Construction Welding Processes for Infrastructure Projects

Trends in Welding Processes in Engineering Construction for Infrastructure Projects

R E Dolby
TWI Ltd, UK

Paper presented at the 56 ^th IIW Annual Assembly, 6 - 11 July 2003, Bucharest, Romania
'Trends in Welding Processes in Engineering Construction for infrastructure projects'

Abstract

The paper examines welding process trends in the construction of buildings, bridges, pipelines and other engineering items necessary for infrastructure in urban environments. Productivity gains in arc welding are reviewed, and the increasing use of laser cutting and welding, as well as laser-arc hybrid welding, particularly in Europe, is discussed and explained. Some friction technologies are already widely adopted, but they could make more impact with further development. Electron beam welding could also be used with economic benefit in some situations, but as with all non-arc processes, it will only be adopted if significant gains in productivity and cost reduction can be created to justify the more specialist welding equipment required. Many productivity improvements can come from non-welding activities in the fabrication process.

1. Introduction

The last 50 years have seen big improvements in productivity for most arc welding processes and arc welding still creates the bulk of welding output in engineering construction today. Increasingly, high power laser, hybridlaser-arc, electron beam (EB) and friction welding techniques are beginning to feature in the fabrication of large scale structures in several countries, and usage is expected to grow. For example, laser and laser-arc hybrid welding are now employed in several European ship panel lines whilst friction stud welding is routinely used for many building and bridge applications worldwide.

It will be some years before newly developed processes are used in bridge, pipeline, building and other public infrastructure constructions and they are only likely to displace arc welding in those applications requiring large assemblies with straight welds, where step improvements in productivity are achievable, where tonnage throughput can justify the initial capital outlay for specialised welding equipment and where distortion control is critical.

This paper reviews current trends in arc welding processes used for C-Mn steel construction in civil infrastructure projects and will discuss some of the newer processes which are beginning to replace arc welding for such steels, or have potential to do so in the next 10 years.

2. Arc welding

Significant improvements in productivity are nowadays hard to gain in this mature process. In gas-shielded MIG/MAG processes, the growth of tubular wire over solid wire is still slow despite the deposition and bead shape benefits ofthe former although tubular wire usage is higher in the USA and Japan compared to other parts of the world. The lower price of solid filler wire, particularly in Europe, is still a dominant factor that continues to favour their use. Self-shielded cored wires are still widely used in the USA but less so in Europe because of the fall-off in fabrication of large offshore installations. Care is always needed in selecting self-shielded wires if weld metal toughness isa critical design factor.

Small inverter controlled electronic power sources are standard today, and gas mixtures are chosen on the basis of price and application. In Europe, argon and helium are cheaper compared to prices in, for example, Japan. European industry uses Ar-CO ₂ mixtures a great deal for MAG welding, while Japan usually employs 100% CO ₂ and has developed more sophisticated power sources to cope with the tendency for greater spatter generation.

Tandem wire MAG welding has become more popular over the last five years and by using a special torch feeding two wires with separate power sources, deposition rates can be more than doubled compared to single wire MIG/MAG, (e.g.15Kg/hour can be achieved at travel speeds of 5m/min). The process ( Fig.1) seems quite suited to long fillet welds where high travel speeds can bring economic benefits. Of course narrow gap techniques offer an additional route for improving productivity in thicker plate but high capital costs remains an obstacle to widespread use.

The tandem wire MAG process is also being investigated for pipeline girth welding with multihead systems under study. ^[1]

The greatest potential for productivity improvements comes from using robots in construction. Japan still leads the work in new robot installations per annum, but generally robot take-up for welding is slow in building and bridge construction because manipulation and good fit-up are difficult on large structures, which may comprise one-off designs. Only in situations where product repetition and/or tonnage throughput are high, combined with relatively standardised designs, are robots being employed. ^[2] Robot profile cutting is increasing in usage and in Japan, some building frames are now being welded on robot lines and agile robots are also used for site welding building column joints at height. The applications most suited to robot use involve butt and fillet welds in the fabrication of beams, welding of stiffeners to beams, welding of end plates and the assembly of panels or decking.

A likely trend is to 'take robots to the work' and use them for large part and sub-assembly manufacture in cells which house autonomous vehicular robots. ^[3] These robots, mounted on moving platforms are manoeuvered around the parts to be welded and are capable of all-positional welding using the MAG process with tubular wires,( Fig.2). The part positions are accurately determined within the cell using cell roof sensors and the robot can then traverse the cell, locate the parts and then the joint to be welded using local sensors. Welding under sensor control then follows. This arrangement is particularly suited to large numbers of parts that are of a modular shape but where dimensions can vary, such as in earth moving equipment, or bridge modules.

Associated with the growing use of robots, there is a trend to use 3D CAD/CAM systems that also generate CNC data and robot motion paths thus creating direct links between design and manufacture.

In submerged arc welding, multi-wire, iron powder additions and narrow gaps are common ways to increase productivity. However, the use of tubular wires is increasing in popularity and can offer better weld quality compared to solid wires in some situations, e.g. when submerged arc welding of primed plate. In building and bridge construction MIG/MAG welding is more suited to automation and is now beginning to replace submerged arc welding, reducing costs and distortion significantly.

3. Lasers

Low power CO ₂ lasers were originally developed in the late 60s for cutting and welding operations, while Nd:YAG lasers entered the market only in the early 1980s. In the shipbuilding and off-road vehicle sectors, higher power lasers (3-10kW CO ₂ and 2-4kW Nd:YAG) have become popular in the last decade because their use brings more precision in dimensional control, and greater accuracy in assembly, as well as the additional advantages of high cutting and welding speeds, and cleaner working.

Laser cutting is now an essential route to accurate assembly in steel fabrication. Parts can be cut to a precision of ±0.3mm in 10 metres compared to ±1-2mm for conventional plasma cutting. Sub assemblies can then be arc welded to tolerances of a few millimetres in 15 metres, with the same lasers being used for marking and hole drilling on the cutting table. Even though arc welding is still regularly used for assembly of laser cut parts, cost reductions can be dramatic because of the improved accuracy of assembly and the potential for eliminating distortion correction and other processing operations downstream from welding. Direct costs have been cut by up to 50% using lasers for cutting in many shipyards, and this shows the extent of benefit which could follow from more widespread use of laser cutting in building, bridge and public infrastructure construction.

Laser welding following laser cutting can bring further benefits. One new opportunity in the construction industry for which laser welding is highly attractive is the manufacture of steel sandwich panels which can give a 10-foldincrease in stiffness and 50% reduction in weight compared to solid panels ( Fig.3). Again, the shipbuilding industry has shown the way and one yard is routinely using sandwich panels for decking and bulkheads in passenger shipping. ^[4]

The success of an installed laser welding system in large-scale assembly depends to a degree on the plate thicknesses involved, and on the variability of fit-up in the joints. With thicknesses of 10mm or more, joint gaps may be variable and not easily controlled by clamping. This problem has been partly solved by the development of hybrid laser-arc systems (discussed in the next section) which also confer additional advantages.

4. Laser-arc hybrid welding

There is tremendous interest in the use of CO ₂ laser-arc or Nd:YAG laser-arc for steel construction at the present time. The concept has been around since 1978 and was actively developed in Aachen in the late 90s using high power CO ₂ lasers in combination with one or two MAG torches directed into the same weld pool in series with the laser. ^[5] Already one shipyard in Germany has such a system in production on a panel line, the process replacing submerged arc welding. ^[4] The advantages of moving from simple arc processes to a hybrid laser-arc system include higher joint completion rates and associated control of distortion. As noted above, laser systems alone are usually not sufficient to cope with variations in joint gap in large structures. However, where plates are thin, it is possible to produce stiffened panels successfully by laser welding. For example, Blohm and Voss has a panel line which uses two 12 kW CO ₂ lasers, producing panels with T stiffeners with no filler wire in plate up to 10mm thickness. Complete ship sections are being manufactured in this way, using both laser cutting and laser welding.

Considerable effort is now being put into gas transmission land pipeline cost reduction by increasing the speed of girth welding and reducing the number of welders and welding stations. Lasers and hybrid systems offer a solution to this challenge and girth-weld procedures are under development using either a high power CO ₂ lasers alone ^[6] or Nd:YAG laser/MAG hybrid welding. ^[7] Advantages of the latter approach are that Nd:YAG lasers offer fibre delivery and are more easily containerised for on-site use than CO ₂ lasers. Further, the hybrid approach offers greater tolerance to fit-up and the increased heat-input gives higher travel speeds or deeper penetration, There is also an opportunity to modify and control weld defects and microstructure by using appropriate filler wires.

The equipment being developed is shown in Fig.4(a) and a typical macro section in Fig.4(b). To date, successful welds have been made which meet the requirements of pipeline specifications such as API 1104 in terms of criteria for imperfection limits, hardness and low temperature toughness.

Whilst the above examples relate to the shipbuilding and pipeline industries, applications in the bridge and building sectors and engineering construction in general are certain to follow.

5. Friction technologies

There have been many exciting developments in friction welding and probably more process innovation in the last decade than in the previous 40 years when friction welding first came on the scene. In the context of metal joining and construction, friction stud, radial friction and friction stir welding are good examples of more recent approaches, with friction stud already being used in the production of sandwich structures comprising steel skins and a concrete core ^[8] and for welding attachments in high volume such as shear connectors and reinforcing bars to end plates. Attachment of anodes under water to offshore installations using steel studs ^[9] and electrical connections to railway lines are other applications of friction stud welding.

Although many organisations have explored rotary friction welding and radial friction welding for making pipeline girth welds in C-Mn, duplex stainless steels and titanium alloys, no company has progressed beyond the machine development stage. The latest publicised approach is developing radial welding for pipe up to 300mm diameter for offshore use. ^[10]

The newest technique, friction stir welding should, in principle, be ideal for applications in building and bridge construction, where long weld lengths are common. However, with current tool technology, it is only feasible to weld around a metre length of common structural steels. Better tools and tool materials are needed before the amazing success of the process in aluminium construction is matched in steel joining applications. However, of note is a recent FSW application connected to infrastructure development involving the joining of Al tubes for containing and protecting high voltage power lines.

6. Electron beam technology

There have been several key improvements in this process in the last decade which have made EB welding increasingly attractive for large-scale fabrication of steel structures such as buildings and bridges. The two main areas of improvement are in gun development and reduced pressure technology. ^[11]

In conventional EB technology, triode guns with directly heated cathodes are prone to short filament lives, beam voltage and current ripple, and a tendency to gun discharges that can create sporadic but serious weld defects. These problems have been solved at TWI by the development of special indirectly heated cathodes, the use of RF excitation for cathode heating (simplifying the cabling), and the use of a diode rather than triode gun design. A switch mode power supply coupled with these changes in gun design have resulted in small, more compact and much more reliable guns, giving enhanced beam quality, Fig.5(a).

Reduced pressure EB welding using chamber pressures of around 1mbar has been another major step forward in simplifying EB welding operations. The beam shape and beam penetration at, say 5mbar, is identical to that at the more conventional pressure for EB welding of 5 x 10 ^-3 mbar. This reduced pressure option does away with the need for large vacuum chambers and worries about leaks and seals. Simple mechanical pumps and local seals are sufficient to achieve ~1 mbar. These systems are also more tolerant to fluctuations in working vacuum pressure, gun to work distance and workpiece cleanliness.

TWI has developed a 200kV 100kW system which can operate over the full range of vacuum, namely, from 1000 mbar to 0.01 mbar. Reduced pressure applications using ~1mbar are being developed for pipeline girth welding, Fig.5(b) and for manufacturing large copper containers for nuclear waste storage. Whilst both applications are not yet using these improved EB technologies in production, the developments are at an advanced stage. The reduced pressure process seems well suited to long lengths of one-pass welding in structural steel for thicknesses up to 100mm, but is obviously restricted to shop welding or to controlled environment cells on site.

It should also be noted that high power in-vacuum EB girth welding of pipelines has been under development in Japan for several years. This system comprises an internal orbital gun with the pipes able to act as the vacuum chamber through the use of temporary internal seals. ^[12]

7. Concluding remarks

Where will engineering construction be in five years time? Productivity improvements can come from many different areas in the total fabrication process, e.g. general design, detailed design, contract drawings, CNC instructions, cutting and profiling, welding, inspection, finishing etc. It needs to be realised that optimising the welding process and procedure in isolation may have only a small effect on overall productivity. As an example, fabricators in the bridge and building sector continue to make big improvements in reducing the cost per tonne of steel fabricated by focussing on efficiency improvements in non-welding operations.

These non-welding areas will continue to receive much attention in the next few years. For example, 3D solid modelling is expected to be at the heart of the improvement process enabling virtual assembly, direct instruction of cutting and welding machines, computer modelling of metal processing and welding operations and optimising of welding sequences. In addition automated inspection and data gathering will increase in usage to give more comprehensive Quality Assurance.

Productivity improvements could also come from greater standardisation in steel specifications and in joint details and from modularisation of large structures to reduce the time spent on site fabrication and erection.

Use of new welding processes and an increased use of automation and robots will take place slowly and gradually, only being justified where the introduction creates significant gains in productivity or cost reduction. As such, these changes can be expected particularly in situations where skilled labour is short, where welding cells can be kept fully occupied or where customised, made to order components are needed.

8. Acknowledgements

The author thanks Mr J Weston, Dr G S Booth, Dr R L Jones and Dr P L Threadgill for their helpful comments on the draft of this paper.

9. References

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