Effect of Build Direction on Fatigue Performance of L-PBF 316L Stainless Steel

doi:10.1007/s40195-019-00983-3

Abstract

Abstract:

The microstructure and fatigue and tensile properties of 316L stainless steel fabricated via laser powder bed fusion (L-PBF) were investigated. Two 316L stainless steel specimens with different loading directions which are either perpendicular to or parallel to building direction were prepared by L-PBF process. The results of X-ray diffraction tomography showed that there was no significant difference in morphology and size/distribution of the defects in the HB and VB samples. Since long axis of columnar grains is generally parallel to the build direction, the fatigue crack encounters more grain boundaries in VB samples under cyclic loading, which led to enhanced fatigue resistance of VB samples compared with HB sample. In contrast to HB sample, the VB sample has a higher fatigue strength due to a higher resistance to localized plastic deformation under cyclic loading. The differences in fatigue properties of L-PBF 316L SS with different build directions were predominantly controlled by solidification microstructures.

Key words: Austenitic stainless steel, Laser powder bed fusion, Tensile properties, Fatigue strength

Chenfan Yu, Yuan Zhong, Peng Zhang, Zhenjun Zhang, Congcong Zhao, Zhefeng Zhang, Zhijian Shen, Wei Liu. Effect of Build Direction on Fatigue Performance of L-PBF 316L Stainless Steel[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(4): 539-550.

Figures/Tables 13

Table 1 Parameters employed in the laser powder bed fusion process

Fig. 1 a Illustration of build orientation and loading direction of HB and VB samples; the loading direction is parallel to build direction for VB sample and the loading direction is vertical to build direction for HB sample; b size and geometry of sample for mechanical tests

Fig. 2 Microstructures of L-PBF 316L stainless steel built vertically: a lateral side (XZ plane); b top side (XY plane); c melt pool morphology; d cellular structures with different orientations near the melt pool boundary

Fig. 3 Electron backscatter diffraction (EBSD) inverse-pole figure (IPF) map of each side of as-built L-PBF 316L SS sample fabricated via different build orientations: a top side of VB sample; b top side of HB sample; c lateral side of VB sample; d lateral side of HB sample. Bottom: IPF legend

Fig. 4 a Tensile engineering strain-stress curves of 316L SS fabricated by L-PBF; bS-N curves showing the relationship between fatigue life (Nf) and total stress amplitude (Δσ/2) of 316L SS specimens fabricated with different build orientations

Fig. 5 Tensile fracture surfaces of L-PBF 316L SS: a VB sample; b HB sample; c tearing surface; d dimple rupture morphology

Table 2 Fatigue properties of L-PBF 316L SS

Fig. 6 Fatigue fracture surfaces of L-PBF 316L SS: a horizontally built specimen, Δσ = 400 MPa, Nf = 24,713 cycles; b vertically built specimen, Δσ = 300 MPa, Nf = 253,003 cycles; c, d typical fatigue striations morphologies of a and b, respectively

Fig. 7 Morphologies and distributions of pore defects detected by X-ray diffraction topography (XRT): a HB sample; b VB sample; insets c and d are magnified views of defects in a and b; e histograms of the defect size distribution in the HB and VB samples

Fig. 8 Fatigue striations morphologies on fatigue propagation region: a HB sample; b VB sample. The black arrows point to the fatigue growth direction

Fig. 9 Illustration of fatigue crack propagation path and typical columnar grains: a VB sample; b HB sample. The double-headed arrow indicates the applied loading direction

Fig. 10 Fatigue crack initiation regions: a, b fatigue crack initiated at the surface of the tested sample; c fatigue crack initiated at the pore defect

Fig. 11 True stress-true strain curves of both HB and VB samples

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