PSI - Issue 53

S. Senol et al. / Procedia Structural Integrity 53 (2024) 12–28

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Author name / Structural Integrity Procedia 00 (2019) 000–000

Keywords: Dual-laser powder bed fusion; Surface roughnes; Fatigue; High-strength aluminum

1. Introduction Laser powder bed fusion (L-PBF), an additive manufacturing (AM) technique, has become one of the most popular production processes in recent years, serving many application areas thanks to its multitude of advantages, such as enabling the fabrication of complex geometries, with customized designs, reduced material waste, and improved performance. However, L-PBF processing of conventional high-strength, lightweight, heat-treatable Aluminium based alloys belonging to the 2xxx, 6xxx or 7xxx series has been challenging, due to the occurrence of solidification cracks at the end of the (rapid) solidification step. Aiming at crack-free high-strength aluminium (Al) parts, many recent studies have focused on the development of modified, L-PBF-specific alloy compositions and one of the most effective solutions is proven to be the aluminium matrix composites (AMCs) (G. Li et al., 2021; X. Liu et al., 2021; Mair et al., 2021; Sullivan et al., 2022; Tan et al., 2020; Zhou et al., 2020). Nomenclature AM Additive manufacturing L-PBF Laser powder bed fusion dL-PBF Dual-laser powder bed fusion MMC Metal matrix composite AMC Aluminium-based metal matrix composite AB As-built AMCs are aluminium-based metal matrix composites (MMCs) produced by the addition of hard and stiff reinforcements, mostly ceramics such as carbide and boride particles, into the aluminium matrix. MMCs are known to offer high specific strength and stiffness, as well as elevated temperature stability (Liao et al., 2019; Mavhungu et al., 2017; Tjong & Wang, 2004). Their enhanced mechanical properties are mainly due to the strengthening through the load transfer from the lower modulus matrix to the stiffer reinforcements (Chawla & Chawla, 2006). Moreover, the ceramic reinforcements incorporated in conventional high-strength Al alloy matrix allow to mitigate the solidification cracking problem during L-PBF, as the ceramic reinforcements can act as heterogeneous nucleation sites, activating also Orowan and grain boundary strengthening (G. Li et al., 2021; Martin et al., 2017). Therefore, processing aluminium-based MMCs by AM, more specifically L-PBF becomes highly interesting as it enables manufacturing high-strength aluminium components with high density, increased homogeneity, and wide design flexibility (Sercombe & Li, 2016). However, the L-PBF parts still suffer from poor surface quality in the as-built (AB) state and the high surface roughness has been recognized as one of the three major factors limiting the fatigue performance of these parts (Brandl et al., 2012; du Plessis & Beretta, 2020; Gockel et al., 2019), in addition to the tensile residual stresses and pores (or defects) induced during L-PBF (Ye et al., 2021). The surface roughness refers to the irregularities at the surface, involving peaks and valleys, forming due to several phenomena, such as stair case effect, balling, semi-attached powder particles, occurring intrinsically during L-PBF (He et al., 2020; Nasab et al., 2018; Pyka et al., 2013). Specifically the critical valleys acting as stress concentration points are deleterious to the fatigue performance because they act as preferential initiation points for fatigue cracks (Dinh et al., 2020; du Plessis & Beretta, 2020). As the most commonly adapted solution, a post-surface treatment is applied to eliminate the surface roughness, especially for fatigue-critical L-PBF parts. A wide variety of surface post-treatments can be applied to improve the surface quality by reducing surface roughness, such as laser polishing and surface re-melting (Qi et al., 2021; Yasa et al., 2011; Yu et al., 2019), machining (grinding, milling), shot peening (Maamoun et al., 2018; Soady et al., 2013) or chemical polishing (Maleki et al., EDM Electric-discharge machining 3PBF Three-point bending fatigue CW Continuous wave PW Pulsed wave