Effects of Composition on Melt Fillability and Impact Resistance of TiAl Alloys for Thin-Blade Turbine Wheels: Laboratory Predictions and Product Verification
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe Authors present a carefully designed and executed experimental work, that aimed to investigate the effect of compositional differences of centrifugal cast TiAl alloys on the fillability and impact resistance. The Authors investigated 10 different alloy compositions, which is a respectable amount of work. The introduction clearly identifies the current state of thin-walled Ti-alloy castings and the difficulties related to increasing the fillability. The experimental methods are well-chosen, the results are clearly presented. The conclusions are supported by the experimental results.
Some minor suggestions are made to further improve the scientific soundness of the article:
-Some more information on the mechanism of reduced fillability of multicomponent alloys would be welcome. The effect of solidification temperature interval is clear, but can you please further elaborate on what type of precipitates are formed in Figure 12b and c?
-Can you please indicate the different phases in Figure 5, with for example, text and arrows?
Author Response
Thank you for your helpful comments. We have added figures and revised the main text based on your comments. The parts of the main text that have been revised in accordance with your comments are highlighted in yellow. Point-by-point responses to your comments are provided below.
Comment 1. -Some more information on the mechanism of reduced fillability of multicomponent alloys would be welcome. The effect of solidification temperature interval is clear, but can you please further elaborate on what type of precipitates are formed in Figure 12b and c?
Response 1. Thank you for your comment. At the end of Section 5.1 we have added the results of the analysis of the precipitates using FE-SEM/EDS as Figure 13. We have also added this to the Evaluation Methods section in Section 3.1.2. The following has been added to the main text in Section 5.1.
The results of the analysis of the microstructure near the mesh of 2Cr using FE-SEM/EDS are shown in Figure 13. Two types of precipitates were observed, produced by the reaction between the mesh and the molten metal. The first type is an Al-rich oxide, as oxygen is concentrated to the same level as Al2O3-1%SiO2 fibers and there is almost no Ti. The second type is thought to be an α2 phase stabilized by oxygen, as oxygen is slightly concentrated and Al is reduced. These precipitates are thought to have been formed as a result of the decomposition of some of the Al2O3-1%SiO2 mesh due to reaction with the molten metal and the resulting oxygen contamination.
Comment 2 -Can you please indicate the different phases in Figure 5, with for example, text and arrows?
Response 2. Thank you for your comment. As you pointed out, we have clearly labelled the phases and microstructures in Figure 5.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThis is a quality article, dealing with the relatively well-researched topic of TiAl intermetallics, but from a completely different perspective. The authors have done a very solid job, and the conclusions are novel. I recommend the article for publication and I have only these minor questions:
1) in section 3.2 it is stated that "The casting test was performed by remelting a master alloy ingot prepared in the laboratory using the method described in Section 3.1.1." I assume that the entire material was used after the Melt Fillability test after removing the mesh, or just the part under the mesh? Please provide a more detailed characterization.
2) Part 3.2 (202) states that the mold preheating temperature was set as a constant (1000 °C), was the heating carried out directly in the casting equipment and measured, or how was the preheating carried out to maintain the same conditions for the individual materials? Please explain.
3) Part 3.2 (212), The filling behavior of the molten metal in the thin turbine wheel blades was evaluated by calculating the volume filling ratio from the weight of each alloy turbine wheel. I would also recommend a more detailed description here.
4) Table 1, is this the nominal composition or the measurement results? Was the chemical composition really as accurate as in the table? I would recommend specifying. I assume that the chemical composition must have been controlled.
Author Response
Thank you for your helpful comments. We have revised the main text based on your comments. The parts of the main text that have been revised in accordance with your comments are highlighted in green. Point-by-point responses to your comments are provided below.
Comment 1. in section 3.2 it is stated that "The casting test was performed by remelting a master alloy ingot prepared in the laboratory using the method described in Section 3.1.1." I assume that the entire material was used after the Melt Fillability test after removing the mesh, or just the part under the mesh? Please provide a more detailed characterization.
Response 1. Thank you for your comment. As you pointed out, we have clarified that the ingot used in the turbine wheel casting test was a newly produced ingot and not a cast material after mesh penetration testing. In Section 3.2, the following text was added
In this case, in order to ensure that the weight of the master alloy ingot for each alloy was constant (900 g) in the turbine wheel casting experiment, new ingots were made instead of casting materials (whose weight was not constant) that had passed through the mesh.
Comment 2. Part 3.2 (202) states that the mold preheating temperature was set as a constant (1000 °C), was the heating carried out directly in the casting equipment and measured, or how was the preheating carried out to maintain the same conditions for the individual materials? Please explain.
Response 2. Thank you for your comment. As you pointed out, we have clearly described the method of preheating the mold and the operations to be carried out during the casting tests. In Section 3.2, the following text was added.
The molds were preheated in a furnace installed outside the casting machine. The molds for each alloy were heated simultaneously in the furnace, which was set at 1000℃, and then the molds were removed from the furnace one by one and installed in the casting machine, where casting tests were carried out quickly. As the time required for this process was not significantly different for each alloy, it is assumed that there was not much difference in mold temperature during the casting tests.
Comment 3. Part 3.2 (212), The filling behavior of the molten metal in the thin turbine wheel blades was evaluated by calculating the volume filling ratio from the weight of each alloy turbine wheel. I would also recommend a more detailed description here.
Response 3. Thank you for your comment. As you pointed out, we have clearly described how to calculate the volume of each alloy turbine wheel. In Section 3.2, the following text was added.
The filling behavior of the molten metal in the thin turbine wheel blades was evaluated by calculating the volume by dividing the weight of each alloy turbine wheel by its specific weight. The specific gravity of each alloy was calculated from the mixing ratio of each element. In this case, the calculation results differed slightly from the actual specific gravity, so a correction factor was obtained by comparing the calculation results for TiAl4822 with its known specific gravity (3.97). The specific gravity of each alloy was estimated by multiplying the calculation results by this correction factor.
Comment 4 Table 1, is this the nominal composition or the measurement results? Was the chemical composition really as accurate as in the table? I would recommend specifying. I assume that the chemical composition must have been controlled.
Thank you for your comment. As you pointed out, We have clearly stated in Table 5 and the accompanying text that the composition is nominal. We also stated that the actual chemical composition of the TiAl alloy melted by the method described in Section 3.1.1 is close to the nominal composition, as shown in a previous study (Reference 14). In Section 3.1.1, the following text was added.
Although the actual chemical composition was not analyzed in this study, previous studies [14] have confirmed that the actual chemical composition of TiAl alloys produced by the above method is close to the nominal composition.
Author Response File: 
Author Response.docx
