Microstructure evolution and mechanical properties of nuclear zircaloy-4 alloy fabricated by laser powder bed fusion at varying laser energy densities

Zircaloy-4 (Zr-4) is widely used in reactor fuel assemblies due to its low thermal neutron absorption properties, superior mechanical properties, and excellent corrosion resistance. Laser powder bed fusion (LPBF) technique provides potential solutions to meet the requirements of fuel assemblies, balancing complex structures with outstanding performance. Therefore, it is of great significance to investigate the microstructural characteristics and mechanical properties of Zr-4 alloy processed using LPBF through a complex solidification process. In this work, the microstructure evolution and mechanical properties of Zr-4 alloy processed under different laser volumetric energy densities (Ev) were investigated, based on an optimized process window for high relative densities of as-built parts. The LPBF-processed Zr-4 alloy exhibited a microstructure of epitaxial columnar β-grains containing highly dislocated α-laths, forming Widmanstätten and basketweave structures. As Ev increased, the width of prior β-grains and α-laths increased by 21.7% and 14.3%, respectively, while the kernel average misorientation and low-angle grain boundary fractions decreased by 40.3% and 46.8%, respectively, indicating a reduction in substructural dislocation density due to lower cooling rates. The medium-Ev specimen demonstrated superior metallurgical quality, featuring approximately 1% higher content of ~57° twin boundaries, along with grain refinement and dislocation strengthening, achieving the highest ultimate tensile strength (725.5 MPa) – approximately 70% higher than the American Society for Testing and Materials’ standard. Nevertheless, limited slip system activation in the hexagonal close-packed structure, deformation constraints from basketweave morphology, and process-induced defects, such as unmelted powder in low-Ev specimens and pores in high-Ev specimens, resulted in reduced ductility. This work provides guidance for tailoring the microstructure and achieving high mechanical properties in LPBF-processed zirconium fuel assemblies.