Simulation Analysis of Nanosecond Laser Processing of Titanium Alloy Based on Helical Trepanning

Round 1

Reviewer 1 Report

The terminology used in the article is inappropriate. For example, "the Rayleigh length" or "Gaussian ligth". According to the generally accepted nomenclature that can be found in books such as: "Lasers" by A.E. Siegman, "Solid-State Laser Engineering" by W. Koechner, or "Principles of Lasers" by O. Svelto, "Rayleigh range" and “Gaussian beam" should be used, respectively. Similarly, in the introduction, the authors wrote "linear scanning (percussion), ring scanning, and spiral scanning", and in accordance with the adopted nomenclature, for example, the book "Laser Material Processing" by W.M. Steen and J. Mazumder, there should be percussion, treapanning and helical trepanning. Particularly ambiguous is the wording "linear scanning (percussion)", because it is not known whether it is a combination of scanning along the line (linear scanning) with a series of pulses at each successive step (percussion), or "percussion" refers to successive pulses in along the entire scan line (linear scanning).

Perhaps this problem would quickly disappear if the authors cited several fundamental works on laser processing of materials from the nineties and earlier, by scientists from research centers in Germany or the USA. Although the reviewed article has a rich list of citations, the vast majority of authors from Asia, and moreover, many of them concern processing with femtosecond lasers, which differs significantly from processing with nanosecond lasers. The latter is the subject of the article.

In the introduction there is also the phrase "the material first absorbs laser energy through the lattice and converts it into the lattice heat" which is not true. The lattice does not absorb and the electrons absorb, and only in the next stage the energy is transferred to the lattice with a certain delay. In the same sentence appears "which is then transferred to the surrounding lattice through thermal radiation" which is completely incomprehensible. Did the authors mean phonon processes?

The sentence "Heat removal and material evaporation, and diffusion are the main removal mechanisms in this type of processing" is also incomprehensible. It takes a long time to think about what the authors meant and it is uncertain whether the interpretation will be correct.

Similarly, in chapter 3.2.1, the following sentence is incomprehensible and inconsistent with the generally accepted laser terminology: "In addition, since the focusing energy of a nanosecond laser changed slightly in the optical axis direction, in the longitudinal axis direction, it changed significantly".

Later in chapter 3.2.3, "the fluid velocity of the metal vapor" is incomprehensible.

The interpretation of the results given by the authors in section 3.2.4 a), presented in Figure 5, is incomprehensible. At this point, it should be mentioned that authors often use the term "morphology" in various contexts, which, due to the lack of an explicit definition, may confuse the reader and make the text incomprehensible. Moreover, there is no explicit definition of the width and depth of morphology, the latter being easy to guess unambiguously.

In the second chapter, the authors present the model for which they conducted numerical simulations. The presentation is too general. There was no mention of any computational algorithms or what software was used. The authors provided six formulas that they used in the calculations, but the explanations of the symbols used in most cases are missing, for example d(j), h(T - T₀), or C_p. Can the liquid phase be neglected in modeling the interaction of nanosecond pulses? After all, the recoil pressure causes the ejection of molten droplets of material and, as a result, a significant loss of mass, similarly but later the Marangoni effect.

A serious shortcoming in the second chapter is the lack of material constants used in the calculations, which makes it impossible to verify the results of the calculations in any way.

In chapter 3.1, the authors provide a number of default parameters, which, however, differ from those for which the calculations were performed. It is completely incomprehensible to give power, not pulse energy and pulse repetition rate. For example, for the power of 18 W given by the authors, we can have a pulse energy of 1.8 mJ and a repetition rate of 10 kHz, and otherwise 1.8 J and 10 Hz, respectively. The power is the same and the interaction is completely different. The description used by the authors would be correct in the case of femtosecond rather than nanosecond processing, which is the subject of the peer-reviewed article. The repetition rate is also very important as it, in combination with the laser spot-size and the scanning speed, provides the so called overlap factor or beam overlap. However, the authors did not provide any of these values ​​in the chapter describing the simulation parameters. These parameters can only be found in chapter 3.3 devoted to the experimental results. But do the values ​​quoted there also apply to the simulation? This cannot be stated unequivocally.

Since chapter 3.1 does not mention the pulse energy or the laser spot-size, the laser fluency cannot be determined, which is one of the most important parameters in the laser-matter interaction.

In chapter 3.2.4 b) the authors write "the increase in the scanning speed reduced the overall energy density". What does "overall energy density" mean in this case? If the authors want to introduce such a concept, it must be defined. I have the impression that it is a parameter called "beam overlapping".

In chapter 3.2.4 c) the authors describe the simulation results for the end path position from 70 µm to 110 µm, while in Figure 8, which presents these results graphically, we have 200 and 240 µm. Where do such discrepancies come from?

From among the presented figures, figures 2, 6 and 9 are hardly legible. In the first two figures, it is difficult to see the isotherm lines, in the last one, the 3D morphology image is blurry, and moreover, it is not known what case it concerns.

Chapter 4 presents the conclusions. They are too general. In two out of three cases, it was not necessary to run these simulations to produce them. Interestingly, the authors conclude the impact of the pulse energy on the laser processing. However, simulations were carried out for power, not to mention the changes in the repetition frequency in the description. In general, two computational cases are, in my opinion, not enough to draw conclusions about any trend. The authors consistently present graphically the simulation results for width and depth versus time for only two parameter values, which were sequentially power, scan speed, and the end path. It would be interesting and telling a lot about the trends to present the width and depth as a function of the above parameters for the specified times.

Comments for author File: Comments.docx

Author Response

Dear Editors and Reviewers:

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Simulation analysis of the nanosecond laser processing titani-um alloy based on spiral scanning” (ID: applsci-1863818). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made corrections which we hope meet with approval. Revised portions are marked in red in the revised manuscript and the revised supporting information. The main corrections and the responses to the reviewers’ comments are as follows:

Responses to reviewer #1:

Thank you for your comments on our manuscript. Your suggestions will definitely improve the quality of the paper. The questions are answered as follows. The revision in the manuscript is marked by red font.

Q1: The terminology used in the article is inappropriate. For example, "the Rayleigh length" or "Gaussian ligth". According to the generally accepted nomenclature that can be found in books such as: "Lasers" by A.E. Siegman, "Solid-State Laser Engineering" by W. Koechner, or "Principles of Lasers" by O. Svelto, "Rayleigh range" and “Gaussian beam" should be used, respectively. Similarly, in the introduction, the authors wrote "linear scanning (percussion), ring scanning, and spiral scanning", and in accordance with the adopted nomenclature, for example, the book "Laser Material Processing" by W.M. Steen and J. Mazumder, there should be percussion, treapanning and helical trepanning. Particularly ambiguous is the wording "linear scanning (percussion)", because it is not known whether it is a combination of scanning along the line (linear scanning) with a series of pulses at each successive step (percussion), or "percussion" refers to successive pulses in along the entire scan line (linear scanning).

A: We changed the name of the laser processing method in the article to the name used in the references

 

Q2: Perhaps this problem would quickly disappear if the authors cited several fundamental works on laser processing of materials from the nineties and earlier, by scientists from research centers in Germany or the USA. Although the reviewed article has a rich list of citations, the vast majority of authors from Asia, and moreover, many of them concern processing with femtosecond lasers, which differs significantly from processing with nanosecond lasers. The latter is the subject of the article.

A: We have greatly revised the citation and introduced the research of researchers in many countries on this issue.

 

Q3: In the introduction there is also the phrase "the material first absorbs laser energy through the lattice and converts it into the lattice heat" which is not true. The lattice does not absorb and the electrons absorb, and only in the next stage the energy is transferred to the lattice with a certain delay. In the same sentence appears "which is then transferred to the surrounding lattice through thermal radiation" which is completely incomprehensible. Did the authors mean phonon processes?

A: We deleted this sentence. We originally wanted to explain that the energy absorption process of nanosecond laser is consistent with that of ultrafast laser, but the short heat transfer process can be ignored in the simulation due to the long pulse time. It seems a little superfluous

 

Q4: The sentence "Heat removal and material evaporation, and diffusion are the main removal mechanisms in this type of processing" is also incomprehensible. It takes a long time to think about what the authors meant and it is uncertain whether the interpretation will be correct.

A: We rewrite this sentence as follows: In the nanosecond laser processing process, the main processing mechanism is the evaporation of the processed material and the re-condensation of the material after melting

 

Q5: Similarly, in chapter 3.2.1, the following sentence is incomprehensible and inconsistent with the generally accepted laser terminology: "In addition, since the focusing energy of a nanosecond laser changed slightly in the optical axis direction, in the longitudinal axis direction, it changed significantly".

A: We rewrite this sentence as follows: In addition, changes in the pit’s width and depth were inconsistent, which can be observed from the energy distribution of the Gaussian light, The laser energy decreases exponentially from the center of the beam to the boundary of the spot

 

Q6: Later in chapter 3.2.3, "the fluid velocity of the metal vapor" is incomprehensible.

A: We revised our interpretation of this: ‘the metal material evaporates under the action of the laser to generate metal vapor, which is sprayed from the bottom of the small hole to the outside of the small hole un-der the pressure difference and diffuses into the surrounding air.’ And ‘The velocity field simulation results show that the metal wall of the hole shaft is much higher in the steam velocity or other areas. This may be related to the Gaussian distri-bution of laser energy. Because the laser energy in the center of the laser beam is high, the central part of the material is easy to gasify. The metal vapor in this part is dense and the generated vapor pressure is also the largest. Therefore, the metal vapor diffu-sion rate in the center of the spot is the largest and the material removal rate is the highest.’

 

Q7: The interpretation of the results given by the authors in section 3.2.4 a), presented in Figure 5, is incomprehensible. At this point, it should be mentioned that authors often use the term "morphology" in various contexts, which, due to the lack of an explicit definition, may confuse the reader and make the text incomprehensible. Moreover, there is no explicit definition of the width and depth of morphology, the latter being easy to guess unambiguously.

A: We modified the 'morphology to the small hole’s shape

 

Q8: In the second chapter, the authors present the model for which they conducted numerical simulations. The presentation is too general. There was no mention of any computational algorithms or what software was used. The authors provided six formulas that they used in the calculations, but the explanations of the symbols used in most cases are missing, for example (), h(T - T0), or Cp. Can the liquid phase be neglected in modeling the interaction of nanosecond pulses? After all, the recoil pressure causes the ejection of molten droplets of material and, as a result, a significant loss of mass, similarly but later the Marangoni effect.

A: We added explanations for missing parameters and explained some complex formula terms. In this paper, the liquid phase is not neglected, on the contrary, the solid phase is regarded as a liquid with extremely high dynamic viscosity with relevant hot blood parameters.  We add ‘In the type, is laser power;  is the laser absorption rate of the material;  is the effective radius of laser beam, z is defocus quantity,  is the path function of laser scanning,  is the level set function which indicates that the energy occurs at the interface,  is latent heat of evaporation,  is Stefan-Boltzmann constant,  is radiation coefficient, is convective heat transfer coefficient,  stands for latent heat of melting,  is universal gas constant,  is gas-liquid transition temperature,  represents heat loss caused by evaporation of metal materials,  and respectively represent energy loss caused by thermal radiation and thermal convection and  stands for heat loss caused by melting of metal materials. refers to the term of heat loss, mainly including thermal radiation and thermal convection loss, material evaporation loss and material melting heat loss. The evaporation loss and thermal convection loss of materials only occur on the surface directly affected by laser, so the level set function is used to control the loss at the gas-liquid interface of materials, and the melting loss and thermal radiation loss of materials affected by heat play a role on the whole surface.’ In chapter 2.

 

Q9: A serious shortcoming in the second chapter is the lack of material constants used in the calculations, which makes it impossible to verify the results of the calculations in any way.

A: We supplement the material parameters used in the simulation as follows:

Q10: In chapter 3.1, the authors provide a number of default parameters, which, however, differ from those for which the calculations were performed. It is completely incomprehensible to give power, not pulse energy and pulse repetition rate. For example, for the power of 18 W given by the authors, we can have a pulse energy of 1.8 mJ and a repetition rate of 10 kHz, and otherwise 1.8 J and 10 Hz, respectively. The power is the same and the interaction is completely different. The description used by the authors would be correct in the case of femtosecond rather than nanosecond processing, which is the subject of the peer-reviewed article. The repetition rate is also very important as it, in combination with the laser spot-size and the scanning speed, provides the so called overlap factor or beam overlap. However, the authors did not provide any of these values in the chapter describing the simulation parameters. These parameters can only be found in chapter 3.3 devoted to the experimental results. But do the values quoted there also apply to the simulation? This cannot be stated unequivocally. Since chapter 3.1 does not mention the pulse energy or the laser spot-size, the laser fluency cannot be determined, which is one of the most important parameters in the laser-matter interaction.

A: We add ‘The laser power was 18 W, the pulse energy is 1.8 mJ, the laser spot-size is 27 μm, the repetition rate is 10 kHz, the scan speed was 2.4 m/s, and the number of iterations was 20.’

 

Q11: In chapter 3.2.4 b) the authors write "the increase in the scanning speed reduced the overall energy density". What does "overall energy density" mean in this case? If the authors want to introduce such a concept, it must be defined. I have the impression that it is a parameter called "beam overlapping"

A: We change "overall energy density" to "beam overlapping"

 

Q12: In chapter 3.2.4 c) the authors describe the simulation results for the end path position from 70 µm to 110 µm, while in Figure 8, which presents these results graphically, we have 200 and 240 µm. Where do such discrepancies come from?

A: The end path position is the end radians of the helical, the width of the hole is the diameter of the helical.

 

Q13: From among the presented figures, figures 2, 6 and 9 are hardly legible. In the first two figures, it is difficult to see the isotherm lines, in the last one, the 3D morphology image is blurry, and moreover, it is not known what case it concerns.

A: We modified the picture, and the thick green line indicates the location of the isotherm. The analysis of isotherm is added. Like:

“The green line indicates the isotherm of the melting temperature (1670k) of the materi-al, and the material between the isotherm and the dark blue (air) is the material still in the molten state at the current time. It can be seen from Figure. 6 that the thickness of the molten pool near the center of the spot at 17.5w laser power is slightly thinner than that of 15W laser power. This happens because the laser power of 17.5w makes the re-moval speed of the material in the center of the spot greater than that of 15W. And there is not much difference in the thermal diffusion rate between the two because the center temperature is close to the evaporation temperature of the material”

 

Q14: Chapter 4 presents the conclusions. They are too general. In two out of three cases, it was not necessary to run these simulations to produce them. Interestingly, the authors conclude the impact of the pulse energy on the laser processing. However, simulations were carried out for power, not to mention the changes in the repetition frequency in the description. In general, two computational cases are, in my opinion, not enough to draw conclusions about any trend. The authors consistently present graphically the simulation results for width and depth versus time for only two parameter values, which were sequentially power, scan speed, and the end path. It would be interesting and telling a lot about the trends to present the width and depth as a function of the above parameters for the specified times.

A: We supplement the advantages of the simulation model over the original drilling model and point out the application scope.

(1) A two-phase flow heat transfer model for nanosecond laser processing of tita-nium thin plate was established. The simulation model of microporous perforation is provided, which makes up the deficiency of deformation geometry simulation for per-foration simulation.

(2) The model described the change of micropore shape under the spiral path and simulated the temperature distribution in the plate. By analyzing the isotherm and the shape of micropores, the movement behavior of molten pool is explained, and the thickness of recast layer is displayed intuitively.

(3) The shape of the morphology depends on the distribution of la ser energy. The shape obtained in the actual machining is consistent with the shape predicted by the simulation model. The flow change in the molten pool in the model explains the for-mation principle of a small number of fillets at the entrance of the material and the mi-cro hole taper in the micro hole machining.

(4) With the increase of laser energy, the material removal rate, the depth and di-ameter of the morphology increase. The effect of laser energy on the morphology is more significant than other process variables. The relationship between laser power and scanning speed and the depth and width of micropores was established.

(5) The simulation model can be used as a prediction tool to select the best optical parameters to ensure the product quality and the lowest cost in laser processing. In or-der to ensure the high efficiency and quality of laser processing, we should first select the appropriate laser energy parameters (such as the type and power of the laser), and then control the scanning speed, scanning path and other parameters to optimize the processing process.

 

We would like to thank you again for taking the time to review our manuscript.

 

We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval. Thank you very much for your comments and suggestions.

Author Response File: Author Response.docx

Reviewer 2 Report

Dear authors, I consider that your manuscript needs major revision. Please see the remarks presented in the attached review document.

Comments for author File: Comments.pdf

Author Response

Dear Editors and Reviewers:

Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Simulation analysis of the nanosecond laser processing titani-um alloy based on spiral scanning” (ID: applsci-1863818). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made corrections which we hope meet with approval. Revised portions are marked in red in the revised manuscript and the revised supporting information. The main corrections and the responses to the reviewers’ comments are as follows:

 

Responses to reviewer #2:

Thank you for your comments on our manuscript. Your suggestions will definitely improve the quality of the paper. The questions are answered as follows. The revision in the manuscript is marked by red font.

Q1:

A: Thank you for your advice. We revised the introduction and the final results are as follows: Titanium alloy has been a commonly used material in the aerospace field, but it often requires high-precision processing in the modern manufacturing industry [1,2]. Laser processing is a new special processing technology with high processing accuracy, which has been developed in recent decades [3-5]. However, different laser processing methods achieve different processing accuracy and effect. Laser scanning methods mainly include doughnut beam mode [6]linear scanning (percussion), helical drilling methods [7], and laser trepanning drilling [8]ring scanning, and spiral scanning meth-ods [6]. Laser trepanning drilling Linear scanning methods have the advantage of high machining efficiency but a drawback of insufficient machining quality. The doughnut beam modering scanning methods can easily lead to the formation of a recast layer and cause the repeated thermal effect. The helical drilling methodsspiral scanning methods have the advantages of controllable energy distribution and good machining quality but a relatively poor processing efficiency. Owing to the advantage of spiral scanning, laser spiral machining has been widely applied to welding [7–9] and drilling [10–12]. Wang Hongyu, Z et al. [13] The quality of a series of micropores, such as recast layer, microcrack, roundness and taper, produced on stainless steel 304 by helical laser drill-ing system was discussed. The formation of recast layer with thickness of about 25 µ m was detected on the side wall, and the taper phenomenon was also observed. A laser spiral drilling technology is proposed, which can be used to effectively manufacture high-quality microporesdiscussed the effects of various parameters of laser path weld-ing on the joint properties and weld pool formation. The results showed that the spiral path had the highest joint strength, which was more than 50% higher than that of the traditional straight-line welding. This was mainly because the weld pool formed by spiral welding had a high aspect ratio. Compared with traditional laser spot welding, the diameter of the fusion surface formed by the spiral laser spot welding was larger, and the upper and lower dimensions of the nugget were more consistent. As for heli-cal drilling methods, Shin, J et al. [7] reports an experimental investigation for the shal-low angle laser drilling of Inconel 718. Trials to improve drilling performance were made. They find a higher laser power, lower speed, and closer focal position to the workpiece surface contributed to the further removal of material by the absorption of more laser energy and larger beam intensity. As for laser drilling, Wang et al. [6] used a femtosecond laser to process the film cooling holes on the K24 superalloy. The exper-imental results showed that the spiral trepanning method could realize the cold pro-cessing with less of a recasting layer and less cracks compared to the three different drilling methods.

However, in the simulation model of nanosecond laser drilling, deformation ge-ometry is often used to track the interface change of micropores, and the change of re-cast layer is tracked by dividing the heat affected zone.the numerical simulation of a laser has been mostly based on single pulse processing [14–16] The deformation geom-etry model can not simulate the flow behavior of molten pool. The simulation of mol-ten pool behavior is mostly used for welding rather than drilling. For the simulation of phase explosion and other behaviors, the mathematical model has just been estab-lished and has not been applied to the actual processing.and linear scanning [17–20]. There have been fewer simulation studies on nanosecond laser helical drilling spiral scanning. In the interaction between the laser and the material, the material first ab-sorbs the laser energy and converts it into heat energy. In this process, the material melts and forms a molten pool to further absorb the laser energy. With the increase of the material temperature, the material vaporizes and the degree of vaporization in-crease. Under the action of the thermal stress field, the material is removed in the form of vaporization fly out and a small amount of liquid splash. This process is the main process of nanosecond laser material removal. The influence range of nanosecond laser is concentrated near the laser spot area.In the interaction between a laser and a materi-al, the material first absorbs laser energy through the lattice and converts it into the lattice heat, which is then transferred to the surrounding lattice through thermal radia-tion and heat transfer. When the lattice temperature within the laser ablation thresh-old exceeds the evaporation temperature of the material, it will vaporize and remove a large amount of energy so that the influence range of the nanosecond laser is concen-trated near the laser spot area. However, changes in the laser scanning path affect the accumulation of laser energy in a material, resulting in different removal effects. Spiral scanning has often been used in actual processing, but it is time consuming. In the na-nosecond laser processing process, the main processing mechanism is the evaporation of the processed material and the re-condensation of the material after melting.Heat removal and material evaporation, and diffusion are the main removal mechanisms in this type of processing. Therefore, analyzing the interaction between laser spiral scan-ning and materials is crucial to explore and master the further improvement in laser processing technology.

Owing to solid heat transfer simulation model can not accurately reflect the inter-face separation generated, there have been fewer simulation studies on laser spiral scanning. Therefore, in order to analyze the morphology forming process with the spi-ral path in the actual long-time processing, this paper simulates through two-phase flow and level set method. It can track the separated interface. Then, the influence of general parameters on the width and depth of morphology molding is analyzed. The simulation model is compared with the actual processing results, and good consistency is obtained.

 

Q2:

A: we add ‘TC4, Two phase flow, Level set,’

 

Q3:

A: We added explanations for missing parameters and explained more complex formula terms

Q4:

A: We added labels for horizontal and vertical axis, width and depth, respectively.

 

Q5:

A:we removed that

 

Q6:

A: We supplement the advantages of the simulation model over the original drilling model and point out the application scope.

 

We would like to thank you again for taking the time to review our manuscript.

 

 

 

We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval. Thank you very much for your comments and suggestions.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Dear authors, I see major improvements of your manuscript. You answered clearly and satisfactory to all my review remarks. I recommend to publish the manuscript in the MDPI journal.