Stainless Steel Powder Research Articles

Dense Uniformed Joints Between Molybdenum Disilicides and Stainless Steel Powders and Their Welding Behavior

The dense uniformed joints between molybdenum disilicides (MoSi2) and stainless steel powders were prepared using the spark plasma sintering (SPS) technique with nine graded materials and their welding behavior was investigated. The results showed that such joints can be achieved using graded interlayers because the coefficient of thermal expansion (CTE) of each interlayer closely matched over a wide temperature range. Furthermore, the compatibility between the graded interlayers prevented MoSi2 with low toughness from the occurrence of microcracks resulted from the residual stresses formed during cooling of the joint. Moreover, the 9-layer joint with a thickness of 1.0 mm for each layer exhibited the minimum residual stress if the compositional exponent was 0.8.

Mechanical properties of additively-manufactured sheet-based gyroidal stochastic cellular materials

The advancements of additive manufacturing enabled progressive investigations of many atomic lattice-mimicking structures, plate- and sheet-based cellular materials. While such materials showed promising mechanical and physical properties, they exhibit anisotropic properties. In this work, a design procedure to create partially closed sheet-based stochastic cellular materials based on implicit functions is presented. Different realizations of the proposed cellular materials have been developed. Finite element analyses (FEA) were employed to assess the mechanical properties and isotropy of the designed cellular materials. Experimentally, several samples were fabricated with a range of relative densities using powder bed fusion and 316 L Stainless Steel powder. The fabricated metallic stochastic samples and other periodic Gyroid samples were investigated for their compressive mechanical properties and their topology-property scaling laws were reported. Results of numerical simulations showed that the stochastic cellular materials exhibit isotropic properties when using nine control points and more. The experimental results showed that the stochastic cellular materials show a stretching-dominated mode of deformation where samples deform collectively with no shear band formation. However, it was found that periodic Gyroid samples exhibited superior mechanical properties to the proposed stochastic cellular materials. The current study opens the doors for further investigation of mathematically-designed metallic stochastic cellular materials and their deployment in different engineering disciplines.

Analysing Powder Injection Moulding of a Helix Geometry Using Soft Tooling.

Freeform injection moulding is a novel technology for powder injection moulding where a sacrificial 3D printed mould (i.e., a soft tooling) is used as an insert in the injection process. The use of 3D printed moulds enable a higher geometrical design flexibility as compared to the conventional injection moulding process. However, there is still very limited knowledge on how the sacrificial soft tooling material and powder suspension handles the increased geometrical complexity during the process. In this study, a stainless steel powder suspension is injected into a geometrically challenging sacrificial mould (viz. a helix structure) that is produced by vat photopolymerization additive manufacturing. Computed tomography is used to quantify the geometrical precision of the mould both before and after injection. In addition, a new numerical model that considers the suspension feedstock is developed to investigate the powder injection moulding process. The numerical results are found to be in qualitative good agreement with the experimental findings in terms of pinpointing critical areas of the structure, thereby highlighting a new pathway for evaluating sacrificial inserts for powder injection moulding with a high geometrical complexity.

Optimization of microwave processing parameters on powder-metallurgical 316L stainless steels

ABSTRACT Ultra-rapid microwave sintering of powder metals is recognized for better process advantages, namely less process time, uniform volumetric heating, and fine microstructural features that will improve mechanical and corrosion properties. In the present study, austenitic (316 L) stainless steel powder compacts were sintered using a multimode microwave hybrid heating system at solidus (1200°C and 1250°C) and super-solidus (1300°C) temperature with different holding times 30, 45 and 60 min. Mechanical as well as electrochemical responses were correlated with the microstructural features and densification characteristics. The samples sintered at 1300°C for 60 min have shown improved densification and higher tensile strength of 474.3 MPa with 22.1% ductility. It is also noted that an increase in sintering temperature resulted in excellent corrosion resistance in the order of ten times. The input process parameters and the output property responses are mathematically modeled with the help of RSM. The developed equations and experimental studies are in good agreement with output responses (sintered density, yield strength, %elongation, microhardness, corrosion rate) with 0.92 to 0.95 desirability. The validation of optimized output responses is evaluated by conducting confirmation tests at two conditions (1300°C for 60 min & 1300°C for 52 min) and are in line with regression model analysis.

Effect of Particle Size and Shape on Wall Slip of Highly Filled Powder Feedstocks for Material Extrusion and Powder Injection Molding.

A necessity to distinguish between the influence of powder shape and size (particle size distribution) is especially demanding for highly filled metal powder feedstocks employed in additive manufacturing and powder injection molding. As their processability is evaluated through rheological behavior, the study focuses on the effect of powder size/shape on a wall slip, which is a typical phenomenon determining flow performance of these materials. Water and gas atomized 17-4PH stainless steel powders with D 50 of about 3 and 20 μm are admixed into a binder containing low-density polyethylene, ethylene vinyl acetate, and paraffin wax. Mooney analysis to intercept the slip velocity of 55 vol. % filled compounds reveals that wall slip effect appears to vary significantly with size and shape of metal powders-round shaped and large particles are the most prone to the wall slip. However, the evaluation is affected by the type of the flow streams resulting from the geometry of the dies-conical dies reduce the slip up to 60% in case of fine and round particles.

Synthesis of the SUS 316L Powders with a Nano-Meso Bi-Modal Structure

Pure stainless steel (SUS 316L) powders with both a nano-grain and micro-structure were prepared using a mechanical milling process. The bimodal microstructure consists of a nano-grain with a size of 50 nm in the surface region and microstructure in the core of the particle. The nano-grain with a bcc structure at the surface and the microstructure of fcc in the core of the particles were prepared by milling at 150rpm for 9h, and those compositions were 14.39% for the nano structure and 85.61% for the micro structure, respectively. Spark plasma sintering (SPS) was carried out for the compaction of the powders. The compacts forming powders with a bi-modal structure have a mesoscopic structure.

A comparison of particle size distribution and morphology data acquired using lab-based and commercially available techniques: Application to stainless steel powder

The particle size distribution (PSD) and particle morphology of metal powders undoubtedly affects the quality of parts produced by additive manufacturing (AM). It is, therefore, crucial to accurately know the PSD and morphology of these powders. There exist several measurement techniques for these quantities, but since each method is based on different physical phenomena, which are sensitive to different aspects of a particle's shape and size, it is unclear how the measured PSDs and morphology compare to one another. In this study, five different techniques are used: sieve analysis, dynamic imaging analysis, laser diffraction analysis, X-ray computed tomography (XCT), and scanning electron microscopy. The first three are commonly used in the powder metallurgy field while the last two are laboratory-based tools capable of providing robust size and shape data. Nominally identical samples of stainless-steel powders were produced via riffling, and each technique was employed to measure effectively the same PSD and in some cases the morphology. In this paper, the differences among these measurement techniques are explored by a comparison of the measured results. Besides the random variations of the various measurement processes, the difference in the results is partly due to the fact that the particles are not perfectly spherical and that there are many multi-particles present. Each of these affect the principle of each method differently. Three-dimensional particle morphology and size data collected via XCT is used to provide insight regarding the discrepancies among other sizing and morphology measurement techniques.(Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.)

Glass-ceramic sealants and steel interconnects: Accelerated interfacial stability and reactivity tests at high temperature

High-temperature reactions between glass–ceramic sealants and Fe-Cr alloy interconnects may lead to the formation of undesirable phases, and consequently degradation of solid oxide fuel/electrolyser devices. In this work, three different glass–ceramic sealants (Na-containing, Ba-containing, Sr-containing compositions) and Fe22Cr stainless steel powders (raw and pre-oxidised) are considered in order to test their chemical reactivity at 750 °C and 850 °C for 500 h in static air. The novelty of this approach is related to the use of powder mixtures instead of studying the reactivity on planar interfaces, which allows a better evaluation the materials’ reactivity. Oxidation tests indicate that the Sr-containing glass–ceramic/steel couple is the least reactive among the aged samples. For the Ba-containing samples, the formation of an undesirable phase of BaCrO4 is observed by diffractometry and photoelectron spectroscopy analyses. The present research explores, for the first time, the effects of exposing the high surface area of the alloy powder and glass–ceramic sealant interface, assessed by oxidation testing and microstructural analysis. The results show that by using mixed powders with large interface surface areas, degradation of the materials can be observed after relatively short times, allowing accelerated screening of the reactivity of materials, and thus their further development.

The Marangoni Convection Effect on Melt Pool Formation during Selective Laser Melting Process

In order to predict the effect of the Marangoni convection and the morphology of melted stainless steel powder, during the selective laser melting (SLM) process, a transient three-dimensional numerical model is developed at the mesoscale. The evolution of the temperature and velocity fields’ is then studied. The initial powder bed distribution is obtained by the discrete element method (DEM) calculation, and the temperature distribution and the molten pool shape deformation are calculated and analyzed by the Ansys-Fluent commercial code. The molten pool shape is obtained by considering the influence of Marangoni convection on the internal flow behavior. The recoil force was not considered in our calculation. As main results, a slight deviation between the position of the maximum temperature of the molten pool and the center of the laser spot is observed. The direction of the heat diffusion is more likely to be horizontal and the flow centrifugal, which causes the melt track to be wide. Finally, the Marangoni convection is the main driver of the flow.

Influence of particle size on powder rheology and effects on mass flow during directed energy deposition additive manufacturing

Metal powders used in directed energy deposition additive manufacturing are subjected to particle-particle and particle-gas interactions not captured through traditional powder characterization. Variations in powder performance were identified through mass flow rate measurements of lean (UNS S32101), standard (UNS S32205), and super (UNS S32750) duplex stainless steel powders specified for directed energy deposition on three commercial powder feeders. Across these different systems, the lean grade consistently displayed the lowest mass flow rates. Differences between the three powders were only identified with advanced powder and rheological characterization tools, which linked particle characteristics and rheological properties to powder flow and performance. A slight shift to a larger size distribution in the lean grade produced a higher percentage of larger particles with higher masses that resisted motion and increased flow resistance. Higher flow resistance was captured through higher cohesion strength and fluidization measurements during rheological testing.

316L FFF binder development and debinding optimization

Fused Filament Fabrication (FFF) technology is used to create metal parts in this paper. A binder formula is developed for 316L stainless steel powder, composed of polypropylene (PP), styrene ethylene butylene styrene (SEBS) and paraffin wax (PW). The binder is mixed with the 316L stainless steel powder to produce mixture which is then extruded into filament. The optimum binder formula, PP:SEBS:PW = 5:2:2, is obtained by orthogonal experiment. After optimization, mixture viscosity is reduced, filament tensile strength is guaranteed, rigidity is improved. The filament can be printed by a desktop FFF printer to obtain green parts. Binder within the green parts can be sufficiently removed by solvent and thermal debinding, and the shape of printed parts can be maintained well. After sintering, shrunken 316L stainless steel parts can be created, some pores distributed inside. With finer metal powder, the relative density of sintered part can be increased to 96%. The research ideas of this paper can provide effective methods for the development and optimization of binder.

Intensive laser repair through additive manufacturing of high-strength martensitic stainless steel powders (I) –powder preparation, laser cladding and microstructures and properties of laser-cladded metals

The intensive high-strength martensitic stainless steel (IHMSS) was designed and prepared. Influences of various parameters of electrode-induced gas atomization (EIGA) on qualities of IHMSS powders were investigated through the orthogonal test design. The IHMSS powders prepared through the optimized technology present a favorable comprehensive quality. The additive manufacturability of IHMSS powder was explored by laser cladding with coaxial powder feeding. Effects of parameters on the forming quality of IHMSS laser-cladded layers were explored. The microstructures and properties of the IHMSS deposit were characterized. Obvious macrosegregation at the interface between the base plate and the deposit and the mechanical properties deliver significant anisotropy. The sample along the Y direction (laser scanning direction) shows the highest tensile strength of 1385 MPa and elongation of 25.7%); the sample along the Z direction (vertical direction) is found to have the lowest tensile strength of 1068 MPa and elongation of 12.2%.

Numerical simulation and experimental measurement of pressureless sintering of stainless steel part printed by Binder Jetting Additive Manufacturing

Binder Jetting-Metal Additive Manufacturing (BJ-MAM) is a powder bed-based additive manufacturing technology which deposits liquid binder droplets to join powder particles to form complex shaped structures (i.e., green parts). As the printing process is done around room temperature, BJ-MAM is largely immune to the distortion and cracking issues that can be prevalent in melting based powder bed processes. However, a main issue for BJ-MAM is the part shrinkage and distortion during high-temperature sintering. The densification and deformation behaviors during pressureless sintering of green parts printed by BJ-MAM were studied in this paper. Experimentally, cantilever- and bridge-shaped coupons with varying beam lengths were printed using 316L stainless steel powder; these coupons produced different extents of deformation after sintering. Based on the existing modeling approaches in the powder metallurgy literature, a finite element model was developed incorporating an elastic-viscoplastic constitutive equation for computing both uniaxial equivalent creep strain and volumetric swelling strain. Two methods, a viscosity based and a power-law creep based, were further evaluated for calculating the uniaxial equivalent creep strain. Material property data used in the constitutive equation such as viscosity and creep parameters were collected from the literature, critically reviewed, and then inputted into the model. Other salient features of the model included thermal-mechanical property data that were dependent on both relative density and temperature as well as frictional contact between the part surface and the furnace wall under gravitational load. The calculated quantities such as shrinkage, final relative density, and deformed shapes were compared with the respective experimental data across different part geometries.

Comparison of Different Additive Manufacturing Methods for 316L Stainless Steel.

In additive manufacturing (AM), the technology and processing parameters are key elements that determine the characteristics of samples for a given material. To distinguish the effects of these variables, we used the same AISI 316L stainless steel powder with different AM techniques. The techniques used are the most relevant ones in the AM of metals, i.e., direct laser deposition (DLD) with a high-power diode laser and selective laser melting (SLM) using a fiber laser and a novel CO2 laser, a novel technique that has not yet been reported with this material. The microstructure of all samples showed austenitic and ferritic phases, which were coarser with the DLD technique than for the two SLM ones. The hardness of the fiber laser SLM samples was the greatest, but its bending strength was lower. In SLM with CO2 laser pieces, the porosity and lack of melting reduced the fracture strain, but the strength was greater than in the fiber laser SLM samples under certain build-up strategies. Specimens manufactured using DLD showed a higher fracture strain than the rest, while maintaining high strength values. In all the cases, crack surfaces were observed and the fracture mechanisms were determined. The processing conditions were compared using a normalized parameters methodology, which has also been used to explain the observed microstructures.

Surface Alloying of Internal Surfaces of Low‐Carbon Steel Castings through Incorporation of Nickel, Nickel and Chromium, and 316L Stainless Steel Powders to Mold and Core Surfaces

A surface alloying technique is developed to enrich the internal surfaces of low‐carbon steel casting with nickel or nickel and chromium. This unique technique involves the incorporation of nickel (Ni), nickel and chromium (Ni + Cr), or stainless steel (SS) powders into a slurry, and then applying the slurry onto the surface of cores used in sand molds which comes in contact with molten metal during casting. Various properties, including coating thickness, composition, microstructural phases, surface hardness, and corrosion resistance of the surface‐alloyed layer, are studied. The internal surface of the substrate metal is enriched with nickel and chromium, up to 16.32 wt% Ni or 7.79 wt% Ni and 15.85 wt% Cr. Optical microscopy shows that the surface‐enriched layer is continuous over the entire surface of the casting and ranges in thickness from 229 to 1342 μm. When alloyed with Ni and SS powders, the surface‐alloyed layers demonstrate a dendritic solidification microstructure, suggesting complete dissolution of powders in steel followed by solidification. While the substrate primarily displays pearlitic and ferritic structures, simulation and X‐ray diffraction confirm the presence of austenite and ferrite in the surface‐alloyed layer when Ni + Cr is used. Microhardness testing and corrosion studies show significant improvement after surface alloying.

A Simulation Study on the Effect of Particle Size Distribution on the Printed Geometry in Selective Laser Melting

Abstract Selective laser melting (SLM) process is a powder bed fusion additive manufacturing process that finds applications in aerospace and medical industries for its ability to produce complex geometry parts. As the raw material used is in the powder form, particle size distribution (PSD) is a significant characteristic that influences the build quality in turn affecting the functionality and esthetic aspects of the product. This article investigates the effect of PSD on the printed geometry for 316L stainless steel pow der, where three coupled in-house simulation tools based on discrete element method (DEM), computational fluid dynamics (CFD), and structural mechanics are employed. DEM is used for simulating the powder bed distribution based on the different powder PSD. The CFD is used as a virtual testbed to determine thermal parameters such as heat capacity and thermal conductivity of the powder bed viewed as a continuum. The values found as a stochastic function of the powder distribution are used to analyze the effect on the melted zone and deformation using structural mechanics. Results showed that mean particle size and PSD had a significant effect on the packing density, melt pool layer thickness, and the final layer thickness after deformation. Specifically, a narrow particle size distribution with smaller mean particle size and standard deviation produced solidified final layer thickness closest to nominal layer thickness. The proposed simulation approach and the results will catalyze the development of geometry assurance strategies to minimize the effect of particle size distribution on the geometric quality of the printed part.

Rheology of dry powders and metal injection molding feedstocks formulated on their base

The influence of powder characteristics on the moldability of metal injection molding feedstock is relatively well understood, but remains difficult to quantify other than by real-scale injections. This study aims to correlate feedstock moldability performances with melt viscosity and dry powder rheology. Four stainless steel powders exhibiting different particle sizes and shapes were tested in dry conditions using an FT4 powder rheometer to obtain thirteen different powder rheology-related metrics. These powders were subsequently mixed with a wax-based binder to formulate four feedstocks that were tested in melt conditions using a rotational rheometer to characterize the injection-related rheology of the feedstock formulations. The moldability of the feedstocks was finally quantified using real-scale injections performed into a spiral mold cavity to measure the injected lengths, and the correlation between the dry and melt rheology metrics and the real-scale injection moldability was evaluated. Results showed that the feedstock melt viscosities and the injected lengths were almost perfectly correlated. Furthermore, two dry powder rheology metrics, namely, the normalized basic flow energy and the conditioned powder bulk density, showed the correlation with the feedstock moldability which is comparable to that obtained with the melt rheology. The results of this study indicate that although the feedstock melt viscosity and the dry powder rheology take fewer variables into account as compared to real-scale injections, these simplified tests (especially dry powder rheology) could be considered as rapid and reliable approaches to assessing the impact of powder characteristics on the moldability of powder-binder feedstocks used in metal injection molding.

Evaluating the Corrosion Resistance of AA7075 Containing Mn-Rich Phases Fabricated By Spark Plasma Sintering

Aluminum alloy 7075 (AA7075) are known to have light-weight and high strength. AA7075 mainly contains alloying elements such as Zn, Mg, Cu, and Fe to increase strength by intermetallic particles (IMPs). However, Cu-containing and Fe-containing IMPs show higher potential relative to the Al-matrix and act as initiation sites of localized corrosion such as pitting in chloride solutions1-2). In particular, Cu-containing IMPs in AA7075 become cathode and strongly affect the initiation and growth of localized corrosion2). As for AA7075, it was reported that breakdown of Al-oxide films occurs around Cu-containing IMPs due to an increase in pH by the oxygen reduction reaction, followed by pitting. Therefore, it would appear that the inhibition of the breakdown of passive films around the IMPs AA7075. Spark plasma sintering (SPS) is a sintering technique which has been applied to fabricate various materials. When stainless steel powders and pure Mo powders are mixed and sintered by SPS, stainless steel containing Mo-rich phases was fabricated3).In this study, AA7075 containing Mn-rich phases was fabricated by SPS. Gas-atomized AA7075 powders and pure Mn powders were mixed and sintered at 773 K for 15 min. After sintering, hot-forging was conducted at 773 K. After hot-forging, heat-treatment was conducted at 688 K for 2 h, followed by furnace-cooling. After that, the specimens were polished with a diamond paste down to 2.5 µm. The surface of specimens was observed using an optical microscope and a field emission scanning electron microscope (FE-SEM) equipped with energy dispersive X-ray spectroscopy system (EDS). The open circuit potential (OCP) of the specimen was measured for 72 h in 0.1 M NaCl at pH 6.0. With the exception of the electrode area, the surfaces of specimens were coated with an epoxy resin. The size of the exposed electrode area was ca. 5 mm × 5 mm. A dip-dry corrosion test was conducted to evaluate the corrosion resistance of specimens. The specimens were dipped in 0.1 M NaCl (pH 6.0) for 20 min and dried at 298 K and 50%RH for 60 min.Mn-rich phases were detected in Mn-added AA7075. Mn-rich phases consisted of pure Mn and Al-Mn intermetallic phases. The size of Mn-rich phases was ca. 20 - 150 µm. In the OCP measurements, OCP of Mn-added AA7075 decreased immediately after the immersion in 0.1 M NaCl, and the minimum value of the OCP of the Mn-added AA7075 was found to be lower than that of Mn-free AA7075. It was observed that the OCP of 5.0 mass% Mn-added AA7075 was lower than that of Mn-free AA7075. After the OCP measurements, the corrosion of Mn-rich phases was confirmed, and the color of these phases changed to dark. In the cyclic dip-dry corrosion test, mass-loss decreased with Mn content. After 40 cycles, mass-loss of 5.0 mass% Mn-added AA7075 was twice as lower as that of Mn-free AA7075. After the cyclic dip-dry corrosion test, pitting corrosion of the matrix was observed and Mn phases were dissolved.

Fatigue of laser powder bed fusion processed 17-4 stainless steel using prior process exposed powder feedstock

The rapid pace of development seen in the metal additive manufacturing (AM) process of laser powder-bed fusion (LPBF) requires in-step advances in processes qualification to enable full-scale adoption. This particularly applies to quantifying how powder feedstock conditions impact end-component quality. This study examines how in-machine 17-4 stainless steel powder feedstocks are affected by prior LPBF processes, and how these effects impact subsequent builds. Examinations of powder morphology, chemistry, flowability, and rheology were conducted to characterize the powder conditions. The resultant effects of powder feedstock condition on produced component quasi-static tensile and high-cycle fatigue properties were analyzed. Fatigue life was analyzed using a reliability modeling approach that enabled a robust statistical comparison of life. Powder characteristics were found to evolve with powder exposure to prior LPBF processes, particularly in the extremes of powder size distribution and measures of bulk flow. No significant effects of these changes on tensile properties were observed. Reliability modeling methods, including the lognormal and Weibull distributions as well as the empirical survival function, are shown to be effective tools for modeling fatigue variability in LPBF manufactured components. Through these tools, fatigue life was found to be invariant with changes in powder condition.

Microwave sintering response of different grade stainless steels and its influence on metallurgical properties

ABSTRACT Ultra-rapid microwave sintering of powder metals will provide fine microstructural features that improve mechanical properties. In present study, three grades of stainless steel powder compacts (316L, 430L, 410) were produced using a uniaxial compaction unit. These compacts were sintered by microwave hybrid heating method at 1300°C (super-solidus region). The densification response, microstructural attributes, and mechanical properties were compared at 30, 45 and 60 min holding times. The compositional analysis was performed with the help of optical emission spectroscopy (OES) and scanning electron microscopy equipped with energy dispersed spectroscopy(SEM-EDS). The results obtained from both spectroscopies are compared for the sintered samples. The correlation of mechanical properties is analysed with evolved microstructural attributes (pore volume, pore shape and pore distribution). The excellent strength of 457 ± 16 MPa, 466 ± 6 MPa and 476 ± 26 MPa with 23 ± 1.3%, 14 ± 1.5% and 11 ± 1% of ductility is observed for sintered AISI 316L, 430L and 410, respectively.