Dependence of mechanical properties and microstructure on solidification onset temperature for Al2024-CaB6 alloys processed using laser powder bed fusion

The addition of a sufficient amount of the potent heterogeneous nucleating agent CaB6 enables the fabrication of crack-free specimens from the solidification-crack susceptible high-strength 2024 (Al-Cu-Mg) aluminum (Al) alloy using laser powder bed fusion (LPBF). The present work investigates the effects of varying addition contents of CaB6 nanoparticles (0.0-2.0 wt%) on the alloys' solidification behavior as well as the specimens' solidification-crack volume, microstructure, and mechanical properties. The findings of X-ray microscopy (XRM) analyses on LPBF specimens and in-situ differential fast scanning calorimetry (DFSC) analyses on single powder particles at LPBF-like high heating and cooling rates reveal decreasing crack volumes with decreasing solidification supercooling. A CaB6 content of equal to or greater than 0.5 wt% effectively suppresses solidification cracking. 1.0 wt% is defined as the optimum CaB6 content in terms of mechanical properties. With this content an average grain size of 0.77 mu m, an ultimate tensile strength (UTS) of 478 +/- 4 MPa and an elongation (A) of 13.2 +/- 0.1% are achieved. When the CaB6 content is further increased, the alloy's average grain size asymptotically approaches a minimum size of similar to 0.7 mu m for the given process parameters. This value corresponds to the nucleation-free zone (NFZ), within which the CaB6 nanoparticles present are not activated as nucleating agents, resulting in deposition along the grain boundaries.