Fatigue crack growth mechanisms at the microstructure scale in as-fabricated and heat treated Ti-6Al-4V ELI manufactured by electron beam melting (EBM)

Abstract

Electron beam melting (EBM) is a metal powder bed fusion additive manufacturing (AM) technology that fabricates parts by selectively scanning consecutive powder layers with an electron beam. Additive manufacturing technologies are increasing in importance for aerospace and medical applications, where the demand for a fundamental understanding and predictability of static and dynamic material properties are high. Ti-6Al-4V is the most widely used and studied alloy for this technology, and is the focus of this work in its Extra Low Interstitial (ELI) variation. The layered manufacturing of metallic components by EBM creates a unique directional microstructure, and consequently, anisotropic properties. Microstructure evolution, and its influence on mechanical properties of the alloy in the asfabricated condition, has been documented by various researchers. However, fatigue crack propagation and the effects of the directional structure have not been sufficiently studied, imposing a barrier for this technology's potential extension to high-integrity applications. In this study, fatigue crack growth (FCG) both parallel and perpendicular to the build directions was studied for different stress ratios and crack growth stages. The interaction between the directional as-fabricated EBM microstructure and FCG was investigated and compared to that of the equiaxed β annealed microstructure obtained by annealing above the β transus temperature. The FCG threshold, ΔK th , was analytically modeled for the two relative crack propagation directions at different stress ratios, and FCG microstructural mechanisms were established for all three regions of crack propagation.

A Review of the As-Built SLM Ti-6Al-4V Mechanical Properties towards Achieving Fatigue Resistant Designs

“…Moreover, this may explain the low FCGR values observed in Cain 3 [40] and Van Hooreweder [52]. However, Cain 1 [40] has the same orientation to Leuders 2 [32] but the FCGR in Cain 1 is comparable to Cain 2, which has a grain and layer orientation, which according to Galarraga et al [55] should produce higher FCGR values. This may be attributed to the influence of residual stresses.…”

“…Nevertheless, more favourable microstructures to crack growth have been shown to be bimodal and lamellar microstructures. In order to obtain a deeper understanding on the influence of grain boundaries and layer orientation, in conjunction with a lamellar microstructure on crack growth, the work of Galarraga et al [55] on EBM-manufactured material has been reviewed. Based on the results arising from this study and the understanding around the effect of SLM-manufactured material build orientation, it can be concluded that it is important crack growth occurs perpendicular to the build layers and, if possible, orientated in the same direction as the elongated grains.…”

“…However, crack propagation may also be dependent of the build orientation. In the recent work of Galarraga et al [55], on Ti-6Al-4V manufactured by electron beam melting (EBM), crack propagation rate parallel to the build direction was lower than that corresponding to the perpendicular build direction. This was attributed to the prior β grain boundaries and the deflection of the crack caused by the scanning layers, which increased the tortuosity of the crack path (reducing the rate of propagation).…”

Tensile behavior of Ti-6Al-4V alloy fabricated by selective laser melting: effects of microstructures and as-built surface quality

“…In the machined and HIP-processed samples, once the cracks nucleated, the cracks could propagate along α/β interfaces or across α colonies. The α phase plates also could defl ect the direction of crack growth depending on the orientation between crack tip and α colonies [26] . Hence, the crack propagation path in Fig.…”

Direct metal laser deposition of titanium powder Ti-6Al-4V

“…It is necessary to provide wide complex of researches and tests. Titanium alloys are used in the following additive technology: melting layer by layer powder by electron beam (EBM) [1,2] or laser (SLM, SLS methods) [3,4], feeding melted powder or wire by plasma arc [5,6] or laser (DMLD) [7,8]. We suggest using laser metal deposition technology for manufacturing of titanium parts.…”