Cold Forming Hybrid Aluminium–Carbon Fibre-Reinforced Polymer Sheets Joined by Mechanical Interlocking

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study demonstrates the feasibility of forming aluminum-CFRP prepreg sheets into complex omega-shaped profiles using a conventional cold stamping process and investigates the effects of the location of CFRP (inside or outside of the formed profile), the number of mechanical joints, the addition of a glass fiber-reinforced polymer (GFRP) interlayer, and adequate lubrication on necking, cracking, springback behavior, and final geometry after curing. The results show that the addition of CFRP to aluminum omega profiles changes the buckling behavior from overall bending to axial folding, which increases the maximum compressive load. The article is well written, but before publishing, I would suggest the following changes:

(1) The Introduction section is divided into several small paragraphs, which may make it difficult for the reader to follow.

(2) In the last paragraph of the Introduction, it is suggested to clarify the main innovation and scientific contribution of the paper.

(3) In Section 2.2, there is an issue with "(Error! Reference source not found.)," which hinders the reader's experience.

(4) The paper seems to use "x" instead of "×." Please revise.

(5) The description of the curing process in the paper is unclear. Please add more details about the process.

(6) Why does not using a lubricant protect the specimen from damage during the stamping process? What is the underlying mechanism?

(7) It is recommended that the authors add schematic diagrams to illustrate the exact position of the aluminum sheet and the composite prepreg in the overall structure.

Author Response

This study demonstrates the feasibility of forming aluminum-CFRP prepreg sheets into complex omega-shaped profiles using a conventional cold stamping  process and investigates the effects of the location of CFRP (inside or outside of the formed  profile), the number of mechanical joints, the addition of a glass fiber-reinforced polymer (GFRP) interlayer, and adequate lubrication on necking, cracking, springback behavior, and final geometry after curing. The results show that the addition of CFRP to aluminum omega profiles changes the buckling behavior from overall bending to axial folding, which increases the maximum compressive load. The article is well written, but before publishing, I would suggest the following changes:

(1) The Introduction section is divided into several small paragraphs, which may make it difficult for the reader to follow.
We have prepared a revised version of the introduction with improved flow, structure, and paragraph organization. The number of paragraphs have been reduced.

(2) In the last paragraph of the Introduction, it is suggested to clarify the main innovation and scientific contribution of the paper.

We have added two sentences in the last paragraph to comply with the reviewer's suggestion

(3) In Section 2.2, there is an issue with "(Error! Reference source not found.)," which hinders the reader's experience.

This typo has been solved

(4) The paper seems to use "x" instead of "×." Please revise.

Solved in the current version

(5) The description of the curing process in the paper is unclear. Please add more details about the process.

The explanantion of the curing prcess in section 2.2.4 has been improved. In addition, the figure 3 has been extended to provide a clearer explanation.

(6) Why does not using a lubricant protect the specimen from damage during the stamping process? What is the underlying mechanism?

There was a typo in the initial manuscript at the end of the first paragraph of section 3.1 that lead to this reviewer's question. The sentence:
"These defects were not present in analogous specimens stamped without lubricant (Figure 4b)."

Should be: 
"These defects were not present in analogous specimens stamped with lubricant (Figure 4b)."
It is well known that the use of lubricant prevents galling in cold stamping of aluminum. Galling involves the transfer of material from the workpiece (in this case, aluminum) to the tool surface due to frictional adhesion, combined with plastic deformation. This leads to surface damage, material buildup on the tool (also known as “pickup”), and can severely degrade part quality and tool life.
Lubricants play a crucial role in reducing or preventing galling during aluminum stamping by reducing friction between the tool and the aluminum sheet, minimizing the adhesive forces that lead to material transfer, and by forming a protective film that acts as a barrier, preventing direct contact between the aluminum and the tool surface.

(7) It is recommended that the authors add schematic diagrams to illustrate the exact position of the aluminum sheet and the composite prepreg in the overall structure.

We appreciate the interest of the reviewer to improve the clarity of the manuscript. To further clarify this point we have substituted Table 1 by a figure (Figure A in the revised version). This figure indicates the exact position of each material sheet in each specimen

Reviewer 2 Report

Comments and Suggestions for Authors

This work provides an important step forward in demonstrating the potential of formed aluminum-CFRP profiles for automotive crashes structures, but open questions remain about the sensitivity to material inputs, dimensional control, energy absorption, microscale mechanisms, corrosion mitigation, and manufacturing scalability. The authors demonstrate forming aluminum-CFRP hybrid sheets into omega profiles, but they only test a single aluminum alloy (5754) and CFRP layup (twill 2x2 prepreg). To establish broader applicability, it would strengthen the work to evaluate a larger matrix of material combinations, including other common aluminum alloys used in auto body panels (e.g. 6xxx series) and different CFRP types (e.g. unidirectional tape, non-crimp fabric). The formability limits may vary significantly based on the materials used.

 

1 The springback and distortion behavior is characterized only by measuring the flange angle. A more comprehensive approach should quantify the full 3D geometry, for example using a coordinate measuring machine or 3D scanning. This would allow the authors to assess if there is twisting, warping or other distortions beyond just the flange region. Automotive manufacturers have tight dimensional tolerances that need to be met.

2 The buckling modes and peak loads in axial compression are reported, but the actual energy absorption is not quantified. For the intended application as a crashworthy structure, the total energy absorbed is a key metric. The authors should calculate this by integrating the load-displacement curves up to a consistent displacement value. Simply reporting peak load does not give the full picture of crash performance.

3 The microscale deformation and failure mechanisms are not investigated. Cross-sectional micrographs of the formed and crushed profiles, especially near the mechanical interlock features, would provide valuable insight into fiber realignment, matrix cracking, delamination growth, etc. The authors hypothesize reasons for the load-displacement trends but do not validate them experimentally.

4 Potential galvanic corrosion between carbon fiber and aluminum is concerning for long-term durability in automotive structures. The glass fiber layer is used as an insulating barrier, but its effectiveness is not assessed. Accelerated corrosion testing in environmental chambers should be conducted on the hybrid profiles to determine if the glass layer is sufficient to prevent degradation over the vehicle life.

5 The scalability of this process to high-volume manufacturing is questionable. Using a slow-curing epoxy prepreg that requires a separate post-cure step in an oven adds significant cost and cycle time. For adoption by the automotive industry, a faster method like thermoplastic composites that can be formed and joined in a highly automated process would be preferred. The authors should comment on pathways to scale up this concept.

Comments on the Quality of English Language

It is fine.

Author Response

1 The springback and distortion behavior is characterized only by measuring the flange angle. A more comprehensive approach should quantify the full 3D geometry, for example using a coordinate measuring machine or 3D scanning. This would allow the 
authors to assess if there is twisting, warping or other distortions beyond just the flange region. Automotive manufacturers have tight dimensional tolerances that need to be met.

We are really thankful to the reviewer for his deep revision of the manuscript and suggestions for improvements. Although it could certainly provide deeper insights on the attained geometrical tolerance, in view of the work emphasis on feasibility and 
demonstration of the potential of a novel methodology, we did not perform a full 3D scan of the geometry. A detailed analysis of the 3D geometry would overcome the extension and scope of this manuscript. We agree with the reviewer that such analysis should be done if this methodology is to be explored at higher TRLs.

2 The buckling modes and peak loads in axial compression are reported, but the actual energy absorption is not quantified. 
For the intended application as a crashworthy structure, the total energy absorbed is a key metric. The authors should 
calculate this by integrating the load-displacement curves up to a consistent displacement value. Simply reporting 
peak load does not give the full picture of crash performance.

We agree with the referee on the importance of assessing the energy absorption for crashworthy structural elements. We computed the absorbed energy for the specimens and did not notice relevant differences among the explored specimens. A paragraph to clarify this topic is included in the section 3.2 of the revised manuscript.

3. The microscale deformation and failure mechanisms are not investigated. Cross-sectional micrographs of the formed and crushed profiles, especially near the mechanical interlock features, would provide valuable insight into fiber realignment, 
matrix cracking, delamination growth, etc. The authors hypothesize reasons for the load-displacement trends but do not validate them experimentally.

This topic was carefully explored in a previous paper for flat specimens: N. Latorre, D. Casellas, J. Costa and N. Blanco, “Failure mechanism of Aluminium – Carbon Fibre Reinforced Polymer interlocking joints through punching,” International Journal of Lightweight Materials and Manufacture, Jan. 2025, doi: 10.1016/j.ijlmm.2025.01.002.

4 Potential galvanic corrosion between carbon fiber and aluminum is concerning for long-term durability in automotive structures. The glass fiber layer is used as an insulating barrier, but its effectiveness is not assessed. Accelerated corrosion testing in environmental chambers should be conducted on the hybrid profiles to determine if the glass layer is sufficient to prevent degradation over the vehicle life.

Although we fully agree on the need to explore this issue for this methodology to reach higher TRLs, our humble opinion is that it is out of the scope of the current manuscript.

5 The scalability of this process to high-volume manufacturing is questionable. Using a slow-curing epoxy prepreg that requires a separate post-cure step in an oven adds significant cost and cycle time. For adoption by the automotive industry, a faster method like thermoplastic composites that can be formed and joined in a highly automated process would be preferred. The authors should comment on pathways to scale up this concept.

This topic has been included in the last paragraph of the discussion of the revised manuscript

Reviewer 3 Report

Comments and Suggestions for Authors

This work proves the feasibility of forming aluminium-CFRP prepreg panels into complex omega shaped profiles following a conventional cold stamping process. The effect of CFRP, the number of mechanical joints, the addition of a Glass Fibre Reinforced Polymer (GFRP) intermediate layer to prevent galvanic corrosion and  adequate lubrication on necking, cracking, springback behaviour and the final geometry after curing was studied. Compression tests were performed to assess the mechanical response of the hybrid profile and results showed the addition of CFRP in the aluminium omega profile changed the buckling behaviour from global bending to axial folding, increasing the maximum compression load.

However, there are a few things that need to be made:

1. The introduction is very short and it would be appropriate to expand it a bit. There are many works on the production of composites. There are many methods of producing this material and it is worth characterizing them briefly.
2. A scheme is needed showing how the sample was loaded.

Overall, the article is very interesting, presents an innovative approach and groundbreaking research.

Once the changes have been made, the article may be considered for publication.

Author Response

1. The introduction is very short and it would be appropriate to expand it a bit. There are many works on the production of composites. There are many methods of producing this material and it is worth characterizing them briefly

We are sincerely greatful to the reviewer for the revision of the mansucript and suggestions. However, our humble opinion is that the introduction is already long and that including a revision on the production methods of composites would be a too broad topic in relation to the scope of the paper. We have updated the Introduction to make it clearer while maintaining its focus.

1. A scheme is needed showing how the sample was loaded.

Following the reviewer's advise, we have included a picture of the compression set up in section 2.4. It is named Figure B in the revised manuscript