PSI - Issue 38

Hugo Roirand et al. / Procedia Structural Integrity 38 (2022) 149–158 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

150

2

instance, SS are processed by AM to repair existing pieces, to create new geometries or to reduce the waste of raw material in machining operations ( AMFG report , 2020; DebRoy et al., 2018; Laverne et al., 2016; Sun et al., 2017). Laser powder bed fusion (LPBF) process is the most advanced AM technology for metallic materials at this day. Many studies have been realized on the influence of LPBF parameters on the usage properties such as monotonic resistance, corrosion behaviour or fatigue resistance (Afkhami et al., 2021; Cruz et al., 2020; Li et al., 2019; Mazzucato et al., 2019; Sun et al., 2017, 2019; Tucho et al., 2018). Laser power (P), laser velocity (V), hatch distance (h), spot diameter (D) or argon flux are such of parameters which influence the final properties of AM materials. In addition to the laser parameters, these studies have pointed out that laser scan strategies used to build AM pieces play a key role in residual stresses control, defect occurrence or microstructure generation. For example, Mercelis and Kruth. (Mercelis and Kruth, 2006) have shown that the choose of an adapted laser scan strategy in LPBF process can lower by more than 100 MPa the residual stresses determined on aluminum as-built samples. Salman et al. (Salman et al., 2019) have shown a clear influence of laser scan strategy on the ultimate tensile strength (UTS) for 316L parts which can be tailored from 675 to 1016 MPa just by modifying the laser scan strategy. The mechanisms by which final properties are modified from a strategy to another one are still not well understood. It is clear that the laser scan strategy controls the thermal history experienced by the material in LPBF process. Among other parameters, thermal history affect the residual stresses distribution, the grain size distribution and texture, which are driven by the local maximum heat gradient, and also the defect population. It is known that these microstructural quantities have a strong influence in regards of the mechanical behaviour, especially for fatigue loadings (Castelluccio et al., 2014; Guerchais et al., 2015; McDowell and Dunne, 2010; Pineau et al., 2016). Their role has to be studied in order to, first, classify the influence of these microstructural parameters on fatigue resistance, then, be able to choose appropriate laser scan strategy depending of the desired application. This paper aim is to quantify the effect of rotations between layers and laser scan strategy on monotonic and fatigue properties of 316L LPBF. The role of crystalline texture, grain morphology and defect population will be separately studied in order to identify the first order parameters influencing fatigue behaviour. 2. Material and methods 2.1. Material 316L AISI SS powders has been provided by Höganas (Germany). Their chemical composition is in range of standard 316L SS as it is detailed in table 1. Powders have been analyzed in scanning electron microscope (SEM) (Figure 1), and their physical properties have been studied (Table 2). ASTM B329-06 and ASTM B213 standards has been respectively followed to calculate the apparent density and the HALL flowability. Measurements are in accordance with data provided by Höganas. SEM observations show that the particles have a good circularity which enhance the flowability and in consequence, allow to have a regular powder bed (Brika et al., 2020).

Table 1. Chemical composition of 316L SS powder Element Cr Ni Mo

Mn 1.5

Si

O

N

P

S

Fe

17.7

11.9

2.3

0.2

0.04

0.01

<0.010

0.004

Bal.

wt%