Achieving high-strength 2195-T8 AlLi alloy double-wire VPPA cladding welded joint by Ag-Nb-Ti-Zr alloying induced grain refinement and T1-Ω phase synergistic strengthening

Abstract

The prevalent strength degradation in fusion welded joints of Al-Cu-Li alloys poses significant challenges in achieving base metal (BM) matching properties like high-strength steel welds. This limitation stems from coarse dendritic structures, eutectic segregation, and insufficient strengthening phases. This study employed double -wire variable polarity plasma arc (VPPA) cladding welding process to weld 2195 AlLi alloy coupled with Ag/In-Nb-Ti-Zr alloying for microstructural and property regulation. With aging treatment, superior strength VPPA welded joints exceeding BM were obtained. A systematic investigation was conducted on grain morphology and precipitation behavior, elucidating microstructural evolution and strengthening mechanisms. The results showed that double-wire feed and arc cladding reduced heat input and suppressed coarse grain growth. Nb-Ti-Zr alloying induced Al₃(Ti, Zr, Nb, V) and β_v' phase. These phases refined grains through peritectic reactions, enhanced nucleation rates, and pinning effect, resulting in fine equiaxed cellular and dendritic grains. θ' phase dominated aged Nb-Ti-Zr alloying welds due to low CuLi vacancy binding energy, yielding minimal T1 phase formation. Indium (In) addition captured quenched vacancies, promoting θ' nucleation via In-Cu-vacancy complexes. Complete T1 phase suppression and PFZ widening degraded strength and elongation. Silver (Ag) addition promoted MgAg co-cluster formation, providing alternative T1 nucleation sites beyond quenching vacancies and promoting precipitation kinetics. The MgAg riched {111}_Al GP zones evolved into the T1-like Ω phase. This dual-phase system achieved breakthrough weld properties in strength (maximum 572 MPa, 101.2 % BM) and hardness (187.9 HV_0.2, 129.9 % BM), exceeding BM for the first time. T1 and Ω phases eliminated universal PFZ at conventional boundaries and enhanced maximum elongation to 6.9 %. Precipitation strengthening from T1/Ω phases contributed 41.2 % of total yield strength enhancement.