316L Stainless Steel Powder Research Articles

Intergranular Corrosion of Feedstock Modified—Additively Manufactured Stainless Steel After Sensitization

Laser powder bed fusion (LPBF), a metal additive manufacturing technique, was conducted on feedstock-modified 316L stainless steel (316L) powder produced by ball-milling of commercial 316L and 1 wt% additive (cerium oxide—CeO2, lanthanum (III) nitrate hexahydrate—La(NO3)3·6H2O, and chromium nitride—CrN). The feedstock-modified LPBF-316L specimens were sensitized at 675°C for 24 h, and the influence of additives on intergranular corrosion (IGC) was investigated following ASTM G108-94 and A262-14 standards. The LPBF-316L with La(NO3)3·6H2O showed higher IGC resistance. The microstructure of the LPBF specimen was investigated and correlated to understand the improved IGC resistance of LPBF-316L with La(NO3)3·6H2O additive.

Fatigue behavior of vacuum-sintered binder jetted fine 316L stainless steel powder

The study examined the impact of surface roughness on the high-cycle fatigue (HCF) properties of vacuum-sintered binder jetted gas atomized fine 316L stainless steel (SS) powder. The microstructure and density of the as-sintered specimens were analyzed using micro-computed tomography, indicating a relative density of ∼99.8 ± 0.1% with equiaxed grains surrounded by delta-ferrite phase at the grain boundaries (volume fraction of ∼2%). The as-sintered specimens had a rough surface with an arithmetical mean roughness of Ra = 6.56 ± 0.58 μm and root-mean-square roughness of Rq = 8.29 ± 0.68 μm. After mechanical grinding, the surface roughness was reduced to Ra = 0.21 ± 0.03 μm and Rq = 0.25 ± 0.04 μm. Microhardness analysis revealed an increase of nearly 70% up to 125 μm beneath the surface of mechanically ground specimens. The enhanced surface hardness was found to be related to an in-plane compressive residual stress on the mechanically ground sample. The fatigue endurance limit was evaluated using the staircase test methodology, with an average value of ∼170 and 225 MPa for the as-sintered and mechanically ground parts, respectively. High-cycle fatigue experiments were conducted at a stress ratio of Rσ = −1, and the results showed that surface grinding improved fatigue life at higher stress levels due to in-plane compressive stress and the reduction in surface roughness. Fractography revealed that the fracture mechanisms in the crack propagation and final fracture zones were striation marks and dimple features. Finally, the electron backscatter diffraction technique was used to study the deformation history at different sites close to the fracture surface.

Optimization and wear behaviors of 316L stainless steel laser cladding on rail material

Laser cladding is a potential technology for repairing the rail with side wear. The laser processing parameters should be optimized and the wear behaviors of clads should be analyzed before its application. Therefore, single-pass and multi-pass laser cladding experiments of 316L stainless steel powder with different laser powers, scan speeds, scan modes (directions), and numbers of layers are conducted on the U71Mn rail to optimize the processing parameters. After that, sliding wear tests are conducted on different depths in clad to explore the wear behaviors. The results show that with the increase in laser power, the clad width is increased, the height is increased first and then decreased. With the increase in scan speed, both the clad height and width are decreased. Scan mode 1# produces the thinnest clads, the mode 3 produces the thickest ones. The microstructure of clad is composed of dendritic grains. With the increase in depth in clad, the grain size decreases and the hardness increases. The grain size is increased with laser power and decreased with scan speed, and the hardness is increased with the power and speed. Smallest grains and highest hardness are produced using mode 1#, and largest grains and lowest hardness using mode 3#. The Cr and Ni elements from clad permeate into the heat affected zone of U71Mn rail material. Wear depth of clad has a positive relationship with the grain size and a negative relationship with the post hardness. A wear resistance coefficient, Xphv2.55/Xgs1.56,where Xphv refers to the post hardness and Xgs refers to the grain size, is proposed to characterize the inherent anti-wear property of 316L stainless steel clad on U71Mn rail.

Discrete element analysis for effects of recoating blade and surface morphology of substrate on powder spreading quality

A discrete element method model was established to simulate 316L stainless steel powder spreading process in selective laser melting. The effects of recoating blade speed, assigned gap width and substrate surface morphology on the powder spreading quality was studied. The results showed that lower recoating speed and larger gap width benefited the quality of powder layer. When the angle between the moving direction of recoater and laser scanning direction was greater than 45 °, the density level of the powder layer on the SLM processing surface was higher than that of the flat substrate and more uniform. The quality of the powder layer on the milled surface varied with position obviously, and the maximum density was between the other two types of surfaces, while uniformity of the powder layer was worse than the other two surfaces. This study shows that the quality of powder layer can be controlled by adjusting the setting parameters of the recoater and the surface morphology of the substrate.

Parameter Optimization and Mechanical Properties of Laser Cladding of 316L Stainless Steel Powder on G20Mn5QT Steel

G20Mn5QT steel has excellent mechanical properties and is widely used in key components of rail vehicles. However, during the operation of high-speed vehicles, wear and tear will inevitably occur. In this paper, laser cladding technology was selected to successfully prepare 316L stainless steel coating. The optimum processing parameters were obtained with a laser power of 2300 W, a scanning speed of 500 mm/min, and a powder feeding speed of 14 g/min. The microstructure of 316L coating is mainly composed of planar crystals, cellular crystals, columnar crystals, and equiaxed crystals. Through range analysis, it is found that the microhardness, wear resistance, and micro-shear strength of the cladding layers increase with the increase of laser power, while the tensile strength and yield strength increase first and then decrease with the increase of laser power. Under the optimized process parameters, the low-temperature impact toughness, elongation, tensile strength, and yield strength of the cladding layer were 97.6%, 24%, 10.9%, and 32.5% higher than that of the G20Mn5QT substrate, respectively. An excellent combination of strength and toughness was achieved by cladding 316L alloy on the surface of the G20Mn5QT substrate, which can meet the requirements of remanufacturing fractional key vehicle parts.

Indirect Induction Sintering of Metal Parts Produced through Material Extrusion Additive Manufacturing.

Avoiding loose powders and resins, material extrusion additive manufacturing is a powerful technique to produce near-net shape parts, being a cheap and safe alternative for developing complex industrial-grade products. Filaments embedded with a high packing density of metallic or ceramic granules are being increasingly used, resulting in almost fully dense parts, whereby geometries are shaped, debinded and sintered sequentially until the completion of the part. Traditionally, "brown" debinded geometries are transported to conventional furnaces to densify the powder compacts, requiring careful tailoring of the heating profiles and sintering environment. This approach is decoupled and often involves time-consuming post-processing, whereby after the completion of the shaping and debinding steps, the parts need to be transported to a sintering furnace. Here, it is shown that sintering via indirect induction heating of a highly filled commercially available filament embedded with stainless steel 316L powder can be an effective route to densify Fused Filament Fabricated (FFF) parts. The results show that densities of 99.8% can be reached with very short soaking times, representing a significant improvement compared to prior methods. A hybrid machine is proposed, whereby a custom-built machine is integrated with an induction heater to combine FFF with local indirect induction sintering. Sintering in situ, without the need for part transportation, simplifies the processing of metal parts produced through material extrusion additive manufacturing.

Effect of Fe-Cr-B on the Microstructure and Mechanical Properties of a Powder Injection Molding Processed Fe-Cr-B/316L Stainless Steel Matrix Composite

Stainless steel-based composites (SMCs) mixed with 316L stainless steel powder and Fe-Cr-B based M alloys powder ed M) powder were manufactured using the powder injection molding (PIM) process, and their microstructure and mechanical properties were investigated. To examine the effect of M content on the properties of SMCs, the M fraction was controlled to 30 vol.%, 50 vol.%, and 70 vol.%. It was observed that PIM-processed Fe-Cr-B/316L SMCs (30 vol.%, 50 vol.%, 70 vol.%) had γ-austenite and minor α-ferrite as a matrix, and (Cr, Fe)2B was formed as a reinforced phase. In tensile tests at room temperature, the yield strengths of the 30 vol.%, 50 vol.%, 70 vol.% M SMCs were measured to be 519.9 MPa, 574.1 MPa, and 604.8 MPa (peak stress), respectively. From the fracture surface observation results, it was found that the deformation behavior changed from the intergranular fracture of boride to intergranular fracture between the matrix and boride, as the M fraction increased. Based on the above results, the deformation and fracture mechanisms of novel Fe-Cr-B/316L SMCs are also discussed.

Powder reusability in Metal Binder Jetting

Metal binder jetting is a potential mass-production additive manufacturing technique that enables faster production of complex structures with isotropic material properties. In metal binder jetting, the quality of parts is heavily influenced by the powder characteristics, and even small variations in powder feedstock can result in poor-quality parts. Powder management is crucial in this technology because the powder is processed in a high-humidity environment in the machine, and the binder is mixed with the powder as well. In this paper, the characteristics of 316l and 17-4PH stainless steel powders under the machine atmosphere and their reusability are studied in detail. Powder samples were taken from the build box at a fixed interval of time and their moisture content, packing density, and morphology were analysed through scanning electron microscopy. Based on the results obtained, it was observed that moisture content of the powder increases with time up to a few hours when it gets constant. Additionally, it was observed that after the first reuse cycle of the powder, the finest powder particles almost disappear, which may have a detrimental effect on the density of the generated parts. Finally, the recycling of broken green parts was also investigated by means of a new approach. Taking into account that broken green parts are generally scrapped in metal binder jetting, the recyclability of this material could reduce the high material costs related to the process.

Suppression of denudation zone using laser profile control in vacuum selective laser melting

A vacuum selective laser melting method with a beam profile control was developed to fabricate the high quality 3D object. To investigate the quality of 3D fabrication, the denudation zone (DZ) as a quality indicator was evaluated to make a single bead. The main challenge is to clarify the correlation between the DZ and beam profiles. The beam profiles, such as a Gaussian mode, a doughnut mode, and a flat-top mode, were formed in several modes, by a dioptric system with a beam shaper. A stainless steel 316L (SS316L) powder was irradiated in arbitrary pressure to form a single bead, and then the DZ was measured. As a result, it was found that the flat-top mode recorded the minimum value of the DZ under atmospheric pressure. Thus, the dynamics of the metal vapor while the laser irradiation were observed by the Schlieren imaging technique under atmospheric pressure. The average velocity of metal vapor with flat-top mode was the slowest, i.e., 0.199 m/s. It was found that the DZ becomes small depending on the dynamics of metal vapor. The ambient pressure was reduced to 300 Pa in order to reduce the gas movement, and the DZ with flat-top mode was improved to record a minimum value of 0.048 mm, 1/3 of the DZ at atmospheric pressure.

Influence of cycle number on the compression behavior of nonlinear periodically gradient porous structures produced by laser powder bed fusion

This research presents a design method of the nonlinear periodic gradient triply periodic minimal surface structures (NL-TPMS). The gradient porosity of NL-TPMS was controlled by a sinusoidal function and the influence of the number of cycles on the compression failure mechanism has been investigated. Homogenous TPMS and NL-TPMS were manufactured by laser powder bed fusion (LPBF) with 316L stainless steel powder. Digital image correlation (DIC) and finite element analysis (FEA) were applied to explore the compression behavior of NL-TPMS. The strain of the plateau end of homogenous TPMS decreases as the porosity increases, and the DIC results show that homogenous TPMS deformed with a 45° diagonal shear band. The compression results show that anisotropy of mechanical properties exists in NL-TPMS. The length of the first plateau and compressive proof strength increase as the number of cycles of porosity increases. The cycle of gradient porosity also influences the compression behavior of NL-TPMS, and a periodic failure deformation was observed in the NL-TPMS rather than a layer-by-layer compression deformation in normal gradient porous structures. The maximum principal strain of NL-TPMS detected by DIC presents a periodic distribution, which corresponds to the sinusoidal period designed function of NL-TPMS.

Cracking behavior of AISI 316L stainless steel powder by gas nitriding and its influence on spark plasma sintering

In this study, the effect of nitriding temperature on the deformation behavior of spherical AISI 316L powders was investigated. The results showed that the nitrogen content in powders was significantly increased by increasing the nitriding temperature under ammonia atmosphere. However, the resulted residual stress led to the deformation or even cracking in powders, the cause of drastic changes in the bulk density and fluidity of powders. Furthermore, the microstructure and properties of alloys synthesized by spark plasma sintering were also analyzed. The obtained results also indicate that the powder deformation has no influence on the consistency of sintered samples, but the nitriding temperature of AISI 316L powder obviously affects the morphology and performance of sintered samples. Nitrogen initially exists as interstitial atom in the steels till the nitriding temperature rises to 500 °C where nitrides begin to precipitate in the sample. Such morphological transition of nitrogen atoms has a significant impact on the mechanical properties of the alloys. The better corrosion resistance of the alloy prepared over 500 °C in 3.5 wt.% NaCl solution shows that the nitriding temperature also matters for corrosion performance.

A novel method of utilizing static mixer to obtain mixing homogeneity of multi-species powders in laser metal deposition

Real-time mixing of multi-species powder challenges Laser Metal Deposition (LMD) of Functionally Graded Materials (FGMs). The current work proposes a novel method of using a static mixer to realize rapid, uniform multi-species powder mixing. Firstly, copper powder and 316L stainless steel powder are selected to complete the powder mixing observation experiment with Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS). Secondly, computational fluid dynamics and particle mixing simulation models are used to analyze the flow field and particle motion characteristics in the static mixer. Finally, LMD experiment and metallographic observation are carried out with 316L stainless steel powder and WC powder to verify the feasibility of the static mixer. This study provides a theoretical and practical basis for powder mixing in laser processing with a static mixer. The conclusions can also be applied to other processing fields requiring real-time and uniform mixing of multi-species powders.

Effects of Mo additions on the potentiostatic corrosion behaviour of multiphase 316L SS-based alloys

ABSTRACT A set of cast alloys was made by alloying 316L stainless steel powder with 5, 10, and 15 wt-% Mo to study the effects of the Mo content on the microstructures and corrosion behaviours. Sigma phases formed in all alloys and Laves phases co-precipitated in alloys with Mo additions of 10 and 15 wt-%. Alloys with higher Mo concentrations had greater volume fractions of both intermetallic phases, which contained higher Mo but lower Ni levels than the austenite. Potentiodynamic and potentiostatic tests were performed in pH 4 solution containing 10 mM NaCl to provide an aggressive chemical environment. All cast alloys had higher corrosion resistance than 316L SS. Electrochemical impedance spectroscopy showed the Mo content had a significant beneficial effect on the electrical properties of the passivating layers under highly oxidising conditions, but only a minor effect under moderately oxidising redox conditions.

Selective Laser Melting of Free-Assembled Stainless Steel 316L Hinges: Optimization of Volumetric Laser Energy Density and Joint Clearance

Selective laser melting technology is one of the metal additive manufacturing technologies that can convert metal powder to complex parts without the assembly process. This study aims to optimize the volumetric laser energy density for printing 3D metal objects with hinges geometry. The material is stainless steel 316L powder. The volumetric laser energy densities ranging from 4.1 J/mm3 to 119.1 J/mm3 are applied to fabricate 3D free-assembled hinges with various clearances of 0.38 mm, 0.39 mm, 0.40 mm, and 0.41 mm and investigate the relationship between volumetric laser energy density and clearance. A multibody model, consisting of nine segments with eight hinges, is proposed to be printed with the optimized volumetric laser energy density. The optical microscope and the hardness test are performed to observe the porosity and hardness property of the SLMed object. The result shows that laser energy densities between 105.5 J/mm3 and 119.1 J/mm3 can produce the high densification of SLMed objects with a porosity defect of 0.24% to 0.20% and hardness in the range of 207 HV to 215 HV. The optimization of laser energy densities is in the range of 105.5 J/mm3 to 119.1 J/mm3, which can be used to fabricate the movable hinges with a minimum clearance size of 0.41 mm. The proposed dinosaur object is printed successfully and all joints are rotatable.

Elucidating the impact of severe oxidation on the powder properties and laser melting behaviors

In this work, commercial 316L stainless steel (SS) powder was used to experimentally determine the impacts of severe oxidation on the powder characteristics and laser powder bed fusion (L-PBF) melting behaviors. The morphology, surface state, and laser absorptivity of both virgin and oxidized powders were systematically characterized. Their impacts on the flowability and powder bed quality were monitored by custom-designed recoating experiments and powder shear tests. The results of an in situ wetting analysis and microstructure evaluation were compared to establish a correlation between the powder characteristics and laser fusion. The powder oxidation enhanced L-PBF processability by improving the homogeneity of powder spreading and the formation of stable, consecutive laser beads. A thin ceramic layer-coated SS alloy reinforced with uniform (Si, Mn)-based oxides was synthesized by the L-PBF processing of severely oxidized powders. The mechanical strength of this alloy was found to be similar to that processed using the virgin powders, whereas the elongation was slightly decreased, likely due to the amorphous oxide feature and the mechanical oxide-Fe interface. This study provides a systematic understanding of powder reuse and new insight into the potential for economically developing high-performance parts by the positive utilization of powder oxidation and the L-PBF process.

The Influence of Powder Reuse on the Properties of Laser Powder Bed‐Fused Stainless Steel 316L: A Review

There is a clear economic benefit for the recycling of metallic powder during additive manufacturing (AM). Laser powder bed fusion (L‐PBF) is one such AM process and research has found that the properties of the powder feedstock can change when the powder is reused through mechanisms such as spatter generation and alterations in chemistry. Such changes to powder properties can accumulate and may lead to significant differences in the mechanical properties of the final component. Often the changes to part properties are reasonably small; however, there is not currently enough understanding of the specific links between powder properties and the characteristics of the end component for the effects of powder recycling to be discounted. Herein, the typical lifecycle of stainless steel 316L powder in L‐PBF, the changes that occur to the powder feedstock, and the effects that this may have on the mechanical properties of components manufactured with recycled powder are reviewed.

Evaluation of injection molded gas and water atomized 316l stainless steel powder properties

This research investigates the features crucial to injection molding via the rheological behavior, injection molding, debinding, and sintering process of water and gas atomised 316L stainless steel powder utilizing a newly developed locally based binder system containing palm stearin. The critical powder loading for injection molding was 65vol% for gas atomisation and 62vol% for water atomisation. Due to the minimal interparticle friction and high packing density, the gas-atomised powder proved easier to shape. In contrast, the water-atomised powder has a high viscosity of the injection molding feedstock, a high interparticle friction, and a low packing density, all of which impede injection molding. The debinding of the binder was accomplished utilizing solvent and thermal methods. After debinding, samples were sintered in a high-temperature vacuum furnace at 1360 degrees Celsius. Results reveal that water-atomised powder can be sintered to 95% of its theoretical density, whereas gas-atomised powder can be sintered to near full density.

Topological and Mechanical Properties of Different Lattice Structures Based on Additive Manufacturing.

The appearance and development of additive manufacturing technology promotes the production and manufacture of parts with more complex designs and smaller sizes and realizes the complex topology that cannot be made by equal-material manufacturing and submanufacturing. Nowadays, the application of tri-periodic minimal surface (TPMS) in topology optimization design has become a new choice, and, because of its excellent structure and properties, has gradually become mainstream. In this paper, the mechanical properties of four different topologies prepared by selective laser melting (SLM) using 316L stainless steel powder were investigated, including two TPMS sheet structures (Primitive surface, Gyroid surface) and two common lattice structures (Bcc lattice, truss lattice). The mechanical properties (Young’s modulus, yield stress, plateau stress, and toughness) were compared by numerical simulation and compression experiment. It can be concluded from the results that the mechanical properties and deformation mechanism of the specimen are mainly related to the type of lattice, though have little relationship with unit thickness at the same relative density. The Gyroid curved structure showed the best mechanical properties and energy absorption capacity, followed by the truss lattice structure. By comparison, the mechanical properties of the traditional Bcc lattice structure and the Primitive surface structure are poor, and the deformation mechanism of these two structures is uncertain and difficult to control.

The Influence of the Structure Parameters on the Mechanical Properties of Cylindrically Mapped Gyroid TPMS Fabricated by Selective Laser Melting with 316L Stainless Steel Powder.

The development of additive manufacturing techniques has made it possible to produce porous structures with complex geometry with unique properties as potential candidates for energy absorption, heat dissipation, biomedical, and vibration control application. Recently, there has been increased interest in additively manufacturing porous structures based on triply periodic minimal surfaces (TPMS) topology. In this paper, the mechanical properties and energy absorption abilities of cylindrical mapped TPMS structures with shell gyroid unit cells fabricated by selective laser melting (SLM) with 316L stainless steel under compression loading were investigated. Based on the experimental study, it was found that tested structures exhibited two different deformation modes. There is also a relationship between the number and shapes of unit cells in the structure and the elastic modulus, yield strength, plateau stress, and energy absorption. These results can be used to design and manufacture more efficient lightweight parts lattices for energy absorbing applications, e.g., in the field of biomedical and bumpers applications. The deformation mode for each tested sample was also presented on the records obtained from the ARAMIS system.

Microstructure and mechanical properties of CNC-SLM hybrid manufacturing 316L parts

In this study, 316L stainless steel powder was deposited on the machined (CNC) 316 L stainless steel using selective laser melting (SLM) technology. The block-shaped hybrid manufacturing samples that combine machining (CNC) and SLM process were prepared. The microstructure and microhardness of the substrate region, SLM region, and interface region were analyzed by different mechanical property characterization and microstructural techniques. The horizontal and vertical hybrid manufacturing tensile patterns were prepared, and the tensile properties of the hybrid manufacturing samples were measured by the tensile test. The results show that the SLM part is relatively dense, and there is no crack at the connection between the substrate and the SLM region. The grains at the interface joint are refined, and the tensile fracture of the hybrid manufacturing part in the vertical direction appears on the substrate region, indicating that the strength of the joint is higher than the strength of the substrate. The yield strength and tensile strength of the horizontal hybrid manufacturing tensile parts reached 497.5Mpa and 607.5 Mpa, and the elongation rate was 45.23%, showing good mechanical properties. This kind of hybrid manufacturing improves the production efficiency of SLM direct production parts and reduces the cost, which has a wide range of application prospects.