Process Modelling Applied to Aluminium-Steel Butt Welding by Hybrid Metal Extrusion and Bonding (HYB)

Round 1

Reviewer 1 Report

This is an excellent research work on the numerical simulation of welding by HYB and confrontation with experimentation (local temperature measurements, hardness in the welded zone, consideration of the existence of IMC). The paper is clear, well written and results presented here are nice and very convincing. A few minor remarks and contribution to the discussion :

- Is there an optimum thickness of IMC to ensure a good welding bond, as there is in FSW? Perhaps clarify this point in the introduction and in the 4.3. part presenting the MEB observations.

- What kind of IMC are present in the welding? Did you perform EDS measurements for instance ?

- Can you give some reference for Eq. (4) ?

- Do you think you could have some recrystallization mechanisms under the pin which can also explain the decrease in hardness observed in Fig. 9 ?

- Concerning yield strength values in Fig.13 and 14, the stress obtained are lower than yield stresses given in Table 3. Can you discuss this point? Again perhaps it could be related to some recrystallization processes?

Author Response

Metals Manuscript No. 1911875 - Authors reply to referees’ comments

In general, the authors find the comments by the referees’ relevant and constructive. The way we have responded to the comments can be summarized below (Referees’ comments in red typescript - authors reply in black typescript/italic style):

This is an excellent research work on the numerical simulation of welding by HYB and confrontation with experimentation (local temperature measurements, hardness in the welded zone, consideration of the existence of IMC). The paper is clear, well written and results presented here are nice and very convincing. A few minor remarks and contribution to the discussion:

• Is there an optimum thickness of IMC to ensure a good welding bond, as there is in FSW? Perhaps clarify this point in the introduction and in the 4.3. part presenting the MEB observations.

Authors reply:

In most Al-steel butt welds the tensile strength is limited by the interfacial bond strength, reflecting the insidious nature of the hard and brittle IMC layer. Therefore, fracture in the IMC layer is particularly a problem in conventional FS and GMA welds because the layer here is quite thick (typically in the range from 2 to 20 µm). However, in the HYB case the situation is different, since bonding in corresponding HYB welds instead occurs by a combination of microscale mechanical interlocking and IMC formation, where the IMC layer is in the sub-micrometer range (< 1µm). This type of microstructure, which provides a high intrinsic resistance against cracking, is unique for the HYB process and reflects its characteristic low interface temperature of about 400 oC preventing excessive growth of the IMC layer. Therefore, in the HYB Al-steel HYB welds dealt with in the present paper the tensile strength is actually determined by the minimum strength level inside either the HAZ or the EZ on the aluminium side of the joint. This has already been documented elsewhere by the authors, see Refs. [14 and 15].

Action undertaken: The introduction to Section 5.2 has now been rewritten to clarify this point.

 

• What kind of IMC are present in the welding? Did you perform EDS measurements for instance?

Authors reply:

The interface microstructure of these Al-steel HYB welds has already been fully characterized both by means of high-resolution TEM and SEM-EDS examinations. Further details can be found in Refs. [13 and 14], which are readily accessible to readers of this journal.

 

Can you give some reference for Eq. (4) ?

Authors reply:

Yes, as an aid to the reader Refs. [28, 29 and 32] have now been included in the revised version of the manuscript before Eq. (4) appears in the text.

 

• Do you think you could have some recrystallization mechanisms under the pin which can also explain the decrease in hardness observed in Fig. 9?

Authors reply:

No, we don’t think recrystallisation plays a role here. It is already well-established the reversion of hardening precipitates is the main cause of the HAZ hardness drop in Al-Mg-Si welds, see for example Refs. [17, 25-29, 30].

 

• Concerning yield strength values in Fig.13 and 14, the stress obtained are lower than yield stresses given in Table 3. Can you discuss this point? Again perhaps it could be related to some recrystallization processes?

Authors reply:

In the Al-BM case there seems to be good agreement between measured and predicted values for the yield strength, showing that WELDSIM adequately captures the major strengthening mechanisms involved. Considering the cold-drawn Al-FW the situation is different. This is because the Al-FW completely loses its microstructural identity already during the passage through the extruder. Hence, at the time when the Al-FM enters the groove it will be fully recrystallized with all alloying elements present in solid solution. This microstructural state is adequately handled by WELDSIM. We feel that this issue now is adequately addressed in the revised version of the manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

1. The correlation study between IMC and peak current of the joints is needed.

2. Heat generation on Al and S should be given quantitative details

3. Heat transfer rate and cooling of the dissimilar welds need to determine and correlate with interface formation and IMCs

4. According to Fig 11, please show the effect of heat distribution in macroimages.

5. The diffusion of Fe into Al, and vice versa phenomena should be ecplained.

 

6. Conclusions can be simplified to understand the key points of the research.

Author Response

Metals Manuscript No. 1911875 - Authors reply to referees’ comments

In general, the authors find the comments by the referees’ relevant and constructive. The way we have responded to the comments can be summarized below (Referees’ comments in red typescript - authors reply in black typescript/italic style):

 

1. The correlation study between IMC and peak current of the joints is needed.

Authors reply:

It is unclear what is meant by peak current. However, if the Reviewer 2 means the peak temperature, its effect is fully accounted for in the model predictions, as Eq. 5 is integrated numerically over the full thermal cycle for each of the three series. We feel that this issue now is adequately addressed in the revised version of the manuscript.

 

2. Heat generation on Al and S should be given quantitative details

Authors reply:

This has now been done in connection with Fig. 8. Thus, in Section 4.1 the following sentence have now been added as an aid to the reader:

“Specifically, in Series I, II and III the calculated values for the net power input were 1350, 1075, and 1150 W, respectively”.

 

3. Heat transfer rate and cooling of the dissimilar welds need to determine and correlate with interface formation and IMCs.

Authors reply:

This is exactly what this process model does. It allows calculation of the thermal field in the three Al-steel welds and uses again that information to calculate the IMC evolution at the Al-steel interface based on Eq. 5. We feel that this issue now is adequately addressed in the revised version of the manuscript.

 

4. According to Fig 11, please show the effect of heat distribution in macroimages.

Authors reply:

The heat distribution in the other two welds is very similar to the one shown in Fig. 11, with exception that these temperature contours will be slightly more oval or elongated, depending on the applied welding speed. However, because of space limitations, it is not advisable to add two more figures just to show that obvious effect. Still, Fig. 11 is a good example, illustrating the characteristic asymmetrical thermal field in such Al-steel welds. We feel that this issue now is adequately addressed in the revised version of the manuscript.

 

The diffusion of Fe into Al, and vice versa phenomena should be explained.

Authors reply:

Yes, interesting topic, indeed, but in the present paper we have used an established diffusion model for predicting the IMC layer thickness in the welds. Since details about the model and how it is constructed can be found elsewhere, e.g. see Ref. [12], it is far beyond the scope of the present paper to go deeper into this topic here.

 

6. Conclusions can be simplified to understand the key points of the research.

Authors reply:

This has now been done, see the new Conclusion section.

Author Response File: Author Response.pdf

Reviewer 3 Report

Review report: Process Modelling Applied to Aluminium-Steel Butt Welding by Hybrid Metal Extrusion & Bonding (HYB)

1.       Abstract section is written well.

2.       Discuss the Novelty and clear application of the work.

3.       The problems related to dissimilar welding need more discussion and also refer to the recently published work on FSW: https://doi.org/10.1007/s11665-022-06832-2; https://doi.org/10.3390/met10050559; https://doi.org/10.1007/s11665-022-06822-4.

4.       Shorten the length of the introduction section and add key published work and try to make a bridge between current and previous published work.

5.       Add the welding parameters in a separate table.

6.       Provide the image of the welded plate.

7.       Discuss about the tool pin profile and their selection.

8.       Add the macrograph of the welded joint.

9.       The mesh and model need more discussion. Add the node elements and boundary conditions with clear discussion.

10.    In the hardness plot mention the each region clearly.

11.    Hardness variation need technical discussion and also try to relate it with microstructure evolution.

12.    Shorten the length of the conclusion section and add only 4/5 key bullet points in the conclusion section.

Author Response

Metals Manuscript No. 1911875 - Authors reply to referees’ comments

In general, the authors find the comments by the referees’ relevant and constructive. The way we have responded to the comments can be summarized below (Referees’ comments in red typescript - authors reply in black typescript/italic style):

 

1. Abstract section is written well.

Authors reply:

Thanks, also we feel that it conveys the message clearly.

 

2. Discuss the Novelty and clear application of the work.

Authors reply:

This is the first time process modelling has been applied to aluminium-steel butt welding by the Hybrid Metal Extrusion & Bonding (HYB) method. As a matter of fact, to the authors knowledge, this type of comprehensive model has not yet been developed neither for GMAW nor FSW of aluminium-steel components. In the authors’ opinion, the novelty and clear application of the work should now be fully explained in the revised Introduction section.

 

3. The problems related to dissimilar welding need more discussion and also refer to the recently published work on FSW: https://doi.org/10.1007/s11665-022-06832-2; https://doi.org/10.3390/met10050559; https://doi.org/10.1007/s11665-022-06822-4.

Authors’ reply:

These three references have now been inculded in the revised version of the manuscript, see Refs. [39-41].

 

4. Shorten the length of the introduction section and add key published work and try to make a bridge between current and previous published work.

Authors reply:

As already explained above, this is the first time process modelling has been applied to aluminium-steel butt welding by the Hybrid Metal Extrusion & Bonding (HYB) method. As a matter of fact, to the authors knowledge, this type of comprehensive model has not yet been developed neither for GMAW nor FSW of aluminium-steel components. In the authors’ opinion, the revised Introduction section should now convey that important message clearly to the unbiased reader.

 

4. Add the welding parameters in a separate table.

Authors reply:

This has already been done in connection with Fig. 8. Thus, in Section 4.1 the following sentence have now been added as an aid to the reader:

“Specifically, in Series I, II and III the calculated values for the net power input were 1350, 1075, and 1150 W, respectively”.

 

6. Provide the image of the welded plate.

Authors reply:

Yes, we agree that perhaps more information about how the experiments were conducted is needed. However, instead of adding new figures which will just make the paper even longer, we have added the following sentence towarsd the end of Section 3.2:

“The whole welding operation, as conducted using the HYB pilot welding machine at NTNU Aluminium Product Innovation Center (NAPIC), can be viewed at the following Web-site: https://www.youtube.com/watch?v=s1gzmN0I_Vs”.

 

7. Discuss about the tool pin profile and their selection.

Authors reply:

As an aid to the reader we have now included the following new paragraph in Section 2.2.

“Note that the pin design outlined in Figure 3 is tailor-made for butt welding of plates being separated from each other by an I-groove, as shown previously in Figure 1. Further details about the HYB pin design and how the pin geometry can be customised to handle a wide range of different applications, ranging from butt, lap, slot, fillet and multi-pass welding to plate surfacing and additive manufacturing have been reported elsewhere [11]”.

The interested reader can consult this very detailed key reference, covering all aspects of the HYB process, if specific information about the pin design how it works in a real welding situation is needed.

 

8. Add the macrograph of the welded joint.

Authors reply:

This is done, see Figure 9. In addition, high-resolution optical/SEM/TEM macrographs of a typical Al-steel HYB butt weld can be found elsewhere, see Refs. [11-16]. The interested reader can consult these key references if specific information about the as-welded joint macro- and microstructures is needed.

 

9. The mesh and model need more discussion. Add the node elements and boundary conditions with clear discussion.

Authors reply:

The numerical model was implemented using the Finite Element code WELDSIM. The geometry was modeled with 47,032 and 17,000 hexahedral finite elements for the plates and the backing plate, respectively. In addition, a finer mesh was created in the vicinity of the weld line in order to capture the high temperature gradients that characterize that region with the smallest element size of 0.75 mm. The effect of the filler material was also taken into account by element activation, while the moving heat source was modeled as a double ellipsoid power density distribution function, as proposed by Goldak [31]. The heat transfer coefficient with the backing plate was considered to be 200 W/m2 °C, while convective heat loss was modeled using a convective heat transfer coefficient equal to 20 W/m2 °C, in agreement with a high air-flow assumption [32].

We feel that this issue now is adequately addressed in the revised version of the manuscript.

 

10. In the hardness plot mention the each region clearly.

Authors reply:

In Figure 9 each region is now marked with different colors and labels: on the left-hand side a dark grey region shows the label S-BM, on the right-hand side a lighter area is marked with the Al-BM label. Boundaries between EZ and HAZ are specifed by arrows.

We feel that this issue now is adequately addressed in the revised version of the manuscript.

 

11. Hardness variation need technical discussion and also try to relate it with microstructure evolution.

Authors reply:

In Section 4.2 of the revised version of the manuscript this topic is now fully addressed. It follows from the extensive discussion that the microstructure evolution on the aluminium side of the joint is essentially similar to that documented, modelled and reported earlier by other researchers for both GMA and FS aluminium welds, see for example Refs. [17, 20, 25-29, 32]. Interested readers can consult theses key references if more specific information about the microstructure evolution during welding and thermomechanical processing of Al-Mg-Si alloys is needed.

 

11. Shorten the length of the conclusion section and add only 4/5 key bullet points in the conclusion section.

Authors reply:

This has now been done, see the new Conclusion section.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Accepted.