Effect of Pulse Current Density on Microstructure of Ti-6Al-4V Alloy by Laser Powder Bed Fusion
Vladimir A. Skripnyak
Round 1
Reviewer 1 Report
The paper deals with the investigation of the microstructure of laser powder bed fusion (LPBF) Ti64 alloy induced by electropulsing (ECP) with different current densities. The effect of the microstructure obtained by treating Ti64 alloy with different current densities on the mechanical properties has been also investigated by the authors providing an exhaustive discussion of the results. The topic and the results of the paper provide new information on the ECP processing additive manufacturing metallic materials.
The paper is well written, the experimental procedures are well explained and the conclusions are supported by the results.
Some minor comments are listed in the following:
- Fig. 3: the optical images report the microstructure in a plane normal to the build direction. In such a plane, it is known the microstructure is mainly equiaxed due to the sectioning of the effective microstructure of AM Ti64 that should be columnar. Without any micrograph in a plane parallel to the build direction, one could infer that the microstructure is equiaxed in any direction. Please, if it is possible, add some micrographs of the microstructure in a plane parallel to the build direction and comment in the manuscript.
- Fig. 8: according to the Hall-Petch model grain refinement lead to an increase in the UTS. The trend observed in the variation of the a' martensite size seems not to be in agreement in with the trend of the UTS by varying the current density. In particular, is the major or the minor axis the size that can most influence the UTS. Can you comment, please?
Author Response
Reviewers 1 (Revisions in the paper are marked in yellow)
Response to comment 1:(Fig. 3: the optical images report the microstructure in a plane normal to the build direction. In such a plane, it is known the microstructure is mainly equiaxed due to the sectioning of the effective microstructure of AM Ti64 that should be columnar. Without any micrograph in a plane parallel to the build direction, one could infer that the microstructure is equiaxed in any direction. Please, if it is possible, add some micrographs of the microstructure in a plane parallel to the build direction and comment in the manuscript.)
Author response: Thank you very much for your valuable suggestion. We have added micrographs of the microstructure in a plane parallel to the building direction and described them analytically in the paper. The microstructure of LPBF-Ti64 alloy in a parallel building direction consists of columnar prior β grains and internal needle-like α' martensite, and the size and aspect ratio of the columnar β grains are described. Additions are marked in yellow in the paper.
Response to comment 2:(Fig. 8: according to the Hall-Petch model grain refinement lead to an increase in the UTS. The trend observed in the variation of the a' martensite size seems not to be in agreement in with the trend of the UTS by varying the current density. In particular, is the major or the minor axis the size that can most influence the UTS. Can you comment, please?)
Author response: We thank the reviewer for raising this question. In our point of view, the length of minor axis of the a' martensite can most influence the UTS. The Hall-Petch relationship is σ=σ0+kd^(-1/2). Gao et al. [1] defined d in the Hall-Petch model as the α lamellar width and studied its relationship with the microhardness of SLM-Ti64 alloy. Therefore, we define σ as the ultimate tensile strength of LPBF-Ti64 alloy, d as the length of minor axis of the needle-like a' martensite, and σ0 and k are material constants. According to the Hall-Petch model, the smaller the length of minor axis of a' martensite, the higher the ultimate tensile strength of LPBF-Ti64 alloy. The length of minor axis of a' martensite and the ultimate tensile strength of ECP-1 samples conform to the Hall-Petch model. When the pulse current density is large (ECP-3), the properties of the alloy will deteriorate [2], resulting in a significant decrease in elongation. According to the strength-elongation trade-off law [3], the decrease in elongation is accompanied by a strength increase. Among them, the Joule heating effect of the pulsed current makes the recrystallized grains coarse [4], which is manifested as a significant increase in the length of minor axis of the a' martensite. And when the pulse current density is more appropriate, the electrothermal coupling effect significantly improves the mechanical properties by acting on defects such as dislocations inside the alloy [5]. This part will be followed by an in-depth study.
References
[1] Gao, J.B.; Ben, D.D.; Yang, H.J.; Meng, L.X.; Ji, H.B.; Lian, D.L.; Chen, J.; Yi, J.L.; Wang, L.; Li, P.; Zhang, Z.F. Effects of electropulsing on the microstructure and microhardness of a selective laser melted Ti6Al4V alloy. J. Alloys Compd. 2021, 875, 160044. https://doi.org/10.1016/j.jallcom.2021.160044.
[2] Wu, C.; Zhou, Y.j.; Liu, B. Experimental and simulated investigation of the deformation behavior and microstructural evolution of Ti6554 titanium alloy during an electropulsing-assisted microtension process. Mater. Sci. Eng. A. 2022, 838, 142745. https://doi.org/10.1016/j.msea.2022.142745.
[3] Yin. P.; Zang, H.; Zhao, Y.; Wu, C.; Xu, X.; Pan D.; Jiang. W. Superior mechanical properties of 40Cr steel obtained by quenching and microstress relieving under electropulsing. Mater. Sci. Eng. A. 2020, 772, 138782. https://doi.org/10.1016/j.msea.2019.138782.
[4] Ao, D.; Chu, X.; Yang, Y.; Lin, S.; Gao, J. Effect of electropulsing treatment on microstructure and mechanical behavior of Ti-6Al-4V alloy sheet under argon gas protection. Vacuum. 2018, 148, 230. https://doi.org/10.1016/j.vacuum.2017.11.017.
[5] Xiao, H.; Jiang, S.; Zhang, K.; Jia, Y.; Shi, C.; Lu, Z.; Jiang, J. Optimizing the microstructure and mechanical properties of a cold-rolled Al–Mg–Li alloy via electropulsing assisted recrystallization annealing and ageing. J. Alloys Compd. 2020, 814, 152257. https://doi.org/10.1016/j.jallcom.2019.152257.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript presents the original results of the microstructure and mechanical properties investigations of the Ti64 alloy samples, which were obtained by laser melting in the powder layer (LPBF). Structural changes in alloy samples induced by electro pulse action (ECP) were investigated.
It is shown that ECP treatment makes it possible to modify α' - martensites and improves mechanical properties.
The results presented in the manuscript expand knowledge about the possibility of modifying the grain and phase and defect structure of bodies created by the LPBF method from titanium alloy Ti64 using the ECP method.
However, the description of the studies in the article should be improved.
It should be specify the specific test equipment that was used for mechanical test of the specimens.
It should be indicated under what loading conditions (strain rate, temperature) and on which specimens in shape and size the data given in subsection 3.4 were obtained.
Author Response
Reviewers 2 (Revisions in the paper are marked in red)
Response to comment 1: (the description of the studies in the article should be improved.)
Author response: Thank you for your comment. Considering the suggestion, we checked the description of the studies for further revisions. We corrected grammatical errors and syntax that existed and optimized the description language. Changes are marked in red.
Response to comment 2: (It should be specify the specific test equipment that was used for mechanical test of the specimens.)
Author response: Thank you for your comments. The mechanical test of the specimens is performed using an AGS-X-10KN electronic universal testing machine at room temperature with a strain rate of 0.8 mm/min. This information is listed at the end of subsection 2.
Response to comment 3: (It should be indicated under what loading conditions (strain rate, temperature) and on which specimens in shape and size the data given in subsection 3.4 were obtained.)
Author response: Thank you for pointing out this problem in manuscript. In order to clarify the shape and size of the mechanical properties test samples and the strain rate and temperature of the experimental process, we have added the following in subsection 3.3 (revision of subsection 3.4 to subsection 3.3): “The mechanical properties of the vertical building direction LPBF-Ti64 samples before and after ECP treatment were tested. The samples used for the performance tests were the same as those used for the microstructure characterization, which were dog bone shaped samples (Figure 1a). The tests were performed using an AGS-X-10KN electronic universal testing machine at room temperature with a strain rate of 0.8 mm/min.”
Author Response File: Author Response.pdf
