Multiple laser beam processes for advanced manufacturing

Abstract

Lasers are used in many material processes, including cutting, drilling, surface melting, and recently, additive manufacturing (AM). These processes use a laser beam, usually with a Gaussian intensity profile, to deliver energy to the desired locations. Currently, the productivity in AM is limited because a faster processing speed is often associated with more product defects and sometimes total failures. The highly localized energy input from a laser beam leads to instability during the melting and solidification process, especially at a high laser scan speed. Therefore, we explored ways to overcome such limitations---by adding another laser beam to provide additional control flexibility and increase manufacturing throughput.This thesis first examines two current AM techniques for metal products---powder bed fusion (PBF) and direct energy deposition (DED). While both methods produced acceptable samples, low throughput and product defects illustrated the limitations of systems with a single Gaussian beam. We then presented a self-built dual-laser set-up for PBF. By running the two laser beams in parallel, the system demonstrated that, besides increasing the throughput, a new regime characterized by periodic coalescence occurred between the two molten pools under certain processing conditions. Such a regime could benefit surface texturing in PBF. Using the same set-up but different spatial arrangements between the two laser beams, we revealed the fluid mechanics for the humping phenomenon, an instability commonly found in PBF with fast processing speed. We further linked this fundamental mechanism to the processing conditions to provide general guidance on avoiding the humping phenomenon. The last part of this thesis employs the dual-laser system to explore laser polishing, a common post-processing step to address the rough surface produced by AM. Our set-up effectively rearranged the laser beam energy from a Gaussian distribution into an elliptical beam, resulting in an elongated molten pool, and showed improved results compared to using a single beam.