# Improving Adhesive Bonding of Al Alloy by Laser-Induced Micro-Nano Structures
## 3 Results and Discussion
### Analysis of Microscopic Morphology and Contact Angle Detection Results  
Shown in Figure 5, the microstructure and contact angle of No.1 and No.2 samples were measured by SEM and contact angle measuring instrument. Alcohol was used to clean oil stains on the surface of the No.1 sample. After cleaning, there was still a layer of loose and uneven oxide film on the surface [29], which reduced the surface energy of Al alloy. The wettability of the No.1 sample was affected. There was a small physical adsorption force between droplet and Al alloy surface results in small mechanical meshing force [30] and a contact angle of 89\({}^{\circ}\). There were obvious rolling cracks on the surface of Al alloy; these cracks were easy to be the source of crack propagation during service. The metal surface of the No.2 sample was coarsened and purified by random collision and repeated scouring of sand particles. Impurities, such as oxide on the substrate surface, were removed. Large bumps and pits appeared on the surface of the No.2 sample, which were in no obvious directionality, with multi-scale and random characteristics. After sandblasting, its contact area increased and the surface wettability improved with a contact angle of 51.4\({}^{\circ}\). Micro-pits, scratches and furrows generated on the sample surface played the role similar to mechanical occlusion, anchoring and nailing [30]. Thus, the wettability of Al alloy was improved after sandblasting. Figure 3: Topography and surface roughness test diagram of No.1–No.6 sample. (**a**) Topography was measured by optical microscope; (**b**,**c**) Surface roughness was measured by white light interferometer. Because the hemispherical melting region impacted by laser beam overlapped with each other, the interaction of stacking and extrusion was enhanced and the surface of the No.3 and No.4 samples forms a uniform rough structure, seen in Figure 6. This increased the contact area between the droplet and the samples. The surface contact angle of No.3 and No.4 sample was 4.88\({}^{\circ}\) and 5.17\({}^{\circ}\), respectively. Figure 6a showed a large number of \"micron clusters\", which formed a uniform \"coral\" rough surface. According to the mechanical interlocking theory [31], the adhesive formed a stronger mechanical interlocking force on the surface, which increased the bonding area and then improved the bonding performance of the sample. Figure 6c,d shows the surface of \"micron cluster\" covered with a large number of nano-floc structures. The micro-nano composite structures further increases the contact area between the droplet and the material. When the droplet filled the pits on the surface of the coarse micro-nano structure of the flocculent, the droplet was completely sucked into the surface and formed infiltration. The surface of Al alloy became hydrophilic. It should be noticed that WLI and SEM methods both provided surface morphology, however, their test diagram did not appear the same thing [32,33]. Because of higher resolution in the \(Z\) direction for the WLI method, the spikes and pits in WLI images seemed taller with sharper appearance than in SEM images. Furthermore, as SEM overcame the limitations of the optical diffraction, the magnification of SEM images was up to 1000 times larger than WLI. As a result, micro-nano composite structures could be seen in SEM images. Figure 4: Surface roughness of No.1–No.6 Sample. (**a**) Values of Sa and Sq; (**b**) Value of Sz. Figure 5: SEM images showing details of surface structure of No.1 sample (untreated) and No.2 sample (sandblasted). Form (**a–d**), the image magnification were monotone increasing. Figure 7a showed that circular pits were regularly arranged on the surface of No.5 and No.6 samples when the distance between laser spots was equal to or greater than the diameter of the melting zone. From the accumulation phenomenon at the edge of pits, it implied that the main reason for the formation of pits was not material evaporation.