Citation: Chludzinski, M.; dos Santos, R.E.; Ortega-Iguña, M.; Churiaque, C.; Porrúa-Lara, M.; Sánchez-Amaya, J.M. Low-Energy Pulsed-Laser Welding as a Root Pass in a GMAW Joint: An Investigation on the Microstructure and Mechanical Properties. Materials 2022, 15, 7741. https://doi.org/10.3390/ ma15217741 Academic Editors: Yong-Cheng Lin, Zhe Zhang, Xin-Yun Wang and Guo-Qun Zhao Received: 13 September 2022 Accepted: 30 October 2022 Published: 3 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 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/). materials Article Low-Energy Pulsed-Laser Welding as a Root Pass in a GMAW Joint: An Investigation on the Microstructure and Mechanical Properties Mariane Chludzinski 1 , Rafael Eugenio dos Santos 1 , Marta Ortega-Iguña 1 , Cristina Churiaque 1 , Manuel Porrúa-Lara 2 and José MaríaSánchez-Amaya 1, * 1 Department of Materials Science and Metallurgical Engineering and Inorganic Chemistry, School of Engineering, University of Cádiz, Av. la Universidad de Cádiz, 10, E-11519 Puerto Real, Cádiz, Spain 2 Navantia S.A., S.M.E., Bahía de Cádiz Shipyard, Industrial Estate s/n, E-11519 Puerto Real, Cádiz, Spain * Correspondence: josemaria.sanchez@uca.es; Tel.: +34-956-483-153 Abstract: Root pass is a fundamental step in multi-pass welding. In gas metal arc welding (GMAW), the weld bead qualities depend on the process parameters, filler materials, and welder abilities. This work investigates the effect of a Nd: YAG pulsed laser as a first pass to reduce the welders’ reliance on the AH36 low-alloy steel with 5.5 mm thickness. This autogenous automatable process delivers reduced thermal impact due to the concentrated high-energy source, pulse overlap, and higher penetration depth-to-power ratio than continuous lasers. The outcomes indicate that the PL as a root welding generated a small HAZ compared to the GMAW condition. In addition, the subsequent arc passes positively affected the microstructure, reducing the hardness from around 500 to 230 HV. The PL + GMAW achieved similar strength results to the GMAW, although its Charpy impact values at −50 ◦ C were around 15% lower than the arc condition. Keywords: gas metal arc welding; pulse laser welding; root pass; AH36 steel; microstructure; mechanical properties 1. Introduction Several industries around the world widely have adopted a great variety of welding techniques. Different areas have constantly invested in innovative joint technologies following the 5.0 industry revolution trends highlighted by social well-being and reduced environmental impacts [1]. In furtherance of this, the combination of the advantages of different welding technologies has allowed to meet this global demand. Advanced studies on alternative energy sources have brought disruptive processes and equipment in recent years. Among them, methods of laser beam welding (LBW) have presented characteristics to supply different industrial segments, such as aeronautical, naval, rail, automobile, and oil and gas [2–4]. In general, its precision and high concentrated energy source delivered by the laser beam melt a small area, creating deep and narrow joints [5]. In addition, it is possible to weld different materials and thicknesses without additional material. Laser beam sources can be divided into two main wave modes, continuous and pulsed. In the continuous mode, laser irradiation is constantly emitted on the material during all welding processes, requiring robust equipment and elevated investments. On the other hand, the pulsed laser (PL) mode adopts an intermittent beam, which is emitted in short pulses with a predetermined duration (usually milliseconds) [6]. As a result, successive overlapped pulses generate a weld bead throughout a sequence of welding spots. Owing to the overlapping periodic thermal exposure, the fusion zone is re-heated above the melting point with a slow subsequent cooling stage. In addition, the average power involved achieves higher penetrations than in the continuous mode [7–9], allowing its application with small and economical apparatus. Furthermore, its accurate energy control permits Materials 2022, 15, 7741. https://doi.org/10.3390/ma15217741 https://www.mdpi.com/journal/materials