Microstructural functionally graded Ti-6Al-4V via advanced laser-based technologies
Geng, Yaoyi
Geng, Yaoyi
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Publication Date
2021-11-10
Type
Thesis
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Abstract
Functionally graded material (FGM) is characterised by an intended spatial variation in material attributes within a single part. This thesis is focused on advanced laser-based manufacturing technologies of microstructural FGMs. Specifically, laser-beam powder bed fusion (PBF-LB) and laser surface modification are investigated. The material evaluated is Ti-6Al-4V, due to its importance in the medical device and aerospace sectors. This work involves the design, fabrication and testing of Ti-6Al-4V FGM through the control of PBF-LB parameters (for bulk FGM) or laser surface modification (for surface FGM) parameters, and crystal plasticity finite element (CPFE) modelling and microscopy of the gradient microstructures. In PBF-LB, two fabrication strategies, namely stepwise FGM and differential equation FGM (DE-FGM), are systematically studied. The PBF-LB process parameters are designed based on the collection of data from the literature. The microstructure of the obtained samples is analysed using cutting edge technologies, including optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Lath thickness, phase content, and crystallography orientation extracted from the experiments are evaluated. Local mechanical properties are measured using nanoindentation, and the tensile response is recorded using digital image correlation (DIC). Gradient grain structure and crystallographic texture are recorded, and the mechanical heterogeneity is also characterized. In parallel, laser surface modification is explored to manufacture a microstructure gradient confined to the region beneath the surface via application of a high power laser to the surface of wrought Ti-6Al-4V. The surface typology, microstructure, and mechanical properties of the surface treated samples were analysed in detail, and compared with the original wrought sample. The microstructure transition from the surface to the original microstructure is observed. These experimental studies have demonstrated the feasibility of FGM fabrication using both PBF-LB and laser surface modification. The microstructure-mechanical properties relation in FGM is studied using a CPFE model. Ti-6Al-4V is a dual phase titanium alloy, with body centred cubic (BCC) β phase and hexagonal close packed (HCP) α phase. Therefore, in order to represent the dual phase microstructure, a dual phase CPFE model is developed to simulate the mechanical response of the microstructure. Two methods are used to generate the models that include representations of the microstructural gradients observed in the fabricated samples, i.e. Voronoi tessellation (VT) and EBSD converting method. The CPFE models are used here to investigate the influence of microstructure features e.g. the lath thickness and crystallographic orientation, on the mechanical properties of the homogeneous material. A linear correlation between the yield strength and microstructure parameters are established, which is also verified in the literature. CPFE models with microstructural gradients are also generated to investigate the local deformation behaviour of the FGM. Using the EBSD converted model, the importance of crystallographic texture on the mechanical properties is verified. The effect of texture is also demonstrated using a VT generated CPFE model.
Publisher
NUI Galway