3.5.1. Effects of Unit Microstructure

Fatigue cracks generally occur at heterogeneous nucleation sites, such as inclusions, pores, or soft spots in microstructures [28]. It can be seen from Figure 15 above that there was more spheroidal graphite in the matrix of nodular cast iron. Similar to a hole, the graphite among the metallic matrix has no intensity. During thermal cycling, cracks are always initiated at the graphite phase, and the main crack often propagates along the graphite and matrix between the nearest graphite. Therefore, the presence of graphite reduces the thermal fatigue resistance of the material matrix to a certain extent. However, no graphite phase was found in the units, hence no bridge connection was produced between the main crack and the microcracks, whose growth depends on graphite, i.e., the crack propagation route was cut off by the unit. Therefore, the units had better thermal fatigue resistance and prevented cracks from propagating. In addition, laser remelting can refine the grains of the structure and improve the unit strength, which prevents crack propagation.

### 3.5.2. E ffects of the E ffective Size and Distance of Units

Schematic diagrams of di fferent-sized units which prevented crack propagation are shown in Figure 17 (Figure 17 A1–D1 is the sample without thermal cycling, and Figure 17 A2–D2 is the sample after 2000 thermal cycles). Since the units had similar surface areas, the depth determined the size of its cross-sectional area. A greater depth of a unit indicates a greater bridging degree of a crack and also a larger cross-sectional area of the strengthened unit; however, the existence of microcracks or holes in the unit reduces the e ffective depth and cross-sectional area. In addition, the existence of such defects greatly reduces the strength of the unit. Under the action of thermal stress, thermal fatigue cracks will continue to expand into the unit, preventing the unit from e ffectively locking cracks. Therefore, when the laser energy density was increased, the unit with a larger e ffective size completely bridged the cracks, and since there were no cracks or holes in the interior, it had a larger tensile strength and could withstand larger thermal tension caused by thermal cycling. This allowed it to greatly reduce the thermal tension on the cracks, thus e ffectively preventing cracks from propagating.

**Figure 17.** Schematic description for cracks blocked by units with di fferent sizes. ( **A**1–**D**1) before thermal cycles; ( **A**2–**D**2) after thermal cycles.

Since thermal stress was uniformly distributed on the crack, the total force of the thermal stress on the crack depends mainly on the crack length. The smaller the distance between adjacent units, the shorter the length of the crack that is blocked by the unit, and therefore the smaller the resultant thermal stress on the crack. In addition, a smaller unit spacing increases the number of units that can share thermal stresses. When subjected to the same magnitude of thermal stress, the crack in specimens with a smaller unit spacing was subject to less thermal stress, and the cracking speed was slower. In summary, a smaller unit spacing resulted in a better crack blocking e ffect.
