**1. Introduction**

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Over the last few decades, cast irons have been widely used in a variety of applications, such as for shafts, axles, engines, and gears in automobile parts or general industrial machinery [1–4], due to their high machinability, good mechanical properties, and low cost compared to other alloys.

Because of the increased production volume, an exhaustive characterization of the mechanical properties and fatigue behavior of cast irons has been extensively reported in the past literature. In addition, several tribological studies have been performed to address the wear mechanisms under diverse sliding conditions, as well as to establish a comparison of their wear rates against other industrial materials.

In general, intense loads in sliding tests reveal higher wear rates in cast irons than in steels. Thus, in extreme wear applications, e.g., mining, marine, aeronautic, or military industries, it is frequently required to improve their hardness and wear resistance in critical points of the material to meet requirements related to corrosion or fracture/fatigue failures. For this purpose, many conventional treatments have been developed and implemented to enhance the performance of cast irons in specific applications. Nonetheless, these techniques usually involve a full transformation of the workpiece, altering many of its beneficial bulk properties.

**Citation:** Catalán, N.; Ramos-Moore, E.; Boccardo, A.; Celentano, D. Surface Laser Treatment of Cast Irons: A Review. *Metals* **2022**, *12*, 562. https://doi.org/10.3390/ met12040562

 Academic Editor: Sergey N. Grigoriev

Received: 16 February 2022 Accepted: 24 March 2022 Published: 26 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In this context, laser surface treatment (LST) has emerged as a novel opportunity to improve the useful life of cast irons. This technique is characterized by a precise, clean, and fast thermal process, where the heat input from a laser beam raises the temperature at the surface of the specimen, and then it is rapidly conducted into the rest of the material. The objective of LST is to achieve temperatures above the point of critical transformation (austenization or melting temperature), following a self-quenching process from the unaffected case bulk. Depending on the cooling and solidification rates to which they are exposed during the treatment, new phases can arise, directly influencing the new mechanical properties of the material.

Past articles and reviews have addressed, in general terms, the benefits and challenges of LST of a variety of cast irons. However, a thorough comparison and analysis of the importance of the initial microstructure and laser properties is ye<sup>t</sup> to be performed. Therefore, this article presents a review of the literature according to LST of industrial iron castings, in order to understand the similarities and differences associated with these parameters and to serve as a guide to a reliable implementation and optimization of this technique.

First, a brief description of typical cast irons is provided, highlighting the different morphologies and conventional heat treatments that are commonly conducted in each one. Secondly, a more detailed theoretical basis of LST is presented, along with a summary of standard laser configurations used in metallic surface treatments, i.e., laser type, power, and scanning velocity. Moreover, the influence of other advanced parameters in the efficiency of LST is remarked. Afterward, an exploration of the results obtained in LST of gray cast irons (GIs) is offered to establish the influence of graphite morphology on the microstructural transformations that the treatment induces, as well as the resulting properties of the samples. Then the most relevant works for ductile irons (DIs) with predominantly pearlitic matrices are exposed, emphasizing the results as a function of laser configurations and characterization tests. Subsequently, investigations on laser-modified austempered ductile iron (ADI) castings are reviewed, mainly highlighting the differences in the scope of the treatment in terms of hardness, wear resistance, and depth of the affected area. Articles dealing with ferritic nodular cast iron treatments are then presented, addressing the main challenges of transforming the microstructure without melting the original matrix. To complete the review, a discussion of LST across all types of cast irons is given, highlighting the differences in achieved microstructure, hardness, and wear-related properties as a function of base material and laser operating parameters. At the same time, the main advantages and limitations of the treatments are presented, noting the improvement in overall performance compared to as-cast specimens. Finally, future technological, economic, and investigation areas related to the LST of cast irons are suggested.
