PSI - Issue 42

2

R. Fernandes et al. / Procedia Structural Integrity 42 (2022) 992–999 Fernandes et al./ Structural Integrity Procedia 00 (2019) 000 – 000

993

Nomenclature  a,t

total strain amplitude elastic strain amplitude plastic strain amplitude fatigue ductility coefficient fatigue strength coefficient fatigue strength exponent fatigue ductility exponent stress amplitude

 a,e  a,p

 f

 a  f

b c

K ’ n ’ N N f

cyclic strain-hardening coefficient cyclic strain-hardening exponent

number of cycles

number of cycles to failure

mechanical properties make them ideal for applications which experience repeated loading histories (Borrego, 2000; Jesus, 2014; Branco, 2019). Although engineering applications are usually designed in a such a way that materials only deform in an elastic manner, local plastic deformation can occur at the geometric discontinuities, and, therefore, an accurate structural integrity assessment requires a deep knowledge of the cyclic plastic behaviour (Carpinteri, 2016; Nejad, 2019; Narayanan, 2021; Sedmak, 2022). The AlSi10Mg aluminium alloy, a classical cast Al-based alloy widely used in aerospace and automotive industries, is also available for laser-beam powder bed fusion. When processed by selective laser melting (SLM), due to the high cooling rates which are generally in the range 10 6 to 10 8 ºC/s, its microstructure differs from its conventional counterpart (Li, 2016). The resulting microstructure exhibits a hierarchical and heterogeneous structure containing a network composed of three levels, i.e. the melting pool boundary network, the grain boundary network, and the cellular network with boundaries that encompass alternate fine eutectic Al and Si phases ( Snopiński , 2022). This cellular network is generally associated with the mechanical properties of the SLMed AlSi10Mg alloy, which encompass high strength and excellent strain hardening rates, but also limited ductility and tensile residual stresses (Bagherifard, 2022). The application of mechanical and thermal post-processing treatments can play an important role in modulating the mechanical properties, namely towards an improved fatigue behaviour, by releasing the detrimental tensile residual stresses and by recovering some degree of ductility [14]. Despite previous research studies have reported a favorable effect of post-processing treatments on static strength of SLM-manufactured AlSi10Mg parts (Brandl, 2012), some antagonistic conclusions have been drawn regarding their fatigue performance. In addition, systematic studies dealing with different post-processing treatments tailored for the AlSi10Mg aluminium alloy manufactured by LPBF and the associated microstructure features and the corresponding cyclic properties are very limited. Thus, in this paper, it is explored the triangular relationship between post-processing routes, microstructure, and cyclic properties. In order to reach this goal, uniaxial strain-controlled fatigue tests are conducted on smooth specimens in low-cycle and high-cycle fatigue regimes for five different material states, encompassing the as-built condition, standard heat treatments (T6 and stress relief), and non-standard heat treatments (stress relief at a non-standard temperature, and stress relief at a non-standard temperature followed by hot isostatic pressure). The tests were conducted at room temperature, under fully-reversed conditions, for strain amplitudes in the range 0.2-1.5%. 2. Materials and methods The material used in this study was an AlSi10Mg aluminium alloy manufactured via laser-beam powder bed fusion. The specimens, which are represented in Figure 1, were built vertically from the platform, with a scan speed of 1.8 m/s a layer thickness of 30  m, and a maximum power of 350 W. After that, the gage sections were polished using different carbide papers and diamond paste resulting in an arithmetic average roughness (R a ) of 0.207 ± 0.012 μm. Next, five batches were prepared: the first in the as-built condition; the second in the T6 condition (annealing at