D S Howse and C H J Gerritsen
Paper presented at 2nd International Conference on Recent Developments and Future Trends in Welding Technology, Cranfield University, UK, 3-5 September 2003.
TWI has been processing materials with high power Nd:YAG lasers since 1997 and has a history of welding structures with lasers extending back some thirty years. Nd:YAG lasers delivered with optical fibres and robot arms have enormous potential for steel structures due to their ability to produce precise, low distortion welds in components very quickly. Components have been welded using this technology for the yellow goods, shipbuilding and pipeline industries. Work carried out at TWI has demonstrated that the laser process can be further enhanced by combining with MAG welding to deliver welds with suitable mechanical properties and good tolerance to variable fit-up. In addition,TWI has now further developed its facility by investing in a 7kW Yb fibre laser.
There are two main types of industrial laser used at high powers for deep penetration keyhole welding, CO 2 gas and Neodymium doped Yttrium, Aluminium Garnet (Nd:YAG) lasers. CO 2 lasers were the first to be developed capable of delivering higher powers needed to process thick structural steels (>6mm thickness) while Nd:YAG lasers lagged behind in terms of power.  However, since the mid 1990s continuous wave Nd:YAG lasers with powers above 2kW have been developed and by 1997, Nd:YAG lasers accounted for 80% of solid state lasers which in turn accounted for 32% of a $1.4 billion laser market.
TWI has been working with high power Nd:YAG lasers delivered by optical fibre since 1997. The facility at TWI consists of three high power Trumpf Nd:YAG lasers which process in three cells alongside the CO 2 lasers ( Figure 1). However, with multiple paths built into a single laser, they are not restricted to working in just one cell but can be switched within a matter of seconds to another safe working area using fibre optic cables of upto 50m in length.
Fig.1. Laser processing facility at TWI
Another feature of the processing facility at TWI is the beam combining unit. This allows the three high power lamp pumped Nd:YAG lasers (10kW power in total) to be optically combined, transmitted via a 1.0mm diameter optical fibre to the workpiece, and re-focused to deliver 9kW at the workpiece. This has enabled TWI to investigate processing up to 9kW in advance of commercial systems becoming available and many projects have been carried out which investigated processing at these high powers.
High power lamp pumped Nd:YAG lasers have been very successfully implemented for processing steel sheet, mainly for the automotive industry.  However, work has been carried out at TWI on a much wider cross section of materials, applications and industrial sectors developing procedures for large structures. This paper describes the use of high power Nd:YAG lasers for structural steel components in the yellow goods, oil and gas and shipbuilding industries, and it evaluates some of the lessons learnt and highlights areas for future development.
Characteristics of high power continuous wave Nd:YAG Lasers
High power continuous wave Nd:YAG lasers differ from CO 2 lasers in terms of processing steels in a number of ways. Firstly, the lasing medium produces light of a different wavelength. This affects the way the laser is transmitted to the workpiece. Nd:YAG lasers produce light of 1.06µm wavelength that can be transmitted to the workpiece by a fibre optic cable. For single fibre processing, the laser energy enters into the fibre (typically 0.6-l.0mm diameter) through a lens system, which focuses the energy on to the fibre end. For beam extraction from the fibre, the light is expanded using a recollimating lens to create a parallel beam that is then focused onto the workpiece using a focusing lens. This is a much more flexible system of beam delivery than for CO 2 lasers (10.6µm wavelength), which are transmitted to the workpiece by more cumbersome reflective or transmissive optical systems. Nd:YAG lasers are, therefore, much more attractive as robot compatible manufacturing tools.
The way the laser light is generated also affects both the efficiency of the process and the quality of the resulting beam. Nd:YAG lasers have a wallplug efficiency of approximately 3%, with most of the energy losses in the form of heat and it is therefore necessary to provide a chiller unit with the laser. The maximum power consumption for a 4kW Nd:YAG laser is 175 kW and the chiller will consume a further 167kW. This compares unfavourably with other laser power sources such as CO 2 and diode, which operate at higher efficiencies. The large heat losses also effectively limit the maximum power that can be achieved using lamp pumped Nd:YAG lasers at a beam quality suitable for welding and commercial systems are only offered up to around 4kW.
Beam quality provides a measure of how much the beam diverges and how easily it can be focused to a given spot size at a reasonable working distance. The measure of beam quality for a Nd:YAG laser is usually given as the term BQ or beam parameter product and is measured in mm.mrad. The smaller the value of BQ as the beam emerges from the laser, the smaller the fibre size that can be used to carry the laser light to the workpiece. The smaller the diameter of the fibre, the smaller the welding head and the easier a small focused spot size at a reasonable working distance is produced. As different terms are used to describe CO 2 and Nd:YAG lasers, it is difficult to achieve a direct comparison in terms of beam quality. However, comparing a Nd:YAG beam with BQ of 25mm.mrad and a CO 2 beam with M 2 value 3, focused through a lens with focal length of 127mm and assuming a raw beam diameter at the focusing optic of 22mm, the diameter of the focused spot for the Nd:YAG laser is 0.58mm. The diameter of the focused spot for the CO 2 laser is 0.23mm.  This is important in terms welding because of the resulting power density that can be achieved. Although the Nd:YAG laser produces a larger spot size, the example above gives a power density for 4kW at the workpiece of 1.5x10 6 W/cm. Although this is less than the calculated power density of a CO 2 laser of equivalent power for the example above (6.5x10 6 W/cm 2), it is still above the threshold for high speed laser keyhole welding in steel, ~10 6 W/cm 2. 
The wavelength of light is also important in that it will affect both the absorptivity of the light by the material being processed, the ability to focus the light to a given spot size and also the interaction with the vaporised plasma or plume during keyhole welding. [5,6] This latter point is particularly important for hybrid Nd:YAG laser/MAG welding as, unlike CO 2 lasers, argon based shielding can be easily used for both laser keyhole welding and MAG welding in the open arc metal transfer condition.
Welding of a complex geometry demonstrator component
The first example discussed in this paper is a complex geometry structural demonstration component ( Figure 2).  It was the intention of this work to demonstrate that a robot mounted Nd:YAG laser was capable of producing fully penetrating, autogenous welds on a large, complex structural component with minimal clamping and jigging. The objectives were to demonstrate the feasibility of using robot mounted lasers for manufacture rather than produce a test piece for procedure qualification.
Fig.2. Schematic of 2200x700x300mm steel demonstration component a) cover plates b) tubes c) curved rectangular plates
The component comprised the following parts:
- Two rectangular C-Mn steel plates of dimensions 2200x700x8mm thick. These were used as base and cover plate (a in Figure 2).
- Two C-Mn steel tubes (b) of height 284mm, internal diameter 305mm, each with a wall thickness of 6.35mm. Each tube had been EDM wire cut into two pieces in a saddle joint configuration, and was to be welded back together around the 360° circumference of the tube. The cutting procedure provided a seam that varied from the normal position to up to 60° to the tube surface. Thus the required penetration over the circumference of the tube varied between 6.4 and 7.3mm.
- Two laser cut, curved, rectangular C-Mn steel plates (c) of 445x284x12mm thick.
The welds to be made were of several types: butt welds in the PC (2G) position (saddle joints on tubes), T butt welds in the PB (2F) position (welding of tubes and curved plates to base plate) and stake welds in the PA (1G) position(welding through cover plate into tubes and curved plates). All welds were single-sided, except for the first of the curved plates, which was welded from both sides.
The welding head was mounted on a 6-axis Kawasaki Heavy Industries JS-30 floor mounted robot and 3kW of laser power was used for the butt and T butt welds. The stake welds through the cover plate were made by combining the output from the three separate lasers in the beam combining unit to achieve 8.9 kW at the workpiece. The parameters used to produce the welds are given in Table 1.
Table 1 Parameters used for the different welds in the demonstration component.
|Weld||Weld type||Weld position||Speed [m/min]||Focal position||Beam angle|
||At surface, on seam
||0° to joint faces; i.e. at constantly changing angle to horizontal
||At surface, ~0.5mm above seam
||18° to horizontal
||At surface, ~0.5mm above seam
||18~ to horizontal
||90° to horizontal
Cross sections of the welds and the final welded component are shown in Figures 3 to 5.
Fig.3. Cross section of double-sided T butt joint at non-optimised conditions, therefore showing only partial penetration. Scale in millimetres
This component demonstrated that autogenous Nd:YAG lasers could be used to fabricate large structural components with minimal clamping and fixturing in a variety of joint configurations and welding positions. The final part also demonstrated one of the major advantages of laser welding, low thermal distortion. The part was extremely flat after welding with no visible rippling or torsional distortion. Figure 4 also demonstrates that the laser process welding in the keyhole mode is tolerant to some linear misalignment of the joint. Also, autogenous fibre delivered laser welding has a very useful feature in that it can be axi-symmetric. This term is used to describe processes that are symmetrical around the tool centre point (e.g. the focal point of the laser beam) and therefore direction independent. As a result of this, the robot used to manipulate the processing head can choose different configurations to follow a specific weld path. Unfortunately this is not the case when peripheral equipment is used (e.g. wire feed nozzles, plasma control jets). The process then ceases to be axi-symmetric, and the tool centre point is no longer a point but a vector, which means the process is direction specific.
Fig.4. Cross section of saddle weld, showing high-low mismatch. Laser beam entry from the right. Scale in millimetres
Fig.5. Picture of finished 2200x700x300mm demonstration component
The work also highlighted some of the limitations of the use of lasers. The first was the very high accuracy required for positioning of the laser, particularly for butt joints. Also, so as not to impair the accessibility, clamping needed to be kept as close to the component as possible. Anything that sticks out increases the likelihood of a collision with the robot arm, or entanglement of the fibre. Similarly, the processing head should be kept as small as possible, which is particularly important for welding in corner areas. Modelling should be used to determine the best robot orientation and welding sequence. Although the fibre optic cable is flexible to some extent, it cannot be twisted or bent over too small a radius. It is generally preferable to pre-wind the fibre in such a manner that it unwinds during the welding sequence.
Welding of stiffened panels for shipbuilding
A further development was the manufacture of a stiffened panel demonstrator component typical of that used in the shipbuilding industry.  This was conceived to demonstrate the advantages of three dimensional laser processing using robot manipulated 4kW Nd:YAG lasers. Again, the focus was on the feasibility of producing large structural components using robot delivered Nd:YAG laser, rather than producing a fully qualified structure. The panel contained stiffener to stiffener and stiffener to baseplate joints. In addition the baseplate came in two halves that had to be butt welded together.Clamping and fixturing were basic, consisting primarily of plates, bars, and G-clamps to hold down the components during welding. A pneumatically operated bridging clamp was also used to ensure close fit-up of the stiffeners to the base plate during manufacture of the full size panel. However, attention was paid to producing welds which met geometrical tolerances and the structure was assessed in terms of the distortion produced. Initial parameter development preceded the fabrication of a 1mx1m test panel and the eventual production of a 4.8mx1.9m full size panel section. The material used during process development, and for the eventual construction of the stiffened panel conformed to BSEN 10025:1993 Grade 275JR. Three thicknesses of steel were used; the base plate (7.5mm thickness) and rolled bulb flats for the longitudinal and transverse stiffener components (8 and 6mm thickness respectively).
A diagram of half the component is shown in Figure 6. Again, the welding head was manipulated on floor mounted robots. The work was carried out in three stages:
- Initial parameter development using small T-joint test pieces.
- Production of 1m2 panels (equivalent to one 'cell' in the full size panel) for demonstration.
- Production of the full size 4.8m x 1.9m panel via two halves of 2.4m x 1.9m.
Fig.6. Testpieces used during procedure development (stages 1 and 2) prior to manufacture of full sized panel (stage 3)
Initial parameter development had produced good results at welding speeds of 0.7 m/min giving acceptable profile and penetration. However, when the 1m 2 panels were produced, problems with fit up were encountered. At processing speeds of 0.7m.min the laser process was not able to cope with the gaps up to 1.0mm seen in the structure. De-focusing the laser by 3mmabove the plate surface solved this but at the expense of processing speed, which had to be reduced to 0.3m/min ( Figure 7).
Fig.7. Weld bead appearance for welds made with 1.0mm joint gap, 0.3m/min welding travel; speed Fig.7a) 0mm focus position;
Fig.7b) +3mm focus position
Another issue identified during production of the 1m 2 panels was the programming time associated with robot manipulation of the laser beam. This was a significant factor in the total process time taken for the production of the test panel when compared with the actual welding time.
For the full sized stiffened panel, a number of steps were taken to improve both the ease of manufacture of the panel assembly and the resulting quality of the laser welds.
The factors responsible for causing variation in fit-up where addressed in turn. To solve the problem of waviness in the under-surface of the stiffeners, the bottoms of the stiffeners were milled flat. This was performed after all the stiffener-to-stiffener welds were completed, prior to welding of the stiffener sub-assembly onto the base plate. Any gaps between the vertical joints in adjacent stiffeners were bridged using shims. The corresponding baseplate surface was prevented from bowing by ensuring an adequate level of clamping was applied. Close fit-up between the stiffeners and the base plate was maintained during tack welding using the pneumatic bridging clamps. The adjoining faces of the baseplates to be butt-welded were also milled square to ensure good fit-up.
Both the tacking and main welding procedures were designed to minimise the level of distortion by careful control of the sequence to spread the heat input and resulting distortion evenly around the area of each panel half-section.
Each half of the panel followed an identical assembly route using the optimum parameters developed during the preceding trials. The stiffener sub-assembly was welded using a vertical up procedure and the curved bulb top sections of the longitudinal stiffeners were arc welded. The base plate and pre-welded/milled stiffener sub-assembly were clamped to a work bed. The stiffeners were tacked into position using laser welds ~40mm in length. Once all of the tack welds were complete, all of the stiffener-base plate joints were welded. Each internal cell was welded in one continuous movement of the robot with some overlap at the stop/start position. The two halves of the panel were then positioned and butt-welded in the PA position giving full penetration. Finally, the four sections of longitudinal stiffener were tacked then fully welded into place. On completion of welding, the panel showed minimal distortion. Figure 8 shows the completed panel.
Fig.8. Completed stiffened panel
Figure 9 shows the distortion of the part due to welding after the clamps were released. It can be seen that although the welding of the part resulted in some distortion, this was not excessive.
Fig.9. Completed panel showing distortion after welding: Fig.9a) view along the edge
Hybrid laser/arc girth welding of pipelines
For a number of years TWI has also been involved in initiatives looking at girth welding of pipelines with high power Nd:YAG lasers. Initially a review project was carried out by TWI for BP to identify potential options for reducing welding costs associated with land lay of pipelines.  This work identified improvements to existing arc welding technology and also highlighted the potential of high power Nd:YAG lasers in reducing costs of pipeline fabrication. Very early studies investigated autogenous laser welding in the 2G position ( Figure 10). Although these welds demonstrated that welding was possible in wall thicknesses up to 12.7mm, the welding speeds were still relatively slow. Higher speeds of around 1.0m/min needed for the process to maintain productivity were used but the process was not tolerant to variation in joint gap. In addition, the autogenous process produced welds with very low impact toughness. In order to solve the problems with poor tolerance to fit up at high speeds and the low toughness, procedures were developed using hybrid Nd:YAG laser/MAG welding. [10,11]
Fig.10. Macrosection of BOP weld made on X52 pipe steel, 324mm outside diameter and 12.7mm wall thickness. Power at the workpiece 8.2kW, travel speed 500mm/min
The proposed fabrication sequence being investigated in the work presently reported is for the MAG pass fill stations to be replaced by a hybrid Nd:YAG laser/MAG welding station. An example of this type of joint, but madeautogenously with 9kW of Nd:YAG laser power, is shown in Figure 11. Two separate laser power sources/hybrid welding heads or a single Nd:YAG laser system with the beam switched between two heads positioned on either side of the pipe, may potentially be used to produce this weld.
Fig.11. Section through pipe wall, showing internal MAG root run and 9.0kW autogenous laser fill made at 0.7m/min. Pipe grade API 1104 X70 14.3mm wall thickness (mm scale shown).
The laser apparatus used comprised two 3kW Trumpf HL3006D Nd:YAG lasers and one 4kW Trumpf HL4006D Nd:YAG laser combined the beam combining unit that delivered 9kW at the workpiece. The output from the lasers was transmitted intothe beam combining unit using a step index fibre optic of core diameter 0.6mm and length 30m. A single fibre optic of core diameter 1mm and length 15m transmitted the combined laser power from the beam combining unit to the laser output housing. A Lincoln Electric Powerwave 455 MAG power source was also used. A synergic pulsed welding programme appropriate for 1mm diameter wire and Ar-CO 2 shielding gas mixtures was selected for the welding trials.
A purpose built fixture was used to hold both the laser focusing head and MAG torch. The fixture permitted accurate and repeatable control of the relative position and orientation of the two heat sources. Figure 12 shows the laser output head and MAG torch attached to the hybrid fixture, with the laser co-axial gas nozzle and MAG shroud removed.
Fig.12. Laboratory demonstration of hybrid Nd:YAG laser/MAG welding for pipelines
Experiments carried out at 9kW highlighted a number of significant factors that can influence weld penetration and flaw levels. Firstly, the application of the hybrid process does not always guarantee an increase in weld penetration for bead-on-plate welds. Autogenous laser welds in the 1.0-1.2m/min speed range attained 7.5-8.8mm penetration with 8.9kW laser power. For the same speed range, the hybrid weld penetration depth ranged between 6.0-9.1mm. This suggests that at higher laser powers, the hybrid process can give both increased and decreased levels of weld penetration compared to the autogenous laser welding process, depending on the parameter/preparation combination applied.
Considering the problem of solidification defects, the results from the experimental work suggested that certain combinations of parameters (laser leading, and higher welding currents) produced sound welds. However, this was a consequence of their reduced level of penetration. The bead-in-groove trials confirmed that higher levels of penetration and/or depth-to-width ratios would increase susceptibility to solidification defect formation. As such, welding conditions that maximised penetration also gave the highest levels of solidification defect formation in the factorial experiment. Whilst the majority of welds below 7.5mm penetration depth showed no solidification defects, if the depth-to-width ratio was high enough, the defect might still occur in welds with penetration depths below this.
However, if the welding conditions used were carefully chosen to give fully penetrating welds, the weld was clear of solidification defects. It was evident that if full penetration is achieved, defect free welds above 8mmpenetration are possible ( Figure 13).
Fig.13. Macrograph of a hybrid Nd:YAG laser/MAG weld in X60 pipeline steel showing an acceptable weld with 11mm full penetration. Laser power at the workpiece 8.0kW, travel speed 1.0 m/min
In summary, the work has shown that it was possible to use high power Nd:YAG laser welding combined with the MAG process to produce deep penetration welding passes in commercially available pipeline steels that met the requirements of pipeline specifications such as BS 4515 and API 1104 in terms of acceptance criteria for imperfection limits. The welds also showed acceptable hardness values and good low temperature toughness ( Tables 2 and 3).
Table 2 Hardness values for a hybrid Nd:YAG laser/MAG weld in API 5L x60 pipeline steel.
|Weld Metal Hardness (HV10)||Heat Affected Zone Hardness (HV10)|
Table 3 Charpy impact results for a hybrid Nd:YAG laser/MAG weld in API 5L x60 pipeline steel tested at -10°C.
|Position||Charpy Impact Energy (J)|
|Weld metal centreline
||106, 130, 91 (average 109)
|Heat affected zone
||43, 140, 90 (average 91)
Although the economics of the process need to be further investigated against other competing arc technologies, it is considered that hybrid Nd:YAG laser/MAG welding potentially offers a viable solution for reducing the costsassociated with the welding of large diameter steel cross country pipelines.
Discussion - where next?
The three projects discussed here have shown the progression of fibre delivered lasers from processing and demonstrating the flexibility of fibre delivered lasers for single pass, low heat input processes, through to beginning toqualify welding procedures in engineering structures with equivalent mechanical properties to arc welded structures.
The projects have highlighted a number of issues with laser processing. The first of which is the tolerance of the process to joint fit up. Although high power Nd:YAG lasers produce larger focused spot sizes than for CO 2 lasers they still have a relatively poor tolerance to joint gap. This can be solved by de-focussing the laser at the point of impingement with the workpiece as was achieved for the stiffened panel, but thiscompromises the welding speed. One very successful method of overcoming this is by combining the laser with the MAG process. This uses the gap bridging ability of the arc to create a molten pool which the laser can focus on to maintaina keyhole. This was successfully used to produce welds with variable fit-up in pipeline steels at around 1.0 m/min. TWI is also involved in a project which led on from the autogenous laser welding work carried out on the stiffenedpanel, SHIPYAG. This is a collaboration with a number of partners including European shipbuilders, Fincantieri and Odense, and aims to deliver low cost, versatile, safe laser welding with fibre delivered Nd:YAG lasers. This also useshybrid laser/MAG processing to achieve high productivity welding in large steel structures with low distortion.
The combination of the laser and the arc welding process also gives other advantages. The addition of a molten filler wire allows oxygen additions to the molten pool which gives fine acicular ferrite microstructures with good lowtemperature toughness.  Another advantage of the combination of the processes is that the laser keyhole appears to stabilise the MAG arc during welding. This allows the arc process to produce stable metal transfer at much higher speeds than would beexpected without the laser to stabilise the arc.
There are however, some drawbacks to processing at very high speeds with hybrid Nd:YAG laser/MAG processes. The first is the propensity of the laser process to solidification cracking when welding structural steel grades.  This problem is made worse in partial penetration welds but can be overcome by either controlling the parent material composition or designing the joint so that fully penetrating welds are achieved. The other problem that occursis related to the high efficiency of the process in melting the steel to produce the weld. Although very high travel speeds can be achieved, the resultant heat input of the process is low. This has benefits in terms of reduceddistortion of the final component, but also leads to rapid cooling of the fusion zone and consequently high hardness. This can be overcome by restricting the composition of the parent steel for laser welding applications, but alsohighlights the need for hardness limits created for arc processes with variable hydrogen potential to be reviewed for more efficient laser processes.
Another issue that still needs to be resolved for the hybrid laser/MAG process is the reduction in processing time through the application of in-process monitoring and control. TWI currently has a number of initiatives in this arealooking at both seam tracking and adaptive control to produce good quality welds.
A final barrier to the full implementation of the process is the cost and lack of portability of the process. Although high power Nd:YAG lasers are relatively compact they are also inefficient processes, only converting around 3% ofthe input energy to produce the laser beam power. This is not a major issue for most manufacturing applications, but the use of lasers for welding cross country pipelines relies on the portability of the process. Although the Nd:YAGlaser process can be containerised, the low efficiency and high capital cost of the process make it very difficult to justify economically. One of the major advances in laser technology in recent times is the introduction of ytterbium(Yb) fibre lasers. The lasing medium for these lasers is contained within the fibre itself and individual units generating 2-300W can be combined to produce single lasers with up to 10kW power. They have a similar wavelength to Nd:YAGlasers and the laser light can be transmitted to the workpiece via a flexible optical fibre. Yb fibre lasers are approximately 25% wallplug efficient and are much more compact than Nd:YAG lasers. This makes them very attractive forapplications such as pipelay where they need to be portable. Although these lasers are just becoming commercially available, at powers in excess of those available for lamp pumped Nd:YAG lasers, there has been very little practicalwork carried out to investigate their suitability for keyhole welding and their performance. Also, although having enormous potential, they need to be thoroughly evaluated for reliable and economic industrial use. To this end, TWI hasrecently added a 7kW Yb fibre system to its laser processing facility. The laser is capable of delivering any power up to 7kW via a 0.3mm diameter, 20m length, optical fibre cable and will have the capability of producing a powerdensity of 2 x 10 6 W/cm 2 at powers equivalent to high power Nd:YAG lasers.
High power laser sources capable of delivering power to the workpiece via flexible optical fibres have found a natural niche in the processing of thin sheet for high volume manufacturing. Within these industries the advantages of ahighly controllable, precisely delivered heat source used to produce products quickly with little or no thermal distortion are obvious. TWI has taken the technology further by investigating the potential of the process for largerstructures. Also by combining the laser process with arc technology some of the pitfalls of using very precise heat sources can be avoided whilst maintaining or building upon the advantages of the laser process. The recent introductionof high power Yb fibre lasers to the materials processing market has also generated much excitement. They offer advantages not only for existing laser processing applications but for widening the scope of laser processing further byvirtue of their potential for portable remote working. The 7kW Yb fibre laser at TWI will be used to build upon the years of manufacturing experience with fibre delivered lasers to produce effective manufacturing solutions for industryworld-wide.
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