PSI - Issue 57

Kalle Lipiäinen et al. / Procedia Structural Integrity 57 (2024) 32–41 Lipiäinen et al. Structural Integrity Procedia 00 (2019) 000 – 000

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Table 1. Material parameters for 4R method

Material

R m [MPa] H

Note

n

MS1 As-built CX As-built

1050 950

1.3 0.03 Includes V and C shaped geometries

1.3 0.03 Unnotched specimens 1.3 0.03 Unnotched specimens 1.6 0.15 Unnotched specimens

CX Heat-treated 1700

316L As-built

600

1000

MS1_UN MS1_EV1 MS1_EV2 MS1_EC1 MS1_EC2 MS1_IV1 MS1_IV2 MS1_IC1 MS1_IC2

Yield strength

CX_VM CX_VA

Notched specimens UN – Unnotched E / I – External / internal geometry V / H – Vertical / Horizontal orientation A / M – As-built / Machined surface HT – Heat treated

CX_HTHM CX_HTVM CX_HTVA

316L_HM 316L_VM 316L_VA 316L_VHFMI

Nominal (net cross-section) stress [MPa]

100

10 000

100 000

1 000 000

Fatigue life

Fig 3. Fatigue test results shown with nominal stress method (net-cross section considered). Quality has an important role on fatigue strength of AM-components. Fractography can be used to define critical defect type, size and shape. Obtained crack length varied from 30 μm to 130 μm depending on specimen and test condition. MS1 specimens included unnotched and relatively sharp and small notches which explains difference in crack size via significantly different critical volume. Machining the surface reduces the crack length by removing or improving the external surface roughness and in case of L-PBF, removing the most significant subsurface defects that are formed just under the built surface. Features that caused failures on machined specimens’ surfaces, were internal defects in manufacturing stage explaining their smaller size. The fractography for this study was performed by the authors based on SEM-images taken while conducting previous studies. Each test specimen has different killer-defect, and its identification and classification may be difficult. During the study, it was found that the depth parameter remained stable even tough defect type changed. Examples of typical crack length in each test series are shown in Figure 4. It can be concluded that the depth of e.g. material-based non-metallic inclusions (NMIs) or process-based lack of fusion determine fatigue properties.