PSI - Issue 19

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

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1. Introduction

The additive manufacturing (AM) is a group of processes that join materials to make objects from three dimensional model data, usually layer by layer. AM is expected as an innovative manufacturing process for various parts with complex geometry that cannot fabricated by traditional process. Many review papers on AM have been also published (e.g. Frazier (2014), Lewandowski (2016), Yadollahi (2017), Liu (2019)). However, AM still remains some problems to be overcome; such as cost, size, quality and reliability. One of the problems is low fatigue strength in comparison with conventional material. Many researchers emphasizes that the tensile strength and hardness of the AM material are the same level as those of the conventional material (e.g. Baufeld (2010), Hrabe (2013), Hayes (2017), Wysocki (2017)). However, it is reported that the fatigue strength of as-built AM material is significantly low because of defects (Günther (2017), Romano (2018), Masuo (2018), Yamashita (2018)). The defects are caused by gas pore and spatter during melting process of metal powder. The occurrence of these defects is not solved completely though the defects decrease with a development of AM technology. The existence of defects disturbs the application of AM to structural components that need fatigue strength. We previously conducted the fatigue tests of Ti-6Al-4V alloy fabricated by AM and revealed that fatigue limit can be quantitatively evaluated using the √ area parameter model proposed by Murakami (1994) (Masuo (2018)). Moreover, we also pointed out that hot isostatic pressing (HIP) is effective to improve the fatigue strength of AM material in the same literature. Another problem is rough surface of as-built AM components. It has been known that surface roughness is one of detrimental factors affecting fatigue strength. In some literatures (Chan (2013), Whcisk (2014), Masuo (2018)), the fatigue strength of as-built AM specimen is lower than that of AM specimen with machined surface. However, the relationship between surface roughness and fatigue limit has not been discussed quantitatively. The surface roughness depends on process parameter such as powder diameter, scan rate, layer thickness and so on. As same as the defects, the surface quality of AM components will become better by improvement of building software. To determine a final goal of surface quality, it is important to evaluate the effect of surface roughness on the fatigue strength of AM component quantitatively. In this study, the effect of surface roughness on the fatigue strength of a Ti-6Al-4V alloy manufactured by AM was investigated. We conducted rotary bending fatigue tests for as-built specimen with different surface roughness. The method of quantitative evaluation of surface roughness for AM component was discussed.

Nomenclature a

depth of an infinite row of circumferential crack pitch of an infinite row of circumferential crack

b

HV

Vickers hardness

N f S a S z

number of cycles to failure

arithmetic surface roughness obtained by area measurement maximum height obtained by area measurement

R

stress ratio

R Sm

mean width of profile elements obtained by line measurement

stress amplitude

 a  w

fatigue limit  w,ideal ideal fatigue limit of a material without defects  K stress factor intensity range  K th threshold stress factor intensity range   applied stress range √ square root of projection area of defect √ R equivalent crack size of surface roughness