## 3 Results and Discussion  
### Microstructure  
In the WAAM process, each deposition is accompanied by thermal input. The continuous thermal cycle results in a differentiated microstructure distribution within the deposition unit. Based on the ultimate microstructure distribution, the deposition unit can be divided into three main regions: the fine crystal region, the columnar crystal region, and the heat-affected region. Figures 6 and 7 illustrate the microstructures of different sections in the WAAM-fabricated invar alloy. Figure 4: Microstructural characterization of the surfaces with different build orientations of WAAM-fabricated invar alloy. Figure 5: Sampling position (**a**) of impact and tensile specimens, size of tensile specimen (**b**), and impact specimen (**c**). In Figure 6a,b, the dark regions and deposition units on the Y-Z section of WAAM-fabricated invar alloy are periodically distributed. It has been confirmed that the dark region in Figure 6a is actually a heat-affected area formed during deposition [23]. Figure 6c shows a singular deposition unit, wherein the microstructures exhibit variation across different locations, primarily associated with the thermal dissipation conditions of the molten pool during solidification and the cyclical thermal input throughout the deposition process. During the cooling of the molten pool, the substantial temperature gradient of the liquid metal at the bottom results in an elevated heat dissipation rate, consequently leading to the formation of fine grains. The liquid metal at the core of the molten pool exhibits a reduced temperature gradient and diminished heat dissipation, resulting in a larger grain size. At the same time, as the molten pool during deposition radiates heat to the air above and to the metal below, a temperature gradient along the \(Z\)-axis is formed, and the crystals grow in the direction of the highest temperature gradient, eventually forming columnar grains along the \(Z\)-axis [24]. The heat-affected zone is located at the top of the deposition unit, and the grains within this zone are predominantly coarse columnar grains. This occurs because when the top area of the deposition unit is exposed to the heat influence from the overlying deposition units, the temperature in this region increases to the austenitizing temperature, leading to the growth of austenitic grains and the subsequent formation of coarse columnar grains [25]. This phenomenon can also be seen in the X-Y and X-Z planes, shown in Figures 6b and 7a. These figures are microscopic images of the continuous  
Figure 6: Deposition unit and its microstructure. (**a**) Macro-morphology of deposition units. (**b**) Schematic of the deposition unit enclosed by the fusion line. (**c**) Schematic of the deposition unit. (**d–f**) Microstructure within deposition unit. Figure 7: Microscopic morphology of adjacent areas of deposition units: (**a**) X–Y section; (**b**) X–Z section. deposition areas near the fusion line in the X-Y and X-Z planes. Two adjacent deposition units, the austenite grains in the earlier-formed deposition unit, will further grow due to the thermal influence of the later-formed deposition unit. Figure 8 shows the microstructures of the alloy 1 specimen and alloy 2 specimen in the X-Y section. It is evident that the grain morphology of both the alloy 1 specimen and alloy 2 specimen is dominated by equiaxial crystals in the X-Y section. As shown in Figure 8a, the grain size distribution in the alloy 1 specimen is uneven, with the smallest grain size being only 29 \(\upmu\)m, the largest grain size being 538 \(\upmu\)m, and the average grain size being 311 \(\upmu\)m. The grain size of the alloy 2 specimen clearly has become finer in Figure 8b, and its homogeneity has somewhat improved. With an average grain size of 206.22 \(\upmu\)m, the alloy 2 specimen shows a grain refinement of almost 33.7% over the alloy 1 specimen. In addition, we can observe some fine cellular substructures, which are regularly arranged within the austenite grains, as depicted in Figure 8c,d.