**3. Results and Discussion**

### *3.1. Microstructure and Sizes of Units*

Figure 8 shows optical microscopy images of the cross-sectional morphology of units after cracks were repaired using different laser parameters. It can be clearly seen that there were obvious differences between the unit and the matrix after laser melting. The unit was parabolic, and there was no spheroidal graphite in the matrix. Due to the different laser parameters, the units had different shapes and sizes. The laser energy density used during laser treatment and the size of the prepared units are shown in Table 3.

**Figure 8.** Optical microscopy images of the cross-sectional morphology of units: (**a**) No. 1, (**b**) No. 2, (**c**) No. 3, (**d**) No. 4, and (**e**) No. 5.



The sizes of units were measured from each sample using an optical microscope. Consequently, along with an increase in the laser energy input, the width of the units were in the order of No. 1 < No. 2 < No. 3 < No. 4 < No. 5, while the unit depths were in the order No. 1 < No. 2 < No. 3 < No. 4 < No. 5. The laser beam energy obeys a Gaussian function, and the energy is mainly concentrated in the central part of the laser spot [23]. The higher the laser energy density, the deeper the heat transferred to the sample, and the depth gradually increased. The width of units was mainly related to the spot diameter, and since it was the same, no significant changes in width were observed. The units gradually changed from a flat crescent shape to a U-shape. Due to the low laser energy densities, samples No. 1–4 had remelting areas that were too small to completely bridge the cracks. Since less metal was melted by heat, the flowing liquid metal could not completely fill the cracks. After solidification, cracks and holes appeared in the remelting zone, which may form new cracks and reduce the thermal fatigue resistance of the material. There were no microcracks or holes in the remelting zone of sample No. 5, and the thermal fatigue cracks were completely repaired.

Figure 9 shows the microstructure of the units under SEM. Due to the rapid increase in the sample surface temperature during laser treatment and then a rapid decline after laser treatment, the microstructure of the melting zone was a dense dendritic structure. Thus, the nucleation of the molten metal during rapid cooling occurred much faster than grain growth, which resulted in a small micron-sized microstructure. Obviously, the microstructure of samples No. 1–5 showed similar dendritic crystals but with different dendrite densities. As the laser energy density increased, the dendrite densification of No. 1–5 cells gradually increased, and the grains were gradually refined. The smaller interdendritic spacing resulted in the higher micro-hardness and the better mechanical properties theoretically [24].

**Figure 9.** SEM images of the microstructure of units: (**a**) No. 1, (**b**) No. 2, (**c**) No. 3, (**d**) No. 4, and (**e**) No. 5.

X-ray diffraction was used to analyze the phase of the matrix and units on the surface of untreated and laser melted samples. The results are shown in Figure 10. Due to the large temperature gradient between the molten pool and the surrounding substrate, liquid metal was supercooled and thus recrystallization occurred at a higher cooling speed to form martensite (M). Graphite and iron formed cementite (Fe3C), and tough residual austenite (γ-Fe) was also found in the unit. In the heat-affected zone (HAZ) between the substrate and the melted zone, a slightly lower temperature below the melting point of the cast iron led to a faster austenitization and partial dissolution of graphite. Due to rapid cooling, carbon could not evenly spread, and the carbon concentration of austenite near graphite increased, causing high-carbon needle martensite (M) and residual austenite to form after rapid cooling. Additionally, a good metallurgical bond was formed between the HAZ and the melting zone, as shown in Figure 11.

**Figure 10.** X-ray diffraction pattern taken from the surface of (**a**) untreated sample and (**b**) bionic blocked unit.

**Figure 11.** The interface between the melting zone and the substrate.

The reported microhardness values in Table 3 are the average of ten measurements taken at different locations in the cross-section of each unit. As the laser energy input increased, the micro-hardness of units followed the order No. 5 > No. 4 > No. 3 > No. 2 > No. 1, and the microhardness of all units were significantly higher than that of the untreated sample. Sample No. 5 displayed the maximum microhardness of 680 HV0.2, which was 128.2% higher than the untreated sample. Fine-grain strengthening was the main reason for this order. Smaller grain sizes resulted in a larger grain boundary area, which increased the resistance of grain plastic deformation, while smaller plastic deformation leads to a higher microhardness [25]. Phase transition strengthening was also involved, as evidenced by the increased dislocation density due to the formation of martensite after laser treatment. High-density dislocations are prone to winding and plugging, which results in deformation hardening. As the crystal defects and microstructural fragmentation increased, the distribution of high pressure on the surface increased the microhardness.

### *3.2. Tensile Tests with Various Laser Parameters*

The stress-strain curves of the tested samples are shown in Figure 12. Samples No. 1–5 showed notably better tensile properties than the unrepaired sample. As the laser energy increased, the ultimate tensile force (UTF) of specimens No. 1 (31.90 kN) < No. 2 (33.24 kN) < No. 3 (35.02 kN) < No. 4 (38.15 kN) < No. 5 (40.68 kN), while the ultimate tension of the unrepaired sample is 29.67 kN. Specifically, when the laser energy density was 165.6+<sup>19</sup> −15 J/mm2, the specimen reached the highest UTF of 40.68 kN, which was 37.11% higher than the untreated sample, demonstrating the best tensile property.
