*3.3. Microhardness*

In this study, the average microhardness is measured from the bottom to the top, the changes in the microhardness of the vertical cross section of the as-deposited component are measured, the results are reported in Figure 9. It can be seen that the maximum value of microhardness is found to be around 290 HV near the bottom part. As the height of deposition increases, the microhardness decreases slightly and fluctuats between 280 HV and 270 HV. Then, the microhardness varies between 270 HV and 260 HV when measured near the top part. Generally, the deposited material goes through complex thermal cycles, which is expected to affect the microhardness [26,27]. It has been confirmed that the fluctuation of the microhardness comes from various thermal cycles and cooling rates in diverse parts of the component, and the microhardness values of multilayers are influenced by the heating at previous layers [28]. Furthermore, the compositions of the microstructure at the bottom, in the middle, and on the top also play a significant role on microhardness. As depicted from the previous analysis on phase transformation, the phase at the bottom is made up of tempered bainite and tempered sorbite, the phase in the middle is composed of tempered bainite, and the phase on the top is comprised of tempered bainite, tempered sorbite, and ferrite. In terms of the microhardness, tempered bainite + tempered sorbite > tempered bainite > tempered bainite + tempered sorbite + ferrite, hence, the microhardness of the components shows such a characteristic, which is in line with the microstructure results.

**Figure 9.** Microhardness along the longitudinal direction for the cross section.

#### *3.4. Tensile Test and DIC Analysis*

The results of transversal and longitudinal tensile tests are listed in Table 3. As summarized from the data, the transversal tensile property obtained is better than that of the longitudinally stretched, indicating an anisotropy behavior, which usually occurs in the process of WAAM [29].


**Table 3.** Transversal and longitudinal tensile properties.

To reveal the anisotropy mechanisms, a DIC analysis is performed on a typical group of specimens and eight typical points are selected to represent the strain evolution during the transversal and longitudinal tensile tests separately. The selected experimental points can be seen from the stress–strain curves, as shown in Figure 10.

**Figure 10.** Transversal and longitudinal stress–strain curves and eight typical points selected. (**a**) Transversal curve; (**b**) longitudinal curve.

Figure 11 shows the strain evolution of the transversal and longitudinal specimens during the tensile testing. Each typical point selected in Figure 10 corresponds to the point in Figure 11. For the tensile test of the transversal specimen, strain evolution can be seen from P1 to P8, as shown in Figure 11a. Before the global strain reaches a certain extent, uniform strain distribution can be observed in the deformation area such as P1 and P2. This means that every area of the gauge section is involved in uniform deformation during the tensile process. As the tensile test proceeds, the applied load begins to increase. Local stress concentrations can be obtained from P3 to P6. Then, when necking takes place in the area of deformation, the local stress concentration intensifies and the macro stress reduces, as shown as P7 and P8. The strain evolution of the transversal tensile process is the same as that of the classic homogeneous material. In contrast, the strain evolution of the longitudinal specimen can be summarized by P1' to P8', as shown in Figure 11b. It can be seen that a high local strain occurred around several areas of the gauge section, shown as P1' to P4'. This means that nonuniform strain distribution happens due to inhomogeneous stress. As the experiment continued, local stress concentration can be seen, but is markedly di fferent from that of the transversal specimen. During this stage of deformation, the maximum strain existes while combined with local high strain near other areas, which can be observed as P5' and P6'. At the end of the experiment, necking emerged, leading to stress concentration, which reduces the e ffect of the inhomogeneous strain distribution of the specimen, seen as P7' and P8'. From the comparison of the strain evolution between the transversal and longitudinal specimens, the latter one shows inferior properties and a nonuniform strain distribution.

**Figure 11.** Strain evolution of transversal and longitudinal specimens during the tensile test. (**a**) Transversal specimen; (**b**) longitudinal specimen.

It is well known that the anisotropy of AM leads to di fferent transversal and longitudinal behaviors and properties, mainly influenced by temperature gradients that are varied in successive layers and the penetration of the molten pool in the previous deposited layer [30–32]. As heat dissipation is higher in the forming direction than that in other directions, its solidified direction results in the anisotropy of grain morphology and mechanical properties. Additionally, longitudinal specimens are parallel with the building direction, implying that more inter-layer areas will be included within the test region. As a weakness, phase mutation and residual stress can usually be found near the inter-layer area, which results from the element interaction and large temperature gradient [33,34]. Researchers have also

pointed out that the inter-layer area is considered as the weak link and that the strain tends to be focused in this region [35]. The cracks and material failure usually take place around these areas in the non-uniform specimen [36].

To give a further illustration of anisotropy, the strain cloud images of the transversal and longitudinal tensile specimens during the first 70 s of the experiment are shown in Figure 12. It can be seen from the differences in Figure 12b that they exhibit a nonuniform and intermittent strain distribution of the longitudinal tensile specimen because several interlayers are included, and that of the transversal specimen is relatively uniform, as shown in Figure 12a.

**Figure 12.** Cloud images of strain of transversal and longitudinal specimens. (**a**) Transversal specimen; (**b**) longitudinal specimen.

Additionally, after investigation on the initial stage of deformation, a comparison of the local strain distributed in the gauge section of the transversal and longitudinal tensile specimens is also made. Referred to as P5, P6, P7 and P5', P6', P7', the results are shown in Figure 13. A high local strain region of the transversal specimen lay in the middle part and the local strain concentration reaches the maximum at P7, as shown in Figure 13a, while more than one region with a high local strain can be observed from the longitudinal specimens. With the experiment proceeding, the local strain difference increases and the local strain concentration is aggravated and reaches a peak at P7', as shown in Figure 13b. Therefore, nonuniform local strain distribution of the longitudinal specimen is obvious, regardless of whether it is at the beginning or end of the deformation.

**Figure 13.** Comparison of the local strain distribution in the gauge section: (**a**) Transversal specimen; (**b**) longitudinal specimen.

The high strain zone is used to refer to all zones of strain concentration and treated as a significant consideration during the tensile test [37]. In this study, the strain distribution condition in the high strain zones before fracture on the transversal and longitudinal tensile specimens is shown in Figure 14.

**Figure 14.** Strain distribution condition in high strain zones before fracture. (**a**) Transversal specimen; (**b**) longitudinal specimen.

Based on the strain distribution condition, high strain zones can be divided into six high strain gradients and a comparison of the duration of high strain gradients is made to reveal the differences between the transversal and longitudinal tensile specimens. The results are shown in Figure 15. It can be seen from Figure 15, that the duration in the high strain zone (from stage I to VI) of the transversal specimen is longer than that of the longitudinal specimen. Furthermore, as high strain varied from stage VI to I (close to fracture), the duration gap between the transversal and longitudinal specimens gradually expands, suggesting that in high strain zones, the transversal specimen performed better, and the higher strain stage the tensile test experiences, the more remarkable the duration superiority of the transversal specimen.

**Figure 15.** Comparison of the duration in high strain stages between the transversal and longitudinal specimen. T—Transversal specimen; L—longitudinal specimen.
