Crystallization Kinetics of Sr-Modified Precipitation Hardening Al-Si-Cu Alloys

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study investigates the impact of strontium addition on the crystallization kinetics, microstructure, and properties of Al-Si-Cu alloys post-solidification and after precipitation hardening. Using thermal-derivative analysis (TDA), microstructure observation, EDS chemical composition analysis, and X-ray phase analysis, it was found that strontium altered the derivative curve of the solidifying alloy. Strontium modified the α+β eutectic morphology from lamellar to fibrous.

Major points:

1.     I will suggest adding a touch of background in abstract which will help readers understand the relevance and context of the research.

2.  The introduction clearly states the background and significance of previous studies related to the topic, showing how the study builds on. I suggest adding a paragraph to clearly state what is the goals and objectives of this current study.

3.     I recommend author review this recent paper (Li, Q., Wang, J., Xue, C. et al. Effects of Sr on Fe-rich intermetallic in recycled Al–Si–Cu alloys. J Mater Sci 59, 11572–11595 (2024). https://doi.org/10.1007/s10853-024-09854-4) as it may offer valuable insights could enhance the discussion and contextualize your finding. They observed similar fibrous morphology transformation with addition 0.03 wt% of Sr to Al-Si-Cu. Based on this paper, first-principal calculation results shown that Sr has strong affinity to vacancy and interfaces, prohibiting the nucleation of the eutectic Si and porosity. This might be able to further support the thermal analysis results.

 

Minor points:

1.     All figures or titles’ font size is greater than main text, please adjust.

2.   I suggest combining Figure 2,3,4 to enhance readability and comparability of the figures. Similarly, Table 3,4,5 could also be combined.

3. Page 3, line 123, ‘using differential DTA curve’, DTA stand for ‘Differential Thermal Analysis’, need to be defined since this is the 1^st time mentioning in the paper.

4.     Page 4, line 142, change ‘differentia curie’ to ‘differential curve’.

5.     Page 9, line 229, change ‘1.1.’ to ‘1.2.’.

6.     Page 9, line 240, change ‘presen’ to ‘present’.

Author Response

Dear reviewer, thank you very much for the review, here are the answers

Answer to review 1

 

Major points:

1. I will suggest adding a touch of background in abstract which will help readers understand the relevance and context of the research.

It was add:

Aluminum alloys are still very useful material for diverse application. The die-casting process of Al

alloys is the most common technology to manufacture cast components. Therefore, the research on Al alloys solidification with high thermal conductivity has an important engineering significance and application value

 

 

2. The introduction clearly states the background and significance of previous studies related to the topic, showing how the study builds on. I suggest adding a paragraph to clearly state what is the goals and objectives of this current study.

 

It was add:

So the goals and objectives of this current study are at first to describe the solidification terms – mainly the temperature values for the modification phases, secondly the optimization of the heat treatment concerning time and temperature can be determined more exactly, so savings in energy can be obtained.

 

 

3. I recommend author review this recent paper (Li, Q., Wang, J., Xue, C. et al.Effects of Sr on Fe-rich intermetallic in recycled Al–Si–Cu alloys. J Mater Sci 59, 11572–11595 (2024). https://doi.org/10.1007/s10853-024-09854-4) as it may offer valuable insights could enhance the discussion and contextualize your finding. They observed similar fibrous morphology transformation with addition 0.03 wt% of Sr to Al-Si-Cu. Based on this paper, first-principal calculation results shown that Sr has strong affinity to vacancy and interfaces, prohibiting the nucleation of the eutectic Si and porosity. This might be able to further support the thermal analysis results.

The reference (Li, Q., Wang, J., Xue, C. et al. Effects of Sr on Fe-rich intermetallic in recycled Al–Si–Cu alloys. J Mater Sci 59, 11572–11595 (2024). https://doi.org/10.1007/s10853-024-09854-4) was add and mentioned in the text as reference [34]

Also the work done by L. Q. Wang and J. C. Xue offer valuable insights and contextualize our finding. The authors have also observed similar fibrous morphology transformation with addition 0.03 wt% of Sr to Al-Si-Cu. Based on this paper, first-principal calculation results shown that Sr has strong affinity to vacancy and interfaces, prohibiting the nucleation of the eutectic Si and porosity. This might be able to further support the thermal analysis results [34].

Minor points:

1. All figures or titles’ font size is greater than main text, please adjust.

Was corrected and adjusted

 

2. I suggest combining Figure 2,3,4 to enhance readability and comparability of the figures. Similarly, Table 3,4,5 could also be combined.

We have tried to combine the figure but the readability was very pure, therefore we have rather decide to leave it as it is

 

3. Page 3, line 123, ‘using differential DTA curve’, DTA stand for ‘Differential Thermal Analysis’, need to be defined since this is the 1^sttime mentioning in the paper.

Was corrected and defined where it is mentioned the 1^st time in the paper

 

4. Page 4, line 142, change ‘differentia curie’ to ‘differential curve’.

Was corrected

 

5. Page 9, line 229, change ‘1.1.’ to ‘1.2.’.

Was corrected

 

6. Page 9, line 240, change ‘presen’ to ‘present’.

Was corrected

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This paper investigates the effects of strontium addition on the crystallization kinetics, microstructure, and properties of Al-Si-Cu alloys. The experimental design is reasonable, research methods are advanced, and the analysis of results is comprehensive and in-depth, providing new insights for improving Al-Si-Cu alloy properties. However, the article needs further improvement in the following aspects:

1. Lines 26-29 outline the role of alloying elements but lack a specific explanation of the role of Cu in Al-Si-Cu alloys. It is suggested to add information on the influence of Cu on alloy properties.
2. Lines 30-44 introduce the role of Fe, occupying a large portion, while the introduction of Sr, the focus of this study, is relatively brief. It is recommended to compress the Fe-related content and expand on the mechanism of Sr's role in Al-Si alloys.
3. Lines 78-82 state the research purpose, but the expression is not clear enough. It is suggested to clearly point out the innovation of this study, such as the first systematic study on the effect of Sr on the crystallization kinetics of Al-Si-Cu alloys.
4. Lines 85-86 list the alloy composition but do not explain the reason for choosing this composition. It is advised to briefly explain the basis for selecting this composition.
5. Lines 92-104 describe the thermal analysis experimental process but do not explain why 750°C was chosen as the heating temperature. Please provide the rationale for selecting this temperature.
6. Lines 109-115 introduce the microstructure observation method but lack a description of etching conditions. It is suggested to add information on the type of etchant, concentration, and etching time.
7. Lines 146-167 analyze the influence of Sr on the alloy crystallization process, but the explanation of the mechanism by which Sr changes the crystallization temperature is not deep enough. It is recommended to further analyze the reasons for Sr's influence on crystallization temperature, combining with Sr's diffusion behavior in liquid Al.
8. Lines 189-213 calculate the latent heat of crystallization of the alloy but do not compare it with reported values in the literature. It is suggested to compare the calculation results with those of other researchers to verify the accuracy of the calculation.
9. Lines 229-256 analyze the effect of Sr on the alloy microstructure but lack quantitative analysis. It is recommended to measure and statistically analyze parameters such as the size and shape factor of the Si phase using image analysis software to quantitatively characterize the modification effect of Sr.
10. Lines 265-291 conduct X-ray diffraction analysis but do not discuss the effect of Sr on the lattice constants of the alloy. It is suggested to calculate and compare the lattice constants of the alloy before and after adding Sr, and analyze the effect of Sr on the crystal structure of the alloy.
11. Lines 293-311 summarize the research results but do not discuss the optimal addition amount of Sr for modifying Al-Si-Cu alloys. It is recommended to provide an optimal range for Sr addition based on the experimental results.
12. The conclusion section lacks an outlook for future research directions. It is suggested to add 1-2 sentences at the end of the conclusion, pointing out the limitations of this study and potential directions for future exploration.
13. The y-axis units in Figures 2-4 should be °C/s, not °C.
14. Tables 2-5 should indicate that the temperature unit is °C.
15. The scale units in Figure 6 should be μm, not mm.
16. The reference format is inconsistent. For example, the journal name formats in references 20 and 21 are inconsistent. It is suggested to uniformly adopt the abbreviated form.

Author Response

1. Lines 26-29 outline the role of alloying elements but lack a specific explanation of the role of Cu in Al-Si-Cu alloys. It is suggested to add information on the influence of Cu on alloy properties

we add to paper (red text):

Copper in aluminum reduces the grain growth rate by segregation and alters the surface tension at the liquid/grain interface. It also affects the distribution and surface contribution of the eutectic in the alloy [1].

2. Lines 30-44 introduce the role of Fe, occupying a large portion, while the introduction of Sr, the focus of this study, is relatively brief. It is recommended to compress the Fe-related content and expand on the mechanism of Sr's role in Al-Si alloys.

our answer:

We do not agree with this view. The iron content was presented in the context of the most common elements affecting the properties of aluminium alloys. The order of the topic required the role of iron to be mentioned. The contribution of strontium was devoted to the next long paragraph.

3. Lines 78-82 state the research purpose, but the expression is not clear enough. It is suggested to clearly point out the innovation of this study, such as the first systematic study on the effect of Sr on the crystallization kinetics of Al-Si-Cu alloys.

our answer:

So the goals and objectives of this current study are at first to describe the solidification terms – mainly the temperature values for the modification phases, secondly the optimization of the heat treatment concerning time and temperature can be determined more exactly, so savings in energy can be obtained. 

4. Lines 85-86 list the alloy composition but do not explain the reason for choosing this composition. It is advised to briefly explain the basis for selecting this composition.

our answer:

The alloy chosen for the study was AlSi6Cu4, which has very good machinability. Due to its properties, it is used for casting machine parts (e.g. vehicles engines), but also for decorative castings.  The disadvantages of this alloy are
to shrinkage porosity and inadequate sealing.
The other two alloys are the 0.2% strontium modified alloy mentioned above and a 1% strontium modified alloy which was added to study the effect on the crystallisation kinetics of Al-Si-Cu alloys.

5. Lines 92-104 describe the thermal analysis experimental process but do not explain why 750°C was chosen as the heating temperature. Please provide the rationale for selecting this temperature.

our answer:

In industrial casting, aluminum alloys are melted in furnaces to a temperature of approximately 700-750°C; the alloy is kept in a liquid state throughout the casting process.  

The temperature of 750°C is the result of technical conditions and long-term studies of the effects that may occur on the liquefaction of all phases of the alloy. 

6. Lines 109-115 introduce the microstructure observation method but lack a description of etching conditions. It is suggested to add information on the type of etchant, concentration, and etching time.

our answer:

For surface preparation, the standard metallographic procedure of grinding, polishing and drying was used, and the specimens were mounted in thermosetting resin. The samples were then etched in 5% HF solution at room temperature for the experimentally chosen time of 25 s for phase etching. For structural etching, the recommended solution is mainly NaOH. 

7. Lines 146-167 analyze the influence of Sr on the alloy crystallization process, but the explanation of the mechanism by which Sr changes the crystallization temperature is not deep enough. It is recommended to further analyze the reasons for Sr's influence on crystallization temperature, combining with Sr's diffusion behavior in liquid Al.

our answer:

Sr is a rather reactive element that can easily form oxides or intermetallic compounds with the alloying elements of the alloy. In fact, in the presence of Sr, some parts of the oxide layer on the surface of the melt and the entrained oxide films inside the molten alloy gradually transform into Sr-containing oxides. Due to the formation of Sr-containing compounds, with time, the modifying effect on the eutectic Si is lost [34]. 

8. Lines 189-213 calculate the latent heat of crystallization of the alloy but do not compare it with reported values in the literature. It is suggested to compare the calculation results with those of other researchers to verify the accuracy of the calculation.

our answer:

Our research is new and no articles have been found to compare with other researchers. Especially with regard to the addition of strontium.

9. Lines 229-256 analyze the effect of Sr on the alloy microstructure but lack quantitative analysis. It is recommended to measure and statistically analyze parameters such as the size and shape factor of the Si phase using image analysis software to quantitatively characterize the modification effect of Sr.

our answer:

Thank you for this comment.  The analysis of Si phase size and form factors is planned as a separate issue and will be published soon. However, it is of secondary interest in the context of the current issue.

11. Lines 293-311 summarize the research results but do not discuss the optimal addition amount of Sr for modifying Al-Si-Cu alloys. It is recommended to provide an optimal range for Sr addition based on the experimental results.

our answer:

Based on the three Sr concentrations in the AlSi6Cu4Sr(n) alloy, where n is 0, 0.2, 1%, it is difficult to determine the optimum alloy composition. Further studies are required to draw such conclusions.

12. The conclusion section lacks an outlook for future research directions. It is suggested to add 1-2 sentences at the end of the conclusion, pointing out the limitations of this study and potential directions for future exploration.

our answer:

In future studies we plan to determine the optimum combination of Al-Si-Cu alloy with the addition of Sr. In perspectives we want to focus on the effect of phase size and shape of the precipitates on the physical properties of the alloy.

13. The y-axis units in Figures 2-4 should be °C/s, not °C.

our answer:

On the left-hand side of the graphs is the Y-axis designation in the form you require (deg C * s-1).

14. Tables 2-5 should indicate that the temperature unit is °C.

our answer:

The relevant units are in the header of the table (in the first or second row of the table). They refer to all the data in the column.

15. The scale units in Figure 6 should be μm, not mm.

our answer:

The size bar in Figure 6. shows microns, not millimetres. Check again.

16. The reference format is inconsistent. For example, the journal name formats in references 20 and 21 are inconsistent. It is suggested to uniformly adopt the abbreviated form.

our answer:

I think the difference in these two articles 20-21 is due to historical problems with the publisher's name. Everywhere I have looked part 1 is published by
SKANALUMINIUM and part 2 by AFS/SKANALUMINIUM.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The draft paper was focused on the experimental study on the impact of strontium (Sr) addition to Al-Si-Cu alloys, focusing on crystallization kinetics, microstructure changes, and mechanical properties after solidification and precipitation hardening.

There are some comments to be considered:

- please add the nomenclature list for all the acronyms and parameters

- please check and revise the structure of the paper

- please check and revise the material section, adding all details for the instruments used

- I suggest to add the quantitative mechanical properties, such as hardness, tensile strength, or wear resistance, in the results section

- there is a lack of a clear statistical analysis (e.g., grain size distribution, porosity percentage)

- please clarify the impact of different Sr concentrations

- the interpretation of the TDA curves results could be expanded

- the discussion could benefit from a stronger link to other studies using TDA for Al-Si-Cu alloys

- there is insufficient explanation regarding the chemical composition changes in different phases

- the porosity analysis is lacking

- the heat treatment process applied to the samples must be considered deeply

 

Comments on the Quality of English Language

fine

Author Response

Answer to review 3

- please add the nomenclature list for all the acronyms and parameters

The list was add in the “Research material and methodology” chapter

- please check and revise the structure of the paper

The paper was revised

- please check and revise the material section, adding all details for the instruments used

Some details were add, especially for the microscope section

- please clarify the impact of different Sr concentrations

The description of Sr impact was extended.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for addressing my comments and suggestions. Regarding the figure 2-4 combination, I understood the concern of losing readability after put into one figure. 

Author Response

Dear reviewer thank You very much for the approuval.

Reviewer 2 Report

Comments and Suggestions for Authors

Accept

Author Response

Dear reviewer thank You for accepting the corrections

Reviewer 3 Report

Comments and Suggestions for Authors

I need to see the answers to my comments

Comments on the Quality of English Language

Could be improved

Author Response

Answer to review 3

 

- please add the nomenclature list for all the acronyms and parameters

The list was add in the “Research material and methodology” chapter

- please check and revise the structure of the paper

The paper was revised

- please check and revise the material section, adding all details for the instruments used

Some details were add, especially for the microscope section

- I suggest to add the quantitative mechanical properties, such as hardness, tensile strength, or wear resistance, in the results section

Wear resistance analysis was add based on porosity results. Analysis of microscopic images was used to analyse the porosity of the alloys. The ImageJ programme was used for this purpose. ImageJ is a freely available multi-platform image processing program commonly used by scientists. The pores in the microscope images were extracted using the ‘Threshold’ filter and then counted using the ‘analyse particles’ function. The following porosity values were obtained in the alloy samples: AlSi6Cu4 - 0.28%; AlSi6Cu4Sr0.2 - 0.35%, AlSi6Cu4Sr1 - 0.05%.

The following diagrams were ad too:

Figure 9. Hardness, HRB test results of the investigated alloys after strontium addition

Figure 10. Wear resistance results of the investigated alloys after strontium addition

 

- there is a lack of a clear statistical analysis (e.g., grain size distribution, porosity percentage)

The porosity analysis was add

- please clarify the impact of different Sr concentrations

The description of Sr impact was extended.

- the interpretation of the TDA curves results could be expanded

The addition of higher amounts of strontium leads to a visible overcooling of the alloy presented in Fig. 5. The reason ro this is the extended formation of intermetallic phases with strontium, which have a lover solidification temperature compared the basic aluminum matric and Al-α phase.

- the discussion could benefit from a stronger link to other studies using TDA for Al-Si-Cu alloys

The discussion was extended: After addition of 1% strontium there is still present porosity (Table 8) in the microstructure of this alloy. Like revealing in other works process innovation is urgently needed. To fundamentally improve the microstructure and reduce defects, we must control the distribution of composition and microstructure morphology in melt solidification and decrease the formation temperature [36]. On possible way could be the appliance of semisolid diecasting, which is a progressive and active near net forming technique that can be used to solve the above problems and to prepare high-quality castings.

- there is insufficient explanation regarding the chemical composition changes in different phases

The addition of Sr to Al-6%Si alloy increases its hardness and wear resistance. On the other hand, the addition of 1%Sr tends to decrease the mechanical properties compared the alloy when there was no addition of strontium to the alloy. However, the effect appears to decline as soon as the addition exceeds the 0.5% level which shows negative effect to the mechanical properties because of the presence of some compounds like for example the brittle Al3SrSi3 phase which are coarsening the eutectic silicon and reduce the properties of the alloy causing them to revert to values more typical of unmodified material.

- the porosity analysis is lacking

The porosity analysis results were add in for of tabularized values:

Table 8. Porosity analysis for the tested alloys after precipitation hardening

 

Alloy	Porosity, %
AlSi6Cu4	0.28
AlSi6Cu4Sr0.2	0.35
AlSi6Cu4Sr1	0.05

- the heat treatment process applied to the samples must be considered deeply

There was not applied any standard heat treatment to these alloys, because a thermos-derivative analysis was carried out, for the reason to melt the entire alloy with all the present phases just to obtain a 100% liquid phase which was cooled down with a determined cooling rate in the UMSA platform.

Author Response File: Author Response.docx

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

the paper was deeply revised and improved

Comments on the Quality of English Language

can be improved

Author Response

Dear reviewer thank you for the acceptance