Effect of Thermal Exposure on Microstructure and Mechanical Properties of A356 Aluminum Alloy

LI Guoxing; GAO Peihu; CHEN Baiyang; ZHAO Daming

doi:10.3969/j.issn.1674-6457.2026.10.009

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Journal of Netshape Forming Engineering ›› 2026, Vol. 18 ›› Issue (10) : 92-102. DOI: 10.3969/j.issn.1674-6457.2026.10.009

Light Alloy Forming

Effect of Thermal Exposure on Microstructure and Mechanical Properties of A356 Aluminum Alloy

LI Guoxing, GAO Peihu^*, CHEN Baiyang, ZHAO Daming

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Abstract

Against the background of automotive lightweighting, A356 aluminum alloy cylinder heads face thermal failure risks during service. The work aims to reveal the thermal stability of microstructures and the degradation mechanism of mechanical properties of T6-treated alloy within the temperature range of 250-385 ℃, providing quantitative theoretical support for the thermal aging resistance design of cylinder heads. Thermal exposure tests were conducted at 250 ℃, 325 ℃ and 385 ℃ for 45 hours to simulate actual engine service conditions. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were adopted to characterize phase microstructure evolution. Combined with Transmission Electron Microscopy (TEM) and Geometric Phase Analysis (GPA), the phase transformation sequence, loss of coherency and evolution law of strain field were clarified at the atomic scale. Room-temperature tensile tests and hardness measurements were performed to verify the variations in mechanical properties. The macroscopic mechanical properties of A356 aluminum alloy were significantly temperature-dependent and exhibited distinct nonlinear characteristics with prolonged thermal exposure time. At 250 ℃, the microstructure remained relatively stable with only slight overaging, resulting in a marginal reduction of 6.7% in yield strength and a moderate increase in elongation. 325 ℃ served as the critical transition temperature for mechanical properties. The coarsening rate constant K rose sharply to 13.455 03, the coherent relationship between precipitates and the matrix was destroyed, and the dislocation pinning capacity decreased by 40%, leading to a sharp drop of 27% in yield strength. Under the extreme condition of 385 ℃, the magnesium content in the matrix was depleted to 0.52%. Metastable strengthening phases completely transformed into coarse equilibrium phases, and the contribution rate of dislocation reinforcement dropped below 10%. Consequently, the yield strength plummeted by 45% to 130.35 MPa, with a 41% reduction in hardness. Meanwhile, the full spheroidization of eutectic silicon fundamentally changed the fracture mode from quasi-cleavage fracture to micropore-aggregated ductile fracture, and the elongation surged to 17.16%. The thermal failure of A356 aluminum alloy is predominantly dominated by the coarsening, destabilization and coherency loss of Mg₂Si strengthening phases. The alloy maintains stable mechanical properties at 250 ℃ and below, while irreversible strength degradation occurs at 325 ℃ and above. This work defines 250 ℃ as the critical temperature threshold for safe service, which can directly support the life prediction and reliability evaluation of complex high-strength aluminum alloy components.

Key words

A356 aluminum alloy / thermal exposure / microstructure evolution / mechanical properties

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LI Guoxing, GAO Peihu, CHEN Baiyang, ZHAO Daming. Effect of Thermal Exposure on Microstructure and Mechanical Properties of A356 Aluminum Alloy[J]. Journal of Netshape Forming Engineering. 2026, 18(10): 92-102 https://doi.org/10.3969/j.issn.1674-6457.2026.10.009

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Funding

National Foreign Expert Program (H20240023); Youth Innovation Team of the Education Department of Shaanxi Province; Key Research and Development Program of Shaanxi Province (2025CY-JJQ-120)