Effect of AlCoCrFeNi2.1 High-Entropy Alloy Reinforcement on the Densification, Microstructure, and Hot-Cracking Behavior of LPBF-Processed AA7075

The application of laser powder bed fusion (LPBF) to 7xxx-series aluminum alloys is fundamentally limited by hot cracking and porosity. This study demonstrates that adding 5 wt.% AlCoCrFeNi2.1 high-entropy alloy (HEA) particles to 7075 aluminum alloy (AA7075) powder can effectively mitigate these issues. Microstructural characterization revealed that the HEA particles remained largely intact and formed a strong metallurgical bond with the α-Al matrix. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis confirmed that this bonding is facilitated via the in situ formation of new intermetallic phases at the particle/matrix interface. X-ray diffraction (XRD) identified these phases as primarily Al5Co2 and Fe3Ni2. A key consequence of this reinforced interface is a significant change in cracking behavior; optical microscopy (OM) showed that long, continuous cracks typical of AA7075 were replaced by shorter, deflected cracks in the composite. While porosity was not eliminated, the addition of HEA stabilized the process, yielding a consistent density improvement of 0.5–1.5% across the processing window. This microstructural modification resulted in a substantial ~64% increase in average microhardness, which increased from 96.41 ± 9.81 HV0.5 to 158.46 ± 11.33 HV0.5. These results indicate that HEA reinforcement is a promising route for engineering the microstructure and improving the LPBF processability of high-strength aluminum alloys.