Substrate modification during laser welding and reaction assisted diffusion bonding: a comparative

The introduction of reactive nanolayered thin films as filler material has potential to join similar and dissimilar materials. These self-reactive filler, having a few microns of thickness, has been showing particularly attractive for microparts/devices. This type of microfillers is essential to join shape memory alloys (SMA) to themselves or with other materials, necessary for different applications, where the health devices must be highlighted. The major problem is the type of new phases that occurs during the process, particularly if there is the formation of liquid phase, but also if the diffusion kinetics in solid state to assume values that led to the formation of undesirable phases Thus, in the bonding of SMA the maximum temperature, holding time, heating and/or cooling rate must be depressed. The use of the reactive multilayer thin films makes possible to avoid the liquid phase formation, to reduce the temperature, pressure and time required to a successful join of the coated materials. Low (Ti/Al, Ni/Ti), medium (Ni/Al) and high (Pd/Al) reaction enthalpy multilayer thin films are being developed to be used in microjoining for other materials applications. The deposition by magnetron sputtering allows reactive multilayer thin films with equiatomic chemical composition and with controlled nanoscale modulation period to be produced. Reaction assisted diffusion bonding of surface modified of NiTi alloys versus other alloys like Ti based alloys has been carried out. The multilayer thin films control the diffusivity and reactivity at the joining interfaces and could also act as localised heat sources. So far, the most promising reactive nanolayers for joining applications are those from the Ni-Al system with intermediate period (between 10 and 20 nm).