Effect of Electric Pulse Current Rapid Aging Treatment on Microstructure and Properties of Al-7Si-0.55Mg Alloy

Round 1

Reviewer 1 Report

The article is devoted to the study of the possibility of improving the mechanical properties of the Al-Si-Mg alloy by applying electric pulse treatment with saving energy. A significant increase in elongation was achieved as a result of research. However, the paper has many shortcomings that need correction to improve it. There are some comments:

Authors write in the experimental section: “One group of three Al-7Si-0.55Mg alloy plates were treated by conventional aging treatment in a box-type resistance furnace at 160 °C for 8 h”. But what about the second group? What time was applied for aging with electric pulse treatment? In fig. 3, the microstructures after casting and heat treatment are compared. However, it seems that these two microstructures are very similar. Neither the form of eutectic Si nor its sizes are visible. Why should eutectic Si change its morphology during heat treatment? On page 4: “There are smaller precipitated phases and some inclusions in the dimples, which reduces the stress concentration between precipitated phase and matrix at grain boundary, and prevents the generation and expansion of microcracks. It significantly improves the morphology of eutectic Si phase and the comprehensive properties of the alloy.” It is unclear. Why does this improve the morphology of eutectic Si phase? On page 4, authors write: “There are only a small part of rich areas and the smaller pulse current value (300 A) provides less diffusion activation energy for the solute atoms. As shown in Figure 5 (b) and (c), with the increase in pulse current, the diffusion activation energy provided for solute atoms is larger.” It should be clarified. In addition, it seems that the microstructural images in fig. 5 and 6 are also similar. And they have a different contrast. Microstructural images with a higher magnification should be presented. What is the difference between microstructures with a pulse frequency of 10 Hz and a frequency of 40 Hz? On page 5: “It can be found that the application of electric pulse provides more diffusion activation energy for the solute atoms and the grains are remarkably refined, which reduces the stress concentration between precipitated phase and matrix at grain boundary, and prevents the generation and expansion of microcracks.” This sentence needs clarification. In addition, a quantitative analysis of the gran sizes should be carried out. What is the reason for grain refining? How does grain refining reduce stress concentration? On page 6, line 163: “It can be obviously seen from Figure 7 that some spherical and short rod precipitates are distributed in α-Al matrix.” What precipitates are these? What are the differences between TEM images in fig. 7? How do modes of pulse current treatment affect the microstructure? What do the SAED patterns and insets in fig. 7 mean? On page 7, line 182: “With the increase of pulse current, the diffusion activation energy provided for solute atoms is larger. It significantly improves the properties of the alloy and the grain structure is more uniform and fine.” Why does elongation decrease with an increase in pulse current peak from 300 to 900 A? Only elongation is improved compared to conventional heat treatment, not strength. On page 8, line 202: “The GP zone is coherent with α-Al matrix and probably have a spherical shape.” Why probably? What about the TEM studies in fig. 7? In line 221: “The main effect of this experiment is the non-thermal due to the short conduction time during the aging treatment of the electric pulse assisted Al-7Si-0.55Mg alloy.” What was the conduction time used? 5 hours? This is a large time to affect the temperature. In this case, it needs to measure temperature during aging while pulse current treatment applies. On page 9, line 247-251: “The thermodynamic energy barriers to be overcome in phase nucleation decrease, which increases the nucleation rate of the β phase.” But this phase is stable and appears at overaging. How does the appearance of a stable β phase affect properties? And further: “Compared with conventional heat treatment, it has a faster nucleation rate and the precipitated phase is uniform and fine.” It needs confirmation by TEM studies in fig. 7. And finally: “The elongation and strength of the alloy is significantly improved after electric pulse assisted aging treatment.” Only elongation is significantly improved compared to conventional heat treatment, while strength is the same. In conclusion (1): “The microstructure of as-cast Al-7Si-0.55Mg alloy is mainly composed of α-Al matrix and eutectic (α-Al+Si). The eutectic Si distributed along grain boundaries is coarse dendrites or needles. The coarse dendrites of Al-7Si-0.55Mg alloy were mainly eliminated after conventional heat treatment and electric pulse assisted aging treatment. The morphology of eutectic Si distributed along grain boundaries is spherical or short rod. The size of eutectic Si is finer and more evenly distributed in α-Al matrix.” Whose coarse dendrites were eliminated? Eutectic Si? But the eutectic Si had to change the morphology during solid solution treatment (540 °C, 10 h), not during aging. In conclusion (2): “The strength and elongation of as-cast Al-7Si-0.55Mg alloy are 183 MPa and 6%, respectively. After conventional heat treatment, the tensile strength and elongation of the alloy are significantly increased to 279.1 MPa and 8.2%.” This is well known to write about this in conclusions. And further: “The grains are remarkably refined, which reduces the stress concentration between precipitated phase and matrix at grain boundary, and prevents the generation and expansion of microcracks. It significantly improves the properties of the alloy and reduces the aging time by 3 h, saving energy.” There is no significant evidence of remarkable grain refining as a result of electric pulse treatment. In addition, it should be added to fig. 8 properties obtained after 5 h of conventional aging and compare them with ones after electric pulse treatment to confirm the effectiveness. So, what are the main reasons of improving in elongation?

Author Response

Dear reviewer, 

Thank you for your comments concerning our manuscript entitled “Effect of Electric Pulse Current Rapid Aging Treatment on Microstructure and Properties of Al-7Si-0.55Mg Alloy” (ID: metals-614138). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are clearly highlighted in the paper. The main corrections in the paper and responds are as following: 

Point 1: Authors write in the experimental section: “One group of three Al-7Si-0.55Mg alloy plates were treated by conventional aging treatment in a box-type resistance furnace at 160 °C for 8 h”. But what about the second group? What time was applied for aging with electric pulse treatment?

Response 1: We are very sorry for our negligence of the applying time for aging with electric pulse treatment. When the box type resistance furnace temperature was 160℃, the alloy plates were put into the furnace. Pulse power supply was turned on and the corresponding parameters were set up. The time of electrical pulse assisted aging treatment is 5 h, in which the time of applying electric pulse is 30 s (pulse width of 100 μs). The revised details can be found in Line 81-82, page 2.

Point 2: In fig. 3, the microstructures after casting and heat treatment are compared. However, it seems that these two microstructures are very similar. Neither the form of eutectic Si nor its sizes are visible. Why should eutectic Si change its morphology during heat treatment? On page 4: “There are smaller precipitated phases and some inclusions in the dimples, which reduces the stress concentration between precipitated phase and matrix at grain boundary, and prevents the generation and expansion of microcracks. It significantly improves the morphology of eutectic Si phase and the comprehensive properties of the alloy.” It is unclear. Why does this improve the morphology of eutectic Si phase?

Response 2: Thank you for your kind advice, we have replaced the microstructural images with higher magnification. The as-cast Al-7Si-0.55Mg alloy is mainly composed of α-Al matrix and eutectics. As is well-known, silicon nucleates at grain boundaries in the process of solidification of the alloy and exists in the eutectic Si. The morphology of eutectics in the alloy is mainly long and thin strip. Tearing effects play a negative role in the matrix because of its sharp end, which limited the properties of as-cast alloys. During the solid solution treatment of Al-7Si-0.55Mg alloy, the long strip Si phases were fused and spheroidized into short strips or spheres. When the load was applied to the alloy, the effect of splitting on the matrix was reduced, which was beneficial to improve the strength of Al-7Si-0.55Mg alloy. The revised details can be found in Line 104-106, page 3.

Point 3: On page 4, authors write: “There are only a small part of rich areas and the smaller pulse current value (300 A) provides less diffusion activation energy for the solute atoms. As shown in Figure 5 (b) and (c), with the increase in pulse current, the diffusion activation energy provided for solute atoms is larger.” It should be clarified. In addition, it seems that the microstructural images in fig. 5 and 6 are also similar. And they have a different contrast. Microstructural images with a higher magnification should be presented. What is the difference between microstructures with a pulse frequency of 10 Hz and a frequency of 40 Hz? On page 5: “It can be found that the application of electric pulse provides more diffusion activation energy for the solute atoms and the grains are remarkably refined, which reduces the stress concentration between precipitated phase and matrix at grain boundary, and prevents the generation and expansion of microcracks.” This sentence needs clarification. In addition, a quantitative analysis of the gran sizes should be carried out. What is the reason for grain refining? How does grain refining reduce stress concentration?

Response 3: Thank you for your kind advice, we have replaced the microstructural images with higher magnification. According to the discussion section in paper, the electric pulse can accelerate the nucleation of phase transformation by decreasing the thermodynamic energy barrier with the increase of pulse current, which increases the nucleation rate of precipitated phase. It can be seen from Figure 6 that the microstructure changes of Al-7Si-0.55Mg alloy after electric pulse aging treatment is not obvious. With the increase of pulse frequency, the convection of atoms in the alloy increases, and the precipitation rate of Mg and Si elements from supersaturated solid solution increases. However, there is an optimal value for pulse frequency. The internal atoms of strike in two directions during electric pulse assisted aging treatment of Al-7Si-0.55Mg alloy because of the combination of electric and temperature field. The contents of silicon and magnesium are constant in Al-7Si-0.55Mg alloy, so the number of precipitates is continuously precipitating from the supersaturated solid solution. Electric pulse increases the atomic diffusion rate, nucleation rate of eutectic and precipitated phase (Mg₂Si), which is beneficial to the vacancy movement. It leads to obtaining more uniform and finer precipitates. We have re-written this part according to your kind suggestion and the detailed revision can be found in Line 147-169, Page 4.

Point 4: On page 6, line 163: “It can be obviously seen from Figure 7 that some spherical and short rod precipitates are distributed in α-Al matrix.” What precipitates are these? What are the differences between TEM images in fig. 7? How do modes of pulse current treatment affect the microstructure? What do the SAED patterns and insets in fig. 7 mean? On page 7, line 182: “With the increase of pulse current, the diffusion activation energy provided for solute atoms is larger. It significantly improves the properties of the alloy and the grain structure is more uniform and fine.” Why does elongation decrease with an increase in pulse current peak from 300 to 900 A? Only elongation is improved compared to conventional heat treatment, not strength.

Response 4: Thank you for your kind suggestion. Figure 7 shows the bright field images and high-resolution transmission electron microscopy (HRTEM) images of Al-7Si-0.55Mg alloy along the direction of [110]_Al, which mainly looks at the morphology and distribution of the precipitated phase. There are some large dark gray Si particles, spherical and short rod precipitates in Figure 7. The spherical precipitates are identified as GP zones, which are coherent with α-Al matrix and the size of GP zone is small. The short rod precipitates are identified as β' phase (Mg₂Si). As shown in Figure 7 (a) that there are some spherical precipitates with a diameter of less than 6nm and a few different lengths of rod precipitates about 15 nm~31 nm in length, 3 nm~6 nm in thickness. As shown in Figure 7 (b) that the Si particles decreases obviously, and the rod precipitated phase of Mg₂Si increases gradually. The length and width of precipitation phase (Mg₂Si) is about 11 nm~32 nm and 4 nm~7 nm. As shown in Figure 7 (c) and (d), Si particles are unevenly distributed in α-Al matrix and the rod precipitated phase of Mg₂Si decreases obviously. The morphology of precipitates and selective electron diffraction (SAED) at conventional heat treatment are shown in Figure 7 (e), which has less spherical and rod precipitates. Compared with the conventional heat treatment, although the time of electric pulse assisted aging treatment is 3 h shorter than that of conventional aging, the spherical precipitates are more evenly distributed in α-Al matrix. The pulse current promotes the nucleation and growth process of precipitates, which provides priority conditions for the nucleation of precipitates phase. We have re-written this part according to your kind suggestion and the detailed revision can be found in Line 199-230 and 251-255, Page 5-7.

Point 5: On page 8, line 202: “The GP zone is coherent with α-Al matrix and probably have a spherical shape.” Why probably? What about the TEM studies in fig. 7? In line 221: “The main effect of this experiment is the non-thermal due to the short conduction time during the aging treatment of the electric pulse assisted Al-7Si-0.55Mg alloy.” What was the conduction time used? 5 hours? This is a large time to affect the temperature. In this case, it needs to measure temperature during aging while pulse current treatment applies. On page 9, line 247-251: “The thermodynamic energy barriers to be overcome in phase nucleation decrease, which increases the nucleation rate of the β phase.” But this phase is stable and appears at overaging. How does the appearance of a stable β phase affect properties? And further: “Compared with conventional heat treatment, it has a faster nucleation rate and the precipitated phase is uniform and fine.” It needs confirmation by TEM studies in fig. 7. And finally: “The elongation and strength of the alloy is significantly improved after electric pulse assisted aging treatment.” Only elongation is significantly improved compared to conventional heat treatment, while strength is the same.

Response 5: We are very sorry for our incorrect writing. What we want to show is that the GP zone is spherical and coherent with α-Al matrix. There are some large dark gray Si particles, spherical GP zones and short rod β' phase (Mg₂Si) in Figure 7. The time of electrical pulse assisted aging treatment is 5 h, in which the time of applying electric pulse is 30 s (pulse width of 100 μs). The main effect of this experiment is the non-thermal due to the short conduction time during the aging treatment of the electric pulse assisted Al-7Si-0.55Mg alloy. The thermodynamic energy barriers to be overcome in phase nucleation decrease, which increases the nucleation rate of the β' phase. Compared with conventional heat treatment, although the time of electric pulse assisted aging treatment is 3 h shorter than that of conventional aging, the spherical precipitates are more evenly distributed in α-Al matrix. The elongation of the alloy is significantly improved after electric pulse assisted aging treatment. The pulse current promotes the nucleation and growth process of precipitates, which provides priority conditions for the nucleation of precipitates phase. Thank you for your kind suggestion and the detailed revision can be found in Line 278 and 325-329, Page 8 and 9.

Point 6: In conclusion (1): “The microstructure of as-cast Al-7Si-0.55Mg alloy is mainly composed of α-Al matrix and eutectic (α-Al+Si). The eutectic Si distributed along grain boundaries is coarse dendrites or needles. The coarse dendrites of Al-7Si-0.55Mg alloy were mainly eliminated after conventional heat treatment and electric pulse assisted aging treatment. The morphology of eutectic Si distributed along grain boundaries is spherical or short rod. The size of eutectic Si is finer and more evenly distributed in α-Al matrix.” Whose coarse dendrites were eliminated? Eutectic Si? But the eutectic Si had to change the morphology during solid solution treatment (540 °C, 10 h), not during aging. In conclusion (2): “The strength and elongation of as-cast Al-7Si-0.55Mg alloy are 183 MPa and 6%, respectively. After conventional heat treatment, the tensile strength and elongation of the alloy are significantly increased to 279.1 MPa and 8.2%.” This is well known to write about this in conclusions. And further: “The grains are remarkably refined, which reduces the stress concentration between precipitated phase and matrix at grain boundary, and prevents the generation and expansion of microcracks. It significantly improves the properties of the alloy and reduces the aging time by 3 h, saving energy.” There is no significant evidence of remarkable grain refining as a result of electric pulse treatment. In addition, it should be added to fig. 8 properties obtained after 5 h of conventional aging and compare them with ones after electric pulse treatment to confirm the effectiveness. So, what are the main reasons of improving in elongation?

Response 6: Thank you for your kind suggestion. Just like what you said, the eutectic Si had to change the morphology during solid solution treatment, not during aging. The microstructure of as-cast Al-7Si-0.55Mg alloy is mainly composed of α-Al matrix and eutectic (α-Al+Si). The morphology of eutectics in the alloy is mainly long and thin strip. Tearing effects play a negative role in the matrix because of its sharp end, which limited the properties of as-cast alloys. During the solid solution treatment of Al-7Si-0.55Mg alloy, the long strip Si phases were fused and spheroidized into short strips or spheres. Compared with as-cast alloy, the morphology of eutectic Si distributed along grain boundaries is spherical or short rod after heat treatment.

The strength and elongation of as-cast Al-7Si-0.55Mg alloy are 183 MPa and 6%, respectively. After conventional heat treatment, the tensile strength and elongation of the alloy are significantly increased to 279.1 MPa and 8.2%. The elongation of Al-7Si-0.55Mg alloy is significantly improved after electric pulse assisted aging, which is higher than that after conventional heat treatment. The strength of Al-7Si-0.55Mg alloy after electric pulse assisted aging is similar to that of conventional heat treatment. When the pulse frequency and pulse current are 10 Hz and 1200 A, the strength and elongation of the alloy are the highest. Compared with the conventional heat treatment, although the time of electric pulse assisted aging treatment is 3 h shorter than that of conventional aging, the spherical precipitates are more evenly distributed in α-Al matrix. The pulse current promotes the nucleation and growth process of precipitates, which provides priority conditions for the nucleation of precipitates phase. The main reason for the increasing of elongation is the more uniform distribution of precipitated phase. We have re-written this part according to your kind suggestion and the detailed revision can be found in Line 332-336 and 342-347, Page 9.

Thank you very much for your comments and suggestions. I found the deficiencies in my current work, and I will improve my scientific research level according to your advice in future work. 

Yours sincerely,

Kaijiao Kang

Author Response File: Author Response.pdf

Reviewer 2 Report

In this study, the Al-7Si-0.55Mg aluminum alloy was subjected to the heat treatment by using electric pulse. The heat treatment response was investigated by tensile test, hardness measurements, microstructural observations using OM, SEM and TEM.

Paper is very well written and research may attract many researchers. Overall quality of the paper is adequate for publishing without major revision.

Author Response

Dear reviewer, 

Thank you very much for your kind comments concerning our manuscript entitled “Effect of Electric Pulse Current Rapid Aging Treatment on Microstructure and Properties of Al-7Si-0.55Mg Alloy” (ID: metals-614138).

Yours sincerely,

Kaijiao Kang

Reviewer 3 Report

Dear Authors,

 

Lines 253-254: The microstructure of as-cast Al-7Si-0.55Mg alloy is mainly composed of α-Al matrix and eutectic (α-Al+Si). The eutectic Si distributed along grain boundaries is coarse dendrites or needles. The coarse dendrites of Al-7Si-0.55Mg alloy were mainly eliminated after conventional heat treatment and electric pulse assisted aging treatment. The morphology of eutectic Si distributed along grain boundaries is spherical or short rod. The size of eutectic Si is finer and more evenly distributed in α-Al matrix. This is obvious (look at the red text).This is not a pioneering discovery of the Authors.  Figure 2. Standard tensile test plate of the Al-7Si-0.55Mg alloy. I doubt that this drawing adds something to the article. Please think about it. Work written correctly. All necessary tests were carried out. Analysis of the current state of knowledge is good. The analysis of the results is good. In my opinion, the article is suitable for publication in the Metals journal.

Yours faithfully
reviewer

Author Response

Dear reviewer, 

Thank you for your comments concerning our manuscript entitled “Effect of Electric Pulse Current Rapid Aging Treatment on Microstructure and Properties of Al-7Si-0.55Mg Alloy” (ID: metals-614138). Thank you for your kind suggestions. We have made correction which we hope meet with approval. We added “As is well-known” in revised manuscript that can be found in Line 102, page 3. 

Thank you very much for your comments and suggestions. Those comments are all valuable and very helpful for revising and improving our paper. 

Yours sincerely,

Kaijiao Kang

Reviewer 4 Report

Paper has a correct structure. This article requires several corrections. In figures 3,4,5,6,7, complete the magnification.

Author Response

Dear reviewer, 

Thank you for your comments concerning our manuscript entitled “Effect of Electric Pulse Current Rapid Aging Treatment on Microstructure and Properties of Al-7Si-0.55Mg Alloy” (ID: metals-614138). Thank you for your kind suggestions, we have replaced the microstructural images with higher magnification in Figures 3,4,5,6,7. We hope meet with approval.

Thank you very much for your comments and suggestions. Those comments are all valuable and very helpful for revising and improving our paper. 

Yours sincerely,

Kaijiao Kang

Round 2

Reviewer 1 Report

I thank the authors for their responses to my comments.