PSI - Issue 52

Tomáš Vražina et al. / Procedia Structural Integrity 52 (2024) 43 – 51 Tomáš Vražina et. al./ Structural Integrity Procedia 00 ( 2022) 000 – 000

45

3

n´ N N f

Cyclic Stress-strain curve fitting exponent

Number of cycles

Number of cycles to fracture Scanning electron microscope

SEM

2. Material and methods The studied material was solution-annealed VDM alloy 699 XA prepared by two different technologies. The 54x45x14 mm 3 block was prepared by using the LPBF method by the Digital Additive Production department at Aachen University and VDM Powder 699 XA with particle size distribution 15-53 µm. The following process parameters of LPBF were used to print the block shown in Fig 1a: laser power of 260 W; scan speed of 1100 mm/s; hatch distance of 110 µm, and a layer thickness of 40 µm. A rotation scan strategy (67°) with a bi-directional scan vector was used without considering the contours. The relative density of 99.87% was established by Archimedes ’ method and confirmed by the optical method. The conventionally produced alloy 699 XA was supplied by VDM Metals in the form of hot rolled (HR) plates with a thickness of 15 mm. The chemical composition of alloy 699 XA is listed in Tab. 1. Both material batches were supplied in solution annealed conditions.

Table 1. The chemical composition of VDM Alloy 699 XA (in wt. %). C Cr Al Si Nb Ti Fe

Zr

Mn

Ni

0.02

29.5

2.1

<0.1

0.1

0.01

0.1

0.03

<0.01

bal.

The room-temperature low-cycle fatigue (LCF) tests were conducted on miniature cylindrical specimens (Fig. 1b) with a gauge length and diameter of 9 mm and 3 mm, respectively. LCF Tests were performed on computer-controlled servo-hydraulic machine MTS 810 in laboratory air under strain control condition. The strain was measured and controlled by a sensitive MTS extensometer. The cyclic loading was uniaxial and fully reversed (R s =-1) with a triangular cycle shape. The fatigue lifetime (cycles to failure) N f was determined as the number of reversals at the time of fracture. The microstructural analysis and fracture surface observation were carried out by field emission gun scanning electron microscope (SEM) LYRA 3 XMU equipped with electron backscatter diffraction (EBSD) detector Symmetry by Oxford Instruments. Grain size estimation was performed using AZtecCrystal software by an approach of equivalent circle diameter; a misorientation of 15° was chosen as a threshold value. The analyzed area of individual EBSD maps for grain size measurement was approximately 1500x2000 μm 2 for both LPBF and HR material.

Fig. 1 Scheme of a) material block with marked position for machining and marked building direction (B.D.) and loading direction (L.D.) b) Drawing of fatigue specimen.