FAST-forge of Titanium Alloy Swarf: A Solid-State Closed-Loop Recycling Approach for Aerospace Machining Waste

Round 1

Reviewer 1 Report

The work discussed the subject of the FAST-forge of titanium alloy swarf: A solid-state closed-loop recycling approach for aerospace machining waste. Presented in the manuscript research shows a high level of significance. The authors put an effort to prepare a very good article in the present form, however, some data is missing and other need revision or proper explanation. For the higher quality of the manuscript, the reviewer recommends a minor revision.

General

The authors should explain why they didn’t prepare all of the specimens by the same equipment at the same parameters for comparison. This should be pointed out more clearly. Why the authors didn’t collet the microstructures of swarf and powder samples after fast and TMC.

The swarf conditioning via grading and cleaning operation should be explained, otherwise, in the proposed recycling closed-loop, this waste preparation procedure remains a black box. More information about the machining operation should be reveled for example dry or wet, tool material, for other possible contaminants presence.

Author Response

Reviewer 1 Comments:

1. The authors should explain why they didn’t prepare all of the specimens by the same equipment at the same parameters for comparison. This should be pointed out more clearly. Why the authors didn’t collect the microstructures of swarf and powder samples after fast and TMC.
   • Microstructures of the starting HDH powder and a FAST parametric study for this feedstock were not included or performed as they have been previously presented in the literature (e.g. references 6 and 7 from this paper). We would reiterate that the major focus of this paper and its novelty lies in successfully demonstrating a solid-state closed‑loop recycling approach for titanium alloy swarf via FAST-forge. We included the additional data, in terms of microstructures and flow curves, from Ti-6Al-4V hydride‑dehydride (HDH) powder to allow the reader to directly compare the performance of the suggested low-cost waste product feedstock with a high-cost commercially available feedstock. We recognise the different cooling rates on the different FAST equipment has resulted in different post-FAST microstructures, which also changed the flow curve behaviour. However, we explicitly discuss and explain reasons for these differences in the text, with reference to well established titanium alloy metallurgical phenomenon reported in the literature. Whilst the comparison may not be ideal, we feel that on balance it gives additional information that is of benefit to the reader.
   • To make our intentions about this additional data clearer we have changed the following paragraphs:

Section 2.1. Materials, lines 101-107:

“Additional hot forging data from previous unpublished work by the authors is also presented in section 3.3. of this paper. This data from a conventional powder feedstock is intended to allow comparison of the forging behavior of the Ti-6Al-4V swarf feedstock after FAST. The powder was Grade 5 Ti-6Al-4V 45-150 µm hydride-dehydride (HDH) from Phelly Materials (U.S.A.) Inc., Upper Saddle River, NJ, USA. The powder was certified to contain 6.22% Al, 4.00% V, 0.03% Fe, 0.01% C, 0.0135% H, 0.01% N, and 0.20% O, which meets specification for ASTM B988-13 and ASTM B381-13.”

Section 2.2.1. Field Assisted Sintering Technology (FAST), lines 118-127:

“The following information is summarized in Table 1. The swarf was consolidated using field assisted sintering technology (FAST) in an FCT Systeme GmbH FAST Furnace Type HP D 25; further details of the equipment can be found in [6]. The HP D 25 system was used to create both 20.0 mm diameter by 4.3 mm thick discs from 6.0 g of Ti-6Al-4V swarf for a parametric study of FAST processing cycle conditions, and a 60.0 mm diameter by 10.3 mm thick disc from 130.0 g of Ti-6Al-4V swarf to extract hot compression test specimens.

The additional Ti-6Al-4V HDH hot forging data presented in section 3.3. was obtained from specimens extracted from a 100.0 mm diameter by 22.0 mm thick disc made from 765.0 g of powder by an FCT Systeme GmbH FAST Furnace Type H-HP D 250 The Type H HP D 250 system, based at Kennametal Manufacturing (UK) Ltd., Newport, Wales.”

2. The swarf conditioning via grading and cleaning operation should be explained, otherwise, in the proposed recycling closed loop, this waste preparation procedure remains a black box. More information about the machining operation should be revealed for example dry or wet, tool material, for other possible contaminants presence.
   • We feel this point is already addressed in the manuscript and have therefore made no changes to the text. As explained (see lines 78-84, and specifically line 81), the swarf was procured from a commercially operating company. The details of the titanium alloy swarf cleaning and grading operations performed are proprietary and commercially sensitive. This is much the same as metal powder manufacturers not divulging the parameters used to create their products. Therefore, further information on cleaning and grading cannot be shared as it us unknown to the authors. Additionally, this company processes many tons of titanium alloy swarf and scrap material from a wide variety of sources every month and does not track the machining processes and conditions used to produce each one. The source of the swarf was identified to us simply as high-quality mill products or forgings that were machined by an aerospace sector supplier, which is stated in the text. It is not possible to give further information on the machining operations and tool materials used as it is unknown. This feedstock variability is reflected in the observed mixed microstructure of the swarf procured for this study as discussed in the text (see lines 212-220). It was also demonstrated that the FAST process was tolerant of this mixed microstructure feedstock when selecting appropriate processing conditions, which is important to allow utilisation of this potentially common and abundant low-cost feedstock (see lines 292-299).

Reviewer 2 Report

The paper proposal is a nice technical report of scientific quality presenting the new recycling approach for titanium machining waste. I recommend the paper for publication (as is).

Author Response

Reviewer 2 Comments:

1. The paper proposal is a nice technical report of scientific quality presenting the new recycling approach for titanium machining waste. I recommend the paper for publication (as is). 
   • We thank the reviewer for their kind comments.

Reviewer 3 Report

The authors present a study on the consolidation of titanium alloy swarf using field-assisted sintering. The need for consolidation of swarf into usable billets is clearly identified in the introduction along with the outline of the experiments and process in materials and methods. The results and discussions are detailed and contain sufficient detail for readers familiar with the research area. I recommend publication with minor revisions, including:

• Add a sentence or two in the introduction on the phase diagram of Ti-6Al-4V and the specific temperature ranges for phase transformation. This might be common knowledge for familiar readers but is important information for readers less acquainted with the subject.
• For a similar reason as above, add some description of how to visibly distinguish between alpha and beta grains (around line 194 in results and discussion)

Author Response

Reviewer 3 Comments:

1. Add a sentence or two in the introduction on the phase diagram of Ti-6Al-4V and the specific temperature ranges for phase transformation. This might be common knowledge for familiar readers but is important information for readers less acquainted with the subject.
   • We have added the following paragraph to section 2.1. Materials, lines 107-115:

“Ti-6Al-4V was selected for this study as it is the most commonly used titanium alloy. It is designated as an α+β alloy, which means at room temperature it typically contains both the α phase with a hexagonal close-packed crystal structure (harder to deform) and the β phase with a body centered cubic crystal structure (easier to deform). As temperature is increased the volume fraction of β phase present increases. The temperature at which there is solely β phase present is known as the β transus temperature; for Ti-6Al-4V this is typically around 990˚C. The mechanical properties and performance of Ti-6Al-4V can be modified by altering the room temperature volume fractions and morphologies of the two phases through thermomechanical processing above or below this transus temperature and selecting an appropriate cooling rate.”

2. For a similar reason as above, add some description of how to visibly distinguish between alpha and beta grains (around line 194 in results and discussion)
   • We have added the following paragraph to section 2.2.3. Metallography, lines 195-202:

“The microstructure of Ti-6Al-4V becomes more prominent when viewed under cross-polarised light conditions. The reflected light diffracts by varying amounts depending upon the local crystallographic orientation of the α phase. This is observed as variations in contrast (from black to white) in the micrographs and allows the size and morphology of the α phase to be seen. There is very little β phase present at room temperature in Ti-6Al-4V, and it is not possible to differentiate the β phase in same manner with polarized light. It is possible to estimate where grains of the β phase were present when the material was above the transus temperature by the propensity for the α phase to form first on and grow along these prior β grain boundaries.”