The Effects of Morphology and Surface Oxidation of Stainless Steel Powder in Laser Based-Powder Bed Fusion

Laser based-powder bed fusion (LB-PBF) is one of the many techniques within additive manufacturing (AM) that allows for near net shape manufacturing of metallic components. By using powder feedstock, it is possible to spread thin layers of powder and further selectively fuse powder in a repetitive manner until the component is completed. The majority of the powder grades for AM are produced using vacuum induction melting and inert gas atomization (VIGA). The process produces spherical powder with high purity, but at a premium cost. In order to improve the utilization of LB-PBF further, more cost-efficient powder grades must be introduced to the market. This work therefore addresses three grades of 316L stainless steel powder with regard to the surface oxide characteristics and processability in AM, a vacuum induction melted and inert gas atomized (VIGA) grade, an air-melted and nitrogen gas atomized (GA) grade and a water-atomized (WA) grade. The chemical surface characterization revealed that the two gas-atomized powders had comparable oxidation states, with minor differences in particulate coverage between the two grades. Since the GA powder contained more Si, it was found in higher concentrations in the oxide particulates. The WA grade, however, had a larger surface coverage of particulates (rich in Cr and Si), yet had a thinner oxide layer surrounding the particulates compared to the gas atomized grades. However, the particulate features on the WA grade did not seem to affect the printability, as densities of > 99.95% were reached without discernible defects in the microstructure. While the printability was comparable with the GA grade at a layer thickness of 20 \ub5m, the limitation of the WA powder were noticed at a higher layer thicknesses (40 \ub5m), where up to about 1 vol.% of porosity was obtained. Furthermore, LB-PBF processing of WA powder was found to result in a rather homogenous precipitation of nanometric oxide inclusions within the microstructure. Consequently, this work investigated whether the oxide inclusions could contribute to an oxide dispersion strengthening effect. The average size of these oxides was found to be 56 nm, with an average number density of 2.8 7 1015 m-3. The oxides were observed to be amorphous with a characteristic core-shell structure. Concerning the mechanical strength, the WA samples had slightly reduced yield strength (~500 MPa) in comparison to the GA samples (~600 MPa). Hence, no oxide dispersion strengthening effect was observed, as the average size and number density of oxides was not optimized

Effect of the powder feedstock on the oxide dispersion strengthening of 316L stainless steel produced by laser powder bed fusion

In this study, the concept of enhancing the in-situ oxide precipitation in laser powder-bed fusion processed parts is investigated using powder produced by water and gas atomization. By using water-atomized 316L powder, compared to gas-atomized powder, more oxide precipitates were introduced into the microstructure with the intent to enhance the strength of the material, as an alternative path to oxide dispersion strengthened materials. The results showed that oxide precipitation was homogenous, with higher-number densities of oxides in the sample built using the water-atomized powder. The oxides were observed to be amorphous and enriched in Si and Cr. The average size of the oxides was ~56 nm. After an annealing heat-treatment at 900 \ub0C, the oxides were observed to remain within the microstructure with only minor changes in size and composition. Mechanical testing at room temperature and at elevated temperature did not show any increase in strength relative to the sample built using gas-atomized powder. However, it was shown that the use of water atomized powder in the L-PBF process provides a viable method of introducing and tailoring the number of oxide particles within a built component relative to a conventional gas atomized powder

Effect of powder variability on laser powder bed fusion processing and properties of 316L

To date, the effects of powder properties, both physical and chemical, on the printed properties of as-built components is still a topic that is poorly understood. A contributing factor is the lack of relevant methods for evaluating the rheological properties of powder, or flowability. This study presents a review of different 316 L powder grades that were produced using various atomization techniques. The physical powder properties were evaluated using powder metallurgical techniques, a powder rheometer (FT4) and a rotating drum analyser (RPA). The results indicate that both the FT4 and RPA are suitable for powder characterization. However, the parameter selection for evaluation must be done keeping in mind the application, in this case thin layer powder spreading. It was found that all bulk powder density measurements, basic flow energy and the break energy were able to both differentiate between powder grades and predict how suitable the powder will be for the laser-based powder-bed fusion process. Despite some printability challenges of the water atomized grades at higher layer thicknesses, it was found that both gas atomized grades performed similarly despite minor differences in particle size distribution. Furthermore, powder variability did not show any detrimental effects on the resulting mechanical properties

Investigation of the strengthening mechanism in 316L stainless steel produced with laser powder bed fusion

Of the many benefits of the additive manufacturing process, laser powder bed fusion (L-PBF) has specifically been shown to produce hierarchical microstructures that circumvent the common strength-ductility trade-off. Typically, high strength materials have limited ductility, and vice versa. The L-PBF microstructure, consisting of fine cells, is formed during the rapid solidification of the laser powder bed fusion process. The cell boundaries are often characterized by the segregation of alloying elements and a dislocation network. While there are a number of works describing the strengthening mechanisms in L-PBF-produced 316L, there are still some gaps in understanding the effect of stress-relief and annealing at various annealing temperatures (400, 800 and 1200 \ub0C) on the plastic strain accumulation during deformation. In this study, the authors evaluated strain partitioning using electron backscatter diffraction and kernel average misorientation maps. The results show strain partitioning to be dependent on both the annealing temperature and the pre-straining of samples. Further, the results indicated that the dislocation structure was stable until 400 \ub0C, whereas at 800 \ub0C strain was no longer detected at the cell boundaries. Similarly, after the heat treatment at 800 \ub0C, elemental segregation at the cell walls was no longer detectable. Upon straining, the boundaries of as-built and annealed samples at 400 and 800 \ub0C registered accumulation of additional strain as compared to the unstrained states. The results demonstrate that even a weak array of dislocations along the cell walls can successfully pin dislocations, albeit at a reduced capability relative to the co-existent dislocation and segregate structures found in microstructures of the as-built and annealed samples at 400 \ub0C

Effect of atomization on surface oxide composition in 316L stainless steel powders for additive manufacturing

The initial oxide state of powder is essential to the robust additive manufacturing of metal components using powder bed fusion processes. However, the variation of the powder surface oxide composition as a function of the atomizing medium is not clear. This work summarizes a detailed surface characterization of three 316L powders, produced using water atomization (WA), vacuum melting inert gas atomization (VIGA), and nitrogen atomization (GA). X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy analyses were combined to characterize the surface state of the powders. The results showed that the surface oxides consisted of a thin (~4 nm) iron oxide (Fe2O3) layer with particulate oxide phases rich in Cr, Mn, and Si, with a varying composition. XPS analysis combined with depth‐profiling showed that the VIGA powder had the lowest surface coverage of particulate compounds, followed by the GA powder, whereas the WA powder had the largest fraction of particulate surface oxides. The composition of the oxides was evaluated based on the XPS analysis of the oxide standards. Effects of Ar sputtering on the peak positions of the oxide standards were evaluated with the aim of providing an accurate analysis of the oxide characteristics at different etch depths

Design and characterization of a cobalt-free stainless maraging steel for laser-based powder bed fusion

This study presents a new Co-free stainless maraging variant for laser-based powder bed fusion developed using a computational alloy design approach. The goal was to develop an easily printable material with similar performance to 18Ni-300. After screening numerous compositions, Fe-13.2Cr-9.1Ni-1.1Al-0.6Mo-0.5Nb-0.23Ti-0.5Mn-0.5Si (wt.%) was selected. This composition showed excellent printability with low porosity levels. The precipitation strengthening response was evaluated by aging at 500 \ub0C for 15 min, 3 h and 18 h, measuring hardness, tensile strength, and by characterization using atom probe tomography. After 15 min of aging, 90% of the maximum hardness was reached, thanks to formation of (Ni, Al, Nb, Ti, Mn, Si) clusters with a density of 1.5 7 1024 m-3. Between 15 min and 3 h, distinct precipitates formed with a radius of ∼1.4 nm. The precipitates underwent a splitting phenomenon after 18 h, forming several unique Ni-rich precipitates including Ni16Si7(Ti, Nb)6 and Ni3(Al, Ti, Nb, Si). The splitting can be a reason for the slow coarsening rate, as the average precipitate radius after 18 h was only 2 nm. Simulations of the precipitation sequence using PRISMA indicated very rapid and dense precipitation of L12-Ni3X precipitates with a slow coarsening rate, in agreement with experimental observations

Laser-based powder bed fusion of stainless steels

The aim of the present work has been to widen the knowledge of how variations within powder manufacturing affect laser-based powder bed fusion processing, and how this processing affects the microstructure and strength of stainless steels. The approach was to keep the processing parameters fixed while the powder feedstock was varied. This methodology enabled an isolation of the powder properties, which were correlated to the residual porosity in the printed samples. After establishing the relationship between printability and powder properties, a careful microstructural investigation was performed to understand what features are responsible for the relatively high strength of austenitic stainless steels. Two different alloying strategies were attempted to boost the strength further, introducing additional oxygen into the processing chamber for an in-situ synthesis of nanometric oxides, and designing a composition that produces strengthening precipitates upon aging.The initial powder investigations revealed that 316L powder produced using vacuum induction melting inert gas atomization (VIGA) and conventional gas atomization (CGA) displayed similar oxidation states despite different atomization gases. The use of water in the atomization process however changed the oxidation state significantly, resulting in more extensive formation of oxide particulates on the powder surfaces. Analysis of the powder properties showed similar trends as the surface analysis, where the VIGA and CGA powder grades had similar flow properties. While water atomized (WA) powder had significantly lower flowability as compared to the other tested grades. The lower flowability caused a significant increase in residual porosity when printing with layer thicknesses above 20 \ub5m.Microstructural characterization of printed 316L specimens revealed a hierarchal structure consisting of elongated grains and within them a fine cellular structure. The cell structure was found to act as soft grain boundaries, hence strengthening the material without sacrificing ductility too much. This structure was found to be stable up to 800 \ub0C.Conceptually, the in-situ synthesis of finely distributed nanometric oxides using water atomized powder was shown to work. However, the size and number densities of the oxides must be further optimized to provide a strengthening effect. Another strategy for increasing the strength was by developing a heat-treatable composition using thermodynamic simulations. This resulted in the development of a novel stainless tool-steel composition. This new material had excellent printability with a fully martensitic structure in the as-printed condition and possessed a yield strength of nearly 1600 MPa after aging. The precipitates were found to have relatively slow coarsening rates and therefore the material retained much of its hardness despite long aging times

Monteringslösning till dörrkarm

Dooria Sweden AB is one of the main door- and doorframe manufacturers inSweden. They deliver door frames for indoor use for project oriented buildingsites, for these types of building sites the doorpost comes pre-assembled. DooriaAB wants to minimize transport to reduce impact on environment and thetransportation costs of pre-assembled doorframes, this thru simplifying themethod of assembly.The main purpose of this thesis has been to develop a prototype solution thatbrings the three parts of the doorframe together with the same properties regardingstability and tightness of joints as the current method of assembly. A profile ofdemands and wishes were created in collaboration with Mårten Dragstedt, DeputyDirector of Dooria Sverige ABBy following the chosen method the authors have been able to produce a fullyfunctional prototype that meet the demands that were put on the product. Catia V5R20 has been used as an aid during prototype construction and virtual tests. Theprototype consists of a fitting, which simplifies and secures the method ofassembly regarding correct alignment of mutual parts and tightness of joints.Involved personnel will be able to assemble the parts of the doorframe easilywithout the need of tighten the doorframe, this prototype solution will also reducedamage to the doorframe caused by abrasion.This thesis has resulted in a prototype solution designed for assemblingdoorframes, it consists of two different types of already made components, aconnection screw with an associated screw-nut, this prototype will further becalled Ikeabeslag. The authors have proven that this solution best meets the stateddemands