Investigation of the Wear Behavior of Surface Welding AZ91 and AZ91+Gd Alloys under Variable Loading Conditions
Round 1
Reviewer 1 Report
The present article presents the investigation of the wear behaviour of surface welding of AZ91 and with the addition Gd at different levels.
Article is well planned with rightly supporting experimental methods. Observed mechanisms have been neatly explained which shows the maturity of the researchers. The readability of the paper could improve further only if the following comments are addressed:
- English, especially in the introduction part needs quite a bit of polishing. For example the lines between 32 to 35 could easily be 2 sentences instead of one long phrase with so many commas.
- Same is true with the subsequent sentences that follow from line 36 till 45. I suggest cutting short the sentences by removing the commas.
- 1. Basic material? What does it refers to?? How is it different from AZ91 ? I think this aspect is not clear from the methods section.
- Refering to Figure 3. How was the volume fraction of each Al2Gd and Mg17Al12 are measured? With the low mag images, it’s highly improbable to say that we have 5.7% of beta and or 4.5% of beta phase. What’s the error bar in the measurements?? How many images were analyzed for the obtaining these statistics??
- Same comments as above. How accurate are these percentages?
- Page 6 – line 175: Friction coefficient remained un-changed irrespective of the Gd content. Why is that?
- Table 3. Addition of Gd has a negative influence on the mechanical properties. Even with 0.5% Gd addition there is a near 9% reduction in the hardness. Why add Gd??
- Page 14, line 450 : You state that Al2Gd is effective in pinning the grain boundary. In your observation in the results section (page 4, line 131) you have the Al2Gd phases that are of 1mm of size! Do you think they are effective grain boundary pinning agents at this size? If no, then how do you explain the reduction in wear rate for 0.5Gd additions when compared to other alloys??
Comments for author File: 
Comments.pdf
Author Response
Response to Reviewers #1’ Comments
Ms. Ref. No.: crystals-1189801
“Investigation of the Wear Behavior of Surface Welding AZ91 and AZ91+Gd Alloys Under Variable Loading Conditions” by Qingqiang Chen, Yalei Yu, Jie Sun, Cainian Jing, Yanhua Zhao and Jia Wang
Comment 1: “English, especially in the introduction part needs quite a bit of polishing. For example the lines between 32 to 35 could easily be 2 sentences instead of one long phrase with so many commas.”
Response 1: We appreciate the Reviewer’s kind suggestions for further improvement of our work. According to the comments from you and the other reviewer, we polished the manuscript with professional assistance. The details can be observed in the revised manuscript. The certificate of english editing is attached to the PDF file.
Comment 2: “Same is true with the subsequent sentences that follow from line 36 till 45. I suggest cutting short the sentences by removing the commas.”
Response 2: Long sentences have been modified in the revised manuscript.
Comment 3: “Table. 1. Basic material? What does it refers to?? How is it different from AZ91 ? I think this aspect is not clear from the methods section.”
Response 3: "Basic Metal" in Table 1 has been changed to "Welding substrate" in the revised manuscript, which refers to the substrate used for surfacing welding.
Comment 4: “Referring to Figure 3. How was the volume fraction of each Al2Gd and Mg17Al12 are measured? With the low mag images, it’s highly improbable to say that we have 5.7% of beta and or 4.5% of beta phase. What’s the error bar in the measurements?? How many images were analyzed for the obtaining these statistics??” And “Table2. Same comments as above. How accurate are these percentages?”
Response 4:The volume percentage of the second phase was obtained not by image analysis of the metallographic images, but by full spectrum fitting of the XRD data using the “Rietveld phase quantitative analysis” (RQPA) technique. Although the basic theory of RQPA is consistent with the traditional quantitative methods (K-Value method and heat insulation method, etc.). The difference is that RQPA uses the whole diffraction spectrum data rather than a few diffraction peaks. Therefore, the accuracy is higher. Based on this advantage, RQPA technology has been widely used in the field of materials science, especially in the field of inorganic materials, as shown in the following literature:
①D. Bish, et al., The first X-ray diffraction measurements on Mars. IUCrJ. 1, 514-522 (2014). doi:10.1107/S2052252514021150;
② E.M. Reynolds, et al., Magneto-structural coupling in SrTcxRu1-xO3 (x=0.25,0.5) perovskites. J. Solid State Chem. 287, 121378 (2020). doi:10.1016/j.jssc.2020.121378.
In the revised manuscript, to prove the credibility of the Rietveld analysis results, Figure 5 shows the analysis results and error information. And reference 14, 15 give detailed description of the RQPA method.
Comment 5: “Page 6 – line 175: Friction coefficient remained un-changed irrespective of the Gd content. Why is that?”
Response 5: In the revision of the manuscript, information about standard deviation of the results in tribological tests are provided in Fig. 6 and 7.
Figure 6 Results of friction and wear tests of surfacing magnesium alloys under different loads: (a) wear rate, (b) friction coefficient.
Figure 7 Results of friction and wear test of surfacing magnesium alloys with varying Gd concen-trations under a load of 100 N
There may be two reasons for this phenomenon: ①Compared with tribological conditions such as load and sliding speed, the influence of material composition on friction coefficient is relatively limited. ②The results in Fig. 7 were obtained at a load of 100N, which was relatively high and caused the machine to vibrate noticeably, thus increasing the experimental error. The influence of load on experimental error was particularly evident in Fig. 6.
As a result of the above factors, the influence of Gd content on the friction coefficient was not obvious in this experiment.
This comment gives us an important reminder. Therefore, we recombed the experimental data and found that the experimental error in Figure 7 was too large to reflect the real effect of Gd content on the tribological properties. Therefore, in the revised manuscript, the author deleted the related content and limited the focus of this paper to the effect of Gd addition on the tribological behavior of surfacing alloy.
Comment 6: “Table 3. Addition of Gd has a negative influence on the mechanical properties. Even with 0.5% Gd addition there is a near 9% reduction in the hardness. Why add Gd??”
Response 6: Although the hardness of surfacing welding AZ91 alloy was reduced by the addition of Gd in this paper, the wear resistance of AZ91 surfacing alloy is significantly improved under most tribological conditions.
In addition, other literatures that the addition of Gd can simultaneously improve the tensile strength, plasticity and rolling capability of as-cast Mg-Al-Zn alloy. For example:
① W. Li, et al. Microstructure and rolling capability of modified AZ31–Ce–Gd alloys. Mater. Charact. 60, 1298-1304 (2009). doi:10.1016/j.matchar.2009.06.003;
② J. Wang, et al. Simultaneously improving strength and ductility of AZ91-type alloys with minor Gd addition. J. Alloy. Compd. 803, 689-699 (2019). doi:10.1016/j.jallcom.2019.06.313.
Therefore, the role of Gd in magnesium alloys is worth exploring systematically.
Comment 7: “Page 14, line 450 : You state that Al2Gd is effective in pinning the grain boundary. In your observation in the results section (page 4, line 131) you have the Al2Gd phases that are of 1μm of size! Do you think they are effective grain boundary pinning agents at this size? If no, then how do you explain the reduction in wear rate for 0.5Gd additions when compared to other alloys?”
Response 7: The reviewer had a very good point. On the one hand, most of the literature showed that the second phase with large size could play more effective role in pinning grain boundaries. As shown in the following literature:
① K. Chang. et al., Effect of second-phase particle morphology on grain growth kinetics. Acta Mater. 57, 5229-5236 (2009). doi:10.1016/j.actamat.2009.07.025;
② B. Pourbahari. et al., Elucidating the effect of intermetallic compounds on the behavior of Mg–Gd–Al–Zn magnesium alloys at elevated temperatures. J. Mater. Res. 32, 4186-4195 (2017). doi:10.1557/jmr.2017.415.
On the other hand, the authors found that some studies suggest the Mg-Gd phase with 2μm and Al3Zr phase with 81nm could play a role in pinning grain boundaries, as shown in the following literature:
① X. Zhang. et al., Super plasticity and microstructure in Mg–Gd–Y–Zr alloy prepared by extrusion. J. Alloy. Compd. 481, 296-300 (2009). doi:10.1016/j.jallcom.2009.03.166;
② C. Zhu. et al., Effect of solid solution state of zirconium and Al3Zr (L12) precipitates on the recrystallization behavior of Al-0.2 wt.%Zr alloy. Materialwiss. Werkst. 51, 1630-1639 (2020). doi:10.1002/mawe.202000035.
In this paper, it is true that the size and density of Al2Gd phases were relatively low. The effect in pinning grain boundaries, if any, was very limited. Therefore, we delete the description in "Conclusion" that Al2Gd phase can pin grain boundaries. In the "Discussion" part, the function of Al2Gd phase is changed from "can effectively pin the grain boundaries during sliding wear" to " may block the dislocation movement and play a strengthening effect".
In addition, the difference in Gd content among AZ91+0.5Gd, AZ91+1Gd and AZ91+2Gd alloys was too small. The real differences in wear rate may be masked by experimental errors in this paper, which was explained in detail in "Response 5". The relevant content in the revised manuscript has been deleted, and the paper will focus on the change of wear behavior of alloy before and after adding Gd element.
Once again, thank you very much for your comments and suggestions. We look forward to hearing from you regarding our submission. We would be glad to respond to any further questions and comments that you may have.
Author Response File: Author Response.pdf
Reviewer 2 Report
The idea of this paper seems interesting and it’s reported in a quite clear way. However, there is a need for certain clarifications and additions:
- What was the roughness (and other surface topography parameters) of discs and pins?
- What was the tip radius of the pin? If it was flat, how adequate contact (with the entire pin surface) to the disc surface in the initial phase was assured?
- There is no information about standard deviation or uncertainties of the results in tribological tests. It is really important in such research. Such information should be provided in Fig. 6 and 7 or in table.
- Authors should point out the sliding directions on the surface texture (Fig. 8).
- Authors mentioned that hardness is a significant parameter in the wear resistance of materials. But what is the role of the hardness of samples in wear rate needs clarification, e.g. proper graph could be provided.
- On what basis the values of the wear parameters during the test were selected (applied load, sliding speed and sliding distance). Please explain.
- The authors stated that the sliding distance was 1.5 km, was taking into account the stage of running-in of working surfaces of the friction pairs? Please explain this point.
- 8. It would be also worth clarifying where the results could be applied. Do the results have only cognitive or also applicable character?
Author Response
Response to Reviewers #2’ Comments
Ms. Ref. No.: crystals-1189801
“Investigation of the Wear Behavior of Surface Welding AZ91 and AZ91+Gd Alloys Under Variable Loading Conditions” by Qingqiang Chen, Yalei Yu, Jie Sun, Cainian Jing, Yanhua Zhao and Jia Wang
Comment 1: “What was the roughness (and other surface topography parameters) of discs and pins?”.
Response 1: In the revised manuscript, Figure 2b has been added to illustrate surface topography parameters of discs and pins.
Comment 2: “What was the tip radius of the pin? If it was flat, how adequate contact (with the entire pin surface) to the disc surface in the initial phase was assured?”
Response 2: The tip of the pin is flat. In order to ensure the adequate contact, we have made two attempts. Firstly, a high precision CNC machining center was used in the processing of the pin and disk to ensure the accuracy of the sample size. Secondly, 1 min of pre-grinding were carried out to ensure the full contact between the pin and the disc. Weighing and formal wear tests were carried out only after the pre-grinding was completed. The above content has been added to the revised draft.
Comment 3: “There is no information about standard deviation or uncertainties of the results in tribological tests. It is really important in such research. Such information should be provided in Fig. 6 and 7 or in table.”
Response 3: Thank you for your valuable advice. Error bars have been added in Figures 6 and 7. The value of the bar is derived from the standard deviation of the experimental data.
Indeed, regarding the data error, the author observed an interesting phenomenon in the process of experimental operation. When the load was low, the vibration and jitter amplitude of the testing machine was very small, and the standard deviation of the experimental data was also low. When the load was high, the vibration of the equipment increases and the standard deviation of the data increases significantly. However, since the tribological properties of AZ91 and AZ91+Gd samples differ greatly, the increase of standard deviation would not affect the analysis conclusion.
Comment 4: “Authors should point out the sliding directions on the surface texture (Fig. 8).”
Response 4: In the revised manuscript, the sliding direction has been added to the picture.
Comment 5: “Authors mentioned that hardness is a significant parameter in the wear resistance of materials. But what is the role of the hardness of samples in wear rate needs clarification, e.g. proper graph could be provided.”
Response 5: In the revised manuscript, Fig. 12 has been added to illustrate the relationship between hardness and wear rate of surfacing alloys.
Comment 6: “On what basis the values of the wear parameters during the test were selected (applied load, sliding speed and sliding distance). Please explain.”
Response 6: Firstly, in order to investigate the effects of Gd on different wear mechanisms, applied load and sliding speed were selected to allow the surfacing alloy to exhibit as many kinds of wear mechanisms as possible. Secondly, longer sliding distance could increase the quality difference before and after the wear test, so that the influence of Gd element on the wear resistance was more obvious, and the influence of instrument error could also be reduced. Therefore, in the design of the experimental scheme, the maximum sliding distance (1.5km) that can be borne by the experimental samples and machine was finally selected.
Comment 7: “The authors stated that the sliding distance was 1.5 km, was taking into account the stage of running-in of working surfaces of the friction pairs? Please explain this point.”
Response 7: The sample was pre-ground before the test, so the sliding distance of 1.5km did not include the running-in stage. In the modified Materials and Methods section, the description of pre-grinding experiment has been added.
Comment 8: “It would be also worth clarifying where the results could be applied. Do the results have only cognitive or also applicable character?”
Response 8: On the one hand, the welding wire used in this paper was developed for repairing the worn surface of magnesium alloy equipment. Therefore, the research results in this paper have direct application value in the field of surface repair and wear modification of magnesium alloy components.
On the other hand, the study on the influence of load and Gd element on the wear behavior of magnesium alloys can increase people's understanding of the wear mechanism of magnesium alloys.
Once again, thank you very much for your comments and suggestions. We would be glad to respond to any further questions and comments that you may have.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The authors have done efforts to answer all the questions addressed by the reviewer. Tha manuscript can now be accepted for publication.
