Laser ultrasonic probing of microstructural evolution mechanisms and thermal damage states in 6061-T6 aluminum alloy

Abstract

Using non-contact laser ultrasonic testing (LUT), this study systematically investigated the acoustic response characteristics of 6061-T6 aluminum alloy at different heat treatment temperatures (25–400 °C) and their multi-scale correlation mechanisms with microstructure evolution and mechanical property degradation. Non-invasive dynamic monitoring of material thermal damage states was achieved through the coupling of optical microscopy analysis, mechanical property testing, and multi-element acoustic feature modeling. The main findings include: (1) A quantitative mapping model relating Rayleigh wave velocity to grain size, elastic modulus, strength, and hardness was established, overcoming limitations of traditional single-parameter detection and enabling synchronous prediction of multiple mechanical properties; (2) Spearman correlation analysis revealed moderate correlations between normalized amplitude variation rate (NAAT), power, weighted peak frequency (WPF), and the evolution/plasticity of precipitated phases; (3) Critical inflection points in NAAT, power, and WPF were proposed as novel criteria for identifying recrystallization temperature (200 °C); (4) Ultrasonic velocity change rate analysis indicated three distinct evolution stages during heat treatment: dislocation recovery and nano-precipitation dominated at 25-200 °C, while recrystallization competed with precipitate coarsening at 200-300 °C, and dynamic equilibrium between grain coarsening and precipitate distribution formed at 300-400 °C.By establishing multi-dimensional quantitative relationships among acoustic characteristics, microstructure, and mechanical properties, this research provided an innovative non-destructive monitoring method for optimizing aluminum alloy heat treatment processes and evaluating the service life of critical components.