High-strength and crack-free welding of 2024 aluminium alloy via Zr-core-Al-shell wire

The 2000 series aluminium alloys are qualified for widespread use in lightweight structures, but solidification cracking during fusion welding has been a long-standing issue. Here, we create a zirconium (Zr)-core-aluminium (Al)-shell wire (ZCASW) and employ the oscillating laser-arc hybrid welding technique to control solidification during welding, and ultimately achieve reliable and crack-free welding of 2024 aluminium alloy. We select Zr wires with an ideal lattice match to Al based on crystallographic information and wind them by the Al wires with similar chemical components to the parent material. Crack-free, equiaxed (where the length, width and height of the grains are roughly equal), fine-grained microstructures are acquired, thereby considerably increasing the tensile strength over that of conventional fusion welding joints, and even comparable to that of friction stir welding joints. This work has important engineering application value in welding of high-strength aluminum alloys.

Achieving reliable and crack-free joints of AA2024 aluminum alloys by means of fusion welding is very challenging, as these alloy is susceptible to hot cracking.In order to overcome this issue, the present manuscript proposes the use of zirconium-core-aluminum-shell wires ('ZCASW') in hybrid laser-arc welding of AA2024.Although the presented topic is basically of high practical relevance, the current version of the manuscript is not suitable for being published in Nature Communications, which is due to the following reasons: (1) English needs major revision.Besides grammatical errors (e.g., incorrect use of was/were) und typing errors (e.g., 'phase fi[e]ld', line 493), the manuscript contains numerous misformulations, e.g.-lines 32-33 "Welding or joining, dubbed 'industrial tailoring', is necessarily ubiquitous..." -lines 126-127: "...characterized as a 'moderate' grain refiner identified with an ideal lattice matching that of to Al to provide a low energy..." -line 149: "Solidified microstructures have long been known to have a substantial influence on cracking during solidification", Do the authors would like to state that "The shape of the solid primary dendrites/crystals is known to influence..." -line 178: "...the concentration of strengthening alloy AA2024 decreased", Do the authors would like to state that "The molten filler alloy locally dilutes the molten AA2024 base alloy."? -lines 181-182: "The use of ZCASW filler material, an alternative method that has substantial potential for thoroughly altering the AA2024 solidification mechanism was employed successfully", Do the authors would like to state that "The use of the ZCASW filler material succesfully altered the solidification mechanism of the AA2024 alloy..."? -line 246 "...melting zones welded with...", line 257 "...the melting zone with..." -many more... (2) The wording is not exact (e.g., When describing the microstructure, the terms 'grains' and 'grain boundaries' were frequently used instead of the terms 'dendrites' and 'interdendritic regions', respectively).
(3) The presented ZCASW was only used in hybrid arc-laser welding of AA2024.Therefore, the statement "This rather simple modification of the filler material of a fusion weld could be generally applied to a wide range of materials that are susceptible to hot cracking, such as nonwelded ['nonweldable'!]nickel superalloys...." is just speculation, as it has not been proven.The authors should avoid purely speculative statements (e.g., lines 17-24 and lines 93-99)!(4) The first paragraph (lines 26-34) is intended to highlight the importance of lightweight design.However, it seems that the authors only cited secondary literature that is not related to the present topic instead of citing the relevant primary literature (e.g., original review papers) on weight reduction, fuel efficiency and lightweight materials.
(5) Hybrid laser-arc welding is not novel.Therefore, the introduction should also credit previous works on beam welding (electron and laser) and on hybrid welding of AA-2024.(6) The introduction of the manuscript should contain fundamentals of grain refinement and epitaxial growth of aluminum alloys, as Figure 1 (c) shows a schematic illustration of the lattice match, but without any sufficient explanation.The reference(s) of the schematic illustration and of the crystallographic data are also missing.(7) The description of hot cracking as given in lines 50-52 is not fully correct.The authors rather describe the formation of shrinkage cavities that form, if melt feeding is prevented during solidification.In simple words, hot cracking occurs during solidification of alloys, if the stress induced by solidification shrinkage locally exceeds the temperature-dependent tensile strength of the already solidified microstructure.As the strength of the solid interdendritic region is less than the strength of the primary dendrites, cracks predominantly form between the dendrites.However, if the feeding capability is sufficient, even these cracks may be fed with highly segregated melt that subsequently solidifies.Therefore, hot cracks do not necessarily appear as "empty" cavities in the final microstructure.
(8) The authors claim that the ZCASW weld did not contain any cracks.For confirmation they present a crack-free 3D reconstruction captured using computed tomography (CT).However, this confirmation is lacking for several reasons: -The dimensions (L x B x H) of the cubic CT 'microvolume' as illustrated in Figure 1 are missing.If the dimensions of the CT volume are small in comparison to the dimensions of the weld, existing (macro)cracks may possibly be overlooked.
-The exact position of the CT volume inside of the weld seam should be provided.Did the authors extract the volume from the root, the top or the side section of the weld, or did they even extract it from the heat affected zone (HAZ)?Stating that "...a microvolume was excised from a region of [the] weld..." is not sufficient.
(9) It is supposed to separate Figure 1 (a, b and c) that illustrates the filler wire and the process from Figure 1 (d and e) that shows the weld seam.Moreover, clear macroscopic images of the weld crosssection and of the weld surface are lacking and must be added to the manuscript!The authors did not present any high-resolution macroscopic image of the weld cross-section.Etching of the polished cross-section would be very beneficial to visualize the different microstructures of the weld, the HAZ and the base metal.Moreover, the cross-sections would enable to identify potential macrodefects, such as cracks or pores.
(10) The authors should be aware that Figure 2 (a, d, g) does not show any 'grains'.The bright areas obviously represent the primary dendritic phase (aluminum dendrites), the dark areas represent the interdendritic phase.The EDX element distribution maps in Figure 2 (k) and in Supplementary Figure 5 do not confirm that Zr, Cu and Mg are present in the weld, because only black squares are shown!(11) Figure 3 (a, c) is unnecessary, as it does not provide any additional information to Figure 3 (b, d). Figure 3 (e, f) should be larger to provide valuable information.The authors claimed that an eutectic formed in the channels between the dendrites.Is it really an eutectic (i.e., it formed by coupled solidification of two phases) or is it just the remaining melt that was highly enriched in Cu, Mg and other elements and that finally solidified as low-melting phase?Are the authors sure about the crystallographic stoichiometry (S-Al2CuMg)?Is there any evidence in the corresponding ternary phase diagrams?
(12) The authors should state where the values of the microstructural features that are summarized in Table 1 were taken from.Supplementary Table 1 should be part of the manuscript.
(13) Supplementary Figure 7 can be removed, as it does not provide any valuable information.The results of the presented spot measurements (high aluminum peak, low Cu and Mg peaks) are trivial, as all spots were located at the primary aluminum dendrites.

Reviewer #3 (Remarks to the Author):
This paper presents a new welding wire consisting of stranded aluminium wires around a Zr wire at the core to weld 'unweldable' 2024 Al alloy.This is a subject particularly interesting and relevant to the academic and industrial community.However, I am raising a number of concerns that need to be addressed before this work can be accepted for publication.
Firs and foremost, the new filler wire is proposed to avoid the formation of cracks in the weld but the authors provide no evidence of the cracks present using traditional filler material.Fig. 1e shows a reconstructed 3D tomography image of the weld.The authors do comment on the absence of crack but it does seem that a number of voids are present.Can the authors comment on the importance of these voids?Are these comparable to what would be obtained in a traditional fusion weld?Would this distribution of voids be acceptable?It would have been interesting to perform a similar tomography experiment with traditional fillers for comparison.
Line 166 -The sentence does not really make sense, maybe 'if' needs to be removed The authors mention the formation of cracks when using alternative fillers, particularly the ER2319 filler.However, no example of such cracks are shown.It would be beneficial to add such images of cracks to validate these claims.It seems difficult to conclude on the morphology of the eutectic phases between fig 3b and 3d.It seems that a large amount of the eutectic phases has been dislodged during the polishing step leaving a high number of voids at the GBs.So it seems difficult to conclude that ' the occurrence and size of the eutectic features were drastically reduced' from these images only.
Again, it is difficult to conclude anything from the CT provided in 3e as there is no discussion on the size and volume fraction of the eutectic features and crucially no comparison with the weld obtained with the alternative filler material.
The solidification behaviour is assessed via simulating the evolution of the liquid channel morphology between the two different fillers.A cracking susceptibility index is calculated as the slope of the T vs fs^0.5 plot.The CSI is said to be higher for the ER2319 vs ZCASW wire.However, the curves are so close to each other that the difference between the two seems almost negligible.How relevant is the observed difference in CSI.Also, could you plot the derivative of T vs fs^0.5 which would make it easier to conclude on whether there is a clear difference.At the moment it seems difficult to conclude form the information provided.
The mechanical properties of the different joints are then analysed using a combination of hardness and tensile tests.
The way the tensile strength is reported is confusing, do you quote the highest obtained strength and then in brackets the average and standard deviation?It would be more standard to only report the average value.How many tensile tests were conducted per condition?
When assessing the different contributions to strengthening, how were the precipitates analysed and what precipitates are we talking about?Is that the coarse Al3Zr phases?

REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): The article reports on the avoidance of cracking in high strength aluminium alloys through an innovative combination of aluminum and zirconium wires.As such, the idea is strong, the characterization detailed, but the science is not new.Therefore, I cannot recommend publication in a high-impact journal such as Nature Communications.

Please see below detailed comments:
Abstract: Line 11: The authors state that the Zr alloy was carefully selected.No reference is made to this selection process throughout the whole manuscript.Simply stating that Zr shows a grain refinement effect is insufficient and does not correspond to a selection process.Line 16ff: The reviewer does not agree that this method will apply to other materials.
There is body of literature that provides evidence for the grain refinement effect of Zr in aluminium, but this is not equivalently the case in other alloys.Thus, the mehtod is less disruptive than suggested by the authors.

Introduction:
Line 44: This is not true; AA2024 is not a eutectic alloy.

Results:
Line 137: Based on the literature, it is actually no surprise that the appraoch chosen works.
Line 197: Please give the definition of Q.The reviewer believes that it is unconvenient for the reader to have to look it up elsewhere.

Methods:
Line 434: Can the authors give the chemical composition of the deposit?As the wires are of identical diameter, it appears that there is a lot of Zr in the deposit.This is an import information.
General comments: The authors state that AA2024 is not weldable, but recent research, which has not been cited, suggests the opposite, provided that advanced fusion welding technques are applied.
English is of insufficient quality and there are several typing errors.
Further, it appears important that Zr is an expensive material.This aspect should be addressed somewhere, as the commercial viability of the approach is questionable.
Reviewer #2 (Remarks to the Author): Achieving reliable and crack-free joints of AA2024 aluminum alloys by means of fusion welding is very challenging, as these alloy is susceptible to hot cracking.In order to overcome this issue, the present manuscript proposes the use of zirconium-corealuminum-shell wires ('ZCASW') in hybrid laser-arc welding of AA2024.Although the presented topic is basically of high practical relevance, the current version of the manuscript is not suitable for being published in Nature Communications, which is due to the following reasons: (1) English needs major revision.
-lines 32-33 "Welding or joining, dubbed 'industrial tailoring', is necessarily ubiquitous..." -lines 126-127: "...characterized as a 'moderate' grain refiner identified with an ideal lattice matching that of to Al to provide a low energy..." -line 149: "Solidified microstructures have long been known to have a substantial influence on cracking during solidification", Do the authors would like to state that "The shape of the solid primary dendrites/crystals is known to influence..." -line 178: "...the concentration of strengthening alloy AA2024 decreased", Do the authors would like to state that "The molten filler alloy locally dilutes the molten AA2024 base alloy."? -lines 181-182: "The use of ZCASW filler material, an alternative method that has substantial potential for thoroughly altering the AA2024 solidification mechanism was employed successfully", Do the authors would like to state that "The use of the ZCASW filler material succesfully altered the solidification mechanism of the AA2024 alloy..."? -line 246 "...melting zones welded with...", line 257 "...the melting zone with..." -many more...
(2) The wording is not exact (e.g., When describing the microstructure, the terms 'grains' and 'grain boundaries' were frequently used instead of the terms 'dendrites' and 'interdendritic regions', respectively).
(3) The presented ZCASW was only used in hybrid arc-laser welding of AA2024.
Therefore, the statement "This rather simple modification of the filler material of a fusion weld could be generally applied to a wide range of materials that are susceptible to hot cracking, such as nonwelded ['nonweldable'!]nickel superalloys...." is just speculation, as it has not been proven.The authors should avoid purely speculative statements (e.g., lines 17-24 and lines 93-99)!(4) The first paragraph (lines 26-34) is intended to highlight the importance of lightweight design.However, it seems that the authors only cited secondary literature that is not related to the present topic instead of citing the relevant primary literature (e.g., original review papers) on weight reduction, fuel efficiency and lightweight materials.
(5) Hybrid laser-arc welding is not novel.Therefore, the introduction should also credit previous works on beam welding (electron and laser) and on hybrid welding of AA-2024.(6) The introduction of the manuscript should contain fundamentals of grain refinement and epitaxial growth of aluminum alloys, as Figure 1 (c) shows a schematic illustration of the lattice match, but without any sufficient explanation.The reference(s) of the schematic illustration and of the crystallographic data are also missing.(7) The description of hot cracking as given in lines 50-52 is not fully correct.The authors rather describe the formation of shrinkage cavities that form, if melt feeding is prevented during solidification.In simple words, hot cracking occurs during solidification of alloys, if the stress induced by solidification shrinkage locally exceeds the temperature-dependent tensile strength of the already solidified microstructure.As the strength of the solid interdendritic region is less than the strength of the primary dendrites, cracks predominantly form between the dendrites.However, if the feeding capability is sufficient, even these cracks may be fed with highly segregated melt that subsequently solidifies.Therefore, hot cracks do not necessarily appear as "empty" cavities in the final microstructure.
(8) The authors claim that the ZCASW weld did not contain any cracks.For confirmation they present a crack-free 3D reconstruction captured using computed tomography (CT).However, this confirmation is lacking for several reasons: -The dimensions (L x B x H) of the cubic CT 'microvolume' as illustrated in Figure 1 are missing.If the dimensions of the CT volume are small in comparison to the dimensions of the weld, existing (macro)cracks may possibly be overlooked.
-The exact position of the CT volume inside of the weld seam should be provided.Did the authors extract the volume from the root, the top or the side section of the weld, or did they even extract it from the heat affected zone (HAZ)?Stating that "...a microvolume was excised from a region of [the] weld..." is not sufficient.
(9) It is supposed to separate Figure 1 (a, b and c) that illustrates the filler wire and the process from Figure 1 (d and e) that shows the weld seam.Moreover, clear macroscopic images of the weld cross-section and of the weld surface are lacking and must be added to the manuscript!The authors did not present any high-resolution macroscopic image of the weld cross-section.Etching of the polished cross-section would be very beneficial to visualize the different microstructures of the weld, the HAZ and the base metal.Moreover, the cross-sections would enable to identify potential macrodefects, such as cracks or pores.
(10) The authors should be aware that Figure 2 (a, d, g) does not show any 'grains'.The bright areas obviously represent the primary dendritic phase (aluminum dendrites), the dark areas represent the interdendritic phase.The EDX element distribution maps in Figure 2 (k) and in Supplementary Figure 5 do not confirm that Zr, Cu and Mg are present in the weld, because only black squares are shown!(11) Figure 3 (a, c) is unnecessary, as it does not provide any additional information to Figure 3 (b, d). Figure 3 (e, f) should be larger to provide valuable information.The authors claimed that an eutectic formed in the channels between the dendrites.Is it really an eutectic (i.e., it formed by coupled solidification of two phases) or is it just the remaining melt that was highly enriched in Cu, Mg and other elements and that finally solidified as low-melting phase?Are the authors sure about the crystallographic stoichiometry (S-Al2CuMg)?Is there any evidence in the corresponding ternary phase diagrams?
(12) The authors should state where the values of the microstructural features that are summarized in Table 1 were taken from.Supplementary Table 1 should be part of the manuscript.
(13) Supplementary Figure 7 can be removed, as it does not provide any valuable information.The results of the presented spot measurements (high aluminum peak, low Cu and Mg peaks) are trivial, as all spots were located at the primary aluminum dendrites.

Reviewer #3 (Remarks to the Author):
This paper presents a new welding wire consisting of stranded aluminium wires around a Zr wire at the core to weld 'unweldable' 2024 Al alloy.This is a subject particularly interesting and relevant to the academic and industrial community.However, I am raising a number of concerns that need to be addressed before this work can be accepted for publication.
Firs and foremost, the new filler wire is proposed to avoid the formation of cracks in the weld but the authors provide no evidence of the cracks present using traditional filler material.Fig. 1e shows a reconstructed 3D tomography image of the weld.The authors do comment on the absence of crack but it does seem that a number of voids are present.
Can the authors comment on the importance of these voids?Are these comparable to what would be obtained in a traditional fusion weld?Would this distribution of voids be acceptable?It would have been interesting to perform a similar tomography experiment with traditional fillers for comparison.
Line 166 -The sentence does not really make sense, maybe 'if' needs to be removed The authors mention the formation of cracks when using alternative fillers, particularly the ER2319 filler.However, no example of such cracks are shown.It would be beneficial to add such images of cracks to validate these claims.It seems difficult to conclude on the morphology of the eutectic phases between fig 3b and 3d.It seems that a large amount of the eutectic phases has been dislodged during the polishing step leaving a high number of voids at the GBs.So it seems difficult to conclude that ' the occurrence and size of the eutectic features were drastically reduced' from these images only.
Again, it is difficult to conclude anything from the CT provided in 3e as there is no discussion on the size and volume fraction of the eutectic features and crucially no comparison with the weld obtained with the alternative filler material.
The solidification behaviour is assessed via simulating the evolution of the liquid channel morphology between the two different fillers.A cracking susceptibility index is calculated as the slope of the T vs fs^0.5 plot.The CSI is said to be higher for the ER2319 vs ZCASW wire.However, the curves are so close to each other that the difference between the two seems almost negligible.How relevant is the observed difference in CSI.Also, could you plot the derivative of T vs fs^0.5 which would make it easier to conclude on whether there is a clear difference.At the moment it seems difficult to conclude form the information provided.
The mechanical properties of the different joints are then analysed using a combination of hardness and tensile tests.
The way the tensile strength is reported is confusing, do you quote the highest obtained strength and then in brackets the average and standard deviation?It would be more standard to only report the average value.How many tensile tests were conducted per condition?
When assessing the different contributions to strengthening, how were the precipitates analysed and what precipitates are we talking about?Is that the coarse Al3Zr phases?Detailed Response to Reviewers' Comments "High-strength and crack-free welding of unweldable aluminium alloys via novel Zr-core-Al-shell wires" We very much appreciate the reviewer's comments and suggestions (listed in black type below), which have significantly helped us improve our paper.We have made substantial revisions to the original manuscript with all these suggestions incorporated.
The detailed responses and the revisions made are given in blue and green type below.
The resulting changes are highlighted in orange in the revised manuscript.
Best regards, and thanks.

General Comments:
The article reports on the avoidance of cracking in high strength aluminium alloys through an innovative combination of aluminum and zirconium wires.As such, the idea is strong, the characterization detailed, but the science is not new.Therefore, I cannot recommend publication in a high-impact journal such as Nature Communications.

Please see below detailed comments:
Response: Thanks for your constructive comments.We are very sorry that we do not make the science of the article clear.Here, we conclude the science of the article as follows.
(1) A novel zirconium-core-aluminium-shell wire is invented and the oscillating laser-arc hybrid welding technique is adopted to synergistically control solidification during welding.Reliable and crack-free welding of 2024 aluminium alloy is achieved ultimately.
(2) The underlying mechanism why the zirconium-core-aluminium-shell wire could inhibit solidification cracking significantly during welding of AA2024 is revealed from the perspective of dendritic structure and interdendritic phase.
(3) An approach combining the phase field simulation and Kou's criterion is developed to reveal how the microstructure morphology and solute segregation affect solidification cracking, and the solidification cracking susceptibility during welding is quantitatively predicted.
(4) The influence mechanism of ZCASW filler on the mechanical properties of welded joint is revealed, and the strength contribution is quantitatively calculated to demonstrate why welded joint fabricated with ZCASW filler has higher tensile strength.

Specific Comments:
1. Line 11: The authors state that the Zr alloy was carefully selected.No reference is made to this selection process throughout the whole manuscript.Simply stating that Zr shows a grain refinement effect is insufficient and does not correspond to a selection process. 1 Response: Thank you very much for your comments.We have added a discussion of the selection process to the introduction of the revised manuscript, as follows.
"Introducing nucleation particles to produce identical ultrafine equiaxed structure has been an effective method.It can enlarge the equiaxed region of the thermal gradientgrowth velocity curve 1 , which could easily assist to generate equiaxed microstructure 2 .Furthermore, the emergence of nucleation particles could increase the undercooling at the solid/liquid interface and decrease the critical nuclear radius 3 , thereby effectively facilitating the grain refinement during solidification.To obtain ultrafine equiaxed microstructures, the nucleation particles need to have similar lattice parameters to α-Al [3][4][5] .Thus, common elements for inoculation treatments, such as Zr, Ti, and Sc, have been chosen to form Al3X (X= Zr, Ti, or Sc) owing to their small lattice mismatch with α-Al.The lattice parameter of Al3Zr, Al3Sc and Al3Ti is 4.08 Ǻ, 4.103 Ǻ and 3.967 Ǻ, which is similar to that of Al of 4.049 Ǻ 4 .Compared to Al3Sc or Al3Ti phase, the lower misfit value (0.765%) between the Al3Zr phase and α-Al phase can decrease the nucleation barrier for precipitation 5 .Moreover, the Al3Zr phase can serve as excellent heterogeneous nucleation sites of α-Al due to the close structural resemblances between the two phases 6 .For example, two kinds of cube-on-cube orientation relationships (OR) exist between L12-Al3Zr and the α-Al: Al(001)//L12-Al3Zr(001), Al[110]//L12-Al3Zr[110] and Al(100)//L12-Al3Zr(100), Al[010]//L12-Al3Zr[010] 7 ." 2 2. Line 16ff: The reviewer does not agree that this method will apply to other materials.
There is body of literature that provides evidence for the grain refinement effect of Zr in aluminium, but this is not equivalently the case in other alloys.Thus, the method is less disruptive than suggested by the authors.

Response:
We are very sorry for our inaccurate representation.Here, we simply want to express that other materials sensitive to solidification cracking can be processed using the filler with novel structure instead of specific zirconium (Zr)-core-aluminium (Al)-shell-wire (ZCASW).the novel filler for Haynes 230 additive manufacturing.Taking the above into considerations, we would make changes to the relevant parts. 3s ZCASW provides a foundation for broad industrial applications because it meets the demands for efficiency in automated welding.This technology (the ZCASW coupled with oscillating laser-arc hybrid welding) also has a great potential for metalbased additive manufacturing of high-strength aluminium alloys, in which solidification cracking is a common issue." 3. Line 44: This is not true; AA2024 is not a eutectic alloy.
Response: This sentence has been corrected in the revised manuscript.
4. Line 137: Based on the literature, it is actually no surprise that the approach chosen works.

Response:
We have corrected this sentence in the revised manuscript, as follows.
"Fig.2g exhibits no cracks as well, indicating that this novel filler material is remarkably effective." 5. Line 197: Please give the definition of Q.The reviewer believes that it is unconvenient for the reader to have to look it up elsewhere.

Response:
We have added the definition of Q to the revised manuscript, as follows.
"the growth-restricting factor (Q), can be expressed as follow 9 : Here, ml represents the gradient of liquidus, C0 represents the initial composition of alloy, and k represents the partition coefficient." 6. Line 434: Can the authors give the chemical composition of the deposit?As the wires are of identical diameter, it appears that there is a lot of Zr in the deposit.This is an import information.
Response: Thank you very much for your valuable comments.We have added the

Response:
We are very sorry for our inaccurate statements.We just want to state that the 2024 aluminum alloy is hard-to-weld.The argument that AA2024 has poor weldability has been stated by leading welding companies such as Lincoln Electric and ESAB.Please see the following website 10-11 .Taking the above into considerations, we will change the title of "High-strength and crack-free welding of unweldable aluminium alloys via novel Zr-core-Al-shell wires" into "High-strength and crack-free welding of 2024 aluminium alloy via novel Zr-core-Al-shell wire".4 8. English is of insufficient quality and there are several typing errors.

Response:
We have carefully modified the language of the article and corrected typing errors.Meanwhile, we have invited professionals to polish the language of the article.
The revised parts are highlighted in orange in the revised manuscript.
9. Further, it appears important that Zr is an expensive material.This aspect should be addressed somewhere, as the commercial viability of the approach is questionable.
Response: Thanks for your constructive comments.We agree with your opinion.As you point out, Zr is a relatively expensive material, which increases the cost of the 10. https://esab.com/us/nam_en/esab-university/blogs/how-do-i-weld-2024-and-7075/11. https://www.lincolnelectric.com/en-us/support/welding-solutions/Pages/aluminum-faqsdetail.aspx#question8ZCASW filler material.We have estimated the cost of the filler material.The price of producing 1 kg ZCASW filler material is about ¥ 704 CNY and the price of 1 kg ER2319 filler material is about ¥ 330 CNY.In the future, we will optimize the structure of our ZCASW filler material and reduce zirconium content, which is help to further decrease the cost.

Response to Referee #2:
General Comments: Achieving reliable and crack-free joints of AA2024 aluminum alloys by means of fusion welding is very challenging, as these alloy is susceptible to hot cracking.In order to overcome this issue, the present manuscript proposes the use of zirconium-corealuminum-shell wires ('ZCASW') in hybrid laser-arc welding of AA2024.Although the presented topic is basically of high practical relevance, the current version of the manuscript is not suitable for being published in Nature Communications, which is due to the following reasons: Specific Comments: 1. English needs major revision.

Response:
We have carefully modified the language of the article and corrected typing errors.Meanwhile, we have invited professionals to polish the language of the article.
As for the numerous misformulations, we also have already made great changes in the revised manuscript.The revised parts are highlighted in orange in the revised manuscript.We have revised each of the misformulations that have been listed below.
"Welding is an important process for assembling lightweight materials." 2) -lines 126-127: "...characterized as a 'moderate' grain refiner identified with an ideal lattice matching that of to Al to provide a low energy..."

Response:
We have corrected this sentence in the revised manuscript, as follows.
"This phase could significantly improve grain refinement efficiency and provide a lowenergy nucleation barrier for α-Al according to classical nucleation theory 3,5 ." 3) -line 149: "Solidified microstructures have long been known to have a substantial influence on cracking during solidification", Do the authors would like to state that "The shape of the solid primary dendrites/crystals is known to influence..." 5 Response: Thanks for your comments.This sentence has been corrected, as shown below.
"The shape of the solid primary dendrites/crystals is known to influence cracking susceptibility during solidification."4) -line 178: "...the concentration of strengthening alloy AA2024 decreased", Do the authors would like to state that "The molten filler alloy locally dilutes the molten AA2024 base alloy."?

Response:
We have corrected this sentence in the revised manuscript, as follows.
"The molten filler alloy locally dilutes the molten AA2024 base alloy, which reduces the concentration of strengthening alloy 6 ."-many more...

Response:
These sentences have been corrected as follows.
"Fig.3a presents the melting zone fabricated with the ER2319 filler material." "…the melting zone fabricated with the ER2319 and the ZCASW filler materials…" 2. The wording is not exact (e.g., When describing the microstructure, the terms 'grains' and 'grain boundaries' were frequently used instead of the terms 'dendrites' and 'interdendritic regions', respectively).

Response:
We have corrected the statements about describing the microstructure, as follows.
"We conducted microstructure analysis from the perspective of the dendrite morphology and the interdendritic region morphology in the melting zones fabricated with different fillers." "Here, we first discuss the effect of the dendrite morphology in detail.Fig. 3a presents the melting zone fabricated with the ER2319 filler material." 3. The presented ZCASW was only used in hybrid arc-laser welding of AA2024.
Therefore, the statement "This rather simple modification of the filler material of a fusion weld could be generally applied to a wide range of materials that are susceptible to hot cracking, such as nonwelded ['nonweldable'!]nickel superalloys...." is just speculation, as it has not been proven.The authors should avoid purely speculative statements (e.g., lines 17-24 and lines 93-99)!
Response: Thank you very much for your insightful comments.We have corrected the corresponding statements, as shown below.
"This ZCASW provides a foundation for broad industrial applications because it meets the demands for efficiency in automated welding.This technology (the ZCASW coupled with oscillating laser-arc hybrid welding) also has a great potential for metalbased additive manufacturing of high-strength aluminium alloys, in which solidification cracking is a common issue." "This new welding technology provides a foundation for broad industrial applications because it could meet the demands for efficiency in automated welding." 4. The first paragraph (lines 26-34) is intended to highlight the importance of lightweight design.However, it seems that the authors only cited secondary literature that is not related to the present topic instead of citing the relevant primary literature (e.g., original review papers) on weight reduction, fuel efficiency and lightweight materials.
Response: Thank you very much for your comments.We have cited the original review papers on weight reduction, fuel efficiency and lightweight materials in the introduction, as follows.
"Today, lightweight materials are an important component in promoting energy and environmental sustainabiliy 12,13 .Every additional 100 kg decrease in vehicle weight leads to a reduction in CO2 emissions of 8.7 g per kilometre and fuel consumption of 0.4 litres per 100 kilometres 14 ." 6ybrid laser-arc welding is not novel.Therefore, the introduction should also credit previous works on beam welding (electron and laser) and on hybrid welding of AA-2024.

Response:
We have added the corresponding contents about electron beam welding, arc welding, laser beam welding and laser-arc hybrid welding of AA2024 to the introduction in the revised manuscript, as shown below.
" welding 16 , electron beam welding 17 and hybrid welding 18 .And they highlighted that one of the primary problems of fusion welding is solidification cracking, which considerably hinders its widespread use." 6.The introduction of the manuscript should contain fundamentals of grain refinement and epitaxial growth of aluminum alloys, as Figure 1 (c) shows a schematic illustration of the lattice match, but without any sufficient explanation.The reference(s) of the schematic illustration and of the crystallographic data are also missing.
Response: Thank you for your valuable comment.We have added the fundamentals of grain refinement and epitaxial growth of aluminum alloys to the introduction of the revised manuscript, and the references of the schematic illustration and the crystallographic data have also been added as follows. 7roducing nucleation particles to produce identical ultrafine equiaxed structure has been an effective method.It can enlarge the equiaxed region of the thermal gradientgrowth velocity curve 1 , which could easily assist to generate equiaxed microstructure 2 .Furthermore, the emergence of nucleation particles could increase the undercooling at the solid/liquid interface and decrease the critical nuclear radius 3 , thereby effectively facilitating the grain refinement during solidification.To obtain ultrafine equiaxed microstructures, the nucleation particles need to have similar lattice parameters to α- necessarily appear as "empty" cavities in the final microstructure.
Response: Thanks for your comments.We are sorry for our inaccurate statements and we have corrected the relevant formulations as follows.
"As the solidification process advances, the proportion of liquid phase decreases.When the tensile stress resulting from solidification shrinkage exceeds the strength of the almost completely solidified microstructure and the liquid feeding is insufficient during solidification, solidification cracking would occur between the dendrites 21 ." 98.The authors claim that the ZCASW weld did not contain any cracks.For confirmation they present a crack-free 3D reconstruction captured using computed tomography (CT).However, this confirmation is lacking for several reasons: -The dimensions (L x B x H) of the cubic CT 'microvolume' as illustrated in Figure 1 are missing.If the dimensions of the CT volume are small in comparison to the dimensions of the weld, existing (macro)cracks may possibly be overlooked.
-The exact position of the CT volume inside of the weld seam should be provided.
Did the authors extract the volume from the root, the top or the side section of the weld, or did they even extract it from the heat affected zone (HAZ)?Stating that "...a microvolume was excised from a region of [the] weld..." is not sufficient.
Response: Firstly, we observed the surface topography, longitudinal and cross section morphology of the welding seam to detect the macro crack defects.If the crack existed, we excised a microvolume with dimensions 2 mm×2 mm×5 mm from the region containing crack defects.If the crack did not exist, we excised a microvolume with dimensions 2 mm×2 mm×5 mm from the upper part of the center of the welding seam for X-ray microtomography analysis.Fig. 1 shows the schematic diagram of the CT samples preparation location.Fig. 2 shows the experimental result.9.It is supposed to separate Figure 1 (a, b and c) that illustrates the filler wire and the process from Figure 1 (d and e) that shows the weld seam.Moreover, clear macroscopic images of the weld cross-section and of the weld surface are lacking and must be added to the manuscript!The authors did not present any highresolution macroscopic image of the weld cross-section.Etching of the polished cross-section would be very beneficial to visualize the different microstructures of the weld, the HAZ and the base metal.Moreover, the cross-sections would enable to identify potential macrodefects, such as cracks or pores.
Response: Thank you very much for your insightful comments.We have separated Figure 1(a, b and c) from Figure 1.Meanwhile, we have added the clear macroscopic images of the welding seam to the ravised manuscript, as shown in Fig. 3. 10.The authors should be aware that Figure 2 (a, d, g) does not show any 'grains'.The bright areas obviously represent the primary dendritic phase (aluminum dendrites), the dark areas represent the interdendritic phase.The EDX element distribution maps in Figure 2 (k) and in Supplementary Figure 5 do not confirm that Zr, Cu and Mg are present in the weld, because only black squares are shown!

Response:
We have corrected the relevant expressions about the bright areas and the dark areas, and replaced them with the dendrites and the interdendritic phase.
Meanwhile, we have added the chemical composition of the welding seam fabricated with the ZCASW filler to the revised manuscript, and the test was conducted using Inductive Coupled Plasma Optical Emission Spectrometer (ICP-OES), the results are shown in Table 1.The content of Zr, Cu and Mg is 1.8066%, 4.1957% and 1.4899%, respectively.11. Figure 3 (a, c) is unnecessary, as it does not provide any additional information to    12.The authors should state where the values of the microstructural features that are summarized in Table 1 were taken from.Supplementary Table 1 should be part of the manuscript. 11 Response: Thanks for your comments.The grain size (d) was 4 µm derived from the EBSD analysis (Fig. 3h and i in the revised manuscript).The concentration of the solute element Mg and Cu (in wt.%) was measured by SEM-EDS (Supplementary Table 1).
The dislocation densities (ρGND) was determined by EBSD analysis (Supplementary Fig. 4).r and f were extracted from the SEM images in BSE mode (Supplementary Fig. 6) using the image processing software.These characteristic values of the microstructure for strength calculation are mentioned in the estimating the strengthening contributions section of the methods in the revised manuscript.Meanwhile, we have added Supplementary Table 1 to the revised manuscript and renamed it as Table 2.

Response:
We have removed the supplementary Fig. 7.

Response to Referee #3:
General Comments: This paper presents a new welding wire consisting of stranded aluminium wires around a Zr wire at the core to weld 'unweldable' 2024 Al alloy.This is a subject particularly interesting and relevant to the academic and industrial community.However, I am raising a number of concerns that need to be addressed before this work can be accepted for publication.
Specific Comments: 1.The new filler wire is proposed to avoid the formation of cracks in the weld but the authors provide no evidence of the cracks present using traditional filler material.
Response: We performed laser welding experiments using ER2319 filler to fuse AA2024 sheets.Meanwhile, to further visualize the 3D nature of internal imperfections, a 2 mm×2 mm×5 mm microvolume was excised for examination by X-ray micro computed tomography technology, the result is shown in Fig. 6. 2. Fig. 1e shows a reconstructed 3D tomography image of the weld.The authors do comment on the absence of crack but it does seem that a number of voids are present.
Can the authors comment on the importance of these voids?Are these comparable to what would be obtained in a traditional fusion weld?Would this distribution of voids be acceptable?It would have been interesting to perform a similar tomography experiment with traditional fillers for comparison.
Response: Thank you for your suggestion.We conducted X-ray micro computed tomography technology analysis on the welds fabricated with the ER2319, the ER4043 and the ZCASW fillers, respectively.The results are shown in Fig. 7a, b and c.The porosity in the welds are 0.02%, 0.06% and 0.15%, respectively.Although the porosity corresponding to the ZCASW filler is slightly higher than that corresponding to the traditional fillers, it is commonly acceptable in engineering (≤1%).How to further reduce the porosity and improve the strength is also the focus of our further work.3. Line 166 -The sentence does not really make sense, maybe 'if' needs to be removed.

Response:
We have removed the 'if' from this sentence, as shown below.
"As the temperature and liquid volume fraction decrease, the long channels could be trapped or hindered by the developing dendritic solid network." 4. The authors mention the formation of cracks when using alternative fillers, particularly the ER2319 filler.However, no example of such cracks are shown.It would be beneficial to add such images of cracks to validate these claims.

Response:
We have added the 3D-reconstruction volumes image of the defects inside the welding seam fabricated with the ER2319 filler to the revised manuscript, as shown in Fig. 6.   9.The solidification behaviour is assessed via simulating the evolution of the liquid channel morphology between the two different fillers.A cracking susceptibility index is calculated as the slope of the T vs fs^0.5 plot.The CSI is said to be higher for the ER2319 vs ZCASW wire.However, the curves are so close to each other that the difference between the two seems almost negligible.How relevant is the observed difference in CSI.Also, could you plot the derivative of T vs fs^0.5 which would make it easier to conclude on whether there is a clear difference.At the moment it seems difficult to conclude form the information provided.
Response: Thank you for your valuable comments.We have plotted the curve of |dT/d(fs 1/2 )| vs T at the final solidification stage in the inset of Fig. 10.It is evident that the cracking susceptibility index corresponding to the ER2319 material is higher than that corresponding to the ZCASW filler.Response: Thank you for your advice.In the previous manuscript, we reported the highest tensile strength and then listed the average and standard deviation in brackets.
In the revised manuscript, we only reported the average value of tensile strength and conducted three experiments under per condition.
11.When assessing the different contributions to strengthening, how were the precipitates analysed and what precipitates are we talking about?Is that the coarse Al3Zr phases?
Response: The precipitates analyzed when assessing the different contributions to strengthening is the coarse Al3Zr phases.We obtained the mean precipitate radius r and the precipitate volume fraction f of the Al3Zr phases from the SEM images in the BSE diffraction mode (supplementary Fig. 6) using the image processing software.Then, we assessed the strength contribution arising from the Orowan bypassing mechanism using the Eq. ( 5) in revised manuscript.
Line 247 and 256.The higher magnification images are Fig 3b and Fig 3d (and not 3a and 3c) The grain sizes from the EBSD in Fig 2 do not seem to match the ones from the microscopy presented in Fig 3.In Fig 3b and 3d, the grain size appears similar for the ER2319 and ZCASW.
Line 247 and 256.The higher magnification images are Fig 3b and Fig 3d (and not 3a and 3c) The grain sizes from the EBSD in Fig 2 do not seem to match the ones from the microscopy presented in Fig 3.In Fig 3b and 3d, the grain size appears similar for the ER2319 and ZCASW.

Fig. 1
Fig. 1 The schematic diagram of the CT samples preparation location.

Fig. 3
Fig. 3 The macroscopic morphology of the welding seam fabricated with the ZCASW filler material.a The surface topography of the welding seam.b The longitudinal section morphology of the welding seam.c The cross-sectional morphology of the welding seam.

Figure 3 (
Figure 3 (b, d).Figure 3 (e, f) should be larger to provide valuable information.The

Figure 3 (Figure 3
Figure 3 (b, d).Figure3 (e, f) should be larger to provide valuable information.The authors claimed that an eutectic formed in the channels between the dendrites.Is it really an eutectic (i.e., it formed by coupled solidification of two phases) or is it just the remaining melt that was highly enriched in Cu, Mg and other elements and that finally solidified as low-melting phase?Are the authors sure about the

Fig. 4
Fig. 4 Interdendritic phase morphology characterization in the melting zone fabricated with the ER2319 and the ZCASW filler materials.a and b Interdendritic phase in the melting zone fabricated with the ER2319 and the ZCASW filler materials.c Comparison of interdendrictic phase area fractions obtained from different melting zones.d EPMA maps showing the chemical constitutions of the interdendritic phase and Al matrix in the melting zone fabricated with the ZCASW filler.e Interdendritic phase observed by TEM-EDX, showing a STEM-HAADF image.f SAED pattern of the interdendritic phase.g EDX mapping of the main elements (Al, Cu, Mg and Zr).h HRTEM image of the interface between the α-Al and Mg 2 Cu phase, with the inset showing the FFT pattern of Mg 2 Cu phase.

Fig. 6 3D
Fig. 6 3D-reconstruction volumes of the defects inside the welding seam fabricated with the ER2319 filler (Color bar denotes pores diameter).

Fig. 7 3D
Fig. 7 3D-reconstruction volumes of the defects inside the welding seam fabricated with different filler materials.a The welding seam fabricated with the ER2319 filler.b The welding seam fabricated with the ER4043 filler, and c The welding seam fabricated with the ZCASW filler (Color bar denotes pores diameter).

Fig. 6 3D 6 .
Fig.6 3D-reconstruction volumes of the defects inside the welding seam fabricated with the ER2319 filler (Color bar denotes pores diameter).

7 .
Fig.9a and b).The results are shown in Fig.9e, the area fraction of the interdendritic phase in the melting zone fabricated with the ER2319 filler is 11.5%, and it increases to 15.95% in the melting zone fabricated with ZCASW filler.More interdendritic phase indicates more liquid exists among dendrites during the terminal stage of solidification 8 , which could sufficiently compensate for solidification shrinkage and thermal contraction.12

Fig. 9
Fig. 9 Interdendritic phase morphology characterization in the melting zone fabricated with the ER2319 and the ZCASW filler materials.a and b The interdendritic phase in the melting zone fabricated with the ER2319 and the ZCASW filler materials.c and d The binarized image of a and b. e Comparison of interdendrictic phase area fractions obtained from different melting zones.

Fig. 10 T
Fig.10 T-(f s) 1/2 curves and cracking susceptibility indexes calculated from the PF simulations for the melting zone fabricated with the ZCASW (blue) and the ER2319 (orange) fillers.
composition of deposit fabricated with the ZCASW filler material to the revised manuscript, and the test is conducted using Inductive Coupled Plasma Optical Emission Spectrometer (ICP-OES).The results are shown in Table1, and the Zr content in the deposit is 1.8066%.

Table 1
Chemical composition of deposit fabricated with the ZCASW filler/wt% can be removed, as it does not provide any valuable information.The results of the presented spot measurements (high aluminum peak, low Cu and Mg peaks) are trivial, as all spots were located at the primary aluminum 22. Chen, S. L. et al.A thermodynamic description for the ternary Al-Mg-Cu system.Metall.Mater.Trans. A. 28, 435-446 (1997).23.Raghavan, V. Al-Cu-Mg (Aluminum-Copper-Magnesium).J. Phase Equilib.Diff.28, 174-179 (2007).