Novel approach to additively manufacture high-strength Al alloys by laser powder bed fusion through addition of hybrid grain refiners

Grain refinement is effective in restraining hot tearing, reducing anisotropy, eliminating defects, improving processability, and enhancing the mechanical properties of high-strength aluminum components additively manufactured by laser powder bed fusion (LPBF). However, achieving the desired strength and ductility in LPBFfabricated high-strength aluminum alloys post grain refinement is a predominant challenge. We have therefore designed and developed a novel hybrid grain refiner (solute/ceramic nanoparticles) which can effectively refine grains and enhance the mechanical properties of LPBF-fabricated high-strength aluminum alloys. Adding a Ti/ TiN hybrid grain refiner to the LPBF-fabricated 7050 alloy can produce ultrafine grains with an average size of 775 nm, resulting in an ultimate tensile strength and ductility of up to 408-618 MPa and 13.2-8.8%, respectively. These tensile properties are comparable to those of conventional wrought 7XXX alloys. During LPBF processing, the hybrid grain refiner exhibited interesting synergistic grain refinements and strengthening mechanisms between the solute and the ceramic nanoparticles. During solidification, not only in-situ particles formed by the chemical reaction of the solute in liquid Al and the externally added ceramic nanoparticles can act as the nuclei of alpha-Al respectively, but also solute can inhibit the agglomeration of ceramic nanoparticles to promote their nucleation efficiency. Moreover, the strength can be further improved by doping the solute atoms at the ceramic nanoparticle/Al interface. The improvement in elongation benefited from the uniform dispersion of the various particles.