Reactive multilayers are desirable for a variety of applications including near-net shape shaping of intermetallic compounds, dissimilar/similar joining, as ignition sources and highly localized heat sources. The production of intermetallic compounds from multilayer thin foils/films is a widespread method. Due to its moderate enthalpy of reaction, the Ni-Ti system has received little attention in the context of low temperature joining; although the reaction product, nickel titanide, has promising mechanical properties, like superleasticity or shape memory effect. The aim of this PhD thesis is to study the feasibility of joining NiTi to Ti6Al4V using only the heat released by Ni/Ti reactive multilayers ignited, using a femtosecond laser.Ni/Ti reactive nano-multilayers with different modulation period (Λ =5 nm, 12 nm, 25 nm and 75 nm) and with a total thickness of around 2.5 µm were deposited, utilizing a dual cathode magnetron sputtering equipment, onto different substrates. These multilayer thin films exhibit a columnar growth, independently of the substrates, and a surface roughness that is directly proportional to the modulation period. Structural evolution was followed using in situ synchrotron x-ray diffraction, observing a single step reaction from Ni/Ti multilayers to B2-NiTi, without the formation of intermediate phases and independently of the substrate and modulation period. The temperature at which the multilayer structure evolves to the final phase varies with the period and the heating rate. When the multilayer thin film is deposited onto Ti6Al4V substrates, the nickel from the NiTi reacted multilayer, at temperatures higher than ≈650◦C, diffuse to the beta phase of the substrate leading to phase growth and NiTi2 formation.Ignition experiments were conducted on several of these multilayers using femtosecond laser pulses. It was observed that the substrates thermal properties plays a very important role: while metallic substrates quench the reaction confining it to the interacted laser volume, for zirconia substrates the reaction affected a wider volume. Based on a theoretical approach, it was confirmed that the produced multilayers did not present sufficient number of bilayers to be used as the only heat source available to promote joining. Concluding that, the number of bilayers had to be increased in order for the reaction to become self-sustained. LIPSS patterning of the multilayer revealed that the energy modulation irradiated on the multilayers surface promotes an athermal ablation on the bottom of the crests while, on the top, a small thickness of amorphous NiTi was observed. This indicates that, on the ridges, some of the multilayer reacted and that the process was not fully athermal.Finally, sound joints between NiTi and Ti6Al4V utilizing Ni/Ti multilayers with 12 and 25 nm periods were achieved at temperature as low as 600◦C for 30 min under a pressure of 10 MPa. The joints were processed, while following the phase evolution, using in situ synchrotron x-ray diffraction in transmission mode, revealing that the formation of the NiTi2 is reduced for the joints producedat lower temperatures. The joints presented good interface quality, without any pores or major defects.
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