Citation: Shah, A.W.; Ha, S.-H.; Siddique, J.A.; Kim, B.-H.; Yoon, Y.-O.; Lim, H.-K.; Kim, S.K. Microstructure Evolution and Mechanical Properties of Al–Cu–Mg Alloys with Si Addition. Materials 2023, 16, 2783. https://doi.org/10.3390/ ma16072783 Academic Editor: Bolv Xiao Received: 12 March 2023 Revised: 27 March 2023 Accepted: 29 March 2023 Published: 30 March 2023 Copyright: © 2023 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 Microstructure Evolution and Mechanical Properties of Al–Cu–Mg Alloys with Si Addition Abdul Wahid Shah 1,2 , Seong-Ho Ha 2, * , Jabir Ali Siddique 1,2 , Bong-Hwan Kim 1,2 , Young-Ok Yoon 2 , Hyun-Kyu Lim 2 and Shae K. Kim 1,2 1 Industrial Technology, University of Science and Technology, Daejeon 34113, Republic of Korea; abdulwahid.shah799@gmail.com (A.W.S.) 2 Industrial Materials Processing R&D Department, Korea Institute of Industrial Technology, Incheon 21999, Republic of Korea * Correspondence: shha@kitech.re.kr Abstract: The aim of this study was to investigate the impact of the addition of a minor quantity of Si on the microstructure evolution, heat treatment response, and mechanical properties of the Al–4.5Cu–0.15Ti–3.0Mg alloy. The microstructure analysis of the base alloy revealed the presence of α-Al grains, eutectic α-Al-Al 2 CuMg (S) phases, and Mg 32 (Al, Cu) 49 (T) phases within the Al grains. In contrast, the Si-added alloy featured the eutectic α-Al-Mg 2 Si phases, eutectic α-Al-S-Mg 2 Si, and Ti-Si-based intermetallic compounds in addition to the aforementioned phases. The study found that the Si-added alloy had a greater quantity of T phase in comparison to the base alloy, which was attributed to the promotion of T phase precipitation facilitated by the inclusion of Si. Additionally, Si facilitated the formation of S phase during aging treatment, thereby accelerating the precipitation-hardening response of the Si-added alloy. The as-cast temper of the base alloy displayed a yield strength of roughly 153 MPa, which increased to 170 MPa in the Si-added alloy. As a result of the aging treatment, both alloys exhibited a notable increase in tensile strength, which was ascribed to the precipitation of S phases. In the T6 temper, the base alloy exhibited a yield strength of 270 MPa, while the Si-added alloy exhibited a significantly higher yield strength of 324 MPa. This novel Si-added alloy demonstrated superior tensile properties compared to many commercially available high-Mg-added Al–Cu–Mg alloys, making it a potential replacement for such alloys in various applications within the aerospace and automotive industries. Keywords: Al–Cu–Mg alloy; Si addition; microstructure; heat treatment; tensile properties 1. Introduction Aluminum–copper–magnesium (Al–Cu–Mg)-based alloys, such as A201 and A206, are the strongest casting alloys of aluminum [1]. The copper content in these alloys varies between 4 and 10% by weight, with most alloys containing around 4.5%. Magnesium is an essential component of these alloys, and depending on the amount of magnesium present, they can be categorized into low-magnesium- and high-magnesium-containing Al–Cu–Mg alloys [1,2]. Aluminum alloys such as A201 and A206, which have a magnesium content below 1%, are renowned for their high strength and toughness. They also possess excellent corrosion resistance, and their precipitation hardening during heat treatment results in the highest tensile strength among all aluminum casting alloys [1–5]. However, these alloys have some disadvantages, including a relatively high tendency to hot tearing and low corrosion resistance [6–10]. Alloys with higher magnesium content (1.5~6%), such as 240, 242, and 243 alloys, are renowned for their superior hardness, high specific strength, and thermal stability at elevated temperatures. These alloys are commonly employed in applications where wear resistance and thermal stability are crucial factors, such as the production of pistons for internal combustion engines (e.g., 242 and A242 alloys) and Materials 2023, 16, 2783. https://doi.org/10.3390/ma16072783 https://www.mdpi.com/journal/materials