316L Powder Research Articles

Study of Mechanical Characteristics of Stainless Steel Samples Obtained by Direct Laser Deposition

The paper presents the results of mechanical tensile tests and microhardness of samples obtained from stainless steel 316L powder by direct laser deposition. The strength characteristics of the deposited samples are better than those of rolled ones obtained in the traditional way. The material strength is reduced and its plasticity is increased with the growth of the laser radiation power during deposition. The obtained regularities are explained by analysis of the microstructure. It is found that the hardness of the cladding is substantially higher than that of the substrate material with the corresponding composition. This is a consequence of hardening of each layer during deposition of the next layer and formation of nanosized spherical inclusions representing oxides of metals that make up the powder in the sample bulk. The density of these particles affects the overall hardness of the material and depends on the radiation power supplied. Individual properties of the material for various applications can be modified by appropriate selection of technological parameters of the printing process.

Systematic approach for determining optimal processing parameters to produce parts with high density in selective laser melting process

Relying on trial-and-error methods to determine the optimal processing parameters which maximize the density of parts produced using selective laser melting (SLM) technique is costly and time consuming. With a given SLM machine characteristics (e.g., laser power, scanning speed, laser spot size, and laser type), powder material, and powder size distribution, the present study proposes a more systematic strategy to reduce the time and cost in finding optimal parameters for producing high-density components. In the proposed approach, a circle packing design algorithm is employed to identify 48 representative combinations of the laser scanning speed and laser power for a commercial Nd:YAG SLM system. For each parameter combination, finite element heat transfer simulations are performed to calculate the melt pool dimensions and peak temperature for 316L stainless steel powder deposited on a 316L substrate. The simulated results are then used to train the artificial neural networks (ANNs). The trained ANNs are used to predict the melt pool dimensions and peak temperature for 3600 combinations of the laser power and laser speed in the design space. The resulting processing maps are then inspected to determine the particular parameter combinations which produce stable single scan tracks with good adhesion to the substrate and a peak temperature lower than the evaporation point of the SS 316L powder bed. Finally, the surface roughness measurements are employed to confirm the parameter settings which maximize the SLM component density. The experimental results show that the proposed approach results in a maximum component density of 99.97 %, an average component density of 99.89%, and a maximum standard deviation of 0.03%.

Rheological properties of two stainless steel 316L powders for additive manufacturing

This study measures the rheological properties of two stainless steel 316L powders which are used for the powder-bed-fusion based additive manufacturing process. The purpose is to evaluate the newly acquired powder in comparison with the used and recycled powder, so that both powders can be mixed with each other to supplement the powder usage. The powder rheology properties, such as dynamic property, bulk property, and shear property, are tested and compared. The results and analysis confirm the compatibility of powder mixing.

Emissivity calibration method for pyrometer measurement of melting pool temperature in selective laser melting of stainless steel 316L

Selective laser melting (SLM) is an additive manufacturing (AM) technique for producing arbitrary work pieces, in which a laser beam is controlled to melt specific regions of a metal powder bed layer by layer so as to build up the required geometric form. In the present study, a method is proposed for calibrating the measurements obtained by a pyrometer for the melting pool temperature in the SLM of stainless steel 316L powder using the estimated values of the emissivity coefficients obtained from finite element heat transfer simulation and experimental tests. The accuracy in temperature prediction by heat transfer simulation is also confirmed by embedding a thermocouple into the powder bed. As a result, the calibration process is applicable to both one-color and two-color pyrometry methods. It is shown that the average error between the temperature measurements obtained from the calibrated pyrometer and the simulated temperature is just 1%. In other words, the feasibility of the proposed emissivity-based calibration method is confirmed. In the author’s knowledge, this is the first proposed idea to calibrate the emissivity of the pyrometer based upon the simulation model for accurately extracting the true melting pool temperature.

3D non-isothermal phase-field simulation of microstructure evolution during selective laser sintering

During selective laser sintering (SLS), the microstructure evolution and local temperature variation interact mutually. Application of conventional isothermal sintering model is thereby insufficient to describe SLS. In this work, we construct our model from entropy level, and derive the non-isothermal kinetics for order parameters along with the heat transfer equation coupled with microstructure evolution. Influences from partial melting and laser-powder interaction are also addressed. We then perform 3D finite element non-isothermal phase-field simulations of the SLS single scan. To confront the high computation cost, we propose a novel algorithm analogy to minimum coloring problem and manage to simulate a system of 200 grains with grain tracking algorithm using as low as 8 non-conserved order parameters. Specifically, applying the model to SLS of the stainless steel 316L powder, we identify the influences of laser power and scan speed on microstructural features, including the porosity, surface morphology, temperature profile, grain geometry, and densification. We further validate the first-order kinetics of the transient porosity during densification, and demonstrate the applicability of the developed model in predicting the linkage of densification factor to the specific energy input during SLS.

XPS, XRD, and SEM characterization of the virgin and recycled metallic powders for 3D printing applications

We characterized the surface and microstructure of both feedstock (virgin) and recycled Stainless Steel 316L powders used for selective laser melting process. Reusing the recycled powders can significantly reduce the powder consumption, production cost and time. We have performed X-ray photoemission spectroscopy (XPS), Scanning electron microscopy (SEM), X-ray Diffraction (XRD) and rheology analysis on steel powders both on the virgin powder and the powders not adhered to the printed parts but recycled in the powder bed. The results confirm that the oxygen level on the surface of the recycled powder is increasing and the metallic oxides diffuse out to the surface of the powder particles during the manufacturing process. Nevertheless, the changes are not significant which promise the reusability of the recycled powders for several cycles. We observed no change on the phase of the recycled powder. Further characterizations are ongoing to measure the chemical composition and morphology of both virgin and recycled powders to correlate them with the mechanical properties of the printed parts. These characterizations allow extending the powder reuse for a number of reusing cycles and reducing the manufacturing time and cost without reducing the mechanical properties of the final parts.

Effects of CeO2 on the Microstructure and Properties of Laser Cladding 316L Coating

Laser cladding was used to distribute a CeO2-modified 316L powder coating onto a 316L stainless steel surface. The cladding layer phase composition was evaluated through x-ray diffraction. An analysis of the microstructure was conducted using an optical microscope and scanning electron microscope, while the micro-hardness was measured using a tester. The coating corrosion behavior in response to 3.5% NaCl solution was analyzed by polarization curves and electrochemical impedance spectroscopy. Finally, the overall performance of coatings with different CeO2 concentrations, 1, 2, and 3%, was compared. Results showed that adding CeO2 significantly refined the grains, reduced the coating porosity, and effectively limited the appearance of local corrosion. Also, additive CeO2 shifted the coating’s electrode potential in a positive direction, minimizing the current density of corrosion, creating a stable passivation film, and improving the coating’s corrosion resistance. The coating performance (hardness and corrosion behavior) was optimal when 2% CeO2 was employed as the powder mass.

Evaluation via powder metallurgy of nano-reinforced iron powders developed for selective laser melting applications

In this work, a gas atomised stainless steel AISI 316L powder was used as metal matrix and SiC was employed as a nano reinforcement. The powders were experimentally characterised to determine the effect of the morphology, size, and levels of reinforcement on the powder flowability. The powder was developed via the powder metallurgy route and the effect of material, process conditions and various levels of reinforcement were investigated through the microhardness of the sintered samples. Sintered samples produced from the 316L + SiC + PVA powder mixes presented improved hardness. Analysis of the Energy Dispersive X-ray measurements detected high intensity levels of carbon and silicon on the surface of the reinforced 316L particles. In terms of measured powder rheology, the 6 wt% SiC coated 316L provided the highest flowability of the prepared SiC coated 316L powders and a much higher flowability than the as received 316L powder. All prepared SiC coated 316L powders showed good flowability and highly repeatable powder rheology. The high degree of flowability was attributed to the particle spherical morphology, the narrow range of particle size distribution and also the coating of nano SiC particles on the 316L particles which were found to act in this case as a solid lubricant. A successful homogeneous and uniform reinforcement of SiC onto the surface particles was resultant from the established mixing technique. While the nano-SiC improved the powder fluidity, the obtained improvement in hardness was also due to the nano-SiC dissolution and resultant precipitates formed during the thermal treatment.

Room Temperature Compressive Property and Deformation Behavior of Microporous STS 316L Stainless Steel Tube Manufactured with Powder Sintering Process.

This study investigated the room-temperature compressive property and deformation behavior of microporous STS 316L stainless steel tube for catalyst manufactured with powder sintering after two-way compression molding. The microporous tube was manufactured using STS 316L powder stocks with an outer diameter of 30 mm, inner diameter of 25 mm and length of 120 mm. In initial microstructure observed from different directions and locations, the porosity was measured as 32%, and the relative density obtained using micro-computed tomography was 0.54. Phase analysis did not identify phases other than γ-Fe. In a room temperature compression test, compressive yield strength measured 32 MPa. Observation of fractpgraphy after compression test revealed that dimples were formed at the powder-powder interface during the process where necks were disconnected. Based on the above findings, this study attempted to identify the deformation behavior of microporous STS 316L material manufactured with powder sintering after two-way compression molding and powder sintering process.

Selective Laser Melting of Diamond-Containing or Postnitrided Materials Intended for Impact-Abrasive Conditions: Experimental and Analytical Study

Materials with higher wear resistance are required in various applications including cutting elements (drag bits) of soft ground tunnel boring machines (TBM) to increase the productivity and to reduce the risk for workers involved in exchange operations (dangerous hyperbolic conditions). In recent work, two types of materials were produced by combining 3D printing (selective laser melting, SLM) of cellular lattice structures and spark plasma sintering (SPS) methods. The lattices were printed from (1) 316L stainless steel with diamond and (2) Ti6Al4V with nitriding. The effect of diamond content (5%, 10%, and 20%; nickel-coated particles) and unit cell size on performance was studied. The titanium alloy lattice was nitrided to increase its hardness and wear resistance. The effect of nitriding temperature (750°C, 900°C, and 1050°C) and lattice volume fraction (6%, 15%, and 24%, vol.) was investigated, and the optimized conditions were applied. The lattices were filled with 316L and Ti6Al4V powders, respectively, and consolidated by SPS. Samples were tested with the help of laboratory impact-abrasive tribodevice. Laboratory results have shown that both reinforcing approaches are beneficial and allow improvement of wear resistance in impact-abrasive conditions with great potential for TBM or similar applications. Modelling with the help of finite element method has shown that lattice structure enables reduction of peak local stresses in scratching and impact conditions.

The third Sandia Fracture Challenge: from theory to practice in a classroom setting

Three computational methods for modeling fracture are compared in the context of a class’ participation in the Third Sandia Fracture Challenge (SFC3). The SFC3 was issued to assess blind predictions of ductile fracture in a complex specimen geometry produced via additive manufacturing of stainless steel 316L powder. In this work, three finite-element-based methods are investigated: (1) adaptive remeshing, with or without material-state mapping; (2) element deletion; and (3) the extended finite element method. Each student team was tasked with learning about its respective method, calibrating model parameters, and performing blind prediction(s) of fracture/failure in the challenge-geometry specimen. Out of 21 teams who participated in the SFC3, three of the seven student teams from this class project ranked among the top five based on either global force-displacement or local strain predictions. Advantages and disadvantages of the three modeling approaches are identified in terms of mesh dependency, user-friendliness, and accuracy compared to experimental results. Recommendations regarding project management and organization are offered to facilitate future classroom participation in the Sandia Fracture Challenge or similar blind round-robin exercises.

Production of corrosion-resistant 316L stainless steel clads on carbon steel using powder bed fusion-selective laser melting

Powder bed fusion-selective laser melting (PBF-SLM) was used to produce corrosion-resistant 316L stainless steel clads on 1018 carbon steel substrates. The PBF-SLM parameters such as laser power, laser-scanning speed, hatch spacing, and layer thickness were optimized to attain maximum clad density and a superior metallurgical bond to the substrate. Typical energy densities that are required for producing dense 3D parts (˜100 J/mm3) was inadequate for cladding operations, and a higher energy density (333–1333 J/mm3) was necessary to produce low-defect clads with good adherence to the substrate. A maximum clad thickness of 133.14 μm was achieved at the lowest tested scan speed of 100 mm/s after ten layers of powder melting. The clads had lower chromium content than the 316L powder due to evaporative losses experienced during laser melting process. However, chromium contents in the range of 13–15% were successfully achieved in all cladded specimens. Increasing laser scan speeds had a negative impact on the nanoindentation hardness of the clads; however, the clad hardness at all scan speeds was found to be higher than AISI 316L SS. Electrochemical tests showed that the corrosion properties of clads produced at low laser scan speeds were comparable to AISI 316L SS.

Effect of Powder Type and Particles Size on Microstructure of Selective Laser Sintered Parts

The goal of this work was to investigate microstructure of the selective laser sintering (SLS) produced parts evaluating effect of powder type and fraction size. Studies have shown that printed samples of 316L and GP1 metal powders had a higher defect content compared to printed components from MP1 powder material. From scanning electron microscopy (SEM), it was found that iron-based printed parts melted worse than Co-Cr alloy components. Iron-based 316L and GP1 metal powders did not get enough energy from laser to perform a better microwelding between particles. Surface roughness Ra numerical values for samples 316L, GP1, MP1 respectively are Ra = 13.7 μm; 11.4 μm; 3.0 μm. Stainless steel powder material contains particles which size varies between 20 – 120 μm. The Co-Cr alloy and the maraging steel powder materials are made of 10 – 80 μm particles. The chemical and elemental composition of powder materials were examined using SEM-EDS technology.

New interpretation for the origin of the anisotropic sintering shrinkage of AISI 316L rings based on the anisotropic stress field occurred on uniaxial cold compaction

ABSTRACTAISI 316L powder mix was sieved in three particle sizes and successively compacted on rings with two different H/(Dext-Dint) ratios. The height and diameters were measured by a CMM at the green and at the sintered state. The shrinkage was calculated to study the influence of particle size and geometry on the anisotropy of dimensional change. As expected, the volumetric shrinkage increases moving from coarser to finer particle size, and the measurements confirmed the anisotropy of the dimensional variation. The measurements revealed that the lowest rings shrink more than the taller rings. The reference to the radial and axial stress during prior cold compaction shows an interesting correlation, in which shrinkage increases on increasing the stress. The further investigation of the deviatoric stress and the deviatoric shrinkage provided a possible explanation of the anisotropy of dimensional change on sintering as a function of the inhomogeneity of the compaction stress.

Технологические особенности изготовления сложнопрофильных изделий методом селективного лазерного плавления из порошкового материала 316L

Технологические особенности изготовления сложнопрофильных изделий методом селективного лазерного плавления из порошкового материала 316L

Localized Changes of Stainless Steel Powder Characteristics During Selective Laser Melting Additive Manufacturing

In laser powder bed additive manufacturing processes, feedstock materials are often recycled after each build. Currently, a knowledge gap exists regarding powder reuse effects on powder size distribution, morphology, and chemistry as a function of part geometry and processing conditions. It was found during selective laser melting (SLM) of 316 stainless steel that a significant amount of (0.100 wt pct) oxygen pickup can occur in molten material (spatter) ejected from the powder bed surface. This value was significantly larger than the oxygen content of the as-received powder feedstock (0.033 wt pct). Furthermore, the powders in the heat-affected-zone regions, adjacent to molten pool, also exhibit oxygen pickup (≥ 0.043 wt pct). The oxygen content in unmelted 316L powder was found to vary as a function of its spatial position in the powder bed, relative to the heat source. Interestingly, the volume of melted material (i.e., thin vs thick walls) did not correlate well with the extent of oxygen pickup. Possible mechanisms for oxygen pickup in the powder during SLM, such as adsorption and breakdown of water, oxygen solubility, spatter re-introduction, and solid-state oxide growth, are discussed.

Microstructural comparison and mechanical properties of stainless steel 316L fabricated by selective laser melting and metal injection moulding processes

Selective laser melting (SLM) process is one of an additive manufacturing technology that has the capabilities of fabricating complex geometries with an excellent accuracy and resolution. Compared to conventional powder metallurgy techniques metal injection moulding (MIM), SLM has several advantages such as low production cost for small batch fabrication, high degrees of geometrical freedom and customisation, and short cycle time. The present work analyses and compares the tensile properties and metallography differences of the stainless steel 316L compacts fabricated by two powder-based manufacturing processes; SLM and MIM, respectively. The SLM compact was built at 0-degree building orientation by SLM 125 HL machine with default optimum processing parameters for stainless steel 316L powder. On the other hand, MIM process produced similar dimension of tensile-shaped compacts differently that by sintered at three different sintering conditions in highly pure argon flow atmosphere. The tensile testing revealed that SLM tensile compact was higher in tensile strength (29% more) and elongation (62% more) at fracture as compared to MIM compact. From microstructure observation, the MIM compact showed the presence of significant amount of porosity which led to moderate mechanical properties compared to SLM compact performances.

大层厚316L选区激光熔化工艺优化及性能研究

大层厚316L选区激光熔化工艺优化及性能研究

Microstructural comparison and mechanical properties of stainless steel 316L fabricated by selective laser melting and metal injection moulding processes

Selective laser melting (SLM) process is one of an additive manufacturing technology that has the capabilities of fabricating complex geometries with an excellent accuracy and resolution. Compared to conventional powder metallurgy techniques metal injection moulding (MIM), SLM has several advantages such as low production cost for small batch fabrication, high degrees of geometrical freedom and customisation, and short cycle time. The present work analyses and compares the tensile properties and metallography differences of the stainless steel 316L compacts fabricated by two powder-based manufacturing processes; SLM and MIM, respectively. The SLM compact was built at 0-degree building orientation by SLM 125 HL machine with default optimum processing parameters for stainless steel 316L powder. On the other hand, MIM process produced similar dimension of tensile-shaped compacts differently that by sintered at three different sintering conditions in highly pure argon flow atmosphere. The tensile testing revealed that SLM tensile compact was higher in tensile strength (29% more) and elongation (62% more) at fracture as compared to MIM compact. From microstructure observation, the MIM compact showed the presence of significant amount of porosity which led to moderate mechanical properties compared to SLM compact performances.

Effect of process parameters on the formation of single track in pulsed laser powder bed fusion

There has been increasing interest in Laser Powder Bed Fusion (L-PBF) of metallic materials as a promising manufacturing technology. Although most L-PBF systems utilize laser beams with continuous wave emission (L-PBF(CW)), the possibility of using pulsed lasers (L-PBF(P)) has become available in some industrial L-PBF machines over the past few years. Previous studies suggest that the use of pulsed lasers could enable larger control of heat input and melt pool formation during the process, and could thus enable improvement of spatial resolution and feature sizes in L-PBF. In this study, the experiments were implemented using a pulsed laser in combination with continuous scanning movement instead of the ‘point-and-shoot’ method typically used by industrial L-PBF(P) machines of today. The experiments were executed using a trial L-PBF system (IPG ytterbium fiber laser, wavelength 1075 nm) for gas-atomized stainless steel 316L powder on compositionally similar substrates. Single tracks were melted with three different pulse lengths (50, 100, and 200 µs) by using a constant layer thickness of 50 µm, while varying pulse repetition rate, scanning speed and laser power based on six preset values of volume energy density (VED) of 36-120 J/mm3. In order to allow a comparison to be made, additional samples were manufactured by using the CW emission of the same laser. It was observed that the L-PBF(P) samples yielded narrower tracks in comparison to the samples manufactured using CW emission. In addition, the results of the experiments show that, while maintaining constant VED values, decreasing the pulse length or scanning speed decreased the widths of the tracks and their penetration into the substrate. Consequently, it was noticed that shorter pulse lengths require more overlap between consecutive pulses in order to produce continuous tracks. Pulsed emission shows potential for improving the spatial resolution of the L-PBF process.