## 3 Results and Discussion
### Effect of Temperature Distribution on Interfacial Bonding Process
In addition to the movement of the plies in the CFRTP composite towards the aluminum alloy, movement of the molten polymer matrix together with the fibers towards the laser scanning direction was also found, based on the position change of the cracking in the observations of the material end face from 7.14 s to 7.43 s.  
To determine the effects on the interfacial bonding process, the \(x\) and \(y\) direction temperature gradients at the corresponding moments shown in Figure 10 were drawn. The color meanings in Figure 10 are the same as those in Table 2, red arrows are used to indicate the temperature gradient directions, and red contours in the CFRTP composite indicate the boundaries of the melting area. In all moments, the \(y\) direction temperature gradients in the CFRTP composite were directed toward the aluminum alloy, while the direction of the \(x\) direction temperature gradients was related to the laser irradiation position. From 7.00 s to 7.14 s, the melting areas surrounded by the contours were basically the same as the areas of the CFRTP composite moving toward the aluminum alloy, indicating that the molten polymer drove the fibers to move toward the aluminum alloy under the large \(y\) direction temperature gradient. Meanwhile, since the second ply of the CFRTP composite was not melted, the first ply moved toward the aluminum alloy, causing delamination between it and the second ply. Once the melting of the polymer in the second ply occurred at 7.24 s, the molten polymer and fibers in the second ply moved toward the aluminum alloy, further proving that the temperature gradient in the melting area drove the CFRTP composite to move and the direction was related to the gradient direction. For the movement toward the laser scanning direction, although the temperature gradients from 7.00 s to 7.14 s were in the \(x+\) direction, the observations showed that the polymer matrix drove the fibers to move in the \(x-\) direction, whereas temperature gradients toward the \(x-\) direction appeared in the observation area at 7.24 s. This was because the flow of the molten polymer in the CFRTP composite was influenced by the fibers; under the extrusion effect of the scanned area and the support effect of the unscanned area, the movement of the molten polymer and fibers in the \(x\) direction was affected by the movement which occurred away from the observation area. Figure 9: Interfacial bonding processes at different moments when the polymer was unfilled. The interfacial bonding processes when the interface was filled with the polymer layer are shown in Figure 11, with a laser power of 600 W. The polymer began to melt at 4.85 s in the observation area, corresponding to the laser scanning direction from left to right. Due to the better melt fluidity of the pure polymer, the gas in the gap between the polymer and the aluminum alloy could not be vented in time, and bubbles were found in the molten polymer near the aluminum alloy in the observation results. As the laser scanned to the right, the molten polymer gradually filled the gap between the polymer and the aluminum alloy, accompanied by the movement of bubbles along the laser scanning direction, while there was no significant change in the position in the thickness direction. Compared with laser direct joining without filling the polymer layer, the polymer layer could rapidly infiltrate the aluminum alloy surface after melting. Once the molten polymer filled the gap at the material interface, the fixed aluminum alloy limited the movement towards the aluminum alloy. This resulted in the molten polymer moving mainly along the laser scanning direction, avoiding cracking and delamination in the CFRTP composite caused by the movement towards the aluminum alloy. Figure 11: Interfacial bonding processes at different moments when the polymer was filled.