Fatigue properties and microstructural analysis of diecast AlSi11Cu2(Fe) alloy: effect of surface finishing

In the present work, a secondary AlSi11Cu2(Fe) alloy was diecast by means of a specifically designed die. Specimens with both the as-diecast surface quality and after sand-blasting were prepared and subjected to uniaxial fatigue tests in order to investigate the effect of surface finishing on S-N curves. The S-N curves for the two different surface finishing were estimated and compared in terms of inverse slope k and amplitude strength sigma(A) at 5x10(6) cycles. Experimental data for the two different surface finishing showed a similar statistical dispersion in fatigue life. Nevertheless, amplitude strength sigma(A)= 73 MPa and S-N slope k= 25.8 were obtained for sand-blasted samples, which are higher than the corresponding sigma(A)= 62 MPa and k= 11.4 showed by the as-diecast samples. After fatigue testing, both fracture profiles and surfaces were studied by optical and scanning electron microscopy to correlate crack initiation and propagation with fatigue strength. The crack initiation sites were localised mainly along the die separation-line in the as-diecast surface finishing condition, while, for the sand-blasted samples, an essential role in crack initiation was played by casting defects such as oxide films and porosities.