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Microstructure, Texture and Mechanical Property Evolution during Additive Manufacturing of Ti6Al4V Alloy for Aerospace Applications
[Thesis]. Manchester, UK: The University of Manchester; 2012.
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Abstract
Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β
Keyword(s)
Additive Manufacturing, α+β Ti6Al4V alloys, Electron Beam Selective Melting, Selective Laser Melting, Wire Arc Additive Manufacturing, Powder Bed, Wire Deposition, Gas Tungsten Arc Welding, Gas Metal Arc Welding, Bulk β Grain Structures, Epitaxial Growth, Columnar Grains, Stray Grains, Bulk Primary β Texture, Transformed bulk α Texture, Fibre Texture, ; Thermal Modelling, Melt pool, Thermal Gradient, Solidification rate, Cooling rate, Fatigue Properties, Tensile Properties, Fractography of AM, Pores, Variant Selection, Rolling Deformation, Influence of Build Geometry and, process parameters during AM.
Bibliographic metadata
- Additive Manufacturing, α+β Ti6Al4V alloys, Electron Beam Selective Melting, Selective Laser Melting, Wire Arc Additive Manufacturing, Powder Bed, Wire Deposition, Gas Tungsten Arc Welding, Gas Metal Arc Welding, Bulk β Grain Structures, Epitaxial Growth, Columnar Grains, Stray Grains, Bulk Primary β Texture, Transformed bulk α Texture, Fibre Texture,
- Thermal Modelling, Melt pool, Thermal Gradient, Solidification rate, Cooling rate, Fatigue Properties, Tensile Properties, Fractography of AM, Pores, Variant Selection, Rolling Deformation, Influence of Build Geometry and, process parameters during AM.