Development of an Ag-Free Sn–Ti–Ni Solder for Low-Temperature Ultrasonic-Assisted Flux-Free Joining of Aluminium Alloys in Air

In this study, an Ag-free, rare-earth-free Sn–Ti–Ni solder was designed, fabricated, and evaluated for ultrasound-enabled flux-free joining of Al alloys in air at 400 °C. Ti was incorporated through Ni–Ti pre-alloying to mitigate the intrinsic immiscibility of the binary Sn–Ti system, producing a more homogeneous solder microstructure with refined Ti-bearing precipitates and suppressing the coarse Ti-rich domains observed in binary Sn–Ti alloys. However, under the present low-temperature joining conditions, no evidence of classical Ti-mediated interfacial reaction-layer formation with Al was observed. This is likely due to the thermodynamic stability of Al₂O₃ Al 2 O 3 and the kinetic immobilisation of Ti within stable Sn–Ti intermetallics. Instead, joint formation was governed by ultrasonic oxide disruption, local substrate dissolution, and molten-solder infiltration. Joints produced on commercially pure Al showed limited adhesion and low shear strength ( ~ 20 ∼ 20 MPa), whereas the use of Al-5754 promoted a favourable substrate-assisted joining pathway: Mg appears to dissolve into the molten solder and is associated with the formation of Mg₂Sn Mg 2 Sn -like phases during cooling, together with deeper grain-boundary infiltration and reinforcement of the solder seam. Ultrasonic activation promoted oxide rupture and interfacial disruption and appeared to have enhanced solute redistribution within the seam, enabling stronger joints with more pronounced mechanical interlocking and seam microstructures consistent with chemical reinforcement. Under these conditions, shear strengths exceeding 50 MPa were obtained for Al-5754. These results show that strong joints in Al alloys can be achieved at low temperature in air without Ag or rare-earth additions, and that ultrasound-enabled flux-free joining with a Sn–Ti–Ni solder depends on the combined effects of solder design, oxide disruption, and substrate-assisted chemistry.