**1. Introduction**

Ductile iron with good wear resistance, heat conductivity, and castability has become a popular cast metal material that is widely used to manufacture train brake discs [1]. Brake discs are important components of trains, and they mainly fail due to thermal fatigue cracks [2–4]. Analyzing the condition of railway train brake discs suggests that their surfaces experienced significant pressures and friction, and they often operate in hot/cold alternating environments due to frequent braking which generates large thermal stresses [5]. When the stress exceeds the limit of the material strength, thermal fatigue cracks appear and expand on the disc surface, leading to the failure and discarding of brake discs and huge wastes of resources [6]. Therefore, it is of grea<sup>t</sup> significance to take effective technical measures to repair the discarded brake discs.

Many efforts have been made to identify the causes of thermal fatigue damage to nodular iron brake discs and repair thermal fatigue cracks on brake disc surfaces. Goo et al. [7] found that the graphite in ductile iron was the source of thermal fatigue cracks and improved the thermal fatigue resistance by regulating its composition and metallurgical structures. Traditionally, during brake disc repair, cracks are first melted with metals using welding equipment and then refilled with liquid metals through spontaneous flow, which completely welds the cracks together. For example, according to the research of Yang et al. [8], after a brake disc was preheated to 300 ◦C, thermal cracks could be repaired by surfacing welding. Li et al. [9] repaired thermal fatigue cracks with CO2 gas shielded welding, and

immediately after welding, the workpiece was heated to about 630 ◦C and held for 4–6 h to eliminate welding stresses.

Such traditional repair processes are tedious, require complex procedures, long repair cycles, and thermal stresses are generated after repair, which decreased the weld strength. In addition, the use of laser cladding technology was shown to repair thermal fatigue cracks by adding powder with the same chemical composition and content as the substrate in a layer-by-layer manner [10]. Gao et al. [11] exploited the electromagnetic heating e ffect to repair hot fatigue cracks inside a hot extrusion die of 3Cr2W8V steel using pulse discharge. However, using this method to repair thermal fatigue cracks on ductile iron brake discs is expensive, which limits its application.

Bionics is a new interdisciplinary field that uses biological mechanisms and laws to solve human needs that has rapidly developed since the 20th century [12,13]. Bionics has had a profound impact on materials science over the past two decades, as the unique structure, composition, and corresponding superior properties of biology provide inspiration for researchers to improve the properties of materials or the reliability of structural components [14,15]. For example, the leaves of plants can resist fatigue cracking caused by storms, which is closely related to the phenomenon of biological coupling. The veins of leaves are composed of vascular bundles and mechanical tissues with strong textures, while the mesophyll is composed of parenchyma cells with soft textures [16]. The force required to sustain the propagation of cracks on the mesophyll is not su fficient to tear the veins, and when a crack encounters a hard vein, the cracking behavior is terminated or the propagation is deflected at an angle (Figure 1a). Coincidentally, the same phenomenon is observed dragonfly wings, whose membranes are covered with crisscross veins that are composed of multi-layer compound mechanisms and are the main force bearing units (Figure 1b) [17]. It has been found that cracks do not propagate along a straight line, but frequently deflect. It shows that the crack growth on the dragonfly wings is greatly hindered.

**Figure 1.** Deflection of crack in the leaf (**a**) and butterfly wing (**b**).

The design of coupled bionic functional materials based on leaves and wings of dragonfly can be used to improve the crack growth resistance of nodular cast iron, which provides a novel idea to repair thermal fatigue cracks. Material surfaces can be laser-treated to produce a region with a di fferent microstructure than the substrate at a predetermined position to form a non-smooth surface in which the softer and harder points are alternately distributed. The surface also has special functions similar to biological surfaces [18]. Laser bionics has been recommended for processing points, strips, and nets on material surfaces to prepare non-smooth surfaces with improved thermal fatigue resistance and

tensile properties [19]. According to Zhou et al. [20,21], lasers with di fferent parameters have di fferent effects on improving the thermal fatigue properties of materials.

All the aforementioned studies were aimed at improving the thermal fatigue resistance of materials, however, how to repair the thermal fatigue cracks of nodular iron brake discs is not mentioned. In this study, inspired by bionics, a new method was proposed to repair thermal fatigue cracks in brake discs using discontinuous laser melting treatment on cracks, which the service life of brake disc is prolonged and the e fficiency of brake disc repair is improved.
