Use of X-ray Computed Tomography for Assessing Defects in Ti Grade 5 Parts Produced by Laser Melting Deposition

Round 1

Reviewer 1 Report

In fact, the publication does not present us with anything particularly new and interesting. The use of the XСT method is not something new in the context of studying the print quality of products made of metals and polymers, as well as various composites. Also, this method is widely used for the selection of 3D printing modes.

1. Extra comma on line 5 on page 1.
2. There is no optimization in the presented publication. When optimizing, a mathematical problem is solved by the condition of which the extremum of the function must be found. The function of optimization itself is not formulated or presented

I recommend abandoning the term optimization in the title and removing it from the text of the publication.

Authors need to seriously revise their manuscript.

Author Response

1. In fact, the publication does not present us with anything particularly new and interesting. The use of the XСT method is not something new in the context of studying the print quality of products made of metals and polymers, as well as various composites. Also, this method is widely used for the selection of 3D printing modes.

The publication presents some analysis on LMD samples, conducted with custom made XCT system with high power of penetration at high spatial resolution, devoted to characterization of relatively large parts in the sense of radiological path. While the XCT method is used for studying the print quality of products, as the reviewer mentioned, one cannot affirm that it is widely used in characterization of these sample types. In fact, while studying the scientific literature we could not find papers on XCT characterization of LMD produced samples, which are denser and larger than usual SLM produced parts. XCT was largely associated with characterization of samples produced by SLM, which are generally small parts with thin walls and lattice structures, easier to analyse by this method.

1. Extra comma on line 5 on page 1.

Thank you very much for pointing out this spelling error. The extra comma has been removed.

1. There is no optimization in the presented publication. When optimizing, a mathematical problem is solved by the condition of which the extremum of the function must be found. The function of optimization itself is not formulated or presented

In fact, there is an optimization presented in this publication. By optimizing the offset between meander planes (ΔZ) and the hatch spacing we succeed to completely remove porosity and other internal defects from LMD produced samples, without any others post deposition treatment. Without XCT the parameters optimization would not have been possible, extensive destructive techniques such as cutting and metallographic preparation in various areas of the samples would have been needed implying a huge amount of work and time consumption. XCT helped with localization of pores and we were able to visualize their evolution, function of parameters tuning, in order to obtain the optimized values.

4. I recommend abandoning the term optimization in the title and removing it from the text of the publication.

In order to avoid misinterpretations, we replaced “optimization” by “quality improvement” / “tuning” / “improvement “/ “manufacturing”, depending on context.

Reviewer 2 Report

Dear author's,

The article is very interesting and I strongly support it. The only think which I can recommend is to explain a little bit in introduction part e.g. line 39 more about the differences between your manufacturing method and similar 3d printing method such as: SLM with support of the article where SLM technology is explained e.g.

The Influence of Printing Orientation on Surface Texture Parameters in Powder Bed Fusion Technology with 316L Steel. Micromachines 2020, 11, 639.

Waviness of Freeform Surface Characterizations from Austenitic Stainless Steel (316L) Manufactured by 3D Printing-Selective Laser Melting (SLM) Technology. Materials 2020, 13, 4372.

second think - please give more information about the materials (particle size, form etc. accuracy of technology what is probably in manual)

third - on fig 2 maybe it is better to use path instead of direction.

fourth - how many samples did you use for tests and analysis in table 3 (maybe is better to add statistical analysis of the dimension deviations)?

Kind regards,

Reviewer

Author Response

1. The article is very interesting and I strongly support it. The only think which I can recommend is to explain a little bit in introduction part e.g. line 39 more about the differences between your manufacturing method and similar 3d printing method such as: SLM with support of the article where SLM technology is explained e.g.

The Influence of Printing Orientation on Surface Texture Parameters in Powder Bed Fusion Technology with 316L Steel. Micromachines 2020, 11, 639.

Waviness of Freeform Surface Characterizations from Austenitic Stainless Steel (316L) Manufactured by 3D Printing-Selective Laser Melting (SLM) Technology. Materials 2020, 13, 4372.

We added a new paragraph on page 1 line 40 which describes the principles of both LMD and SLM. We inserted in this paragraph the references suggested by the reviewer. The paragraph reads:

In this technique, the laser irradiates a substrate and locally melts it, while powder is injected into the molten pool. The path of the laser beam also represents the contour of the part to be printed. Layer by layer, a 3D part can be constructed in this manner. Another common 3D printing technique for metallic materials is Selective Laser Melting (SLM). In this method, a bed of powder is irradiated by a laser beam. The powder is melted in the areas scanned by the laser beam and is rapidly solidified after the action of the laser beam stops. A new layer of powder is applied on the top via a leveling set-up, followed by a new laser irradiation step. Thus, layer by layer a 3D shape is produced [7, 8].

2. second think - please give more information about the materials (particle size, form etc. accuracy of technology what is probably in manual)

The authors added new information about the powder on page 3 line 102.

The paragraph reads as:

The powder was obtained using the gas atomization technique. In order for the particles to have the desired diameters range, the powders were sieved after the production process. The alloy contains Al (min 5.5% and max 6.5%), V (min 3.5% and max 4.5%), and the rest up to 100%Ti. The density of this alloy is the range of 4.43 – 4.5 g/cm3 [46].

3. third - on fig 2 maybe it is better to use path instead of direction.

We replaced “direction” by “path” in fig 2

4. fourth - how many samples did you use for tests and analysis in table 3 (maybe is better to add statistical analysis of the dimension deviations)?

Each sample was produced in triplicate. We added the standard deviations in table 3 for the experimental height. This information was added in page 4 line 144.

Reviewer 3 Report

The presented text for review is very interesting and valuable. The use of X-ray rays to determine the filling density (porosity) of an element made by sintering is presented.

1. The literature review should also include studies on the measurement of the change in the proportion of metallic phases under the influence of plastic deformation, using X-ray, and other machining processes, e.g.
-The application of laboratory X-ray micro-diffraction to study the effects of clinching process in steel sheets;
-3D characterization of dry powder inhaler formulations: Developing X-ray micro computed tomography approaches;
-X-ray-induced transformations on the surface of MoVNbTe mixed oxide catalyst: An XPS study;
-Preparation, luminescent properties and X-ray imaging result of Lu2O3: Eu structured scintillation film on sapphire substrate by LCVD method, etc.
2. Please explain why such values ​​of the change of distance in the z-axis direction were adopted (Table 2). What was the assumed product (process) model - initial - for which the errors were determined?
3. What optimization algorithm has been adopted for the data from Table 2?
4. Please present the coordinate system for measurements in the X-ray method (alignment of the sample with respect to the coordinate system of the measuring machine.
5. X-ray measurement parameters should be presented in the table.
6. Please describe the properties of the alloy material used.
7. The scales in figures 6, 7, 8d are illegible. I am asking for improvement.
8. How many measurements have been made for how many samples?
9. The discussion of the results should be expanded.

Author Response

1. The literature review should also include studies on the measurement of the change in the proportion of metallic phases under the influence of plastic deformation, using X-ray, and other machining processes.

-The application of laboratory X-ray micro-diffraction to study the effects of clinching process in steel sheets;
-3D characterization of dry powder inhaler formulations: Developing X-ray micro computed tomography approaches;
-X-ray-induced transformations on the surface of MoVNbTe mixed oxide catalyst: An XPS study;
-Preparation, luminescent properties and X-ray imaging result of Lu2O3: Eu structured scintillation film on sapphire substrate by LCVD method, etc.

We inserted a new paragraph on page 2 line 84 in which we describe the possible interaction between X-rays and some materials. We introduced in this paragraph the references proposed by the reviewer.

2. Please explain why such values ​​of the change of distance in the z-axis direction were adopted (Table 2). What was the assumed product (process) model - initial - for which the errors were determined?

The optimal distance between planes was selected based on trial and error. We produced samples with various distances on Z-axis and the specific offset between meander planes ΔZ corresponding to samples without internal defects (in the resolution limit of our XCT set-up) was considered optimal.

Initially, we optimized the process for a single traced line. With the conditions found optimal for tracing a line, we produced bulk samples in form of prisms. We found out that the conditions for a single line were no longer suitable for producing the prisms. Thus, we varied two parameters: Hatch spacing between horizontal lines and offset between meander planes. Two problems had to be solved: elimination of internal defects and the height growth in accordance with the theoretical values (e.g.10 layers x 0.5 mm between planes equals a theoretical value of 5 mm). Multiple samples displayed the lack of porosity, but experimental height was not close to the theoretical value. We selected in the end the hatch distance and the offset distance that provided the lack of porosity and the closest value from the theoretical one.

3. What optimization algorithm has been adopted for the data from Table 2?

We started from the conditions found optimal for tracing a single line (parallelism between borders of the line, lack of defects, homogeneity of the deposition, low amount of residual material) and changed the hatch spacing with a step of 0.25 mm from 0.5 to 1.5 mm between lines. We continued even further, but the samples displayed a high amount of defects (see figure below) that were not only concentrated in a plane but extended in the whole bulk of the sample. In the same time, we varied the offset between meander planes. Thus, a matrix of parameters was generated and the XCT results helped to eliminate the inappropriate parameter combinations.

4. Please present the coordinate system for measurements in the X-ray method (alignment of the sample with respect to the coordinate system of the measuring machine).

The reference system was redone in order to be clearly visible in figs 6 and 7. Moreover, we added a paragraph on page 9 line 246: The samples were aligned with respect to the X-ray tomography system by placing the substrate interface parallel to the detector and the long side along direction of rotation. In other words, the Z axis from the sample coordinates is perpendicular to the X-ray detector.

5. X-ray measurement parameters should be presented in the table.

We inserted a new table 3 that contains the XCT measurement parameters in case of both, powder and LMD samples.

6. Please describe the properties of the alloy material used.

For our current study, we consider that it is of the interest for the readers to know the chemical composition of the alloy and the density of the bulk. This information was added in page 3 line 108.

7. The scales in figures 6, 7, 8d are illegible. I am asking for improvement.

The scale bars have been redrawn in order to be more visible in figs 6, 7 and 8.

8. How many measurements have been made for how many samples?

Each sample in table 2 has been produced in triplicate, corresponding to 24 samples. Besides these samples, we also conducted experiments with hatch spacing of 1.75 and 2 mm which were filled with internal defects and were excluded from this study (six more samples). This information was added in page 4 line 144.

9. The discussion of the results should be expanded.

The discussion section has been expanded with new information related to the 3D printing methods and XCT characterization technique.

Round 2

Reviewer 1 Report

The presented version takes into account the previously mentioned comments. The article can be published in its current form.

Reviewer 3 Report

Thank you for the improvements made. The final decision is made by the chief editor.