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Open AccessArticle

Peer-Review Record

FAST-forge of Diffusion Bonded Dissimilar Titanium Alloys: A Novel Hybrid Processing Approach for Next Generation Near-Net Shape Components

Reviewer 1: Anonymous
Reviewer 2: Anonymous
Reviewer 3: Anonymous
Reviewer 4: Anonymous
Reviewer 5: Anonymous
Metals 2019, 9(6), 654; https://doi.org/10.3390/met9060654
Received: 19 May 2019 / Revised: 1 June 2019 / Accepted: 3 June 2019 / Published: 4 June 2019
(This article belongs to the Special Issue Titanium Alloys: Processing and Properties)

Round  1

Reviewer 1 Report

The article presents sound experiment and simulation results on near-net shaping of bonded Ti-based alloys by forging. A convincing step-by-step approach has been employed, starting by a determination of the flow curves in simple compression tests, followed by experiments and simulations of macrostructures of bonded materials deformed in compression, and finishing with elaboration of near-net shape of complex geometry. The methodology employed is rigorous, the presentation of the results is clear, and the discussion is convincing. Therefore, I have no particular advice to improve the quality of the manuscript, which can be accepted in its present form.

Here are a few minor comments:

- Fig. 3 is not cited in text

- Define "TMC."

Author Response

The authors acknowledge the feedback from the reviewer and thank him/her for their time.

We agree with the minor amendments suggested by the reviewer.

1) We have now cited Fig3 in the body text, by adding the following sentence:

The photographs in Figure 3 show the stages of the manual lay-up of dissimilar titanium alloy powders prior to large-scale FAST processing.  [see page 4, lines 109-110]

2) Also we have defined "TMC" in the text, by modifying the following sentence:

Macrographs of the uni-axially forged thermomechanial compression (TMC) samples... [see page 5, lines 177-178]

Reviewer 2 Report

The paper can be accepted.

Author Response

We thank the reviewer for their time and are glad that the paper can be accepted without minor revisions.

Reviewer 3 Report

The article ‘’FAST-forge of diffusion bonded dissimilar titanium 2 alloys: A novel hybrid processing approach for next 3 generation near-net shape components ‘’ is an experimental and numerical exploration of the capacity to obtain bi-materials complex shape objects via the forging of a bi-material specimen obtained by spark plasma sintering.

This work is very interesting as it represents a first step for advanced multi-material forging process which can end up with the optimization of object properties, weight, material reduction and offers various design possibilities. In addition, they present a multi-material finite element tools which were quite successful in the description of the high temperature specimens deformation. The paper is well written, and I recommend his publication in "metals" after the following minor revisions.

-          Page 3 line 100: it is said the SPS specimen is cooled in ‘’air”. Is it really air or the author mean cooled by gas convection (for instance in argon)? Because at 1200°C the graphite tools would burn of the titanium oxidize.

-          In page 4, If I am not mistaken, Figure3 reference does not appear in the text.

-          Still about figure3, Is the rigid Ti sheet introduced in the die appears to be incompressible compared to the surrounding powder, Is this generates a splitting or rough interface after SPS?

-          In page 4: The experimental procedure is difficult to follow, in particular; what was done with the SPS specimen; where was the bi-material machined in this SPS specimen; It could help to show by a scheme or improve the text.

-          In page 7 line 229-230, It would be better to briefly discuss in the text the origin of


Author Response

We thank the reviewer for their useful comments and feedback.

We agree with their suggested amendments and the responses are below:

Comment 1) Page 3 line 100: it is said the SPS specimen is cooled in ‘’air”. Is it really air or the author mean cooled by gas convection (for instance in argon)? Because at 1200°C the graphite tools would burn of the titanium oxidize.

Response 1) We have added the sentence:

The chamber was then evacuated to approximately 2 x10-3bar and pulsed current was applied through the electrodes to begin heating. Due to the size of the specimen, a slow heating rate of 25°C/min was required, and the average cooling rate in the water-cooled vacuum chamber was 4°C/min. [see p4, line 115-118].

Comment 2) In page 4, If I am not mistaken, Figure3 reference does not appear in the text.

Response 2) We have now cited Fig3 in the body text, by adding the following sentence:

The photographs in Figure 3 show the stages of the manual lay-up of dissimilar titanium alloy powders prior to large-scale FAST processing. [see page 4, lines 112-113]

Comment 3) Still about figure3, Is the rigid Ti sheet introduced in the die appears to be incompressible compared to the surrounding powder, Is this generates a splitting or rough interface after SPS?

Response 3) We have added the sentence:

To reduce contamination, Ti-6-4 sheet acts as a temporary divider between dissimilar alloy powders during the manual lay-up. [see page 4, lines 113-114]

NB: - In reference 18 (https://doi.org/10.1016/j.jmatprotec.2019.02.011) which is referred to on page 2, lines 66-72 - smooth bonds of approximately +/- 50microns are produced using this technique.

Comment 4) In page 4: The experimental procedure is difficult to follow, in particular; what was done with the SPS specimen; where was the bi-material machined in this SPS specimen; It could help to show by a scheme or improve the text.

Response 4) We have added an additional sentence:

Forging preforms of dimensions 22 mm diameter by 160 mm length were electro-discharge machined from the 250 mm diameter, 35 mm thick FAST billet. The forging preforms were extracted from the FAST billet with the diffusion bond running along the preform length at the centreline. [page 5, line 162-164]

Comment 5) In page 7 line 229-230, It would be better to briefly discuss in the text the origin of hardening and softening in Ti64.

Response 5) This is characteristic flow stress response of Ti-64 during hot working.  We have expanded the following sentence to make the origins of hardening and softening more explicit to the reader.

After rapid work hardening up to a yield point flow softening, due to the break-up of fine-scale plate-like secondary alpha, is observed particularly at the lowest test temperature of 850°C. [See page 7, lines 247-249].

Comment 6)  In page 7 line 244, it is said the true stress and true strain are “friction corrected”; please explain how.

Response 6) We have expanded to the following the sentence to make the this more explicit.  

The true stress data was friction corrected, by applying a shear friction factor  to account for the characteristic barreling of the TMC sample, and adjusted for machine compliance during compression. [page 5, lines 155-157]

Comment 7). This bi-material forging is closed to the bi-powder SPS compaction process called “deformable interface approach” (http://dx.doi.org/10.1016/j.powtec.2017.07.048), please mention this in the state of the art for comparison.

Response 7) This is a key piece of work that we intended to include in refs 3-15.  We have added the sentence:

FAST has been demonstrated to be an effective sintering route for complex shapes, most notably a novel deformable interface approach produced fully dense CoNiCrAlY turbine blades [18]. [See page 2, lines 59-61]

(New reference [18] is http://dx.doi.org/10.1016/j.powtec.2017.07.048)

Reviewer 4 Report

The manuscript is very well prepared – I have no objections.

Author Response

We thank the reviewer for their positive feedback and acceptance without minor revisions.

Reviewer 5 Report

I have three principal remarks:

Clearly highlight the purpose of research.

Justify the choice of grades of titanium alloys Ti-6Al-4V and Ti-5Al-5Mo-5V-3Cr for next generation near-net shape components.

Prescribe the benefits of modeling deformation processes in the DEFORM program.

Author Response

We thank the review for their useful feedback.

Comment 1)  Clearly highlight the purpose of research.

Response 1) We feel that we highlighted the motivation for the work in the following sentences in the introduction:.  See two paragraphs from lines 37-51.

Comment 2) Justify the choice of grades of titanium alloys Ti-6Al-4V and Ti-5Al-5Mo-5V-3Cr for next generation near-net shape components.

Response 2) We have added the sentence:

The alloys Ti-6-4 and Ti-5553 were selected for the large-scale NNS forgings due to their increasing usage in the aerospace sector, making them ideal alloys to demonstrate the FAST-forge technology on dissimilar alloys.

[see page 2, lines 86-88].

Comment 3) Prescribe the benefits of modeling deformation processes in the DEFORM program.

Response 3) DEFORM is a commercial package used by the aerospace supply chain to model deformation processes.  There are no benefits of using DEFORM, other packages such as FORGE and LS-Dyna etc could also produce similar strain predictions.


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