Laser powder bed fusion (LPBF) technology can shape metal components into almost any complex shape. Casting is the most commonly used method for fabricating magnesium (Mg) alloy components. However, because of the high vapor pressure and susceptibility of Mg to reacting with air, cast Mg alloys often exhibit defects and relatively poor mechanical properties. Mg alloys manufactured using methods such as forging, rolling, and extrusion are deformed to improve performance; however, forming complex magnesium alloy components is difficult. Although traditional manufacturing methods have laid the foundation for improving Mg alloy properties and their applications, these alloys struggle to meet the demands of complex components in fields such as aerospace, biomedicine, and electronic communication. The application prospects are widened by combining the characteristics of Mg alloys such as high specific strength, high specific stiffness, good electromagnetic shielding performance, and extremely low weight. Currently, the density of Mg alloys formed by LPBF is generally low, and the mechanisms underlying defect formation and microstructural evolution remain unclear. In this study, the densification behavior, defect formation mechanism, and microstructure of LPBF-formed AZ91D magnesium alloy are investigated under different process parameters, and high-quality samples of AZ91D magnesium alloys are formed using LPBF technology.

AZ91D magnesium alloy specimens were formed using LPBF equipment with a fiber laser wavelength of 1064 nm and a maximum laser power of 500 W. During processing, the laser spot size was adjusted to 80 μm, and two preheating temperatures of 25 ℃ and 200 ℃ were applied. Square specimens measuring 10 mm×10 mm×10 mm were formed on cast AZ91D magnesium alloy substrates. An optical microscope (OM) was used to capture metallographic images, and ImagePro Plus software was used for defect shape and type statistics analysis. A scanning electron microscope (SEM) was employed to analyze the internal morphology of the defects and the microstructure of the formed specimens. The elemental composition of the specimens was characterized using a SEM with energy-dispersive X-ray spectroscope (EDS). An universal material testing machine was used to conduct room-temperature static tensile tests on relevant specimens.

Under a preheating condition of 200 ℃, the forming interval of AZ91D magnesium alloy can be distinctly divided into porosity, transition, dense forming, and unmelted zones. In the dense forming area, high-density (99.9%) AZ91D specimens without voids or lack-of-fusion defects were successfully formed. Under the processing conditions used in this study, the forming density was sensitive to hatch space such that when the hatch space increased to 0.08 mm, the overall forming density decreased significantly. In Fig. 5, when the hatch space is 0.08 mm, the range of input energy density (150 J/mm³≤ρE≤280 J/mm³) in the dense forming zone decreases significantly compared with that (120 J/mm³≤ρE≤380 J/mm³) when the hatch space is 0.06 mm, and the unmelted range significantly increases. An increase in hatch space leads to more inter-track lack-of-fusion defects, resulting in a larger unmelted zone. Under the preheating condition of 25 ℃, several specimens cannot be formed simultaneously, and the formation is unstable. During the formation of Mg alloys, evaporation of elements leads to a large number of porosity defect variations formed according to the Stokes law in Equation (4). At a high laser power and low scanning speed, keyhole and vapor pores appear simultaneously. In samples with a hatch space of 0.08 mm, unmelted powder is observed in the unmelted zone, presenting a typical inter-track lack-of-fusion shape. At a high laser power and low scanning speed, a large number of elements evaporate during the forming process of magnesium alloys, which removes heat and shields the laser. Under a preheating condition of 200 ℃, the evaporation of elements is the dominant factor affecting the grain size of specimens formed using different parameters. Under the same processing parameters, specimens formed under a preheating temperature of 25 ℃ exhibit finer grains than those formed at 200 ℃, with the cooling rate being the dominant factor. The microstructures of the formed AZ91D magnesium alloy specimens are determined based on the cooling rate and evaporation of the elements. However, under a preheating temperature of 25 ℃, a higher temperature gradient predisposes the specimens to cracking, interfering with their formation.

High-density specimens (99.9%) were successfully formed without porosity or lack-of-fusion defects under a preheating condition of 200 ℃, with a laser power of 200 W, scanning speeds ranging from 300?400 mm/s, and a hatch space of 0.06 mm. With an increase in hatch space to 0.08 mm, inter-track lack-of-fusion defects increased, leading to a decrease in overall forming density and a reduction in the forming window, as the dense forming input energy density range changed from 120 J/mm³≤ρ_E≤380 J/mm³ to 150 J/mm³≤ρ_E≤280 J/mm³. The specimens were characterized by keyholes and evaporative pores with different morphologies on the inner surfaces of their walls. The evolution of the specimen microstructures was jointly determined by the cooling rate and element evaporation. At the same preheating temperature, specimens with severe element evaporation usually had finer grains. For the same processing parameters, the specimens with lower preheating temperatures had finer grains because of the higher cooling rate. With the dense forming parameters, the ultimate tensile strength (UTS) reached (322.04±6.72) MPa, and the yield strength reached (258.07±4.72) MPa, and the mechanical properties far exceeded those of cast Mg alloys, reaching forging standards.