PSI - Issue 37

G. Macoretta et al. / Procedia Structural Integrity 37 (2022) 632–643 G. Macoretta, B. D. Monelli / Structural Integrity Procedia 00 (2019) 000 – 000

634

3

and continuous laser beam, but in the present work only the continuous mode was employed. For all the specimens, the build plate was heated at 170°C and the process chamber was filled with gas argon, reaching an oxygen concentration in the machine process camera lower than 10 ppmw, guaranteeing to avoid oxidation phenomena. The specimens were built in a vertical direction, namely with the longitudinal direction aligned with the z machine direction (the direction of motion of the built plate), in order not to have traces of the support structures in the specimen gauge length and have uniform residual stresses on the specimen surfaces. No significant distortion was found in the specimens after the removal from the built substrate. In the present investigation the material was tested in the as-built condition, namely no aging or solution heat treatment was applied after the printing phase.

Table 1. Powder chemical composition.

Element

Min %

Max %

Element

Min %

Max %

Ni Cr

50 17

55 21

Co Mn

1

0.35 0.35

Nb Mo

4.75

5.5 3.3

Si

2.8

Cu Fe

0.3

Ti Al

0.65

1.15

Bal.

0.2

0.8

2.2. Feasible region definition and process parameters selection The simplified approach introduced by Moda (2021) introduced a first-order prediction of the thermal field produced by a single line laser scan, which allows defining a material independent process feasible region, depending on the principal process parameters characterizing the SLM process (laser power, scan speed, layer thickness, hatch distance, substrate temperature) and the thermal properties of the material. Starting from the simplified hypothesis of modeling the laser beam as a steady-state moving point heat source along a straight line with constant velocity v on the flat surface of a semi-infinite solid with constant thermophysical properties, whose temperature field is described by the Rosenthal solution, Mendez et al. (2018), an analytical expression of the thermal field produced by the heat source was obtained, equation (1). The coordinates of the isotherm at the metal melt temperature ( T m ) can be considered as the boundary of the meltpool, moving at the scan laser velocity, Fig. 1 (a). ( , , ) = 0 + 2 ∙ (− 2 ( + )) (1) where p is the laser power, λ the laser absorptivity, v the scan speed, α and k the thermal diffusivity and conductivity of the substrate, r the distance from the point heat source, and T 0 the reference temperature far from the heat source. It yields a prediction of the meltpool geometry, shown in Fig. 1 (a), and thus an evaluation of its radius ( r) and aspect ratio ( A r ), equation (2) and (3). It is thus possible to infer the geometrical relations that describe the boundaries representing the onset of lack of fusion or meltpool humping, Soderstrom and Mendez (2006). ≈ [1 + ( 32 ( − 0 ) ⁄2 ] 1⁄ (2)