Microstructure and mechanical response of LPBF-fabricated AZ91D magnesium alloy and TiB₂/AZ91D composite

AZ91D magnesium alloy and 2 wt.% TiB₂/AZ91D composite were fabricated by laser powder bed fusion (LPBF) to improve the stiffness of AZ-series magnesium alloys. The effects of LPBF processing parameters, TiB₂ addition and T4 heat treatment on defect characteristics, microstructure, Young’s modulus and tensile response were investigated. Under the best-performing LPBF condition within the investigated parameter range, the AZ91D alloy exhibited limited visible defects, fine equiaxed grains with an average size of approximately 3–4 µm, and discrete β-Mg₁₇Al₁₂ particles in the Mg matrix. The alloy fabricated under this condition achieved a yield strength of 219.1 ± 13.5 MPa, an ultimate tensile strength of 329.5 ± 0.4 MPa, and an elongation of 7.3 ± 1.4%. TiB₂ addition increased the Young’s modulus from 44.2 ± 1.2 GPa for AZ91D to 56.4 ± 1.0 GPa for the as-printed TiB₂/AZ91D composite, corresponding to an increase of approximately 28%. However, the as-printed composite showed reduced ultimate tensile strength and elongation compared with the AZ91D alloy fabricated under the best-performing condition, which can be associated with residual pores and local TiB₂-rich regions. After T4 treatment at 420 °C for 6 h followed by water quenching, the representative composite retained a high Young’s modulus of 56.2 ± 1.3 GPa. Although the yield strength decreased to 187.3 ± 2.1 MPa, the ultimate tensile strength and elongation increased to 317.3 ± 3.8 MPa and 6.7 ± 1.3%, respectively. The improved tensile deformation capability is likely related to the partial dissolution of β-Mg₁₇Al₁₂ during solution treatment. These results indicate that TiB₂ reinforcement combined with T4 treatment is an effective strategy for improving the stiffness of LPBF-fabricated AZ91D alloy while maintaining acceptable tensile ductility.