High strength aluminum-cerium alloy processed by laser powder bed fusion

Abstract

Readily available high strength Al-alloys for laser power bed fusion (LPBF) typically require post-processing such as mechanical working or heat treatment to evolve strengthening phases. Through the calculation of phase diagrams (CALPHAD) and a method based on Scheil solidification, the Al-8 wt%Ce-10 wt%Mg alloy was evaluated for LPBF to be microstructurally favorable to yield components with full volumetric density, without the need for post-processing. Through an exhaustive LPBF parametric study, documentation of defects was performed for the use of a wide range of laser power and scan speed combinations, i.e., varying the energy density, followed by microstructural characterization with X-ray diffraction, optical and electron microscopy, and mechanical testing with both Vickers hardness and quasi-static uniaxial tension. Overall, defects were observed with use of high and low laser powers and scan speeds, but a processing window for LPBF was observed at both high and low laser powers, where > 99% volumetric density was achieved. Besides defects, decreasing the scan speed, i.e., increasing the energy density, resulted in decreases in the concentration of Mg, down to a maximum of 4.5 wt% due to vaporization. Moreover, a decrease in Mg led to a lower Vickers hardness, attributed to a decrease in the solid solutioning of Mg in the α-Al matrix. Samples selected for tensile mechanical testing were produced with a LPBF laser power of 200 W traversing at 800 mm/s, which yielded a density > 99% and Mg concentration of approximately 7 wt%. A yield strength and ultimate tensile strength of 377 and 468 MPa, respectively, was found for those produced with the power of 200 W. However, the ductility of the alloy was less than 2% elongation at failure. A fine (< 1 µm) sub-grain cellular solidification structure was found distributed throughout the α-Al matrix, with ribbons of the eutectic Al11Ce3 intermetallic distributed along the intercellular boundaries. The high strengths achieved in this alloy were attributed to contribution from Orowan dislocation looping of the Al11Ce3 phase, the high degree of solid solutioning of Mg in the α-Al matrix, and cellular boundary strengthening due to impeding dislocation movement.