Titanium stands trial

Materials World magazine
,
2 Feb 2011
Examples of a Ti-6Al-4V additively manufactured monolithic acetabular cup with pre-existing holes and an outer mesh structure for enhanced bone integration. Image courtesy of the University of Sheffield

There is still much to learn about titanium and its alloys for improved properties, processing and application. Dr Martin Jackson of the University of Sheffield, UK, reviews the UK effort.

Over the last few decades, many investigations into titanium and its alloys have been conducted at UK universities, exploring processing, microstructure and property development. Several of the research outputs are established in high performance components and industrial sectors. Today, there continues to be a vibrant, collaborative titanium research community in the UK, where activities range from modelling deformation mechanisms through to supporting the aerospace industry with critical fatigue lifing data. Importantly, the different research groups complement each other, providing cradle-to-grave analysis.

Processing behaviour

At the University of Birmingham, research is underway on the influence of carbon on the precipitation behaviour and kinetics of some high strength, high toughness titanium alloys. The properties of these alloys when processed via net shape hot isostatic pressing (HIPping) are also explored, as is additive laser manufacturing with an emphasis on components made from alloy Ti-6Al-4V.

The importance of metallurgical defects in various process routes is being assessed in a study that is aimed at defining the efficiency of material usage and the quality of products made using well established manufacturing and joining technologies. This will be compared to state-of-the-art additive and net shape manufacturing technologies.

The UK Engineering and Physical Sciences Research Council (EPSRC) and the Technology Strategy Board (TSB) have recently awarded projects to the Advanced Forming Research Centre of the University of Strathclyde. These initiatives have been funded through a major research programme – Strategic Affordable Manufacturing in the UK with Leading Environmental Technology (SAMULET). The collaborative programme brings together a consortium of high profile manufacturers such as Rolls-Royce and BAE Systems, as well as Regional Development Agencies and several universities.

A major project concerns methods to obtain and apply fine grained and ultrafine grained Ti-6Al-4V for superplastic forming/diffusion bonding (SPF/DB) with industrial partners Rolls-Royce and Timet, a titanium metals corporation heardquartered in the USA with operations in Birmingham UK. The purpose is to lower the process temperature in order to save energy, increasing tool life and productivity.

Another area of investigation is hot-die forging of titanium alloys, where the focus is on establishing flow stress characteristics and tool/material interface behaviour at forging conditions. This data can be used in reliable finite element simulation of different forming operations of aerofoils. Other projects explore flow-forming.

Researchers at Imperial College London and the University of Sheffield are working on the forging of landing gear near bita titanium alloys and on the superelastic beta-Ti Gum metal family of alloys.

A macrozone in Ti-6Al-4V: microstructure imaged using (from left to right) electron backscatter diffraction, polarised light and conventional microscopy. A major EPSRC project on crystal plasticity effects has been co-ordinated from Swansea University, in collaboration with Birmingham, Imperial College, Manchester and Oxford, and sponsored by Rolls-Royce and Timet

Microstructural evolution

Fundamental studies into the fatigue and creep behaviour of various conventional titanium alloys and gamma titanium aluminides are underway at Swansea University. This understanding is then applied to service applications in the aero-engine and aerostructure sectors, increasing the work’s technology readiness levels.

A major project on crystal plasticity effects has been coordinated from Swansea, in collaboration with the following universities – Birmingham, Imperial College, Manchester and Oxford – and sponsored by Rolls-Royce and Timet. The effects of microtexture and the subsequent control of mechanical properties have been assessed in selected commercial products. In addition to bulk property information, the programme has offered an invaluable insight into the early stages of plasticity and fatigue crack initiation via the so called ‘quasi-cleavage’ faceting mechanism, long implicated at Swansea as a contributing factor behind ‘cold dwell’ in certain alloy variants.

Automated quantitative tilt fractography techniques have been developed to interrogate fracture surfaces in titanium, and define crystallographic grain orientations without the need for destructive metallographic sectioning. However, the vital role of electron backscatter diffraction in characterising titanium microstructures, and, in particular, the presence of ‘macrozones’, has been affirmed. Future research planned under the EPSRC/Rolls-Royce Strategic Partnership in Structural Metals for Gas Turbines will be aimed at the development of new titanium alloys for fan disc and blade applications.

In parallel, a team at The University of Manchester is looking at the fundamental deformation mechanisms in titanium and how these change with temperature, strain rate and alloy composition, with particular emphasis on the understanding and modelling of twinning. The projects bring to bear the unique capabilities of synchrotron diffraction to study deformation in real time and in individual grains in the bulk. The aim is to determine the rules for new crystal plasticity models, which are essential to understand not only fatigue but also the formability of titanium products.

Stability, superelasticity and shear transformations are also the subject of further study under Dr David Dye’s recent EPSRC Leadership fellowship at Imperial College London.

Powder works

A consortium led by Metalysis, based in Rotherham, secured a £862,000 grant from the TSB on a match-funding basis in July 2009. This has enabled scale up of its technology for the production of titanium powders, exploiting the company’s FFC Process, which was developed by Fray, Farthing and Chen at the University of Cambridge a decade ago. Metalysis owns the global intellectual property and commercial exploitation rights to this process.

Universities in the consortium include Sheffield and Newcastle, while Warwick Manufacturing Group (WMG) has a small-scale experimental FFC reduction cell that enables the reduction process to be studied in situ using synchrotron X-ray diffraction. The group at WMG, along with researchers at Imperial College London, has exploited the FFC process and tomography techniques to produce foam structures for biomedical applications.

A titanium research group in the Department of Materials Science and Engineering at The University of Sheffield has significant EPSRC/TSB-funded activity in powder processing, thermomechanical processing, surface engineering and high performance machining. The group is working on a number of solid-state powder processing techniques and it is metal injection moulding components that have final strengths, elongations and chemistries within ASTM specifications 2 to 5. Research into ALM techniques is also continuing for automotive, aerospace and biological applications and it is hoped the technology will allow for novel, high performance and highly effective structures to be produced. Furthermore, powder routes are being exploited to form titanium sponge structures. These are potentially of interest for their high specific stiffness, as well as being used as high surface area electrodes that resist aggressive electrolytes, and as implants into the human body.

Finally, in collaboration with the Advanced Manufacturing Research Centre, the group is understanding the effects of emerging high performance machining on service properties. And a surface engineering group, as part of a TSB project involving Tecvac and Airbus, has developed a treatment for Ti alloys that makes the surface more resilient and hard-wearing to increase the life of critical bearing components.

Further information

The article was coordinated by Dr Martin Jackson (Sheffield) on behalf of the IOM3 Light Metals Committee. Jackson would like to acknowledge the following contributors from UK universities Professor Mike Loretto (Birmingham), Professor Martin Bache (Swansea), Dr João Fonseca (Manchester), Dr David Dye (Imperial College) and Dr Andrzej Rosochowski (Strathclyde).