Anisotropy in Microstructure and Mechanical Properties of Pure Zinc Fabricated by Laser Additive Manufacturing

Objective Selective laser melting (SLM) is a widely popular metal additive manufacturing technique that offers distinct advantages in fabricating bone implants with customized shapes and internal bionic porous structures. In particular, using a very high cooling rate (10(3)-10(8) K. s(-1)) during the SLM process can inhibit the grain growth of pure Zn and confer good mechanical properties. This study reveals the internal relationship between the microstructure and mechanical anisotropy of SLM-fabricated pure Zn. We also report the influences of the grain characteristics and texture on the anisotropy. Methods The purity (mass fraction) of Zn powder used in this experiment is 99.9% and the sizes of particles are 7.2-29.7 mu m. Pure Zn samples are fabricated using a commercial SLM printing device equipped with a 200 W fiber laser. The density of a pure Zn sample is greater than 99.5% when using optimized forming parameters (laser power P=80 W, and scanning speed V-S=900 mm. s(-1)). To investigate the mechanical anisotropy, the fabricated Zn samples with dimensions of 8 mmx8 mmx8 mm are microscopically characterized in the horizontal and vertical directions. After etching with the 4% (volume fraction) nitric acid solution for 5 s, the microstructures on both the horizontal and vertical planes of the Zn samples are characterized using a metallographic optical microscope (OM) and scanning electron microscope (SEM). The grain orientation, grain size, and texture information are analyzed using electronic backscattered diffractometer (EBSD). Moreover, tensile samples with a gauge length of 22.0 mm, width of 3.0 mm,and thickness of 2.8 mm are fabricated for tensile tests. Results and Discussions Significant differences are observed in the microstructures of Zn samples formed by SLM on horizontal and vertical planes. A large number of equiaxed grains are observed on the horizontal plane in the OM and SEM images. In contrast,fish-scale molten pools with depth of 30-50 mu m and width of 100-150 mu m are found on the vertical plane. Furthermore, most of the grains exhibit preferred orientations along <(1) over bar2 (1) over bar0 > and < 01 (1) over bar0 > perpendicular to the building direction (BD) on the horizontal plane. In contrast, on the vertical plane, a majority of the grains display preferred orientations along (sic) 0001 (sic) (red region) parallel to the BD.Notably, the average grain size (10.21 mu m) on the horizontal plane is 40.8% smaller than that (17.24 mu m) on the vertical plane. The statistical distribution of grain boundary misorientation angles (Fig.6) and analysis results of the Kernel average misorientation (KAM)(Fig.7) indicate that low-angle grain boundaries (LAGBs) are more prevalent on the horizontal plane and these areas also exhibit a higher dislocation density. The KAM value on the horizontal plane is 0.84(degrees), which is marginally higher than the value of 0.79 degrees observed on the vertical plane. Finally, we report the examination results of the tensile properties of the SLM-fabricated Zn in both the horizontal and vertical orientations (Fig.8). The strain hardening rate of the specimens in the horizontal direction exceeds that of the specimens in the vertical direction. A quantitative analysis of the tensile properties reveals distinct mechanical characteristics for Zn specimens fabricated on different planes. The yield strength, ultimate tensile strength, and elongation of the specimens fabricated on the horizontal plane are 108.0 MPa,123.5 MPa, and 11.7%, respectively. However, Zn specimens fabricated on the vertical plane exhibit a yield strength of 90.2 MPa, an ultimate tensile strength of 108.0 MPa, and an elongation of 14.1%. Although specimens fabricated on the horizontal plane demonstrate yield and ultimate tensile strengths that are 16.5% and 12.5% greater than their vertical counterparts, respectively, their elongation rate is 17% lower than that of the vertical specimens. The aforementioned results collectively indicate the presence of anisotropy in the mechanical properties of SLM-fabricated Zn. Conclusions This study reports the investigation results on the microstructure and mechanical properties of SLM-fabricated Zn in both the horizontal and vertical directions, with a particular focus on the grain morphology and orientation. Furthermore, the relationships between these microstructural aspects and mechanical properties are discussed. The SLM-fabricated Zn exhibits pronounced anisotropy in its tensile strength and ductility. Specimens fabricated on the horizontal plane exhibit a higher yield strength and ultimate tensile strength but a lower elongation rate compared to those fabricated in the vertical direction. The greater strength of horizontally fabricated specimens is primarily attributed to their finer grain size and higher initial dislocation density, which hinder dislocation movement. Conversely, specimens fabricated on the vertical plane demonstrate enhanced ductility because they contain a higher proportion of high-angle grain boundaries, which effectively impede crack propagation and thereby prevent premature fracturing.