Thermal stability of gradient microstructure and property of Ni-Ti based laser cladding layer on Ti6Al4V alloy fabricated via laser powder bed fusion

Annealing effects on Ni-Ti based gradient laser cladding layer on the surface of Ti6Al4V alloy fabricated via laser powder bed fusion (LPBF) were studied in this paper. Ti₂Ni growth was driven by low transformation activation energy (11.80 kJ/mol) and thermodynamic spontaneity (ΔG < 0) which resulted in a 3.94 % increase in hardness in the top layer at 873 K. The amorphous phase in nickel-enriched area of middle layer (NEAm) resisted full crystallization even at 1123 K due to multi-element composition, atomic size mismatch, negative mixing enthalpies, and its low amorphous Gibbs energy (−33.619 kJ/mol). Ti₂Ni grain size increased 328 % and α-Ti precipitated by interfacial diffusion imbalance in the bottom layer at 1123 K. The static recrystallization (SRX) was triggered by higher dislocation density from non-equilibrium solidification and thermo-mechanical constraint imposed by the substrate in the fusion zone at 1123 K. Avrami plate growth model (n = 1) for kinetic analysis predicted 62.18 % SRXed fraction, matching experimental 60.83 %. Top-layer hardening was dominated by the phase transformation strengthening after 773 K and 873 K annealing, while grain coarsening caused softening above 973 K. Excepting NEAm and HAZ, excellent thermal stability resulted from thermally stable second phases (Y₂O₃/TiNi/Ti₅Si₃) and their pinning effect. The hardness of NEAm declined sharply due to the combination of amorphous crystallization and grain coarsening. Hardness in HAZ approached substrate level as a result of coarsening of α’-Ti. Ti₂Ni/TiNi grain growth remained negligible below 973 K but accelerated sharply at 1123 K. Despite significant softening of HAZ at 1123 K, hardness of the other layers remained almost unchanged, confirming high heat resistance of this Ti-Ni cladding layer.