316L Stainless Steel Specimens Research Articles

Effect of micro-defects on fatigue lifetime of additive manufactured 316L stainless steel under multiaxial loading

The statistical distribution of the geometric characteristics of microscopic defects, stress concentration factor (SCF) caused by microscopic defects and multiaxial fatigue lifetime of additive manufactured 316L stainless steel were investigated. The samples were machined after manufactured by Selective Laser Melting. Based on the X-ray computer tomography of typical 316L stainless steel specimen, the distribution laws of the geometric characteristics of microscopic defects in the material were studied, and the results indicated that the size and shape characteristics distributions of defects were subjected to lognormal rules well. The 3D volumetric models of typical microscopic defects under uniaxial, pure torsional, proportional and 90° non-proportional loading were imported into a finite element software to calculate SCFs caused by defects. The degree of stress concentration located at the edge of the microscopic defect under uniaxial loading was more serious than that under pure torsional loading with same nominal Von Mises equivalent stress (VMES). Simultaneously, greater SCF was arisen at the edge of microscopic defect under non-proportional loading instead of under proportional loading. The above conclusions were proved to be correct through analyzing the experimental data of additive manufactured 316L stainless steel under multiaxial fatigue loading. It showed that the fatigue lifetime under uniaxial loading was shorter than that under pure torsional loading with same nominal VMES. Meanwhile, the specimens subjected non-proportional loading had shorter fatigue lifetime than that under proportional loading with same nominal VMES.

Synergistic Effect of Solid State Hydrogen and Cold Work Pretreatment on Oxide Films Grown on 316L Stainless Steel during Short Term Immersion in Deaerated High Temperature Water at 300 °C

The properties of the oxide films formed on solution-annealed and cold-worked 316L stainless steel (SS) specimens with and without charged hydrogen in deaerated pressurized water reactor primary water at 300 °C were investigated. The outer oxide layers of all specimens were composed of magnetite (Fe3O4) and NiFe2O4. Charged hydrogen resulted in larger outer iron-bearing oxide particles forming due to hydrogen-enhanced outward diffusion of iron cations. Prior cold-work accelerates the oxidation was observed. Charged hydrogen led to local cracks in the oxide film and enhanced the penetration oxidation beneath the metal/oxide interface. The Cr-rich inner oxide layer grown on the prior cold-worked specimen with charged hydrogen was thicker than that on the cold-work specimen or the hydrogen-charged specimen, revealing the combined effects of charged hydrogen and prior cold-work on the acceleration of the oxidation process. The working mechanism of the solid-state hydrogen effect on the oxide film was discussed.

Evaluation of material degradation using phased array ultrasonic technique with full matrix capture

This paper presents a new approach based on phased array ultrasonic technique (PAUT) with full matrix capture (FMC) to effectively measure acoustic properties such as ultrasonic wave velocity and attenuation coefficient and to evaluate material degradation. This paper also proposes the use of PAUT with FMC, to simultaneously generate and detect ultrasonic waves such as longitudinal, shear, and Rayleigh waves in the material. A theoretical model is developed to derive the analytical solution of ultrasonic waves induced in the material by an ultrasonic phased array probe. Based on the theoretical prediction, experiments are carried out on AISI 316L stainless steel specimen of 3 mm thickness, and a 2D finite element model is built to verify the generation and detection of these waves. Furthermore, measurements of acoustic properties are carried out on the specimen using the proposed approach as well as the conventional ultrasonic testing (CUT) technique and their sensitivities are compared. The measurements of time-of-flight and back-wall echo amplitude are more accurate with the proposed approach than the CUT technique, which makes the former technique better than the latter for the evaluation of material degradation. Finally, experiments are performed on AISI 316L specimen degraded with different levels of tensile stress to verify the capability of PAUT with FMC. The results show that the velocities and attenuation coefficient increase with an increase in tensile stress until material fracture. This phenomenon is interpreted as resulting from microstructural changes such as dislocations during material degradation. From the experimental results, it is proposed that PAUT with FMC can potentially be used to evaluate early-stage materials degradation.

Laser Directed Energy Deposition of Bulk 316L Stainless Steel

Laser directed energy deposition (DED) of high-density parallelepiped 316L stainless steel specimens has been performed while varying laser power, scanning velocity, powder mass flow rate and layer thickness. An extensive set of analyses has been performed into the resulting dimensions, surface roughness, microstructure, hardness, tensile strength, build substrate temperature, deposition efficiency, production time and specific energy input with the aim of differentiating variations in component performance and process efficiency for components that have already been optimized in terms of density. Average layer height is found to be an approximately linear function of the powder line mass flow rate, while samples with the same powder feed per unit length have similar microstructures despite differences in scanning velocity and total powder feed rate. The hardness of specimens is generally higher than bulk 316L stainless steel, while yield and ultimate tensile strength are similar or higher than the bulk material but with dependence on the load direction. Deposition efficiency is strongly influenced by the scanning strategy and laser off time, with values as low as 42% with a “raster” scanning strategy and up to 84% with a “snake” scanning strategy. The large number of analyses performed on high-density components provides important new insight into factors affecting build quality and process efficiency that go beyond simply maximizing density, suggesting that significant further improvements can be obtained. Such knowledge is exploitable for application-specific process optimization and implementation of open and closed-loop height control strategies for production of bulk 316L stainless steel components.

Tensile Properties of Built and Rebuilt/Repaired Specimens of 316L Stainless Steel Using Directed Energy Deposition

One of the most exciting possibilities brought forward by some additive manufacturing (AM) processes is their ability to deposit materials on existing parts. Mainly achievable with directed energy deposition systems, it is thus possible to repair worn or broken components. Two applications were studied in this research: (1) complete construction and (2) repair of tensile specimens made of 316L-Si stainless steel. The latter series was produced by adding material on existing half specimens made of wrought 316L stainless steel. All specimens were built along the Z-axis. Tensile specimens machined from wrought were also tested for comparison purposes. Three conditions were tested for each series of AM specimens: as-built, stress relieved and HIPed. The results show that yield strength, ultimate tensile strength (UTS) and elongation are higher than the typical tensile properties reported for annealed 316L. Repairs show excellent bond resistance with the wrought material and good mechanical properties with a mean UTS of 647 MPa in the as-built condition.

Effect of Processing Atmosphere and Secondary Operations on the Mechanical Properties of Additive Manufactured AISI 316L Stainless Steel by Plasma Metal Deposition

Plasma metal deposition (PMD) is an interesting additive technique whereby diverse materials can be employed to produce end parts with complex geometries. This study investigates not only the effects of the manufacturing conditions on the final properties of 316L stainless steel specimens by PMD, but it also affords an opportunity to study how secondary treatments could modify these properties. The tested processing condition was the atmosphere, either air or argon, with the other parameters having previously been optimized. Furthermore, two standard thermal treatments were conducted with the intention of broadening knowledge regarding how these secondary operations could cause changes in the microstructure and properties of 316L parts. To better appreciate and understand the variation of conditions affecting the behavior properties, a thorough characterization of the specimens was carried out. The results indicate that the presence of vermicular ferrite (δ) varied slightly as a consequence of the processing conditions, since it was less prone to appear in specimens manufactured in argon than in air. In this respect, their mechanical properties suffered variations; the higher the ferrite (δ) content, the higher the mechanical properties measured. The degree of influence of the thermal treatment was similar regardless of the processing conditions, which affected the properties based on the heating temperature.

Influence of Pore Characteristics on Anisotropic Mechanical Behavior of Laser Powder Bed Fusion–Manufactured Metal by Micromechanical Modeling

In recent times, additive manufacturing (AM) has proven to be an indispensable technique for processing complex 3D parts because of the versatility and ease of fabrication it offers. However, the generated microstructures show a high degree of complexity due to the complex solidification process of the melt pool. In this study, micromechanical modeling is applied to gain deeper insight into the influence of defects on plasticity and damage of 316L stainless steel specimens produced by a laser powder bed fusion (L‐PBF) process. With the statistical data obtained from microstructure characterization, the complex AM microstructures are modeled by a synthetic microstructure generation tool. A damage model in combination with an element deletion technique is implemented into a nonlocal crystal plasticity model to describe anisotropic mechanical behavior, including damage evolution. The element deletion technique is applied to effectively model the growth and coalescence of microstructural pores as described by a damage parameter. Numerical simulations show that the shape of the pores not only affects the yielding and hardening behavior but also influences the porosity evolution itself.

Very High Cycle Fatigue Behavior of Additively Manufactured 316L Stainless Steel.

The present paper is focused on an experimental study of the damage-to-failure mechanism of additively manufactured 316L stainless steel specimens subjected to very high cycle fatigue (VHCF) loading. Ultrasonic axial tension-compression tests were carried out on specimens for up to 109 cycles, and fracture surface analysis was performed. A fine granular area (FGA) surrounding internal defects was observed and formed a “fish-eye” fracture type. Nonmetallic inclusions and the lack of fusion within the fracture surfaces that were observed with SEM were assumed to be sources of damage initiation and growth of the FGAs. The characteristic diameter of the FGAs was ≈500 μm on the fracture surface and were induced by nonmetallic inclusions; this characteristic diameter was the same as that for the fracture surface induced by a lack of fusion. Fracture surfaces corresponding to the high cycle fatigue (HCF) regime were discussed as well to emphasize damage features related to the VHCF regime.

Effect of initial microstructure on the deformation heterogeneities of 316L stainless steels fabricated by selective laser melting processing

The selective laser melting (SLM) is a popular additive manufacturing (AM) technique used for the fabrication of metal parts. In the present study, two 316L stainless steel specimens (SLM-I and SLM-II) with different microstructures were fabricated with different levels of energy density by changing the laser power and scanning speed, which are the main SLM process conditions. The deformation and fracture behavior of miniature SLM specimens under uniaxial tension were experimentally measured via optical microscopy (OM), field emission scanning microscopy (FE-SEM), and an electron back-scattered diffraction (EBSD) technique. In order to analyze the deformation heterogeneities under uniaxial tension, the inverse pole figure (IPF) map, kernel average misorientation (KAM) map, Taylor factor (TF) map, grain boundaries (GBs), Σ3 twin boundaries (TBs), and melt pool boundaries (MPBs) developed in deformed SLM specimens were analyzed at different strain levels. The effect of microstructural factors on the deformation heterogeneities of SLM specimens was explained by the evolution of KAM, GBs, MPBs, and Σ3 TBs. The initial microstructures of the SLM specimens significantly influenced the generation and propagation of cracks under uniaxial tension.

Self-heating behavior during cyclic loadings of 316L stainless steel specimens manufactured or repaired by Directed Energy Deposition

The purpose of this article is to assess a self-heating testing method for the characterization of fatigue properties of single-track thickness additively manufactured specimens. It also evaluates the impact of the microstructure orientation with respect to the loading direction on the dissipative behavior and the initiation of microcracks. The 316L stainless steel specimens under scrutiny were manufactured by Directed Energy Deposition in two configurations: (i) fully printed specimens (2 orientations) and (ii) repaired specimens. The paper first presents a morphologic and crystallographic texture analysis and second, a series of self-heating tests under cyclic loading. The microstructural analysis revealed elongated grains with their sizes, shapes and preferred orientations controlled by process parameters. The self-heating measurements under cyclic tensile loading proved that the dissipation estimation through infrared measurements can be performed on small scale, thin specimens. The self-heating curves could successfully be represented by the Munier model. Moreover, several links between the printing parameters and self-heating results could be established. For example, a smaller vertical increment between successively deposited layers leads to higher mean endurance limits in all configurations. Repaired specimens had a lower mean endurance limit when compared with fully printed or conventionally manufactured substrate specimens. Finally, anisotropy was highlighted during these cyclic tests: specimens loaded orthogonally to the printing direction (PD) showed higher fatigue limits when compared with the ones tested along the PD. Additionally, post-mortem observations revealed characteristic microcracking patterns initiated during the self-heating experiments. Loading along the printing direction induced a classical dominating crack, whereas orthogonal loading generated a network of microcracks along the printing direction. This suggests that the damage, such as void opening, where concentrated at the interlayers. Additionally these damage patterns can be correlated with patterns of plasticity at the grains scale observed in a previous study.

Additive manufacturing of fracture fixation implants: Design, material characterization, biomechanical modeling and experimentation

Recent advancements in additive manufacturing (AM) have motivated researchers to consider this fabrication technique as a solution for challenges in patient-specific orthopaedic needs. Although there is an increasing trend in the applications of AM in medical fields, there is a critical need to understand the biomechanical performance of AM implants. In particular, design opportunities, anisotropic material properties and resulting stability of AM implant constructs for large bone defects such as osteosarcoma, comminuted fractures and infections are unexplored. This study aims to evaluate metal AM for complex fracture fixation using both computational and experimental methods. In addition, this research highlights the role of AM in the entire workflow to fabricate metal AM fixation plates for treatment of comminuted proximal humerus fractures. A new AM-centric patient-specific implant design for reducing common postoperative complications such as varus collapse and screw cutout is investigated. Biocompatible 316L stainless steel specimens processed in laser-powder bed fusion (L-PBF) is subjected to tensile testing and post-hoc microhardness to obtain orthotropic material properties of the AM implants. Subsequently, risk of screw cut-out is evaluated using finite element modelling (FEM) of AM implant-bone constructs. Parallel experiments included synthetic bones that are evaluated using a 3D motion capture system. The biomechanical tests are analyzed to quantify the medial fracture gap displacement among study groups subject to different loading conditions. The outcomes of this study suggest that the proposed AM-centric fixation plate design reduces average varus collapse (i.e. medial fracture gap displacement) by 47.2 % and risk of screw cut-out by 14.6 % when compared to the conventional plate design. Findings from this study can be extended to other patient anatomy, loading conditions, and AM processes.

An investigation of the corrosion behavior of 316L stainless steel fabricated by SLM and SPS techniques

The microstructure and corrosion characteristics of 316L stainless steel specimens fabricated by spark plasma sintering (SPS) and selective laser melting (SLM) techniques were investigated. SPS specimens have a relatively low density compared with SLM specimens. Microstructural analysis showed that SPS specimens consisted of fully and partially sintered regions and contained initial pores, which are located at the powder boundaries (PBs). SPS specimens have relatively low density and high kernel average misorientation (KAM) values near the interface of the pores and the matrix. In the case of SLM specimens, the scanning strategy did not affect the spatial distribution of the KAM, while it affected the extent of surface defects. Electrochemical analysis revealed that the corrosion behavior of SPS specimens was similar to localized corrosion, and weak levels of passive protection were observed in the anodic response curves due to the initial surface pores. SLM specimens with well-connected scan tracks showed passivation behavior similar to that of as-rolled specimens. Moreover, due to a homogeneous microstructure distribution and lower porosity, in NaCl solutions the SLM specimens showed corrosion performance that was significantly enhanced compared with that of SPS specimens. Cross-sectional analysis of corroded SPS specimens revealed that the configuration and connectivity of pores (or small cavity) affected the morphology of open spaces under the corroded surface.

The effects of hot isostatic pressing (HIP) and solubilization heat treatment on the density, mechanical properties, and microstructure of austenitic stainless steel parts produced by selective laser melting (SLM)

Despite ongoing optimization, selective laser melting (SLM) technology still cannot guarantee the production of completely defect-free components. This work investigates hot isostatic pressing (HIP) in a nitrogen environment and solubilization heat treatment as methods for improving the quality of 316L stainless steel components produced by SLM. The characteristics of HIP-treated specimens are firstly correlated with the initial density of samples obtained with different SLM process parameters, showing that HIP is effective at eliminating pores in components with high initial density (above 99%) but not in those with low initial density (approximately 94%). Subsequently, the mechanical properties and microstructure of 316L stainless steel specimens produced by SLM are examined in the as-built state and after various post-process conditions including solubilization heat treatment and HIP at pressures from 50 to 2000 bar. The observed effects of post-processing on the porosity and microstructure of each specimen are consistent with hardness and tensile test results, with the benefits and limitations of HIP clarified for future implementation.

Influence of heat treatment temperature on the microstructural, mechanical, and wear behavior of 316L stainless steel fabricated by laser powder bed additive manufacturing

A metal component fabricated by additive manufacturing (AM) is generally required to be heat treated to enhance microstructural and mechanical aspects. This present study aims to contribute to the literature in understanding the effect of heat treatment and various heat treatment temperatures on as-built components fabricated by AM. In this study, various heat treatment temperatures were applied to 316L stainless steel specimens produced by selective laser melting (SLM) and the effects on the microstructure, microhardness, XRD response, porosity, and wear behavior were investigated. The microhardness, XRD, and wear response of SLM 316L were compared with those of wrought 316L. The results illustrate that the heat treatment temperature has a substantial effect on the evolution of microstructure, XRD response, and porosity. Our results also support the argument that the effect of porosity on wear behavior is more dominant than the effect on microhardness. It should also be noted that the wrought 316L stainless steel specimen shows much better wear resistance than SLM 316L specimen.

In-Situ Defect Detection in Laser Powder Bed Fusion by Using Thermography and Optical Tomography—Comparison to Computed Tomography

Among additive manufacturing (AM) technologies, the laser powder bed fusion (L-PBF) is one of the most important technologies to produce metallic components. The layer-wise build-up of components and the complex process conditions increase the probability of the occurrence of defects. However, due to the iterative nature of its manufacturing process and in contrast to conventional manufacturing technologies such as casting, L-PBF offers unique opportunities for in-situ monitoring. In this study, two cameras were successfully tested simultaneously as a machine manufacturer independent process monitoring setup: a high-frequency infrared camera and a camera for long time exposure, working in the visible and infrared spectrum and equipped with a near infrared filter. An AISI 316L stainless steel specimen with integrated artificial defects has been monitored during the build. The acquired camera data was compared to data obtained by computed tomography. A promising and easy to use examination method for data analysis was developed and correlations between measured signals and defects were identified. Moreover, sources of possible data misinterpretation were specified. Lastly, attempts for automatic data analysis by data integration are presented.

Build Orientation Effects on Mechanical Properties of 316SS Components Produced by Directed Energy Deposition

Laser Directed Energy Deposition (DED) is an additive manufacturing process whereby a laser fuses metal feed stock to the surface of a part. The process can be complete once to form a cladded layer or can be carried out numerous times to build complex three-dimensional structures. One major obstacle to the widescale acceptance of DED technology is the tightly coupled relationship between mechanical performance and process parameters.This study investigated the effects of layer orientation on the mechanical properties of 316L stainless steel (SS) parts fabricated via DED process. Quasi-static tensile tests were conducted on DED 316L SS specimens fabricated in orientation increments of 15 degrees with respect to the loading direction. Specimens were tested in their as-built condition without post process heat treatments. Yield, ultimate tensile strength (UTS), and maximum elongation measurement were taken, contributing to the ultimate finding that: Young’s Modulus is largely unaffected, yield stress may vary by 73 MPa corresponding to a 30 degree change in layer orientation, ultimate tensile strength does not show a strong statistical outcome based on layer orientation, and ductility shows a strong positive correlation to increases in layer angle with respect to axis of tensile stress.

Nondestructive quantitative characterisation of material phases in metal additive manufacturing using multi-energy synchrotron X-rays microtomography

Metal additive manufacturing (MAM) has found emerging application in the aerospace, biomedical and defence industries. However, the lack of reproducibility and quality issues are regarded as the two main drawbacks to AM. Both of these aspects are affected by the distribution of defects (e.g. pores) in the AM part. Computed tomography (CT) allows the determination of defect sizes, shapes and locations, which are all important aspects for the mechanical properties of the final part. In this paper, data-constrained modelling (DCM) with multi-energy synchrotron X-rays is employed to characterise the distribution of defects in 316L stainless steel specimens manufactured with laser metal deposition (LMD). It is shown that DCM offers a more reliable method to the determination of defect levels when compared to traditional segmentation techniques through the calculation of multiple volume fractions inside a voxel, i.e. by providing sub-voxel information. The results indicate that the samples are dominated by a high number of small light constituents (including pores) that would not be detected under the voxel size in the majority of studies reported in the literature using conventional thresholding methods.

Additive/subtractive hybrid manufacturing of 316L stainless steel powder: Densification, microhardness and residual stress

316L stainless steel specimens were manufactured by additive/subtractive hybrid manufacturing (ASHM). The densification level, mi-crostructure, microhardness and residual stress characterization in the different zones of the part were investigated. The density was determined by the Archimedes method, and the density measurement was divided into three regions, namely, the bottom, middle and top zones. The results show that the middle zone has a much smoother melting surface and that a relative density of nearly 100 % was achieved for the part in this study. The hardness profiles at room temperature, along the width and height directions of the cross-section of the top and bottom zones, were also studied. The residual stress was evaluated by X-ray diffraction (XRD) for the 316L SS specimen fabricated by the ASHM process, and it was compared with a specimen manufactured by the additive laser directed energy deposition (DED) process. The results show that the top and bottom zones exhibit tensile stress, and compressive stress occurs in the middle area. Moreover, the residual stress of ASHM shows a slightly smaller trend than that of the simplex DED due to the stress relaxation of the subsequent subtractive milling. These results may offer guidance for ASHM-fabricated 316L parts.

Directional Crystallization of 316L Stainless Steel Specimens by Direct Laser Deposition

Advantages of direct laser deposition of metal providing the directional crystallization of 316L stainless steel specimens are shown. It is found that direct laser deposition of metal offers exciting possibilities for obtaining monocrystalline structures by directional crystallization of the deposited metal. The metal structure of the obtained specimens is represented by dendritic crystals oriented toward the thermal center. The cross-section view of the structure, formed by the crystal axes of the first order, is of the form of honeycombs. The thickness of the applied layer (200 µm) in the central part of the specimen does not prevent the development of the dendrite structure and it contributes to the formation of the monocrystalline structures (having no pores and cracks). It was stated that the growth orientation of dendritic crystals is driven by the thermal center determined by the position of the laser spot. A technique of manufacturing stainless steel products by direct laser deposition using the developed algorithm which allowed connecting the movement of the robotic center and specimen, laser switching, and feeding of the gas-powder mixture was presented. The possibilities of estimating the inclination angle of dendritic crystals in the metal relative to the symmetry axis of the specimen using a metallographic software package for analysis of digital images of the microstructures, as well as the quality control of the items obtained by the X-ray tomographic quality control, determination of the parameters of the laser spot on the substrate surface using high-speed photography, and estimation of the phase composition of the deposited metal using X-ray diffraction analysis, are shown. In such a case, the roughness of the specimens determined by the size of the powder particles may be removed by mechanical treatment.

A heat energy dissipation approach to elastic-plastic fatigue crack propagation

A heat energy dissipation approach has been formulated to assess the crack propagation rate of mode I fatigue cracks in AISI 304L stainless steel specimens. The proposed experimental approach relies on the heat energy converted from the plastic strain hysteresis energy at the crack tip. Practically, the heat energy is estimated starting from temperature measurements in the vicinity of the crack tip, by means of an infrared camera. The approach evaluates the heat energy dissipated in a properly defined control volume surrounding the crack tip, reminiscent of Neuber’s structural volume, to account for stress gradient effects caused by notches. The investigation performed has revealed that the control volume size varies with fatigue lifetime, in that the longer the fatigue life is, the smaller the control volume size. However, a fixed control volume size proved to correlate well the available crack growth data. The proposed experimental method has also been compared with the theoretical approach, which assumes the plastic strain hysteresis energy evaluated inside the cyclic plastic zone as a crack driving force.