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-CuTi-Ag-Mg alloy is processed using LPBF. The solidification path, which is necessary to achieve sufficient supercooling to exceed the critical nucleation supercooling (delta T-n) required for heterogeneous nucleation on Ll(2) 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 mu m, solidifies first and forms on the bottom of the semicircular melting pool. It is dominated by columnar alpha-Al grains, which contain numerous precipitated cube shaped Al-Cu-Ti-Ag nanoparticles. During the solidification of Area 1, the constitutional supercooling (delta T-CS) and the thermal supercooling (delta T-therm) gradually increase. The Ti and Al atoms in the residual melt react to form numerous primary Ll(2) Al3Ti particles, which are activated for heterogeneous nucleation and serve as nuclei for alpha-Al grain growth once delta Ttotal (delta T-CS + delta T-therm) exceeds delta T-n. Area 2, formed by heterogeneous grain refinement, occupies the remaining part of the melting pool and consists of fine equiaxed alpha-Al grains. The cube-shaped Al-CuTi-Ag nanoparticles precipitated from the supersaturated alpha-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%.