PSI - Issue 19

Masanori Nakatani et al. / Procedia Structural Integrity 19 (2019) 294–301 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

296

3

2. Experimental procedures

The round bars and specimens were fabricated by AM processes; electron beam melting (EBM) and direct metal laser sintering (DMLS) methods. The particle sizes used in AM processes are about 80  m for EBM and about 40  m for DMLS. The bars and specimens were built in the direction of the specimen axis. For EBM process, preheating was applied in every layer. On the other hand, stress relief heat treatment was applied to specimens built by DMLS. To separate the effects of defect and surface roughness on the fatigue strength, the HIP was applied to several series of specimens. HIP was carried out at 920 °C and 100 MPa for 2 hours and the specimens were cooled in the furnace. For as-built specimen with/without HIP, only chuck part was machined. The polished specimen with HIP was machined from AM round bar after HIP and the surface was polished with #600 emery paper. Table 1 shows the mechanical properties of the Ti-6Al-4V alloys manufactured by AM. Tensile strength and Vickers hardness are almost the same between EBM and DMLS. The AM specimens were softened slightly by HIP treatment. Figure 1 shows the microstructures fabricated by DMLS process. HIP eliminated the internal defects and made the grain size large. On the other hand, the surface roughness remained after HIP. The similar microstructures were also observed in DMLS specimen. To investigate the difference of as-built surface between EBM and DMLS, the surface roughness and morphology were measured using an optical three-dimensional profilometer (KEYENCE, VR-5000). As explained later, the surfaces of as-built specimen have irregular and three-dimensionally complex morphology. Thus, the area roughness was measured at 10 areas of 4.5 × 6.0 mm to evaluate the maximum value of roughness parameter. In this study, the maximum height S z and the arithmetical mean height S a were focused on. In addition, mean width of profile elements R Sm was measured from roughness profiles parallel to building direction. Figure 2 shows the shape and dimension of fatigue test specimens for rotating bending fatigue. Rotating bending fatigue tests with a stress ratio R of -1 were carried out at 60Hz. The test were run out at 10 7 cycles. After fatigue tests, the fracture surface was observed by a scanning electron microscope.

Table 1. Mechanical properties. AM process

EBM

DMLS

HIP

No

Yes 986

No

Yes 980

Tensile strength (MPa)

1046

1176

Elongation (%)

20

22

14

22

Vickers hardness HV 0.3 (kgf/mm 2 )

369

345

378

340

Fig. 1. Microstructure of Ti-6Al-4V alloy fabricated by EBM process.