Effects of Ultrashort Pulsed Direct Laser Writing on Ni/Al Reactive Multilayer Foils

Abstract

Featured Application: Reactive multilayer foils (RMFs) possess the capability for self-propagating exothermic reactions, making them suitable as a source of localized heat for joining applications. Laser structuring offers the possibility of cutting arbitrary shapes into foil which change the direction of reaction front propagation. Consequently, the spatiotemporal heat release of the reaction inside the joint can be altered, thus providing a means to control the joining process. Furthermore, a flow of molten material through the RMF plane becomes possible, potentially strengthening the joint. Possible areas of application are diverse and extend across industries such as polymer manufacturing, automotive, aerospace, mechanical and electrical engineering, and household and housing technology. Companies that previously relied on adhesive bonding or plastic welding to manufacture their products may find it beneficial to switch to an alternative method due to the drawbacks associated with these processes, such as the need for pre-treatment, long hardening times, and thermal stress. Reactive multilayer foils (RMFs) for joining processes have attracted a great deal of attention over the last few years. They are capable of exothermic self-propagating reactions and can serve as localized heat sources for joining applications when ignited by suitable means. Using short and ultrashort pulsed lasers with carefully selected parameters, cutting and shaping of RMFs makes it possible to tailor heat release characteristics without triggering the reaction. The present study is an investigation of microstructural changes induced by femtosecond laser machining of a commercially available Ni/Al-based RMF. The effects of the specific laser parameters pulse duration and repetition rate on the heat-affected zone (HAZ) are investigated by scanning and transmission electron microscopy. Debris consisting of oxide deposits can be found at a distance of several tens of microns from the cut edge. A negligible HAZ extending to less than 100 nm was observed for all parameters tested and no signs of ignition of a self-propagating reaction were observed. These results underline the suitability of femtosecond lasers for metal machining with minimal heat input.