Abstract
In Additive Manufacture (AM), with the widely used titanium alloy Ti–6Al–4V, the solidification conditions typically result in undesirable, coarse-columnar, primary β grain structures. This can result in a strong texture and mechanical anisotropy in AM components. Here, we have investigated the efficacy of a new approach to promote β grain refinement in Wire–Arc Additive Manufacture (WAAM) of large scale parts, which combines a rolling step sequentially with layer deposition. It has been found that when applied in-process, to each added layer, only a surprisingly low level of deformation is required to greatly reduce the β grain size. From EBSD analysis of the rolling strain distribution in each layer and reconstruction of the prior β grain structure, it has been demonstrated that the normally coarse centimetre scale columnar β grain structure could be refined down to <100μm. Moreover, in the process both the β and α phase textures were substantially weakened to close to random. It is postulated that the deformation step causes new β orientations to develop, through local heterogeneities in the deformation structure, which act as nuclei during the α→β transformation that occurs as each layer is re-heated by the subsequent deposition pass.
Highlights
Near-net-shape fabrication of metallic components by Additive Manufacture (AM) is an important new technological area with many potential applications in the aerospace industry (e.g. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17])
In Wire–Arc Additive Manufacture (WAAM) a consumable wire is fed at a controlled rate into an adapted electric arc welding torch that is translated by a robot [9,10,11,12]
The main features that can be noted are the prior β grain structure that developed on solidification, before transformation to an α–β lamellar microstructure on cooling to room temperature, and the regularly spaced horizontal white bands
Summary
Near-net-shape fabrication of metallic components by Additive Manufacture (AM) is an important new technological area with many potential applications in the aerospace industry (e.g. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]). A range of AM processes are available, mainly based on laser or electron beam systems, that use powder or wire feedstock [1,2,3,4,5,6,7,8,9,10,11,12] Of these techniques, powder bed methods allow more geometrically complex components to be produced, but the part size is restricted by slow build rates and the limited dimensions of the working chamber [1,2,3,4]. The WAAM process has a much higher deposition rate than most other metal additive manufacturing techniques (up to 10 kg/h) It provides better material utilization than powder based methods [9,10,11,12], but is restricted to wider wall thicknesses and cannot produce as fine scale features. This low cost process is most suited to producing larger scale parts with less complex geometries
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