Predictive analytical microstructural and defect modeling of as-solidified 316L manufactured through laser powder bed fusion

Abstract

Additive manufacturing presents a novel fabrication method for construction of complex geometry parts along with reducing waste material. The different methods of additive manufacturing create an adaptable process space where processes and materials can be tailored to different applications. Additive manufacturing results in unique solidification behavior that creates microstructures and defects that can have large effects on the material properties and behavior. This work explores two analytical modeling methodologies, one for pore formation and one for as-solidified microstructure, toward providing a better understanding of 316L stainless steel microstructure evolution produced by laser powder bed fusion. The defect modeling methodology was investigated to predict keyhole porosity, as well as lack-of-fusion that forms between melt pools and within melt pools by utilizing or modifying criteria proposed through literature. The keyhole porosity and lack-of-fusion between melt pool criteria were taken from literature, but a laser shock pressure model was modified to predict the conditions under which spatter or powder is ejected from the melt pool and causes lack of fusion porosity within melt pools. The models were solved to create maps that indicate combinations of laser power and laser travel speed where keyhole, lack-of-fusion, and gas porosity are expected form as well as where no porosity is expected. Predictions of the resulting defects maps were compared against volumetric energy density criteria often cited in additive manufacturing literature. The map predictions were tested by comparing defect and melt pool geometries in builds built at the same volumetric energy density, and the results show that volumetric energy density is not a reliable predictor of porosity, but the defect map is. An analytical microstructure map was created to predict the primary solidifying phase, primary dendrite arm spacing, and the transition from columnar to into equiaxed dendritic solidification. The primary dendrite spacing model was modified to account for decreases in dendrite tip temperature that occur at high solidification velocities typical to laser powder bed fusion. A computational heat transfer finite element model was implemented to predict the temperature gradients and solidification velocities during solidification in laser powder bed fusion. The heat source in the heat transfer model was adjusted so that the predicted melt pool geometry matched a single experimental melt pool, and then solidification velocities and thermal gradients were extracted from the heat transfer model. In general, the combination of analytical microstructure models and heat transfer simulation results were able to predict the primary solidifying phase and primary dendrite arm spacing in laser powder bed fusion. In particular, the modified primary dendrite arm spacing model showed much improved agreement with experimental results compared to previous versions of the model that assume a constant dendrite tip temperature.