## 3 Results and Discussion  
### Consistency Verification of Observation and Simulation Results  
To verify the relationship between the temperature distribution and the melting of the CFRTP composite, the observed interfacial bonding processes and the corresponding temperature distributions at the material end faces in the simulation were compared as shown in Figures 5 and 6. In the observed results, the melting of the polymer matrix caused the thickness change in the CFRTP composite, and the melting depth of the CFRTPcomposite was difficult to accurately determine; therefore, the number of melted plies of CFRTP composite was used to compare the simulated and observed results. In the simulation results, the melting temperature (343 \({}^{\circ}\)C) of the PEEK polymer was used as the melting boundary criterion, and the corresponding melting area was indicated in gray. When the interface was not filled with polymer, Figure 5a shows the results at a laser power of 400 W. The position change of the fibers in the CFRTP composite and the cracking between the matrix and fibers were observed in the first ply, whereas the other piles were unchanged, indicating that melting only occurred in the first ply. Continuing to increase the laser power to 500 W and 600 W, the melting of the three and six piles, as shown in Figure 5b,c, respectively, was found in the observed results using a similar criterion. The observed number of melted plies was the same as the number of melted plies in the gray area in the simulation, indicating that there was a correlation between the number of melted plies and the temperature distribution. With the increase in laser power, the time corresponding to the maximum number of melted plies was delayed because the temperature of the aluminum alloy was higher at higher laser powers, and the CFRTP composite was still heated by a high-temperature aluminum alloy after the laser stopped irradiating the observation area. However, when the number of melted plies increased, the fibers in the direction of the parallel end faces were extruded, which affected the observation of the interfacial bonding process. Figure 5: Observed interfacial bonding processes and the corresponding temperature distributions when the interface was not filled with polymer: (a) laser power 400 W, (b) 500 W and (c) 600 W.  
The observed results and corresponding temperature distributions after filling the polymer layer at the interface are shown in Figure 6. The molten polymer layer filled the gap between the end face of the material and the refractory glass, leading to a more pronounced color change compared with the melting. However, this did not mean that melting of the polymer matrix occurred, and it was still necessary to use the position change of the fibers or cracking between the matrix and fibers as the melting criterion. Due to the lower thermal conductivity of the polymer than the CFRTP composite, the number of melted plies was lower under the same laser power. When the laser power was 500 W, cracking was not observed in Figure 6a, but the observations at 11.58 s and 11.89 s showed that the position of the fibers in the first ply changed, indicating that the polymer layer was completely melted and the first ply was partially melted, which was consistent with the simulation. With similar criterion, it was found that the number of melted plies in the CFRTP composite increased to three plies when the laser power was 600 W, and it was accompanied by thermal decomposition in the molten polymer layer. When the laser power was 700 W, the number of melted plies in the CFRTP composite was still observed to be three plies, which was different from the simulation result of five plies, because bubbles in molten polymer induced by the thermal decomposition hindered the heat transfer. The above results proved that the temperature distribution during laser direct joining was correlated with the melting of the polymer matrix and the interfacial filled polymer.