Circular manufacturing of sustainable additive parts using mechanically-generated Ti-6Al-4V feedstock from machining chips

Baramate, Karan 2025. Circular manufacturing of sustainable additive parts using mechanically-generated Ti-6Al-4V feedstock from machining chips. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Sustainable manufacturing is gaining increasing momentum in response to the global net zero targets and circular economy goals. While additive manufacturing (AM) offers design flexibility and material efficiency, its sustainability is significantly undermined by energy intensive powder production routes such as gas, water, and plasma atomisation. To address this issue, this PhD project develops a Circular Hybrid Manufacturing (CHM) process chain, integrating solid-state recycling of machining chips into AM feedstock, followed by additive manufacturing of the parts using the mechanically-generated powders and post-processing of these parts. Ti-6Al-4V (Ti64) swarf/chips were utilised in this study to establish a viability of the developed CHM process chain. Using a two-stage ball milling process, Ti64 machining chips were converted into Ti64 powder with a yield of ~95-96% within 2.5 hours. The resulting powders exhibited irregular morphology and broad particle size distribution (~95 wt.% <100 µm), while demonstrating free flow characteristics with an angle of repose (AOR) of 31-32°, indicating suitability for AM processing. These powders were utilised in fabricating 3D test-pieces using both Directed Energy Deposition (DED) and Binder Jetting (BJT) processes. DED processing revealed that the commercial gas-atomised (GA) Ti64 powders produced near fully dense parts (98–98.5%), whereas ball-milled (BM) Ti64 powders resulted in lower densities (75–78%) due to irregular morphology and unstable flow of the latter powder particles. However, BM-derived DED parts exhibited refined equiaxed microstructures, higher β-phase fraction, weaker texture, and significantly increased microhardness (800–1000 HV0.1), compared to that of GA parts. High porosity and low density in BM-DED parts prevented further mechanical testing. In contrast, BJT processing showed low sensitivity to powder particle morphology. BM derived BJT Ti64 parts achieved relative densities of 91-95%, nearly comparable to GA parts, but exhibited 15-25 % higher surface roughness and almost double the hardness (~900 HV0.1). Microstructural analysis confirmed fine equiaxed grains and increased β-phase content in BM parts, while GA parts exhibited lamellar α+β structures. Despite similar densities, BM parts showed much lower compressive strength (450-550 MPa) and brittle failure behaviour compared to GA parts (925-1100 MPa), attributed to prior work and strain hardening of the BM popwder particles. Tensile testing was not conducted on the BJT specimens as they exhibit lower densities compared to other metallic AM processes. Furthermore, as the BJT-BM parts exhibited brittle failure during the compression testing they deemed unsuitable for the tensile testing. Post-processing heat treatments (625 °C and 850 °C) and hot isostatic pressing (927 °C/150 MPa/4 h) of the BJT parts resulted in improved density (~95-96%), reduced porosity (particularly with GA parts) and slight increase in the compressive strength, though BM parts retained high hardness (up to 920 HV0.1) and limited ductility. Specific energy consumption analysis (76.9 MJ/kg) indicated lower energy demand (~25-30 % less) of the BM process than the gas atomisation process, with further reductions possible through process scale-up. In summary, this project demonstrates the feasibility of the CHM framework and establishes solid-state recycling as a potential and a viable route for sustainable AM feedstock production, particularly for applications requiring high hardness and wear resistance of the AM parts.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Engineering
Uncontrolled Keywords:	1. Circular economy 2. Additive manufacturing 3. Binder jetting 4. Directed energy deposition 5. Ball milling 6. Ti-6Al-4V
Date of First Compliant Deposit:	19 June 2026
Last Modified:	19 Jun 2026 15:43
URI:	https://orca.cardiff.ac.uk/id/eprint/186591

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