Strategies to Reduce Porosity in Al-Mg WAAM Parts and Their Impact on Mechanical Properties

Round 1

Reviewer 1 Report

Additive technologies are a modern method for the production of parts for various purposes. However, despite all the advantages, it is still a very expensive method of manufacturing parts. In addition, it is not always possible to achieve the desired level of properties for a number of reasons, which makes the work on the study of additive manufacturing processes very relevant. In this work, the authors use a fairly cheap method for the production of parts. There are a number of comments to the article that do not diminish the practical significance of this study
1) Dimension mark in figures 4 and 5
2) Grain size estimation has not been carried out? There should definitely be some differences in the grain size along the height of the workpiece.
3) "Moreover, temperatures determined for Argon shielding gas, gave 256 rise to comparatively higher weld bead temperatures that for equivalent conditions with 257 Stargold® shielding gas"
With what it can be connected?
4) What is the reason for the increase in the confidence interval in Figure 17
five)    . "This was due to the un-uniformly distributed wall chemical 394 composition and dendritic microstructure with elongated grains along building direction" these conclusions are not supported by experiments. Analysis of the chemical composition is necessary
6) In addition, it is necessary to more clearly show the novelty of the article and the advantages of the proposed method. What is the difference from previous work in this area? Show practical value. Please provide quantitative and qualitative results. Conclusions should reflect the purpose of the article

Author Response

Point 1: Dimension mark in figures 4 and 5.

Response 1: The scale bar has been amplified keeping the same scale relation. 

Point 2: Grain size estimation has not been carried out? There should definitely be some differences in the grain size along the height of the workpiece.

Response 2: During the metallographic examination, the grains demonstrated to be equiaxed and not columnar throughout the height of the sample. The grain size has not been measured, but it seemed similar along the length of the wall without any significant grain growth at the bottom of top layers. 

Point 3: "Moreover, temperatures determined for Argon shielding gas, gave 256 rise to comparatively higher weld bead temperatures that for equivalent conditions with 257 Stargold® shielding gas"
With what it can be connected?

Response 3: The origin of this difference is related to the different ionization energy of both gases. The melt pool temperatures measured for Argon shielding gas were cnstantly higher than for Stargold. The measurements were carried out in a recurrent manner from the obtained experimental data by the pyrometer relating both shielding gases. 

Point 4: What is the reason for the increase in the confidence interval in Figure 17

Response 4: This is due to the scattering of the experimental data. 3 samples were tested for each condition; this explanation has been included in the text (line 210).

 Point 5: "This was due to the un-uniformly distributed wall chemical 394 composition and dendritic microstructure with elongated grains along building direction" these conclusions are not supported by experiments. Analysis of the chemical composition is necessary

Response 5: This statement is not from the work carried out in this paper, it is from the author previously mentioned in the text. Our work demonstrated to have equiaxed grains. No chemical composition analysis was carried out.

Point 6: In addition, it is necessary to more clearly show the novelty of the article and the advantages of the proposed method. What is the difference from previous work in this area? Show practical value. Please provide quantitative and qualitative results. Conclusions should reflect the purpose of the article

Response 6: 

The novelty and the progress beyond the state of the art of this article relies on:

• Circling deposition strategy enables a stable growth and smooth surface in Al WAAM part Nobody has tested this novel strategy of deposition.
• Influence of the shielding gas flow rate has been deeply studied. It has been demonstrated that gas flow rates equal or below 25 l/min can give rise to porosity in long weld beads. The increase to 30 l/min provides a better shielding which reduces the porosity in this long deposits. This has been related to an increase of the melt pool temperature due to heat accumulation. Other authors usually limit the gas flow rate to 25 l/min or lower.
• A direct relationship between the type of gas and deposition strategy has been found ir order to get the lowest porosity values and lowest pore sizes; Argon should be used with circling and Stargold® is recommended for hatching.

Quantitative results are given in terms of porosity percentage, maximum pore diameter and pore size distributions. Also, the influence of the manufacturing conditions and the resulting porosity on mechanical properties and anisotropy is studied. The reasons for explaining the good mechanical properties and low anisotropy levels are introduced:

• Grain refinement due to re-melting the previous layer.
• Lack of clustered pores in the interlayer region.

 

Reviewer 2 Report

Dear Authors,

I have read your paper "Strategies to reduce porosity in Al-Mg WAAM parts and its impact on mechanical properties" carefully.

This paper describes the investigation of the tensile strength of the WAAM fabricated Al-5Mg samples. The general novelty is the new data about the effect of the shielding gas, gas flow rate, and deposition strategy on the porosity and tensile strength.

The paper is easy to read.

But the methods are not properly described, so that other research groups may not reproduce them.

The paper is interesting. However, it requires few corrections.

1. What type of the welding modes did you use (pulse, cold metal transfer or any other)?
2. Please describe acronyms: CMT.
3. Please, add more information about test equipment. It’s recommended to add the paragraph with the type of the optical and electron microscopes, test machines for tensile test of this work to the second section 
4. Please, add the heat input formula. Line 172. Is this average heat input value?
5. Please, add the information about X-ray method and results of the analysis. 

The paper can be accepted for publication only after major improvements.

Author Response

Point 1: What type of the welding modes did you use (pulse, cold metal transfer or any other)?

Response 1: Only CMT welding mode was used as specified in line 147.

Point 2: Please describe acronyms: CMT.

Response 2: Acronym has been specified in line 147.

Point 3: Please, add more information about test equipment. It’s recommended to add the paragraph with the type of the optical and electron microscopes, test machines for tensile test of this work to the second section.

Response 3: Microscope information has been added in line 205 and tensile test machines specifications has been added in line 214.

Point 4: Please, add the heat input formula. Line 172. Is this average heat input value?

Response 4: The heat input has been calculated with the overall parameters. Meaning, this value was not calculated for each instance, general intensity and tension were used for the calculus. The formula has been included in line 197. 

Point 5: Please, add the information about X-ray method and results of the analysis.

Response 5: For the X-rays, a Titán x-ray emission equipment is used with 100 kV and 15.7 mA (information included in line 212). "Manufactured walls showed no presence of pores greater than 0.3 mm of diameter, lack of fusion or other internal defects."(line 355). Minimum measurable pore diameter in x-ray analysis is 0.3 mm. Do you recommend to include the images of x-ray analysis?

Reviewer 3 Report

This paper presents a study on the influence of shielding gas and building strategy on the porosity and mechanical properties of Al-Mg WAAM parts. The subject is interesting since the WAAM method could be a very good additive manufacturing method for large parts.

Many results are presented in this paper but there are two main issues:

. a lot of practical aspects are not sufficiently described

. there is a lack of discussion on the major results of the paper

Concerning the first point, please find the comments

. Figure 4 and 5 : the legend is not clear enough

. Table 4 : I think there are too many “nonmeaningful” numbers in the porosity results. Moreover, what is the accuracy of the measurement

. What is the minimum pore size measurable ?

. I am not sure that the distribution of the pores is changing with gas flow rates (form aligned to isolated). Isn’t it due to the smaller amount of pores that it appears this way ?

. concerning the mechanical properties : how many tests have been carried out by condition ? For the strain, it seems that the authors did not choose the true values so I am not sure about the comparison between the curves….

For the discussion :

. I think the properties of the base metal should be added for the sake of comparison

. I do not understand the sentence line 401 about the fact that remelting increased the solubility of the alloying elements (it does not make sense to me)

. I think there is a lack of discussion about why the porosity fraction changes with the manufacturing parameters

Author Response

Point 1: Figure 4 and 5 : the legend is not clear enough

Response 1: The scale bar has been amplified keeping the same scale relation.

Point 2:  Table 4 : I think there are too many “nonmeaningful” numbers in the porosity results. 

Response 2: These numbers are part of a qualitative analysis to show relative improvement that support what show figures 9 and figure 10. The improvement of porosity results because of the increase of the shielding gas flow rate can be seen.

Point 3: What is the minimum pore size measurable ?

Response 3: Only pores greater than 10 µm in diameter were measured. It has been included in the text (line 204).

Point 4: I am not sure that the distribution of the pores is changing with gas flow rates (form aligned to isolated). Isn’t it due to the smaller amount of pores that it appears this way ?

Response 4: Explanation of this effect has been improved in the text (line 271):

“For the lowest shield gas flow rates, high dimensioned pores tended to be aligned and concentrated in the interlayer bands with smaller pores distributed randomly in the part, whereas for 30 l/min only the smaller, isolated and randomly distributed pores appeared.”

Moreover, images of this effect have been added (Figure 11).

Point 5: concerning the mechanical properties : how many tests have been carried out by condition ? For the strain, it seems that the authors did not choose the true values so I am not sure about the comparison between the curves….

Response 5: Three samples were tested for each condition; this explanation has been included in the text (line 210). For the strain, the values shown are the Engineering values. This has been included and specified in Figure 18.

Point 6: I think the properties of the base metal should be added for the sake of comparison

Response 6: Base metal does not influence in the part manufactured. Moreover, base metal is not included in the mechanical property testing, since first and last deposited layers are dismissed in the extraction of the specimens and the “reduced” section of the sample is far from the first layers.

Point 7: I do not understand the sentence line 401 about the fact that remelting increased the solubility of the alloying elements (it does not make sense to me)

Response 7: This statement has been extracted from the reference previously mentioned in the text. We have not carried out this testing.

Point 8: I think there is a lack of discussion about why the porosity fraction changes with the manufacturing parameters

Response 8: Working with flow rates below 30 l/min can give rise to high porosity, especially when long tracks and many layers are supposed in height.

Reviewer 4 Report

The article is interesting and seems to me to be well written, however it contains some flaws that I pointed out in the attached pdf.

Comments for author File: Comments.pdf

Author Response

Response 1: Mistake has been corrected in the text (line 45).

Response 2: This paper does not study the CMT arc modes, therefore it does not add value to explain them.

Response 3: In the .word uploaded, these line numbers do not appear. They are the line numbers that for some reason were misplaced.

Response 4: In the .word uploaded, these line numbers do not appear. They are the line numbers that for some reason were misplaced.

Response 5: The letter “l” has been changed to capital letter “L” for better comprehension and misunderstanding with the number 1.

Response 6: This mistake has been erased from the text.

Response 7: This sentence can be confusing. It has been modified to “The impact was especially relevant for the Ar circling and Stargold® hatching conditions that shown the lowest porosity levels below 06% and 0.12% in area, respectively.”

Response 8: The origin of this difference is related to the different ionization energy of both gases. The melt pool temperatures measured for Argon shielding gas were constantly higher than for Stargold. The measurements were carried out in a recurrent manner from the obtained experimental data by the pyrometer relating both shielding gases.

Response 9: It was observed that because of the geometry of the part, the horizontal part had a higher temperature accumulation and therefore the porosity is increased. Explained in the line 344: “Whereas for the vertical wall, the average melt pool temperature keeps constant in different layers, for the horizontal one with longer weld beads, the temperature increases constantly after initial layers”.

Response 10: Naming sequence has been specified in the title of the figure.

Response 11: These are the layer numbers. The interval value has been changed to reduce the number of labels in the horizontal axes.

Response 12: 3 samples were tested for each condition; this explanation has been included in the text (line 210).

Reviewer 5 Report

The submission has enough novelty to get priority for publication in Metals. However, some minor points need to be addressed/modified within the revised text:

(1) The number of reviewed refs is limited and must be extended by adding relevant refs.

(2) For Fig. 18, it is recommended to add stress-strain curves for all fabricated samples rather than just presenting only one example.

(3) In Table 6, units are missing.

(4) In Table 5, the unit for porosity measurement is missing.

(5) The applied scale bars for Figs. 9 and 10 are not visible enough.

(6) Scale bar for images in Fig. 7 is missing.

(7) Acceptable variation range of measurements for values in Fig. 6 are necessary. 

(8) It is recommended to add some parts on the effect of alloying elements on the porosities formation considering the applied processing parameters.

 

Author Response

Point 1: The number of reviewed refs is limited and must be extended by adding relevant refs.

Response 1: The following additional references have been added:

Wang and Y. Zhang, “A review of aluminum alloy fabricated by different processes of wire arc additive manufacturing,” Medziagotyra, vol. 27, no. 1, pp. 18–26, 2021, doi: 10.5755/j02.ms.22772.

R. Cunningham, J. M. Flynn, A. Shokrani, V. Dhokia, and S. T. Newman, “Invited review article: Strategies and processes for high quality wire arc additive manufacturing,” Addit. Manuf., vol. 22, pp. 672–686, 2018, doi: 10.1016/j.addma.2018.06.020

Pan, D. Ding, B. Wu, D. Cuiuri, H. Li, and J. Norrish, “Arc Welding Processes for Additive Manufacturing: A Review,” no. 2, pp. 3–24, 2018, doi: 10.1007/978-981-10-5355-9_1

Ivántabernero, A. Paskual, P. Álvarez, and A. Suárez, “Study on Arc Welding Processes for High Deposition Rate Additive Manufacturing,” Procedia CIRP, vol. 68, no. April, pp. 358–362, 2018, doi: 10.1016/j.procir.2017.12.095

S. Derekar et al., “Effect of pulsed metal inert gas (pulsed-MIG) and cold metal transfer (CMT) techniques on hydrogen dissolution in wire arc additive manufacturing (WAAM) of aluminium,” Int. J. Adv. Manuf. Technol., vol. 107, no. 1–2, pp. 311–331, 2020, doi: 10.1007/s00170-020-04946-2

Horgar, H. Fostervoll, B. Nyhus, X. Ren, M. Eriksson, and O. M. Akselsen, “Additive manufacturing using WAAM with AA5183 wire,” J. Mater. Process. Technol., vol. 259, no. March, pp. 68–74, 2018, doi: 10.1016/j.jmatprotec.2018.04.014

Fang et al., “Correlations between Microstructure Characteristics and Mechanical Properties in 5183 Aluminium Alloy Fabricated by Wire-Arc Additive Manufacturing with Different Arc Modes,” Materials (Basel)., vol. 11, no. 11, p. 2075, 2018, doi: 10.3390/ma11112075

J. Gu, B. Cong, J. Ding, S. W. Williams, and Y. Zhai, “Wire-arc additive manufacturing of aluminium_Cranfield_2014,” 2014

Point 2: For Fig. 18, it is recommended to add stress-strain curves for all fabricated samples rather than just presenting only one example.

Response 2: If all fabricated samples results are included, an overlapping of the curves happens and no clear result can be seen. There are no outstanding differences between the curves. The main reason why one curve example was included has been to prove and show a case of the Portevin Le-Chantellier effect. This happened in every tested sample.

Point 3: In Table 6, units are missing.

Response 3: In table 6 units are specified in the title and in next to the name of the measurement “Anisotropy (%)”.

Point 4: In Table 5, the unit for porosity measurement is missing.

Response 4: In table 5, units were specified in the title but the percentage symbol (%) has been included next to “porosity”.

Point 5: The applied scale bars for Figs. 9 and 10 are not visible enough.

Response 5: The scale bar has been amplified keeping the same scale relation.

Point 6: Scale bar for images in Fig. 7 is missing.

Response 6: A new image with a ruler has been taken.

Point 7: Acceptable variation range of measurements for values in Fig. 6 are necessary.

Response 7: These measurements are single welding beads dimensions. Only one measurement has been taken from the samples shown in figure 4 and 5 from the steady zone of the welding (beginning and ends are dismissed).

Point 8: It is recommended to add some parts on the effect of alloying elements on the porosities formation considering the applied processing parameters.

Response 8: Only one aluminium alloy has been studied (5356). The chemical composition has not been measured. We are currently working on the manufacturing of other aluminium alloys (2219 and 2319) with 6% copper content with greater influence on porosity since the solubility coefficient of hydrogen is reduced.

Round 2

Reviewer 1 Report

Thanks for the changes you made. article can be accepted

Reviewer 2 Report

Dear Authors,

I have read your modified paper "Strategies to reduce porosity in Al-Mg WAAM parts and its impact on mechanical properties" carefully.

The materials and methods are properly described, so that other research groups may reproduce them. Explanations are clear and the paper is easy to read.

I can recommend the Editor to accept this revised manuscript to be published in Metals