High Nb–TiAl Intermetallic Blades Fabricated by Isothermal Die Forging Process at Low Temperature

Round 1

Reviewer 1 Report

- Set-up of the experimentation has to be improved. More details should be provided about the flow stress curves shown in Fig. 2 (billet size, temperature control, strain rate control, etc). How have the flow stress curves for this alloy been obtained? How was the strain rate measured, in order to obtain the flow stress curves? Moreover, the employed equipment should be shown.
- The quality of Figures 2 and 3 has to be improved. These figures have poor quality. Furthermore, these figures should be analysed in the manuscript and the main conclusions that can be drawn from them should be indicated.
- FEM Section is poorly explained. How has the contact been simulated? Friction coefficient? Initial meshing (elements type) size? Solver employed, etc.
- The authors should add a figure showing the filling of the die in FEM simulations.
- References should be provided for equations 1-3.
- How have the properties shown in Table 1 been obtained?
- Figures 4 and 5 have poor quality. These figures have to be significantly improved and these figures should be analysed in the manuscript. In addition, the main conclusions that can be drawn from them should be indicated.
- Indicate (add a figure) to which areas of the blades the micrographs shown in figure 8 correspond.
- Indicate how samples were prepared for analysis using SEM and EBSD
- The authors should describe in greater detail the isothermal forging equipment. A figure should be included.
- The initial billet should be shown, before forging.
- Detail the type of lubricant. Moreover, detail how the alloy was heated.
- What equipment exists for temperature control?
- Detail how the blades have been manufactured? How the strain rate was measured?
- Indicate to which zones the figures shown in the micrographs correspond exactly.
- Conclusions are not well justified with the text. This has to be improved and the conclusions should be justified.

Author Response

Dear Reviewer and Editor，

Thank you for your attention to my submitted paper. Now according to the Reviewer’s comments, we revised the manuscript carefully and all the revisions are write in blue.

- Set-up of the experimentation has to be improved. More details should be provided about the flow stress curves shown in Fig. 2 (billet size, temperature control, strain rate control, etc). How have the flow stress curves for this alloy been obtained? How was the strain rate measured, in order to obtain the flow stress curves? Moreover, the employed equipment should be shown.

√ Yes. Isothermal hot compression experimentation has been improved between line 71 and line 81 in the revision. The billet used in hot compression experimentation is cylindrical samples with dimension of Φ8×12mm and Gleeble 1500D equipment is employed. The temperature and strain rate both are controlled by Gleeble 1500D equipment and the particular test parameters are listed in table 1 in the revision. The flow stress curves for this alloy are obtain according to standard equations and the details are described as “the particular test parameters are shown in table 1. Graphite plates were used as the lubricant between specimens and platens in hot compression tests. The samples were deformed to a true strain of 0.4. From the stress–strain data, true stress–true strain curves were evaluated using standard equations. The flow stress data were also corrected for the adiabatic temperature increase at different temperatures and strain rates [23]. The compressed specimens were sectioned parallel to the compression axis.”

- The quality of Figures 2 and 3 has to be improved. These figures have poor quality. Furthermore, these figures should be analysed in the manuscript and the main conclusions that can be drawn from them should be indicated.

√ Yes. Quality of Figures 2 and 3 has been improved. and the further conclusions are drawnin the revision during line 121 to line 150 use blue words.

- FEM Section is poorly explained. How has the contact been simulated? Friction coefficient? Initial meshing (elements type) size? Solver employed, etc.

√ Yes. The rigid viscoplasticity finite element method was used in this FEM, and friction coefficient was 0.15 and the initial meshing, etc were drawn in the Section 2.2..

- The authors should add a figure showing the filling of the die in FEM simulations.

√ Yes. Model of blade isothermal die was shown in Figure 1.

- References should be provided for equations 1-3.

√ Yes. Equations 1 was a general formula, so I do not think it is necessary to provided reference

References were provided for equations 2 and 3 shown as following:

1138. Sellars, W. McTegart, On the mechanism of hot deformation, Acta Metall. 14(1966) 1136–1138.
1139. Gupta, S.N. Murty, B. Pant, V. Agarwala, P. Sinha, Hot workability of γ + α2 titanium aluminide: development of processing map and constitutive equations, Mater. Sci. Eng. A 551 (2012) 169–186.

- How have the properties shown in Table 1 been obtained?

√ Yes. The properties shown in Table 1 were our long-term research results.

- Figures 4 and 5 have poor quality. These figures have to be significantly improved and these figures should be analysed in the manuscript. In addition, the main conclusions that can be drawn from them should be indicated.

√ Yes. The quality of figures 4 and 5 has been improved and the figures have been analysed.

- Indicate (add a figure) to which areas of the blades the micrographs shown in figure 8 correspond.

√ Yes. Figure 8 e) has been added to indicate the sample position.

- Indicate how samples were prepared for analysis using SEM and EBSD

- The authors should describe in greater detail the isothermal forging equipment. A figure should be included.

√ I am sorry that the forging equipment could not be supplied due to the whole equipment are self designed prototype.

- The initial billet should be shown, before forging.

√ Yes. The initial billet was shown in Figure 6

- Detail the type of lubricant. Moreover, detail how the alloy was heated.

√ Yes. The lubricant was glass lubricant as drawn in manuscript and the detail of alloy heat treatment was shown” After forging, no heat treatment is required and due to the poor plasticity of high Nb-TiAl alloy at room temperature, it is necessary to draft immediately”

- What equipment exists for temperature control?

We use furnace with silicon molybdenum rod heater to control the temperature.

- Detail how the blades have been manufactured? How the strain rate was measured?

Manufactured detail of blades has been drawn in section 3.4 and the strain rate was measure by the forging equipment.

- Indicate to which zones the figures shown in the micrographs correspond exactly.

√ Yes. Figure 8 e) has been added to indicate the sample position.

- Conclusions are not well justified with the text. This has to be improved and the conclusions should be justified.

√ Yes. Conclusions have been rewrite and we think they are justified with the text now.

The manuscript has been revised according to your comments and  resubmitted. We look forward to your positive response.

Sincerely yours,

Wang Xiaopeng

Reviewer 2 Report

The presented manuscript concerns comprehensive numerical and technological research with the analysis of the structure and mechanical properties of high Nb-TiAl intermetallic blades after isothermal die forging process.

The introduction contains very key pieces of information and is well written, but sentence in lines 57-60 requires some explanation. It is well understood, that at a lower temperature of isothermal die forging the dynamic recrystallization is limited and the mechanical properties of the material are higher due to strain hardening. But how do the authors understand the formation of a refined microstructure with limited dynamic recrystallization (DRX) ? This requires explanation, the more that in the their results the authors show that under certain isothermal die forged process conditions at reduced temperature, it is DRX that is the reason for grain refinement.

In addition, the explain requires:

• What based the thermo-physical parameters of the Nb-TiAl alloy shown in Table 1 was determined ?
• The information presented in chapter 3.4 should be in Experimental (chapter 2.3). They present practically no discussion of the research results, only the course of the Isothermal die forging experiment. Figures 6 and 7 with appropriate commentary can be presented in chapter 3.5.
• After what treatment state was the TiAl alloy in its initial state (before isothermal die forged process), with the microstructure shown in Fig. 8a ?
• There is no evidence of the presence of γ/α2, γ and B2 phases described in specific grains of the microstructure in Fig. 8. These results should be confirmed by XRD study, especially applies to it the B2 phase. Therefore, it is important to know what the treatment state was in the starting TiAl alloy before isothermal die forged process ? Only then the discussion of the EBSD results presented in Fig. 9 in the center position and on the edge of the forged blade should not be raise any doubts.
• Some explanation needs to be supplemented in the discussion of the results presented in Fig. 9, which was the reason for the irregular DRX in the blade structure in the isothermal die forged process condition. For this reason, in Conclusions point 4, information that "the lamellae were effectively broken down into fine γ grains, and a relatively complete DRX occurred" is not precise in the light of the presented results.

Author Response

Dear Reviewer and Editor，

Thank you for your attention to my submitted paper. Now according to the Reviewer’s comments, we revised the manuscript carefully and all the revisions are write in blue.

The introduction contains very key pieces of information and is well written, but sentence in lines 57-60 requires some explanation. It is well understood, that at a lower temperature of isothermal die forging the dynamic recrystallization is limited and the mechanical properties of the material are higher due to strain hardening. But how do the authors understand the formation of a refined microstructure with limited dynamic recrystallization (DRX) ? This requires explanation, the more that in the their results the authors show that under certain isothermal die forged process conditions at reduced temperature, it is DRX that is the reason for grain refinement.

√ Yes. Sentence in lines 57-60 is not expressed clearly. Now we rewrite it again.“An advantage of operation with low isothermal die forging temperatures could be limited the growth of dynamic recrystallization grains in the initial extruded or forged billets, thereby improving the mechanical properties of the forged blades. In addition, forging at low temperature could reduce the fabrication costs”

What based the thermo-physical parameters of the Nb-TiAl alloy shown in Table 1 was determined ?

√ Yes. The properties shown in Table 1 were our long-term research results.

The information presented in chapter 3.4 should be in Experimental (chapter 2.3). They present practically no discussion of the research results, only the course of the Isothermal die forging experiment. Figures 6 and 7 with appropriate commentary can be presented in chapter 3.5.

√ Yes. I thought that information presented in chapter 3.4 was obtain based on the simulated result above. Figure 6 and 7(experimental results) validated by comparison with the simulation results.

After what treatment state was the TiAl alloy in its initial state (before isothermal die forged process), with the microstructure shown in Fig. 8a ?

√ Yes. The TiAl alloy in its initial state (before isothermal die forged process) is forged billet as we drawn in line 72.

There is no evidence of the presence of γ/α2, γ and B2 phases described in specific grains of the microstructure in Fig. 8. These results should be confirmed by XRD study, especially applies to it the B2 phase. Therefore, it is important to know what the treatment state was in the starting TiAl alloy before isothermal die forged process ? Only then the discussion of the EBSD results presented in Fig. 9 in the center position and on the edge of the forged blade should not be raise any doubts.

√ Yes. Thank you for your question. The initial state of TiAl alloy used in the study is as-forged billet. Moreover, in our published papers and others papers about as-forged high Nb-TiAl alloys have been proved the presence of of γ/α2, γ and B2 phases, so in this manuscript we do not do the XRD test.

Some explanation needs to be supplemented in the discussion of the results presented in Fig. 9, which was the reason for the irregular DRX in the blade structure in the isothermal die forged process condition. For this reason, in Conclusions point 4, information that "the lamellae were effectively broken down into fine γ grains, and a relatively complete DRX occurred" is not precise in the light of the presented results.

Generally, the angle boundaries could reflect the DRX, high angle boundaries generally exist in dynamic recrystallized grains and some subgrains. In this study, the high angle boundaries in the center position and edge position were 93% and 95.5%, respectively, so we summarized that a relatively complete DRX occurred

The manuscript has been revised according to your comments and resubmitted. We look forward to your positive response.

Sincerely yours,

Wang Xiaopeng

Round 2

Reviewer 1 Report

Figures 2 and 3 still have poor quality. In addition, how these flow stress curves were obtained? If experimental data is available, these figures should be drawn to improve quality. Furthermore, how have the authors measured the strain rate?

Line 148, "...alloys[]...." Reference is mised?
The type of elements is still missing, remeshing criteria are not indicated. Morevover, why a fricction coeeficient of m=0.15 is employed?

- The authors should add a figure showing the filling of the die in FEM simulations.
This has not been done.

- References should be provided for equations 1-3.
"...Equations 1 was a general formula, so I do not think it is necessary to provided reference."
Yes, but in any case you should either justify how this equation is obtained as well as the validity of this approach for your study or add a reference.

- How have the properties shown in Table 1 been obtained?
"... The properties shown in Table 1 were our long-term research results."
Then, add some references.

- Figures 4 and 5 have poor quality. The quality of these figures has to be improved.

- Detail how the blades have been manufactured? How the strain rate was measured?
"...Manufactured detail of blades has been drawn in section 3.4 and the strain rate was measure by the forging equipment...."

This is not well justified. How can you measure a specific strain rate in the forged part?

Author Response

（1）Figures 2 and 3 still have poor quality. In addition, how these flow stress curves were obtained? If experimental data is available, these figures should be drawn to improve quality. Furthermore, how have the authors measured the strain rate?

Yes. Flow stress curves were obtained according to the experimental data tested by Gleeble 1500D. The strain rate was set according to our experimental studies in the early and based on the general analysis and summary, we predicted that the strain rates between 0.005s^-1 and 0.05 s^-1 could meet the isothermal die forging of high Nb-TiAl alloy, and the strain rates were controlled by Gleeble 1500D device.

In addition, Figures 2 and 3 were redrawn according to the experimental dates by originPro 8 and the quality was improved.

（2）Line 148, "...alloys[]...." Reference is mised?
The type of elements is still missing, remeshing criteria are not indicated. Morevover, why a fricction coeeficient of m=0.15 is employed?

Yes. Reference is supplemented as Reference 27 “Cheng Liang, Chang Hui, Tang Bin, Kou Hongchao, Li Jinshan. Simulation of Microstructure for Hot Pack-Forging of a High Nb Containing TiAl Alloy, Rare Metal Materials and Engineering, 2014; 43(1): 36-41”

Also the FEM Section is explained now.“The spatial position and contact area of contact surfaces between blade billet and forging dies would change with the process of die closing, thermal expansion and constraint, so the contact between forging die and blade billet was considered to a non-linear problem and the finite element method of isothermal die forging of high Nb-TiAl alloy used the algorithm for contact interface pairs and set the corresponding constraint relationship. Modified second-order element (10-node tetrahedron, C3D10M) was used in the finite element simulations processing. The mesh division of the three parts of isothermal die forging blade blank and die is as follows: (1) the global dimension mesh seed density of blade billet was set to 0.5, the maximum curvature deviation factor was 0.02, and the minimum size was 0.01; (2) due to the complexity of the blade shape; the primary meshes were regionally divided to improve the calculation accuracy and area division of blade billet was adopted. the density of the area mesh seed at the fillet and direct contact with the edge of the dies was set to 0.05, and the adaptive mesh division function of ABAQUS was enabled; (3) global mesh seed density of the upper and lower isothermal forging dies was set to 0.9, the maximum curvature deviation factor was 0.04, and the minimum size was 0.02. The die adopts the equal density mesh division method; (4) die adopts the absolute mesh division to reduce the remeshing during the finite element simulation. According to these principles, the blade billet and isothermal forging dies were divided into finite element mesh, and the assembly is depicted in Figure 1. The blade billet was designed into a plastic mold and it discretized into 205518 mesh units, as shown in Figure 1b). The forging dies, made by K403 nickel-based superalloy, were regard as rigid dies. The upper and lower dies discretized into 55247 and 54676 mesh units, respectively and distance between the upper and lower dies was set at 15mm.”

The friction coefficient was employed according to the glass lubricant and our pre-experiment experience.

（3）- The authors should add a figure showing the filling of the die in FEM simulations.
This has not been done.

Yes. Figure 4 showing the filling of the die in FEM simulations was added in the manuscript

（4）- References should be provided for equations 1-3.
"...Equations 1 was a general formula, so I do not think it is necessary to provided reference."
Yes, but in any case you should either justify how this equation is obtained as well as the validity of this approach for your study or add a reference.

Yes. Reference was provided for equation1 “Debin Shan, Xu Yi, Lu Yan. Three-dimensional rigid-plastic ®nite-element analysis of the isothermal precision forging of a cylindrical housing, Journal of Materials Processing Technology,2000; 102:188-192”

（5）- How have the properties shown in Table 1 been obtained?
"... The properties shown in Table 1 were our long-term research results."
Then, add some references.

Yes. The properties shown in Table 1 were obtained from our research results and our cooperative company test results. Reference was provided “Peng Xu.Fundamental Research on Isothermal Die Forging of High Nb-TiAl alloy, Harbin Institute of Technology,2015”

（6）- Figures 4 and 5 have poor quality. The quality of these figures has to be improved.

Yes. Results of Figures 4 and 5 were provided according to the simulated results and the quality has been improved

（7）- Detail how the blades have been manufactured? How the strain rate was measured?
"...Manufactured detail of blades has been drawn in section 3.4 and the strain rate was measure by the forging equipment...."

This is not well justified. How can you measure a specific strain rate in the forged part?

Yes. The fabrication details were provided. “the blades isothermal forging were performed on hydraulic press with the maximum load of 6300kN, which allocates resistance furnace with an accuracy of 1K and the isothermal die forging temperature was set at 950℃. The upper die pressure speed was set at lower than 0.01mm/s respectively. Before forging, the high Nb-TiAl alloy billets must be sand blasted and sprayed by glass lubricant and the forging dies were heated treatment at 950℃ in furnace for 20 min, then the billet was assembled with the isothermal forging die, the purpose of these operations was to prevent the internal structure of forged high Nb-TiAl alloy from over burning. At the same time, the resistance furnace on the 6300kN isothermal forging machine was heated to 950℃ in advance.The isothermal forging die assembled with high Nb-TiAl alloy billets were placed on the 6300kN isothermal forging machine preheated to 950°C and maintained at that temperature for 2h., and sets the forging parameter information by computer. Then the isothermal die forging is performed.The isothermal die forging process was then carried out using a forging load of about 200 kN. The maximum load, when the dies were closed, was about 300 kN.”

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

Dear authors:

Please, revise writing of units throughout the manuscript. There are some examples:

Write: 6300 kN instead of 6300kN; 950 ºC instead of 950ºC, 2 h instead of 2h., etc

Sincerely,