PSI - Issue 13

S.M.J. Razavi et al. / Procedia Structural Integrity 13 (2018) 74–78 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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performance of the components. AM technologies allow the direct conversion of digital designs into physical products with direct production leading to avoid the setup time and the use of tools. Dealing with AM techniques, a significant problem stand up in failure assessment. Considering the conventional manufacturing technologies (such as casting, milling, etc.), one utilizes a given material with defined and well-known material properties and removes material to obtain the desired geometry. In contrast, the material properties of additively manufactured components change during the fabrication process making it to be dependent on the geometry of the components. This means that every change in the geometry will change the way that the AM machine performs its fabrication routine affecting the properties of the resulting solid. The AM components are no longer isotropic, commonly not fully dense, the surfaces roughness is considerable and there is a high risk of inclusions, inhomogeneities and impurities, all related to the underlying manufacturing strategy, which, in turn, is a function of the input geometry (Wu et al., 2014; Lindgren et al., 2016; Todai et al., 2017). Although AM technology is undoubtedly of high potential and can fulfil the needs of modern digital manufacturing, however, due to poorly known material properties and lack of appropriate failure criteria their failure prediction is not fully guaranteed. The latest topology optimization routines can be used and developed further for better usability and better interaction with the AM manufacturing process chain fulfilling stringent requirements of aerospace, automotive and biomedical applications. To develop experimental and theoretical understanding of the structural integrity of these advanced geometric complex components is then a fundamental step for taking advantage of AM processes in structural components. Currently, no specific design criteria are presented for complex AM components considering stress concentration phenomena arising from geometrical discontinuities. Furthermore, only limited fatigue data are presented in technical literature by testing notched components made by AM metals. Hence, it is a very strategic point to fill this knowledge gap allowing future applicants to take full advantage of the unique features of AM, which will be key to integrate AM in every-day manufacturing. Among various methods of additive manufacturing, a particular attention is paid on Selective Laser Melting (SLM) which is a powder bed fusion laser method (Kruth et al., 2005). Due to excellent corrosion resistance, high specific strength, low density and low elastic modulus of Ti-6Al-4V, attracted considerable attention in aerospace and biomedical applications. Both in aerospace and biomedical applications, fatigue is the primary mechanism of rupture in components such as turbine blade, hip prosthesis and mechanical heart valve (Cherolis, 2008; Song et al., 2014; Sun et al., 2014). Hence, the fatigue strength of additive manufactured parts is widely studied in literature (Leuders et al., 2013; Spierings et al., 2013; Kasperovich and Hausmann, 2015; Riemer et al., 2015; Yadollahi and Shamsaei, 2017; Razavi et al., 2018). For example, Leuders et al. (2013) found that porosity acts as strong stress raiser and lead to failure of SLM produced TiAl6V4 samples. Reduction of porosity during fabrication process was considered by some researchers and defined to be more important than microstructure optimization for improving the fatigue strength of Ti 6Al-4V samples manufactured by SLM. Kasperovich and Hausmann (2015) found a reduction in fatigue strength of SLM processed TiAl6V4 compared to the wrought alloy due to a combination of the unfavorable martensitic microstructure, unmolten particles, pores, and microcracks. They reported that the Hot Isostatic Pressing (HIP) process should be used for SLM-processed TiAl6V4 samples in order to restore the fatigue strength. The HIP process reduces porosity of printed material and subsequently enhance the fatigue life of the components. Finally, they proposed that heat treatment methods don’t give a considerable improvement on high cycle fatigue strength of AM samples. The majority of engineering components and structures contain notches of different shapes (e.g. V-, blunt V-, U-, O-, semi-circular and key-hole notches), mainly used for connecting various components together. By their utility, notches are prone to crack initiation due to the intensified stress at their neighborhood. Microcrack(s) in the vicinity of the notch root may extent somehow and lead to final failure of the component. Hence, it is commonly attempted in design of notched components to prevent or delay the crack initiation from the notch edge under static and fatigue loading (Ayatollahi et al., 2014; Ayatollahi et al., 2017). Dealing with AM notched components, no specific design criteria have been proposed so far to take into account stress concentration phenomena arising from geometrical discontinuities/features. In this context, this paper aims to contribute to the fundamental understanding of the fatigue behavior of additively manufactured Ti-6Al-4V specimens weakened by circular notches and blunt V notches. For this aim, the fatigue data related to notched AM samples available in two recently published researches by the authors (Razavi et al., 2017; Razavi et al., in press) are presented and compared. In the first part of the paper, details of the manufacturing method that was used for AM fabrication of the samples are described. Afterwards, the fatigue results of notched samples are presented and compared.