PSI - Issue 7

2

Hiroshige Masuo et Al./ Structural Integrity Procedia 00 (2017) 000–000

20 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of the 3rd International Symposium on Fatigue Design and Material Defects. Keywords: Fatigue, Additive Manufacturing (AM), Defects, S tatistics of extremes, the √ area parameter model 1. Introduction The advantage of additive manufacturing (AM) has been emphasized especially for high strength or hard steels which are difficult and costly to manufacture by traditional machining to complex shapes. Many literature on fatigue properties of AM materials have been published in recent years. For example, Berreta, S and Roman, S (2017) reviewed the papers related to the fatigue strength of AlSi10Mg and Ti-6Al-4V from the viewpoint of small crack. Günther, J et als. (2017) carried out precise experimental investigations on Ti-6Al-4V in high cycle fatigue and very high cycle fatigue and discussed the problems from the viewpoint of statistical scatter of defect size based on the microstructural observation. They pointed out the problem raised by the interaction between defects and specimen surface. Thus, the disadvantage or challenge of AM is presence of defects which are inevitably contained in the manufacturing process. They also pointed out the advantages of HIP on the improvement of fatigue properties of Ti 6Al-4V. This paper discusses fatigue properties of a Ti-6Al-4V manufactured by AM in terms of the effect of defects, surface roughness and HIP. The guide will be presented for the safe fatigue design and development of high quality Ti-6Al-4V by AM processing based on the combination of the statistics of extremes analysis on defects, surface roughness and the √ area parameter model. 2. Material, Specimen and Experimental method The raw materials and as-built specimens having the final shape were prepared by AM processes; Electron Beam Melting (EBM) and Direct Metal Laser Sintering (DMLS) methods. The particle size is about 80 µ m for EBM and about 40 µ m for DMLS. The specimens were built in the direction of specimen axis. The preheating was applied to EBM in every layer and the stress relief heat treatment was applied to DMLS after building specimens. HIP was applied to several series of specimens. Table 1 classifies the specimens in terms of AM methods (EBM or DMLS), surface polishing (as-built or surface polish) and HIP. Table 2 shows the mechanical properties of the Ti-6Al-4V manufactured by AM. Figure 1 shows the shape and dimension of fatigue test specimens for rotating bending fatigue. Rotating bending fatigue tests were carried out under 60Hz. The surface of machined specimens was polished with #600 emery paper. HIP was applied to some series of machined specimens and as-built specimens in order to separate the effects of surface roughness and defects. The Vickers hardness HV ( P =0.3kgf) was measured at 6 points. The fatigue fracture origins were mostly at surface or at defects near surface. Hiroshige Masuo et al. / Procedia Structural Integrity 7 (2017) 19–26

Table 1 Specimen classifications

Table 2 Mechanical properties

AM process

EBM

DMLS

Process

Surface condition HIP

HIP

No

Yes 986

No

Yes 980

σ UTS (MPa)

1046

1176

No Yes No Yes No Yes No Yes No Yes

Elongation (%)

20

22

14

22

As-built

HV 0.3

369

345

378

340

EBM

Surface polish (#600)

AM type

φ 12 0

-0 .02

As-built

R18

φ 6

R18

DMLS

Surface polish (#600) Surface polish (#600)

25

12

25

Rolled material

82

Fig. 1 Shape and dimension of specimen (in mm)

3. Results and Discussion When we discuss the fatigue properties of materials manufactured by AM, we should first pay attention to the ideal