A unique microstructure and its formation mechanism induced by laser cladding of Ti6Al4V alloy produced by laser powder bed fusion

A nickel based gradient laser cladding layer was successfully prepared on Ti6Al4V alloy produced by laser powder bed fusion (L-PBF). The nickel-enriched area of middle layer (NEAm) in the cladding layer had a unique structure. NEAm was composed of many large strip-like Ti2Ni grains. The sub-grain inside a single Ti2Ni grain presented a fine dendrite structure (these small dendrite units belong to the same grain, which was different from the traditional dendrite structure, so it was called "pseudo-dendrite"), the interdendrite was composed of amorphous phase or alternating beta-Ti, Ti5Si3 nano-grains or mixtures of amorphous phase and the alternating structure. There were many massive TiNi and dispersed Y2O3, TiC and other fine second phase particles in the dendrite stem. Some of the Si-rich amorphous phase in the interdendrite transformed into alternating beta-Ti and Ti5Si3 nano-grains. Thermodynamic calculation showed that the Gibbs free energies of amorphous phase in the top layer and NEAm were -13.353 and -33.619 kJ/mol, respectively. The glass-forming ability (GFA) values gamma(ABC)* of the top layer and NEAm were 1.813 and 3.067, respectively. Obviously, the amorphous phase formation was easier in the NEAm than in the top layer, which explains the reason that although the cooling rate in the NEAm is slower than that in the top layer, the amorphous phase is only generated in the NEAm. The two- dimensional mismatch between of Y2O3 and Ti2Ni reached 15.43%, which was larger than the limit value of heterogeneous nucleation (12%), so Y2O3 in the NEAm did not play a role in heterogeneous nucleation. An unreported metastable phase was identified in a grain of Y2O3 in the NEAm. Since this phase was sensitive to electron irradiation, extremely limited photo-information was acquired. Accordingly, the Bravais lattice of the metastable phase was reconstructed; its most likely structure was simple cubic. Its lattice constant was anew phase =3 root a(origin) (Y2O3) =1.8369 nm, alpha = beta = gamma = 90 degrees. Its orientational relationship with the parent-phase Y2O3 was {1 (1) over bar1} (origin Y2O3)parallel to{110} (new phase), <<(1)over bar>01>(origin Y2O3)parallel to;<<(1)over bar>10>(new phase). The large increase of the phase interface, the pinning effect of the second phase, and the formation of amorphous phase significantly increased the hardness(up to 1100HV).