PSI - Issue 7

L. Lattanzi et al. / Procedia Structural Integrity 7 (2017) 505–512 L. Lattanzi et al./ Structural Integrity Procedia 00 (2017) 000–000

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b Fig. 1. (a) Geometry of the die casting, the investigated specimens are indicated by the dashed line rectangle; (b) technical design of the tensile specimen.

a

In the present study, the flat specimens with a nominal thickness of 3 mm underwent uniaxial fatigue tests. The gauge length and width are 35 and 10 mm, respectively. The average static tensile properties of these specimens in terms of yield strength, ultimate tensile strength and elongation to fracture are 165 MPa, 315 MPa, and 4 %, respectively (Timelli et al. 2011). 2.2. Fatigue tests Before being tested, the as die-cast components were stored at room temperature for several years and consequently were in a T1 condition, i.e. cooling from the casting temperature and natural aging. Uniaxial fatigue tests were performed on LM10 Italsigma testing machine, powered by linear electric motors, at load control with a frequency of 50 s -1 and with a stress amplitude of 80 MPa. A stress ratio of R = 0.1, which corresponds to a tension-tension cycle with min =0.1 ∙ max , was applied. This stress cycle leads to severe conditions since consists exclusively in traction loads. For this reason, it is often used in automotive and aircraft component testing, as stated by Roylance 2001. 2.3. Microstructural investigations Preliminary microstructural investigations were performed on some specimens by means of different techniques. In particular, microstructural observations were carried out by means of optical (OM) and scanning electron microscopy (SEM), while the identification of the phases was performed by X-ray diffraction (XRD) technique. Moreover, the evaluation of the grain size and texture was performed by electron back-scattered diffraction (EBSD) investigations. After fatigue testing, the fracture surface and profile were studied to evaluate the crack initiation and propagation by both optical and scanning electron microscopy.

3. Results and discussion 3.1. General microstructure

The typical microstructure of the alloy near the surface is shown in Fig. 2a and Fig. 2b depicts the microstructure at the center of the specimen. It is worth noting that the difference between the observed microstructures is due to different cooling rates that occur on the surface and in the center. The α -Al dendrites are not developed in length and show a rosette-like morphology, which is typical of a globular grain structure where the grain size is comparable with SDAS values. The grain structure was further investigated at the center of the cross-section of the specimens and the distribution of grain size is plotted in Fig. 3a. The equiaxed grains are size-distributed according to a three-parameters log-normal distribution. The average values of grain size ( d ) and secondary dendrite arm spacing (SDAS) at the core region of the samples are very similar, as shown in Table 2.