Scientists from academia and industry have identified four ways in which pores form in copper laser welding, thanks to in-situ X-ray imaging experiments at the ESRF combined with multi-physics simulation. The results provide clues for optimisation of the manufacturing of copper components through this method. The results are out in the International Journal of Machine Tools and Manufacture.
Copper is widely used for components in electric vehicles, energy storage or electronic devices because of it is excellent electrical and thermal conductivity. However, these same properties—high thermal conductivity and low infrared light absorption—pose challenges for traditional welding techniques, often leading to inconsistent results or defects.
The technique of laser beam welding overcomes some of these difficulties by delivering high-intensity, focused energy that rapidly heats and melts copper, enabling deep, narrow welds with minimal heat-affected zones. Its high processing speed and ability to be finely tuned for different material thicknesses and conditions make it ideal for modern manufacturing demands.
However, defects such as pores still happen, especially microscopic pores. Now scientists led by Technische Universität Ilmenau and TU Wien, and with the collaboration of the company COHERENT, leader in laser solutions and photonics technologies, and the ESRF, have unveiled the dynamics behind pore formation.
The findings show that pore formation is driven by four different mechanisms: bulging, spiking, upwelling waves at the keyhole rear wall and melt pool ejections. In particular, bulging takes place at the rear keyhole wall due to dynamic melt flow; spiking occurs when rapid keyhole penetration causes detachment and solidification at the tip; pores travel due to chaotic vapor flows and bulges in the melt pool and finally, depression pores are linked to melt pool ejection and dynamic keyhole pressure.
A special laser beam shape, called a concentric core-ring profile, was also analysed and found to help make the welding process more stable and reduce defects like pores.
The team used high-speed synchrotron X-ray imaging at beamline ID19, where they acquired 20,000 images per second to identify the processes in pore formation during laser beam welding.
The experiment was very challenging, with 12 engineers to install the laser welding instrument, which contained two lasers and an ESRF in-house developed gas nozzle. Alexander Rack, scientist in charge of the beamline ID19, explains the complexity of the set-up: “This is among the most demanding experiments done so far: we had to install dedicated power lines to feed the big laser needed for welding. Experiments with sample environments are core expertise of ESRF, and thanks to our experience with complex set-ups such as furnaces, gas launchers or high-pressure cells; we are perfectly adapted for this study”. He adds: “The white undulator light with the new EBS is bright enough to shine light through 3 mm of steel while welding, and we were able to take high-speed X-ray movies with microseconds exposure time”.
They then compared these results with multi-physics modeling simulations. This physics-based model, closely aligned with experimental data, allows researchers to validate elusive phenomena with unprecedented accuracy.
Read more on ESRF website
Image: Leander Schmidt, from Technische Universität Ilmenau, during the experiment on ID19.
Credit: S. Candé.
