Abstract
Laser powder bed fusion (LPBF), an additive manufacturing (AM) technology, shows its potential in the fabrication of low-volume and high-geometrical complexity productions, owing to the unique vector-layer-part manufacturing manner. The process involves intense energy inputs that create melt pools characterised by steep thermal gradients and high cooling rates ranging from 104 to 106 K/s. This rapid solidification triggers the development of epitaxial grain growth patterns aligned with the build direction (BD), leading to unique microstructures when compared to conventional cast and wrought alloys.Generally, the microstructure of LPBF-fabricated components mainly depends on scanning strategies and laser parameters, which control the preferential grain growth directions and form specific crystallographic textures. To evaluate the microstructure evolutions of different rotational angles, Alloy 718 samples with different crystallographic textures were fabricated by LPBF via three laser rotation angles, namely 0°, 67° and 90°. Moreover, different vector lengths are also widely used in the LPBF process to fill in the corners near the fabricated geometry borders. However, the effect of vector lengths on the microstructure is not well understood. Therefore, various vector lengths with unidirectional scanning direction were used to further investigate the effect of vector lengths on the microstructure. For the first time, multi-vector-length scanning strategies, namely Long-vector (4000 µm), Short-vector (80 µm), Long-Short vectors (4000 µm and 80 µm), and Incremental vectors (ranging from 80 µm to 400 µm), are employed in this work for controlling the microstructure of as-built samples.
It was shown that the melt pool depth for Short-vector printing (about 138 µm) exceeded that of Long-vector printing (about 85 µm). This was due to that the energy densities were not the same at different vector lengths when considering the actual laser motion. The slow scanning speed caused excessive energy input in short vectors, resulting in narrow and deep melt pools. In addition, with the increase in vector length, grain size correspondingly increased from less than 20 µm in Short-vector printing to over 50 µm in Long-vector printing. However, this increase in grain size was accompanied by a reduction in microhardness, declining from approximately 347.6±7.5 to 298.2±4.4 HV(0.2). This is the first time in the literature to report the vector length effect on the grain structure characteristics in LPBF.
Since high dimensional accuracy and surface finish are required for the end-use of LPBF-fabricated parts, post-machining is employed to improve the surface quality of the as-built components. To further understand the machining-induced deformation and chip formation of components fabricated by LPBF, the Alloy 718 samples with typical textures (fabricated by 0°, 67° and 90° rotational scanning strategies) are employed for the post-machining tests. A “quasi-in-situ” grain deformation investigation method and a quick-stop cutting method are used with the pendulum-based orthogonal cutting machine to investigate the machined surface deformation and chip formation mechanisms, respectively.
The crystallographic level deformation history for hundreds of microns during a high strain rate shear removal deformation was illustrated. Due to the carefully retained deformation history (i.e., typical bulges and slip bands) on the surface, a repeated deformation pattern was observed, attributing to the non-homogeneous deformation of typical build-directional blocks. The most active slip trace of deformed grain was calculated and verified based on the dominated slip bands within individual grains. The slip trace direction and intensity were quantified for different textured Alloy 718. Since the slipping-based deformation for an orientated grain was represented by its most active slip trace, a deformation tendency map was obtained by combining the shear direction, slip system and grain morphology. It was revealed that grains in high texture intensity workpieces generally followed the macro shear-based deformation, while with the decrease in texture intensity, the plastic anisotropy was significant at the grain scale.
By retaining chips on the workpieces in the quick-stop cutting test, it was found that the elongated grains in LPBF-fabricated Alloy 718 significantly influenced the material pile-up behaviour along the shear direction. This leads to an increase in shear angle and a decrease in chip ratio when compared with the machining of equiaxed grains in the wrought sample. This is because the transition of shear deformation from the cutting edge to the free surface is hindered by long grain boundaries that are perpendicular to the cutting direction. Since the shear bands are hard to cross long grain boundaries, the deformation of LPBF-fabricated Alloy 718 is governed by grain boundary bending in the primary shear zone.
| Date of Award | Jul 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Hao Chen (Supervisor), Zhirong Liao (Supervisor) & Dragos Axinte (Supervisor) |