Optimization of upcycling of Ti-6Al-4V swarf
Graphical abstract
Introduction
The increasing application of titanium alloys in aerospace industry leads to huge losses of metals in form of swarf. In some instances, as much as 50 to 90% of the part's weight ends up as swarf. An investigation conducted by Ezugwu (2005) revealed that machining industry converts about 10% of all the metal produced into machining chips. Bridges and Magnus (2001) stated that in aerospace industry over 50% of the cast net shape Ti alloys aerospace component of complex geometry is wasted during machining. Most recently, Murr et al. (2009) found that in biomedical applications, a typical Ti alloys knee implant component machined from a bar stock results in as much as 80% of material wasted in the form of swarf. Traditionally recycling of Ti swarf has been a very expensive process, therefore there exists an opportunity for the development of an innovative process to convert the swarf to bulk titanium or titanium powder for additive manufacturing. This paper will describe a method which has the potential to produce bulk titanium from Ti swarf.
Solid state recycling has been drawing increasing attention since the pioneering work conducted by Lazzar and Atzori (1992) and Gronostajski and Matuszak (1999) in consolidation of aluminum swarf using conform process and extrusion, respectively. The progressing in terms of solid state recycling techniques has been recently reviewed by Topolski et al. (2017) and Wan et al. (2017). The reasons for developing solid state recycling techniques are twofold. Firstly, when compared with the conventional recycling technique, which includes re-melting and casting, solid state recycling is conducted at a much lower temperature than the melting point of the material, leading to significant energy and cost savings. The investigation conducted by Allwood et al. (2005) showed that compared with conventional procedures, only 5% of energy would be consumed by recycling Al at room temperature using cold-bonding. Moreover, the inferior properties of cast ingots due to inevitable contamination could be avoided in the solid state processing techniques. Secondly, swarf can be regarded as a valuable source of ultrafine grained (UFG) metals. Indeed, Shankar et al. (2006) have found that during machining the swarf would have undergone an intensive shear strain that commonly results in grain refinement. Furthermore, as compared to metal powders, a lower content of oxide contamination is present in machining chips owing to their smaller specific surface area. Based on an extensive literature review, Cooper and Allwood (2014) concluded that the intervening of oxides would hinder the formation of sound solid state bonding. Therefore, swarf could be used as potential precursors to manufacture bulk UFG or multicomponent materials if sufficiently high density and strength can be achieved by compaction (Lapovok et al. 2014). This process is typically called ‘upcycling’ due to attainment of advanced properties and adding values, which is beyond reuse and recycle, in the view of Braungart (2013).
Equal channel angular pressing (ECAP) compaction is a novel approach for compaction of particulate metals, built on the notion that severe shear deformation under high hydrostatic pressure triggers several physical mechanisms contributing to improved consolidation at low temperatures. There has been extensive research in the recent years into ECAP process (Estrin and Vinogradov 2013). The parameters that are examined typically include: number of passes, processing route, processing temperature, back pressure and the effect of these parameters on the evolution of the microstructure and properties of the final product (Valiev and Langdon 2006). Using ECAP compaction to recycle swarf was firstly proposed by Lapovok and Thomson (2004). Since then, due to the capacity of processing hard-to-deform materials, this technique has been studied to compact Ti and Ti alloys swarf. For example, Luo et al. (2013) conducted two passes of ECAP compaction at 450 °C on pure Ti swarf and achieved a yield strength of 650 MPa. Regarding to recycling of Ti-6Al-4V swarf using ECAP compaction, a number of studies have been conducted and showed promising results. However, a common issue in these studies is, to achieve good mechanical strength, a series of processing routes were conducted, thus the processing is too complicated and expensive to be implemented by industry. Three recent studies of recycling of Ti-6Al-4V swarf using ECAP compaction are analyzed in following paragraphs.
Influence of the number of ECAP passes, processing temperature, magnitude of back-pressure on the density, hardness, homogeneity and microstructure evolution of Ti-6Al-4V compacts was studied by Shi et al. (2016). It was concluded that working at 500 °C with a back-pressure higher than 100 MPa produced compacts having satisfactory density. Specifically, using a back-pressure of 250 MPa, 1 pass of ECAP compaction resulted in a theoretical density of 99.0%. To further densify the compact, higher number of ECAP passes was carried out. It was found that the maximum theoretical density of 99.9% was achieved after 4 and 8 passes with corresponding back-pressures of 150 MPa and 100 MPa. This indicated that high back-pressure enhances the densification process during ECAP compaction. However, the microstructure evolution in terms of average grain size of the overall compact and detailed machining chips boundaries characterization was not presented in that study. Moreover, heat treatment, as an essential step in the processing of deformed titanium alloy for the optimization of microstructures and improvement of mechanical properties was not investigated.
Dissolution of the oxide layers at Ti-6Al-4V machining chips boundaries during post-ECAP compaction heat treatments and conventional mill-annealing processing was investigated by McDonald et al. (2014). The ECAP compaction was conducted at 590 °C with a back-pressure of 50 MPa for 4 passes. The study stated that the oxide dissolution took place from several minutes to less than one second at annealing temperatures between 700 °C and 1050 °C. Furthermore, after a conventional mill-annealing processing, the compacts obtained via described recycling route possessed improved properties with superior yield strength and equivalent ductility compared with the commercial mill-annealed Ti-6Al-4V. However, the experiments were conducted at a high processing temperature of 590 °C using multiple ECAP passes. In addition, the study focused on the dissolution of oxide layer with heat treatment, but did not investigate the recovery and recrystallization in detail, which is an important aspect associated with the formation of equiaxed and strain-free microstructure and restoration of ductility following deformation.
Most recently, Lui et al. (2016) conducted an investigation on the effects of chip initial condition on the ECAP compaction of Ti-6Al-4V machining chips. Using same processing route as the work conducted by McDonald et al. (2014), after the mill-annealing treatment that comprises three steps: i) β homogenization at 1050 °C; ii) hot deformation at 900 °C; and iii) annealing at 735 °C, similar properties were achieved for all compacts, regardless of their original condition. However, the fundamental examination of the microstructure evolution and possible improvement of the processing route were not assessed in the study.
In order to develop a more economic and efficient processing route, a systematic and detailed microstructure study of the Ti-6Al-4V compact is required. In the current study, Ti-6Al-4V swarf were compacted using ECAP at a relatively low temperature of 500 °C, with a high back-pressure of 250 MPa, with just one ECAP pass. Then the compacts went through a series of annealing treatments well below the β transus temperature for better bonding between machining chip boundaries, higher ductility and good strength. The economic benefits of the ECAP process presented here are apparent: with reduction of the processing temperature and number of ECAP passes, it is more attractive for industry to adopt the process for Ti-6Al-4V swarf recycling. Also in absence of the 3-steps mill-annealing treatment, the manufacturing procedure is simplified and the cost is further reduced.
The primary objective of this paper is to present the results of the systematic study of properties and microstructure evolution of Ti-6Al-4V machining chips compacts produced by economically-viable ECAP compaction. The microstructure evolution of the compacts after ECAP and annealing treatments is examined using advanced characterization techniques and the optimization of the upcycling process is suggested.
Section snippets
Sample preparation using ECAP compaction
The chips used in this study were machined by turning (cutting speed of ∼80 m/s and feed rate of ∼0.1 mm/revolution) from damage-tolerant grade Ti-6Al-4V Extra Low Interstitial (ELI) plates. The plates were beta annealed at 997 °C for 31 min and mill annealed at 732 °C for 2 h. The microstructure of heat-treated plates consisted of large colonies of aligned Widmanstätten α platelets and the prior β grains (Fig. 1)
The swarf was cleaned to remove the residual coolant using acetone for 15 min and
Mechanical properties
The relative density of ∼99% that of theoretical density was found for all ECAP compaction and annealed samples (Fig. 4), which is comparable to recently published research by Shi et al. (2016). However, the density of the compacted samples after annealing was a slightly higher (Fig. 4) than after ECAP with the highest value of 4.377 ± 0.003 g/cm3 after 732 °C for 120 min treatment, which is 99.3% of the theoretical density (Fig. 4). It could be due to the internal voids shrinkage through
Conclusions
A systematic and detailed properties and microstructural characterization was conducted on a series of Ti-6Al-4V swarf compacts, after ECAP compaction and post annealing treatments. The main conclusions are:
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A simple ECAP process using a single pass with annealing was successfully applied and resulted in compacts with near 99.3% theoretical density with minimal porosity and mechanical properties.
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Grain refinement of α phase was evidenced after the ECAP compaction due to static and dynamic
Acknowledgements
The authors acknowledge use of facilities with the Monash Centre for Electron Microscopy. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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