Browsing by Author "Dhansay, N. M."

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    Fatigue crack growth rate threshold of laser powder bed Fusion Ti-6AI-4V
    (Stellenbosch University, 2021-12) Dhansay, N. M.; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Typically, producing Ti-6Al-4V through the laser powder bed fusion (LPBF) technique, results in the material having large residual stresses and martensitic microstructure. These stresses and microstructure have been shown to result in Ti-6Al-4V having poor fatigue properties. However, the insight into the fatigue failure mechanisms caused by the residual stress and microstructure has been limited. LPBF is one of many additive manufacturing (AM) techniques in which parts are built in a layer- wise manner with the use of powdered material and consolidated through high power laser melting. This allows for complex geometries and previously impossible geometries to be manufactured with minimal material wastage. Many industries are aware of the potential in this manufacturing technique and have shown interest in it becoming a viable option for the manufacturing of some of their components. In particular, the use of LPBF produced Ti-6Al-4V is of interest to the aerospace and biomedical industries. This is because Ti-6Al-4V is already well established in many existing industries. For LPBF produced Ti-6Al-4V to be a viable option in industry, researchers need to have an adequate understanding of the material and the implications for its mechanical properties. Fatigue property investigations have largely focused on the fatigue life approach (crack initiation) and Paris regime (region II, crack propagation). However, in recent years, the near-threshold regime (region I, crack propagation) has become of interest, albeit limited in approach. The consensus within literature shows that the large tensile residual stresses, martensitic microstructure and porosity results in poor fatigue properties. Unfortunately, the insight into the fatigue fracture mechanisms brought about by the residual stress and microstructure is not yet well established. Furthermore, the near-threshold fatigue crack growth rate regime (FCGR) experiences crack closure mechanisms which result in premature near-threshold values. The majority of the near-threshold investigations on LPBF produced Ti-6Al-4V do not account for the crack closure mechanisms and therefore produce premature results. As a result of the low crack growth rates achieved in the near-threshold regime, a window into observing the fatigue crack initiation mechanisms is obtained. More specifically, how the fatigue crack initiation mechanisms are influenced by residual stress, martensitic microstructure and changing microstructural morphology. Literature has shown that porosity acts as crack initiation sites and reduce the fatigue life of a component. Furthermore, surface and near-surface porosity have been shown to have a more severe impact on fatigue life than internal porosity. It is through using the near-threshold FCGR approach in which one can calculate the allowable pore size under operational loads. Using the load-shedding technique to obtain near-threshold FCGRs, the results showed anisotropic behaviour dependent on residual stress levels and R-ratios. The fatigue fracture mechanisms were predominantly governed by transgranular quasi-cleavage mechanisms. Furthermore, the fracture is directed by PBG morphology which results in anisotropic crack- closure effects. In addition, primary α lath orientation is governed by the crystallographic texture of the PBG which influences the mechanisms of crack propagation. With the increase in grain size, the presence of β improves the near-threshold FCGRs in the duplex anneal (DA) condition due to the superior plastic flow abilities more than in the as-fabricated and stress relief conditions. This study investigates the near-threshold FCGRs of LPBG produced Ti-6Al-4V in the as- fabricated, stress relief and bi-modal conditions in three build orientations. In addition, crack closure mechanisms are accounted for by implementing variable R-ratio testing. This research presents the influencing mechanisms of residual stress and microstructure on fatigue behaviour.