Search

Technological developments in the area of functionally graded multi-material manufacture are poised to disrupt the aerospace industry, providing the means for step-change improvements in performance through tailored component design. However, the challenges faced during the downstream processing, i.e., machining of such functionally graded multi-materials are unclear. In this study, the challenges involved when face-turning billets consisting of multiple alloys are assessed. To achieve this, a cylindrical billet consisting of Ti-64, Ti-6242, Ti-5553 and Beta C alloys was manufactured from powder feedstock using field-assisted sintering technique (FAST) and termed MulTi-FAST billets. A detailed study of the structural integrity during machining at the diffusion bond interfaces of multiple titanium alloy bond pairings in the MulTi-FAST billet was conducted. The machining forces were measured during face-turning to investigate the impact and behaviour of different alloy pairings during a continuous machining operation. The results showed the significant differences in force machining response, surface topography and the type of surface damage was dependent on the direction the titanium alloy graded pairings were machined in. In terms of subsurface microstructural damage, regardless of the machining direction, no critical damage was found in the vicinity of the bonded alloys. The findings provide an insight into the deformation characteristics and challenges faced in the machining of functionally graded components with multiple titanium alloys.

Combining the FFC-Cambridge process with field-assisted sintering technology (FAST) allows for the realisation of an alternative, entirely solid-state, production route for a wide range of metals and alloys. For titanium, this could provide a route to produce alloys at a lower cost compared to the conventional Kroll-based route. Use of synthetic rutile instead of high purity TiO2 offers further potential cost savings, with previous studies reporting on the reduction of this feedstock via the FFC-Cambridge process. In this study, mixtures of synthetic rutile and iron oxide (Fe2O3) powders were co-reduced using the FFC-Cambridge process, directly producing titanium alloy powders. The powders were subsequently consolidated using FAST to generate homogeneous, pseudo-binary Ti–Fe alloys containing up to 9 wt.% Fe. The oxide mixture, reduced powders and bulk alloys were fully characterised to determine the microstructure and chemistry evolution during processing. Increasing Fe content led to greater β phase stabilisation but no TiFe intermetallic phase was observed in any of the consolidated alloys. Microhardness testing was performed for preliminary assessment of mechanical properties, with values between 330–400 Hv. Maximum hardness was measured in the alloy containing 5.15 wt.% Fe, thought due to the strengthening effect of fine α phase precipitation within the β grains. At higher Fe contents, there was sufficient β stabilisation to prevent α phase transformation on cooling, leading to a reduction in hardness despite a general increase from solid solution strengthening.

Machining is often the most expensive manufacturing process step when producing titanium alloy components. With a move towards higher metal removal rates to meet aircraft orders, it is important to develop rapid, low-cost methods that assess the machinability of titanium alloys and cutting tool combinations. A cost-effective, small-scale methodology has been developed and validated on the industrial scale using high-speed turning to inform on the machining characteristics of commercial and emerging titanium alloys to aid companies in future developments of new titanium alloys and cutting tool materials. The article demonstrates, using the titanium alloys Ti-6Al-4V (Ti-64) and TIMETAL 407® (Ti-407), that a series of early stage, small-scale methods can identify key machinability characteristics including chip form, tool wear, cutting force and surface damage. It can be concluded using these low-cost machinability assessment methods that Ti-407 exhibits better machining properties to Ti-64 for the aspects of machinability focused on cutting forces and tool wear, whereas the contrary is found for subsurface microstructural features and chip control.

The emergence of metal additive manufacturing (AM) processes offer manufacturers a promising alternative to traditional forging and casting techniques for the production of near net shape titanium alloy components. However, limitations in both the surface finish quality and the geometric accuracy of parts produced by AM means that post-build finish machining of the part remains to be a requirement to produce high precision components. Furthermore, the fatigue performance of material produced directly by these processes is often limited by both the poor surface finish and porosity related defects which occur within the material. This study investigates the implications of machining stock allowance on the surface integrity of Ti-6Al-4V specimens produced by selective electron beam melting (SEBM) followed by post-build finish machining. The study revealed that the exposure of porosity related defects on the newly machined surface varied depending on the depth of material removed from the as-built specimen surface during machining. Four point bend fatigue testing of the specimens was carried out to determine the effect of the exposed surface defects on the fatigue performance of the material. This study highlights that the non-uniform distribution of pores within SEBM Ti-6Al-4V means that careful considerations must be given regarding machining stock allowance in the design of these components due to the implications of material removal depth on surface integrity.

The surface response to drilling of the high performance titanium alloy Ti-5Al-5Mo-5V-3Cr (Ti-5553) with WC/Co tools cannot be assumed to similar to that of Ti-6Al-4V (Ti-64). Comparison of surface roughness for holes drilled in Ti-5553 with Ti-Al-N PVD coated and uncoated WC/Co tools at low and high cutting feed and speed was investigated. Cutting surface speed was shown to have more of an impact on surface roughness than that of feed, with high cutting speeds causing higher and more varied Ra value. In addition using coated tools at lower cutting speeds resulted in a lower average Ra. In most cases, the coated tools achieved lower Ra values compared to the uncoated tools. This is thought to be due to a difference in cutting edge radius and better stability associated with the coated tools. High variance in RMax was observed for all tested conditions and tools, in some cases RMax was above 10 µm. Alicona and SEM imaging identified re-adhered material pickup as the probable cause. A correlation between torque and surface roughness was identified for the drilling of Ti-5553 and further investigation is proposed for using torque measurement as an alternate way of quantifying surface roughness without probe measurement or image analysis.

Titanium alloys have a high strength-to-weight ratio, fatigue performance and excellent corrosion resistance, and therefore are widely used in the aerospace sector due to their ability to withstand severe mechanical and thermal stresses in service. There are numerous cases where it would be desirable to use different titanium alloys in defined subcomponent regions to improve performance and efficiency. Conventional processing routes do not permit components to be produced with multiple titanium alloys and thus, design efficiency and optimization of component properties is compromised or over-engineered. In this study, a hybrid solid-state consolidation route is presented whereby field assisted sintering technology (FAST) is exploited to diffusion bond (DB) dissimilar titanium alloy powders in defined regions—a process termed FAST-DB. Titanium alloy powders Ti-6Al-4V (Ti-64) and Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) were bonded using FAST in order to study the tensile deformation behavior and strain localization across a dissimilar alloy solid-state bond. FAST-DB was carried out at the sub- and super- beta transus temperatures of both alloys to generate dissimilar microstructure morphologies across the bond. In all cases, diffusion bonds showed excellent structural integrity with no defects and a smooth hardness profile across the bond. The deformation characteristics of the bonds was studied using two different tensile test approaches. The first approach used ASTM standard specimens to measure the mechanical properties of FAST-DB samples and study the location of the tensile failure. The second approach used a microtester and optical Digital Image Correlation to capture the grain interaction in the bond region under tensile loading. The work demonstrated that the diffusion bond remains intact and that tensile failure occurs in Ti-64 (i.e. the lower strength alloy) and is independent of the grain crystal orientation. The results from this study will provide materials engineers confidence in nesting FAST-DB technology in future near net shape manufacturing routes.

Titanium alloy components are subjected to challenging conditions in high performance applications, consisting of complex loads and thermal gradients. To improve the performance and efficiency of such components, it is desirable to introduce different microstructures into subcomponent regions which cannot be achieved by the conventional melt-wrought processing route. Instead, a solid-state consolidation route using dissimilar titanium alloy powders is proposed. In this study commercially pure Ti (CP-Ti), Ti-6Al-4V (Ti-6-4), and Ti-5Al-5Mo-5V-3Cr (Ti-5553) have been diffusion bonded using field assisted sintering technology (FAST) for dwell times of 10, 20, and 60 min. The effectiveness of FAST for diffusion bonding (DB) of dissimilar alloy powders has led to the authors terming this hybrid process as "FAST-DB". Excellent bond integrity was produced with no cracking, unbonded regions or voids in the dissimilar bond combinations, at all dwell times. Furthermore, reliable control and prediction of the bond characteristics and degree of elemental diffusion was demonstrated through commercial thermodynamic software. Elemental diffusion was characterised across the bonds and the chemical diffusion bond width increased linearly with dwell time. Peak hardness values occurred directly on the interface for Ti-5553 and CP-Ti bonds, whilst slightly into the Ti-6-4 alloy for the Ti-5553 and Ti-6-4 bonds, which can be attributed to fine scale alpha at the interface. In Ti-6-4 with CP-Ti bonds, a smooth transition was observed. Mechanical testing demonstrated that the FAST-DB interface had excellent structural integrity, with necking and fracture occurring in the lower strength alloy.

Metastable beta titanium alloys such as Ti-5Al-5Mo-5V-3Cr (Ti-5553) are used for large aero-structural components. During processing these alloys are subject to costly machining operations. There is an industry demand to machine at improved metal removal rates (MRR) to meet increasing aircraft orders. To understand the effects of MRR on subsurface deformation and fatigue performance, fatigue coupons were end milled under two different industrial conditions. The surface roughness, residual stress and subsurface deformation was characterised and a custom four point bend fatigue testing was designed to determine the effect of MMR on cycles to failure. The study has demonstrated that an increase in MMR of 50% resulted in a LCF reduction of 57.37%. This was a direct result of increased subsurface damage and residual stresses.

Starting from a historic perspective, we discuss the use of second sound in experimental investigations of quantized vorticity and quantum turbulence in the two-fluid temperature regime of superfluid $$^4$$ He. Starting with the theoretical prediction of second sound by Tisza and Landau and its experimental discovery by Peshkov, we briefly review the pioneering experiments of Hall, Vinen and others that have contributed in an essential way to our current understanding of quantum turbulence in superfluid $$^4$$ He, with an emphasis on relevant research performed over the last two decades in our laboratory in Prague. We then propose further dedicated experiments where second sound can be used both as a generator and detector of quantum turbulence.

The current status of field assisted sintering technology (FAST) of structural metals from powder is critically reviewed. Recently, there have been significant increases in the uptake of FA...

Porosity related defects within metal additive manufactured components has made it challenging to certify their use in fatigue critical applications. In this study, the fatigue behaviour of selective electron beam melted (SEBM) Ti-6Al-4V specimens following hot isostatic pressing (HIP’ing) and finish machining has been investigated. Employing a sufficient material removal depth during machining was shown to significantly improve fatigue performance. This was attributed to be a direct result of removing material rich in lack-of-fusion defects, concentrated in the sub-surface. HIP’ing was shown to enhance fatigue performance to a lesser degree, as the treatment was only effective at reducing gas porosity.

During the manufacture of critical aerospace components from metastable β titanium alloys, excessive tool wear and machining cost can be significant obstacles. Although tools are highly optimised for drilling the α+β alloy Ti–6Al–4V (Ti-64), insufficient information is available to efficiently improve tools for use on higher strength alloys due to the cost intensive processes required to design and optimise new drills. To reduce this cost, a novel method is presented which enables rapid comparisons of machinability between different alloys, (Ti-64; Ti–6Al–2Sn–4Zr–6Mo (Ti-6246); and Ti–5Al–5Mo–5V–3Cr (Ti-5553)), to quickly assess the performance of specific tool characteristics using industrially relevant drilling parameters. Rapid assessment and comparison of microstructural damage at selected forces was made possible through the development of 3D tool velocity force maps (VFM) for coated TiAlN and uncoated WC/Co tools at different cutting feeds and speeds. Microstructural damage depth has been shown to decrease with increasing alloy strength even for cases where the thrust force between the work piece and the tool was the same ~1600 ​N. In addition, a lower damage depth was measured when using uncoated tools. The machinability evaluation of tool coating and its interaction with force, torque, and material properties showed that TiAlN coated tools responded differently in the case of each alloy, displaying significantly different torque profiles, thus indicating different optimum cutting speeds and feeds. Abrasive and adhesive wear mechanisms were identified in the coated tools, while the majority of wear in uncoated tools was abrasive.

Titanium alloy components require several machining stages of forged billets which are supplied in a range of annealing conditions. Generally, the machining performance is influenced by the heat treatment and changes in billet microstructures are often overlooked by tool manufacturers and machinists. Due to the non-linear strain path during primary forging, titanium alloy billets are anisotropic in nature and require ex-situ non-destructive evaluation (NDE) during the manufacturing stages to ensure excellent service performance, particularly in safety-critical aerospace components. In this study, the local analysis of the fluctuations presented in the force response during face-turning operations is directly linked to the billet heat treatment condition and presented as microstructure fingerprint plots. The evolution of cutting forces in four different billet conditions of the alpha + beta titanium alloy Ti–6Al–2Sn–4Zr–6Mo (Ti-6246) was measured. The magnitude and fluctuations in force were directly correlated to microstructural features derived from the heat treatments. In addition, local spatial high-resolution synchronization of the cutting forces was used to determine the effects of microstructure from the heterogeneous upstream forging process and subsequent heat treatment. These rapidly produced microstructure fingerprint plots are an important development step for providing manufacturers with an in-process machining NDE method: this will help to qualify material upstream prior to expensive secondary forging or finish machining stages.

The Mechanisms of fatigue failure in Ti-6Al-2Sn-4Zr-6Mo forged discs are investigated: the effects of forging and machining operations on fatigue are decoupled. A four-point bend fatigue testing approach enabled the crack initiation and propagation characteristics to be studied at multiple locations around the disc periphery. Fatigue performance variation (of ~60%) at different positions, and crack initiation and propagation behaviour were linked to the heterogeneous crystallographic texture - developed during upstream forging. Downstream machining processes were found to increase fatigue life, regardless of the cutting speed. However, circumferential fatigue heterogeneity, inherent from the forging stage was still evident even after machining.

Over the last 20 years, there has been growing research and development investment to exploit the benefits of wire deposition additive manufacturing (AM) for the production of near-net shape components in aircraft and space applications. The wire feedstock for these processes is a significant part of the overall process costs, especially for high-value materials such as alloyed titanium. Powders for powder-based AM have tight specifications regarding size and morphology, resulting in a significant amount of waste during the powder production. In the aerospace sector, up to 95% of forged billet can be machined away, and with increasing aircraft orders, stockpiles of such machining swarf are increasing. In this study, the continuous extrusion process&mdash, ConformTM&mdash, was employed to consolidate waste titanium alloy feedstocks in the forms of gas atomised powder and machining swarf into wire. Samples of wire were further cold-drawn down to 40% reduction, using conventional wiredrawing equipment. As close to 100% of the waste powder can be converted to wire by using the ConformTM process. This technology offers an attractive addition to the circular economy for manufacturers and, with further development, could be an important addition as industries move toward more sustainable supply chains.

Utilising novel extraction and processing technologies allows for the realisation of an alternative titanium alloy production route, with many benefits over the traditional Kroll-based one. The route proposed has the potential to reduce the cost of titanium and offers the ability to create alloys which are difficult to make conventionally. It combines the Metalysis Process, an electrolytic metal extraction technique, with field assisted sintering technology (FAST), a rapid and effective solid-state sintering technique. The Metalysis Process reduces metal oxide powders directly into metal powders, which can then be consolidated using FAST. Using synthetic rutile (SR) as the feedstock, compared to pigment grade rutile and TiCl4, further reduces the cost of titanium produced via this route. This research investigates the use of this route to create a range of pseudo-binary Ti-Fe alloys, by co-reducing SR with iron (III) oxide (Fe O ). Various techniques were used to analyse the feedstock, reduced alloy powders and consolidated material post-FAST to 23 determine chemistry and microstructure.

We present preliminary results of the visualization of a submerged coflow jet of liquid helium produced by a fountain pump. The jet propagating inside the bulk superfluid is visualized with particle tracking velocimetry using hydrogen particles. We compare the characteristics of the coflow jet with those measured in classical fluids such as helium gas or water. In contrast to the classical experiments, a temperature-dependent angle of the jet is observed, suggesting that the flow may not be described quasi-classically, despite the strong coupling between normal and superfluid components by mutual friction. We report on the statistics of the velocities inferred from the particle trajectories recorded by a high-speed camera at 1.68 and 1.95 K, for jet velocities ranging from 47 to 4500 mm/s.

It is widely accepted that crater wear is the primary tool wear mechanism observed in titanium alloy machining. An investigation was carried out utilising a small scale, low cost diffusion couple method to determine the effect of chemistry on tool crater wear characteristics. Using a range of commercial titanium alloys, it was found that as the β stability at RT increased, the quantity of TiC formation at the reaction interface decreased. This trend allowed for the determination of the machinability characteristics of two emerging aerospace titanium alloys: TIMETAL 407 and TIMETAL 575. The former was found to be more machinable than Ti-6Al-4V, and the latter more machinable than Ti-6Al-2Sn-4Zr-6Mo. As the chemistries of titanium alloys are directly related to the rate of tool wear, the second part of this investigation examined the effect of alloying titanium with Al, Mo, Cr and V. It was found that as the concentration of these elements in the binary titanium alloys increased, the quantity of TiC formed at the reaction interface decreased. The formation of TiC has a key role on the tool crater wear mechanism during the machining of titanium alloys.

The high strength titanium alloy Ti-5553 has been fully consolidated and thermomechanically processed from powder using the FAST-forge process, in only three steps, both at the small and the pilot scale. Titanium alloy components are conventionally produced using a time-consuming process, which involves carbo-chlorination extraction of TiO2, triple vacuum arc re-melting, and multiple thermomechanical and heat treatment steps, before machining. The proposed FAST-forge processing route for titanium alloy components cuts out or significantly reduces these stages, and uses field-assisted sintering technology (FAST) to consolidate powder. This paper assesses the effectiveness of the process for a conventionally used high-strength beta titanium alloy, Ti-5553. Ti-5553 has been fully consolidated by the FAST process at two different dwell temperatures, 850 and 1000 °C, and for a 30 min dwell time. Small-scale upset forging of cylinders machined from each FAST condition has been performed at forging temperatures 785, 810 and 835 °C, and strain rates 0.01, 0.1, 1 and 5 s−1, in order to examine the flow stress behaviour. The flow behaviour of both FAST-produced Ti-5553 specimens was found to be very similar to conventionally produced Ti-5553, and the forged microstructures were also comparable. Large-scale forging has been performed on three double truncated cones machined from a FAST specimen produced at a temperature of 1000 °C with a 30 min dwell time. The cones were forged at 785, 810 and 835 °C, and at a strain rate of approximately 3 s−1. Each forged specimen was heat treated for 4 h at a proprietary temperature between 750 and 850 °C, and aged for 8 h at a proprietary temperature below 650 °C in an inert atmosphere, before air cooling to room temperature. The forged microstructures and heat treated microstructures were found to be comparable to that of conventionally produced Ti-5553. Microhardness measurements of the heat treated specimens, with averages of between 410 and 417 Hv, were higher than that of conventionally produced Ti-5553, and were very consistent despite variations in strain and forging temperature. Successful scale-up of the process for a metastable beta titanium alloy indicates its potential to be utilised at an industrial scale.

The primary wear mechanism of WC-Co tools when machining aerospace titanium alloy components is crater wear. The diffusion controlled dissolution and thus the crater wear rate is more apparent when machining the high strength, fatigue resistant metastable β titanium alloys: an expensive class of titanium alloy used in aerostructural components such as twin aisle aircraft landing gear parts. To characterise the mechanistic process of crater wear progression, outer diameter turning trials were conducted on the metastable β alloy, Ti-5553. Uncoated WC-Co inserts were used for time steps of 30, 60 and 900 s at a cutting speed of 70 m/min. The crater region was analysed using high-resolution SEM, X-EDS, WDS and STEM/TEM. Such targeted characterisation enabled the detailed mechanism of progressive crater wear in titanium machining to be further understood. In the crater wear process, rapid diffusion of Co into adhered Ti-5553 occurs within the crater zone, proceeded by decarburisation of the WC (hcp). Thermodynamic modelling and high-resolution imaging of the interface suggests Co facilitates the formation of Ti2Co, and possibly a liquid, within prior Co rich regions at WC grain boundaries. Subsequently, this creates regions that exhibit similar characteristics to a Kirkendall morphology due to accelerated C depletion and W–Ti dissolution. The formation of abrasive titanium carbides in the workpiece material increases as C is made available from the WC transforming to W (bcc) and through binder regions. Once the W (bcc) layer is formed the rapid decarburisation process is concentrated at prior Co-rich (heavily β stabilised) Ti regions, this degradation dictates the crater wear rate during the machining of Ti-5553. Now an understanding of the crater wear mechanism has been characterised for Ti-5553, tooling manufacturers can engineer the Co-binder volume fraction and distribution, to improve tool life and provide significant cost savings when machining metastable β titanium alloy components.

Pure titanium has an HCP structure and lacks mechanical properties for many industrial purposes. The BCC phase of Ti is required to make alloys with increased strength compared to pure Ti. Iron is the most potent element for stabilising the BCC phase. However, the addition of Fe to Ti causes segregation issues during solidification, which can be avoided by diffusion-driven solid-state alloying. To predict the diffusion kinetics, the interaction mobility parameters of Ti and Fe in the disordered BCC phase of Ti are necessary. In this work, these parameters are optimised based on new experimental data from Ti-Fe diffusion couples produced by the Field Assisted Sintering Technology (FAST). Diffusion couples were held at 1173K and 1273K for one hour. High-resolution Fe concentration profiles are obtained from Electron Probe Micro Analyser (EPMA). Ternary mobility interaction parameters are assessed based on binary endmembers with a DICTRA sub-module, and results are compared to earlier assessments of mobilities of the disordered BCC TiFe system.

The evolution of residual stress and microstructure has been investigated in electron-beam welded Ti-6Al-4V alloy rings in order to develop an understanding of how the distribution of through-thickness residual stress correlates with microstructural evolution. A multiple technique approach to residual stress measurement was employed using a combination of different measurement techniques including X-ray diffraction (XRD), hole drilling method based on electronic speckle pattern interferometry (ESPI), and the contour method. It was found that there is a strong correlation between the change in residual stress and alpha phase morphology across the weld. The fusion zone exhibited highly tensile residual stress which was typified by an entirely acicular α′ microstructure formed by a displacive transformation within prior β grains on cooling. The tensile residual stress in the centre of the weld reduced towards the heat affected zone, transitioning to a compressive residual stress upon increasing distance from the weld centre. The transition from tensile to compressive residual stress correlates with a significant decrease in the volume fraction of α′ and an increase in the bimodal morphology of equiaxed primary alpha in a diffusional transformed beta matrix leading to elongated alpha in the base material.

Mechanical property data of a low-cost titanium alloy derived directly from synthetic rutile is reported. A small-scale testing approach comprising consolidation via field-assisted sintering technology, followed by axisymmetric compression testing, has been designed to yield mechanical property data from small quantities of titanium alloy powder. To validate this approach and provide a benchmark, Ti-6Al-4V powder has been processed using the same methodology and compared with material property data generated from thermo-physical simulation software. Compressive yield strength and strain to failure of the synthetic rutile-derived titanium alloy were revealed to be similar to that of Ti-6Al-4V.

Tool crater wear is a major cause of accelerated tool failure during the machining of titanium alloy components. This paper presents a static diffusion couple method which replicates the complex reaction mechanisms occurring at the tool-chip interface during machining and is validated with dynamic turning trials. The diffusion couple tests are realised via the use of a bespoke vacuum compression rig. The rig consists of two graphite rods which secure the sample under a clamping load at the required temperature and pressure. Titanium alloys bearing a high molybdenum content are found to be less susceptible to TiC formation at the tool-workpiece interface. A TiC layer during machining coats and protects the tool from excessive crater wear therefore increasing tool life. Strong agreement was found between the diffusion couple technique and turning trial. This low cost test strategy will be used to design machinable titanium alloys and more compatible tools.

Material reductions, weight savings, design optimisation, and a reduction in the environmental impact can be achieved by improving the performance of near-net shape (NNS) titanium alloy components. The method demonstrated in this paper is to use a solid-state approach, which includes diffusion bonding discrete layers of dissimilar titanium alloy powders (CP-Ti, Ti-6Al-4V and Ti-5Al-5Mo-5V-3Cr) using field-assisted sintering technology (FAST), followed by subsequent forging steps. This article demonstrates the hybrid process route, firstly through small-scale uni-axial compression tests and secondly through closed-die forging of dissimilar titanium alloy FAST preforms into an NNS (near-net shape) component. In order to characterise and simulate the underlying forging behaviour of dissimilar alloy combinations, uni-axial compression tests of FAST cylindrical samples provided flow stress behaviour and the effect of differing alloy volume fractions on the resistance to deformation and hot working behaviour. Despite the mismatch in the magnitude of flow stress between alloys, excellent structural bond integrity is maintained throughout. This is also reflected in the comparatively uncontrolled closed-die forging of the NNS demonstrator components. Microstructural analysis across the dissimilar diffusion bond line was undertaken in the components and finite element modelling software reliably predicts the strain distribution and bond line flow behaviour during the multi-step forging process.

Minibasins are fundamental components of many salt-bearing sedimentary basins, where they may host large volumes of hydrocarbons. Although we understand the basic mechanics governing their subsidence, we know surprisingly little of how minibasins subside in three-dimensions over geological timescales, or what controls such variability. Such knowledge would improve our ability to constrain initial salt volumes in sedimentary basins, the timing of salt welding, and the distribution and likely charging histories of suprasalt hydrocarbon reservoirs. We use 3D seismic reflection data from the Precaspian Basin, onshore Kazakhstan to reveal the subsidence histories of 16, Upper Permian-to-Triassic, suprasalt minibasins. These minibasins subsided into a Lower-to-Middle Permian salt layer that contained numerous relatively strong, clastic-dominated minibasins encased during an earlier, latest Permian phase of diapirism; because of this, the salt varied in thickness. Suprasalt minibasins contain a stratigraphic record of symmetric (bowl-shaped units) and then asymmetric (wedge-shaped units) subsidence, with this change in style seemingly occurring at different times in different minibasins, and most likely prior to welding. We complement our observations from natural minibasins in the Precaspian Basin with results arising from new physical sandbox models; this allows us to explore the potential controls on minibasin subsidence patterns, before assessing which of these might be applicable to our natural example. We conclude that due to uncertainties in the original spatial relationships between encased and suprasalt minibasins, and the timing of changes in style of subsidence between individual minibasins, it is unclear why such complex temporal and spatial variations in subsidence occur in the Precaspian Basin. Regardless of what controls the observed variability, we argue that vertical changes in minibasin stratigraphic architecture may not record the initial (depositional) thickness of underlying salt or the timing of salt welding; this latter point is critical when attempting to constrain the timing of potential hydraulic communication between sub-salt source rocks and suprasalt reservoirs. Furthermore, temporal changes in minibasin subsidence style will likely control suprasalt reservoir distribution and trapping style.

We present a technical report on our tests of a double-paddle oscillator as a detector of quantum turbulence in superfluid $$^{4}$$ He at low temperatures ranging from 20 to 1100 mK. The device, known to operate well in the two-fluid regime (Zemma and Luzuriaga in J Low Temp Phys 166:171–181, 2012), is also capable of detecting quantum turbulence in the zero temperature limit. The oscillator demonstrated Lorentzian responses with quality factors of order $$10^5$$ in vacuum, and displayed negative-Duffing resonances in liquid, even at moderate drives. In superfluid He-II at low temperatures, its sensitivity was adversely affected by acoustic damping at higher harmonics. While it successfully created and detected the quantum turbulence, its overall performance does not compare favourably with other oscillators such as tuning forks.

Powder produced by gas atomisation of metastable beta alloy Ti-5Al-5Mo-5V-3Cr has been consolidated using field-assisted sintering technology (FAST) to examine the suitability of the process to produce pre-forged billets as opposed to the conventional multi-step Kroll-VAR-forging route. FAST testing was performed using an FCT Systeme GmbH spark plasma sintering furnace type HP D 25 at The University of Sheffield. The dwell temperature was varied between 800 and 1200°C and the dwell time between 3 and 60 minutes. The process was found to allow very precise control over the beta grain size in Ti-5Al-5Mo-5V-3Cr by variation of the dwell time and dwell temperature, producing grain sizes between ~40 and ~350 μm. However, it was found that the grain size was increased more by increasing the dwell temperature than increasing the dwell time. The porosity was found to be between 99.5 and 100 % for all dwell temperatures at dwell times of 30 minutes or above. FAST has the ability to produce almost fully consolidated Ti-5Al-5Mo-5V-3Cr pre-forged material from powder with a controlled beta grain size, which is not possible by other methods.

Typically, pure TiO2 in pellet form has been utilised as the feedstock for the production of titanium metal via the solid state extraction FFC process. For the first time, this paper reports the use of loose synthetic rutile powder as the feedstock, along with its full characterisation at each stage of the reduction. The kinetics and mechanism of the reduction of synthetic rutile to a low oxygen titanium alloy have been studied in detail using a combination of X-ray diffraction, scanning electron microscopy, oxygen analysis, and X-ray fluorescence techniques. Partial reductions of synthetic rutile enabled a reaction pathway to be determined, with full reduction to a low oxygen titanium alloy occurring at 16 h. Major remnant elements from the Becher process within the feedstock were followed throughout the process, with a particular emphasis placed on the reduction behaviour of iron within the alloy. Although impurities such as Fe, Al, and Mn are found in the feedstock and alloy, no major deviations from previously reported reaction mechanisms and phase transformations utilising a pure porous (25–30 % porosity) TiO2 precursor were found. Following reduction, the titanium alloy powder produced from synthetic rutile (approx. 3500 ppm oxygen) has been consolidated via an emerging rapid sintering technique, and its microstructure analysed. This work will act as the baseline for future alloy development projects aimed at producing low-cost titanium alloys directly from synthetic rutile. Producing titanium alloys directly from synthetic rutile may negate the use of master alloy additions to Ti in the future.

In recent years, new titanium alloys with improved “machinability” have been developed, to provide manufacturers with alloys that can be machined to final dimensions at higher metal removal rates. One such alloy, TIMETAL® 407 (Ti-407), was developed to replace Ti-6Al-4V (Ti-64) in applications where energy absorption prior to failure is a key property requirement. In this study, a combination of large scale turning trials was used to characterise the full range of machining criteria, including: (1) tool life, (2) chip formation and (3) subsurface microstructural deformation. The machining response of the two alloys was summarised in easy-to-visualise machinability maps: to aid the machining supply chain in selecting feed rates and cutting speeds to generate certain chip forms, to extend tool life and minimise subsurface deformation for specific titanium alloys. Assessment of these key characteristics has provided an understanding of the effects of mechanical twinning on the poor chip control through increasing the accommodation of strain prior to fracture.

The fluctuating forces on the cutting tool generated during machining of β processed Ti-17 alloy are shown to contain sufficient information to enable measurement of β grain size to an equivalent accuracy of standard etching methods. Three orthogonal forces were gathered, cutting force tangential to the rotation, the force in the feed (radial) direction, and the normal force in the longitudinal axis. Each individual force produced a microstructure image with a high level of contrast but in some cases did not fully highlight all features as shown in the optical image of the equivalent area. By normalising and combining the three forces into a vector, followed by noise reduction, a high-resolution image with sufficient detail to undertake grain size measurements using the linear intercept was produced. The measured grain size differed by no more than 5% with respect the grain size measured in the etched micrograph. It is believed that the forces which have a higher proportion of elastic response in their total values, i.e., the feed and normal forces, produced the higher contrast images, indicating that elastic stresses produce the highest contrast between grains and plastic strains smear out the grain to grain variation.

Titanium alloys have excellent properties, but components are very expensive due to the high levels of processing required, such as vacuum melting, multi-stage forging, and machining. As a result, forged titanium alloy components are largely exclusive to the aerospace industry, where a high strength-to-weight ratio, corrosion resistance, and excellent fatigue resistance are essential. However, a typical buy-to-fly ratio for such components is approximately 9:1, as much of the forged billet is machined to swarf. The quantity of waste titanium alloy swarf generated is increasing as aircraft orders, and the titanium components contained within them, are increasing. In this paper, waste swarf material has been recycled using the two-step solid-state FAST-forge process, which utilizes field assisted sintering technology (FAST) followed by hot forging. Cleaned Ti-6Al-4V swarf was fully consolidated using the FAST process at sub-transus and super-transus temperatures, followed by hot forging at sub-transus temperatures at different strain rates. It was demonstrated that swarf-derived Ti-6Al-4V FAST billets have equivalent hot forging flow behaviour and resultant microstructures when directly compared to equivalently processed conventional expensive hydride&ndash, dehydride powder, and previously reported Kroll-derived melt-wrought material. This demonstrates that titanium swarf is a good quality feedstock for downstream processing. Additionally, FAST-forge is a viable processing route for the closed-loop recycling of machining waste for next-generation components in vehicles and non-aerospace applications, which is game changing for the economics of titanium alloy components.