# Improving Adhesive Bonding of Al Alloy by Laser-Induced Micro-Nano Structures
## 3 Results and Discussion  
### Analysis of Surface Roughness Detection Results  
Figure 3 showed the morphology and surface roughness of No.1, No.2 and No.3-No.6 samples, which were untreated, treated by sandblasting and laser ablation, respectively. The test area for roughness was 1.5 mm \(\times\) 1.5 mm. It was seen from the No.1 sample that  
\begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**NO.**} & **Power** & **Frequency** & **Scanning Speed** & **Spot Line Spacing** & **Spot Horizontal Spacing** \\  & **(W)** & **(kHz)** & **(mm/s)** & **(tum)** & **(tum)** \\ \hline No.3 & 30 & 40 & 400 & 10 & 10 \\ No.4 & 30 & 40 & 1200 & 30 & 30 \\ No.5 & 30 & 40 & 2000 & 50 & 50 \\ No.6 & 30 & 40 & 2800 & 70 & 70 \\ \hline \end{tabular}
\end{table}
Table 1: Experimental parameters for laser surface treatment.  
Figure 2: Experiment setup for laser surface treatment. The inset showed pictures of Al alloy samples before treatment (No.1), after sandblasting (No.2) and laser surface treatment (No.3–No.6).  
the surface of the original substrate was relatively smooth with Sa = 0.37 \(\upmu\)m. Parallel stripes formed on the No.1 sample surface, because the surface line groove morphology was rolled during the manufacturing process. The surface of No.2 sample had concave-convex structures in different directions, instead of rolling streaks in the fixed orientation. Under laser irradiation, the Al alloy sample was heated intensively and melted at the laser spot, which produced a localized melt pool. The center of the melt pool was rendered as pit, where some of melted material were vaporized and removed by high-intense laser ablation. Because of the laser impact force, the remaining melted part was squeezed out from the pits and extruded each other, which finally formed a spike at the edge of melt pool. As the laser spot travelled under a designed track, a large number of spikes and pits occurred on the surface of No.3-No.6 samples, which were consistent with the reported experiment results [27; 28]. Observed from No.5 and No.6 samples, the diameter of a single pit was around 40 \(\upmu\)m, which implied that the laser ablation area was a little bigger than the laser spot. When the spot spacing was smaller than or equivalent with the diameter of the laser spot, pits on the No.3 and No.4 sample surface were much smaller than those of No.5 and No.6 samples. Due to a small overlap of successive laser tracks, multiple laser pulses acted upon the same melt pool, which made more spikes and reduced the area of each pit. Thus, the constructed structure on the surface of No.3 and No.4 samples were different from those of No.5 and No.6 samples.  
Figure 4 showed the Sa, Sq and Sz test maps of surface roughness of No.1-No.6 samples. The surface roughness Sa, Sq and Sz values of the original substrate were the lowest, while that of the No.2 sample treated by sandblasting was the highest. The surface roughness of samples No.3-No.6 decreased gradually by increasing the horizontal and line spacing of laser spot. The laser-treatment results showed that the morphology and surface roughness of the sample could be controlled by adjusting the laser spot distribution without changing the laser power. Compared with the No.1 sample, the surface roughness of the sample after laser treating was increased. However, compared with No.2 sandblasted samples, the surface roughness of laser treated samples was relatively lower.