A brief review of recent developments in laser welding processes for ferritic pipe steels
Paper published in the Australasian Welding Journal, Vol.52, 4th quarter 2007, pp.21-22.
Laser welding and the pipeline industry
Shortly after the advent of CO2 gas lasers in the late 1960s, laser welding has been considered as a process of interest for industrial pipeline welding.  Attractive features of laser welding for pipeline applications include:
- Consistent quality welds with high productivity.
- Higher welding speeds or fewer welding passes compared with arc welding, given the penetrating nature of laser welding.
- As a low hydrogen process, comparable with TIG welding, the risk of cold cracking can be reduced.
- The low heat input can be beneficial, e.g. in fine grained steels. Nevertheless, very low heat inputs should not be used, to avoid unacceptably hard microstructures. In certain steels, hard microstructures can lead to low toughness, or a tendency towards cracking in sour service.
- Electrode consumption can be reduced, or even eliminated.
- Unlike electron beam welding, X-rays are not generated, although appropriate shielding from the laser radiation is necessary.
- Laser welding is performed out of vacuum. Weld bead gas shielding may be required, but this is in much the same way as for TIG or MIG/MAG arc welding.
Nevertheless, laser welding has not been widely implemented by the pipeline industry, owing to a number of limiting factors, including:
- Power output being insufficient for thickness or productivity requirements.
- The limited positional flexibility of beam delivery systems.
- A poor tolerance to joint alignment and fit-up gaps.
- Limited equipment portability.
- High equipment costs.
- High running costs.
- Equipment reliability issues.
However, recent technological developments have now led to laser welding becoming more worthy than ever of serious assessment for pipeline welding. How these developments have addressed the factors limiting widespread implementation are described below.
High power CO2 gas laser welding
In terms of power output, CO2 gas lasers were the highest power industrial lasers available up until the beginnings of the current decade. For example, in 2001 Ono et al.  reported the installation of a 25kW CO2 laser for seam welding of pipe with a wall thickness up to 16mm. Welding speeds were as high as 8m/min on 5mm pipe, reducing to 2m/min on 16mm pipe. However, HF induction pre-heating was successfully demonstrated to increase laser welding speeds further, by a factor of at least three. Vigreux et al.  have also identified HF pre-heating as a means to reduce weld zone cooling rate, and hence weld metal and HAZ hardness. Such preheating avoids having to otherwise reduce welding speed to meet hardness and/or toughness requirements.
In terms of tolerance to alignment, sub-millimetre joint tracking accuracy is required. This is due to the small diameter of the laser beam when focussed, and the narrow weld that results. In the case of Ono's work, tracking was successfully achieved using an optical seam-tracking device. This is just one of many examples in industry where such sensors facilitate laser welding.
However, the usefulness of high power CO2 lasers for pipe welding remains restricted. Firstly, CO2 lasers are not insignificant in size, and they are not considered readily portable. Secondly, beam delivery systems are relatively inflexible. CO2 laser radiation can only be delivered to the work by mirror systems, typically mounted on gantries or articulated arms. These systems are best suited to seam welding, or girth welding if the pipe can be rotated. Orbital welding requires rotating mirror systems.  Lastly, tolerance to joint gaps is poor. In general, gaps must be <10% of wall thickness or the diameter of the focussed beam, whichever is the smaller. Welding outside these tolerances can produce unacceptable weld profiles, lack of fusion, or even failure to make a weld. Cold wire addition can relax tolerances slightly, but reduces welding speed.
Nd:YAG laser welding
Around the turn of the current decade, solid state Nd:YAG lasers were considered for pipe welding. When compared with CO2 lasers, the shorter wavelength of Nd:YAG laser radiation allows it to be delivered down an optical fibre, resulting in more flexible beam delivery, facilitating orbital girth welding. This shorter wavelength also couples more efficiently in to the material being welded, allowing higher welding speeds compared with CO2 lasers. However, commercially available Nd:YAG lasers are limited to powers of between 4kW and 6kW. Consequently, two or more Nd:YAG laser beams have to be combined either in to a single focussing optic  or in to a single optical fibre [5,6] to weld pipe much in excess of 8mm wall thickness. With this approach, 12-14mm pipe can be welded successfully, an example being shown in Figure 1. Again however, the tolerance to joint gaps is poor, unless filler wire addition is used.
Fig.1. A cross-section through an API 5L X70 pipe wall, showing an internal MAG root run, autogenous 9kW Nd:YAG laser fill, and MAG capping pass. The MAG root and capping passes have been made using conventional techniques. However, the single laser fill pass leads to a significant reduction in welding time, compared with the multiple MAG fill passes which would be required otherwise
Hybrid laser-MAG welding
Regarding tolerance to joint gaps, progress has been made by hybrid laser-MAG welding. Hybrid welding combines the laser beam process and an arc welding process in a single zone. Figure 2 shows an experimental hybrid welding head, with a schematic of the process shown in Figure 3. Such systems are now commercially available.
Fig.2. Hybrid welding head, showing the laser focussing optic mounted on the end of a 6-axis robot arm, and a conventional MAG welding torch mounted at an angle 40° off the laser axis
Fig.3. Schematic of the hybrid laser-MAG process
In hybrid welding the penetrating laser weld combines with the gap bridging ability of arc welding. For example, joint gaps of up to 1.6mm have been bridged in 8mm steel plate using the hybrid process.  In addition, the filler wire addition from the arc process can be used to control weld metal properties, as is achieved in conventional MAG welding. Hot cracking susceptibility can be reduced,  as shown in Figure 4, or weld metal toughness improved. Some examples of hybrid weld properties achieved by TWI are summarised in Table 1. In general, weld qualities and properties have proven acceptable. Where unacceptably hard microstructures have resulted, these can be addressed by preheating or changing welding conditions, as noted earlier.
Fig.4. Solidification cracking encountered in an autogenous 9kW Nd:YAG laser fill pass (left), versus a hybrid fill pass which produces consistently crack free welds (right). In both cases, a MAG root pass has been made previously using conventional techniques. A subsequent capping pass by the MAG process would be required for a full penetration weld
Table 1. Examples of hybrid weld properties achieved by TWI.
|API 5L grade, and wall thickness||Hybrid process||Welding speeds 2||WM 3 porosity content||Hardness, HV5||Charpy impact energy at -10°C, J|
|Parent||WM 3||HAZ 4||WM 3||HAZ 4|
||'No porosity' reported
- = not recorded
1 performed using three Nd:YAG lasers combined in to a single beam
2 dependant on processing conditions and size of root face chosen in Y joint preparation
3 WM = weld metal. Hardness will depend on processing conditions and position around circumference.
4 HAZ = heat affected zone. Hardness will depend on processing conditions and position around circumference.
5 at -10°C
6 =performed on sub-size specimens
Recent developments in laser welding sources
In the last 3-5 years the final barriers to serious implementation of laser welding of pipe steels are starting to be removed. High power Yb fibre laser and Yb:YAG disc lasers are now available on the market. The wavelength of their radiation is similar to that of Nd:YAG lasers, allowing beam delivery via an optical fibre. In the case of Yb fibre lasers, powers can be in excess of 8kW. Equipment costs are in line with existing laser technologies, kW for kW. Running costs can be reduced, as the operating efficiencies are ten times higher than certain other lasers. As already proven with Nd:YAG lasers, the solid state design of these lasers will lend them reliability and robustness in industrial operation. Particularly in the case of the Yb fibre laser, the compact laser design and higher efficiency with reduced chiller requirements results in a system size one quarter that of an equivalent power Nd:YAG laser. The Yb fibre laser is thus approaching a portable system in the near future, as designs inevitably evolve.
Being fibre delivered, these lasers are suited to orbital welding  in the same way that Nd:YAG lasers have been considered in the recent past. Beam qualities are better than Nd:YAG lasers and more comparable with CO2 lasers. Therefore, requirements for edge preparation and joint tracking are likely to be at least as demanding as when CO2 laser welding. In this respect hybrid welding is again of interest, and can easily be achieved using these new lasers. Figure 5 shows an example of a hybrid weld made using a Yb fibre laser, with Table 1 listing typical properties achieved. 
Fig.5. A cross-section through a hybrid Yb fibre laser-MAG weld in API 5L X80 pipe, made at 1.8m/min, in the overhead (PE/4G) position. A subsequent capping pass by the MAG process would be required for a full penetration weld
Laser welding is a high speed welding process, capable of automated production of consistent quality welds. Compared with many other arc welding processes, fewer passes or higher welding speeds can be used, with lower usage of welding consumables. With proper optimisation of welding procedures, full advantage can be taken of the low heat input nature of laser welding for a wide variety of materials, producing welds with acceptable hardness and toughness properties. Process variants such as hybrid laser-MAG welding or induction heating are of interest in this respect, the former also demonstrating significant improvements in tolerance to joint gap.
With the recent advent of higher power, fibre-delivered lasers, with equipment and running costs comparable with established laser technologies, and certain designs becoming more portable than ever before, research and development of laser welding for flexible, all-positional pipe welding is set to continue, apace with equipment developments.
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