Fabrication of Functionally Graded Materials Using Aluminum Alloys via Hot Extrusion

Round  1

Reviewer 1 Report

The paper is in the second round of evaluation. Despite some improvement, the paper must be fundamentally revised again.

1. Introduction.

1.1. Lines 35-46 contain a very general discussion of Al alloys and Al-based composite application. This is well known information, which is not related to the topic of presented paper. At the same time, there are many papers considering production of FGM by extrusion. In fact, this process is not new and was considered for long time. The authors should discuss these papers in introduction to justify the novelty of their research.

1.2. Line 50. What the authors had in mind with “by adding other elements”. Please, make this sentence clear for any reader.

1.3. Line 57. The sentence starting with “Bahr et al….” does not relate to the paper content and must be omitted.

1.4. Lines 59-71. The authors discussed different methods for FGM production. This discussion should be replaced by analysis of FGM manufacturing by extrusion (see my comment Nr. 1.1).

1.5. The authors do not justify in the paper the aim of their research. They did not mention the possible application. They did not explain in the paper why the can was made from 6063 alloy and the core from 3003 alloy. The authors partly answered these questions in the reply to reviewer, but they did not include the corresponding explanations in the paper. The authors must also justify the choice of can’s dimensions (inner and outer diameters and length).

1.6. Line 72. The authors confused the definitions “composite material” and “FGM”. This is completely different structures. Please correct here and through the entire text.

1.7. Line 74. How the core can provide a better machinability? Please, explain this statement or exclude.

1.8. Lines 87-95 and Fig. 1. The statement that a bulk/bulk contact has a smaller contact area comparing with a bilk/powder contact is wrong. In first case we have a surface/surface type of contact. In second case we have a surface/point type of contact. Obviously, that in first case (bulk/bulk contact) the total contact area is larger. The lines 87-95 and Fig. 1 must be removed from the paper. An advantage of the use of powder could be a large deformation of particles during extrusion leading to better joining of contacting surfaces.

2. Materials and methods

2.1. Figure 2. How the bottom and the tube were joined in the can? Please explain this. The capture should be changed as “Schematic view of Al 6063 can”.

2.2. What was the mass of 3003 powder poured into the can?

2.3. Line 105. 49 kN is a force and not a pressure. Was any lubricant during extrusion used? Please, add the corresponding information.

2.4. Fig. 3 can be replaced just by SEM image of 3003 particles.

2.5. How you can use Archimedes method for density measurement? The FGM consists of a dense external cover and a powder core. What was measured by Archimedes method? Please, use the image analysis or any other suitable technique to determine the porosity of powder core. By the way, the pores in 3003 part are clear visible in Figs. 5 and 6.

2.6. Line 121. Please, type Czech Rep., not just Czech.

2.7. The type the specimens used for tensile test must be provided in the paper and not just in the response to the reviewer. Also the relation between the 6063 and 3003 area in samples must be provided in the paper.

3. Results

3.1. Lines 136-138. The influence of particle size distribution on packing density is well known. Please, omit the sentence starting with “Desmond…” and corresponding reference [12].

3.2 . Line 142. Remove “The”.

3.3. Lines 145-150. How the chemical composition of 3003 powder was determined? Please, explain this.

3.4. From Fig. 4(b) is obviously, that all mechanical properties of FGM are determined by 6063 material. An area fraction of 3003 is only around of 1.3%.

3.5. Please explain the large amount of Si in Fig. 5 (f). It looks like this is the rest of a polishing paste. The same for white phase in Fig. 5. This particularly means that the samples were not good enough prepared.

3.6. Fig. 8. Please replace all Vickers indentations by a graph or a table. These pictures are not informative.

3.7. Lines 216-219. Please, remove the sentence starting with “A greater…”.

3.8. Fig. 9. Change the vertical axe title as “Tensile stress”, not strength.

3.9. The strength analysis of FGM by Hall-Petch or Kelly-Tyson equation is meaningless because tensile strength is nearly fully determined by 6036 properties (see my comment Nr. 3.4).

Concluding remark

The paper must be fundamentally revised.

Author Response

Review answers

Fabrication of Aluminum Alloys Functionally Graded Materials by Hot Extrusion Process

Dasom Kim, Kwangjae Park, Minwoo Chang, Sungwook Joo, Sanghwui Hong, Seungchan Cho, Hansang Kwon^*

E-mail: [email protected], [email protected]

First of all, thank you for your careful reading our research. We have revised the manuscript based on the comments of reviewers. We would like to ask that you kindly proceed with a second review. We here address the comments made by the reviewers:

Dear reviewer #1:
Thank you very much for your careful review about our research. We agree with your opinion that some part in this study should be more defined clearly. We have revised the manuscript based on your comments. The paper has become much more complemented and scientific due to your careful comments. I appreciated again sincerely.

1. Introduction

1.1. Lines 35-46 contain a very general discussion of Al alloys and Al-based composite application. This is well known information, which is not related to the topic of presented paper. At the same time, there are many papers considering production of FGM by extrusion. In fact, this process is not new and was considered for long time. The authors should discuss these papers in introduction to justify the novelty of their research.

(RESPONSE)

We totally agree with your opinion. This paragraph 1 (35-46) was inserted to introduce the aluminum as a lightweight material and some aluminum composites studied by other researchers recently. It needs to be shorten because it is general and not focus in this study.

As you mentioned, the introduction about process is in other paragraph 2 (59-71). It need to be justified about the novelty of this research.

(paragraph 1)

Before)

In recent years, lightweight materials have been extensively developed for use in transport machineries, especially for aircraft, automobile, and marine applications. In particular, aluminum is usually used to realize lightweight in materials. Therefore, aluminum has been used in the form of an alloy by adding other elements by many researchers. Kelkar et al. tried to use can aluminum as automobile bodies and analyzed the costs which can be safe when aluminum is alternative to steel [1]. Mechanical superiority of Al-stainless steel(SUS) composites which have lightweight and high strength fabricated by spark plasma sintering method was also reported by Park et al. [2]. He confirmed that Al-50vol.% SUS has very high hardness (almost 630 HV), this value is 21 times higher than pure Al.

Revised)

In recent years, lightweight materials have been extensively developed for use in transport machineries, especially for aircraft, automobile, and marine applications. In particular, aluminum is usually used to realize lightweight in materials. Therefore, aluminum has been used in the form of an alloy by adding other elements such as stainless steel, copper and CNTs by many researchers [1-3].

(paragraph 2)

Before)

Researchers have attempted new fabrication methods to solve this problem. Watanabe et al. tried to fabricate FGMs by centrifugal casting method, which is a casting process to produce cylindrical parts [5]. The advantage of this method is that the microstructure can be controlled and good mechanical properties can be obtained. However, it was reported that nanoparticles were distributed only on the surface, and thus, the mechanical properties were not evidently improved. Liu et al. attempted to fabricate MnSb/Sb-MnSb FGMs using a semi-solid forming process under a magnetic field [6]. Although the FGMs was controlled effectively using a magnetic field, it was difficult to control the field gradient. Kwon et al. attempted to fabricate FGMs using spark plasma sintering (SPS) [7]. Although the FGMs was superior mechanical properties, it was difficult to control Al carbides. However, that hot extrusion process improved the tensile strength and hardness of the Al-CNTs composite, even though only 1 vol. % of CNTs was added reported by Kwon and Leparoux [8]. In addition, Li and Xie fabricated the W/Cu FGMs which was fabricated via multi-billet extrusion exhibited a crystalline structure with improved density [9].

Revised)

Researchers have attempted new fabrication methods such as centrifugal casting, semi-solid forming under magnetic field and SPS to solve this problem [5-7]. The extrusion process such as Co-extrusion, indirection, constant extrusion has been used as one of the method of fabrication for over 30 years [8,9]. Fan et al. tried to joining of Al1060 and Al6063 by welding following die extrusion [10]. There are many cracks in the interfaces due to the difficulty of controlling stress. Kwon et al. tried hot extrusion process to fabricated Al-CNT composite [11]. The microstructure was shown elongated with the parallel to extrusion direction with no cracks and pores. Even though the composite includes only 1 vol., the mechanical properties were improved much.

(In P. 1, line 35-38, the paragraph was revised.)

(In P. 2, line 57-64, the paragraph was revised.)

(In P. 12, line 385-387, the reference was added newly.)

(In P. 12, line 400-407, the references were added newly.)

1.2. Line 50. What the authors had in mind with “by adding other elements”. Please, make this sentence clear for any reader.

(RESPONSE)

I’m so sorry about that the sentence is miswritten, so that the part was deleted.

(In P. 1, line 43, the part was deleted.)

1.3. Line 57. The sentence starting with “Bahr et al….” does not relate to the paper content and must be omitted.

(RESPONSE)

We agree that this sentence is irrelevant to the flow. Sentence has been deleted.

(In P. 2, line 54-56, the sentence was deleted.)

1.4. Lines 59-71. The authors discussed different methods for FGM production. This discussion should be replaced by analysis of FGM manufacturing by extrusion (see my comment Nr. 1.1).

(RESPONSE)

We totally agree with your opinion that researches about extrusion should be discussed here. The paragraphs were revised like below.

Before)

Researchers have attempted new fabrication methods to solve this problem. Watanabe et al. tried to fabricate FGMs by centrifugal casting method, which is a casting process to produce cylindrical parts [5]. The advantage of this method is that the microstructure can be controlled and good mechanical properties can be obtained. However, it was reported that nanoparticles were distributed only on the surface, and thus, the mechanical properties were not evidently improved. Liu et al. attempted to fabricate MnSb/Sb-MnSb FGMs using a semi-solid forming process under a magnetic field [6]. Although the FGMs was controlled effectively using a magnetic field, it was difficult to control the field gradient. Kwon et al. attempted to fabricate FGMs using spark plasma sintering(SPS) [7]. Although the FGMs was superior mechanical properties, it was difficult to control Al carbides. However, that hot extrusion process improved the tensile strength and hardness of the Al-CNTs composite, even though only 1 vol. % of CNTs was added reported by Kwon and Leparoux [8]. In addition, Li and Xie fabricated the W/Cu FGMs which was fabricated via multi-billet extrusion exhibited a crystalline structure with improved density [9].

Revised)

Researchers have attempted new fabrication methods such as centrifugal casting, semi-solid forming under magnetic field and SPS to solve this problem [5-7].

Extrusion is one of the methods of manufacturing materials that have been used for a long time. However, there are no many research about FGM fabricated by hot extrusion although other manufacturing methods such as sintering, powder metallurgy have not been widely used in producing inclined functional materials [1-3]. Fan et al. tried to joining of Al1060 and Al6063 by welding following die extrusion [4]. There are many cracks in the interfaces due to the difficulty of controlling stress. Kwon et al. tried hot extrusion process to fabricated Al-CNT composite [8]. The microstructure was shown elongated with the parallel to extrusion direction with no cracks and pores. Even though the composite includes only 1 vol., the mechanical properties were improved much.

Therefore, in this study, we investigated the improvement of the microstructure and mechanical properties of the sloping functional material by hot extrusion.

(In P. 2, line 57-64, the paragraph was revised.)

1.5. The authors do not justify in the paper the aim of their research. They did not mention the possible application. They did not explain in the paper why the can was made from 6063 alloy and the core from 3003 alloy. The authors partly answered these questions in the reply to reviewer, but they did not include the corresponding explanations in the paper. The authors must also justify the choice of can’s dimensions (inner and outer diameters and length).

(RESPONSE)

First of all, the dimensions of the Al6063 can have been selected based on the tries of several times and the most optimized dimensions for the machine conditions was selected as shown in Fig. 2.

Secondly, the Al3003/Al6063 FGM in this study has a rod shape, the center axis of which is consisted of Al 3003 and serves as a skeleton of the material, and the center is surrounded by Al6063. Al6063 has high strength and high hardness that protects the material from external shocks. On the other hand, the Al3003 alloy is an aluminum-based alloy with excellent ductility, which gives flexibility to the material itself, thereby enhancing workability. Thus, the FGM is suitable for use as a wire material for an industrial electric pole, and can be used for a skeleton of a steel structure and an automobile structural material. According to your opinion the paragraph was revised like below.

Before)

Where the outer surface was made from Aluminum 6000 series owing to its high strength, and the interior was composed of the Aluminum 3000 series alloy to provide high elongation and better machinability to the composite. Al6063 was used on the outside in the form of a bulk material and Al3003 in powder form was used as the inner core to improve the adhesive strength at the interface between the two materials.

Revised)

The Al3003/Al6063 FGM in this study has a rod shape, the center axis of which is consisted of Al 3003 and serves as a skeleton of the material, and the center is surrounded by Al6063. Al6063 has high strength and high hardness that protects the material from external shocks. On the other hand, the Al3003 alloy is an aluminum-based alloy with excellent ductility, which gives flexibility to the material itself, thereby enhancing workability. Thus, the FGM is suitable for use as a wire material for an industrial electric pole, and can be used for a skeleton of a steel structure and an automobile structural material.

(In P. 2, line 66-72, the paragraph was revised.)

1.6. Line 72. The authors confused the definitions “composite material” and “FGM”. This is completely different structures. Please correct here and through the entire text.

(RESPONSE)

We totally agree with the “FGMs” is proper to our material than “composite”.  All of the word “composite” and “composite materials” was changed to “FGM” or deleted in whole text.

1.7. Line 74. How the core can provide a better machinability? Please, explain this statement or exclude.

(RESPONSE)

Al3003 has high elongation about 30%, which was used to FGMs in the core in this study. It make the FGMs have high machinability. The Fig S1 and S2 show that the FGMs rod has high machinability. There are no cracks even though the rod was bended around with 90 degree. If only Al6063 was made to rod, cracks will be formed where the rod is bend. Thus, Al3003 contribute to machinability of the material. However, the word of ‘machinability is not adaptable to express this properties of Al3003. The word was changed to ‘workability’.

Fig. S1. Al3003/Al6063 FGMs which are bended or curved with diverse direction.

Fig. S2. S Al3003/Al6063 FGMs which was bended; (a) the digital image of Al alloy FGMs, and (b) X-ray image of the FGMs where the bended region (marked as C and D in (a)).

(In P. 2, line 70, the word was changed.)

1.8. Lines 87-95 and Fig. 1. The statement that a bulk/bulk contact has a smaller contact area comparing with a bilk/powder contact is wrong. In first case we have a surface/surface type of contact. In second case we have a surface/point type of contact. Obviously, that in first case (bulk/bulk contact) the total contact area is larger. The lines 87-95 and Fig. 1 must be removed from the paper. An advantage of the use of powder could be a large deformation of particles during extrusion leading to better joining of contacting surfaces.

(RESPONSE)

We agree with the contact area between bulk and bulk is wider than that of bulk and powder as shown in Fig 1. There are some mistake to explain it. Actually, when the bulk and powder was contacted each other, it could be more actively react due to the larger contact area.

The reason why we express the wider contact area is explained below.

In fact, bulk material has a somewhat rugged surface, not a perfect plane. When these bulk and bulk are in contact, it is difficult to make perfect contact without any vacancy, so that there is a large gap between the materials. On the other hand, in the case of the powder, it is possible to flexibly contact with the surface of the bulk and reduce the gap between them. In this respect, it can be seen that the use of powder increases the contact area.

Thus, the Fig. 1 was revised to show this.

Figure 1. Illustration of the interface between two dissimilar materials. (a) bulk–bulk interface (b) bulk–powder interface, and (c) powder–powder interface.

(In P. 2, Fig. 1 was changed.)

2. Materials and methods

2.1. Figure 2. How the bottom and the tube were joined in the can? Please explain this. The capture should be changed as “Schematic view of Al 6063 can”.

(RESPONSE)

The 6063 consists of a can and a lid. After filling the can with 3003 powder, the lid was closed. At this time, the lid of the can has a tolerance of 3mm. The Fig.2 has been revised considering of your review like below.

Figure 2. Schematic view of the Al6063 bulk cover and can.

(In P. 3, Fig.2 was changed and the caption was revised from ‘Schematic’ to ‘Schematic view’)

2.2. What was the mass of 3003 powder poured into the can?

(RESPONSE)

The Al6063 can was filled with Al3003 powder fully and then, closed with the Al6063 cover. The total amount of Al3003 powder which was poured into the can is around 965 g. This value was inserted also in the text.

(In P. 3, line 99, the ‘of 965g’ was inserted.)

2.3. Line 105. 49 kN is a force and not a pressure. Was any lubricant during extrusion used? Please, add the corresponding information.

(RESPONSE)

Thank you for careful review. The 49 kN had been revised according to opinion of previous from 5000 kgf. It was revised to original value of 5000 kgf.

(In P. 3, line 101, the ‘49 kN’ has been changed to ‘5000kgf’)

2.4. Fig. 3 can be replaced just by SEM image of 3003 particles.

(RESPONSE)

We totally agree with the SEM image should be highlighted in this figure. Since the FGM manufactured in this study is made of powder material and filling density and density after extrusion can be changed according to the degree of particle size dispersion of the powder, the SEM image is enlarged without deleting the particle size dispersion graph.

Figure 3. (a) SEM image of Al3003 powder and (b)Typical PSA graph (close square figure) and accumulate PSA graph (open triangle figure) of Al3003 powder used in this study.

(In P. 4, Fig3 has been improved and the caption was changed)

2.5. How you can use Archimedes method for density measurement? The FGM consists of a dense external cover and a powder core. What was measured by Archimedes method? Please, use the image analysis or any other suitable technique to determine the porosity of powder core. By the way, the pores in 3003 part are clear visible in Figs. 5 and 6.

(RESPONSE)

In order to measure the overall density of the FGM, the material was cut in the transverse direction, and the density was measured by the Archimedes method as it was in the cross-sectional shape of the FGM (3003 of about 1 mm circle was embedded in a circle of 8 mm). Further, only the 3003 part of the extruded material was densely measured in the same manner.

As a result, it was confirmed that the relative density was almost 100%.The pores in 3003 shown in the SEM image are generated by deep etching and the material immediately after extrusion has almost no pores.

2.6. Line 121. Please, type Czech Rep., not just Czech.

(RESPONSE)

Thank you for your correction. Rep. was added as you said.

(In P. 4, line 117-118, 119, the word of ‘Rep.’ was inserted.)

2.7. The type the specimens used for tensile test must be provided in the paper and not just in the response to the reviewer. Also the relation between the 6063 and 3003 area in samples must be provided in the paper.

(RESPONSE)

We also agree with that the information about tensile specimen should be informed to readers. The no.4 tensile specimen was adapted for the FGMs shown in the below Figure.

In the case of rod type materials, when the diameter is under 25mm, the No.2 tensile test specimen is used. In the specimen the W (inner width is 6 mm).

(In P. 4, line 125-126, the information was added.)

3. Results

3.1. Lines 136-138. The influence of particle size distribution on packing density is well known. Please, omit the sentence starting with “Desmond…” and corresponding reference [12].

(RESPONSE)

We agree that this is fairly general. Therefore, this sentence is deleted.

(In P. 4, line 134-135, the sentence was deleted.)

3.2. Line 142. Remove “The”.

(RESPONSE)

Thank you for kindly modifying even small parts. "The" has been deleted.

(In P. 5, line 145, ‘The’ was deleted.)

3.3. Lines 145-150. How the chemical composition of 3003 powder was determined? Please, explain this.

 (RESPONSE)

As specified in the experimental procedure section, the chemical composition of the 3003 powder was determined by X-ray fluorescence (XRF). To clarify this part, we have corrected the following sentence.

“By comparison, Si (1.10 %), Mn (0.96 %), Fe (0.87 %), Zn (0.23 %), Cu (0.17 %), and Al (96.66 %) which is determined by XRF are present in the Al3003 powder used in this study.”

3.4. From Fig. 4(b) is obviously, that all mechanical properties of FGM are determined by 6063 material. An area fraction of 3003 is only around of 1.3%.

(RESPONSE)

The area ratio of 3003 and 6063 was recalculated. As a result, the area ratio of 3003 after extrusion was about 7%. We apologize that there was a mistake in the calculation. The area ratio of 7% was substituted to calculate the tensile strength and elongation as a rule of mixture

Actually, it can be considered that the mechanical properties (hardness and strength, elongation) are determined by the material properties of approximately 6063, while the area ratio of 3003 in the cross section is 7% and that of the tensile specimen is 18%. The area ratio is very small, but it can be considered to affect the overall physical properties.

Table 2 general mechanical properties of Al3003 and Al6063

Table 3 Experimental mechanical properties of FGMs

Table S1 Experimental Tensile stress and elongation and theoretical values calculated with rule of mixture and Kelly-Tyson eqaution

In the above table, the results of the calculation using the rule of mixture and the Kelly-Tyson equation are compared with the experimental results. As a result, the actual results were significantly different from those calculated by the rule of mixture equation (similar to the properties of 6063). This means that there is a factor that strengthens this FGM. In this study, the Kelly-Tyson equation was analyzed by the grain size refinement (introduction of Hall-Petch formula), precipitation hardening and strengthening by strengthening phase (Kelly-Tyson equation) is used to calculate the strength when the reinforcing phases are aligned in one direction.

 The FGM of this study is not actually fiber-reinforced composite material, but one material is considered to be reinforced, and the material produced by extrusion is stretched in one direction so that it is applied in one way. As a result, we found that the Kelly-Tyson equation can be applied to some extent.

3.5. Please explain the large amount of Si in Fig. 5 (f). It looks like this is the rest of a polishing paste. The same for white phase in Fig. 5. This particularly means that the samples were not good enough prepared.

(RESPONSE)

In Al3003 region, the Al contents was measured about 70 wt.% and Si was 27 wt.%. when the sample was etched to observe microstructure, Al was more etched than Si. Si elements stand out by this phenomenon. As well as, EDS was measured in local area in Al3003, then it would be measured in area where Si elements are collected. However, it was confirmed that the Al3003 which is used in this study, belongs to the generally known Al3003 content range shown in Table 1 in manuscript. 

This material used alumina polishing paste, as you said however, there seems to be a clean sample that is not processed. We apologizes for the inconvenience and not showing a clear result for this point.

3.6. Fig. 8. Please replace all Vickers indentations by a graph or a table. These pictures are not informative.

(RESPONSE)

We agree with that the Fig.8 is needed to change to a form of a graph or a table. We selected graph form to show the Vickers hardness of each region in the one section and also it could be useful to compare of the values between that on cross section and longitudinal section.

Figure 8. Vickers hardness graph measured on the transvers cross section and longitudinal section of FGMs at 5 region from center (Al3003) to Al6063 including the interface between Al3003 and Al6063.

(In P. 8, Fig.8 was changed and the caption was revised.)

3.7. Lines 216-219. Please, remove the sentence starting with “A greater…”.

(RESPONSE)

We agree that this sentence is irrelevant to the flow. Therefore, this sentence has been deleted.

(In P. 7, line 212-214, the sentence was deleted.)

3.8. Fig. 9. Change the vertical axe title as “Tensile stress”, not strength.

(RESPONSE)

Thank you very carefully. The tensile strength in the drawing has been changed to ‘Tensile stress’.

(In P. 9, Fig.9 has been revised.)

3.9. The strength analysis of FGM by Hall-Petch or Kelly-Tyson equation is meaningless because tensile strength is nearly fully determined by 6036 properties (see my comment Nr. 3.4).

(RESPONSE)

We agree with your opinion that the Hall-Petch equation couldn’t explain perfectly the improved strength of FMGs. However, we just want to show that we tried to calculate the strength of Al3003/Al6063 FGM with the Hall-Petch equation and Kelly-Tyson equation to explain the strengthening effect occurred in FGM.

In the case of Hall-Petch equation, which is used to expect the strength with the grain size. The Al3003/Al6063 FGM is fabricated with Al3003 powder and Al6063 bulk by hot extrusion process. The powder could give the strength to FGM due to the finer grain size than that of bulk. Second of all, the grain size would be smaller after extrusion process because the material have been took compressive stress perpendicular to the extrusion direction. This point also considered as a factor causing strengthening of FGM.

Some of researchers have been used XRD to measure grain size of materials. They inserted crystallite size to grain size of Hall-Petch equation. Thus, we expected that the strengthening effect by fine grain size would be occurred in FGM and tried to calculated the strength. we just calculate the strength with crystallite size measured in XRD analysis. Thus, the result would not be matched with experimental results. As we mentioned in manuscript, other researchers used to Hall-Petch eqaution to expect the strength of materials, so that we would show the adaption possibility of Hall-Petch equation in FGM also, although the precise calculation is needed more specific analyzations.

In the case of Kelly-Tyson equation, as we noted at 3.4 of review answer, the strength of FGMs couldn’t be explained by rule of mixture. We tried to find the strengthening factors, and tried to adapt Kelly-Tyson equation to our FGMs as one of the method. Even if the equation is not adapted to FGMs generally, we considered it is meaningful to show new approach method to readers.

Reviewer 2 Report

This paper describes a study of the fabrication of aluminum alloys used graded materials in a hot extrusion system. The work is carried out in an appropriate manner. The experimental work is of high quality and the characterization techniques are state of the art. The following changes need to be made before acceptance.

Define FGM in the text as well as in the abstract.

Figure 3. Add error bars if possible.

Line 141. Typographical error? ‘The When’ ??

Line 247. Change ‘proved’ to ‘showed’.

Line 259. I do not understand the use of the word ‘insisted’ in this sentence.

Line 267. Add ‘as given by Equation (1)’

Line 279. Add ‘as given by Equation (2)’

Near line 312. Give the Kelly-Tyson equation that was used in the text.

For Table 2, give the error bars as done for Table 3. The inclusion of error bars in Table 3 is very good.

In the Conclusions, there are too many small paragraphs. Use a fewer number of paragraphs.

Line 358. Change ‘strengthening’ to ‘strengthened’.

Author Response

Review answers

Fabrication of Aluminum Alloys Functionally Graded Materials by Hot Extrusion Process

Dasom Kim, Kwangjae Park, Minwoo Chang, Sungwook Joo, Sanghwui Hong, Seungchan Cho, Hansang Kwon^*

E-mail: [email protected], [email protected]

First of all, thank you for your careful reading our research. We have revised the manuscript based on the comments of reviewers. We would like to ask that you kindly proceed with a second review. We here address the comments made by the reviewers:

Dear reviewer #2:

Thank you so much for your positive opinions and careful review about our research. We agree with your opinion that some part in this study should be more defined clearly. We have revised the manuscript based on your comments. The paper has become much more complemented and scientific due to your careful comments. I appreciated again sincerely.

1.      Define FGM in the text as well as in the abstract.

(RESPONSE)

Thank you so much your revision. The FGM was defined in the text.

 (In P. 1, line 41, the FGM was defined in the text.)

2.      Figure 3. Add error bars if possible.

 (RESPONSE)

We apologizes that all of the particle size distribution data are displayed as a single value, not the error range. However, the error range on the device is known to be less than 5%.

3.      Line 141. Typographical error? ‘The When’ ??

 (RESPONSE)

Thank you for your careful review sincerely. ‘The When’ is mistake, the word ‘The’ was deleted in the sentence.

 (In P. 5, line 145, ‘The’ was deleted.)

4.      Line 247. Change ‘proved’ to ‘showed’.

(RESPONSE)

 Thank you for your comment. The word has been revised to ‘showed’ as you mentioned.

(In P. 8, line 248, the ‘proved’ was changed to ‘showed’)

5.      Line 259. I do not understand the use of the word ‘insisted’ in this sentence.

 (RESPONSE)

We agree that the word of ‘insisted’ is vague to understand. Thus, the word was changed to ‘showed’.

(In P. 9, line 269, the ‘insisted’ was changed to ‘showed’)

6.      Line 267. Add ‘as given by Equation (1)’

 (RESPONSE)

Thank you so much about your comment. It was revised according to your advice.

(In P. 10, line 281, ‘as given by equation (1) was inserted)

7.      Line 279. Add ‘as given by Equation (2)’

(RESPONSE)

 Thank you so much about your comment. It was revised according to your advice also.

(In P. 9, line 279, ‘as given by equation (2) was inserted)

8.      Near line 312. Give the Kelly-Tyson equation that was used in the text.

 (RESPONSE)

We agree that Kelly-Tyson is should be shown in this paper. The equation has been inserted near related text.

(In P. 10, line 317, Kelly-Tyson equation was inserted)

9.      For Table 2, give the error bars as done for Table 3. The inclusion of error bars in Table 3 is very good.

 (RESPONSE)

We apologize that the values in Table 2 are typical property values of the material, so that the error range is not indicated in the reference documents and therefore couldn’t be inserted.

10.  In the Conclusions, there are too many small paragraphs. Use a fewer number of paragraphs.

 (RESPONSE)

Thank you for your comment sincerely. We totally agree that the sentences in conclusion have to be improved as your opinion. The conclusion was revised like below.

Before)

The Al3003/Al6063 FGMs was successfully fabricated via hot extrusion to realize multi-functionality including high strength and low weight.

The interface between the two materials is clear, but almost no cracks are observed. Further, the hardness value at the interface is superior. It can be considered that the extruded composite has excellent interface properties. This improvement resulted from combination of powder and bulk. When powder and bulk are contact, the contact area could be wider than that between bulks.

The tensile strength of the final composite material subjected to T-5 heat treatment was improved to twice that generally observed for Al3003 and Al6063. The elongation was more than 10%. From these results, it can be confirmed that the final product has a high strength and adequate elongation, which are objectives of this study.

To analyze the strengthened mechanism, we considered precipitation hardening and grain refinement; strengthening via grain refinement was emphasized more in this study. The grain size of powder is finer than bulks generally and it is difficult to occur grain growth due to presence of many grain boundaries. Therefore, the FGMs fabricated including Al3003 powder could be strengthening by fine grain size. As well as, the grain size would be reduced by extrusion process because the grains were subjected to mechanical stress. The Hall–Petch equation was used to explain the tendency for improved strength. We could conclude that this equation is valid for explaining the tendency of the increase in strength when the composite was extruded as part of the studies of other researchers as well as our results.

In addition, when we used the Kelly–Tyson equation assuming that Al6063 was a fiber, the theoretical results nearly corresponded to the experimental values. Therefore, the Kelly–Tyson equation could also be used in the prediction of strength of the extruded composite.

In summary, Al-alloy-based FGMs with lightweight and high strength properties were successfully fabricated via hot extrusion. Thus, we can confirm that hot extrusion is useful to fabricate FGMs with improved mechanical properties.

Revised)

The Al3003/Al6063 FGMs was successfully fabricated via hot extrusion to realize multi-functionality including high strength and low weight.

The interface between the two materials is clear, but almost no cracks are observed. It can be considered that the extruded compositesFGMs has excellent interface properties base on the high Vickers hardness of interface. When powder and bulk are contact, the contact area could be wider than that between bulks, so that the interface was improved.

The tensile strength of the final composite material subjected to T-5 heat treatment was improved to twice that generally observed for Al3003 and Al6063, and the elongation was more than 10%. From these results, it can be confirmed that the final product has a high strength and adequate elongation, which are objectives of this study.

To analyze the strengthened mechanism, we considered precipitation hardening and grain refinement; strengthening via grain refinement was emphasized more in this study. The grain size of powder is finer than bulks generally and it is difficult to occur grain growth due to presence of many grain boundaries. Therefore, the FGMs fabricated including Al3003 powder could be strengthening by fine grain size. As well as, the grain size would be reduced by extrusion process because the grains were subjected to mechanical stress. The Hall–Petch equation was used to explain the tendency for improved strength, which is proper to explain the tendency of the increase in strength when the compositesFGMs was extruded as part of the studies of other researchers as well as our results.

In addition, when we used the Kelly–Tyson equation assuming that Al6063 was a fiber, the theoretical results nearly corresponded to the experimental values. Therefore, the Kelly–Tyson equation could also be used in the prediction of strength of the extruded compositesFGMs.

In summary, Al-alloy-based FGMs with lightweight and high strength properties were successfully fabricated via hot extrusion. Thus, we can confirm that hot extrusion is useful to fabricate FGMs with improved mechanical properties.

(In P. 11-12, line 355-376, the sentences were revised)

11.  Line 358. Change ‘strengthening’ to ‘strengthened’.

(RESPONSE)

Thank you for your correction. The word was changed to ‘strengthened’.

(In P. 11, line 363, the word was changed to ‘strengthened’)

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper shows an interesting approach and useful results for the field of application – please find following comments to improve the quality.

Author Response

Review answers

Fabrication of Aluminum Alloys Functionally Graded Materials by Hot Extrusion Process

Dasom Kim, Kwangjae Park, Minwoo Chang, Sungwook Joo, Sanghwui Hong, Seungchan Cho, Hansang Kwon^*

E-mail: [email protected], [email protected]

First of all, thank you for your careful reading our research. We have revised the manuscript based on the comments of reviewers. We would like to ask that you kindly proceed with a second review. We here address the comments made by the reviewers:

Dear reviewer #3:
Thank you very much for your careful review about our research. We agree with your opinion that some part in this study should be more defined clearly. We have revised the manuscript based on your comments. The paper has become much more complemented and scientific due to your careful comments. I appreciated again sincerely.

1.      In P. 1, line 41, I would recommend   not to use: he, she, we …. rather the authors name again, here “Park et al. [2) confirmed…”

(REPONSE)

We agree with your opinion totally. The words of ‘He’ and “She” are not proper to use in the paper. In the revision process, some part of introduction was revised, then the sentences including ‘he’ and ‘she’ were deleted. As well as, the other region also checked again. Thus, the words of ‘she’ and ‘he’ were not present in revised manuscript.

2.      In P. 2, line 72, Please avoid “we”

(REPONSE)

Thank you for correction. The use of “we” is not adequate. The sentences including “we” have been revised properly.

(In P. 2, line 65, the sentence was revised without “we”)

(In P. 10, line 303-304, the sentence was revised without “we”)

(In P. 11, line 333,334,362,370, the sentences were revised without “we”)

(In P. 12, line 374, the sentence was revised without “we”)

3.      In P. 2, line 83, …the surface   roughness was 0-1µm. 0 µm is impossible… I would recommend to change it in:  <1µm.< strong="">

(REPONSE)

Thank you for your carful correction. We agree with your opinion that 0-1µm is not adequate. The part was revised to ‘< 1µm’.

(In P. 2, line 79, the part was revised to “<1µm”)< span="">

4.      In P. 3, line 85, The sketch in Figure   2  does not agree with the relation of   inner and outer diameter seen in Figure 4… there the inner part (Al3003) look   much smaller in relation

(REPONSE)

We agree with that reduction from the area ratio before extrusion and after extrusion can be sufficiently misunderstood. Before extrusion, 6063 cans are filled with around 1 kg of 3003 powder and then, be extruded.

Actually, the compression ratio of bulk and powder is different. Powder has a high surface friction due to its large surface area, leaving plenty of voids between fillings. As a result, when compressed with the same force, the powder portion compresses much more, resulting in a reduced area ratio of the powder. Thus, the dimension in Fig. 2 could be explained with this view.

5.      In P. 5, line 151, Please add here the   final inner and outer diameters

(REPONSE)

Thank you for your careful advice. The information about diameter of inner (Al3003 part) and outer (Al6063 part, actually diameter of FGMs) was inserted.

“The inner diameter is around 1 mm, and the outer is 8 mm.”

(In P. 5, line 156, a sentence was inserted)

6.      In P. 8, line 213, There is some   statistics missing for the hardness results: a range is shown – this is very   unusual… it would be wise to measure further lines from the middle to the outside   and show some standard deviation, I guess mostly the interface shows the   highest scattering, depending how accurate the interface is measured

(REPONSE)

In measuring the hardness, the hardness was measured while maintaining a constant distance from the center to the outside. However, the indentations of the longitudinal section in the figure have been modified to improve understanding according to the previous reviewer's opinion.

In addition, the Vickers value of the interface was measured 5 times and the error range was considerably smaller. The reason is that the difference in Vickers hardness between the two materials is small.

We hope to show the interface has similar hardness compared to Al6063 region and Al3003 region because it is indicated that the interface was bonded well without cracks or flaws showing high mechanical properties.

7.      In P. 10, line 276, Figure 9: there is   only one curve shown, are this mean values?

(REPONSE)

Actually, the graph shown in the figure shows the average of the results of the two tensile tests that are the most reliable among the results of the actual tensile tests.

Because there are many other information such strength and elongation of Al3003 and Al6063 etc. we considered that it is more clear showing the average one graph.

8.      In P. 11, line 343, Table 3: the   standard deviation is given in the headline, that cannot be right, the   deviation belongs to the mean value and cannot be the same for T0 and T5

(REPONSE)

We totally agree with your opinion. In previous manuscript, the large error range was written at the headline between properties of T-0 FGMs and T-5 FGMs.

The table has been revised adding the error range respectively beside the values like below.

Table 3. Experimental mechanical properties of Al6063/Al30063 FGM (T-0, T-5).

(In P. 10, Table 3 was revised.)

Author Response File: Author Response.pdf

Round  2

Reviewer 1 Report

The English must checked more carefully.

Author Response

Dear Reviewer,

Thank you for your kind comments.

I have attached the revised manuscript from the ELSEVIER English editing service.

However, this edited manuscript is fine for further processing of review.

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round  1

Reviewer 1 Report

1. Main comments

1.1. The aim of the paper must be better justified. What applications of this FGM have authors in mind? Why they use 6063 alloy as a can and 3003 as a core? Why it is not opposite, for instance.

1.2. The authors must justify the choice of 3003 alloy in a powder form. This is done in the section Results. Please, move Fig. 9 and related text to Introduction.

1.3. The strength analysis (lines 269-323) by Hall-Petch relation did not give any new information. This analysis must be fully excluded from the paper.

1.4. The same for strength analysis by Kelly-Tyson equation (lines 324-349). Firstly, the formula is not provided in the manuscript. Secondly, this formula is relevant for fiber-reinforced composites. The discussed FGM does not belong to such type of composites. It is much better to use the mixture rule, taking into consideration 6063/3003 area ratio. Please, do this in the revised version.

1.5. The points 1.2 and 1.3 must be considered in other parts of manuscript including Abstract and Conclusions.

1.6. The SI units must be used through the whole text (kN for force, line 85 and line 105).

1.7. Line 84. The outer diameter was rather 100 mm (see Fig. 1) and the inner diameter was 60 mm (see Fig. 1 again). Please, correct in the text.

1.8. Please explain the used heat treatments (T-5 etc.) or give a reference to standard.

1.9. How the specimens for tensile tests were manufactured? What was relation between 6063/3003 areas in such specimens?

1.9. Please, add the value of packing density (intensively discussed in the paper) and d₁₀, d₅₀, d₉₀ data for particles size distribution.

1.10. What was the density of 3003 core after extrusion? This can be done by image analysis, for instance, but not as an average with 6063 cover.

1.11. Fig. 4. Please, indicate by arrows the spots of EDS analysis. Why Al content was so low in Fig. 4f? Please, explain this result in detail.

1.12. Figs. 7 and 8 must be reduced to 7a and 8a and then united in one figure. Other parts of these figures do not provide any useful information.

1.13. Table 3. Please add a capture.

1.14. The number of references is too large. Many of them are not really needed. Please, reduce their number to approx. 20-25.

2. Other comments

2.1. Please, add the Al-powder manufacturer and the information on device used for extrusion.

2.2. What kind of SEM device was used: TESCAN or HITACHI? Please, make this clear.

2.3. Line 24-25. What does it mean “strength”? Probably, the authors mean “tensile strength”? Please, writhe this clear. This phrase (line 24: Strength….) must be edited with better English.

2.4. Many strange sentences and definitions. Lines 36-37: Everybody knows that steel is stronger than aluminum. Please omit this sentence. Line 39: Save not safe. Line 41: What is “airport component”? Please correct. Line 42: What is SUS? Probably, the authors mean stainless steel. Then write SS, at least. Line 55: Please replace the definition “shrinkage stress” by relevant one. There is not such definition in engineering. Line 103: UTS test is wrong, just tensile test.

2.5. Please, make the scale bar in Fig. 2 larger and thus visible.

2.6. Fig. 3a. Please expand to see the names of alloys.

2.7. Lines 143 …has a high… and line 150 ...higher. Please edit or finish these sentences.

2.8. Line 175. The elongation of grains in extrusion is well-known. Please, remove the reference [19]. Now it is looking a bit unprofessional.

2.9. Please provide the values of hardness measured in longitudinal cross-section.

2.10. Line 262. What is Al12-15, Si1-2 and Al6? Please correct, explain or add, at least, a reference.

2.11. Please, check the paper on subscript and italic, and correct if necessary.

3. Overall conclusion

3.1. Too many corrections are needed. The paper can be published after major revision.

Reviewer 2 Report

It is not possible to determinate the grain size by Scherrer, because is not account the deformation. The material after hot press extrusion have high microdefects and microdeformations that it is not included in Scherrer formulation.

There are not in the work any XRD data.

From the XRD data, 285 the crystallite size of the Al3003 powder is determined to be 48.85 nm and that of Al3003/Al6063 286 FGMs is determined to be 44.87 nm