Laser Melted 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

Nondestructive evaluation of microstructure and mechanical property of post-processing annealed selective laser melted 316L stainless steel by laser ultrasonics

By using the laser ultrasonics nondestructive technology, combined with the microstructure and mechanical property of post-processing annealed selective laser melted 316L stainless steel characterized by scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and mechanical property testing, selective laser melted 316L stainless steel after post-processing annealing was evaluated. The results show that the frequency domain attenuation coefficient of ultrasonic waves is positively correlated with the average grain size; the time domain attenuation coefficient of ultrasonic waves is negatively correlated with the low angle boundary content. The tensile strength has a good correlation with the time domain attenuation coefficient and wave speed, the correlation coefficients R2 are 0.95 and 0.89 respectively; the yield strength correlates with the time domain attenuation coefficient, the correlation coefficient R2 is 0.76; the elongation has a good correlation with the frequency domain attenuation coefficient and wave speed, the correlation coefficient R2 is 0.90 and 0.86 respectively.

Experimental Study on Parameters Optimization of Selective Laser Melting 316L Stainless Steel Based on the Response Surface Methodology

Abstract The use of 3D printing technology can prepare flexible and varied special-shaped complex structures while realizing resource-saving and cost reduction. For this purpose, a 316L stainless steel sample was formed by selective laser melting technology, and the quality of samples was optimized by the Box–Behnken surface response method. Taking the surface roughness Sa as the response value, a regression analysis of four parameters of selective laser melting (laser power, scanning speed, scanning spacing, and scanning strategy) was designed using design expert software. The results showed that the scanning spacing and scanning speed have the greatest influence on the surface roughness, while the laser power and scanning strategy have no significant influence on the surface roughness. Meanwhile, the established surface roughness response surface model is effective and can be used for quality optimization of 316L structural trim. When the laser power was 185 W, the scanning speed was 615 mm/s, the scanning spacing was 110 μm and concentric scanning strategy was adopted, the surface adhesion powder was less, and the minimum surface roughness Sa was 9.001 μm.

Surface nanocrystallization mechanism of additive manufactured 316L stainless steel via high-energy shot peening

High-energy shot peening (HESP) is an effective way to achieve surface nanocrystallization modification of additive manufactured metal parts, significantly improving their mechanical properties. Yet, there is still a lack of deep understanding of the surface nanocrystallization evolution mechanism of additive manufactured metals. For that, this study focuses on the surface nanocrystallization mechanism of selective laser melted 316L stainless steel treated by HESP treatment. The detailed microstructural characteristics are investigated via optical microscope (OM) and transmission electron microscopy (TEM). The experimental results show that the matrix layer, the low-strain matrix layer, the elongated ultrafine grain layer, the short rod-like ultrafine grain layer, and the equiaxed nanograin layer occurred in sequence from the matrix to the topmost surface. HESP treatment enables the gradient grain refinement from the matrix with a micrometer structure of about 120 mm to the topmost surface with an average grain size of about 25 nm. The evolution of surface nanocrystallization has resulted from the interaction of high-density dislocation, dislocation tangles (DTS), and twins. This study could provide a deep understanding of the surface nanocrystallization evolution mechanism of additive manufactured metal parts treated by HESP.

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.

Study on Laser-Electrochemical Hybrid Polishing of Selective Laser Melted 316L Stainless Steel.

The process of forming metal components through selective laser melting (SLM) results in inherent spherical effects, powder adhesion, and step effects, which collectively lead to surface roughness in stainless steel, limiting its potential for high-end applications. This study utilizes a laser-electrochemical hybrid process to polish SLM-formed 316L stainless steel (SS) and examines the influence of process parameters such as laser power and scanning speed on surface roughness and micro-morphology. A comparative analysis of the surface roughness, microstructure, and wear resistance of SLM-formed 316L SS polished using laser, electrochemical, and laser-electrochemical hybrid processes is presented. The findings demonstrate that, compared to laser and electrochemical polishing alone, the laser-electrochemical hybrid polishing exhibits the most significant improvement in surface roughness and the highest material wear resistance. Additionally, the hybrid process results in a surface free of cracks and only a small number of tiny corrosion holes, making it more suitable for polishing the surface of 316L SS parts manufactured via SLM.

Probabilistic prediction of the size effect on the fatigue strength for variable length specimens of selective laser melted 316L stainless steel

The paper presents an approach for modelling of the size effect in the high-cycle fatigue for an additively manufactured stainless steel. The analyses verify the probability of a critical defect, significantly speeding up fatigue crack initiation. An assessment of the fracture surface failure mechanism shows the presence of a void due to lack of fusion. The selected defect parameters significantly decrease the fatigue strength with increasing specimen length. The implemented linear model of the normalized Crossland stress provides a good fit to the experimental data. The probabilistic approach to the scatter of the selected variables shows the correct prediction of fatigue properties.

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.

Microstructure of Selective Laser Melted 316L under Non-Equilibrium Solidification Conditions

In this study, the microstructural properties of selective laser melted 316L stainless steel were investigated using optical, scanning and transmission electron microscopy as well as X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy. The results show a very fine microstructure with visible melt pool boundaries and austenite as the predominant phase. Extremely fine sub-grain structures can be found within the grains, consisting of colonies of round or elongated cellular structures depending on orientations. Due to the prevailing cooling and solidification conditions, micro-segregations occur, leading to enrichment of the sub-grain boundaries with alloying elements such as silicon, chromium, manganese and molybdenum. The presence of ferrite could be detected in this area using TEM analysis.

Dynamic recrystallization behavior of selective laser melted 316L stainless steel

The present work deals with the hot deformation behavior of selective laser melted 316L stainless steel with an emphasize on the involved recrystallization mechanisms. Toward this end, a series of hot compression test have been conducted in the temperature range of 700–1000 °C under the various strain rates of 0.001, 0.01 and 0.1s−1. The flow pattern analysis through Jonas-Poliak method verifies the occurrence of dynamic recrystallization in whole investigated temperature range. The intensified substructure development and continuous recrystallization is identified as the main softening mechanism in the course of thermomechanical processing. This causes appearance of a broad peak and low softening fraction in corresponding flow curves. The cellular substructure in the as-printed state facilitates low-angle into high-angle boundaries conversion, which provide a proper condition for the occurrence of continuous mechanisms and increases the rate of recrystallization. However, detailed investigation of the sub-boundary maps indicates the contribution of discontinuous and geometric dynamic recrystallization mechanisms. Dynamic recrystallization causes substantial refinement; and the percentage of the grain holding size lower than 10 μm increases from 2 % in printed microstructure to 16.8 % and 24 % in the microstructure deformed at 700 and 1000 °C.

Correction to: Effect of layer‑by‑layer laser remelting process on the microstructure and performance of selective laser melting 316L stainless steel

Correction to: Effect of layer‑by‑layer laser remelting process on the microstructure and performance of selective laser melting 316L stainless steel

Effect of layer-by-layer laser remelting process on the microstructure and performance of selective laser melting 316L stainless steel

Effect of layer-by-layer laser remelting process on the microstructure and performance of selective laser melting 316L stainless steel

Three-dimensional printed 316L stainless steel cardiovascular stent's electrolytic polishing and its mechanical properties

The interventional therapy of vascular stent implantation is a popular treatment method for cardiovascular stenosis and blockage. However, traditional stent manufacturing methods such as laser cutting are complex and cannot easily manufacture complex structures such as bifurcated stents, while three-dimensional (3D) printing technology provides a new method for manufacturing stents with complex structure and personalized designs. In this paper, a cardiovascular stent was designed, and printed using selective laser melting technology and 316L stainless steel powder of 0-10 µm size. Electrolytic polishing was performed to improve the surface quality of the printed vascular stent, and the expansion behavior of the polished stent was assessed by balloon inflation. The results showed that the newly designed cardiovascular stent could be manufactured by 3D printing technology. Electrolytic polishing removed the attached powder and reduced the surface roughness Ra from 1.36 µm to 0.82 µm. The axial shortening rate of the polished bracket was 4.23% when the outside diameter was expanded from 2.42 mm to 3.63 mm under the pressure of the balloon, and the radial rebound rate was 2.48% after unloading. The radial force of polished stent was 8.32 N. The 3D printed vascular stent can remove the surface powder through electrolytic polishing to improve the surface quality, and show good dilatation performance and radial support performance, which provides a reference for the practical application of 3D printed vascular stent.

Selective laser melted 316L stainless steel: Influence of surface and inner defects on fatigue behavior

In additive manufacturing, despite its several indisputable advantages, detrimental variables to fatigue strength still remain poor surface finishing and porosities. However, because the majority of defects locate close to the surface, their mechanical removal is expected to appreciably improve fatigue strength. Considering SLMed 316L, fully-reversed rotating-bending fatigue tests in both as-built and machined conditions are performed. Fatigue failures are discussed using the Kitagawa-Takahashi diagram. In each condition, the fatigue stress is related to the equivalent micro-notch length of the killer defect. Then, this work analyses the possibility of predicting fatigue limits at 50 % probability considering the equivalent micro-notch length.

Investigating the effect of laser shock peening on the wear behaviour of selective laser melted 316L stainless steel

Selective Laser Melting (SLM) is an emerging additive manufacturing technology that produces metallic parts with high customization. The applications of stainless steel 316L have been restricted owing to their poor tribological performance, especially in the as-printed condition. In this study, SLM-made SS 316L is subjected to laser shock peening (LSP), one of the surface modification techniques, to improve its characteristic microstructure, mechanical and wear behaviour. X-ray diffraction and residual stress analysis show that the laser shock peening promotes micro-level grain refinement and compressive residual stresses in the severely deformed layer, resulting in increased micro-hardness up to 22%. Pin on disk wear test is carried out, and the optimum condition is determined using the Taguchi method for the polished as-printed specimen. The wear rate of polished as-printed and laser shock peened specimens at optimized conditions were 2.4156 × 10−3 mm3 m−1 and 1.7871 × 10−3 mm3 m−1, respectively. The laser shock peened specimen showed higher wear resistance with a 26% reduction in wear rate compared to the polished as-printed specimens.

Passivation Behavior of Different Building Planes of Selective Laser Melting 316L Stainless Steel in 3.5% NaCl Solution

The passivation behavior of selective laser melting 316L stainless steel (SLM 316L SS) on different building planes is studied in 3.5% NaCl solution. The results show that the XOZ and YOZ planes have a semicircular molten pool structure, and the XOY plane shows a columnar molten pool structure. Meanwhile, the XOZ planes have relatively smaller grain sizes and higher low‐angle grain boundaries (LAGBs) proportionate than that of YOZ and XOY planes. The electrochemical testing results illustrate that the XOZ plane has a lower corrosion tendency than that of YOZ and XOY planes, as the thicker passive film with higher content of Cr2O3 is detected on the XOZ plane. The XOZ plane, which has a higher Crox + hy /(Feox + hy + Crox + hy ) ratio, has excellent passivation performance and better electrochemical properties than the other two planes.

Electrochemical study on the effect of hydrogen on the passive film of selective laser melted 316L stainless steel in a proton exchange membrane water electrolyzer environment

The effect of hydrogen on the passivation behavior and electrochemical characteristics of selective laser melted (SLMed) 316L stainless steel in a simulated anode environment for a proton exchange membrane water electrolyzer (PEMWE) was studied. The results indicate that hydrogen charged into the sample increased the ratio of superficial Fe2+/Fe3+ and OH−/O2−, increased the concentration of point defects, reduced the film thickness, and weakened its protective effect. The film near 0.6 VSCE showed n-type semiconductor behavior. Hydrogen charging resulted in a higher defect density and thinner space charge layer in the film, which promoted the invasion of aggressive ions.

Comparing the sensitization behavior and the corrosion resistance of the wrought and selective laser melted 316L stainless steels

The degree of sensitization of conventional solid solution 316L stainless steel increased with the increasing of sensitization time at 800 °C. However, the degree of sensitization of selective laser melted 316L stainless steel increased first and then decreased with sensitization time, which was supported by surface corrosion morphology. Moreover, the defect point density in the passive films formed on selective laser melted 316L stainless steel also increased firstly and then decreased with the increasing of sensitization time. Refined grain and better recrystallization synergistically prevented the diffusion of chromium and the formation of chromium depletion areas for selective laser melted 316L stainless steel.