*3.3. Tensile Mechanism*

In the laser melted zone, the metal melted and solidified at an extremely rapid rate and formed a smaller and denser microstructure. According to the research of Aqida et al. [26], microstructure improvements enhanced the mechanical properties. As can be seen from Figure 13, during the initial stage of tension application, the units resisted external force, and the stress acting on the crack was reduced. After fracture, the propagation path of the crack inside the unit presented a folded line, which indicated that during tensile tests, units with good toughness concentrated most of the tensile stresses, which hindered crack propagation; therefore, units with greater strengths led to specimens with greater tensile strengths. The relationship between grain size and yield strength can be expressed by the Hall–Petch formula [27]:

$$
\delta\_\mathbf{y} = \delta\_\mathbf{i} + \mathbf{k}\_\mathbf{y} \mathbf{d}^{-1/2} \tag{1}
$$

where δy is the yield strength of the material, δi represents the resistance to dislocation movement; ky is a constant related to the grain size, and d is the average diameter of grains. The formula indicates that the finer the grain size, the higher the strength. In addition, the larger effective cross-section size of the units led to the more significant enhancement effect on the tensile strength of specimens. For samples No. 1 to No. 5, as the laser energy input increased, the unit size was increasingly enlarged, but the grain size gradually decreased, so the strength exhibited a trend of increasing.

As shown in Figure 5, the units of samples No. 1–4 did not completely lock the cracks, and microcracks and shrinkage cavities were observed in the sample. On the one hand, these defects reduced the effective cross-sectional area of the unit; on the other hand, when samples were subjected to loads, these residual defects will serve to create new cracks, thus reducing the tensile strength of the sample. However, No. 5 displayed the highest tensile strength because there were no defects in the unit, and the cracks were completely locked.

**Figure 12.** The stress-strain curves of the tested samples.

**Figure 13.** The fracture morphology of units. (**a**) Tensile specimen after fracture; (**b**) partial magnification of the tensile fracture.
