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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.

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.

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.

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.

Reducing the high cost of titanium to a level where it can compete with currently used commodity metals offers opportunities to many industries to exploit its excellent combination of properties to improve performance or reduce weight. The key to decreasing cost is to reduce the number of processing steps to go from ore to component, as well as material wastage from excessive machining. This paper describes a new solid-state hybrid manufacturing route, termed by the authors as FAST-forge, for converting titanium alloy powder into components with wrought properties in two steps; utilising field assisted sintering technology (FAST) to produce a shaped preform billet that is finished to near net shape by a one-step precision hot forge. The route has been demonstrated at the laboratory scale using Ti-6Al–4V hydride-dehydride powder by producing fully consolidated, microstructurally homogeneous, double truncated cone specimens directly through FAST, which were then upset forged at a range of temperatures and strain rates. The microstructural evolution and forging behaviour of the Ti-6Al–4V after FAST consolidation is similar to conventional melt, multi-step forged product. Break up of primary α at high strains was observed at 950°C and 0.01 s−1, 0.1 s−1, and 1 s−1. There is good agreement between finite element modelling of the hot forging and the experimental data, which will enable more complex shaped geometries to be produced via the proposed FAST-forge route in future. Such a route could be used to consolidate powder from a lower-cost alternative extraction method to become a disruptive technology that will enable a step-change in the economics of titanium components.

We present proof-of-concept measurements of the vortex line density generated by a quartz tuning fork resonator probed by the attenuation of second sound in superfluid $$^4$$ He at 1.6 K. The force–velocity response of a quartz tuning fork operating at a frequency of 31 kHz exhibited the onset of extra damping at a velocity of 0.5 ms $$^{-1}$$ . Attenuation of the 5th resonant mode of second sound was observed at the same velocity, indicating the production of vortex lines. Our measurements demonstrate that an increase of the drag coefficient corresponds to the development of quantum turbulence.

In some commercial titanium extrusion practices, twisting of the extrudate can occur, which can result in the need to crop the back and front end of the extruded material, thereby reducing yield and increasing material losses. Understanding more about the behaviour of material during the extrusion process, and investigating the cause of defects such as twisting by use of finite element (FE) modelling techniques could help to reduce these losses, improve the productivity of the extrusion process, and the overall quality of the material produced. One of the most important components of FE techniques for hot deformation is the type of flow stress model that is used in the simulations. In this investigation isothermal uniaxial compression testing was performed on cylindrical specimens of Ti-6Al-4V at temperatures ranging from 950 °C to 1200°C and strain rates of 0.1 s-1 to 50 s-1, to produce true stress against true strain and load against die travel curves which were subsequently used to develop a new specific flow stress model for use in hot deformation above the beta transus, which can ultimately be applied to the hot extrusion of Ti-6Al-4V. From analysis of this data it was concluded that flow softening and work hardening do not occur during deformation, and that low friction conditions exist between the material and the tooling. The activation energy for deformation was found to be 193178 J.mol-1, and the flow stress model was shown to give a good fit to the raw data at low strain rates, but this relationship broke down at higher strain rates. Finally the importance of generating a flow stress model specific to a particular operation, and set of experimental data, rather than relying on existing data available in the literature is demonstrated.

Emerging lower cost titanium metal powder produced via an electrolytic method has been fully consolidated using spark plasma sintering (SPS) generating microstructures comparable to those observed in Ti–6Al–4V PM product. This is the first time powder from an alternative titanium extraction method has been processed via SPS and it is benchmarked with commercial alloys (CP–Ti, Ti–6Al–4V, and Ti–5Al–5V–5Mo–3Cr). The effect of powder feedstock size, morphology, and alloy chemistry on the consolidated density and microstructure is presented. Through a design of experiments (DoE) type approach the effect of SPS processing conditions on these alloy powders, including maximum sintering temperature, pressure, heating rate, and dwell time were investigated. The SPS process is found to be largely insensitive to feedstock size and morphology, although very large or highly porous powder particles are more difficult to fully consolidate. The maximum sintering temperature and pressure have the largest contribution to achieving full consolidation, with higher pressures and temperatures increasing the final density. Increasing heating rate increases the final grain size, despite less time being spent at the higher temperature and it is thought this is due to bypassing the traditional first phase of sintering. This paper shows that SPS is a viable step for a low-cost manufacturing route, for example to produce preform billets to be finished with a one-step forging operation, especially when combined with the possibility of lower cost powder. In the long-term, SPS will allow a significant reduction in the processing cost, contributing to an increased usage of titanium powder feedstock for a range of applications. This is reinforced by the successful large scale production of a 5 kg SPS Ti-6-4 billet, demonstrating the potential industrial scalability of the process, particularly for the aerospace industry.

The decomposition of the beta phase during rapid cooling of the near beta titanium alloy Ti-5Al-5Mo-5V-3Cr has been studied using in situ X-ray synchrotron diffraction combined with ex situ conventional laboratory X-ray diffraction and transmission electron microscopy (TEM). Evidence is found supporting the suggestion by De Fontaine et al. (Acta Mater. 1971:19) that embryonic omega structures form by the correlation of linear (111)beta defects at high temperatures. Further Cooling causes increased correlation of these defects and the formation of athermal omega structures within the beta matrix at temperatures similar to 500 degrees C. Post-quench aging at 570 degrees C resulted in the nucleation of alpha laths after similar to 90 s at temperature, with the laths all initially belonging to a single variant type. Aging for 30 min produced an even distribution of alpha precipitates with a lath morphology similar to 1.5 mu m x 0.2 mu m in size composed of both the expected Burgers variants. Mechanical property data suggests that the omega structures alone have no real effect: however, hardness increases were observed as the alpha phase developed. The utilization of thermal regimes similar to those presented in this paper could offer a method to engineer the alpha phase in near beta titanium alloys and hence control mechanical properties. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We discuss the Ricci flow on homogeneous 4-manifolds. After classifying these manifolds, we note that there are families of initial metrics such that we can diagonalize them and the Ricci flow preserves the diagonalization. We analyze the long time behavior of these families. We find that if a solution exists for all time, then the flow exhibits a type III singularity in the sense of Hamilton., 32 pages

Chemical wear during high speed machining of titanium alloys is a serious problem which affects the surface integrity of both the tool and workpiece. A low cost, novel diffusion couple method is presented which allows for thorough analysis of the tool-workpiece interface at the high temperatures reached during conventional machining operations. X-EDS analysis reveals that no less than seven distinct diffusion zones arise between Ti-6Al-4V and a WC-Co tool which are home to different phases and reaction species. Loss of cobalt binder coupled with a deficit of carbon results in a brittle η-phase leading to catastrophic fracturing of the tool. DICTRA is employed to thermodynamically model the diffusion mechanisms and verify the X-EDS results.