Materials 2023, 16, 301. https://doi.org/10.3390/ma16010301 www.mdpi.com/journal/materials Article Material Defects in Friction Stir Welding through Thermo–Mechanical Simulation: Dissimilar Materials with Tool Wear Consideration Debtanay Das 1 , Swarup Bag 1 , Sukhomay Pal 1 and Abhay Sharma 2, * 1 Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, India 2 Faculty of Engineering Technology, KU Leuven, Campus De Nayer, 2860 Sint‐Katelijne‐Waver, Belgium * Correspondence: abhay.sharma@kuleuven.be Abstract: Despite the remarkable capabilities of friction stir welding (FSW) in joining dissimilar ma‐ terials, the numerical simulation of FSW is predominantly limited to the joining of similar materials. The material mixing and defects’ prediction in FSW of dissimilar materials through numerical sim‐ ulation have not been thoroughly studied. The role of progressive tool wear is another aspect of practical importance that has not received due consideration in numerical simulation. As such, we contribute to the body of knowledge with a numerical study of FSW of dissimilar materials in the context of defect prediction and tool wear. We numerically simulated material mixing and defects (surface and subsurface tunnel, exit hole, and flash formation) using a coupled Eulerian–Lagrangian approach. The model predictions are validated with the experimental results on FSW of the candi‐ date pair AA6061 and AZ31B. The influence of tool wear on tool dimensions is experimentally in‐ vestigated for several sets of tool rotations and traverse speeds and incorporated in the numerical simulation to predict the weld defects. The developed model successfully predicted subsurface tun‐ nel defects, surface tunnels, excessive flash formations, and exit holes with a maximum deviation of 1.2 mm. The simulation revealed the substantial impact of the plate position, on either the advancing or retreating side, on the defect formation; for instance, when AZ31B was placed on the AS, the surface tunnel reached about 50% of the workpiece thickness. The numerical model successfully captured defect formation due to the wear‐induced changes in tool dimensions, e.g., the pin length decreased up to 30% after welding at higher tool rotations and traverse speeds, leading to surface tunnel defects. Keywords: friction stir welding; defects in FSW; CEL approach; finite element model; dissimilar welding; solid state joining 1. Introduction Friction stir welding (FSW) is a multiphysics welding technique that produces joints primarily through frictional heat generation and material plasticization. FSW, being a solid‐state process, is tremendously effective in joining several dissimilar alloys that are difficult to join with arc welding. The dissimilar materials’ welds obtained with FSW, such as aluminum and magnesium alloys, have been applied in lightweight automobiles. The FSW of dissimilar materials (commonly known as dissimilar FSW) has been investigated with the help of experimental and numerical studies. The FSW process has been numeri‐ cally modeled mainly to predict heat generation, material flow, stress formation, and strain, etc. [1]. The numerical prediction of such results can lead to an easy and cost‐effec‐ tive prediction of weld quality, which can further improve the application of dissimilar welds in the manufacturing industry. Citation: Das, D.; Bag, S.; Pal, S.; Sharma, A. Material Defects in Friction Stir Welding through Thermo–Mechanical Simulation: Dissimilar Materials with Tool Wear Consideration. Materials 2023, 16, 301. https://doi.org/10.3390/ ma16010301 Academic Editor: Hamed Aghajani Derazkola Received: 24 November 2022 Revised: 23 December 2022 Accepted: 25 December 2022 Published: 28 December 2022 Copyright: © 2022 by the authors. Li‐ censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con‐ ditions of the Creative Commons At‐ tribution (CC BY) license (https://cre‐ ativecommons.org/licenses/by/4.0/).