agreement is obtained irrespective of whether the specimens were heat treated or not." Where in the manuscript is this comparison discussed?
11. Name of authors for reference 13 is not formatted properly. 

Author Response

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Reviewer 3 Report

The presented paper deals with the preparation, metallurgical quality testing and mechanical properties analysis of high strength aluminium EN AW-7075 manufactured by laser metal deposition. All results are presented in a comprehensible way. The scientific soundness of the paper is on a good level. Please refer to the below mentioned points to the paper before publishing:

• Please enlarge the view of the porous structures in Figure 3; 
• Please provide LMD parameters on Figure 4 and 5 - that they become more visible;
• Please improve Figure 5 - axis markings and units;

Author Response

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Reviewer 4 Report

line 43: you mention that an important source for hydrogen is the ambient air - please add a citation therefore

line 80: use subscripts for D10 etc.

line 111: 'artificial aging'

line 127, Fig. 3: you are showing error bars. Please provide the number of samples used therefore.

You show several measured data with a '±'. Also here provide the number of samples for the specific measurements you have done.

line 174: 'artificially aged'

Author Response

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Reviewer 5 Report

This manuscript presents a study on laser deposition of aluminum. Additional shielding gas shroud is designed to reduce the porosity of the manufactured parts.

1. The porosity was measured at cross sections in this study. The porosity could vary significantly at different locations. How many cross sections were selected for each experiment case? How were the locations selected?
2. There should be more systematic experimental study to confirm the conclusion of this study. To show the effect of new shielding gas setup, there is only one paragraph saying the porosity is reduced from 7% to 5.9%, compared with the common setup. This is not enough to draw the conclusion. More experimental results, such as the effect of shielding gas setup at different laser power, gas flow rate (center and shroud) are needed.
3. What is the mechanism for the new shielding gas setup to reduce the porosity, if it is confirmed? If the key is to protect the manufactured part from the air, is the gas flow rate or cover area more important?
4. What is the distance between the central gas and the shrouds? What is the coverage area of each?  
5. This study says that the hydrogen is the main cause of the pores, and high laser power could reduce the porosity. However, at this power level, improperly closed keyholes could also induce a lot of pores. Can the authors explain why hydrogen is the dominating factor?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The revision is satisfactory. Therefore the paper can be accepted.

Reviewer 2 Report

The authors have addressed my concerns of the first version. However, there still some technical and grammatical errors that I hope can be fixed during the final editorial process.

Reviewer 5 Report

All issues have been addressed.