The selection of laser beam diameter in directed energy deposition of austenitic stainless steel: A comprehensive assessment

One focus of laser directed energy deposition (L-DED) is to increase productivity, and this can be achieved by using a larger beam diameter and higher laser power. However, it is difficult to build complex metallic components with fine structures using a laser beam of large diameter. Under the trade-off between formability and productivity, the choice of beam diameter needs to be thoroughly evaluated. Herein, three beam diameters (0.75 mm, 1.5 mm, and 3 mm) in building austenitic stainless steel parts have been investigated and compared in four aspects: microstructure, mechanical properties, powder utilization, and as-built part surface quality. Specifically, cooling rate and powder splashing in the building process, and phase composition, grain size and dislocation density of as-built parts are revealed by means of numerical simulation, high-speed camera, electron back scattered diffraction, and X-ray diffractometry. Results show that for a small beam diameter, the microstructure formed has finer grains and less amount of ferrite due to higher cooling rate of the molten pool, and grain refinement and high density of dislocations lead to superior mechanical properties of the as-built part; conversely, for a larger beam diameter, more stable molten pool and less amount of powder splashing brings higher material utilization rate and better surface quality. A brief evaluation of beam diameter selection is accordingly proposed, and a novel hybrid beam diameters building strategy is presented for building complex metallic components with high quality and efficiency.