SPONSORED: BEAM ANALYSIS
Safe laser welding in the production of batteries
B
attery modules are the beating heart of electric cars. They ensure mobility
and security, and their reliability determines whether a vehicle is accepted or not. The BMW Group launched the i3, the first fully-electric production vehicle, in 2013 – so the company is well aware of the pitfalls of battery production, which grow in complexity with every uptick in capacity.
Up to 144 laser welds are
required for contacting a 5th- generation battery module. Thanks to Ophir BeamWatch Integrated systems, the auto- maker can check the laser beam before manufacturing each module, independent of cycle times and with success. Up until now, in order to confirm ideal weld-in depths, spot checks were carried out using micrographs on a weekly basis.
Cycle-time-neutral testing Laser welding in the production of battery cells requires absolute precision. Regularly checking the key parameters of the laser beam before welding contributes significantly to the quality of the finished battery pack. From the very first discussion with the experts from the BMW Group, the focus was on how to do this as completely – and with as little impact on cycle time – as possible. In the process, the key metrics to be captured were: focus diameter; laser power;
focus position; and focus shift The latter two are particularly crucial for single-mode lasers, but measuring them quickly proved difficult. The only method capable of this is non-contact measurement of the laser beam based on the principle of Rayleigh scattering – developed by MKS and employed in the Ophir BeamWatch product family. The company designed the Ophir BeamWatch Integrated system specifically for automated manufacturing. This combined beam profiler and power gauge, has various interfaces that permit direct implementation in industrial networks.
The Ophir BeamWatch Integrated system was specifically designed for automated manufacturing
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Non-contact detection of focus shift The first Ophir BeamWatch Integrated system was put into operation in 2019 as part of the prototyping and basic definition for the battery production line. A test phase in technology development followed. It became clear that the device could ascertain all the required parameters. Moreover, the results revealed that the thermal focus shift in the process had been underestimated. Measurements with BeamWatch Integrated showed a thermal focus shift in the millimetre range that the engineers had not expected; however, once the magnitude of this shift was known, adjustments could be made in the manufacturing process. For the auto-maker, this was an important step towards attaining a consistent weld depth. In the development phase, micrographs of the contact welds were used to determine the relationship between the focus shift and the weld depth. Based on this, and working in close cooperation with the Ophir experts, the team determined the limit values for the focus shift.
Diminished weld penetration depth due to weld spatter Another finding that emerged was about the optics of the
The technology contributed to the optimisation of both the development of the BMW Group’s laser welding process as a whole and the quality of the manufactured modules
single-mode fibre lasers used for contact welding. They are affected by spatters on the protective glass. The spatters made their mark not only in terms of focus shift but even more directly on the focus diameter. The effects of laser beam defocusing, for example, due to a smudged protective glass, could also be analysed. Because of these deviations in the laser beam parameters, the team also checked the weld penetration depth of the respective weld seams. Although the seam looked completely flawless, the weld depth was clearly too shallow. It was concluded that measurement technology had to be integrated into the series production process – as only then can the quality of the contact welds be guaranteed.
Reproducible quality Today, the Ophir BeamWatch Integrated system is built into all the automated production lines where BMW 5th-generation battery modules are made worldwide. During the loading and unloading process, the laser is briefly operated at full power to determine focus shift. Only after confirmation (IO signal) that the parameters of the laser beam correspond to specs does the welding begin. While production was still in
the start-up phase, measurements were taken after each module. For series production at full capacity, however, the decision was made
November 2022 Electro Optics 27
to measure the laser beam after every 10th module. Should the BeamWatch Integrated detect a deviation, it displays a warning message. The operator then checks the protective glass of the laser and cleans or exchanges it, as necessary. In addition to this proactive testing, all modules are subjected to an electrical function test after the welding process.
Essential tool for monitoring The production of a battery module is a very complex process. The individual battery cells are stacked, pressed and placed into a frame. Then, all positive and negative poles must be welded in the cell contacting system – upwards of 15,000 spot welds per hour are performed in each system. Ophir BeamWatch Integrated is an essential tool for monitoring the laser beam in the production of the battery modules. MKS measurement technology has contributed to the optimisation of both the development of the BMW Group’s laser welding process as a whole and the quality of the manufactured modules. EO
To learn more visit:
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