In order to obtain a good joint, a highly corrosive specially formulated flux is necessary to be used. [PERSON] et al. [PERSON] et al. (2018) used Al-Si-Ge, Al-Si-Zn and Al-Si-Ge-Zn filler metal with Al\({}_{3}\)-KF-KCl-CsF flux to join 6061 aluminum alloys at higher joining temperature (570 \({}^{\circ}\)C), and the joining strengths reached 87.5, 103.6 and 138.2 MPa, respectively. The flux can cause environmental pollution, and the residual flux may also cause corrosion of the joint. In the study, the flux-less joining strength with Al10 Si10 Cu4 Ti0.1 RE filler alloys (57.3 MPa) is higher than the NaOH surface-treated adhesive bonding (21.8 MPa) [PERSON] et al. (2018), the adhesive bonding with mechanically pretreated rough surface (34.4 MPa) [PERSON] et al. (2018) and gas metal arc welding (53.3 MPa) [PERSON] et al. (2018). Figure 7 presents the microhardness profiles measured across the brazing interfacial region after bonding at 530 \({}^{\circ}\)C for 10 min. Due to the large number of intermetallic compounds and Si-rich particles in the filler alloys, the hardness values of the filler alloys were higher than that of the AA6061 substrate. The hardness values of the Al10 Si10 Cu4 Ti and Al10 Si10 Cu4 Ti0.1 RE filler alloys were higher than that of the Al10.8 Si10 Cu filler alloy due to Al\({}_{5}\)Si\({}_{12}\)Ti\({}_{7}\) precipitation hardening and Al\({}_{2}\)Cu grain refinement. Large increments of the hardness values of the Al10 Si10 Cu4 Ti0.1 RE filler alloy resulted when the trace rare-earth elements were added. Fig. 4.9 The shear strengths of AA6061/AA6061 joints bonded with Al10.8 Si10 Cu, Al10 Si10 Cu4 Ti and Al10 Si10 Cu0.1 RE filler alloys. Figure 8a-c show the fractography of the AA6061/AA6061 joints brazed with Al10.8 Si10 Cu4 Ti and Al10 Si10 Cu4 Ti and Al10 Si10 Cu4 TiO.1 RE filler alloys, respectively. All of the fractographs of the joints indicated that the fractured surfaces were covered with filler alloys with intermetallic compounds and Si-rich solid solution particles, suggesting that the fractures of the joints occurred mainly in the interior of the filler alloys. Mainly intergranular fracture occurred at the joint with Al-10.8 Si-10 Cu filler metal due to the coarse Al\({}_{2}\)Cu intermetallic compounds at the grain boundaries. In the joints brazed with Al10.8 Si10 Cu4 Ti and Al10.8 Si10 Cu4 TiO.1 RE filler alloys, mixed intergranular and transgranular fractures were found at the brazing interfaces. Cross-sectional SEM micrographs under BSE mode of the AA6061/alumina joints brazed with Al10.8 Si10 Cu, Al10 Si10 Cu4 Ti and Al10 Si10 Cu4 Ti0.1 RE filler alloys at 530 \({}^{\circ}\)C for 10 min are presented in Figure 9a-c, respectively. Satisfactory bonding interfaces formed in the AA6061 and alumina joints. No obvious defects were found at the joint interfaces. A few Si-rich and Al\({}_{2}\)Cu intermetallic compound particles were found at the interface between the Al10.8 Si10 Cu filler alloy and alumina, as shown in Figure 9a. Large grains were observed in the Al10 Si10 Cu4 Ti filler metal, as were Al\({}_{2}\)Cu and Al\({}_{5}\)Si\({}_{12}\)Ti\({}_{7}\) intermetallic compounds on the grain boundaries, as shown in Figure 9b. Compared with the Al10 Si10 Cu4 Ti filler alloy, the Al10 Si10 Cu4 Ti0.1 RE filler alloy had a smaller grain structure, and rare-earth elements accumulated at the interface of the Al10 Si10 Cu4 Ti0.1 RE filler alloy and alumina. According to the EDX results, the composition of La segregates was 53.4 at.% of Al and 46.6 at.% of La, which corresponded to the AlLa intermetallic compound [27; 28]. Fig. 4.0 Microhardness profiles for the interfaces of AA6061/AA6061 joints brazed with Al10.8 Si10 Cu, Al10 Si10 Cu4 Ti and Al10 Si10 Cu4 Ti0.1 RE filler alloys at 530 \({}^{\circ}\)C for 10 min. Fig. 4.1 Fractography of the AA6061/AA6061 joints brazed with (**a**) All0.8 Si10 Cu, (**b**) All0 Si10 Cu4 Ti, and (**c**) All0 Si10 Cu4 Ti0.1 RE filler alloys.