# Improving Adhesive Bonding of Al Alloy by Laser-Induced Micro-Nano Structures
## 2 Materials and Methods
### Experimental Method  
The experiment setup for surface treatment was presented in Figure 2. Nanosecond laser pulses were emitted by a commercial fiber laser system (IPG Company, New York, NY, USA), and then delivered to a scanning Galvanometer (Scanlab Company, Puchheim, Germany). The maximum laser power was 30 W, the average laser wavelength was 1064 nm and the pulse width was 100 ns. Passing through an F-\(\theta\) field lens with 160-mm focal length, the laser beam was focused to be a spot with diameter 30 \(\upmu\)m at 1/e\({}^{2}\) of the maximum laser intensity. To ensure the laser focusing on the top surface of samples, the distance between the laser spot and the samples was adjusted by \(Z\)-axis lifting stage. The line spacing of laser spot (ranged from 10 \(\upmu\)m to 100 \(\upmu\)m) and the scanning speed (ranged from 100 mm/s to 10,000 mm/s) were accurately controlled with the scanning Galvanometer. The inset of Figure 1 showed pictures of an untreated sample (No.1), a sandblasted sample (No.2) and four laser-treated samples (from No.3 to No.6).  
Figure 1: Schematic diagram of adhesive bonding of Al alloy samples.  
To explore the influence of multiple laser pulses on the bonding performance Al alloy, the laser-treating experiment was conducted under the same condition except spot spacing. The laser power was 30 W and laser frequency was 40 KHz, considering energy utilization and processing efficiency. By adjusting the scanning speed of laser pulses, the spot horizontal spacing was increased from 10 \(\upmu\)m to 70 \(\upmu\)m, with a step of 20 \(\upmu\)m. To be simplified, the spot line spacing was set as same as the spot horizontal spacing. The experimental parameters for No.3-No.6 samples were briefly summarized in Table 1. The laser track was consisted of a matrix of spots with different spacing.  
In order to test the adhesive bonding strength of Al alloy, the sample surface was treated with sandblasting or laser treating and two samples were handled for each experimental parameter. The detection instruments used in the experiment include the following. Firstly, the samples were examined with an optical microscope (Olympus Company, BX51, Tokyo, Japan). By white light interferometer (WLI, ZYGO Company, Nexview9000, California, USA), surface roughness including Sa, Sq and Sz were measured, where Sa was the average absolute value of height difference of all points in the measured region, Sq was the root-mean-square value of height, and Sz was the sum value of the maximum peak height and the maximum valley depth. To characterize wettability, the contact angle of substrate surface was explored with JC2000C4 Contact Angle tester. Scanning electron microscope (SEM, Leo Company, Leo-1450, Berlin, Germany) was used to measure the surface morphology and energy spectrum of the samples. Then, 944 adhesive was coated on the treating area (shown in Figure 1) and the two samples of the same treatment were pasted with the following processing of RT \(\rightarrow\) 80 \({}^{\circ}\)C/2 h \(\rightarrow\) 100 \({}^{\circ}\)C/2 h \(\rightarrow\) 150 \({}^{\circ}\)C/6 h \(\rightarrow\) natural cooling. Finally, the shear strength test was conducted on the technical standard of G4-853-81.