Heat treatment of laser welded dissimilar Al-Cu-Li joints with significant Li/Cu ratio difference

BACKGROUND: Joining dissimilar materials is a complex but important task for industry as dissimilar materials are necessary in complex structures. This paper presents the results of microstructure optimization of a laser welded dissimilar Al-Cu-Li joint with different Cu/Li ratios. AIM: The aim was to study the structure of a laser welded dissimilar Al-Cu-Li joint and to develop methods to improve this structure based on heat treatment. METHODS: We used scanning electron microscopy, synchrotron radiation diffraction, and tensile testing. Synchrotron radiation (SR) diffraction allowed to detect lithium-containing phases, where they cannot be fully detected using less bright radiation sources due to their volume fraction and scattering. RESULTS: The study shows that welding promotes creation of a mechanically unfavorable microstructure; dendritic grains of aluminum surrounded by an eutectic network of intermetallic compounds T1Al2CuLi, T2Al6CuLi3, and T3Al5CuLi3. In this state, the weld seam had low mechanical properties; tensile strength σВ was 252 MPa, yield strength σ0.2 was 184 MPa, and linear strain δ was 2.9%. Thermal quenching at 530 °C allowed to almost completely dissolve the eutectic network, which was confirmed by scanning electron microscopy. Artificial aging at 170 °C for 16 hours allowed for the recrystallization of the strengthening phases. As a result, the welded joint tensile strength σВ increased to 344 MPa, yield strength σ0.2 increased to 230 MPa and the linear strain δ increased to 4.8%. CONCLUSION: The study allowed to identify the structure of laser welded dissimilar Al-Cu-Li joints and showed that this structure can be improved by heat treatment, ensuring a significant increase in both the strength properties and ductility of the joint.