Experimenting with handheld laser welding

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Leonardo Caprio, Claudio Orlandi and Ali Gӧkhan Demir assess the applicability of handheld laser welding for the flexible production of components with aesthetic requirements

Less than 15 years after the invention of the laser, handheld laser welding (HHLW) was first conceived as an application [1]. However, it is not until recently that the application has actually become viable for industry uptake.

The intervening years have seen lasers experience widespread adoption in manufacturing with the assistance of remote scanners, gantry solutions and automated robotics.

For example, laser cutting has become well-established for processing thin metal sheets, and is continually gaining market share in the cutting of thicker materials thanks to ongoing increases in laser power.

Concurrently, due to its exceptional flexibility and precision, laser welding has been increasingly implemented within manufacturing chains such as that of the automotive sector – most recently in the production of electric vehicles [2]. 

However, sectors using conventional manual arc welding operations have been notoriously resilient to the adoption of laser-based solutions. Historically this has been due to challenges such as cost, as well as the ruggedised requirements of industrial environments. However, with the cost of fibre laser sources having come down each year, and with continual advancements taking place in the design and performance of laser welding heads, HHLW systems have now become readily available for industrial use. Consequently there is a great potential for the technology to disrupt standard production chains in current markets.

HHLW systems are typically composed of a fibre laser source with a continuous or modulated emission profile and power levels between 1 and 2kW. The laser light is delivered to a gun-shaped welding head by means of a transport fibre through a QBH connector. The diverging beam from the fibre is collimated by a lens and diverted to one (or two) reflective mirrors, which enable spatial oscillation of the beam (as shown in Figure 1a). The light is then focused by a final optical element and directed towards the workpiece. Such configuration implies that the HHLW systems are generally categorised as Class-4 laser machines, according to the standard EN 60825-1.

Figure 1: (a) Typical beam propagation of a HHLW system. System operated in different configurations (a) handheld and (b) with a linear axis [3]

Further fundamental knowledge required

Currently, within scientific literature there is a lack of information regarding HHLW technology. Fundamental aspects that require further attention are related to the safety of using such systems (given their classification and manual use) as well as the mechanical properties of parts produced with this technology. 

In order to promote investigations on such themes, a flexible experimental set up has been integrated within the SITEC Laboratories for Laser Applications at the Department of Mechanical Engineering of the Politecnico di Milano. The HHLW system developed could be either operated manually (Figure 1b) or on a linear axis (Figure 1c), thus providing reference conditions for the experimental runs. The experimental equipment has been developed to promote a greater understanding of the dynamics of this new process, as well as for didactical purposes to promote knowledge regarding laser safety.

The first research work on the topic conducted by the research team at SITEC was presented this year in Orlando, Florida, at the International Congress on Applications of Lasers & Electro-Optics (ICALEO) [3]. The authors worked in collaboration with the company Sice Previt to investigate the influence of different operators over the mechanical and aesthetic properties of stainless-steel joints performed with a HHLW system, and compared them with an automatically driven system. 

Figure 2: Luxury retail shop with furniture details manufactured by Sice Previt using handheld laser welding systems

Sice Previt operates in the construction and high-end furnishing sector, supplying retail shops serving the customers of this luxury industry (see figure 2). Tailor-made products are required with aesthetic properties playing an equally important role alongside the durability of the products. Production lots are typically restrained to a single or few parts with a dedicated design and production process. Stainless-steel alloys are typically employed for the realisation of such components with both aesthetic and mechanical requirements. 

HHLW systems have been introduced within the production lines of Sice Previt as an alternative to conventional arc welding systems. The weld bead characteristics achievable with laser welding could help shorten the production cycle compared to arc-based welding, since currently post-processing is required to provide mirror finishing to the components and compensate for any thermally induced deformations. In addition, the ease of use of HHLW systems resides amongst the promising aspects of the technology, thus it was also of interest to evaluate the training experience of the operators. 

Professional vs rookie operators

A benchmarking experimental campaign was designed to investigate the differences in terms of both mechanical and aesthetic properties of butt welds of AISI301LN stainless steel performed by operators with different skill levels. Two welders from Sice Previt who habitually employ arc-based welding systems were taken as the ‘professional’ operators, while the ‘rookie’ operators were two students from the Master of Science in Mechanical Engineering at the Politecnico di Milano, who received basic training with the system. As a reference term for the comparison, the HHLW head was adapted to a linear axis to perform welds under controlled conditions. The weld bead width variability was employed as an indicator for the aesthetic compliance, while tensile testing of the samples was used to assess the mechanical properties.

Figure 3 shows the main results from this investigation. Surprisingly, statistical analysis indicated that the mechanical properties (yield stress and ultimate tensile stress, shown in Figure 3a) were not significantly affected by the operator skill. On the other hand, the professional welders were capable of obtaining a significantly lower weld bead width variability with respect to their rookie counterparts, as confirmed by the results of Figure 3b and the metallographic cross-sections of Figure 3c. As previously mentioned, the reference levels shown were provided by welds performed with a linear axis representative of conditions typical of automated machines. These weld beads showed in general a lower variability in terms of weld bead width, indicating that robotised or motorised actuators yet remain the best option from an aesthetic perspective. 

Figure 3: (a) Mechanical and (b) aesthetic properties of handheld laser welds performed by a “professional” and “rookie” alongside (c) the metallographic cross-sections of the welds [3]

With regards to the mechanical properties, operations under controlled conditions reported higher average levels of ultimate tensile stress. On the other hand, the yield stress of the joints was not significantly affected by the configuration of the welding systems. 

Conclusion

It is a reported fact that HHLW systems are undergoing a significant adoption throughout the industry. However, this introduction of the technology within production environments requires in-depth discussions regarding both the technological and safety aspects. Careful evaluations regarding the safety of such systems should be conducted prior to their implementation on the production floor. 

The collaborative investigation between Sice Previt and SITEC reports a case study that may be useful for users belonging to different sectors in evaluating the applicability of such systems to their production needs. It must be considered that this work provides preliminary indications regarding the process for a specific material and joint configuration. Further studies are envisaged in order to provide a more comprehensive analysis exploring a wider range of experimental conditions with further replications and more operators.

Possibly, the compromise between the safety issues and the production flexibility requirements may be found in soft automation solutions such as collaborative robotics equipped with HHLW torches. Such solutions are currently taking shape and have been recently presented at Fabtech 2022 [4], thus providing an interesting outlook on the future developments of lasers as a driving component for welding processes.

Leonardo Caprio is Assistant Professor at the Department of Mechanical Engineering of the Politecnico di Milano

Ali Gӧkhan Demir is Associate Professor at the Department of Mechanical Engineering of the Politecnico di Milano

Claudio Orlandi is Product Designer at Sice Previt SpA 

This article was co-authored by Giulio Borzoni and Barbara Previtali, of the Department of Mechanical Engineering at the Politecnico di Milano, and Gianluca Di Matola, of Sice Previt SpA

References

[1] G. Nath, ‘Hand-held laser welding of metals using fibre optics’, Opt. Laser Technol. 6 (1974) 233–235. https://doi.org/10.1016/0030-3992(74)90064-4.

[2] M. Kirchhoff, Laser Applications in Battery Production - ‘From Cutting Foils to Welding the Case’, in: 3rd Int. Electr. Drives Prod. Conf., IEEE, 2013: pp. 16–18.

[3] L. Caprio, G. Borzoni, B. Previtali, A.G. Demir, ‘Hand-Held Laser Welding of AISI301LN for components with aesthetic requirements: towards the integration of machine and human intelligence’, in: Proc. ICALEO 2022, 2022.

[4] Cobot-assisted handheld laser welding solution be shown fabtech-2022’, Laser Systems Europe: https://www.lasersystemseurope.com/news/cobot-assisted-handheld-laser-we...

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