Selective Laser Melting 316L Stainless Steel Research Articles

Recovery-Assisted Abnormal Grain Evolution of Selective Laser-Melted 316L Stainless Steel at Intermediate Temperatures

Recovery-Assisted Abnormal Grain Evolution of Selective Laser-Melted 316L Stainless Steel at Intermediate Temperatures

Damage behavior of additive manufacturing 316L stainless steel irradiated with Fe ions

The irradiation damage behavior of 316L stainless steel (SS) fabricated using selective laser melting (SLM) and traditional manufacturing (TM) was investigated in terms of microstructural changes in hardness and swelling at RT under irradiation with 3 MeV Fe ions at doses of 0.3 to 6 dpa. At a dose of 6 dpa, the swelling heights of the SLM and TM samples were 39.1 nm and 30.7 nm, respectively, corresponding to swelling rates of 3.0 % and 2.4 %. The irradiated SLM 316L SS exhibited a lower degree of hardening at continuous fluences than the TM 316L SS. At a dose of 6 dpa, the hardening rates of the SLM and TM 316L SS specimens were 102.1 % and 84.1 %, respectively. Furthermore, TEM results showed that the density of dislocation loops in the TM 316L SS was 8.36 × 1023/m3 while that in the SLM 316L SS was 6.15 × 1023/m3. The average sizes of the dislocation loops were 1.35 and 1.22 nm, respectively. According to calculations using the dispersed barrier hardening (DBH) model, the dislocation loops were the primary factor influencing the irradiation hardening behavior. Under Fe-ion irradiation at RT, SLM 316L SS showed superior resistance to irradiation hardening and swelling compared with TM 316L SS.

A new phenomenon in a selective laser melted 316L stainless steel with a wider and higher laser energy density

Defects, boundaries, textures, tensile properties, fractographies, sub-grains, nano-oxide particles, and deformation twins in selective laser melted 316L stainless steel (SLM316LSS) were systematically characterized as functions of laser energy density. The defects, boundaries, textures of the as-built SLM316LSS were shown to depend largely on the applied laser power. The porosity of the SLM316LSS significantly decreased with the increase laser power from 70 to 140 W (corresponding to the laser energy density from 116.7 to 233.3 J/mm3), while exhibiting a slight increase in the laser power range of 140–280 W (the laser energy density of 233.3–466.7 J/mm3). Additionally, the SLM316LSS specimen at a laser power of 140 W reached the hardness of ∼252 HV, the yield strength of ∼605 MPa and the ultimate tensile strength of ∼710 MPa, along with the elongation to fracture of ∼32.5%, respectively, due to the lowest defect concentration and the presence of twin boundaries and textures, combined with SiO2 nano-oxide particles and deformation twinning. However, the crack appeared in the SLM316LSS within the laser power range from 210 to 280 W (the laser energy density from 350.0 to 466.7 J/mm3), and the mean crystallite size was more than 50 μm, which insignificantly affected the mechanical properties of the steel compared with a laser power of 140 W.

Fatigue behavior of selective laser melted 316L stainless steel: Experiments and modeling

Selective laser melting (SLM) is extensively applied in fabricating 316L stainless steel structures. In this study, a series of mechanical tests were conducted on SLM 316L stainless steel, including tensile testing, high cycle fatigue (HCF) testing at various stress ratios and mean stresses, and low cycle fatigue (LCF) testing at different strain amplitudes. A efficient method of processing CT images was employed to analyze the porosity and fatigue failure behavior of material. Additionally, we proposed an elasto-plastic fatigue damage evolution model takes porosity into account, with relevant parameters determined through experiments. The predicted life under experimental conditions was calculated using the UMAT and DISP subroutines in ABAQUS software. Results indicate that mean stress significantly reduces fatigue strength, and irregularly shaped pores are the primary cause of material failure and reduction in strength. The predicted life of the elasto-plastic fatigue damage model is in good agreement with the experimental results. These results have important implications for the design and maintenance of SLM 316L stainless steel structures.

Corrosion behavior of different building planes of selective laser melting 316L stainless steel in 0.1 M HCl solution

This study aimed to investigate the impact of building planes on the microstructure and corrosion behaviors of 316L stainless steel (SS) manufactured using selective laser melting (SLM) when subjected to a 0.1 M HCl solution. The investigation of material corrosion behavior involved the utilization of a wide array of surface characterization techniques, as well as various electrochemical measurements. The findings were subjected to a comparison with those obtained from a commercially rolled 316L SS (R 316L SS). The analysis revealed that the XZ and YZ planes displayed a distinctive fish scale melt pool structure, while the XY plane exhibited a columnar melt pool structure. The densities of the low-angle grain boundaries for different building planes were observed as YZ < XY < XZ. SLM 316L SS showing a broader passivation region and a higher breakdown potential compared to the commercially available R 316L SS, especially the XZ and XY planes. Meanwhile, the passive film on the XZ plane was the more compact and had a lower carrier density. The observed factors contributing to these findings could be attributed to the SLM sample containing an increased proportion of O2−/OH− and Cr2O3 in the passive film. This resulted in the formation of a thick and dense passive film, which is particularly evident on the XY and XZ planes. In addition, it was observed that the nucleation mechanism of the passive films remained unchanged following the SLM process.

Improving surface integrity and wear resistance of selective laser melted 316L stainless steel using ultrasonic nanocrystal surface modification

In this study, ultrasonic nanocrystal surface modification (UNSM) was employed to process 316L stainless steel fabricated using selective laser melting (SLM). The surface integrity, microstructure evolution, and mechanical behavior of this alloy before and after UNSM treatment were systematically characterized. The results show that the surface integrity and wear resistance of the alloy has been significantly improved due to the synergistic effect of grain refinement, work hardening, and beneficial compressive residual stresses that resulted from UNSM processing. The effect of the gradient structure and residual stress introduced by UNSM processing on the strengthening mechanism and its effect on friction and wear behavior have been discussed. The improved surface finish, the grain refinement layer, and the presence of compressive residual stress of the UNSM SLM 316L stainless steel alloy successfully demonstrated UNSM to be a promising post-processing surface treatment technology to enhance the fatigue performance of additively manufactured metallic materials.

Relationship among grain morphology, texture and sliding wear resistance of ZrB2/316L SS composites processed by SLM

Pure 316L stainless steel and 5 wt% ZrB2-reinforced 316L stainless steel were prepared by selective laser melting (SLM) in this paper. As compared to 316L SS, ZrB2/316L SS displays smaller columnar grains as well as less anisotropic microstructure. EBSD results show that the SLM 316L stainless steel exhibits a maximum texture index of 13 for the brass-like texture, while the ZrB2/316L SS composite exhibits a maximum texture index of 7 for the cubic texture. The hardness of the ZrB2/316L SS composite is significantly improved, reaching 415HV, which is much higher than the hardness of 316L SS (215HV). In addition, the wear resistance of ZrB2/316L SS increases, implying broadened applications of the stainless steel composites.

Experimental and numerical study of 3D printed steel plates stiffened by sinusoidal waves for enhanced stability

This paper reports the first experimental tests on additively manufactured steel plates stiffened by sinusoidal waves, followed by a detailed numerical modelling programme to account for the imperfection sensitivity of these shapes. This innovative study focuses on stiffened plates (plates simply supported along both longitudinal edges), which were experimentally tested employing square hollow section (SHS) stub columns. The experimental study comprised of 5 tensile coupon tests, 15 stub column tests and measurements on geometric accuracy and residual stresses on 316L stainless steel samples made by selective laser melting (SLM). Validating against the experimental results, numerical models accounting for realistic manufactured dimensions and geometric imperfections have been developed. The validated numerical models were used to perform a parametric study considering a wider range of geometries and plate slendernesses. Based on the results of the parametric study, the efficiency of this stiffening method across a range of plate slendernesses has been assessed and optimum wave patterns identified, which were shown to be able to enhance buckling strength up to 50% compared to the flat counterparts. Provisional design equations have also been proposed for SLM 316L stainless steel plates stiffened by the optimum stiffening patterns selected.

A Novel Method for Early Fatigue Damage Diagnosis in 316L Stainless Steel Formed by Selective Laser Melting Technology.

Early fatigue damage is an important factor affecting the service safety of 316L stainless steel parts formed by selective laser melting (SLM) technology. Nonlinear ultrasonic testing for early fatigue damage in SLM 316L stainless steel specimens was carried out. A new method for evaluation of early fatigue damage based on nonlinear ultrasonic testing was proposed. Empirical mode decomposition (EMD) was applied to the unsteady ultrasonic testing signal, and the signal was decomposed into multiple intrinsic mode functions (IMFs) that meet certain conditions; then, the specific IMF (ESI) containing the effective fatigue damage information was extracted. Lastly, fast Fourier transform (FFT) was applied to the specific IMF signal to obtain the required information to evaluate the damage in the measured part caused by fatigue. The results of nonlinear ultrasonic testing agreed well with transmission electron microscope experimental analysis and theoretical model of acoustic nonlinearity caused by dislocations. The change in nonlinear ultrasonic testing results reflected the generation and evolution of dislocation structure during the low-cycle fatigue regime of the SLM 316L stainless steel specimen and revealed the early fatigue damage mechanism of this metal part. Compared with the classical FFT method, the EMD-ESI-FFT method is more sensitive in identifying the early damage in SLM 316L stainless parts induced by fatigue loading, which is equivalent to improving the early fatigue damage identification and diagnosis ability and can better ensure the service safety of important metal parts.

Microstructure and mechanical properties of SS316L prepared by selective laser melting

In this work, 316L stainless steel samples were prepared by selective laser melting (SLM) and the effects of the bulk laser energy density on the resulting density and mechanical properties were evaluated. The results show that the density and mechanical properties of SLM 316L stainless steel first improved and then deteriorated with increasing bulk laser energy density. When the bulk laser energy density was 70.2 J/mm3, the obtained tensile strength and yield strength vertical to the building direction were 720 and 546 MPa, respectively, which surpass the design requirements for forged 316L stainless steel. The influence of the microstructure on the mechanical properties of SLM 316L stainless steel is also discussed. The purpose of this research was to provide experimental data and a theoretical basis for high-performance 316L stainless steel fabricated by SLM.

Anisotropic tension-compression asymmetry in SLM 316L stainless steel

Selective laser melted (SLM) metallic materials commonly possess typical columnar grains growing epitaxially along the building direction and substantial microscale internal stresses. However, the coupling impact of columnar grains and residual stresses on the mechanical behavior of SLM materials needs further research. Here, by combining experiments and crystal plasticity finite element (CPFE) simulations, tension and compression tests in different directions are performed to investigate the anisotropic tension-compression asymmetry of SLM 316 L stainless steel. The comparison of the data and model predictions shows that the simulated true stress-strain behavior under uniaxial tension and uniaxial compression can well predict the experimental trends qualitatively. The CPFE simulation results further reveal that the tension-compression asymmetry in the building direction results from directional internal stresses. In addition, the tension-compression asymmetry perpendicular to the building direction is correlated with the microstructure and internal stresses. It is inferred that the heterogeneous grain distribution induces plastic anisotropy in the stress level of tensile and compressive behaviors. This work establishes a comprehensive intrinsic connection between materials and properties of SLM stainless steel.

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.

Twinning behavior in deformation of SLM 316L stainless steel

The present work investigates the microstructures, mechanical properties and deformation behaviors of the 316L stainless steel fabricated by selective laser melting (SLM). The initial dislocation density of as-SLM and conventional samples is measured using x-ray diffraction (XRD). It is found that the high dislocation density is obtained by SLM, contributing to the enhancement of yield strength. Twinning has occurred at the early stage of SLM deformation, which undergoes obvious grain rotation and twinning development during the tensile tests. However, only a few transformed martensite is presented in the deformed samples, and no significant grain orientation changes are observed.

Comparison of the Properties of Additively Manufactured 316L Stainless Steel for Orthopedic Applications: A Review

Owing to the low cost, ease of fabricability, good mechanical properties, corrosion resistance and biocompatibility of the 316L stainless steel (SS), this material is considered a suitable choice for orthopedic applications. Based on its properties and large utilization in orthopedics, this review focuses on the importance of additively manufactured (AM) 316L stainless steel. Owing to the large flexibility of the additive manufacturing process, the microstructure of the 316L SS can be easily tuned to modify the mechanical, corrosion and biological properties. To elucidate the benefits of additively manufactured 316L stainless steel, the properties of the selective laser melted (SLM) 316L stainless steel and wrought 316L stainless steel are compared. Particularly, the unique features of the SLM 316L stainless steel have been discussed in detail. The existing challenges associated with the additive manufacturing processes and implications of their widespread application are also highlighted. A brief overview of the biological properties and reactions sequence of the host immune system, i.e. tissue response, the activation of acute and chronic inflammatory processes and immunological reactions, is also provided to understand the reasons for implant failure or rejection by the body.

The Long-term Corrosion Behaviors of SLM 316L Stainless Steel Immersed in Artificial Saliva

The Selective Laser Melting (SLM) is an effective method to produce highly corrosion resistant Oral Alveolar Implants (OAI) by changing the scanning speed. Nevertheless, the long-term corrosion behavior of SLM 316L orthodontic bracket alloys in artificial saliva is overlooked in previous studies. In this study, 316L SS samples fabricated by SLM at different scanning speeds (800 mm/s, 1200 mm/s, and 1600 mm/s) were conducted a long-term (90 days) immersion test by being put into artificial saliva with three different pH values (pH 1.5, pH 4, and pH 6). The morphology, XRD and especially corrosion behavior of the samples fabricated by SLM and forging were investigated. The XRD results exhibit that high-speed scanning accelerates the cooling rate and inhibits the δ → γ phase transformation proceeding. Corrosion results show that the corrosion attack induced by pre-existing pores will expand through the crevices and break down the surface structure around the pores, leading to corrosion damage area to expand to several times of the original pore area. Besides, the mean corrosion rate of the sample at a scanning speed of 800 mm/s shows better corrosion resistance in artificial saliva of pH 4 and pH 6. Compared with wrought 316L SS samples, the SLM 316L SS samples had an obvious difference in the performance of corrosion rate and corrosion behaviors.

Three-Body Abrasion-Corrosion Behavior of As-Printed and Solution-Annealed Additively Manufactured 316L Stainless Steel

Selective laser melting (SLM) or powder bed fusion is a type of additive manufacturing technology with applications in, e.g., the orthopedics, energy, and aerospace industries. Several studies investigated the localized corrosion behavior of SLM-fabricated Type 316L (UNS S31603) stainless steel. However, little is known about the effects of tribocorrosive conditions on the response of stainless steels fabricated by SLM. In this study, the effects of third-body abrasive particles on the tribo-electrochemical behavior of SLM 316L stainless steel produced by SLM were investigated and compared with wrought counterparts (including UNS S31703, 317W) in 0.6 M NaCl. It was found that the presence of Mo played a more decisive role in the tribocorrosion behavior than the manufacturing method, i.e., 317W revealed the best tribocorrosion behavior vis-a-vis wrought 316L and the SLM-fabricated specimens. The improved tribocorrosion behavior contrasted with the much higher breakdown potential of the SLM-fabricated samples. Nano-scale secondary ion mass spectroscopy was used to investigate the effects of Mo on passivity. The implications of passivity and tribocorrosion behavior are discussed.

Evolution of helium bubbles in SLM 316L stainless steel irradiated with helium ions at different temperatures

The temperature dependence of helium bubble evolution in 316L stainless steel (SS) fabricated by selective laser melting (SLM) has been studied by performing 500 keV helium ion irradiation at temperatures between 350 and 900 °C. The number density of helium bubbles decreased steadily whereas the bubble diameter gradually increased with increasing temperature. The material swelling caused by helium bubbles had no obvious change at the temperature below 800 °C. However, at the maximum temperature of 900 °C, the swelling sharply increased to 0.17% mainly due to the rapidly growing bubble diameter. In particular, heterogeneous nucleation of bubbles occurred in the vicinity of dislocation structures at high temperatures of 700–900 °C, and the formed helium bubbles rapidly grew at the oxide-matrix interface at 900 °C. It was confirmed various intrinsic structures (dislocation structures or oxide particles) differently affected the bubble evolution process. The apparent activation energy (Eact) of helium bubbles determined by the Arrhenius model were used to elucidate the most probable helium bubble evolution mechanisms at various temperatures, as it is an experimentally observed activation energy, which contained information about the activity of helium atoms. It was found that the low-temperature regime with low apparent activation energies (ECbact=0.15eV,ERbact=0.04eV) and high-temperature regime with high apparent activation energies (ECbact=1.50eV,ERbact=0.58eV) favored different helium bubble evolution mechanisms. In the low-temperature regime, helium bubble evolution was controlled by the diffusion of helium atoms via a vacancy mechanism in the SLM 316L SS. On the other hand, the migration and coalescence of helium bubbles also occurred in the high-temperature regime which caused the apparent activation energy of density was higher than the theoretical value of helium atoms diffuse via a replacement mechanism.

Effect of Laser Speed and Hatch Spacing on the Corrosion Behavior of 316L Stainless Steel Produced by Selective Laser Melting.

In this work, the corrosion properties of 316L stainless steel (SS) obtained by selective laser melting (SLM) are analyzed. The electrochemical results of samples manufactured with an energy density between 40 and 140 J/mm3 are compared using different hatch distances and laser speeds. The analysis correlates the impact of the microstructure and processing defects of SLM 316L stainless steel on its behavior against corrosion. The optimal manufacturing conditions were selected considering the electrochemical results. Although the samples obtained with an energy density close to 90 J/mm3 show a high resistance to corrosion, their performance depends on the combination of selected parameters, obtaining the best results for an intermediate laser speed and a low hatch distance. These manufacturing conditions produce a higher breakdown potential, a faster repassivation of the steel and reduce the current density on electrochemical test.

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.