Femtosecond laser approach to cut Ni/Al reactive foils without ignition of a self-propagation reaction
Reactive multilayer foils (RMFs) for joining processes have attracted a great deal of attention over the last few years. The manufacturing of joints based on RMFs offers some crucial advantages for application. The extremely fast joining is conducted at room temperature with the heat source locally in the joining zone, as opposed to standard soldering. The RMFs comprise alternating nanoscale layers of two different metals that, when intermixed, form a product phase through a highly exothermic reaction. With the advent of reliable ultrashort-pulsed lasers, their wide range of properties found application in different fields of science, especially in materials science and engineering. Previous works have shown that it is possible to machine RMFs with pulsed femtosecond (fs) laser ablation with minimal risk of igniting them, due to its limited thermal dissipation into the material being machined. Additionally, fs-laser has been proposed as a cutting method for the fabrication of shaped foils for joining.
In the present work, a Ti:Sapphire femtosecond laser operating at 780 nm wavelength was used to machine commercial Ni/Al Nanofoils®. A careful optimization of the laser process parameters such as pulse-length, focus diameter and repetition rate was conducted in order to avoid the ignition of the foils during the process. Different shapes were cut out of the freestanding foil with micron-scale precision aiming at enabling form-locking joining for improving joint strength and reliability and guiding the reaction front. Moreover, a systematic characterization of the microstructure modification by laser treatment was conducted by using electron and optical microscopy. These characterizations are important to understand what happens to the microstructure close to the cut edge and the material removing mechanisms.