Autogenous laser welding and laser hybrid welding

Autogenous or keyhole welding is the dominant welding process in laser welding applications as it can produce a deep and narrow weld at higher welding speeds than conventional arc welding processes such as GMAW or even submerged arc welding (SAW). With the high power density of a focused laser beam, the required energy can be directed precisely onto the fusion zone. The heat conduction losses are smaller and the thermal load of the workpiece is decreased (Katayama, 2013). High power solid state lasers, especially fiber lasers, continuously increased their share in material processing since the beginning of the 1990s, benefiting from a higher beam quality and a shorter wavelength compared with CO2 lasers (Hügel, 2000). The shorter wavelength allows a higher absorptivity in metals (Dausinger, 1995) and a lower sensitivity to laser induced plasmas (Shcheglov, 2012). These two effects result in a wider processing window (Vollertsen, 2009), increased process stability (Quintino, 2007) and better weld quality (Kawahito, 2007).

The combination of an autogenous laser welding process with a GMAW process is called laser arc hybrid welding and will be referred to as laser hybrid welding in this work. The development of laser hybrid welding started in the late 1970s with the idea of Steen and Eboo to utilize a laser beam and an electric arc in the same processing zone (Eboo, 1978), (Steen, 1979), (Steen, 1980). In this common zone of interaction both processes share and contribute to a variety of parameters. These parameters can be grouped into parameters for the combined laser hybrid process with sub-processes for the laser beam and the arc, material parameters and design parameters, as shown in Figure 2.15 (Olsen, 2009). The characteristic properties of the laser hybrid welding process arise from the benefits and limitations of both individual processes (Victor, 2011), (Unt, 2018). Table 2.2 compares the advantages and disadvantages of autogenous laser and laser hybrid welding as regards the welding process, and also considers economic aspects.

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Figure 2.15: Schematic drawing of a hybrid welding process with full penetration and an overview of principle parameters, based on Olsen (2009).

Table 2.2: Comparison of advantages and disadvantages of autogenous laser welding and laser hybrid welding as regards the welding process and economic aspects.

Advantages Disadvantages

Autogenous laser welding

− High power density provides deep penetration and narrow welds at high welding speeds

− Lower heat input with less distortion

− All welding positions possible

− Precise preparation of workpieces and alignment required

− Fast cooling cycles may result in brittle microstructure and formation of hot cracks

Laser Hybrid welding

− Greater tolerance to joint gaps and joint preparation

− Provision of filler metal allows control of mechanical and metallurgical properties

− Increased energy utilization from laser radiation into the material

− Complex process with larger number of process parameters

− Higher cost for filler material, shielding gas, equipment and maintenance

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Laser hybrid welding has been intensively studied for scientific purposes and, as a result of international research and development activities, the technique is now utilized in many different industrial applications. Numerous publications on the topic can be found in scientific journals as research articles, review papers, books and book contributions.

These publications present, in comprehensive detail, descriptions of laser hybrid welding processes, process parameters and their effects on the welding process and the resulting welds, available welding systems, and applications for welding, see Table 2.3.

Table 2.3: Overview of publications reviewing laser hybrid welding processes, parameters and applications.

Seyffarth et. al 2002 Book about processes and applications in welding and material treatment such as development of combined laser arc processes for joining of different materials, laser beam arc plasma interaction and combined discharge, integrated plasma torches for laser arc processes and practical applications.

Shelyagin et. al 2002 Review article on scientific publications on hybrid and combined laser arc processes.

Bagger et. al 2005 Review article of fundamental phenomena in laser arc interaction, on governing process parameters, and examples of industrial applications.

F. O. Olsen 2009 Book summarising research on laser hybrid welding processes and its applications such as fundamental characteristics on plasma interaction, dynamics and stability of the weld pool, effect of shielding gases and joint properties, quality control and weld quality assessment, heat sources for hybrid welding processes and applications for shipbuilding, magnesium and aluminium alloys and steels.

Hübner et. al 2010 Review article of laser hybrid welding with different arc sources for practical applications.

Casalino et. al 2010 Book chapter reviewing laser hybrid welding processes for different materials such as stainless steels, mild steels, and aluminium and magnesium alloys with a focus on the process parameters of welding speed, shielding gas and laser arc distance.

Victor et. al 2011 Review article of laser hybrid welding processes summarising the modes of operation and giving a detailed overview of the influence of process parameters such as welding speed, laser

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power, laser arc distance, arc orientation, shielding gas, wire feed rate and arc current and voltage, as well as joint design with gap size and joint mismatch.

P. Kah 2012 Review article comparing autogenous laser welding processes and conventional arc welding processes with laser hybrid welding processes describing the combination of different types of lasers with arc welding sources.

S. Katayama 2013 Book chapters on topics related to laser hybrid welding and combined laser beam technologies, with a focus on thick section materials, including modelling and simulation of autogenous laser and hybrid laser welding.

B. Acherjee 2018 Review article on laser hybrid arc welding systems with GMAW, gas tungsten arc welding (GTAW) and plasma arc welding (PAW) sources and their arrangements relative to the laser source. Discussion of the influence of parameters such as laser power, welding speed, relative position of the laser beam and arc electrode, focal point position, electrode angle, shielding gas composition, modulation of the arc welding system, wire feed rate, joint gap and joint configuration on the hybrid welding process. Description of the performance characteristics and weld quality and industrial applications.

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