## 4 Conclusions  
To investigate the interfacial bonding process between the CFRTP composite and aluminum alloy during laser direct joining, an online observation device for the interfacial bonding process was designed, and the temperature field simulation model for laser direct joining was established. Based on the observed and simulated results, the relationship between the temperature distribution and the melting and flow of the polymer was analyzed, as well as the effect of the filled polymer at the interface on the interfacial bonding process. The main conclusions are as follows:  
(1) The melting and flow of the polymer matrix and filled polymer at the interface were observed, as well as the position change of the bubbles and fibers therein. As the laser power increased, the number of melted plies of the CFRTP composite increased when the interface was not filled with polymer. Once the interface was filled with polymer, the number of melted plies increased and then remained constant as the laser power increased, because bubbles caused by the thermal decomposition of the polymer layer at high laser powers hindered the heat transfer into the CFRTP composite. (2) The number of melted plies in the CFRTP composite obtained with the simulation were the same as the observed results, when thermal decomposition of the polymer did not occur. Affected by the low thermal conductivity of the CFRTP composite and polymer,  
Figure 12: Bubble position change and \(x\) direction temperature gradient in single-laser scanning. the largest temperature gradient occurred at the contact area with the aluminum alloy. Regardless of whether the interface was filled with a polymer layer or not, the temperature gradient along the material thickness direction was larger than that along the laser scanning direction, which was about 400 \({}^{\circ}\)C/mm and 40 \({}^{\circ}\)C/mm, respectively. (3) When the interface was not filled with polymer, the molten matrix together with the fibers moved to the aluminum alloy under the temperature gradient along the material thickness direction. This resulted in filling the gap between the CFRTP composite and the aluminum alloy, coupled with cracking between the matrix and fibers and delamination in the CFRTP composite. For the movement along the laser scanning direction, less movement of the molten matrix and fibers was observed because of the lower temperature gradient and the support effect of the unscanned area. (4) When the interface was filled with polymer, the gap between the polymer layer and the aluminum alloy was filled rapidly once the polymer started to melt, after which the aluminum alloy limited the flow of the molten polymer towards the aluminum alloy, thus avoiding cracking and delamination. However, because of the better fluidity of the molten polymer, the gas in the interfacial gap tended to remain in the molten polymer. The molten polymer and bubbles moved in the laser scanning direction, and the motion was in the positive direction of the temperature gradient. In the future, optimization and modification of the interface-filled polymer layer and development of the matched laser direct joining process are still needed. Those studies are expected to promote the flow of the molten polymer and bonding with aluminum alloy based on the suppression effect of the polymer layer on joining defects. The promoted interfacial bonding and reduced joining defects will contribute to the improved performance and application of laser direct joining between CFRTP composites and aluminum alloys. Conceptualization, Q.W., R.F. and F.W. ; methodology, Q.W. ; software, Q.W. and C.L. ; validation, R.F. and J.L. ; formal analysis, J.L. ; investigation, Q.W. and C.L. ; resources, R.F. and F.W. ; data curation, R.F. ; writing--original draft preparation, Q.W. and J.L. ; writing--review and editing, F.W. ; visualization, Q.W. and C.L. ; supervision, F.W. and Z.J. ; project administration, Z.J. ; funding acquisition, R.F., F.W. and Z.J. All authors have read and agreed to the published version of the manuscript.