316L Powder Research Articles

Microstructure and dynamic deformation mechanism of laser powder bed fusion of 316L-containing Ti–6Al–4V with dual-phase heterostructure

In this paper, 316L-containing Ti–6Al–4V alloy was fabricated by laser powder bed fusion (LPBF) through mixing Ti–6Al–4V and 4.5 wt% 316L powders, and the dynamic compressive mechanical properties and deformation mechanism at different strain rates were studied. The results show that the volume fraction of β phase sharply increases from 1.8% (in TC4) to 61.4% (in HST), and micro- and submicron-sized grains coexist in the HST alloy, producing heterostructure in the form of bimodal grain size distribution. The presence of the heterostructure led to superior heterogeneous deformation induced stress, thus facilitating the additional strain hardening and deformation coordinating ability, thus made the HST alloy achieved ultimate compressive stress of ∼1750 MPa and fracture strain of ∼17%, showing excellent comprehensive mechanical properties. Under quasi-static load, the deformation mode of HST alloy is mainly attributed to progressive deformation-induced martensitic transformation. However, at high strain rates, the impact loading inhibited the activation of progressive DIMT, and thus the deformation mode changed to combination of dislocation slipping and deformation twinning.

Evolutionary Mechanism of Solidification Behavior in the Melt Pool during Disk Laser Cladding with 316L Alloy

Laser cladding is an emerging environmentally friendly surface-strengthening technology. During the cladding process, the changes in molten pool temperature and velocity directly affect the solidification process and element distribution. The quantitative revelation of the directional solidification mechanism in the molten pool during the cladding process is crucial for enhancing the quality of the cladding layer. In this study, a multi-field coupling numerical model was developed to simulate the coating process of 316L powder on 45 steel matrices using a disk laser. The instantaneous evolution law of the temperature and flow fields was derived, providing input conditions for simulating microstructure evolution in the molten pool’s paste zone. The behavior characteristics of the molten pool were predicted through numerical simulation, and the microstructure evolution was simulated using the phase field method. The phase field model reveals that dendrite formation in the molten pool follows a sequence of plane crystal growth, cell crystal growth, and columnar crystal growth. The dendrites can undergo splitting to form algal structures under conditions of higher cooling rates and lower temperature gradients. The scanning speed of laser cladding (6 mm/s) has minimal impact on dendrite growth; instead, convection within the molten pool primarily influences dendrite growth and tilt and solute distribution.

Improved wear and corrosion resistance of additively manufactured SS316L by laser remelting process

Directed energy deposition is an efficient and flexible additive manufacturing technique that has broad application prospects due to its ability to fabricate intricate structures with heterogeneous properties. In this study, the laser-based directed energy deposition (L-DED) technique is employed to deposit an austenitic stainless steel 316L powder at a significantly high deposition rate. In recent developments, laser remelting (LR) has been explored as a superior alternative for enhancing the surface integrity of the components produced by L-DED. This study endeavours to examine the impacts of LR, emphasizing the improvement of wear and corrosion resistance. The grain refinement is achieved through laser remelting, which eventually leads to a 34 % improvement in micro–hardness over the as-deposited sample; as a result of increased micro–hardness, wear resistance improved by 66 % when compared to the as-deposited specimen. Furthermore, the corrosion resistance performance of the laser-remelted samples is compared with as-deposited samples by electrochemical impedance spectroscopy (EIS) in a solution of 3.5 % NaCl solution. The remelted sample showed improved corrosion resistance, which is attributed to the grain refinement due to rapid solidification in LR. This study demonstrates that laser remelting can be an effective way to improve the wear and corrosion resistance of SS316L, which enlarges its application in harsh environments.

THEORETICAL APPROACH FOR DETERMINING AN EMISSIVITY OF SOLID MATERIALS AND ITS COMPARISON WITH EXPERIMENTAL STUDIES ON THE EXAMPLE OF 316L POWDER STEEL

The work used Maxwell's electromagnetic theory to quantitatively describe the emissivity of solid materials through electrical resistivity and temperature. An equation is proposed for recalculating the emissivity of smooth surfaces into powdery or rough surfaces. The obtained theoretical characteristics of the change in the emissivity of 316L powder steel were compared with experimental ones. As a result of the comparison, it was established that the experimental results obtained correlate with theoretical calculations and do not go beyond the limits of the expanded uncertainty of measurement.

Thermal behavior of coated powder during directed energy deposition (DED)

In powder-based additive manufacturing (AM), the quality of the feedstock material is critical for obtaining enhanced mechanical properties. Recently, the application of coated powders during directed energy deposition (DED) has been prompted by the goal of fabricating composite and functional materials in-situ. The complex temperature and momentum fields established during DED render direct experimental characterization of coated powder behavior challenging. To address this challenge, this study reports on the thermal behavior of coated powders during interactions with the molten pool by constructing three-dimensional heat transfer and phase distribution models using the finite elements method (FEM). Transient temperature and phase distributions were calculated for coated and uncoated stainless steel 316L and ZnAl powders under various particle size, coating thickness, molten pool temperature, and coating material conditions. Particle residence time values were extracted from the calculations, defined as time spent by the particle before a phase change. The results show large variations in particle residence time (85 μs to 2670 μs for stainless steel 316L particles, and 48 μs to infinity for ZnAl particles) as a function of the variables considered, especially the thermal diffusivity of the coating materials, thereby highlighting the potential value of coatings as an additional design parameter in DED. Significant increases in particle residence time for both stainless steel 316L and ZnAl particles were found when contact angle increases from 0° (submergence regime) to 180° (floating regime).

The effect of building strategies on tensile strength of additively manufactured parts by laser metal deposition

Abstract Laser Metal Deposition (LMD) is a powder-based technology. This technology can be used for surface coating, repairing and additive manufacturing. The property of the part is dependent on the building parameters of the process. In the research work, we use austenitic steel (316L) powder to produce blocks with different building strategies, from which we machine tensile test specimens. The patterns were prepared according to three strategies: the welded bands were parallel to the longitudinal axis of the test specimen, perpendicular to it, and finally the parallel and perpendicular directions were alternated. We perform hardness and tensile tests on these specimens and search for correlations between the strength and the strategies.

Influence of overlapping process on the distribution of Cr element in laser cladding 316L powder on 45# steel substrate

To investigate the influence of the overlapping process on the distribution of Cr during the laser cladding process, a novel three-dimensional melting and solidification model based on the volume-average method coupled with Eulerian-Eulerian multiphase was established to simulate the process of laser cladding on a 45# steel substrate with different overlapping processes of 316 L powders. The reliability of the model was verified by comparing the simulation results with the melt pool morphology and melt flow rate measured by a high-speed camera. The height and depth of melt pool were verified through the cross-sectional metallographic diagram. The deviations of the melt pool height and depth for single-path cladding were 2.49 % and 6.35 %, respectively, while the deviations for overlapping process were 8.26 % and 6.88 %, respectively. The deviations of melt flow rate were 12.8 % for single-path cladding and 10.0 % for overlapping, respectively. The distribution of chromium (Cr) was verified based on the results of energy-dispersive spectroscopy (EDS), which demonstrated the accuracy of the model. The results demonstrate that reducing the overlapping rate during the overlapping process will increase the dilution rate of the second path of the molten cladding layer which in turn will decrease the concentration of Cr in the lapped area. Reverse overlapping, on the other hand, increases the temperature of the molten pool and the flow rate of the melt, resulting in a more homogeneous mixing of the elements within the second path of the cladding layer. This reduces the difference between the elemental concentration in the lapped area and the elemental concentration of the first and second paths of the cladding layer.

Phase transformation mechanisms occurring during spark plasma sintering elaboration of new duplex composite stainless steels

This study focuses on the elaboration of duplex stainless steels (DSS) from powder mixtures using spark plasma sintering (SPS). Different mass fractions of an austenitic 316L powder and a ferritic 410L one were blended and then sintered by SPS. Microstructural characterizations of the sintered samples obtained from different powder mixtures were performed. They were coupled with marking experiments of the powder particles’ surface. The results showed the formation of martensite within the ferritic powder and at the austenite/ferrite interfaces, following two different mechanisms. In addition, it was found that the width of the martensitic regions is mainly influenced by the diffusion of Cr and Ni from the austenitic to the ferritic powder during sintering. The characterizations revealed that the originality of this approach lies in the particular microstructure obtained after sintering. The characteristic size of the ferritic and austenitic domains in the final material is that of the initial powder particles (up to some hundred microns). Moreover, each domain is formed by equiaxed and isotropic grains, having a size ranging from some microns to some tens of microns. This particular microstructure justifies the use of the term “composite duplex stainless steels” (COMPLEX) for this kind of new DSS.

Multi-targeting optimization of cladding process parameters based on NSGA-II algorithm and entropy weight method

Abstract Laser cladding is a new technology that can be used in the field of manufacturing and surface modification. The parameters of cladding 316L powder on 45 steel are studied by designing orthogonal experiments. Three optimal variables including laser power, scanning speed and cladding height are established. The mathematical model of target response with coating width, height and cladding area as indexes is constructed. Based on the Pareto set of NSGA-II, the optimal process parameters are found. The optimal solution on the Pareto front is obtained by the entropy weight method and data normalization. According to the results, when the laser power is 1607.0079 W, the scanning speed is 6.0033 mm/min, and the cladding height is 11.9961 mm, the optimized predicted cladding width is 1.3776 mm, the cladding height is 0.1847 mm, and the cladding area is 2.2907 mm2.

Ultrafine Grain 316L Stainless Steel Manufactured by Ball Milling and Spark Plasma Sintering: Consequences on the Corrosion Resistance in Chloride Media

This paper reports experimental results concerning the corrosion of 316L austenitic stainless steels produced by ball milling and spark plasma sintering in NaCl electrolyte. Specimens with grain sizes ranging from 0.3 µm to 3 µm, without crystallographic texture, were obtained and compared with a cast that is 110 µm in grain size and an annealed reference. The potentiodynamic experiments showed that the reduction in grain size leads to a degradation of the electrochemical passivation behavior. This detrimental effect can be overcome by appropriate passivation in a HNO3 concentrated solution before consolidation. The Mott–Schottky measurements showed that the semiconducting properties of the passive layer do not vary significantly on the grain size, especially the donor density, which is responsible for the chemical passivation breakdown by chloride anions. The total electrical resistance of the layer, measured by impedance spectroscopy is always lower than the one of a cast and annealed 316L, but it slightly increases with a reduction in grain size in the ultrafine grain range. This is followed by a slight increase in the thickness of the oxide layer. The effect of chloride ions is very pronounced in terms of passivation breakdown if the powder is not passivated prior to sintering. This leads to the nucleation and growth of subsurface main pits and the formation of secondary satellite pits, especially for the smallest grain sizes. Passivation of the 316L powder before sintering has been found to be an effective way to prevent this phenomenon.

Fabrication of a Plate-Type Porous Stainless Steel 316L Powder Filter with Different Pore Structures without Use of a Binder

Generally, when fabricating porous filters using metal powder, about 1 to 2 wt% of a binder is added to increase the formability of the metal powder. If the binder is not completely removed through a debinding and sintering process, however it can cause defects such as discoloration and the generation of fine particles. In this study, a study was conducted to fabricate a plate-type porous stainless steel powder filter with a different pore structure without the use of a biner. First, the metal powder is charged into a ceramic mold to form a plate-shaped powder filter, and then covered with a ceramic top plate. In order to observe the pore properties according to the pressure, the unpressurized specimen and the specimen pressurized with a pressure of 30 MPa were then sintered at 1050℃ for one hour in a high vacuum atmosphere furnace. The microstructure of the sintered plate-shaped powder filter was observed through an optical microscope and in order to analyze its pore properties as a filter, gas permeability and porosity were measured using a capillary flow porometer and Archimedes’ law.

The effect of powder recycling on the mechanical performance of laser powder bed fused stainless steel 316L

Powder recycling refers to the reuse of unused powder feedstock in the laser powder bed fusion (PBF-LB/M) process. This approach is crucial for the economic viability and sustainability of PBF-LB/M, as powder accounts for a large proportion of the total production cost. However, through powder recycling, the physical and chemical properties of powder are liable to change. This variation in powder properties can subsequently lead to knock-on effects on the mechanical properties of a fully built component.This research has investigated the changes that occur to stainless steel 316L (SS316L) powder as a result of recycling. This includes changes to powder size distribution (PSD), flowability, chemistry and phase composition. Likewise, the impact that these changes have will also be assessed in PBF-LB/M SS316L components manufactured from powders after different levels of recycling and subjected to alternative post processing routes such as hot isostatic pressing (HIP). This comprehensive investigation involves a thorough examination of both macro- and microstructures, encompassing detailed analyses of chemical composition, microstructural features, and defects. The study aims to elucidate differences in mechanical behaviour through a series of experiments, including uniaxial tensile tests, Charpy impact assessments, and low cycle fatigue (LCF) experiments. Additionally, the investigation will be complemented by pitting potential tests, providing a holistic understanding of the material's performance and characteristics.Although moderate changes to powder were observed for both PSD and chemistry, this was found to be negligible and not enough to result in any adverse changes to part performance. In addition, the microstructure of SS316L remained stable across differing levels of powder recycling. Whereas the porosity content increased marginally as the fine particle content of powder was reduced, this was not found to be sufficient to affect the LCF performance of the material. After powder recycling, increases in ductility and Young’s modulus were attributed to a reduction in oxides present in the microstructure, which were sources of localised damage and deformation.

Capillary effects and consolidation kinetics during selective laser melting of 316L powder

Selective laser melting (SLM) technology has the advantage of quickly producing complex-shaped parts. To achieve good mechanical properties, it's vital to minimize defects that can occur because of high residual porosity if incorrect processing techniques are used. One effective way to prevent defects is by using computer simulations of underlying processes before printing in the industry. This paper presents a reduced-order numerical model of SLM processing that accurately predicts material porosity by focusing on the key mechanisms that affect the melting and consolidation processes. The focus is on the formation of defects and the expected time that is required until the consolidation of a powder bed is completed. Then the elasticity of the SLM processed materials near defects is analyzed. The modeling results for powder consolidation are shown for comparison with experimental data on stainless steel 316L powder during SLM. This information can be further used for proper selection of SLM parameters such as the scanning speed and the power of the laser source.

On the potential of eddy current characterization of the ferritic content of recovered 316L powders after LaserPowder bed fusion fabrication

An original container was designed to measure the ferritic content of powder batches by the Eddy Current (EC) technique. As opposed to the standard X-Ray Diffraction (XRD) or Electronic BackScatter Diffraction (EBSD) methods, the EC measurements can be implemented on powder batches of significant sizes, say a hundred grams or so. Using a methodology based on the multiple recycling of an initially ferrite-free virgin powder, it was shown that the EC signals are sensitive to the ferritic content of the recovered powder. On the other hand, in the frequency range scanned by the sensor, the EC signals are virtually independent on the oxygen concentration within the tested powder.

Effects of Porosity and Boron Reinforcement in AISI 316L Stainless Steel for Biomedical Applications

AISI 316L stainless steel (SS) is one of the most widely used biomaterials in the manufacture of implants and biomaterials. It has advantages over equivalent biomaterials such as low cost, good mechanical properties and biocompatibility. The pores found in porous biomaterials provide mechanical interlock, ensuring strong attachment of the implant to the tissue. In this study, 20%, 30% and 40% by volume of polyvinyl alcohol (PVA) and Boron powder were added into 316L powder to obtain porous SS implant. To investigate the effect of porosity and boron effect on the stainless-steel implant material, the samples produced in PVA and Boron added groups, were sintered at 1180 oC under an argon atmosphere. With the evaporation of PVA in the structure, porous and boron added samples were obtained in two groups. Finally, the samples were subjected to Brinell hardness and compression tests and analyzed by SEM, EDS and XRD. As a result of the hardness tests, the highest values were measured as 37.006, 31.32, 25.28 HB. 39.5, 34.5, 26.2 MPa strengths were measured for 20%, 30% and 40% porous samples respectively.

Analysis of the causes determining dimensional and geometrical errors in 316L and 17-4PH stainless steel parts fabricated by metal binder jetting

This work aims at investigating the causes affecting the dimensional and geometrical accuracy of holes in metal binder jetting stainless steel parts. Parallelepiped samples with a through hole were produced using AISI 316L and 17-4PH powders, differing for diameter (3, 4, 5 mm), and position of the axes with respect to the building plane (6, 9, 12 mm distance). Dimensions and geometrical characteristics were measured at green and sintered state by a coordinate measuring machine, determining the dimensional change and the geometrical characteristics. As expected, the shrinkage of linear dimensions is anisotropic; moreover, change in volume and sintered density are significantly affected by the position in the printing chamber. Higher shrinkage was measured along building direction (Z) – 18.5 ÷ 19.5%, than in the building plane – 16.5 ÷ 17.5%, and slightly higher shrinkage – 0.5 ÷ 0.8% was measured along powder spreading direction (X) than binder injection direction (Y). A variation up to 3% in relative density of sintered parts depending on the position in the building plane was observed in 316L. The dimensional change of diameters generally confirmed the shrinkage predicted by the model previously developed—difference between real and expected dimensional changes lower than 3%, except for three geometries (4 ÷ 6%). The cylindricity form error of sintered parts was strongly underestimated by the prediction model (up to 0.15 mm), but underestimation was considerably reduced (generally lower than 0.05 mm) adding the cylindricity form error due to printing. Dimensional and geometrical accuracy of holes are strongly affected by shape distortion of the parallelepiped geometry, in turn due to layer shifting and inhomogeneous green density during printing, and to the effect of frictional forces with trays during sintering. Gravity load effect was also observed on the holes closest to the building plane. Future work will improve the reliability of the prediction model implementing the results of the present work.

Multi-Physics Modeling of Melting-Solidification Characteristics in Laser Powder Bed Fusion Process of 316L Stainless Steel.

In the laser powder bed fusion process, the melting-solidification characteristics of 316L stainless steel have a great effect on the workpiece quality. In this paper, a multi-physics model was constructed using the finite volume method (FVM) to simulate the melting-solidification process of a 316L powder bed via laser powder bed fusion. In this physical model, the phase change process, the influence of temperature gradient on surface tension of molten pool, and the influence of recoil pressure caused by the metal vapor on molten pool surface were considered. Using this model, the effects of laser scanning speed, hatch space, and laser power on temperature distribution, keyhole depth, and workpiece quality were studied. This study can be used to guide the optimization of process parameters, which is beneficial to the improvement of workpiece quality.

Factors affecting particle characterization of powders used in additive manufacturing

The present study analyze the effects of sample preparation and data analysis techniques for different experimental methods of metal powder characterization. The aim is to propose a robust strategy for quantifying powder particle size distribution and morphology of relevance for metal additive manufacturing (MAM) processes, in particular for powders with nearly spherical particle shape. Laser diffraction (LD), scanning electron microscopy (SEM) and high-resolution X-ray computed tomography (XCT) are used to analyze three different stainless-steel 316L powders. The results obtained demonstrate that the powder sample itself and the sample preparation, both have a significant effect on the obtained values. For the imaging processing methods, the post-processing route is an important part of the characterization and strongly affects the results. However, proper sample preparation is shown to ease this post-processing. Given the high sphericity of the powders used, it is shown that 2D and 3D methods generally lead to the same result when the 2D data are properly transformed to 3D space. Guidelines to overcome the shortcomings of the different techniques are suggested.

CT scan, EBSD and nanoindentation analysis of 3D-printed parts with post-process heat-treatment

Heat treatment is vital for improving the characteristics of Laser Powder Bed Fusion (LPBF) components. The technique has the potential to change the microstructure of the material as well as its mechanical properties, such as yield strength, hardness, and ultimate tensile strength. To avoid undesirable impacts on the microstructure, temperature, heating, and cooling rates must be precisely controlled. Several parts were printed using LPBF from Steel 316L powder and went through post-process heating. The CT scan analysis revealed that heating the 3D printed parts for 40 min at 900 °C and 950 °C increased the porosity level across the parts although the porosity then decreased after 950 °C. From 850 °C to 1050 °C, EBSD analysis resulted in inverted pole figure maps demonstrating a relative increase in grain size. ImageJ software was used to determine the actual grain size and phase, revealing a grain size growth. Furthermore, as heat treatment temperatures increased, the ferrite phase enlarged. The cellular structure and high temperatures had a major impact on mechanical characteristics. Hardness test findings revealed a decreased mechanical characteristic as heat treatment temperature rose represented by increased porosity population and grain size. To increase the mechanical properties of these materials, an effective strategy is to achieve an even distribution of micro grains while limiting the porosity population.

In-process monitoring of track geometry as a control approach for laser metal deposition

Laser metal deposition (LMD) is an additive manufacturing process, which is used to generate large parts or for reconditioning machine components. During the process the welding tracks are affected by complex dynamic temperature-time curves that results in geometric deviations, which limits the build-up process. Therefore, complex structures are more difficult to manufacture with constant parameters. The present work describes an in-process monitoring system and first approaches of a control concept for LMD processes. It is set up using a newly developed laser triangulation sensor from Falldorf Sensor GmbH, which is trailing mounted. Due to the resistance to diffused radiation, e.g. from process light, it is possible to place the sensor close to the process zone which offers the possibility of a height control system. For the experiments, single tracks were welded from 316L powder by using a diode laser. The results show that the track width and height increase with rising laser power. Depending on the process parameter the height changes up to 29% and the width up to 58% if only the laser power is modified by 1800 W. The monitoring system forms a good basis for a later control concept, which can be used to make the process more stable in the case of complex structures.