Dimensional analysis for tuning Selective Laser Melting parameters for near-full density of Inconel 718

Abstract

Selective laser melting is a powder bed fusion process that allows the production of metallic pieces of high geometrical complexity. Full densification is regarded as fundamental to achieve mechanical integrity. Nevertheless, doing so for a new material requires an intensive, in time and resources, experimentation stage in order to set proper manufacturing parameters. In this work, dimensional analysis is used to develop a general mathematical model on bulk density of SLMed components taking volumetric energy density, scanning speed and powder’s thermal conductivity, specific heat capacity and average grain diameter as independent variables. Strong relation between dependent and independent dimensionless products was observed. Bulk density is found to be proportional to volumetric energy density and be affected by scanning speed by a factor of negative two. Inconel 718 probes were produced and a particular expression, in the form of a first order polynomial, for its bulk density,in the independent dimensionless product π1 range from 3.17x10−8 to 4.6 x10−8 was obtained. In this range, better densification is achieved at lower scanning speed and lower laser power. The first is related to higher exposure time and ensuring full melt of the powder, and the second may be due to powder particle sublimation / ejection due to improperly large laser power conditions. An average relative density of 95.218% was measured. An average error percentage of 1.6503% between experimental and predicted bulk density (and dimensionless density) was achieved. A mathematical tool for tuning scanning speed to achieve full densification, with respect to laser power, was developed. Moreover, particular conditions for achieving so for Inconel 718 in the π1 range was provided.