PSI - Issue 38

Alok Gupta et al. / Procedia Structural Integrity 38 (2022) 40–49

43

4

Author name / Structural Integrity Procedia 00 (2021) 000 – 000

2.3. Microstructure and Fracture Characterization

For the Electron backscatter diffraction (EBSD) scanning, the specimen section was cut and hot mounted in bakelite resin, which was polished first with the SiC papers (p240, 400, 800 and 1200 grit sizes) and then finely polished using the plane-cloth (6  m) and alpha-cloth (1  m) in diamond suspension and finally using the chemi-cloth in colloidal silica solution. A JEOL 7100F Field Emission Gun Scanning Electron Microscope (FEG-SEM) was used to perform EBSD scans for a detailed microstructure characterization. The EBSD scans were conducted at 15 kV with a step size of 0.25  m. The AZtecCrystal software was used for post-processing. The grains were identified based on a 10  mis orientation criteria i.e. continuous presence of more than 10  boundary were demarcated as grains. The fracture surface of LCF test specimens were scanned using a Quanta FE600 scanning electron microscope. 3. Test Results 3.1. Uni-axial tensile tests

The engineering stress-strain curves of the two tensile tests are shown in Fig. 4 and the derived mechanical properties are listed in Table 1. The mechanical properties i.e., Young’s Modulus, 0.2% Proof Strength ( 0.2% PS - MPa), Ultimate Tensile Strength (UTS - MPa) and Strain to Failure (Ductility - %) values from the tensile test results are presented in Table 1. These mechanical strength values for the SLM Ti-6Al-4V material are also compared with those for the annealed Ti-6Al-4V bar material in Table 1. From Table 1, the two tensile strength values (0.2% PS and UTS) for SLM Ti-6Al-4V material are higher but the ductility values are slightly lower than the corresponding values of the conventionally manufactured annealed Ti-6Al-4V bar. Table 1 also presents the values of Ratio of Tensile Strengths (RTS) for TT1 and TT2 tests, where: RTS = 0.2 % (1)

As shown in Table 2, for both the tests performed, the RTS value is less than 1.2 which indicates that some level of softening in cyclic performance of the SLM material can be expected [Prasad et al. (2011)]. There is a slight improvement in strength and ductility values with increase in SR. Table 1. Mechanical properties from tensile tests. Fig. 4. Tensile stress-strain (engineering) curves

Temperature (°C)

SR (s -1 )

Young’s Modulus (MPa)

0.2% PS (MPa)

UTS (MPa)

Ductility (%)

RTS

Test Identification Number (#)

0.0002 0.0265

TT1 TT2

20 20 20

116000 121816

1020 1125

1114 1171

6.4 8.7

1.09 1.04

Annealed Ti-6Al-4V Bar [MMPDS-15, 2020]

861

930

10

3.2. Uniaxial LCF tests Cyclic performance of SLM Ti-6Al-4V material under the SWT waveform loading at RT is shown in Fig. 5. The fracture happened after 504 cycles and 294 cycles for LT1 and LT2 tests respectively. The plots of variation of maximum and minimum stress values for LT1 and LT2 tests are shown in Fig. 5(a) and Fig 5(b) respectively, where a drop in stress levels with increasing number of cycles represents cyclic softening. Since the LT2 test did lower number of cycles, it showed an increased rate of cyclic softening compared to the LT1 test (Fig, 5(a) and Fig. 5(b)