title: Novel Scan Strategies for Selective Laser Melting
Creators: name: Revanur, Ramkumar
Publisher: name: University of Liverpool Repository
Year: 2024

description: This work investigates novel scan strategies for Selective Laser Melting with the powder bed fusion process. The capabilities and limitations of the additive manufacturing machine play a vital role in the design of the scan strategies. A scan strategy is highly dependent on the capability/execution of the scan hardware and the ability of the control system to steer the laser beam through the optical drives and deliver focused laser energy at the precise location on the powder bed. This work identifies novel scan strategies that overcome the limitations of current techniques relying on discrete point exposures for consolidating material. 
The discrete point exposure strategy delivers laser energy at pre-determined locations in a sequence along the scan path to consolidate and bind regions in a layer with layers below the current layer. The scan strategies for discrete point exposures are discussed, and novel strategies are proposed, characterised and evaluated in this research. The available technology (Laser(s), steering optics, mechatronics, firmware, and control architecture) used to build modern additive manufacturing (AM) machines has its limitations. The key considerations in developing scan strategies are the steering mirror dynamics and the challenges in controlling the position of the laser beam on the powder bed.
The mirrors steering the laser during the execution of scan strategies are typically subjected to very high acceleration. Discrete scan strategies often start and stop the mirror movement, thus exacerbating the acceleration problems. This research explores the potential limitations of discrete scan strategies used by Laser powder bed fusion (LPBF) and proposes enhancements, such as sky-writing, that overcome some limitations. Sky-writing is studied in detail to improve repeatability and mark consistent exposures. This research also explores the reasons for local energy concentration in the build layers. Multi-layer scan strategies, introduced in this work, consolidate material over many consecutive layers and reduce/eliminate local energy concentration.
This work investigates the broad control hardware, including steering and focus optics, software architecture and development methods to extend the understanding of LPBF to produce consistent metal parts with fewer defects. The scan strategies catering to the core and border areas are different as they are typically processed with different laser parameters. Novel techniques, such as blended borders, reduce defects between core and borders. Energy density, rate of energy delivery, gas flow and layer geometry influence the consolidation process and the material properties in the LPBF process. Local energy density varies based on the location of each exposure and the proximity of such exposures with other neighbouring exposures. The scan strategies proposed in this work distribute energy uniformly. Hexagonal discrete point exposure strategies demonstrated and evaluated in this work show higher part densities with a broader process regime. These strategies can be used alongside vector scan strategies.
Novel metrology techniques to measure LPBF part distortion and the methods to compensate for such distortion are presented. A hexagonal multi-layer scan strategy using non-overlapping exposures is introduced to minimise residual stresses. Another proposed technique (independent of the scan strategy) relies on inverse compensation of the measured distortion. 
This work (for the first time) introduces sky-writing for discrete point exposures and combines this with vector scan strategies. Multi-layer hexagonal point exposure strategies are shown to improve part density and have a broader process regime, enabling fasting material parameter development.