## 3 Results and Discussion
### Joining of CFRP to Aluminum Lising RMFs  
For the RMF joining processes, freestanding Ni/Al RMFs with different thicknesses were used as the local heat source. The thermoplastic CFRP used in this experiment was an epoxy matrix, thus before joining the laser treated CRFP and aluminum surface were coated with molten PC as a filler material. PC is a thermoplastic material, and due to its attractive processing and physical properties such as low weight, flame-retardation, and high impact resistance, it has been widely used in electronic, automotive, and aircraft components [20; 21]. The deflection temperature of PC is around 130 \({}^{\circ}\)C, which can deform after absorbing heat released from the reacted RMFs. Due to the rough surface treated with the laser, molten PC can fill in the rough surface to form a mechanical bond. When the RMFs were ignited, the heat release of the reaction melts the PC again on both the CFRP and aluminum side and thus form a joint. Considering RMF as a fast-moving heat source that releases heat at a constant speed simplified the complex situation to a heat conduction model, avoiding the complicated reactions between the RMF layers. The moving speed was the same as the reaction speed, and the heat releasing speed can be calculated by the DSC result. Figure 6 shows snapshots of the ANSYS simulation of the temperature evolution and distribution in the joint at various time steps. The simulation explained the cross-section temperature distribution of the joint. In the middle of the joint was the RMF, which as it is ignited by electrical heating can reach over 208 \({}^{\circ}\)C in 100 \(\upmu\)s. The PC around the RMF absorbed the heat released from the reaction and thus reached over 150 \({}^{\circ}\)C, which is higher than the heat deflection temperature of PC at around 130 \({}^{\circ}\)C. Due to the presence of 5 MPa pressure on the aluminum and CFRP plate, molten PC wrapped around the reacted RMF or permeated from the cracks on the reacted RMF to the other side of the PC, and after cooling down, the PC solidified again and formed a joint. Figure 7 shows the shear strength of the joints using RMFs with different thicknesses. For each processing parameter, at least three trials were conducted. The difference is reflected by the error bar. As seen in Figure 7, thicker RMF can improve the shear strength of the joint, since the thicker film can generate more heat than a thin film, therefore the PC on both sides of the thicker RMF can melt more completely, resulting in a higher shear strength. Using two pieces of RMF definitely increased the heat release of the reaction, thus forming a much stronger joint. The highest shear strength of 9.5 MPa was obtained using two pieces of RMF, with the RMF thickness of 48 \(\upmu\)m.  
Figure 8 shows the fracture surfaces of both the CFRP and aluminum sides. As seen in Figure 8a,b, the PC has embedded into both the carbon fiber and aluminum and formed a sound mechanical joint. The fracture happened partially on the aluminum side and partially on the PC, which can be proved by the dimples seen in Figure 8c, indicating a ductile fracture was formed. Some parts of the reacted RMF still remained in the fracture surface.  
Figure 6: ANSYS simulation of the temperature evolution and distribution in the joint at various time steps.  
Figure 7: Shear strength of the joints using RMFs with different thicknesses.