Unique microstructure evolution of a novel Ti-modified Al-Cu alloy processed using laser powder bed fusion

Numerous studies on laser powder bed fusion (LPBF) have already demonstrated the evolution of out-of-equilibrium microstructures with metastable phases. In the present work, a self-designed, pre-alloyed Al-Cu-Ti-Ag-Mg alloy is processed using LPBF. The solidification path, which is necessary to achieve sufficient supercooling to exceed the critical nucleation supercooling (ΔT_n) required for heterogeneous nucleation on L1₂ Al₃Ti nuclei, is derived from the microstructure. This unique microstructure can be divided into two areas: Area 1, with a thickness in the building direction of 5–10 µm, solidifies first and forms on the bottom of the semicircular melting pool. It is dominated by columnar α-Al grains, which contain numerous precipitated cube-shaped Al-Cu-Ti-Ag nanoparticles. During the solidification of Area 1, the constitutional supercooling (ΔT_CS) and the thermal supercooling (ΔT_therm) gradually increase. The Ti and Al atoms in the residual melt react to form numerous primary L1₂ Al₃Ti particles, which are activated for heterogeneous nucleation and serve as nuclei for α-Al grain growth once ΔT_total (ΔT_CS + ΔT_therm) exceeds ΔT_n. Area 2, formed by heterogeneous grain refinement, occupies the remaining part of the melting pool and consists of fine equiaxed α-Al grains. The cube-shaped Al-Cu-Ti-Ag nanoparticles precipitated from the supersaturated α-Al in Area 1 cannot be observed in Area 2. The novel alloy with a fine-grained microstructure exhibits a tensile strength of 475 ± 7 MPa in combination with an elongation to fracture of 8.7 ± 0.5%.