Laser and Hybrid Laser-MAG Welding of Steel Structures

Laser & Sheet Processes Group, TWI Ltd

Paper presented at 2007 International Forum on Welding Technologies in Shipping Industry held in Shanghai on 16-18 June 2007.

Abstract

The recent development in laser technologies and applications of laser welding and hybrid laser-MAG welding in shipbuilding are discussed. The characteristics of laser welding and hybrid laser-MAG welding for welding stiffened structures were briefly compared. Research and development work carried out for ship building applications at TWI was briefly introduced. These work demonstrated that the fibre delivered Nd:YAG lasers have great potential for welding stiffened structures for shipbuilding industry. Hybrid laser-MAG welding has advantages over autogenous laser welding for welding stiffened steel structures in terms of the preferred weld profile, high productivity and great tolerance to gaps. In process monitoring sensors could help to detect factors inducing weld imperfections during welding.

1. Introduction

Industrial lasers have been used for a wide range of technical applications, ranging from drilling and cutting to heat treatment and welding. The main benefit of keyhole laser welding in comparison to conventional (arc) welding processes is that the deep penetration keyhole welding mode allows welds with very large depth-to-width ratios to be made. This consequently leads to narrow joints made with very little heat input, few weld passes and, as a result, little distortion. Thus the main justification for changing from conventional arc welding used traditionally for shipbuilding applications, mainly submerged arc welding (SAW) and metal active gas welding (MAG), is an expected increase in travel speed and/or productivity and, most importantly, a reduction in thermal distortion. Since rectification of distortion has been reported to cost up to 30% of the overall man-hours for manufacture of steel (sub) assemblies ^[1] , for some applications the reduction in re-work alone can justify the high investment cost of a laser welding system. Laser welding in the shipbuilding industries commenced in the late 1980s, for welding steel in thickness upto 20mm using mainly CO ₂ lasers. ^[2-3]

In the past few years, main shipyards in Europe have replaced some of their traditional arc welding panel production lines by lines employing CO ₂ lasers for full-scale production (Meyer Werft ^[4,5] and Blohm+Voss ^[6,7] , both in Germany). In addition to these two yards, laser welding is also in use or under investigation at Fincantieri in Italy for butt welding of panels ^[8,9] and Odense Steel Shipyard in Denmark for welding of subassemblies. ^[10] Outside Europe, Bender Shipbuilding and Repair in the USA is also active in this field. ^[11]

2. Development of laser technologies

Initially, only CO ₂ laser sources could provide the power levels required (several kilowatts) for welding of stiffened panels for shipbuilding. Nowadays, though, solid-state Nd:YAG lasers are commercially available at power levels typically up to 6kW, making these laser sources also suitable for thick section welding. Although the cost per kilowatt for these solid-state lasers is higher than for CO ₂ lasers, and they are not readily available up to the same power levels (commercially available CO ₂ lasers typically up to 20kW), they do have one important advantage over CO ₂ lasers. Whereas the wavelength of CO ₂ -generated laser light (10.6µm) can only be realistically guided and directed using mirrors, the laser beam from an Nd:YAG laser (1.06µm wavelength) can easily be guided using a flexible, optical fibre, making robotic manipulation of the welding torch far more straightforward. Lamp pumped Nd:YAG lasers have established themselves as reliable processing tools capable of delivering a precise heat source in high volume, flexible manufacturing environments such as the automotive industry. Over recent years, developments in Nd:YAG laser technology have resulted in higher power systems, up to 6kW, becoming commercially available.

Although Nd:YAG lasers compare favourably to CO ₂ lasers in terms of reliability and ease of processing, they have a significant drawback for some manufacturing applications in that although they are relatively compact they are inefficient, only converting around 3% of the input energy to produce the laser beam power. Although the Nd:YAG laser process can be containerised, the low efficiency and high capital cost of the process make it difficult to justify economically. One of the major advances in laser technology in recent years is the introduction of ytterbium (Yb) fibre lasers. The lasing medium for these lasers is contained within the fibre itself and individual units generating 200-400W or more can be combined to produce single lasers with up to 17kW power and beyond. Alternatively, higher power single mode fibre lasers are also commercially available at powers up to 1kW. These lasers have a similar wavelength to Nd:YAG lasers and the laserlight can be transmitted to the workpiece via a flexible optical fibre. Yb fibre lasers have approximately 20% wallplug efficient and are much more compact than Nd:YAG lasers. The material processing industry has shown particular interest in this new laser technology as an addition to, or a possible replacement for, the more conventional CO ₂ and Nd:YAG lasers currently used. Fibre laser technology seems to allow, for the first time, the manufacture of easily scalable lasers, in a compact form, with no obvious limit to the output power. Since the first industrial high power fibre laser was delivered a few years ago, the output power of fibre laser has far exceeded that achievable for commercially available Nd:YAG lasers, whilst also offering a better beam quality that even exceeds those of CO ₂ lasers in certain cases. Fibre lasers will create more potential applications for lasers with their benefits of compact design, high energy efficiency and high beam quality apart from the fibre delivery capability. The high output power and high beam quality compared with Nd:YAG lasers mean that the laser beam can be focussed to a small diameter spot and welding can be carried out at higher speeds. For the same spot diameter, laser welding can be carried out at a longer stand-off distance, reducing thermal effects of welding fumes on the optics.

3. Laser welding and hybrid laser-arc welding for shipbuilding

For all autogenous keyhole laser welding, a very accurate joint fit-up is required, which is linked to the fact that a very small focused laser spot is used, producing very narrow welds. For autogenous keyhole welding, the maximum joint gap that can be tolerated is smaller compared with arc welding, as shown in Fig.1, and is normally in the region of 10% of the thickness of the material to be welded, with an upper limit of a couple of tenths of millimetres. The resulting level of accuracy required in part fit-up may be achievable for small components, for which machining of the joint surfaces is practicable, but it is impractical for hull welding applications in shipbuilding, where the components - and therefore welds - are often up to 20m in length.

Fig.1. Cross sections of autogenous CO ₂ laser welds in 8.0mm C-Mn steel plate produced with 4.0kW laser power and 1.2m/min travel speed:
a) 0mm joint gap

b) 0.2mm joint gap

The tolerance to fit-up inaccuracy of keyhole laser welding can be extended to perhaps 0.5-1mm or greater using laser welding with the addition of filler wire, although a large part of the laser energy is then used to melt the filler wire. This reduces the process efficiency and consequently causes a reduction in penetration and/or travel speed ( Fig.2a), and potentially lack of fusion ( Fig.2b).

To increase the gap bridging capability of laser welding, different techniques have been investigated over the years. A development originally proposed in the late 1970s ^[12,13] , but more recently of renewed interest both for thick and thin section applications, is the use of laser-arc hybrid welding. In this process, a laser beam and an electric arc process impinge simultaneously on one common weldpool. Typical process combinations are tungsten inert gas welding (TIG), plasma arc welding (PAW) and, most commonly used, metal inert or active gas welding (MIG/MAG) in conjunction with either CO ₂ or Nd:YAG or fibre laser keyhole welding. When welding thick parts and/or in the presence of a gap, hybrid laser using the MAG (GMAW) process is most advantageous because it is integrated in the process and it offers easy addition of filler material. In heavy section laser-MAG (GMAW) hybrid welding penetration is determined by the laser alone, but due to the arc the welding speed may be maintained at a high level even in the case of a relatively large gap. Thus an increased ability to bridge a gap as well as a significant increase in speed may be taken advantage of by using the hybrid process as compared to a laser alone welding process, and surprisingly the heat input per unit length (and therefore also the distortion) is not increased significantly. A cross section of an arc-laser hybrid weld in 8mm structural steel is shown in Fig.3. Hybrid-MAG laser welding was shown tolerate joint gaps up to 1mm in this case without jeopardizing the weld quality.

Fig.2. Sections of square edge butt joint welds in 8.0mm C-Mn steel produced with 4.0kW laser power, 1.5mm joint gap and 4.2m/min wire feeding rate:

a) Travel speed 0.6m/min

b) Travel speed 0.4m/min

Fig.3. Hybrid laser/MAG welds in 8.0mm C-Mn steel in a square edge butt joint with 1.0mm constant gap, produced with 4.0kW laser power, -2.0mm focus, MAG pulling 1.5mm separation, 4.0kW MAG power, 9.0m/min wire feeding rate and a travel speed of 1.0m/min

Some of the other advantages that have been claimed for laser-arc hybrid welding over autogenous laser welding are ^[14] :

• Lower capital investment, because part of the 'expensive' laser power can be substituted by 'cheaper' arc welding power. The arc energy can then be used to melt the filler metal and bridge any joint gap, whereas the laser provides the penetration.
• Sometimes even less heat input and distortion than with a laser-only keyhole weld can be achieved (mainly for thin sections).
• An improvement in joint quality (for example in terms of porosity), because of the wider, more slowly solidifying weld pool may be realised.
• In the case of filler metal addition, the chemical composition of the weld can be tailored to requirements.

However, the hybrid process set-up includes the parameters of the separate processes, plus those that result from the combination of the processes (e.g. relative orientation of the laser and arc process torches), making it a more complex process to set-up and develop. The process generally becomes non-axi-symmetric when compared to autogenous laser welding, as the two processes are generally not orientated co-axially (although some co-axial systems are available). This may necessitate an extra axis for welding of non-linear seams to maintain proper orientation of the process head with regard to the joint line. In addition, the hybrid process generally produces a weld profile with a wide top bead and narrow laser root (not unlike the nail-head or wineglass profile), as the weld width cannot be maintained through the full thickness of the material. This means there is still a risk of lack of fusion, and it may increase distortion due to the 'heavy' weld cap.

In shipbuilding, the use of laser-arc hybrid welding rather than autogenous laser welding is of interest, mainly because it is able to tolerate the larger fit-up tolerances in shipyards (already demonstrated at Meyerwerft in Germany ^[5] and Odense Steel Shipyard in Denmark ^[10] with hybrid CO ₂ laser MIG/MAG welding). With CO ₂ laser and hybrid CO ₂ laser MIG/MAG welding now proven for shipbuilding applications, the interest is moving on to hybrid Nd:YAG or fibre laser MIG/MAG welding, because of the benefits the flexible fibre beam delivery can give interms of the ease of welding head manipulation. This would better facilitate welding of non-linear welds and welding inside confined spaces.

4. Laser welding and hybrid laser-MAG welding for shipbuilding structures at TWI

4.1. Autogenous laser welding of stiffened panels for shipbuilding

The objective of this work was to manufacture 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. 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 base plate joints. In addition the base plate 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 BS EN 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 Fig.4. 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 1m ² 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.4. Schematic illustration of the stiffened panel

 

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.

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 were 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 base plate 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 base plates 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 5 shows the completed panel. Visual inspection indicated that although the welding of the part resulted in some distortion, this was not excessive.

 

Fig.5. A laser welded stiffened panel

 

4.2. Hybrid Nd:YAG laser-MAG welding of stiffened structures

4.2.1. Procedure development for hybrid MAG welding of stiffened structures

As hybrid laser-MAG welding often needs to weld into and out of corners in stiffened structures (T-joints), the equipment set-up needs to be as compact as possible. Work has been carried out at TWI to develop the optimised set-up for hybrid laser-MAG welding of T-joints. The two torches were placed above one-another in a plane normal to the joint line (see Fig.6).

Fig.6. Hybrid bracket showing MAG torch on top and laser optics and viewing camera underneath

The main parameters considered during procedure development were:

• Angle of laser beam and MAG torch to the vertical (work angle).
• Longitudinal separation between the processes, i.e. separation along the joint line. A positive longitudinal separation was defined as the laser leading.
• Transverse separation between the processes, i.e. separation transverse to the joint line (a vertical separation in the case of T joints).
• MAG contact tip to workpiece distance (electrode extension).
• Laser focus position. A positive focus position was defined as the focus position above the workpiece surface.

4.2.2. Partially penetrated hybrid laser-MAG welding

The work within this project was to develop hybrid laser-MAG welding procedures for specific shipbuilding components (T joints in all cases). At the outset, boundary conditions were specified, based on economic justification, the equipment available, the need for classification society approval and the requirements for shipyard implementation. The main boundary conditions specified were:

• Materials:

4mm and 5mm thickness AH355 C-Mn steel.

• Travel speed:

1.5m/min minimum.

• Laser power::

4kW maximum at workpiece.

• Throat size:

3.5mm minimum.

• Joint gaps:

up to 0.5mm.

As reduction of distortion is generally one of the main reasons for moving to laser or laser-arc hybrid welding, the throat size was re-defined specifically for the project. Unlike traditional arc welding, where generally only the external fillet size is considered, the fused laser penetration was included in the calculation of the effective dimensions of the weld, allowing the fillet size to be reduced. Fig.7 illustrates the definition of throat size used. It was measured as the shortest distance from the deepest point of fusion between the two plates to the straight line connecting the weld toes of the fillet (or the tangent of the weld face in the case of a concave fillet). A disadvantage of including the fused laser penetration in the effective weld dimensions of the T joints was that the throat size could only be determined by destructive transverse cross-section sampling, since partial penetration welds were made.

Fig.7. Illustration of the definition used for the throat size (arrow)

Procedure development was then undertaken with the laser focus position, the travel speed and the wire feed rate as principal parameters. Apart from achieving the required throat size of 3.5mm at a high travel speed, the procedure development had two specific aims with regard to weld shape and dimensions:

• Minimisation of the external fillet size. Since the fillet is the main cause of angular distortion in a T joint as it is located furthest from the neutral axis it was aimed to minimise the size of this fillet, leading more to a T butt weld.
• Maximisation of the weld root width. To improve the tolerance to laser beam-to-joint alignment, it was attempted to create as wide a weld root as practical.

The second of these aims, maximising the weld root width, is a direct result of the fact that the laser beam had to be at an angle to the joint plane. Thus, the wider the weld, the more of the joint line could be fused, maximising the throat size for the same level of penetration.

The developed welding procedures were tested and, where required, further adjusted in terms of wire feed rate and travel speed for welding over samples with joint gaps. The joint gaps were introduced using shim material to keep the stiffener and flange plate apart. The shims were generally located at the start and end of the weld and held in place by a tack weld. Both a continuously increasing joint gap and a constant joint gap were investigated.

Initially, procedures were developed that produced welds with a smooth, mitre-shaped fillet, with a laser root extending from the fillet, as can be seen in Fig.8. A reduction in the fillet size, with the aim of reducing distortion, was then achieved by reducing the amount of filler wire per unit weld length. The filler wire feed rate was reduced from 11m/min ( Fig.8) to 6.5m/min ( Fig.9), reducing the arc power from ~4.1kW to ~2.7kW. Simultaneously, the focus position was changed from 3mm (below surface) to +3mm (above surface), in order to widen the weld root width, and the longitudinal process separation from -3mm (laser trailing) to 0mm. The welding speed was kept at 1.5m/min.

Fig.8. Transverse cross-section of laser hybrid weld showing large external fillet and narrow laser root. Scale in millimetres

 

Fig.9. Transverse cross-section of laser-MAG hybrid weld showing reduced fillet and wider laser root. Scale in millimetres

 

To further increase the width of the weld root, the focus position was moved further above the surface to +6mm. To maximise the laser penetration, the work angle (angle from vertical) of the laser beam was further increased from70°, to 73°, which meant a reduction in the angle of the beam with the joint plane from 20° to 17°, the minimum attainable with the equipment available for this project. At a lower angle, the laser focusing optics collided with the base plate. With this increased work angle, the welding procedures were re-optimised, simultaneously increasing the wire feed rate again to 8m/min. The main other change was the use of continuous, rather than pulsed welding current, as it was felt this would improve repeatability in the shipyards.

4.2.3. Fully penetrated hybrid laser-MAG welding

This work was again carried out as part of a European project to develop procedures for achieving fully penetrating and visually acceptable welds on 6mm to 8mm T-joints and 8mm to 12mm T-joints, in S355J2G3 (EN10025) C-Mn steel using single pass hybrid laser-MAG laser welding with 4kW Nd:YAG laser.

Fig.10. Cross sections of welds produced using laser welding and hybrid laser-MAG welding. The laser power used was 4kW:

a) autogenous weld on a 6-to-8mm T-joint produced at 0.8m/min;

b) hybrid laser-MAG weld on a 6-to 8mm T-joint produced at 0.75m/min;

c) autogenous weld on an 8 to 12mm T-joint produced at 0.3m/min; 

d) hybrid laser-MAG weld on an 8-to-12mm T-joint produced at 0.28m/min.

Initially, welding trials were carried out to develop conditions for achieving full penetration. The initial conditions were then optimised on zero gap T-joints in terms of weld visual appearance, weld profile and porosity. It was noticed that fully penetrating T-fillet welds in thick section steels could be achieved at similar travel speeds to those obtained for autogenous laser welding. However, the hybrid laser-MAG welding produced better weld profiles than the laser alone process ( Fig.10). The hybrid Nd:YAG laser-MAG welding process also had a much larger tolerance to variations in joint conditions. The hybrid process could tolerate variable joint gaps up to 1.2mm.

4.2.4. In-process monitoring of hybrid laser-MAG welding

Notwithstanding the capabilities of both the laser and hybrid laser-arc processes, imperfections can still occur during welding and a current primary interest in industrial applications is the detection of imperfections using real-time monitoring methods. ^[15] This work was conducted to assess the performance of commercially available photo-diode sensors for detecting weld imperfections occurred during hybrid laser-MAG welding.

Initially weld imperfections, likely to occur during laser welding of stiffened structures, were selected in terms of their frequency of occurrence, effects on joint performance and ease of detection. The selected weld imperfections, such as lack of penetration, lack of fusion, excessive spatter and incorrect weld toe, were artificially engineered into the weld by varying process parameters or introducing changes in joint fit-up during hybridlaser-MAG welding. Three laser process monitoring sensors (monitoring at the infrared, ultraviolet and Nd:YAG laser wavelengths), were used to monitor the weld pool temperature, plume radiations and reflected laser light to infer the weld quality during laser-MAG hybrid laser welding ( Fig.11).

Fig.11. Set-up of laser welding with in - process monitoring

a) Schematic illustration 

b) Experimental set-up

The responses of the sensor were measured during the production of a series of imperfection free 'reference welds' for the hybrid laser welding process and were compared to responses from the detectors to the simulated weld imperfections.

It was noticed that there were clear correlations between the output signals from the sensors and variations in joint fit up and process parameter changes that affected weld quality when hybrid laser welding, as shown in Fig.12. For this particular joint/material combination, it was possible to detect various factors causing weld imperfections during hybrid welding. The temperature and plasma sensors were able to detect changes in arc parameters (current, voltage and shielding gas flow rate) causing excess weld metal, excessive weld spatter and incorrect weld toe, changes in beam to joint alignment and variation in beam axial focus position causing undercut and incomplete penetration. The plasma sensor also showed responses to insufficient gas shielding, causing porosity in the weld. Both sensors exhibited the largest response to variations in arc parameters. These variations caused excess weld metal and spatter. The reflectivity sensor showed significant response to changes in laser focus, resulting in incomplete penetration.

Fig.12. Sensor response from a hybrid weld made at the reference conditions but with two small pieces of silver solder introduced into the joint line. 8mm thickness stiffener plate and 12mm thickness base plate:

a) Weld top bead

b) Temperatures sensor response

 c) Reflectivity and plasma sensor response

The monitoring system evaluated can be used to assess welds in production in an automatic way which would register an alarm if any particular sensor detected a signal which was outside a certain (user-set) tolerance of the reference signal (the reference weld), although this capability was not tested in the these trials. In production, safe working tolerance limits would be set up by producing welds with well established procedures and conditions which would determine the limit of acceptable weld quality. The data achieved in this work can be used as a database for the response of the sensors to various imperfections. When a received signal exceeds the pre-set limits, a failure alarm is displayed. In this way, the response of all welds can be recorded for QA purposes and any weld found to contain imperfections, can be isolated and examined. Welds with imperfections can be identified at the earliest stage so that remedial action can be taken promptly. It should be noted that as the response of the detectors is similar for a range of imperfections, analysis of the recorded signals is required for the operator to predict the imperfection detected.

5. Concluding remarks

Laser and hybrid laser-arc welding have been introduced into major shipyards in Europe. Most of these welding systems, to date, are based on CO ₂ lasers. The set-up is suitable for long and linear welds like those typically welded on panel lines. Work at TWI has demonstrated that the flexible and fibre delivered solid state lasers have clear advantages over CO ₂ lasers for more complex structures for shipbuilding application. The hybrid laser-arc welding process, combining the advantages of laser welding and arc welding, clearly exhibited advantages over laser welding in terms of weld profile control and gap bridging capability. The hybrid process could cope with fit-up tolerances typically to be found in shipbuilding applications and reduce the cost for the edge preparation fit-up control required for autogenous laser welding. The photo diode sensor could detect variations in joint fit-up and process parameters causing weld imperfections during hybrid laser-MAG welding. With the development of new high power laser sources and increased use of advanced processes such as hybrid laser-arc welding for large section fabrication, this on-line weld monitoring system will be useful for laser and hybrid laser-arc process monitoring and quality control.

6. Acknowledgement

Part of the work reported here was funded by the European Union through ShipYAG and IPCIM projects. The authors would like to thank C H J Gerritsen for carrying out some of the work when he was working at TWI.

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