- Selective laser melting (SLM) is an additive manufacturing (AM) process that uses a laser beam to melt metal powder as it is spread onto the build surface using a roller or scraper, followed by rapid solidification to manufacture a 3D component, one layer at a time.
In SLM process, density of manufactured components is directly influenced by the density of powder bed. Remaining voids between particles can result in low relative densities, poor mechanical behavior and warping due to non-uniform shrinkage during melting. One approach for addressing the challenge of producing fully dense metal parts via SLM is by maximizing the packing density of the powder feedstock before/during melting.
The powder bed density of these metal feedstock powders is influenced by variations in powder size, powder size distribution, powder morphology, and chemical composition. The purpose of this thesis is to investigate the influence of particle size distribution (PSD) in 316L stainless steel powder feedstock on the density, microstructure and hardness of SLMed components. This austenitic steel is desirable for its strength and ductility, as well as good corrosion resistance.
Spherical powder feedstock with single and bimodal PSD (with coarse to fine particle size ratio of approximately 7:1) were used as feedstock. First, the commonly practiced methods of measuring flow properties and density of powder bed were identified in order to establish a relationship between powder characteristics and component density. This effort included designing and testing a simple method for measuring density of layer-wise spread powder to measure the density of a powder bed during the SLM process. Then, spread density was compared with other powder density measurements to determine if apparent density (ρa) and tap density (ρt) were practical indicators of powder bed density in SLM. Using the tap density of each, the optimal quantity of coarse and fine powder for each bimodal mixture was calculated.
The packing density and flowability of the mixed bimodal PSDs were determined by measuring tap density and the Hausner ratio (ρt/ρa). SLM was performed in nitrogen atmosphere at volumetric energy densities (VED) ranging from 35.7 J/mm3 to 116.0 J/mm3 using feedstock with single mode PSD where D90 <50µm and bimodal PSD with a primary large and small powder size of ~35µm and 5µm. The density of bulk samples made from each powder type and VED was measured using the Archimedes method. Metallography, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the porosity, microstructure and phase identification of samples manufactured from different powder feedstocks. Furthermore, the Vickers microhardness of select samples produced from both single mode and bimodal feedstock was measured. Samples from both single mode and bimodal powder size distribution, SLMed at VEDs of 74 and 89 J/mm3, respectively, were annealed at 1020°C for two hours and were then characterized.
It was demonstrated that bimodal PSD could provide denser particle packing in a powder bed that is mechanically tapped to maximum packing density. This is because tap density of the bimodal size distributions was up to 2% greater than single mode powder from the same supplier. Relative density of SLMed parts as a function of VED at each power level revealed that for low laser power (107-178W) where relative density is below 99%, bimodal feedstock resulted in higher density than single mode feedstock. However, at higher power (>203W), the density of bimodal-fed components decreased as the VED increased, likely due vaporizing of the fine powder in bimodal distributions at higher energy levels.
Hardness was approximately similar for single mode and bimodal samples at microhardness values of ~225-245 HV using the same melting parameters. Grain size did not appear to change significantly between single mode and bimodal powders with the approximate width of columnar grains ranging from ~30-70µm. A fully austenite phase was maintained in SLMed components from both PSD types, both before and after annealing. Annealed samples showed recrystallization primarily adjacent to melt pool boundaries were observed prior to annealing.
In summary, this thesis demonstrates that despite higher powder bed densities in powder feedstock with bimodal PSD, differences in conduction melting and vaporization points between the two primary particle sizes would practically limit the maximum achievable density of SLMed components produced from bimodal powder.