Particle Ejection by Jetting and Related Effects in Impact Welding Processes

Round 1

Reviewer 1 Report

The article entitled "Particle ejection by jetting and related effects in impact welding processes" written by Bellmann et al. conducts an investigation into the effects of ambient pressure/vacuum conditions and material selection/geometry/morphology on thermal and physical phenomenon involved in metal jetting during the collision-based welding processes of similar/dissimilar metallic pairs. Therefore, this article is fully in the scope of the “Metals” journal as it helps better understand the underlying physical phenomena involved in an advanced group of metal welding technologies.

The article is overall well written and well structured. However, an editing of English language and style is required to help the reader better understand and thus appreciate the merits of the paper.

From comments made above, I do recommend the publication of this article. However, interested in this topic, I would recommend some minor complements and corrections, which are mentioned in what follows:

It is not clear that the term “jetting” in the wider literature was ever intended to be strictly limited to relatively narrow streams of material that are ablated from the surfaces in a collision weld process, despite what analytical/computational models (that allow for jetting) have shown. The cloud of particles does seem a useful concept to describe experimental observations, but it may clarify matters to define the CoP to be a subset of jetting, applicable to situations where a collision weld jet manifests as finely dispersed particles, rather than as a categorically separate phenomenon. Some discussion in the introduction defining the “CoP” term in relation to the more commonly used “jet” should be done to ensure clarity throughout the work.

Page 4: Before the first appearance of the PDV acronym in the main text at line 230, it appears in Table 1 without definition.

Page 5: More description of the laser processing of the test rig’s contact surfaces is needed. Alternatively, a reference to a previous work where an identical procedure was used may be suitable.

• Laser surfaced Al alloy parent sheets were used in the test rig, with given roughness heights. What was the measured roughness height of the laser ablated ‘control’ surface with no additional laser surfacing?
• What type of laser(s) was used for the surfacing/structuring?

Page 7: Figure 3 should have mm indicated for the length units shown.

Pages 9-10: For the test rig, the 30-micron laser structured Al alloy parent sheet produced no successful welds, and the reader may surmise that that is why photography for that trial was not included in Figure 5. However, the ultrafine grained Cu parent material also did not produce a weld, and yet photos for that experiment were included in Figure 6. The unsuccessful trial for the 30-µm rough Al should be shown, or its exclusion should be accounted for.

Line 31: Please add the word “CoP” before the word “interaction”.

Lines 32-33: In the abstract, the sentence reads “The CoP formed during the collision of the joining partners is compressed by the closing joining gap…”. I suggest rewording this sentence as follows: “The CoP formed during the collision of the joining metal pair is compressed by the air entrapped inside the closing joining gap...” as a gap cannot cause compression but air will. Please make a similar change in lines 56, 63, 103, 128, 156, 159, 161, 165, 175, 280, 445, 461, and 468-469.

Lines 34-36: The abstract summarizes what has been done but fails to list the important findings of the work. Instead of “More generally…conditions.”, please include more of the results and conclusions of this work in the abstract. (Line 35: Please replace the phrase “supported by” with “met by the use of”.)

Lines 26-27, 87, 209, 381, 463-464: Two distinct yet similar concepts named Cloud of Particles (CoP) and jet (which is already established in the impact welding research field) are emphasized and compared in numerous occasions. However, this work and the previous work (reference [12]) fail to clearly define each phenomenon and explain exactly how different these two phenomena are. A clear visual comparison (even if it is in the form of two side-by-side schematics) is needed to help the reader understand this crucial distinction.

Lines 51-55: The introduction lists only the older collision-based welding techniques such as explosive welding (EXW) and magnetic pulse welding (MPW) and does not mention the more modern related technologies such as vaporizing foil actuator welding (VFAW) and laser impact welding (LIW). Similar to EXW and MPW, VFAW and LIW processes revolve around the jetting phenomenon and therefore are related topics to be mentioned in the introduction. In addition to reference [2], the work of Sadeh et al. (https://www.mdpi.com/2075-4701/9/11/1196) contains a comprehensive review of the literature on the related topics and would be a helpful reference.

Line 68: In meshless simulations of the jetting phenomenon, the recently published work by Gleason et al. (https://www.sciencedirect.com/science/article/pii/S2351978920314906) includes an interesting discussion on the topic and is worth mentioning as a reference.

Line 148: In Table 1, the capital letter S is used as the symbol for two parameters. Please use two different symbols if possible to avoid causing confusion.

Line 239: Please thoroughly explain the parameter l_w working length. This article and the reference [23] fail to do so.

Line 357: Please change the word “Wolfram” to tungsten for consistency.

Line 466: Typo: please change “were” to “where”.

Section 2. Materials and Methods: Please include the brand, model and make/version of all enlisted equipment in full detail.

Page 14: Similar results were obtained for steel and tungsten witness pins, which were different from those of graphite pins. Authors need to make sure that this is not due to the placement of the pins shown in Figure 11a. In this figure, the graphite pin is centered while the other two pins are placed on the sides of the graphite pin with different radial angles/positions. This possibility needs to be considered and discussed (preferably through experiments with different pin arrangements).

Author Response

Dear reviewer,

thanks for your helpful and very detailed comments. We agree with you that the term “jetting” was not commonly and strictly defined in literature as the relatively narrow stream of material that is ablated from the surfaces in a collision weld process. However, several researchers have described different phenomena that are summarized with the phrase “jetting”. First, it was used in the analytic material flow models based on the fluid hydrodynamics for the material stream (e.g. Walsh et al., 1953 - Limiting Conditions for Jet Formation in High Velocity and Cowan and Holtzman, 1963 - Flow Configurations in Colliding Plates: Explosive Bonding). The first experimental investigation by high speed imaging could not detect a compact material stream which is pushed ahead of the point of collision, instead a fine spray of particles was identified (Bergmann et al., 1966 - Experimental evidence of jet formation during explosion cladding). In the following a first differentiation of the occurring phenomena as “jet” in the case of a cumulative stream and “cloud of particles” was done in dependence on the process conditions (Deribas and Zakharenko, 1974 - Surface effects with oblique collisions between metallic plates). Two reasons for the formation of the cloud of particles were described in this context. It is either caused by the breakdown of the cumulative stream in particles due to disturbances on the surfaces or it is formed by individual particles removed from the surfaces by the collision, even though the process conditions do not lead to the formation of a solid stream. Furthermore, it has been mentioned several times that the material stream, called “jet” by the authors, also removes oxide layers and other surface films (e.g. Cowan et al., 1971 - Mechanism of bond zone wave formation in explosion-clad metals or Carpenter and Wittman, 1975 - Explosion Welding). Considering the facts that high strain rates occur and that most of the oxides are brittle, it can be assumed that the surface layers break up into small particles during the removal and form or contribute to the formation of the cloud of particles. To our knowledge, the latter phenomenon has not been taken into account in analytical or computational models (except for Bellmann et al., 2019 - Thermal Effects in Dissimilar Magnetic Pulse Welding; however, in this study the formation of the cloud of particles and its thermal influence on bond formation was investigated, but not the formation of a material stream).

Hence we support a clear definition of the phenomena and suggest a differentiation between the formation of the metal stream and the cloud of particles as follows: during the oblique collision of two joining partners a material stream is formed by the plastic deformation of the colliding surface layers due to the prevalent high pressure at the point of collision. This stream can either remain in a cumulative shape or disperse in particles during the further progression of the collision, depending on the collision conditions and involved materials (like shown in the papers recommended by you). Additionally, brittle oxide layers and other surface contaminations spall from the surfaces due to high strain rates at the point of collision. The cloud of particles is a result of the dispersed material stream, the spalled surface layers or both. Therefore we created a new figure 1 for visual comparison. If a cumulative metal stream is formed and sustained during the collision, it might partly be hidden by the cloud of particles (Mori et al., 2019 - Observation for the High-Speed Oblique Collision of Metals). In case of comparable low energy input, it might even be too small to be visually detectable during the high speed observation ([12] Groche et al., 2019 - Process boundaries of collision welding at low energies).

Concerning the historical origin of the term “jet” from fluid hydrodynamics, we would like to define the material stream as “jet”, whereas the cloud of particles is a second result from the collision and can be a subset of the occurring jet, too. Both phenomena strongly influence bond formation. The role of the cloud of particles is not just that it is ejected by the closing joining gap, it also interacts with the surfaces in front of the point of collision and heats them up (Bellmann et al., 2019 - Thermal Effects in Dissimilar Magnetic Pulse Welding). These effects are also discussed in the companion paper that is currently submitted to this special issue (Niessen et al. - Interface Formation during Collision Welding of Aluminum).

According to your suggestions, we:

• Improved English language
• explained the PDV acronym at the first appearance in line 230
• deepened the description of the laser structuring process on page 5
• indicated length units in Figure 3 (now 4)
• included the results of the test rig with 30 µm laser structure in Figure 5 (now 6)
• insert the word “CoP” before the word “interaction” in line 31
• added important findings in the abstract
• included VFAW and LIW and the corresponding references in the introduction
• added the relevant reference for meshless simulations of the jetting phenomenon in line 68
• changed the parameter for the sheet thickness from “S” to “s” in Table 1
• explained the parameter working length l_w. to support the indication in Figure 4 (now 5)
• changed the word “Wolfram” to tungsten for consistency in line 357
• corrected the typing error “were” to “where” in line 466
• included brand, model and version of the enlisted equipment in chapter 2.1
• explained that the influence of the pin position can be neglected in Figure 11a (now Figure 12 a)

Thanks also for your suggestion regarding a rewording of lines 32-33 (and lines 56, 63, 103, 128, 156, 159, 161, 165, 175, 280, 445, 461, and 468-469). We discussed internally about this topic, but we think our formulation is correct, since the cloud of particles is also compressed by the closing joining gap in the absence of the surrounding air in vacuum-like conditions. We agree that also the air entrapped inside the closing joining gap at normal ambient pressure is compressed, but this is not the reason for the compression of the CoP. Closing the joining gap means a reduction of the entrapped volume, no matter which medium is located between the colliding metal pair (air or/and CoP)

With kind regards,

Jörg Bellmann

Reviewer 2 Report

The paper makes interesting reading and the results and discussions are quite conclusive. However, I recommend the following minor revisions to be made before the paper is considered for publication in the Metals journal:

He authors mentioned " <<<three different joining setups were utilized; their specific characteristics are summarized in Table 2." Add please the quantity of the samples fabricated and used for the different experiments/tests and measurements for each welding process.

Peel-tests  are mentioned in Table 2, no results related to this QC method are presented in the text. Please elaborate and add the pertinent results.

The authors should add scale-bars and/or the welding direction to the relevant figures, e.g. to Fig.5 a,b – welding direction and Fig.5 c-f scale-bars. Please check and correct figures 6,7,8,9.

Figures 5a,b and 6a display the microstructures of selected joints; please elaborate and add explanations regarding the microstructures shown in the micrographs.

I hope above comments help to improve a future version of the paper.

FINAL COMMENT: Important data and interpretations, the manuscript is recommended for publication.

Author Response

Dear reviewer,

thanks for your helpful comments. We included the quantity of the samples fabricated and used for the different experiments and measurements for each welding process. The manual peel test indicates whether a weld was formed or not at both MPW setups. Thus, the results of the peel tests are already implemented in table 4 and 6. We inserted a footnote to explain these columns. Polished micro sections were prepared from successful welding experiments. We added scale-bars, welding directions and explanations to the relevant figures 5…9 (now 6…10).

With kind regards,

Jörg Bellmann

Reviewer 3 Report

The detailed comments are as follows:

1. No explanation of symbols and abbreviations as PCO, PST, and others.
2. Fig. 7 - in the figure caption there is no a) and b).
3. SI units should be used in the work.

Author Response

Dear reviewer,

thanks for you helpful comments. According to your suggestions we explained all symbols and abbreviations when they occur in the text for the first time. In case of proper names like the manufacturer PST, an explanation is not necessary from our point of view. The caption in Figure 7 (now 8) includes now a) and b). We checked all the units in the manuscript and they are now compliant with SI base units.

With kind regards,

Jörg Bellmann