The Effect of Scan Speed on Lack of Fusion Formation During Electron Beam Melting of Ti-6Al-4V Alloy and Crack Growth Behaviour of the Alloy Under Impact and Fatigue Loading
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Electron beam melting (EBM) is a powder-bed fusion additive manufacturing process suitable for fabricating metallic parts with a high degree of shape complexity, opening EBM’s application potential with the Ti-6Al-4V alloy for biomedical and aerospace components. Due to the additive nature of part building track-by-track (TbT) and layer-by-layer (LbL), track size and track spacing coordination is required to coalesce tracks via overlapping and to prevent the formation of lack of fusion (LOF) for EBM to be efficient. To date, how Scan Speed (v) affects the morphology (size and shape) of the melt pool during EBM of Ti-6Al-4V has been insufficiently understood hence, this thesis opens with the expansion of knowledge on this. Heat flow direction during EBM of Ti-6Al-4V contributes to the columnar growth of β (bcc) phase predominantly along the build direction (BD) during solidification. Subsequently, upon cooling, β → α (hcp) + β in colony form takes place with α phase lining along the prior solidified β grain boundaries. Heat cycles imparted from TbT and LbL consolidation means transformation takes place multiple times. Thus, the microstructure formed during EBM is complex, with its influence on crack propagation under impact and fatigue loading yet to be sufficiently understood. This knowledge is important for microstructure control during EBM and for producing parts of satisfactory mechanical properties. Hence, understanding how EBM build orientation affects crack propagation under impact and fatigue loading are covered in the second and third part of this thesis respectively. In the first series of EBM experiments, the speed function was decoupled so that v could vary from 5 to 18 m/s for constant beam current of 28 mA. Samples were then examined metallographically with defect levels measured and quantified. In the second part of experimental/testing work, impact test samples were built applying EBM conditions common for Ti-6Al-4V. Samples were made with notches orientated in 0, 45 and 90° to the EBM BD. After impact testing, the fracture surface and cross sections of test samples were analysed, and defect measurements were made. In the third part of experimental/testing/analytical work, fatigue crack growth (FCG) samples were built and machined with notch orientations also at 0, 45 and 90° to EBM BD. After FCG testing and obtaining the fatigue curves of crack increment per cycle (da/dN) versus the change of stress intensity factor (ΔK), fracture surface cross-sections were examined in detail with an emphasis on colony dependent crack growth. ii A direct relationship between the LOF frequency and v has been established and will be explained in detail. It has been found that increasing v not only reduced the size of melt pool but also the stability of the shape of pool and that the reduction of melt pool size and shape stability resulted in insufficient overlapping, increasing the amount of LOF. Varying v is also found to impart a shape change of the melt pool, as well as influencing the height of the crown and depth of the melt pool. Explanation of the change of melt pool shape and size instability as a function of v is discussed in detail, including consideration of the Marangoni flow’s effect on the melt pool through varying v. Impact energy (IM) values of a range of samples have been found to vary widely from 26 J to 49J. The variation has been identified to be caused from two sources: BD dependent microstructure and batch dependent fraction of LOF. The dependence of IM on BD appears to originate from the mode of solidification during EBM. Cracks have been found to propagate through α + β colonies in vertically built samples with propagation barely affected by the α phase lining normal to the crack growth, along the prior β phase grain boundaries during columnar solidification. The effect of the α phase lining in samples built with other orientations on lowering IM will be shown and explained. The batch effect will be explained by LOF size variation in different batch and by LOF orientation which aids crack growth. FCG tests showed similar behaviour in region II for all notch orientations, with exponential constants determined to be m = 3.1 and C = 1×10-8mm/cycle. It will be shown that LOF in favourable orientation may affect crack growth but the low level of LOF at up to 1% has not contributed to an overall faster growth rate. The low level of porosity at 0.1% has affected crack growth little. It is also found that locally crack advancing in a specific orientation in a α+β colony and thus the striation orientation in one colony is different from another. The combined growth direction is then the same as the direction that the global crack front moves during crack growth. This crack growth feature is suggested to be the reason for the lack of the dependence of Paris law constants on notch/BD relationship. In summary, the effect of v on LOF formation and on fraction of LOF area have been quantified, along with introducing Type 4 LOF. Two major factors that affect IM the most, which are BD and maximum LOF area, have been identified. Furthermore, it has been found that the fatigue crack growth region II is independent of LOF, porosity and BD. In addition, a significant role of β phase in α+β colonies has been revealed.