The first four years - TWI teams up with University in postgrad training partnership

TWI Bulletin, May/June 2000

Roger Wise joined TWI in 1986 having graduated in physics from the University of Exeter. After working in the Electron Beam Welding and Plastics Joining Groups, he became part of TWI's Innovation Unit in 1997. He gained a PhD from Cambridge University in 1999 on ultrasonic welding of thermoplastic polymers and in the same year became TWI's PTP Co-ordinator.

Stephanie Sutton joined TWI in April 1995 and is a Project Manager in the e-commerce Group. Its activities cover regional and national technology transfer projects and services including the JoinIT ^TM internet based information and advice service.

As Assistant Co-ordinator for the Postgraduate Training Partnership scheme she is responsible for the administration of TWI's activities within the TWI/University of Cambridge partnership. She is currently studying for a degree in Technology and Science with the Open University.

Rob Wallach is a lecturer in the Department of Materials Science and Metallurgy at the University of Cambridge and also a Senior Tutor at King's College. His main research interests are in joining using advanced processes and modelling of the behaviour of materials.

A fruitful collaboration between TWI and Cambridge University is now in its fourth year. The co-ordinating team of Roger Wise, Stephanie Sutton, Rob Wallach and Rosie Ward, take a look at what it has achieved and at future prospects.

TWI and Cambridge University have an association going back 50 years. In fact some of the early work on fatigue of welded structures started at Cambridge University in the 1940s was transferred to the organisation which was to become TWI when it became too noisy for the dons. Since then, there have been many collaborative projects and topics but perhaps none as large as the Postgraduate Training Partnership (PTP) which was established four years ago.

The Partnership, one of only eight in the UK, is a joint Department of Trade and Industry (DTI) and Engineering and Physical Sciences Research Council (EPSRC) initiative. The idea is that PhD projects are co-supervised by an academic and a member of staff at an RTO such as TWI, so that a commercial perspective is applied to the work. Industrially relevant courses are given to the students (or Associates as they are called) to set the significance of their research in an industrial context.

The collaboration is not only of value to the Associates. TWI and Cambridge University are both benefiting from increased interaction as a result of the Partnership and in some cases, this has acted as a catalyst for generation of other projects.

Taking general themes of materials joining and surfacing, and the analysis and testing of these technologies, the PTP currently spans three University Departments, Engineering, Materials Science and Metallurgy and Applied Mathematics. Two Associates have completed their projects both of whom have found challenging jobs in industry, one at TWI and one at National Power.

This article reviews the Partnership, including its operational aspects and a description of its projects.

Operational aspects

Funding is available for prospective PTP PhD students with a good first degree or a further degree in an engineering, maths, physics or materials science discipline. Funding is only available for European Union nationals.

Project areas

The project areas are shown in the Table with their start date, technology area and the main anticipated sectors for exploitation.

Table

Activated fluxes in TIG welding

Richard Segar

Supervisors: Dr W Lucas and Mr D Howse (TWI), Dr E R Wallach (CU)

Activated fluxes applied to a steel surface, have the effect of increasing the weld penetration compared with conventional TIG welding. This project has been able to determine the primary mechanisms altering the welding characteristics when using activated fluxes. A broad set of principles has been produced to apply activated flux technology to other metals.

Corrosion behaviour of NiAl intermetallic high velocity oxy-fuel (HVOF) sprayed coatings (project completed)
James Hearley
Supervisors: Dr A Sturgeon (TWI), Dr J Little (CU)
For components in severe environments, deposition of low porosity aluminide protective coatings using the high velocity oxyfuel (HVOF) spraying process offers an alternative route to diffusion based protective coatings. This project has developed an understanding of factors affecting the quality and performance of aluminide coatings deposited using this process.

Modelling of chemically-active, supersonic, multi-phase flows and its application to HVOF spraying
David Hunt
Supervisors: Dr A Sturgeon (TWI), Dr N Nikiforakis (CU)
Spray guns for high velocity oxyfuel (HVOF) use a combustion process to heat a flowing gas stream containing the coating material and then accelerate this gas/particle two-phase flow to high, supersonic velocities. Detailed modelling of chemically-active, supersonic, multi-phase flows to provide an in-depth understanding of the spray process is necessary to support future developments in gun design and process control systems aimed at improving coating consistency.

Fundamental aspects of friction stir welding (project completed)
Mike Russell
Supervisors: Dr P Threadgill (TWI), Dr H Shercliff (CU)
The fundamentals of friction stir welding are now understood qualitatively, but a more detailed understanding was believed to be very important for further development of the process. Within this project, an analytical heat flow model capable of predicting heat flow during friction stir welding in all materials was developed. These data were then used as an input to a further model to predict the microstructure and hardness in aluminium alloy grade 2014.

Application of low stress no distortion techniques to the TIG welding of aluminium aerospace structures

Robin Preston

Supervisors: Dr S Smith (TWI), Dr H Shercliff, Prof P Withers and Dr S Williams (CU)

Finite element models for TIG welding the 'unweldable' aerospace alloy 2024-T3 have been validated with neutron and synchrotron diffraction experiments. The models have been used to explore and optimise pro-active residual stress control techniques.

The work derived from the project won Best Student Research Paper at the 13th ABAQUS Users Group Conference, Chester in 1998, and Best Graduate Poster at CIM99 in Quebec City, Canada, 1999.

The industrial application of ultrasonic wave propagation models to the NDT of surface engineering processes

Rachel McCarthy

Supervisors: Dr G Georgiou (TWI), Prof J Willis (CU)

The main objective is to extend the capabilities of computer models developed at TWI for ultrasonic surface wave interaction with various geometries.

A range of wave sources has been incorporated into the suite of computer models and a hybrid model which will improve accuracy and computational efficiency is under development.

Micro-mechanical modelling of cleavage failure in structural steel

Sylvain Bordet

Supervisors: Dr A Karstensen (TWI), Dr D Knowles (CU)

This project investigates the applicability of current local approaches to understanding fracture using a modern, high strength structural steel (BS Grade 450EMZ), with an emphasis on their micro-mechanical foundations. This is done through the use of FE modelling, low temperature CTOD, microstructural and fractographic analysis.

Strength of polymer matrix composite-metal joints

Stephen Clifford

Supervisors: Dr C Manger (TWI), Prof W Clyne (CU)

The strength of structural co-cured polymer composite to metal joints is being investigated. Failure modes under static and fatigue loading have been determined. Recent work has concentrated on understanding the toughness properties of the composite metal interface. The effects of steel surface pretreatment, surface roughness and heat treatment of the joint have been evaluated.

Ceramics reinforced active metal braze alloys for joining silicon carbide

Damien Ormston

Supervisors: Dr W Hanson (TWI), Dr K Knowles (CU)

The use of active metal braze alloys (based on Ag-Cu-Ti) to join ceramics to themselves and to metals is well established. In this work, silicon carbide particulates have been incorporated into two commercially available active metal braze alloys to produce in-situ metal-matrix-composites. The strengths of the resultant joints are up to 50% greater than is achieved with unreinforced brazes.

Glenn Parry

Supervisors: Dr A Taylor (TWI), Dr I M Hutchings, (CU)

The sol-gel technique produces thin, dense, ceramic/polymer nanocomposite coatings at temperatures well below those normally associated with ceramic materials, <280°C as opposed to >600°C. A number of testing techniques have been developed, these allow transparent coatings on transparent substrates to be evaluated. The effect of variations in formulation and processing have been studied, and the coatings can now be optimised for hardness, scratch and wear resistance.

Long-term durability of adhesive joints

Richard Court

Supervisors: Dr S M Tavakoli (TWI), Dr M Sutcliffe (CU)

The aim of this project is to model the long-term durability of adhesive joints and particularly aspects affecting the initiation and propagation of fracture as a function of temperature and moisture degradation of the adhesive joints. A new test method for observing the environmental degradation of adhesive joints allows the observed damage to be directly related to the stress state within the joint. The results of ageing tests are being used to develop a theoretical model for prediction of joint life-times.

Design of composite adhesive joints

Stefanie Feih

Supervisors: Dr G McGrath (TWI), Dr H Shercliff (CU)

Prediction of initial damage and failure modes of adhesive composite joints cannot yet be achieved by a simple, executable program. A user subroutine has been developed which performs an implicit analysis of the stress-state with progressive damage modelling at every calculated increment of a non-linear analysis. Common composite failure modes included are fibre breaking, fibre buckling, matrix cracking in tension/compression and the onset of delamination using a 3-D extended progressive Chang-Chang model.

Continuum modelling of mesoscale polymer structure and the welding of structured polymers

Marc Hamm

Supervisors: Dr R Wise (TWI), Dr G Goldbeck-Wood (CU)

The diffusion and structural evolution of many widely used semi-crystalline polymers is not described by the established reptation model. In this project it is intended to propose a model that takes account of the tendency of the polymer chains to align during the diffusion process by which welding takes place. It will increase the qualitative understanding of the role of chain alignment in the welding process.

Characterisation of interfaces formed in solid state bonding of dissimilar materials

Ton van Helvoort

Supervisors: Dr J Fernie (TWI), Dr K Knowles (CU)

The primary aim of this project is to specify the mechanism of electrostatic bonding and establish how the process might be improved and extended to other dissimilar materials. In the first experiments Si/pyrex electrostatic bonds have been made successfully and specimens from these bonds have been prepared for transmission electron microscopy (TEM). The project may be broadened to examine interfaces in other ceramic-metal combinations produced in the solid state by friction welding.

Modelling the reliability of solder joints

Rollo Cooper

Supervisors: Dr J Fernie (TWI), Dr E R Wallach (CU)

Materials to be considered will include Pb-containing and Pb-free solders, in accordance with current industry and legislative trends. Apparatus has been constructed which measures the rate of creep deformation of several solder alloys under varying load and temperature (as would be in service). The project will also produce new data on the fatigue behaviour and on elastic modulus for specific alloys. The ultimate aim of this project is to integrate the above data into a generic model, for use with finite element analysis.

Fabrication and properties of phosphate matrix composites

Cara Mulcahy

Supervisors: Dr K Ironside (TWI), Dr W Clegg (CU)

There is demand from the electronics industry for new substrate materials in integrated circuits (IC). The aim is to conduct (and dissipate) heat away from the semiconductor devices more effectively, so providing better 'thermal management'. The aim of this project is to establish methods for the manufacture of aluminium phosphate composites containing carbon-fibres. Such a composite would possess very high thermal conductivity and low thermal expansion, both of which are highly advantageous to industry.

Improvements to high power Nd:YAG laser welding

Jose Greses-Juan

Supervisers: Dr P Hilton (TWI), Dr C Y Barlow (CU)

This project is attempting to determine the causes and effects of plume/ plasma formation above the weld pool. The first stage is a comparison between CO ₂ and Nd:YAG laser welding at high powers.

Development of competence (focusing on technological innovation)

Markus Erlich

Supervisors: Mr J Weston (TWI), Dr J Mills (CU)

Manufacturing organisations are more and more aware of the shortcomings of traditional approaches to sustain competitive advantage. To help counteract these concerns, companies are adopting strategies based on 'competence' and their associated concepts. This project will involve development of a research method to analyse a firm's technological innovation capability, and demonstrate this method's potential use as a data collection tool for future research.

Carbon migration at welds in dissimilar Cr-Mo steels

Dorothy Elder

Supervisors: The recently deceased Dr T Gooch (TWI), Prof H Bhadeshia and Dr E R Wallach (CU)

The project aims to increase understanding of the behaviour of welds between different grades of steel used in electricity generation plant operating at high temperatures. Changes in decarburised and carburised zone widths formed after ageing dissimilar steel diffusion bonds have been recorded along with variations in local hardness across the bond interface. These measurements have been compared with predictions from a simple computer model (which is based on carbide dissolution and carbon diffusion). Unlike welding, no unwanted microstructures or mixing of the component steels occur during diffusion bonding.

Development of microstructure in electron-beam welded (EBW) ferritic steels

Alan Bates

Supervisors: Dr P Hart (TWI), Dr E R Wallach (CU)

Electron beam welding offers a number of advantages over arc welding when applied to heavy section ferritic steels, but these advantages are cancelled out by a fusion zone microstructure that is lacking in toughness. This project sets out to understand some of the factors that produce this poor microstructure and will go someway in enabling tough microstructures to be produced with EBW.

Welding metallurgy of low carbon 13%Cr stainless steel

Dominique Carrouge

Supervisors: The recently deceased Dr T Gooch (TWI), Prof H Bhadeshia (CU)

Recent years have seen the introduction of 13%Cr martensitic stainless steels as an alternative to duplex grades. The 13%Cr types have extremely low carbon contents about 0.01%, to improve weldability. The aim of this project is to devise a system for predicting the microstructure formed at welded joints in low carbon 13% Cr steels, recognising especially formation of ferrite and austenite and transformation during post weld heat treatment.

Next steps

The last intake of PTP Associates will be in October 2000. The DTI and EPSRC are now operating an initiative called Faraday Centres which can include PhD projects along similar lines to the PTP. TWI and Cambridge University intend to submit a proposal for a Faraday Centre on the theme of joining. This would offer a suitable focus for a collaboration under Faraday and this could include topics such as light weighting, product life extension and the design of joints for disassembly. Details of the current PTP, TWI's Core Research Programme and any information about the proposed Faraday Centre are regularly updated on TWI's website at www.html.co.uk.

Summary

A PTP scheme between TWI and Cambridge University has been operating for four years, the last intake of Associates being in October 2000. The jointly supervised PhD projects have covered a wide range of topics in the areas of joining, surfacing and the associated testing and analysis of the techniques.

The first indications are that the Associates produced by the scheme are finding employment in good positions having completed projects of industrial relevance.

Plans are underway to develop further the collaboration between TWI and the University of Cambridge by the application for Faraday Centre status.

For more information contact Dr Roger Wise, rjwise@twi.co.uk , Mrs Stephanie Sutton, stephanie.sutton@twi.co.uk or Dr Rosie Ward on admin@msm.cam.ac.uk