Microstructure and Mechanical Property of Al6Si2Cu Alloy Subjected to Double-Solution Heat Treatment

Round 1

Reviewer 1 Report

1. In Figure 3 the images (e,f,g) are not described in the caption. Moreover, there are no references to the images in the text. The discussion of figure 3 is poor.
2. The structure evolution is not obvious according to figure 6.
3. The two-stage heat treatment is well known to avoid the melting of non-equilibrium θ-phase after casting and therefore the novelty of current research is poor.
4. The second conclusion is too superficial. Obviously after annealing the diffraction peaks for the θ-phase should be eliminated.

Author Response

REVIEWER 1:

Comments and Suggestions for Authors

1. In Figure 3 the images (e,f,g) are not described in the caption. Moreover, there are no references to the images in the text. The discussion of figure 3 is poor.

In the caption of Fig.3, in addition to the (e), (f), (g), all of the captions for figures have been revised. Also, the description of the figures has been explained in detail in the text and revised.

 

 

2. The structure evolution is not obvious according to figure 6.

All of the micrographs in Fig. 6 are the typical microstructure at each heat-treated sample. Of course, as the reviewer’s comments, the microstructure evolution seems to be not obvious, but the long blocky and needle shape Si phase are fragmented and sherardized as shown in these figures. Table 3 shows the quantitative data for the Si phase at each heat treatment condition.

 

 

3. The two-stage heat treatment is well known to avoid the melting of non-equilibrium θ-phase after casting and therefore the novelty of current research is poor.

In the present industries, a lot of Al-based alloys have been applied concerning chemical composition. Especially, many Al-based alloys are used for automotive materials. Because there is no standard and specification about double solution heat treatment in this type of alloy, it does not apply to the industries. Therefore, this study has originality for the standardization of data for double solution heat treatment of Al6Si2Cu alloy.

 

 

4. The second conclusion is too superficial. Obviously after annealing the diffraction peaks for the θ-phase should be eliminated.

As the reviewer's comments, the diffraction peaks should be disappeared after annealing heat treatment following the phase diagram when the heat treatment time is unlimited and for a long time. But, this phase diagram is a very ideal condition for pressure, temperature, an unlimited time for heat treatment. Therefore, the diffraction peaks may be present depending on the heat treatment condition, temperature, and time. In this study, we investigate the presence of the Al2Cu phase during heat treatment.

 

Reviewer 2 Report

Microstructure and Mechanical Property of Al6Si2Cu Alloy Subjected to Double-solution Heat Treatment

1. The chemical composition was 6.01Si, 2.09Cu, 1.03Fe, 0.19Mn, and Al balance. Atomic or weight percent?
2. What is the melting point for the studied alloy?
3. After the first annealing step, “the θ-Al2Cu phase was partially dissolved”. After that samples were heated higher the melting point of the θ phase, which should provide melting. What is the reason to overheat the alloys causing the melting?
4. The error bars are required for the mechanical properties. In the present form, the results and conclusions are not reliable. How many samples were tested for each point?
5. The mechanical properties are poorly analyzed. The authors studied multiple regimes and it is not clear which of them is optimal. In conclusion, the authors mentioned the T44B-DS8B regime. Is it optimal? This regime provided the highest UTS but the YS is not the highest compared to other regimes. It is known that the YS is a more important characteristic for the construction materials. What was the criterium?

Author Response

REVIEWER 2:

Comments and Suggestions for Authors

Microstructure and Mechanical Property of Al6Si2Cu Alloy Subjected to Double-solution Heat Treatment

1. The chemical composition was 6.01Si, 2.09Cu, 1.03Fe, 0.19Mn, and Al balance. Atomic or weight percent?

Weight percent

 

 

2. What is the melting point for the studied alloy?

The melting temperature is calculated by thermal simulation in our study. It is about 620 °C.

 

3. After the first annealing step, “the θ-Al2Cu phase was partially dissolved”. After that samples were heated higher the melting point of the θ phase, which should provide melting. What is the reason to overheat the alloys causing the melting?

As the reviewer's comments, after the first-step annealing, the coarse Al2Cu phase is partially dissolved into the Al matrix. But, it is not enough to dissolve the Al2Cu phase to obtain the great effect of the solid solution strengthening in this Al6Si2Cu alloy. Therefore, we try to dissolve thoroughly this Al2Cu phase as possible as we can without any incipient melting of the Al2Cu phase, which may generate a defect (pore) in the site of the Al2Cu phase.

 

4. The error bars are required for the mechanical properties. In the present form, the results and conclusions are not reliable. How many samples were tested for each point?

The statistical and quantitative analysis for the mechanical properties is very important and reliable to understand the physical phenomenon. As the reviewer's comments, the error bar of the data is already applied to the hardness data. As already shown in the experimental details, the hardness test was achieved 20 times for each specimen, and then averaged the data. In the case of the tensile test, we tested 3 times at each specimen, and also the data was averaged as shown in figure 9.

 

5. The mechanical properties are poorly analyzed. The authors studied multiple regimes and it is not clear which of them is optimal. In conclusion, the authors mentioned the T44B-DS8B regime. Is it optimal? This regime provided the highest UTS but the YS is not the highest compared to other regimes. It is known that the YS is a more important characteristic for the construction materials. What was the criterium?

As the reviewer's comments, the mechanical properties are discussed in more detail and revised in the text. Of course, the yield strength is a very important parameter for structural materials and analysis the physical integrity. However, in the automotive industry, the tensile strength of Al alloy is also important to qualify the mechanical stability. Most of all, for the lifetime and fatigue damage, elongation and toughness are also important parameter. Therefore, in this study, the optimal heat treatment condition should be the double solution treatment, T44B-SS8B.

Reviewer 3 Report

This work tried to apply two stage homogenization treatments on Al6Si2Cu alloy to optimize its mechanical property controlled by the microstructure. The microstructures were characterized via OM, BSE, EPMA and XRD, additionally, its equilibrium phase diagram was also calculated basing on thermal-calc. The results can support the discussion and conclusions.

1: What is the threshold temperature for the overburning to take place in this Al6Si2Cu alloy?

2: What is the influence of the solution treatment time? Will extending the solution treatment time improve the mechanical properties?

3: Please clarify why there is such a huge difference of elongation between T44A and T44B alloy?

Author Response

REVIEWER 3:

Comments and Suggestions for Authors

This work tried to apply two stage homogenization treatments on Al6Si2Cu alloy to optimize its mechanical property controlled by the microstructure. The microstructures were characterized via OM, BSE, EPMA and XRD, additionally, its equilibrium phase diagram was also calculated basing on thermal-calc. The results can support the discussion and conclusions.

 

1. What is the threshold temperature for the overburning to take place in this Al6Si2Cu alloy?

In the case of solution treatment, overburning of microstructure and mechanical properties is about over 510. We usually only consider the temperature for the heat treatment condition. But, heat treatment time is also very important. The phase diagram is based on unlimited time.

 

 

2. What is the influence of the solution treatment time? Will extending the solution treatment time improve the mechanical properties?

As we explain in the second response, the phase diagram is based on unlimited time, pressure, and temperature. Also, the solution heat treatment is for the dissolution of alloy elements into the matrix metal. To enhance the mechanical property, there are many mechanisms for an increase in the mechanical strength of materials. In most cases, the solution heat treatment is not the only technique to enhance physical strength. However, in this study, the coarse Al2Cu is not good for mechanical strength in this type AlSiCu alloy. Therefore, we are trying to dissolve thoroughly with a double solution without the incipient melting of Al2Cu in the Al matrix. There is an optimal condition of heat treatment time at each sample condition and alloy types of aluminum.

 

3. Please clarify why there is such a huge difference of elongation between T44A and T44B alloy?

The difference of elongation between T44A and T44B alloy is not huge as the reviewer’s comments. In Fig. 9, the elongation of T44A and T44B is 000 and 000, as well. There are various microstructural features to influence the elongation of materials. In this study, the major microstructural parameter is brittle phases, several secondary phases, Fe-rich phase, Cu-rich phase, and Si particles. Most of all, in this study, Si particle is a great effect on the elongation of Al alloy, the mean area, and aspect ratio of Si particle as shown in Table 3. For this physical reason, we can concentrate on the Table 3 results, mean area, and aspect ratio of Si particle. Therefore, we can get a higher elongation in the T44B-SS8B sample than the T44A-SS8B sample as shown in Fig. 9(b).

Round 2

Reviewer 1 Report

The paper may be published in current state

Author Response

Thanks for your comments.

Reviewer 2 Report

The paper can be published

Author Response

Thanks for your comments.