Comparison of the microstructure, mechanical properties and distortion of stainless steel 316 L fabricated by micro and conventional laser powder bed fusion

Abstract

Micro laser powder bed fusion (μLPBF) technology offers great benefits to industries as it enables fabrication of complicated metallic components with greater accuracy and minimum feature size as small as 50 µm. Employing finer laser beam and smaller metal powder in μLPBF leads to many variations from the conventional LPBF (cLPBF) in terms of microstructure, mechanical properties and distortion, which have not yet been well understood. This work provides a comparative study of the μLPBF and cLPBF of the well-known material, stainless steel 316 L based on the surface quality, crystal structure, solidification microstructure, tensile properties and distortion of as-printed parts, and their sensitivities to μLPBF process parameters are also studied. Results show that lower surface roughness (Ra= 3.4 µm for top surfaces) is obtained after μLPBF. Stronger < 110 > texture along building direction is developed in the μLPBFed samples, accompanied with smaller grain size, higher density of low-angle grain boundary (LAGB) and geometrically necessary dislocation (GND). μLPBF creates a cellular microstructure with smaller cell size and cell wall thickness compared with cLPBF. The yield strength of μLPBFed samples is marginally lower than cLPBFed ones, which is dominated by the difference of compositional microsegregation in the cellular structures. Both cLPBFed and μLPBFed samples show a strong anisotropy in terms of yield strength, ductility and deformation behavior. The distortion measurement of the printed cantilever design suggests a lower level of macroscopic residual stresses in the μLPBFed samples due to the smaller molten pool and more thermal cycles. Moreover, the microstructure, mechanical properties and distortion of μLPBFed samples remain at the same level with variation of laser power and scanning speed. Overall, better surface finish, finer microstructure, more desirable mechanical properties and smaller part distortion can be obtained by μLPBF.