Influence of processing and microstructure on the local and bulk thermal conductivity of selective laser melted 316L stainless steel

Abstract

Material properties of parts made via selective laser melting are not the same as the well-established properties for bulk base materials, due to the unique processes used to produce the parts. Meanwhile, additive manufacturing is increasingly being used for heat exchangers and heat removal devices, which demand high thermal conductivities. The thermal properties are also important for many non-destructive testing technologies. The thermal conductivity of selective laser melted 316 L stainless steel was studied as a function of processing conditions and build orientation. The porosity and thermal conductivity were measured versus processing conditions. A critical energy density of 44.4 J/mm3 was observed below which the porosity increased and the thermal conductivity decreased. For the lowest-porosity sample, the local thermal conductivity map taken with frequency domain thermoreflectance showed a variation in the stainless steel thermal conductivity between 10.4 and 19.8 W/m-K, while the average thermal conductivity of 14.3 W/m-K from the thermal conductivity map agreed, within measurement uncertainty, with the bulk thermal conductivity measurements. The thermal conductivity trend was not fully explained by the porosity, as effective medium models fail to predict the trend. Amorphous stripes in the selective laser melted stainless steel grains were identified by transmission electron microscopy. These amorphous regions also resulted in decreased x-ray diffraction intensities with increasing porosity. The amorphous regions are hypothesized to lower the thermal conductivity at faster laser scanning speeds due to less time at elevated temperatures. We also found that in-print plane and through-print plane thermal conductivities have the same value when the energy density is greater than this critical amount. When the energy density reduces below this critical amount, the in-plane conductivity exceeds the through-plane.