The magazine of TWI Driving inspection around the bend TWI ... · PDF fileAker Solutions Woodfield Systems Ltd UK Manufacture and maintain marine loading arms and swivels Aker Solutions

Driving inspection around the bend

Issue 181 November/December 2012

TWI Events

Opportunities for addressing unmet clinical needs in major trauma and emergency careMedical23 January 2013London

IMechE: Residual stresses: When do they matter?Construction and engineering31 January 2013Manchester

NDT & Condition Monitoring Technical Group MeetingPower and oil and gas12 February 2013Glasgow

Trade Mission to South Africa, Johannesburg and Cape TownNDT18-22 February 2013South Africa

IMechE: Tribology at seaShipping6 March 2013London

Advanced Structures Technical Group MeetingConstruction & engineering7 March 2013Great Abington

Emerging manufacturing techniques for metallic structuresConstruction & engineering14 March 2013London

Linear Friction WeldingAerospace14 March 2013Great Abington

T h e m a g a z i n e o f T W I

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Friction stir welding enables worldwide engineering success

Invented by Wayne Thomas and a team of researchers at TWI in the UK in 1991, FSW has evolved from its beginnings as a novel method proven for joining thin-section aluminium alloy components in the automotive and aerospace industries, into a revolutionary and adaptable engineering process with far-reaching industrial potential for joining light metal alloys including titanium, as well as steel and other hard metals in thicker sections.

The benefits of FSW on processing speed, weld strength and integrity, welding environment safety, energy efficiency and cost reduction are being realised by an increasing number of fabricators across the globe. So far the joining technique has been chosen as an engineering design solution or for process development by 231 organisations in 24 countries. Around half these organisations are end-users; the rest research organisations, equipment suppliers and academia - with the majority based in the USA, Japan, China, Germany and Sweden.

As process inventor TWI holds the patent to the basic method of FSW plus several variants and continues to drive the innovation in its variations and improvements through an internal research and development programme. TWI is also active in the preparation of world standards and guidance relating to the use of the process.

In terms of advanced applications demonstrating the success of the process, FSW has been applied in many of the world’s space launch vehicles, including the Space Shuttle, Delta II and IV, SpaceX Falcon 9 and Ariane.

In the rail industry, the process is used in the production of aluminium-bodied rail cars, including Hitachi super-fast trains (Shinkansen), which can reach speeds of 320kph

Continued overleaf.

Friction stir welding (FSW) re-enters the spotlight with the launch of the new Apple iMac – illustrating the adaptability of the process and its potential for the future of electronics design.

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FSW is also commonly used in the aerospace industry by Boeing, Lockheed Martin, BAE Systems and EADS and specifically for some applications in Eclipse, Boeing, Spirit and Airbus aircraft.

Other uses of the enabling welding process include specialist car manufacture by companies such as Audi, Ford and Mazda, as well as in the manufacture of large heatsinks and in the build of ship superstructures.

New applications are still being discovered and Apple Inc’s endorsement of FSW during its recent product launch presentation and in promotional literature about the

design of the new iMac is the latest high-profile industry example of the worldwide success of the UK-patented joining process.

For further information concerning FSW patents and licences, please contact [email protected] or visit the friction stir welding Intellectual Property Licensing page on our website.

For technical information about the process, please contact us or visit the Friction Stir Welding section of our website.

Aker Solutions Woodfield Systems Ltd UKManufacture and maintain marine loading arms and swivels

Aker Solutions Workover SystemsNorwayInstallation, workover and intervention of subsea wells

Aleva Neurotherapeutics SASwitzerland Medical devices

Alstom Grid - StaffordUSAElectrical engineering

Daewoo Shipbuilding & Marine Engineering Co LtdRepublic of KoreaShipbuilding, offshore, plant, wind power

Friede & Goldman LtdUSAMarine engineering design

Haldor Topsoe A/SDenmarkCatalyst manufacture, R&D, petrochemical technical support

Khartoum Refinery CompanySudanOil refinery - Sino-Sudan joint venture business

Murata Boring Giken Co LtdItalyThermal spray, laser engraving, mirror finishing, grinding

Pearson Engineering LtdUKMilitary engineering

Smith & Nephew - Trauma Division UKMedical device manufacturing

Spirax Sarco SrlItalyProvider of steam system solutions

Tocalo Co Ltd JapanThermal spray coating

Tyco Electronics Corporation via TE Aerosapce, Defense & Marine businessUSAAerospace

YKK CorpJapanFastening products manufacturing

New Members of TWITWI is pleased to welcome the following as Industrial Members

We are always open to applications from Materials Scientists, Metallurgists and Welding Engineers, in particular experts in corrosion and ferritic steels with knowledge of the oil and gas sector.

For current vacancies, please see our careers page www.twi.co.uk/careers or to apply speculatively email your CV and covering letter to [email protected]

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Built in new premises adjacent to TWI’s headquarters, NSIRCTM will be established and managed by structural integrity specialist TWI, working closely with lead academic partner Brunel University, alongside the Universities of Cambridge and Manchester. Key industrial organisations including BP, Lloyd’s Register and Network Rail have also indicated their support. The academic partners will provide NSIRC with fundamental science, academic supervision and governance for the degree awards. Industry will provide the long-term technical challenges that the Centre will address through its research programmes.

Network Rail’s Technical Director of Asset Management Services, Steve Yianni, said: ‘Network Rail supports the creation of the National Structural Integrity Research Centre and is currently in discussion with TWI about how it can be involved. By taking an active role in developing the engineers of the future, a foundation like this can not only give the railway the talent it needs, but also provide the rest of British industry with a stream of well-qualified engineers to drive the country forward.’

In parallel with this award of funding from the Regional Growth Fund, Brunel University has also secured funding from the Higher Education Funding Council for England (HEFCE) for specialist research equipment for the Centre.

The aim of the Centre will be to train, qualify and award higher degrees specifically related to structural integrity and at the same time to innovate by developing technologies and approaches to enhance the safety of new and existing engineering structures. The facility will provide UK industry with world-class engineers who can lead the development of new, safe, world-beating products in diverse industries, including oil and gas, energy generation, aerospace, road transport and medical devices.

The new Centre is expected to offer capacity for 200 postgraduate students and will create around 48 new jobs.

TWI estimates that in addition to the environmental and social benefits resulting from the avoidance of engineering failures, the direct economic benefit brought by those graduating from the Centre after the first ten years of its operation will be in excess of £350m. This can be

multiplied to more than £3.5bn when considering the benefits of the work the qualified engineers will undertake for industry.

MP for South Cambridgeshire, Andrew Lansley CBE said: ‘Structural integrity is a technology field critical to many sectors of engineering and advanced manufacturing. I applaud TWI in securing this major funding to enable such a remarkable extension to its high-level research into structural integrity and support for specialist skills across the country.’

TWI Chief Executive, Dr Christoph Wiesner added: “This is a key strategic initiative for UK engineering. It will ensure we have the qualified engineers that industry needs, together with the underlying science that will keep the UK as worldwide leader in this hugely important technical area.”

For further information, please contact [email protected], Associate Director, TWI.

Award for new higher-education engineering research centre at TWITWI welcomes the announcement by the Department of Business, Innovation and Skills to support its plans to form a National Structural Integrity Research Centre (NSIRCTM). The latest round of the Government’s Regional Growth Funding awards will enable TWI and a consortium of leading academic and industrial partners to realise the establishment of the new postgraduate engineering facility at Granta Park, South Cambridgeshire.

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Technology Transfer

Job Knowledge

121 Power source characteristics

The prime objective of an arc welding power source is to deliver controllable welding current at a voltage demanded by the welding process. The arc welding processes have different requirements with respect to the controls necessary to give the required welding conditions and these influence the design of the power source. To understand how the requirements of the processes affect the design of the power source it is necessary to understand the interaction of the power source and the arc characteristics.

If the voltage of a welding arc at varying arc lengths is plotted against the welding current the curves illustrated in Fig. 1 are obtained. The highest voltage is the open circuit voltage of the power source. Once the arc is struck the voltage rapidly falls as the gases in the arc gap become ionised and electrically conductive, the electrode heats up and the size of the arc column increases. The welding current increases as the voltage falls until a point is reached where the voltage/current relationship becomes linear and begins to follow Ohm’s Law. What is important to note from Fig.1 is that as the arc length changes both the voltage and welding current also change – a longer arc giving higher voltage but with a corresponding drop in welding current and vice versa. This characteristic of the welding arc affects the design of the power source since large changes in welding current in manual metallic arc (MMA) and tungsten inert gas (TIG) welding is undesirable but is essential for the metal inert gas/metal active gas (MIG/MAG) and flux cored arc welding (FCAW) processes.

MMA, TIG and submerged arc power sources are therefore designed with what is known as a drooping

output or constant current static characteristic, MIG/MAG and FCAW power sources with a flat or constant voltage static characteristic. On most power sources the slope of the characteristic can be changed either to flatten or make

steeper the curves shown in Figs2 and 3

Fig2 shows drooping or constant current power source static characteristics such as used for the MMA or TIG process, superimposed on the arc characteristic curves. When manual welding the arc length is continually changing as the welder cannot maintain a constant arc length. With a constant current power source as the arc length changes due to the welder’s manipulation of the welding torch there is only a small change in the welding current – the steeper the curve the smaller the change in current so there will be no current surges and a stable welding condition is achieved. Since it is primarily the welding current that determines such features as the penetration and electrode consumption this means that the arc length is less critical, making the welder’s task easier in achieving sound defect free welds. Typically, a ±5volt change would result in around a ±8 amp change at 150amp welding current.

In some situations, for example when welding in the overhead position or when the welder is faced with variable root gaps, it is an advantage if the welder has rather more control over deposition rates by enabling him to vary the rate by changing the arc length. In such a situation a flatter power source characteristic will be of benefit.

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Submerged arc welding also uses a drooping characteristic power source where the welding current and the electrode feed rate are matched to the rate at which the wire is melted and transferred across the arc and into the weld pool the ‘burn-off rate’. This matching of parameters is carried out by a monitoring system which uses the arc voltage to control the electrode feed speed – if the arc length/voltage increases the wire feed speed is increased to restore equilibrium.The constant voltage power source characteristic is illustrated in Fig.3. This shows that as the arc length and hence the voltage changes there is a large change in the welding current – as the arc lengthens the welding current falls, as the arc shortens the current increases.

With MIG/MAG and FCAW power sources the welding current is controlled by the wire feed speed, the welding current determining the rate at which the welding wire is melted and transferred across the arc and into the weld pool the ‘burn-off’ rate. As the current decreases the burn-off rate also falls, less wire is melted and the wire tip approaches the weld pool. In doing so, the voltage decreases, the welding current and hence the burn-off rate increase. Since the wire feed speed is constant there is a surplus of burn-off over wire feed such that the desired arc length, voltage and current are re-established. The converse also occurs – a shortening of the arc causes a reduction in voltage, the current rises, the burn-off rate increases, causing the arc to lengthen, the voltage to increase and the welding current to fall until the preset welding conditions are re-established. Again, a typical figure for the change in welding current for a constant voltage power source would be in the region of ±40amps for a change in arc length of ±5volts. This feature gives what is known as a self-adjusting arc where changes in arc length, voltage and current are automatically returned to the required values, producing stable welding conditions. This makes the welder’s task easier compared with MMA

or TIG welding. Although in principle it may be possible to use a constant voltage characteristic power source for MMA welding it is far more difficult for the welder to judge burn-off rate than arc length so arc instability results and the method is not practicable.

In addition to this voltage control of the welding arc the speed at which the power source responds to short circuiting is important, the power source dynamic characteristic. Short circuits occur during arc striking and in MIG/MAG welding during dip transfer. As the voltage drops to zero when a short circuit occurs the current rises. If this increase in the current is fast and uncontrolled then the electrode tip blows like an electrical fuse resulting in excessive spatter – too slow a rise and the electrode may stub into the weld pool and extinguish the arc. This is not too significant when using the MMA process since the maximum current at zero voltage is controlled by the slope of the static characteristic curve and the welder can easily establish an arc gap. It is important in the MIG/MAG process where a flat static characteristic power source is used and the current could rise to an extremely high value,

in particular when welding in the dip transfer or short circuiting condition.

An electrical component called an inductor is therefore introduced into the power source electrical circuit. This device opposes changes in the welding current and hence slows the rate at which the current increases during a short circuit. Inductance is variable and can be adjusted to give a stable condition as shown in Fig. 4. Inductance in the welding circuit also results in fewer short circuits per second and a longer arc-on time, giveing a smoother better shaped weld bead. Too much inductance may result in such a slow rise in the welding current that there is insufficient time for the arc to re-establish and melt the wire tip so that the welding wire then stubs into the weld pool. Inductance during spray transfer is also helpful in providing a better and less violent arc start.

This article was written by Gene Mathers.

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November/December 2012

TWI hosted the first summer school for the mathematical modelling of welding in late summer 2012. This event ‘Leading Edges in Welding’ organised in collaboration with the University of Leicester, was attended by students, engineers and visiting academics even from as far afield as China. Talks were given by universities and companies from all over the world on the modelling at length scales, from quantum scale up to millimetres, combined with practical workshops to illustrate the techniques taught.

The event was a great success, with most of attendees giving positive feedback and indicating interest in future workshops. It is planned to run a similar event in 2014, hosted by the University of Leicester and open to students and engineers of all levels of experience.

Leading Edges in Welding: Mathematical modelling of welding

Six engineers from Sellafield Nuclear celebrated gaining a CSWIP certification in Butt Fusion and Electrofusion welding of polyethylene (PE) pipes, having successfully completed TWI’s tailored training course.

Ian Noble, a Quality Manager, from Sellafield Ltd approached TWI when a new project came up to design and install a large diameter polyethylene pipeline to carry cooling water. The successful and safe installation of the pipeline requires essential knowledge of the installation processes and procedures. As the industry’s leading expert in plastics welding, TWI was best placed to deliver the training required to Sellafield’s engineers.

Andy Knight and Scott Andrews from TWI’s Polymer Section delivered the two-day training and examination course at the Sellafield Nuclear Reprocessing Site in Seascale, Cumbria. The course was designed for the specific needs of the engineers and tailored around their existing skills and experience, ensuring each candidate completed the training with a thorough knowledge of the principles and best practice of butt fusion and electrofusion welding of PE pipes.

The course involved a mixture of classroom and practical sessions and covered an introduction to common thermoplastics, welding equipment, avoidance of joint defects, health and safety, welding parameters, joint preparation, process control and inspection and testing. The training course concluded in the successful completion of a theoretical and practical examination exercise, which led to each candidate gaining a CSWIP certification in Butt Fusion and Electrofusion welding of PE pipes. For further information contact [email protected]

Sellafield Limited upskill engineers with CSWIP certification

Q

Aregister now www.twi.co.uk

Are there any training courses for plastics welding for chemical tank fabricators?

How long does the coding for underwater welding last?

What are the benefits of ribbon bonding?

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November/December 2012

News in briefWelding sparks interest at WorldSkills UK

The Welding Institute would like to congratulate the competitors and winners at the recent WorldSkills UK event held at the NEC in Birmingham for their demonstrations of skill and high standards of achievement. The Welding Institute, in conjunction with the Skills Council for Science, Engineering and Manufacturing Technologies (SEMTA), sponsored three categories of event for young engineers and apprentices:

Construction Metal Works, SkillPIPE: Heating and Ventilation, plus a Welding demonstration competition.

TWI agrees research and training collaboration with GMRI, China TWI has signed a major research agreement with one of its Chinese Members (GMRI) to carry out R&D projects and technical training on structure integrity, materials performance testing and inspection. The three-year contract will see TWI provide R&D technical support and carry out training to GMRI on materials performance in different environment conditions, structural integrity of joints and non-destructive testing activities in support of the manufacture of components such as the pressure vessels, piping systems and offshore structures.

TWI Technology South East Asia (SEA) seal new partnership to expand their training services

TWI Technology (SEA) sealed a partnership with The Global Skills International Co.Ltd. in Myanmar by signing a memorandum of understanding in Kuala Lumpur. The Global Skills International Co. Ltd will now be able to deliver TWI’s training and certification programmes, effectively responding to customer demand.

For further information on TWI visit the website at www.twi.co.uk

An expansion in demand for plastics welder training and certification is being driven by global changes in legislation that require certification to guarantee the skill and knowledge of welders when installing plastic fabrications.

TWI’s plastics welder training and certification programme, which includes Hot Gas and Extrusion Welder, Lining Membrane Welder, and Butt Fusion, Electrofusion and Socket Fusion Welder training courses, is finding new markets in the Middle East, Malaysia Africa and the Asia-Pacific region. The first successes outside Europe for the plastics welder programme has been the training and qualification at CSWIP Entry Level Extrusion Welding of trainee welders in Nigeria, Uganda, New Zealand, Malaysia and New Caledonia.

For example, one of the companies involved in the training programme, Baywood Dextron Ventures, enlisted TWI to deliver training and certification for six of its staff at the Chevron site at Escravos in Nigeria for a large oil storage tank rehabilitation contract. The six trainees had little or no experience in the welding process and TWI instructors led the group through a series of practical and theory sessions at CSWIP Entry Level Extrusion Welding to great success. For more information or to discuss plastics welder training, email [email protected]

TWI’s bespoke plastics welder training courses go worldwide

Guided wave inspection using Teletest®

provides full volumetric coverage of tens of metres of pipe from a single test location and is widely used for pipeline inspection particularly for unpiggable sections of pipeline such as at cased road crossings. TWI has recently developed techniques to derive more quantitative information from the inspections including an estimate of the through wall extent of flaws. However, bends in the pipe distort the guided wave signals, making quantitative analysis problematic. TWI has researched methods for inspecting beyond pipe bends using a combination of finite element analysis (FEA) and practical trials to understand the behaviour of guided waves in a relatively tight pipe bend example.

Initially FEA was used so the mode conversion that occurs through the bend and the orientation of the wave modes after the bend were investigated and experimental validation carried out to confirm the findings. Building on the understanding of the guided wave behaviour gained in the first stage of the project, analytical methods were developed to predict the behaviour and be used as a tool to undo the signal distortion. The analytical distortion correction methods were verified against finite element solutions. The aim of the analytical method is to provide computationally fast techniques to give quantitative rather than qualitative measurements of flaws beyond bends in the field.

The finite element modelling showed that although the incident wave mode can be made to pass through a pipe bend, mode conversion occurs and the orientation of the wave modes are often altered. The

orientation of these modes can be used to determine the location of the flaw so this knowledge is an important step forward in ensuring inspection results from beyond a pipe bend are valid.

Next, an analytical distortion correction method was developed and tested against experimentally validated finite element modelling results. It was shown to work well; however, the distortion correction method works for one journey through the pipe bend whereas guided wave inspection is normally used in a pulse-echo configuration which means the waves will travel twice through the pipe bend. Therefore, a method was developed to alter the excitation algorithm to generate a non-standard mixture of wave modes. The mixture is selected so that, after propagating around the bend, the guided wave is the same as a conventional excitation propagating in a straight pipe. This was successfully demonstrated in a validated finite element model.

This combination of pre- and post-processing techniques has opened up the potential for more refined inspection beyond pipe bends. Compensating for the distortion of guided waves also allows flaw sizing techniques to be used beyond pipe bends.

Contact: [email protected]

Driving inspection around the bend

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Connect is the bi-monthly magazine of TWIEditor Candy SmelliePhotography Simon Condie Production Candy Smellie © Copyright TWI Ltd 2012

Articles may be reprinted with permission from TWI. Storage in electronic media is not permitted.

Articles in this publication are for information only. TWI does not accept responsibility for the consequences of actions taken by others after reading this information.

This publication is also available in alternative formats. Please contact [email protected] to request a copy.

Published by TWI Ltd, Granta Park, Great Abington, Cambridge CB21 6AL, UK Tel: +44(0)1223 899000 E-mail: [email protected] www.twi.co.uk

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Issue 181 November/December 2012