Application of the Theory of Critical Distances to As-Built Selective Laser Melted Ti-6Al-4V

Abstract

Metal additive manufacturing (AM) techniques such as selective laser melting (SLM) have become an increasingly present feature of new and novel manufacturing methods in recent years. This is owed to further technological advancements in a number of areas which have provided for increased understanding and finer control of the process as a whole. Such progressions have enabled the process’ capability to engineer components with complex shapes. Consequently, SLM has acquired a variety of applications in various sectors, ranging from aerospace to biomedical industries. Herein, metal alloys such as that of Ti-6Al-4V are employed, due to their characteristically high strength-to-weight ratios along with other exceptional material properties. Despite its growing acceptance however, many defects can arise throughout the SLM manufacturing process. Coupled with the potential to impose greater design complexity within AM parts, the extensive and sporadic nature of such sources of geometrical discontinuities can lead to a greater degradation of mechanical properties in comparison to those of conventionally manufactured materials. Furthermore, given the adoption of AM Ti-6Al-4V material in industries whereby a lengthy and reliable service life is imperative, a strong knowledge of fatigue behaviour is essential. The response of such material compositions to fatigue therefore needs to be quantified in a way that is as timely and efficient as possible. This work explores the potential to further verify and subsequently develop improved modified fractures mechanics based techniques as a way to predict the fatigue performance of AM Ti64 components. The hypothesis states that the availability of improved forms of theoretical assessment surrounding the dynamic capabilities of AM Ti64 are facilitated. An investigation in to the available literature revealed an experimentally heavy approach to the fatigue validation process of AM Ti64 material to date. This gave motivation towards evaluating the possibility of encompassing a more theoretical approach to fatigue appraisal. However, a review of traditional material failure theory identified certain gaps within the domain of a universally applicable fatigue prediction approach being available. Developments in the area of modified fracture mechanics (MFM) techniques served to breach these shortcomings somewhat. The Theory of Critical Distances (TCD) was earmarked as being the most promising out of all MFM methods to fulfilling these inadequacies. However, it was postulated that some key aspects still required further attention. This work therefore represents attempts to attend to such needs. The first practical task of this research was to verify the validity of applying TCD methods to forecasting the fatigue strength of notched SLM Ti64 specimens. To achieve this, an experimental study was formulated which involved evaluating various different sets of notched specimen designs which contained gradually decreasing feature sizes. This included the scenario of a plain specimen design with a rough SLM surface finish being considered as a micro-notch. In doing so, the accuracy of TCD methods with regards to predicting the effect of process-inherent defects (i.e. surface roughness) on resulting fatigue performance was evaluated. A review of results obtained revealed that TCD methods could satisfactorily estimate the dynamic response of AM Ti64 material in the presence of a range of differing notch (Kt) feature sizes. In an effort to develop TCD approaches beyond their current limitations, the ability of such methods to perform in increasingly challenging environments was assessed. This involved the analysis of specimen designs which incorporated progressively complex arrangements of stress- rising elements within their composition. Novel ways of executing TCD methodology were evaluated in an attempt to establish a superior universally predictive tool. Optimal ideologies were identified and earmarked as being potential successors to original TCD methods going forward. On the basis of the work that was accomplished, the hypothesis that the availability of improved forms of theoretical assessment surrounding the dynamic capabilities of AM Ti64 could be facilitated was accepted. Additional concluding remarks were made and suggestions for further work which could potentially improve the limitations of current advancements were given.