Laser powder bed fusion (LPBF) is capable of fabricating aluminum (Al) alloy parts with great geometrical flexibility and rapid prototyping for various industries. However, most commercially available high-strength Al alloys, like AA 2024 or AA 7075, are not suitable for additive manufacturing due to their high susceptibility to solidification cracking, whereas commonly compromised for a higher printability, Al-Si or Al-Si-Mg trade off strength. Here, we systematically demonstrate our efforts on developing a high strength Al alloy Al92Ti2Fe2Co2Ni2 (at%) applicable to LPBF.

 

First, we report several strategies to mitigate the hot crack susceptibility of a high strength Al92Ti2Fe2Co2Ni2 alloy. Routine processing parameter optimization based on varying laser power and scanning speed has to trade off porosity for producing crack-free parts, making it not suitable for load-bearing structural applications. Crack morphology and residual stress measurements indicate that the cracks are generated in the solid state driven by large tensile residual stress, instead of solidification cracking or liquation cracking. Thus, an attempt was made to alleviate the residual stress in a controlled manner. By properly introducing a compliant, sacrificial, scaffold support structure to regulate crack propagation, near fully dense, crack-free parts can be successfully printed. The results are further verified by micro computed tomography, showing that cracking can be arrested in the support before propagating through the parts. This method can be readily applied to other alloy systems without modifying the chemistry.

 

Second, the microstructure and mechanical behaviors were examined. Heterogeneous nanoscale medium-entropy intermetallic lamella form in the as-printed Al alloy. Macroscale compression tests reveal a combination of high strength, over 900 MPa, and prominent plastic deformability. Micropillar compression tests display significant back stress in all regions, and certain regions have flow stresses exceeding 1 GPa. Post-deformation analyses surprisingly reveal that, in addition to abundant dislocation activities in Al matrix, complex dislocation structures and stacking faults form in monoclinic Al9Co2 type brittle intermetallics. Apart from good mechanical properties at room temperature, this alloy exhibits high tensile strength and thermal stability up to 500 ℃ as revealed by in-situ tension studies in a scanning electron microscope. This enhanced high temperature performance can be attributed to a substantial volume fraction of well-dispersed, nanoscale stable

 

intermetallic phases. This study shows that proper introduction of heterogeneous microstructures and nanoscale medium entropy intermetallics offer an alternative solution to the design of ultrastrong, deformable Al alloys via additive manufacturing.