## 2 Innovative Processing on Micro/Nanojoining  
Weight saving of products with keeping strength is a recent world mission in industrial fields. Dissimilar joining is one of the solutions. Joining of steel with aluminum is the most important and challenging technical issue in the automotive industry, because brittle Fe-Al intermetallic compounds (IMCs) formed between steel and aluminum decrease the mechanical properties of the joint. Controlling the IMCs is the key factor to achieve a strong steel-aluminum joint [16; 17; 18; 19]. [PERSON] et al. investigated the microstructure and mechanical properties of dissimilar Al/steel joints with and without nickel coating [20]. They found that the Ni coating acted as a physical barrier to avoid the direct mixing of Fe and Al elements, which inhibited the formation of brittle Fe\({}_{2}\)Al\({}_{5}\). Hence, it led to the improvement of joint strength.  
Direct joining of metal with polymeric material is also an important technical issue in many industrial fields, in particular, the automotive and aeronautical industries [21]. [PERSON] et al. produced hybrid joints of Ti6 Al4V and glass fiber-reinforced polyamide using a direct laser joining method [22]. Surface texturing treatment was performed on Ti6 Al4V before the laser joining process, producing a strong mechanical anchor effect between Ti6 Al4V and PA66-GF30 after the solidification of PA66-GF30 melts. A modified direct laser joining process was proposed to effectively eliminate the generated micropores in the joints. The fracture strength of Ti6 Al4V-PA66-GF30 joints was improved to 41.5 MPa, and fractures of the joints occurred at the bonding interface betweenTi6 Al4V and PA66-GF30.  
Figure 1: Inconel 718 Joint with Ni Nanomaterials as the filler metal: (**a**) SEM image; (**b**) forescatter detector image.  
Focused ultrashort laser pulse induces a variety of nonlinear phenomena inside transparent materials, producing many attractive novel applications [23; 24; 25]. [PERSON] et al. applied the light-emitting diode (LED) array microscope to visualize femtosecond laser-induced structural modifications [26]. Structural changes induced inside BK7 glass using a femtosecond laser were observed using the LED array microscope. They demonstrated bright-field, dark-field, and differential phase contrast (DPC) images of the structural changes using an LED array as a light source and by temporally changing the illumination pattern. They also demonstrated simultaneous acquisition of bright-field and dark-field images of structural modifications in glass by color-coded Rheinberg illumination and subsequent computational image processing.  
Seeing is believing. Ultrasonic bonding is widely used in industrial fields; however, there are still many unresolved phenomena. [PERSON] et al. directly observed the interfacial adhesion behavior during ultrasonic bonding between an Al ribbon and transparent silica substrate using a high-speed video camera with differing frame rates [27]. Initially, friction slip occurred, producing multiple island streaks in the direction parallel to the ultrasonic vibration. The island streaks were formed as a scratch, due to surface waviness of the Al ribbon. Momentarily, a belt-shaped bond zone was formed at the center, normally due to the ultrasonic vibration. The positional relationship between the island streaks and the central belt zone was confirmed from in situ observation results of a twist and peel test of Al ribbon bonded to silica substrate. The central belt zone was between the island streaks and the silica substrate.