_Applied_  
_sciences_  
_Article_  
Effects of Ultrashort Pulsed Direct Laser Writing on Ni/Al  
Reactive Multilayer Foils  
**[PERSON] 1, [PERSON] 1, [PERSON] 2, [PERSON] 3, [PERSON] 3, [PERSON] 1,4 and [PERSON] 1,*  
Footnote 1: [[http://www.mdpi.com/journal/applsci](http://www.mdpi.com/journal/applsci)]([http://www.mdpi.com/journal/applsci](http://www.mdpi.com/journal/applsci))  
Footnote 2: [[http://www.mdpi.com/journals/mdpi](http://www.mdpi.com/journals/mdpi)]([http://www.mdpi.com/journals/mdpi](http://www.mdpi.com/journals/mdpi)). beam parameters; therefore, a proper understanding of laser-matter interaction is essential for successful machining. Apart from laser wavelength, pulse duration and repetition rate also play a decisive role. Continuous wave and pulsed lasers down to >10 picosecond (ps) regime operate in the thermal ablation regime, which occurs when the duration of the pulse is longer than the time for electron-lattice relaxation, thus generating a significant heat-affected zone (HAZ) [4]. Going towards the sub-picosecond regime, the laser pulse is shorter than all major relaxation processes, including electron-to-lattice energy transfer and heat diffusion. This means that, during the pulse, atomic movement is insignificant in the laser-affected solid and the atomic structure remains intact [5], which minimizes or eliminates thermal damage [6; 7]. Therefore, laser ablation using ultrashort laser pulses produces cleaner features compared to long pulses [8; 9] and enables the processing of sensitive materials with micrometer precision, high quality, and a negligible HAZ [10; 11; 12; 13]. An example of such sensitive materials is reactive metallic multilayers built from at least two alternating metallic layers with individual layer thicknesses of ~5-200 nm and total film thicknesses of up to some tens of microns [14]. These materials are known for exhibiting exothermic self-propagating reactions upon local ignition with reaction front velocities of up to 100 m/s [15] and are capable of achieving reaction temperatures of well above 2000 \({}^{\circ}\)C. The best-studied reactive multilayer system is the Ni/Al system, though other material combinations such as Co/Al, Ru/Al, Nb/Si, Ti/Si, Pd/Al, and others have been identified and characterized over the past 30 years [16; 17; 18; 19; 20; 21; 22]. Reactions can be ignited in multiple ways, including direct heating, spark discharge, laser irradiation, and mechanical shock [23]. The foundation for the self-propagating reaction is the presence of one or more intermetallic phases in the respective phase diagram with a sufficiently large negative heat of formation [24]. A possible application for these reactive multilayer foils (RMFs) is as a local heat source in reactive joining processes [25] and a commercial product based on Ni/Al is available in the market (Indium Corporation, Clinton, NY, USA). Driven by innovative lightweight applications, there is a steadily increasing demand for new materials and material combinations, as well as associated joining methods. Hybrid plastic-metal composites and their application are becoming increasingly popular, as they provide sufficient load-bearing capacity along with a significant reduction in weight. The self-propagating reaction supplies a high amount of heat for a very short time, making it interesting for joining with minimum thermal influence [26; 21]. As an energy source introduced directly into the joining zone, RMFs allow for the joining of material combinations that were previously difficult or impossible to join, with increased process flexibility and additional degrees of freedom at joining locations where accessibility is unfavorable to alternative joining processes. Previous work related to non-structured reactive foils is mainly concerned with a general description of the reaction process, the applicability as an energy source, possible material pairings that can be joined, and characterization of their composite strength after joining [15; 25; 26; 27; 28; 29].