Heart of the matter - research at TWI
TWI Bulletin, March/April 2005
Fundamental to TWI's operation and its contribution to industrial research is the Core Research Programme. As its title suggests it constitutes the lifeblood of TWI's frontier R&D output.
Geoff Booth joined TWI in 1975, with a first degree in metallurgy from Cambridge University and a doctorate from Birmingham University. Initially, he worked in areas now covered by Structural Integrity and from 1986-89 was on assignment to EWI, becoming Manager, Engineering. He returned to TWI as Deputy Head, Engineering and since 1994 has been involved in managing process groups and is now Manager, Laser and Sheet Processes. He is a chartered engineer, a member of the Institution of Mechanical Engineers and a fellow of The Welding Institute.
Christoph Wiesner. After obtaining his engineering degree in Germany and his doctorate at the Swiss Institute of Technology, Dr Wiesner joined TWI in 1991. Following 12 years (as Manager since 1998) in the Structural Integrity Technology Group which provides services related to the performance, properties and inspection of engineering structures, Dr Wiesner became Director, Research and Technology in 2002/3.
The CRP is funded principally by Industrial Member subscriptions and is intended to serve the interests of all TWI Members. Geoff Booth and Christoph Wiesner review the recent programme and begin with a reminder of its specific aims:
- to create new technical knowledge and information
- to develop new technical capabilities
- to enhance staff skills for providing advisory and consultancy services
- to maintain TWI's position as a highly regarded centre for materials joining technology
The CRP is run as a series of projects in a three year rolling programme, and Member funds of approximately £1.8m per year were committed to the 2001-2003 programme. Where appropriate these were associated with other public funding sources in matching programmes to give a total value of some £3.3m per year.
The distribution of the programme by sector is shown in Fig.1 which indicates the value of work related to each industry sector. (Note: when considering the values in Fig.1 several projects are of relevance to more than one sector.)
Fig.1. CRP value by industry sector
Figures 2 and 3 show that the programme also achieved a balanced distribution in terms of material type and TWI technologies. The distribution of effort by material type shows some differences from the distribution of global consumption of material; this is deliberate and reflects the requirements of Members who tend to be biased towards new developments in materials and applications.
Fig.2. CRP effort by material type
Fig.3. CRP effort by technology
This paper summarises the main achievements of the 2001-2003 programme. These are identified by industry sector, but there are many overlaps and it is often the cross-sectoral work which adds real value. Nevertheless, in the text, project achievements are ordered by prime sector.
Technical highlights
Oil and gas sector
Greater tolerance to defects in pipe girth welds
The fracture toughness of pipe girth welds may reasonably be determined using single edge-notch tension specimens, instead of the conventionally used single edge-notched bend specimens. This gives an advantage in allowing a greater tolerance to flaws in pipe girth welds than would otherwise be the case.
Local post weld heat treatment
Recommendations have been made for the use of patch and band heating of local branch connections in pipes that will produce effective post weld heat treatment yet avoid the problems associated with current full band procedures.
Corrosion resistance of welded stainless steels
In supermartensitic stainless steel welds, weld toe geometry and degree of oxidation at the weld root control pitting corrosion resistance in sweet and mildly sour produced waters. Weld roots without any crevice-like features and with a low level of oxidation have the greatest corrosion resistance.
Sustained load cracking of titanium alloys
There have been several expensive sustained load cracking (SLC) failures in recent years associated with weldments in Ti-6Al-4V. Recommendations have been made for avoiding SLC failures by control of microstructure, chemistry and static and loading conditions.
High-velocity wire flame spraying
The performance of a new low cost and portable high-velocity wire flame spraying system has been established. Coatings of aluminium deposited by this system possess improved quality compared with coatings deposited by conventional arc and flame spraying systems. Wire flame sprayed aluminium coatings may therefore provide more reliable and longer-term corrosion protection to steel structures and components.
Performance of welded polyethylene pipes
A test has been developed that can consistently assess the long-term performance of butt-fusion welded polyethylene pipes. This test subjects whole welded pipes to a constant axial tensile stress at elevated temperature and is believed to be the most representative test currently available.
Construction and engineering sector
Validation of low-stress-no-distortion welding
A combined experimental and analytical programme of work has given a new capability to characterise low-stress-no-distortion (LSND) welding. For a given welding procedure, recommendations can be made regarding the optimum forced cooling conditions to minimise distortion.
Long-range ultrasonic inspection
The characteristics of ultrasonic waves have been determined for structures of regular cross section (eg rails). This work has identified opportunities for long-range ultrasonic testing using guided waves in a range of applications.
Hybrid Nd:YAG laser-arc welding with adaptive control
A hybrid laser-arc welding system has been developed in which the effect of joint gap variations can be compensated for in real time, via adaptive control of the process travel speed. With this system, good and consistent weld quality has been achieved with joint gap variations well outside those normally tolerated by hybrid laser-arc welding.
Friction stir welding of steel
An exhaustive review of friction stir welding (FSW) of steels, based on TWI's experience and the published literature has been carried out. This work has provided a definitive 'statement of the art' and has identified future research needs.
Development of hole filling technologies
The influence of the key process parameters for friction-taper-plug welding (FTPW) of aluminium alloys (rotation speed, applied force, material burn-off and joint design) have been established. The formation of a good quality FTP weld has for the first time been linked to the achievement of a critical energy input during processing and a high degree of FTPW process reproducibility has been demonstrated.
Aerospace sector
Design guidance for joints in composites
The static load resistance of joints between glass and carbon fibre composites that have been damaged by an impact load has been measured. Similar performance data have been obtained for joints containing pre-existing flaws. Additionally, an information database that will be useful to designers has been established.
Welding nickel alloys for turbine applications
Nickel superalloys are extremely difficult to weld because of their susceptibility to cracking. The application of forced cooling during TIG welding of aged Waspalloy and Alloy 718 resulted in the reduction or elimination of microfissures and in a substantial reduction of distortion and buckling. Using electro-spark deposition local repairs free from cracking, microfissures and recrystallisation were obtained, in a single crystal alloy.
Control of porosity during laser welding of aluminium alloy
A procedure has been developed for the Nd:YAG laser welding of square butt joints in 2024 aluminium alloy that meets demanding porosity criteria. Large-scale porosity was controlled using a twin spot laser beam approach, while small-scale porosity was found to be strongly dependent on the mixture content in the process gas.
Tool materials and design for friction stir welding
Advanced tool materials and composite tool designs have been developed for friction stir welding of up to 25mm thick aluminium alloys. Outline procedures are available for producing good quality welds in all the major Al-alloy groups.
Corrosion protection of metallic substrates
A significant reduction in the corrosion/degradation suffered by stainless steel, steel, silver, aluminium and brass was achieved by the use of thin (~1mm) sol-gel derived coatings. A range of coating compositions was examined including the addition of fluorine to make the coatings more hydrophobic. All the coatings studied gave significant corrosion protection to the metal substrates.
Power sector
Exemption from post weld heat treatment for steels
A succinct comparison has been provided of the limiting thickness below which post-weld heat treatment is not required for C-Mn and low alloy steels in fabrication codes. Fracture mechanics calculations show that the degree of conservatism is different in the different fabrication codes, and in some instances there may be no conservatism for higher strength grades of steel intended for service at low temperatures.
Consumables for 9-13%Cr steels in elevated temperature
Recent developments in the composition of chromium-containing, creep resistant steels for elevated temperature service have led to improved creep performance and there is a need for new welding consumable compositions for use with these steels. Additions of Ni and Co to 9%Cr and 10.5%Cr steel weld metal were shown to have a beneficial effect on both weld metal toughness and creep rupture strength.
Pulsed electron beam welding for thick section welding
A locally coupled grid supply and novel gun design have been successfully developed for Reduced Pressure and Non-Vacuum Electron Beam welding. Pulsed beams of up to 30kW (at 50% duty ratio) have been projected into the atmosphere, thus extending the range of industrial application for electron beam welding.
Electronics, sensors and medical sectors
Development of novel sealants for fuel cells
A novel one-step joining and sealing operation has been proposed which has the potential to speed up fuel cell production, increasing the likelihood that polymeric proton exchange membrane fuel cells will become a more viable energy source for the future. Developments include a barrier layer to silica poisoning and a dense glass/ceramic sealant that shows significant promise for allowing zirconia electrolyte to operate for extended periods at ~1000°C.
Generation and application of high intensity electron beams
The factors controlling the brightness and intensity of a 150kV electron beam have been analysed, and a novel indirectly heated EB gun and column has been designed and manufactured. Cutting, drilling and surface texturing trials have been conducted with encouraging results in a range of workpiece materials.
Development of the Clearweld ® process
To assist development of the Clearweld ® process, a computer program has been developed to enable rapid selection of processing conditions for potential applications. The software can be used to give guidance in the selection of process parameters and absorber requirements for laser welding of numerous polymer types.
Assembly of solder-bumped flip-chip components
Flip-chip technology offers high input/output (I/O) capability, excellent radio frequency transmission characteristics and the smallest possible device footprint. The capability to assemble high I/O, 200µm pitch solder-bumped flip-chip devices has been demonstrated using both high accuracy manual and high volume automated assembly equipment. A full reliability assessment has proved that the process is both reliable and controllable.
Road transport sector
Deposition of metallic coatings using the cold-spray process
Information on coating characteristics and properties has been generated for coatings of copper, aluminium and 316L stainless steel, produced by the cold-spraying process. A high quality copper coating, with very low coating porosity and oxide levels, high adhesion and deposition efficiency, can be achieved without heating the helium process gas.
Resistance spot welding of high strength TRIP steel
Resistance spot welds made in Transformation Induced Plasticity (TRIP) steels using conventional procedures possess poor interface properties. New procedures, with post weld tempering, have been developed to reduce weld hardening and improve weld performance in TRIP steels. By using the tempering procedure, the resultant shear strength of the welds could be improved by 25-30% and the cross tension strength by about a factor of four.
Joining of aluminium to steel using laser processing
An initial evaluation of a relatively new laser-based process for joining aluminium to steel has been carried out. A defocused laser is used to heat a steel sheet, and the conducted heat melts an adjacent aluminium alloy sheet, leading to wetting and bonding of the aluminium to the steel. Static mechanical properties in shear were good but in tension were relatively low. Further process optimisation should lead to an improvement in tensile properties.
Friction stir welding of lightweight magnesium components
Friction stir welding has been successfully applied to a range of wrought and cast magnesium alloys. Common Mg-alloys were welded to themselves and friction stir welds between Mg and Al-alloys were produced. Consolidated welds were produced, although a brittle Al-Mg grain boundary phase was commonly formed, and this limited weld mechanical properties.
Welding equipment and consumables, gases and raw materials sectors.
Microstructural modelling of heat-affected zones in microalloyed C-Mn steels
Integrated software has been developed for predicting weld thermal cycle, precipitate evolution, austenite grain growth, phase transformations, hardness and toughness in the grain coarsened heat-affected zone of power beam and arc welds in plain and microalloyed C-Mn steels.
Risk assessment and control of exposure to welding fume
International regulations require that the risks to health arising from work activities be assessed before work commences. For arc welding, this includes assessing exposure to fume and initiating control if exposure is above prescribed limits. A number of risk assessment schemes, ranging from complex to simple in their approaches, are available to assist in this task. It was found that these schemes generally arrive at a correct conclusion, but that simpler schemes were preferred. Risk assessment during MMA and MAG welding of steel and stainless steel can be reduced to a simple statement: fume control will be required in almost all situations.
High productivity arc welding
AC-pulsed MIG/MAG welding procedures have been established for butt welding thin sheet steel and aluminium with gaps that could not be accommodated with DC pulsed MIG. Tandem wire MIG/MAG welding procedures have been developed for high speed welding of thin sheet steel and aluminium. The process has been demonstrated to be capable of welding at speeds of two to three times greater than that which can be achieved using the conventional single wire MIG/MAG process.
Friction stir weld modelling
Two useful techniques have been developed for the modelling of friction stir welding (FSW) processes: a simple mathematical model for prediction of torque during FSW, and a computational fluid dynamics based model for prediction of material flow during welding. These solutions provide a range of modelling tools allowing both simple and detailed investigation of the process. This work also included a review of forces generated by FSW tools, which led to the design of a new FSW tool design, the Trivex tool.
Dissemination of results
Eighty-four TWI Research reports have been prepared based on the 2001-2003 programme. The results are all available to TWI Members as hard copy or via the TWI website www.twi.co.uk. Additionally, after an embargo period on publication to enable Members to have preferential access to the work, staff have prepared approximately two hundred papers and presentations, for technical journals and conferences.
The following Best Practice Guides and Toolkit software packages have been produced and are available on JoinIT.
- Best practice guides:
- Mechanical integrity of plastic joints
- NDT of thermosetting and thermoplastic polymer composites
- Statistical analysis of fatigue data
- Arc welding of duplex stainless steels
- Wire bonding for microelectronic interconnection
- Toolkit software packages:
- Charpy - toughness correlations
- Fatigue design to British Standard BS7608
- Welding preheat
- Yield strength hardness temperature correlations
- Ferrite predictor
Research board
The CRP is monitored by a Research Board, which comprises senior technical staff drawn from the entire Industrial Membership base. Research Board is charged with ensuring the relevance of the CRP to the broad interests of members and overseeing its planning and execution.
In reviewing the 2001-2003 CRP, Research Board considered that approximately 75% of the objectives set at the outset of the programme were met, with a small number of objectives still being pursued in ongoing projects. This was felt to be a satisfactory outcome given that it is the nature of research projects to set challenging objectives.
Concluding remarks
The highlights of the 2001-2003 TWI Core Research Programme (CRP) have been summarised. Main achievements have been identified by relevant industry sectors as has the distribution of value by sector, material and technology. The programme created significant technical output for the use of TWI Members and an assessment of the research and development objectives of the various projects has shown that 75% of them were met.
The CRP is a cornerstone of TWI's membership benefits and a distinguishing feature of TWI from other technology providers. Through the use of annual fees to co-finance the CRP, Industrial Members assist in maintaining the position of TWI as the leading edge of joining technology. Continued access to up-to-date knowledge, skill and experience is ensured for Industrial Members to act as an extension to in-house technology resources.
TWI is currently engaged in the 2005-2006 CRP, details of which are available on the TWI website. Member participation in individual projects is welcome and encouraged. Please contact either of the authors if interested in one or more specific projects.
