PSI - Issue 31

David Liović et al. / Procedia Structural Integrity 31 (2021) 86– 91 David Liovi ć et al. / Structural Integrity Procedia 00 (2019) 000–000

87

2

1. Introduction Because of its favorable mechanical properties, great biocompatibility, high strength to weight ratio and low corrosion rates, titanium and its alloys are widely used in many fields, such as automotive, aeronautical, space medical implants industry. In these industries, the use of components of high topological complexity is often required as Babić et al. (2020) and Juechter et al. (2018) have shown in their work, which is usually difficult to achieve by conventional processing methods. However, by using the powder bed fusion technologies, the manufacturing of products with complex topology is not an issue as shown by Tallon et al. (2020). Selective laser melting (SLM) is one of the powder bed fusion technologies, which is based on the principle of melting stationary metal powder in a so-called powder bed, using a laser as an energy source. After the first layer of the metal powder has been melted, the metal powder is again spread with special blades across the powder bed in a predefined thickness and the melting process starts over again, in a layer-by-layer manner. Product is formed by repeating this process of spreading and melting of the metal powder. The most researched and widely used, both conventionally and additively manufactured titanium alloy is Ti6Al4V as shown by Viespoli et al. (2020); Jesus et al. (2020); Ortega et al. (2020); Nabhani, Shoja Razavi, and Barekat (2019). By alloying titanium with aluminum and vanadium mechanical properties can be increased. However, according to Domingo (2002) and Tomljenovic (2011), when using these alloying elements in human body applications, Alzheimer’s disease, as well as hematological and biochemical alternations, loss of body weight, nephrotoxicity, immunotoxicity, etc. may develop. Additionally, Bryers (2008) states that 60 – 70% of nosocomial infections are associated with some type of implanted medical device as. Nevertheless, by alloying titanium with 5 and 10 wt. % of copper, the number of bacteria (Staphylococcus aureus) can be reduced by 50% and 65% respectively compared to commercially pure titanium produced by casting as shown by Zhang E. et al. (2016). Systematic experimental data and knowledge on the modeling of monotonic and cyclic elastoplastic behavior of SLM-ed titanium and its alloys are poorly available in the literature and very limited. Therefore, in this paper mechanical properties of most researched and widely used SLM-ed Ti6Al4V alloy, researched by different groups of authors, are given. Further, simple Ramberg – Osgood (R – O) material model is used on available experimental data to obtain monotonic and cyclic elastoplastic material parameters, that will serve as the first step for further comparison between this simple R – O model and more advanced material models capable of capturing additional phenomena that may occur during cyclic loading. 2. Ramberg – Osgood material model One of the simplest phenomenological models for describing the monotonic and cyclic elastoplastic behavior of a material is the R – O material model. To fully describe monotonic elastoplastic behavior using this model, it is necessary to identify three material parameters, which are strength coefficient ( K ), strain hardening exponent ( n ), and Young’s modulus ( E ) which can be identified from simple and low - cost monotonic tests. The assumption within this model is that the total deformation consists of an elastic and a plastic part:

1 n

t E K σ σ ε ε ε   = + = +     t t e p

(1)

,

where, t ε represents true total strain, e ε true elastic strain, p ε true plastic strain and t σ true stress. If a monotonic true stress – true strain curve is known (Fig. 1. a), the plastic part of the curve can be plotted in a logarithmic scale to determine monotonic strength coefficient and strain hardening exponent by using nonlinear regression (Fig. 1. b), while Young’s modulus ( E ) can be determined directly from stress – strain curve. The data used in this paper were gathered from two distinct groups of authors Zhang et al. (2019) and Agius et al. (2017). It is important to emphasize that the engineering stress – strain data is used up to the point of maximum tensile stress. The reason is that beyond maximum tensile stress, the true stress – strain curve should be based on actual diameter measurements.