An Overview of Surface Engineering Techniques for Industrial-Grade Aluminum Alloys—Thermochemical, Precipitation Hardening, and Thermomechanical Process

Aluminum alloys, particularly those in the Al-Cu and Al-Mg-Si series, are extensively employed in aerospace, automotive, and structural applications owing to their favorable strength-to-weight ratio. However, optimizing their mechanical and surface properties to meet advanced performance requirements remains a critical challenge. Over the past three decades, extensive research has explored thermochemical treatments, precipitation hardening, and thermomechanical processing, yet most studies have examined these methods in isolation. This review systematically analyzes the influence of each treatment route on microstructural evolution, precipitation behavior, and mechanical performance, with emphasis on grain refinement, precipitation kinetics, surface hardening, and fatigue resistance. Particular attention is given to severe plastic deformation, advanced surface modification techniques, and aging behavior under different conditions. The review also highlights gaps in the current literature, including limited integration of hybrid treatment cycles, insufficient understanding of coupled diffusion-precipitation mechanisms, a lack of high-temperature performance data, and minimal industrial-scale validation. Future research directions are proposed to develop optimized hybrid processing strategies, predictive computational models, and scalable treatment cycles. This consolidated review provides a comprehensive foundation for advancing aluminum alloy design, aiming to achieve tailored surface-to-core property gradients suitable for next-generation aerospace and automotive applications.