## 2 Materials and Methods  
### Experimental Device and Setup for Interfacial Bonding Process Observation  
To observe the interfacial bonding process during laser direct joining of CFRTP composite with aluminum alloy, an online observation experimental device as shown in Figure 1 was designed. Principle of observation of interfacial bonding process: Firstly, the polished end faces of the CFRTP composite and the aluminum alloy were pressed onto a refractory glass, thus equating them to the confined material interface during laser direct joining; subsequently, a laser was used to irradiate the aluminum alloy close to the refractory glass, and a camera with a microscope was used to observe the interfacial bonding process through the refractory glass. The observation device consisted of the fixture, refractory glass, microscope, camera, focus adjuster, height adjuster, and laser baffle. The camera with microscope had a spatial resolution of 1.23 \(\upmu\)m/pixel and a frame rate of about 40 Hz, satisfying the observation requirements of the interfacial bonding process. To accurately observe the interfacial bonding process, thermal insulation was made at the contact area between the fixture and the materials. The experimental materials were carbon fiber reinforced polyether-ether-ether-ketone (CF/PEEK) thermoplastic composite, 6061 aluminum alloy, and polyether-ether-ether-ketone (PEEK) polymer, which have been used in aviation and other fields. The lengths and widths were 100 mm \(\times\) 25 mm, and the thicknesses were 2 mm, 3 mm, and 0.25 mm, respectively. The mechanical and thermal properties of the experimental materials  
Figure 1: Observational principle and device for interfacial bonding process during laser direct joining of CFRTP composite with aluminum alloy. are shown in Table 1. The CFCTP composite consisted of 16 plies alternating at 0\({}^{\circ}\) and 90\({}^{\circ}\), and its fiber volume fraction was 60%. As shown in Figure 2, the laser direct joining experimental device consisted of a laser head, motion platform, observation device, and acquisition computer. Since the quality of the end faces of the materials directly determines the effect of observation, 120#, 400#, 1000#, and 3000# sandpapers were used to polish the end faces of the CFCTP composite and aluminum alloy until the end faces were smooth and without obvious scratches. Once the materials were prefixed to the fixture, it was necessary to apply pressure on the refractory glass. After ensuring that the polished end faces of the materials were in close contact with the refractory glass, the materials were clamped under the same torque using a torque wrench, and the observing position was adjusted to the center of the interface using the height and focus adjusters. Finally, to prevent the laser scattering from affecting the observation effect, the laser baffle was attached to the fixture. Because of the high fiber volume fraction of the CFCTP composite, insufficient polymer matrix may lead to poor interfacial bonding during laser direct joining, and filling the \begin{table}
\begin{tabular}{c c c c} \hline
**Parameters** & **CF/PEEK** & **6061 Aluminum Alloy** & **PEEK Polymer** \\ \hline Density (kg/m\({}^{3}\)) & 1580 & 2700 & 1300 \\ Tensile strength (MPa) & 980 & 320 & 100 \\ Melting temperature (C\({}^{\circ}\)C) & 343 & 615-655 & 343 \\ Decomposition temperature (C\({}^{\circ}\)C) & 520 & - & 520 \\ \hline \end{tabular}
\end{table} 
Table 1: Mechanical and thermal properties of the experimental materials [25]. Figure 2: Laser direct joining experimental device. polymer layer at the interface between the CFRTP composite and the aluminum alloy is a common solution [33]. To this end, laser direct joining experiments with and without polymer layer filling were performed as shown in Figure 3. In the experiments, a fiber laser with a wavelength of 1064 nm was used to scan the aluminum alloy surface reciprocally six times.