Microstructure Evolution and Mechanical Performance of Al-Li Alloy Manufactured via Solid-state Friction Stir Additive Manufacturing

Aluminum-lithium (Al-Li) alloys are highly competitive and attractive lightweight structural materials commonly utilized in the aerospace industry owing to their combination of low density, high damage tolerance, and excellent formability, which enhance the comprehensive mechanical properties of transportation aircraft. Fusion-based additive manufacturing (AM) techniques often suffer from elemental burn-off in high-strength aluminum alloys and similar materials, which can lead to various defects and structural deformations such as porosity, cracks, shrinkage, and micropores. These defects typically arise during the melting and solidification processes. In contrast, friction stir additive manufacturing (FSAM), which is characterized by low-temperature processing and substantial plastic deformation, encourages the formation of fine grains and results in a narrower heat-affected zone, thereby improving the mechanical properties of the fabricated components. FSAM demonstrates significant advantages in the additive manufacturing of lightweight materials, such as aluminum and magnesium alloys. In this study, single-pass multilayer Al-Li alloy samples were fabricated using FSAM. The macroscopic morphology and microstructure of the cross sections were examined using a Leica optical microscope. The fracture mechanisms of the additive samples were analyzed by observing the tensile fracture surfaces using a ZEISS Gemini 500 scanning electron microscope (SEM). The same SEM equipment was used for electron backscatter diffraction (EBSD) mapping to assess the grain size and recrystallization. Microhardness tests were conducted using an SHV-1000Z microhardness tester to produce hardness distribution curves and contour maps of the sample cross sections. Tensile tests were conducted using a WHVS-1 M-AXYZF universal testing machine at a rate of 1.0 mm / s to record the maximum tensile shear force and displacement of each sample. The average values were calculated as the evaluation criteria. Optical microscopy results indicated that the effective additive region of the FSAM samples exhibited a dense structure without visible defects. The material in the stir zone (SZ) experienced the highest peak temperatures and the most intense plastic deformation, undergoing dynamic recrystallization and transformation into finer equiaxed grains. The EBSD analysis revealed that the SZ was entirely composed of fine equiaxed grains. Statistical analysis of grain size demonstrates a gradual decrease in average grain size from the top to the bottom of the SZ, with values of 3.1, 2.9, 2.2, and 2.1 μm, respectively. These variations were strongly influenced by the thermal cycling, stirring, and cooling conditions in each region. Grain-type analysis within the SZ showed that recrystallized grains accounted for 24.349% in the first layer, and the proportion increased to 45.462% in the second layer. In the third and bottom layers, the recrystallized grains comprised 28.441% and 27.053%, respectively. The degree of recrystallization in different regions corresponded to the statistical data of the high-angle grain boundary percentages and geometrically necessary dislocation densities. Microhardness testing reveals a “W”-shaped or “V”-shaped hardness distribution across the sample cross-section, symmetric to the centerline. Hardness progressively decreased from the base material (BM) to the SZ. The average hardness of the BM was 188 HV, whereas the SZ exhibited a relatively lower hardness, indicating a significant reduction due to thermal cycling. Tensile test results showed that the longitudinal specimen (LS) has an ultimate tensile strength (UTS) of 341 MPa and an elongation at break of 5.7%, whereas the transverse specimen (TS) has a UTS of 280 MPa and an elongation at break of 1.1%. The fracture modes in both longitudinal and transverse tensile specimens exhibited typical ductile fracture characteristics. In conclusion, FSAM effectively prevented the formation of multiple defects, promoted the development of fine and dense microstructures, and significantly enhanced the mechanical properties of the additive samples. The study of single-pass multilayer FSAM samples provides insights into the grain characteristics in different regions and their effects on the microhardness and mechanical properties. These findings offer a theoretical basis for further research on FSAM of lightweight, high-performance materials such as aluminum and magnesium alloys.