Influence on micro-geometry and surface characteristics of laser powder bed fusion built 17-4 PH miniature spur gears in laser shock peening

Micro-geometrical errors, surface roughness, and surface integrity (microstructure, residual stresses, microhardness)
play an important role in defining the quality of the gears as they directly affect their noise, vibration
characteristics and service life during their use. In the present work, underwater laser shock peening (LSP) is
employed to improve the quality of the laser powder bed fusion built 17-4 PH small-size spur gears (12 mm
outside diameter). LSP was employed near the spur gear root, and effects were measured in terms of residual
stresses, variation in microgeometry errors, surface roughness, porosity, microstructure, and microhardness. It
was observed that LSP could impart compressive residual stresses up to 0.4 mm of measured depth, while the
surface roughness has improved by 32%. Microgeometry and microhardness of gears showed minor variations.
Additionally, LSP has shown an impact on the microstructure as the grain orientation was altered and grain size
reduced by 15.6% due to shock wave transmission. The study paves the way to use LSP as a post-processing
technique to modify the surface characteristics of LPBF-built miniature spur gears with minimal impact on the
gear microgeometry.