Beam-current-controlled interfacial evolution in TiAl/Ti-6Al-4V bimetals fabricated via electron beam melting hybrid manufacturing

Hybrid manufacturing of TiAl alloys on Ti-6Al-4V (TC4) substrates via electron beam melting (EBM) holds great promise for lightweight components, yet achieving robust joints is impeded by the tendency to form brittle interfacial phases. More critically, the specific role of beam current in governing non-equilibrium interfacial evolution during EBM-based TiAl/TC4 hybrid manufacturing remains insufficiently understood. This study systematically investigates how beam current (12–15 mA) affects the interfacial microstructure evolution and mechanical response of Ti-48Al-2Cr-2Nb/TC4 bimetals. The key advancement lies in revealing the competition and interplay between elemental diffusion and phase transformation kinetics under rapid solidification. Increased beam current promotes Ti/Al interdiffusion, thickening the interfacial interlayer from ∼50 μm to 200 μm. More importantly, the prolonged high-temperature duration coarsens the prior β grains. This coarse initial microstructure dictates the subsequent β→α/α₂ transformation, driving a microstructural transition within the interlayer (composed of α₂-Ti₃Al and B2 phases) from fine columnar to coarse lath-like grains. Furthermore, within the Partially Melted Zone (PMZ), rapid cooling kinetically freezes a microscopic Al gradient, resulting in a distinctive non-equilibrium mixture of α-Ti and α₂-Ti₃Al within prior β grains. The tensile strength of TiAl/TC4 bimetals is significantly improved from 61.0 MPa to 300.5 MPa. Fracture analysis reveals a shift in failure initiation: from lack-of-fusion defects at low currents to transgranular cleavage within the interlayer at high currents. This indicates that sufficient energy input first eliminates process defects and then makes the interlayer microstructure the performance-limiting factor. This work provides a mechanistic framework linking beam current to non-equilibrium microstructure selection, offering guidance for manufacturing high-performance dissimilar alloy components via electron beam melting.