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 (ΔTn) 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 (ΔTCS) and the thermal supercooling (ΔTtherm) 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 ΔTtotal (ΔTCS + ΔTtherm) exceeds ΔTn. 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%. [Display omitted] •Pre-alloyed powder of Al-Cu-Ti-Ag-Mg was prepared by inert gas atomization.•The Al-Cu-Ti-Ag-Mg alloy was processed crack-free by LPBF.•A unique microstructure consisting of two different areas was observed.•Star-shaped L12 Al3Ti particles induced effective heterogeneous nucleation.•A solidification path of 5–10 µm was required to exceed ΔTn of the L12 Al3Ti nuclei.