Comparative Study on the Mechanical Improvement of Aluminum Piston Alloys

Aluminum piston alloys are extensively employed in automotive engines because of their low density, excellent thermal conductivity, good castability, and favorable strength-to-weight ratio. However, repeated thermal fluctuations and cyclic mechanical loading during engine operation can accelerate crack initiation, reduce fatigue life, and compromise long-term reliability. The present study comparatively investigates the effect of different heat-treatment conditions on the structural and mechanical performance of aluminum piston alloys. Four representative conditions were considered: as-cast alloy (S1), solution-treated alloy (S2), T6-treated alloy consisting of solution treatment followed by artificial aging (S3), and over-aged alloy (S4). Optical microscopy, SEM/EDS, XRD, and FESEM analyses were employed to correlate microstructural evolution with mechanical behavior. XRD results revealed that the average crystallite size increased from 28.05 nm for S1 to 31.00 nm for S2 and reached 37.27 nm for S3 before decreasing to 32.12 nm in S4. Concurrently, the microstrain decreased from 2.63–4.89 × 10-3 in S1 to 1.93–3.81 × 10-3 in S3, accompanied by a reduction in dislocation density from 1.17–1.39 × 10-3 nm-2 to 0.66–0.78 × 10-3 nm-2. Tensile testing showed that the yield strength increased from 128 MPa in S1 to 182 MPa in S3, while the ultimate tensile strength improved from 185 MPa to 265 MPa, and elongation increased from 4.8% to 7.1%. The impact strength rose from 85 kJ/m2 to 134 kJ/m2, whereas the fatigue life increased markedly from 1.2 × 105 cycles to 6.8 × 10⁵ cycles after T6 treatment. FESEM fractography revealed a transition from brittle cleavage fracture in the as-cast alloy to ductile microvoid coalescence in the T6-treated condition. These findings demonstrate that controlled thermal processing, particularly T6 treatment, effectively tailors the microstructure of aluminum piston alloys to achieve enhanced strength, toughness, and resistance to cyclic loading, making such alloys highly suitable for advanced automotive piston applications.