Laser Melted 316L Stainless Steel Research Articles

Enhanced Strength-Ductility Synergy Properties in Selective Laser Melted 316L Stainless Steel by Strengthening Grinding Process.

Selective laser melted (SLM) 316L stainless steel (SS) has been widely employed in the fields of designing and manufacturing components with complex shapes and sizes. However, the low yield strength, low ultimate tensile stress, and low hardness of SLM 316L SS components hinder its further application. In this work, the strengthening grinding process (SGP) was used to enhance the mechanical properties of SLM 316L SS. The microhardness, residual stress, microstructure, and tensile properties of all the samples were analyzed. The results demonstrate that the SGP induced higher compressive residual stress and microhardness, as well as higher tensile properties. The maximum hardness and residual stress reached 354.5 HV and −446 MPa, respectively, indicating that the SGP resulted in a plastic deformation layer over 150 μm. The possible mechanisms have been discussed in further detail. Compared to the untreated sample, the SGP sample shows a significant improvement in yield strength (YS), ultimate tensile stress (UTS), and elongation (EL), increasing 30%, 25.5%, and 99.1%, respectively. This work demonstrates that SGP treatment could be an efficient approach to simultaneously improving the strength and ductility of the SLM 316L SS, which makes it more suitable for engineering applications.

Improving corrosion resistance of selective laser melted 316L stainless steel through ultrasonic severe surface rolling

Selective laser melting (SLM) is one of the common methods of additive manufacturing and herein, ultrasonic severe surface rolling (USSR) is applied to enhance the corrosion resistance of SLM 316L stainless steel (SS) in the 3.5% NaCl solution. USSR produces a thick (∼1 mm) gradient surface layer with a nanostructured, chemically homogeneous, and high-quality surface but a small amount of martensite. The USSR processed 316L SS exhibits significant enhancement in the corrosion resistance compared to the SLM one, as manifested by the greatly improved pitting potential, decreased passive current density, and reduced corrosion current density. The superior corrosion properties stem from the unique gradient structure with a high density of grain boundaries and chemically homogeneous and high-quality surface facilitating the formation of a thicker and more protective passive film that contains larger contents of oxides and hydroxides of Fe and Cr in spite of almost the same total Cr concentration. In addition to the high strength and ductility, the unique gradient structure endows the SLM 316L SS with superior corrosion resistance and long service lifetime in the marine environment, boding well for application to marine equipment and engineering.

Impact of the additive manufacturing process on the high-temperature corrosion of 316L steel in the presence of NaCl–KCl–ZnCl2 molten solar salt

A comparative study of the high-temperature oxidation and corrosion resistance of wrought and additive manufacturing selective laser melting (SLM) 316L stainless steel was performed in presence of molten NaCl–KCl–ZnCl2 salt used for thermal energy storage in the new generation of concentrated solar plants. Isothermal tests at 650 and 700 °C in a dry air atmosphere were conducted to evaluate (i) the effect of the manufacturing process and (ii) the effect of molten salts on the corrosion behavior of the steel by dimensional metrology analysis, scanning electron microscopy and X-ray diffraction characterization. The manufacturing process had a significant influence on the oxidation resistance. The wrought samples had a mass gain after 48 h that was four times greater than that of SLM samples at 650 °C, and 50% greater at 700 °C. The presence of the salt primarily increased the corrosion rate of all the samples and, again, the wrought pieces suffered 13% and 6% more corrosion in terms of metal loss than the SLM ones at 650 and 700 °C, respectively. The presence of more pathways for Cr diffusion associated with a higher number of defects in the microstructure of the SLM sample, as dislocations and grain boundaries, facilitated the formation of Cr2O3 scales in the SLM sample, avoiding the intergranular attack shown in the wrought one.

Effect of laser shock peening on tensile properties and microstructure of selective laser melted 316L stainless steel with different build directions

316L stainless steel tensile samples were fabricated via selective laser melting (SLM), wherein the build directions (BD) deviated from the tensile directions by 0°, 45°, and 90°. The effects of BD and laser shock peening (LSP) on compressive residual stress distribution, microstructure, texture, and tensile properties were investigated. The intensity and volume fraction of strong <110> fibre texture decreased after LSP, and the sample with a build angle of 90° had the weakest intensity and the least volume fraction. A tensile test was conducted at 25 °C temperature to investigate the effects of the BD and LSP on strength and ductility. LSP resulted in a noticeable improvement in the strength of all the samples, but the ductility of the samples with 0° and 45° build angles declined. Additionally, it was found that 316L fabricated via SLM with a build angle of 90° and subjected to LSP had the optimal combination of strength and ductility.

Deformation faulting and dislocation-cell refinement in a selective laser melted 316L stainless steel

The selective laser melted (SLM) 316L stainless steel (316L SS) has shown superior tensile ductility and doubled yield strength compared to its wrought counterpart. The significantly improved yield strength has been attributed to the unique cellular substructures featured by Cr/Mo-segregation and trapped dislocations. The excellent ductility of SLMed 316L SS has been mainly understood from the pronounced deformation twinning, which, however, is just one of the dominant plastic deformation mechanisms in 316L SS. Here instead, we report two other fundamental deformation micro-mechanisms, i.e., deformation faulting and dislocation cell refinement, in the SLM 316L SS. Our results showed that deformation faulting and dislocation cell refinement characterize the whole tensile deformation significantly except for deformation twinning. These newly found mechanisms synergistically dominated the deformation behavior at low strain levels while deformation faulting and twinning play a crucial role at medium and high strain levels. The pre-existing stacking faults (SFs) in cellular substructure is mainly responsible for the significant deformation faulting. At the early deformation stage, these pre-existing SFs provide faulting nuclei for deformation faulting, leading to wide SFs. These wide SFs in different cellular interiors penetrate the cellular boundaries and overlap as the strain increases, resulting in long stacking fault ribbons (SFRs). We attributed the formation of DCs to the measured medium stacking fault energy (SFE) of ∼18 mJ/m2. Upon straining, equiaxed dislocation cells (DCs) were generated by new dislocation cell walls (DCWs) inside the cellular substructure, and refined by either the dissociation of fully developed DCWs or the continuously formed new DCWs. Together with deformation twinning, these two fundamental deformation mechanisms jointly led to a steady strain hardening rate during tension and thus a superior tensile ductility of the SLM 316L SS with high yield strength. These findings provide new insights into the excellent strength-ductility combination of SLMed 316L SS and the experimental basis for crystal plasticity modeling and simulation, as well.

Material removal characteristics of various surface features on selective laser melted 316L stainless steel during electropolishing

This study aims to establish the relationship between material removal characteristics and various surface features on selective laser melted 316L stainless steel after electropolishing (EP). Morphology and roughness were characterized on top, face up, side and face down surfaces of 316L stainless steel samples prepared by selective laser melting (SLM). Three types of un-sintered powders and two types of sintered area were categorized and further investigated based on the differences of morphology, microstructure and location. Material removal mechanism of each surface feature was also studied based on experimental and simulation results. Impacts of these surface features on the reduction of roughness during EP were thus discussed. The study reveals that different microstructures and feature locations lead to diverse material removal behaviors during EP process. Understanding the characteristics of un-sintered powders and sintered area on SLM surfaces is crucial in achieving satisfactory surface finish in EP.

The influence of heat treatment on the microstructure and corrosion behavior of selective laser melted 316L stainless steel in Ringer’s solution

The aim of this study is to investigate the effect of heat treatment on the microstructure and corrosion behavior of 316L stainless steel (SS) in Ringer’s solution. The corrosion performance of the as-received, heat-treated at 450 °C (HT1), and 1050 °C (HT2) SS alloys manufactured by selective laser melting was evaluated. The heat treatment at 450 °C and 1050 °C relieved about 38% and 62% of the compressive residual stress derived from the selective laser melting (SLM) process, respectively. The charge transfer resistance of the as-received, HT1, and HT2 was 102, 140, and 12 kΩ cm2 in the Ringer’s solution, respectively. This severe reduction in corrosion resistance for the HT2 sample can be attributed to the high possibility of harmful phases such as MnS forming during heat treatment and possibly depletion of chromium content in the inclusions compared to the alloy matrix.

Constructing the high temperature efficiency and instability maps of selective laser melted 316L stainless steel through artificial neural network modeling

The present work focuses on the high temperature flow behavior of additively manufactured 316 L stainless steel. Toward this end, a series of hot compression tests were conducted in the temperature range of 700–1000 °C under the strain rates of 0.001, 0.01 and 0.1s-1. The high temperature flow curves were accompanied with considerable fraction of strain softening at various thermomechanical conditions. The strain rate sensitivity (m) maps were constructed which comprised the negative and positive m-values from −0.01 to 0.14. Accordingly, the conventional phenomenological models were un-capable of precise predicting the level and trend of the flow stress. In this respect, a single layer Artificial Neural Network (ANN) model was employed for assessment of flow behavior at elevated temperature. The training of the feedforward model was performed through a backpropagation algorithm. The statistical factors were employed to assess the model's reliability. Flow curves were also predicted at thermomechanical conditions which were completely different from those utilized for construction of the model. Interestingly, the developed ANN model not only efficiently tracked the strain hardening portion, but also well predicted the flow softening behavior of the material.

Mechanical Properties and Constitutive Model of Selective Laser Melting 316L Stainless Steel at Different Scanning Speeds

To investigate the effects of different scanning speeds on the mechanical properties of selective laser melting 316L stainless steel (SLM 316L SS), process specimens with scanning speeds at 350 mm/s, 650 mm/s, 950 mm/s, and 1250 mm/s were prepared using the SLM technique in this paper. The quasistatic and dynamic compressive mechanical properties of the four specimens were tested by electrohydraulic servo experimental machine and Hopkinson compression bar experimental device, and the mechanical differences of the specimens were analyzed by microscopic observation. Finally, the modified Johnson-Cook (J-C) model was adopted to describe the dynamic mechanical properties of SLM 316L SS. Results showed that the four process steel specimens exhibited typical viscoplastic characteristics and significant strain rate strengthening effects in the mechanical property tests. Moreover, the scanning speed significantly affects the internal defects and melt pool characteristics of the SLM 316L specimens, and the yield strength decreases significantly with the disappearance of the melt pool characteristics. Finally, the modified J-C model can better describe the mechanical behavior of SLM 316L SS material more accurately. This study can provide theoretical references for improving the fabrication process of SLM 316L SS to broaden the practical application of this material.

Effect of cold deformation on corrosion behavior of selective laser melted 316L stainless steel bipolar plates in a simulated environment for proton exchange membrane fuel cells

This work studied the influence of cold deformation on the microstructure evolution and corrosion behavior of selective laser melted (SLMed) 316L stainless steel in a simulated cathode environment for proton exchange membrane fuel cells (PEMFCs). Results indicated that at < 50% deformation levels, the corrosion resistance was improved with an increase in deformation level, which was attributed to the deformation twins, refining sub-grains, and the formation of stable passive films. At 70% deformation level, martensite nucleation decreased the stability of the passive film and consequently reduced the corrosion resistance.

On the exceptional stress corrosion cracking susceptibility of selective laser melted 316L stainless steel under the individual effect of surface residual stresses

SCC and electrochemical behaviour of selective laser melted 316L stainless steel was studied under the combined effect of surface residual stresses (RS) and energy input variation. A significant correlation was found between columnar subgrains width and pore size with the electrochemical response and SCC susceptibility. There existed a critical pore size affecting local corrosion behaviour which decreased with an increase in RS magnitude. The critical RS needed for SCC occurrence was lower compared to wrought counterpart and the SCC mechanism was hypothesized as the synergistic effect of exceptional selective corrosion attack at subgrains and mechanical rupture of subgrain boundaries.

An experimental investigation into influences of build orientation and specimen thickness on quasi-static and dynamic mechanical responses of Selective Laser Melting 316L Stainless Steel

Additive Layer Manufacturing (ALM) processes like Selective Laser Melting (SLM) enable the conception of complex designs with a high precision and equal or enhanced mechanical properties compared to Conventionally Manufactured (CM) structures. Nevertheless, this process, which consists in melting metallic powders layer by layer with a laser beam, greatly influences the microstructure and therefore the mechanical properties. While some studies have considered the effects of the thickness and/or the building direction of 316L Stainless Steel (SS) specimens produced by SLM on the quasi-static mechanical behavior, the strain rate effect for crash or impact applications on these two parameters has not been fully investigated. To complete the actual knowledge, the present work proposes to analyze the mechanical behavior of 316L SS tensile specimens produced by SLM with different build orientations (0°, 45° and 90°) and thicknesses (0.5, 0.75, 1 mm) and submitted to dynamic loadings at various strain rates up to 103 s−1. In addition, the microstructure and the fracture surfaces are analyzed to give a more detailed comprehension of the mechanical tests. It results that the SLM 316L SS achieves better Yield Stress (YS), similar Ultimate Tensile Stress (UTS) and equal or lower failure strain compared to the CM material. This is mainly a result of microstructure refinement. Anisotropy is observed at the macroscopic level with higher tensile stress and lower failure strain for horizontal specimens, which is explained by the different shapes, orientation and size of the grains at the microscopic level. The mechanical properties greatly decrease as the thickness reduces from 1 to 0.5 mm, by 14% for the YS and 16% for the UTS for a quasi-static loading. A minimum thickness of 0.75 mm is advised to at least recover the mechanical properties of the CM 316L SS. A positive strain rate sensitivity, higher than the CM material, is observed for all configurations, with the exception of 0.5 mm thickness. For strain rates ranging from to 10−3 to 103 s−1, there is an increase of 20% of the UTS. The material anisotropy is not affected by the strain rate sensitivity whereas the latter increases with the thickness.

Process optimization of selective laser melting 316L stainless steel by a data-driven nonlinear system

Process optimization of selective laser melting 316L stainless steel by a data-driven nonlinear system

State of the Art of Selective Laser Melted 316L Stainless Steel: Process, Microstructure, and Mechanical Properties

State of the Art of Selective Laser Melted 316L Stainless Steel: Process, Microstructure, and Mechanical Properties

A Multi-Scale Experimental Investigation for Fatigue Limit and Fatigue Crack Initiation Behavior of Selective Laser Melted 316l Stainless Steel

A Multi-Scale Experimental Investigation for Fatigue Limit and Fatigue Crack Initiation Behavior of Selective Laser Melted 316l Stainless Steel

Effect of Laser Scanning Angle on Shear Slip Behavior Along Melt Track of Selective Laser Melted 316l Stainless Steel During Tensile Failure

Effect of Laser Scanning Angle on Shear Slip Behavior Along Melt Track of Selective Laser Melted 316l Stainless Steel During Tensile Failure

Tuning strength-ductility combination on selective laser melted 316L stainless steel through gradient heterogeneous structure

We report a strategy of gradient heterogeneous structure (GHS) architecture to achieve a superior strength-ductility combination in additively manufactured (AM) 316L stainless steel prepared by selective laser melting (SLM) and ultrasonic severe surface rolling (USSR). The GHS is a microstructure gradient system, which is composed of a gradient structured outer layer and a hierarchically heterostructured interlayer. The gradient structured outer layer exhibits a variation in grain size, phases, dislocations, twins, and cellular structure with depth and has a large thickness of approximately 1000 µm. The average grain size is approximately 93.9 nm in the topmost surface layer and increases with increasing depth from the treated surface. The hierarchically heterostructured interlayer has the heterogeneous grain structure, cellular structure, high-density dislocations, and nanoprecipitates, with length scales spanning several orders of magnitude. Due to the high microstructural heterogeneity arising from the combination of two types of heterogeneous structures, the GHS strengthens the synergistic effect, and thus results 316L stainless steel with a superior strength and high ductility. Furthermore, the mechanical properties of the GHS 316L stainless steel can be tuned by tailoring the microstructure: the larger the volume fraction of the gradient structured outer layer is, the higher the strength, but the lower the ductility. Molecular dynamics simulations indicate that deformation twinning, in addition to dislocation slipping, is an important deformation mechanism for nanocrystalline 316L samples at a large strain and thus plays an important role in maintaining strain hardening and ductility. We also discuss the potential application of USSR to AM parts to improve their mechanical properties and surface integrity. Our work demonstrates the potential of the proposed highly heterogeneous microstructure design strategy to prepare high-performance AM parts.

Strengthening mechanisms in selective laser melted 316L stainless steel

The microstructure and chemical composition of a 316L stainless steel prepared by selective laser melting have been characterized using electron backscatter diffraction, transmission electron microscopy and atom probe tomography (APT). A multi-scale microstructure in the 316L stainless steel was observed in the as-built samples, consisting of equiaxed and columnar grains, dislocation cell blocks, dislocation cells, individual dislocations and nano-sized particles. The misorientations across dislocation cells were determined based on local crystallographic orientation measurements using a Kikuchi pattern method. The dislocation cells have very small misorientation angles with an average angle of 0.9°, and are arranged to form dislocation cell-blocks with cell-block boundary misorientation angles generally larger than 2°. APT data reveal that alloying elements are evenly distributed in the matrix as well as a high nitrogen content in the as-built material. Based on quantification of the microstructural parameters, good agreement is achieved between the yield strength as calculated from a linear sum of different strengthening contributions, and the experimentally measured value, with significant contributions from dislocation strengthening and solid solution strengthening effects.

Effect of High Laser Energy Density on Selective Laser Melted 316L Stainless Steel: Analysis on Metallurgical and Mechanical Properties and Comparison with Wrought 316L Stainless Steel.

The austenitic 316L stainless steel (SS) is used extensively for marine applications as well as in construction, processing, and petrochemical industries due to its outstanding corrosion resistance properties. This study investigates the density, microhardness, and microstructural development of 316L SS samples fabricated by selective laser melting (SLM) under high laser energy densities. The selective laser melted (SLMed) specimens were fabricated under high laser energy densities (500, 400, and 333.33 J/mm3) and their metallurgical and mechanical properties were compared with the wrought specimen. SLMed 316L SS showed excellent printability, thereby enabling the fabrication of parts near full density. The porosity content present in the SLMed specimens was determined by both the image analysis method and Archimedes method. SLMed 316L specimens fabricated by the SLM process allowed observation of a microhardness of 253 HV1.0 and achieved relative density up to 98.022%. Microstructural analysis using optical microscopy and phase composition analysis by X-ray diffraction (XRD) has been performed. Residual stresses were observed using the XRD method, and compressive stress (-68.9 MPa) was noticed in the as-printed specimen along the surface of the build direction. The microstructure of the as-built SLMed specimens consisted of a single-phase face-centered cubic solid solution with fine cellular and columnar grains along the build direction. The SLMed specimens seemed to yield better results than the wrought counterpart. IRB approval and Clinical Trial Registration Number are not applicable for this current work.

Effect of Annealing Temperature and Strain Rate on Mechanical Property of a Selective Laser Melted 316L Stainless Steel

Effect of Annealing Temperature and Strain Rate on Mechanical Property of a Selective Laser Melted 316L Stainless Steel