Research on the Warping and Dross Formation of an Overhang Structure Manufactured by Laser Powder Bed Fusion
Round 1
Reviewer 1 Report
The paper investigates warping and dross formation in the LPBF process through a combination of experiments and finite element simulations. Overall, the paper lacks novelty, the computational models used are overly simplified, and the conclusions drawn regarding the mechanisms of warping and dross formation are well-understood and not novel. The only part of the paper that is novel is in section 3.6, in my opinion. Some additional comments are as follows:
1. Check clarity and grammar: lines 76-77, 276.
2. Please correct citations in lines 103, 104, 105, and 109.
3. The introduction is too lengthy and needs to be reworded to highlight the state-of-the-art, knowledge gap in the field, and novelty of the work. Simply listing excessive references and results is not recommended, and the author needs to summarize and synthesize these references at a higher level.
4. Line 104: What is the median, D10, and D90 of the particle size?
5. Figure 1a: With the current placement of samples on the build plate, how do the authors account for the influence of sample location? It is well known that sample quality can vary based on the location on the build plate, even when using the same laser parameters.
6. Line 152: The laser absorption rate used in the calculation is the absorption rate of AlSi10Mg powder, which is incorrect. The absorption rate of liquid AlSi10Mg should be used.
7. Line 160: How does the simplified model ensure the accuracy of the calculation? This seems not reasonable. Please rephrase this statement.
8. Lines 262-264: This explanation is unclear. How is the heat transfer rate (downward) related to the laser energy absorption rate? Please clarify.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript "Research on Warping and Dross Formation of Overhang Structure Manufactured by Laser Powder Bed Fusion" presents an experimental approach to evaluate warping and dross formation processes to solve the forming difficulties caused by the overhang structure during Laser Powder Bed Fusion (LPBF) additive manufacturing (AM) processes. In the study, the authors presented the FE model by combining a Gaussian moving heat source to confirm the difference in the thermal physical properties between the AlSi10Mg powder and solid and a layer addition feature. Through experiments and numerical simulation, the Authors studied the influence of the number of forming layers and the overhanging length on the warping and dross formation in the forming process of an overhang structure. The authors provided theoretical support for the subsequent optimisation of the forming overhang structure and proposed an optimisation method to improve the quality of the elements. To realise the optimisation an experimental study was carried out on the LPBF equipment Dimetal-100H developed by the South China University of Technology. AlSi10Mg alloy powder was used as research material. After LPBF manufacturing, the parts were separated from the substrate by EDM cutting and analysed with the optical microscopy. The macro- and microstructure effect was not fully explained by the Authors. It should be more widely analysed, with Light microscopy and SEM to connect the results with the simulations and stress analysis after LPBF. Such a presentation would give a deeper insight into the method and material’s characterisation after the LPBF. The next step of the research was using the finite element method to simulate the temperature field. Temperature is the key factor affecting the size of the molten pool. To study the temperaturę-changing process in different support areas, six points were equally spaced on the first track of the first and third layers. It was found that the temperature gradient in the powder support zone is large, which drives the high residual stress in the material. Moreover, the phases of AlSi10Mg powder and solid phases were also considered in the simulation model and thermophysical properties, including thermal conductivity, density, specific heat capacity, and enthalpy. The materials' thermal, elastic, and plastic strains were also considered for the stress analysis model. It was found that the forming defects are mainly dross formation and warping in the printing process. The dross formation primarily appeared at the bottom of the overhang layer, while the warping especially appeared at the edge of the overhang layer. Another result is that with the increase in moulding layers, the overall distortion of the parts increases while the top warping gradually decreases. The simulation results also showed that the maximum full warping occurs in the second layer.
Generally, the article is well written; however, sometimes, it is hard to follow. Authors should proofread and improve some sections (Introduction, Materials and Methods, Conclusions). The Authors cite literature accordingly. Considering the strengths above, though, as I read the manuscript, some comments arose that I would appreciate when the authors explained. Moreover, before the publishing process starts, the Authors should also improve some editorial aspects to present their results. Some of the editorial comments I have highlighted below:
1. Have the Authors investigated the microstructure of AlSi10Mg after AM process? It should also be emphasized in the introduction or the Results and Discussion Section to compare the results with FE simulation. The introduction should be extended for more examples of the aluminium alloys AM processes, which will introduce the Reader to the research and show need to optimise the LPBF process.
2. Authors should also present some more information on the mechanism of layers formation during LPBF process.
3. The conclusions in the article are too general. I suggest re-editing the conclusions and extending the description of the investigation’s effect on AM processes development and application; Please also highlight the main outlook for the future here.
4. Please format the units correctly.
5. Please describe the formulas’ legends and description more clearly
6. Figure 3. scale needs correction. Alloy samples should be presented more clearly and with higher magnification.
7. It would be good to correlate the simulations with the effect on Al alloy samples and show the structures of the layers after LPBF.
8. Please format the references correctly according to Journal’s Guide for the Authors
I have no more comments that I would like to point out. The article requires some editorial refinement and proofreading. Nevertheless, the manuscript, after corrections, should be interesting to readers from the AM methods field. The issues presented in the article are suitable for publication in the Applied Sciences. I recommend the paper for publication after minor revision.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
I review the paper entitled ” Research on Warping and Dross Formation of Overhang Structure Manufactured by Laser Powder Bed Fusion” and the subject is very interesting and of the actuality.
In order to be clearer, please take into account the following observations:
- Can you explain why is necessary this overhang or we can avoid it? When we print a part, we try to find the best orientation of the part. If there is a part like yours it can be printed with orientation like is presented in figure 3.
- Please check the references: Error! Reference source not found..(lines 103, 104, 105, 109)
- Figure 13 - please change the resolution
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I appreciate the authors' efforts in addressing my comments, and I am mostly satisfied, except for their response to Point 6. The article they cited, "Numerical simulation and parametric analysis of selective laser melting process of AlSi10Mg powder," did not mention that the laser absorption rates in the powder and liquid are similar. In fact, the absorption rates used in that article were based on the authors' assumptions, not on facts. They stated, "Since the absorptivity of aluminum alloy powder is two times higher than that of polished bulk material [49], the absorptivity of 2αA was used in the simulation." However, even in reference [49], there is no evidence to prove that the absorptivity of aluminum powder is twice that of polished surface material. Additionally, it should be noted that the authors only compared the absorption rates of the powder and polished surface in the citation, and did not mention the liquid absorption rate. Due to the aforementioned issues, I cannot agree with the changes made by the authors on this issue unless they provide more compelling evidence.
(p.s., even if the absorption rates in the powder and liquid are similar, the authors' statement in the current version is not accurate. They said, "The laser absorption rate is divided into solid and liquid, but the difference between the two is very small." However, the small difference should be between the absorption rates of the powder and liquid, not between the solid and liquid's absorption rates. This is because the solid absorption rate can vary over a wide range due to differences in surface roughness.)
Author Response
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Reviewer 3 Report
Dear authors,
Thank you for your response and I have only a little recommendation to be made: please change the term of ”molding” in the paper. As I know, molding or moulding process consist in shaping a material in a die, mold or matrix so, in the AM process you don’t need a die (mold or matrix).
The authors take into account about the recommendations from the first review so, after the above minor correction, I suggest that the paper is suitable for publication.
Author Response
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