Numerical Analysis of Laser Pattern Effects to Residual Stress on Metal 3D Printing

Round 1

Reviewer 1 Report

The paper addresses FE simulation of powder-bed based laser additive manufacturing, and the results of FE simulation of AM have been compared with the experimental investigations of a sample manufactured by AM process on Ti64 material. The paper presents a good approach which can sufficiently shorten the lead time. The paper is worth publishing after addressing the following questions.

Title of the paper can be improved to reflect the topic covered. The current title is too general and the authors may consider adding the investigated effects instead of ‘effects’. It can better guide the reader as to which type of effects are being discussed in this particular study.

It seems that only three layers have been simulated and manufactured experimentally for the comparison, which seems rather sceptic towards the application of this study for process optimization. As the process is not a nano-manufacturing process; but a micro-level process, the number of layers in my opinion should be high enough so as to avoid the effects of distortion and to ensure structural integrity. Though 300 um may be considered enough but in the case when it covers 7-10 layers. In the current case, layer size is 100 um. In my opinion, the study should have included another three layers so as random effects could have been avoided. The authors should address the issue, and relate it to the separation of the part from the base plate, which may also affect the magnitude of residual stresses.

Author Response

Thanks for your comments. Please find the attachment for the answer.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper presents a numerical study on the laser pattern effects on metal 3D printing. Both the numerical and experimental details are well presented and analyzed. The paper can be recommended for publication after addressing the following questions.

1. In introduction section, it is mentioned ‘cracks occur owing to the stress and distortion generated by the laser path 44 when printed at a length longer than the thickness’. It is not clear why this statement stands.

2. In the literature survey, the inherent strain-based fast simulation method should be stated since it is the main-stream for practical process simulation and residual stress control. The following publications are recommended.

Takezawa, A., To, A.C., Chen, Q., Liang, X., Dugast, F., Zhang, X., Kitamura, M. (2020) Sensitivity analysis and lattice density optimization for sequential inherent strain method used in additive manufacturing process. Computer Methods in Applied Mechanics and Engineering, 370, 113231.

Xu, S., Liu, J., & Ma, Y. (2022). Residual stress constrained self-support topology optimization for metal additive manufacturing. Computer Methods in Applied Mechanics and Engineering, 389, 114380.

3. Figure 1 is unclear to read. The meanings of the differently-colored arrows should be annotated and explained.

4. It is mentioned that phase changes were considered, but it is unclear about how?

5. Analysis should be added to explain the phenomenon in Fig. 6.

6. In Fig. 8c, it is unclear why the disordered residual stresses happen to the top surface.

7. In Fig. 9, the location of the CT scanning section should be explained. The current statement ‘CT scan results at a height of 1 mm from the base plate:’ is unclear. It seems only three layers of materials are printed with the total height of only 0.3 mm.

8. Simulation time information is better provided.

Author Response

Thank you for your careful review.

Thanks to this, I was able to write a better and more scientific paper.

I have attached and replied to your inquiry and what you pointed out.

--------------

Reviewer1

1. In introduction section, it is mentioned ‘cracks occur owing to the stress and distortion generated by the laser path 44 when printed at a length longer than the thickness’. It is not clear why this statement stands.

• The description was inaccurate and the content was corrected as follows.
• The residual stress generated causes deformation and cracking in the entire material, and cracks occur owing to the stress and distortion generated by the laser path when printed at a high aspect ratio shape.

2. In the literature survey, the inherent strain-based fast simulation method should be stated since it is the main-stream for practical process simulation and residual stress control. The following publications are recommended.

• The contents of the recommended papers were added to the literature survey.
• Takezawa analyzed the residual stress of metal 3D printing materials using the inherent strain method and conducted optimization studies to minimize deformation by applying a porous grid structure.[7] However, the deformation of the structure by heat was applied only to the initial inherent strain. Since the strain application per layer and heat transfer to adjacent structures are not considered, detailed analysis is limited.

3. Figure 1 is unclear to read. The meanings of the differently-colored arrows should be annotated and explained.

• The meaning of the arrow added to the caption.

4. It is mentioned that phase changes were considered, but it is unclear about how?

• The density, specific heat, coefficient of thermal expansion, elastic modulus, and plasticity curve were set to be applied differently depending on the temperature when the temperature change in the solid state and the phase change from solid to liquid. Due to the lack of space, it was not possible to prepare all the information on physical properties. For more details, refer to Reference No. 10.

5. Analysis should be added to explain the phenomenon in Fig. 6.

• As advised, the analysis on the change in residual stress has been added to the text as follows.
• When the Laser was irradiated at the second and third layer, residual stress was relaxed by the heat of the upper layer instantaneously and restored to a different value. Based on these results, it was confirmed that the cooling rate was changed depending on the location and the temperature of the element, thereby affecting the residual stress.

6. In Fig. 8c, it is unclear why the disordered residual stresses happen to the top surface.

• Fig. 8c is set to a rotation angle of 67 degrees, and a pattern with a similar angle does not appear up to a maximum of 8 layers. Under the conditions of 180 degrees and 90 degrees, the same pattern was applied in the adjacent layer and the residual stress was amplified. In the 67 degree condition, the pattern angles of each layer were different and overlapped irregularly. The 45 degree condition looked similar to the 90 degree condition because of the scale setting, but based on the maximum stress and average stress, it had a dispersion value similar to 67 degrees. In this regard, content has been added to the text.

7. In Fig. 9, the location of the CT scanning section should be explained. The current statement ‘CT scan results at a height of 1 mm from the base plate:’ is unclear. It seems only three layers of materials are printed with the total height of only 0.3 mm.

• The processed specimen was printed at a height of 5 mm for the convenience of work, and the data on the size of the workpiece were prepared in Table 2. The CT measurement result was confirmed at the lower 1mm height. The reason for this is to ignore the cracks that occur when the lower support is separated from the base plate. In this regard, content has been added to the text.

8. Simulation time information is better provided.

• Contents related to analysis and required time have been added as follows.
• The CPU used for the analysis was Intel's i9 product, and it was performed in an environment equipped with 6 cores of 3.4 GHz and 64 GB of RAM. The analysis model took an average of 80 hours per case.

Reviewer 3 Report

The topic of this article is very current. 3D metal printing is a promising technology and numerical modeling certainly has something to offer in this regard.

The technique used to create the FEM model is very suitable, I have only a few questions and comments about your work:

• Did you try to use the described technique also for a problem on a larger scale?
• Occasionally there are minor errors in the work, such as a lowercase letter at the beginning of a sentence in line 64. Occasional language formulations are cumbersome.
• In the abstract, you mentioned that the results were compared with the measurement. I also recommend stating how the results were measured here. This is explained later, but it would be appropriate to mention it (method) in the abstract as well.
• Is your numerical model applicable, for example, for the laser cladding technique (also additive technology)?
• Did you tested also different angles of rotation?
• The results presented in Figure 4 are not well readable. The size and also color combination are not good.

Author Response

Thank you for your careful review.

Thanks to this, I was able to write a better and more scientific paper.

I have attached and replied to your inquiry and what you pointed out.

------------

1. Did you try to use the described technique also for a problem on a larger scale?

• As mentioned in the conclusion, I think that it can be applied to larger-scale shapes in the future. In particular, in the case of PBF processing using the Chess board technique having a unit grid of 5 mm * 5 mm as shown in this analysis, it will be easily accessible. We will analyze it through future research.

2. Occasionally there are minor errors in the work, such as a lowercase letter at the beginning of a sentence in line 64. Occasional language formulations are cumbersome.

• The typo pointed out was corrected and the entire document was checked.

3. In the abstract, you mentioned that the results were compared with the measurement. I also recommend stating how the results were measured here. This is explained later, but it would be appropriate to mention it (method) in the abstract as well.

• As recommended, it was added to the abstract that the residual stress was measured by XRD.        

4. Is your numerical model applicable, for example, for the laser cladding technique (also additive technology)?

• It is expected that it can be applied to research confirming the heat distribution of laser cladding through more research. In particular, it seems necessary to study the method of increasing physical property information and convergence.

5. Did you tested also different angles of rotation?

• A test was conducted on angles that are typically used in several metal 3D printing machines, and it was composed based on how many times a pattern of a similar angle appeared after several times. 180 degrees is the condition that it should be laminated once, 90 degrees is twice, 45 degrees is 4 times, and 67 degrees is 8 times or more. The other angles were not progressed because the difference from these four angles was not large.

6. The results presented in Figure 4 are not well readable. The size and also color combination are not good.

• Edited to make it bigger and more visible.