scholarly journals Defect Analysis of 316 L Stainless Steel Prepared by LPBF Additive Manufacturing Processes

Coatings ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 1562
Author(s):  
Zhijun Zheng ◽  
Le Peng ◽  
Di Wang
Keyword(s):  
Stainless Steel ◽  
Energy Density ◽  
Orthogonal Experiment ◽  
Scanning Speed ◽  
Processing Parameters ◽  
Powder Bed ◽  
X Ray ◽  
Optimum Parameters ◽  
316 L Stainless Steel ◽  
X Ray Computed

The 316 L stainless-steel samples were prepared by laser powder bed fusion (LPBF). The effects of processing parameters on the density and defects of 316 L stainless steel were studied through an orthogonal experiment. The density of the samples was measured by the Archimedes method, optical microscopy (OM) and X-ray Computed Tomography (XCT). The microstructures and defects under different LPBF parameters were studied by OM and SEM. The results show that the energy density has a significant effect on the defect and density of the structure. When the energy density is lower than 35.19 J/mm3, the density increases significantly with the increase of energy density. However, when the energy density is larger than this value, the density remains relatively stable. The process parameter with the greatest influence on energy density is the hatch distance D, followed by laser power P, scanning speed V and rotation angle θ. In this paper, the optimum parameters consist of P = 260 W, V = 1700 mm, D = 0.05 mm and θ = 67°, in which the density is as high as 98.5%. In addition, the possibility and accuracy of the XCT method in detecting the discontinuity and porosity of 316 L stainless steel were discussed. The results show that XCT can provide the whole size and variation trend of pores in the different producing direction of LPBF.

scholarly journals Exploring the Correlation between Subsurface Residual Stresses and Manufacturing Parameters in Laser Powder Bed Fused Ti-6Al-4V

Metals ◽  
2019 ◽  
Vol 9 (2) ◽  
pp. 261 ◽  
Author(s):  
Tatiana Mishurova ◽  
Katia Artzt ◽  
Jan Haubrich ◽  
Guillermo Requena ◽  
Giovanni Bruno
Keyword(s):  
Energy Density ◽  
Residual Stresses ◽  
Processing Parameters ◽  
Synchrotron Diffraction ◽  
Support Structure ◽  
Powder Bed ◽  
X Ray ◽  
Laser Velocity ◽  
Effect Of Support ◽  
Manufacturing Parameters

Subsurface residual stresses (RS) were investigated in Ti-6Al-4V cuboid samples by means of X-ray synchrotron diffraction. The samples were manufactured by laser powder bed fusion (LPBF) applying different processing parameters, not commonly considered in open literature, in order to assess their influence on RS state. While investigating the effect of process parameters used for the calculation of volumetric energy density (such as laser velocity, laser power and hatch distance), we observed that an increase of energy density led to a decrease of RS, although not to the same extent for every parameter variation. Additionally, the effect of support structure, sample roughness and LPBF machine effects potentially coming from Ar flow were studied. We observed no influence of support structure on subsurface RS while the orientation with respect to Ar flow showed to have an impact on RS. We conclude recommending monitoring such parameters to improve part reliability and reproducibility.

scholarly journals Microstructure, Solidification Texture, and Thermal Stability of 316 L Stainless Steel Manufactured by Laser Powder Bed Fusion

Metals ◽  
2018 ◽  
Vol 8 (8) ◽  
pp. 643 ◽  
Author(s):  
Pavel Krakhmalev ◽  
Gunnel Fredriksson ◽  
Krister Svensson ◽  
Igor Yadroitsev ◽  
Ina Yadroitsava ◽  
...  
Keyword(s):  
Thermal Stability ◽  
Stainless Steel ◽  
Laser Scanning ◽  
Scanning Speed ◽  
Powder Bed Fusion ◽  
Laser Powder Bed Fusion ◽  
Powder Bed ◽  
316 L Stainless Steel ◽  
Solidification Texture ◽  
Thermal Stability Of

This article overviews the scientific results of the microstructural features observed in 316 L stainless steel manufactured by the laser powder bed fusion (LPBF) method obtained by the authors, and discusses the results with respect to the recently published literature. Microscopic features of the LPBF microstructure, i.e., epitaxial nucleation, cellular structure, microsegregation, porosity, competitive colony growth, and solidification texture, were experimentally studied by scanning and transmission electron microscopy, diffraction methods, and atom probe tomography. The influence of laser power and laser scanning speed on the microstructure was discussed in the perspective of governing the microstructure by controlling the process parameters. It was shown that the three-dimensional (3D) zig-zag solidification texture observed in the LPBF 316 L was related to the laser scanning strategy. The thermal stability of the microstructure was investigated under isothermal annealing conditions. It was shown that the cells formed at solidification started to disappear at about 800 °C, and that this process leads to a substantial decrease in hardness. Colony boundaries, nevertheless, were quite stable, and no significant grain growth was observed after heat treatment at 1050 °C. The observed experimental results are discussed with respect to the fundamental knowledge of the solidification processes, and compared with the existing literature data.

Surrogate Pore Generations in L-PBF Ti64 and Effects on Mechanical Behavior

2020 ◽  
Author(s):  
Shanshan Zhang ◽  
Sabina Chertmanova ◽  
Kevin Chou
Keyword(s):  
Energy Density ◽  
Pore Formation ◽  
Laser Energy ◽  
Process Conditions ◽  
Laser Powder Bed Fusion ◽  
Middle Section ◽  
Powder Bed ◽  
X Ray ◽  
Tomography System ◽  
X Ray Computed

Abstract In this study, surrogate pores were designed and generated at specific locations inside tensile specimens fabricated by laser powder bed fusion (L-PBF) processing and further evaluated in porosity characteristics and mechanical properties. The objectives are to demonstrate the feasibility of pore generation and to characterize the pores and examine the effect from various process conditions. The pore-generated specimens were fabricated in an L-PBF system using Ti-6Al-4V (Ti64) powder. Overall, specimens were made using default settings. But, during processing the middle section of the tensile specimens, single track exposures were applied to induce keyhole pores with different energy density levels as well as different scan track numbers and layers. An X-ray computed tomography system was utilized to measure and analyze surrogate pores in the fabricated specimens in pore counts, volumes and sphericity related to process conditions. The results showed that, as expected, pore formation increases with the laser energy density applied and the number of tracks and layers exposed, although individual tracks exhibit a high variability. Specimens evaluated by tensile testing and fractography show that surrogate pores produced in this study so far influence only the ductility of the specimens noticeably, but not Young’s modulus, nor the yield and tensile strengths.

Optimization of built-part distortion in laser powder bed fusion processing of Inconel 718

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
You-Cheng Chang ◽  
Hong-Chuong Tran ◽  
Yu-Lung Lo
Keyword(s):  
Cooling Rate ◽  
Cantilever Beam ◽  
Laser Power ◽  
Scanning Speed ◽  
Processing Parameters ◽  
Laser Powder Bed Fusion ◽  
Simulation Framework ◽  
Content Type ◽  
Powder Bed ◽  
Simulation Results

Purpose Laser powder bed fusion (LPBF) provides the means to produce unique components with almost no restriction on geometry in an extremely short time. However, the high-temperature gradient and high cooling rate produced during the fabrication process result in residual stress, which may prompt part warpage, cracks or even baseplate separation. Accordingly, an appropriate selection of the LPBF processing parameters is essential to ensure the quality of the built part. This study, thus, aims to develop an integrated simulation framework consisting of a single-track heat transfer model and a modified inherent shrinkage method model for predicting the curvature of an Inconel 718 cantilever beam produced using the LPBF process. Design/methodology/approach The simulation results for the curvature of the cantilever beam are calibrated via a comparison with the experimental observations. It is shown that the calibration factor required to drive the simulation results toward the experimental measurements has the same value for all settings of the laser power and scanning speed. Representative combinations of the laser power and scanning speed are, thus, chosen using the circle packing design method and supplied as inputs to the validated simulation framework to predict the corresponding cantilever beam curvature and density. The simulation results are then used to train artificial neural network models to predict the curvature and solid cooling rate of the cantilever beam for any combination of the laser power and scanning speed within the input design space. The resulting processing maps are screened in accordance with three quality criteria, namely, the part density, the radius of curvature and the solid cooling rate, to determine the optimal processing parameters for the LPBF process. Findings It is shown that the parameters lying within the optimal region of the processing map reduce the curvature of the cantilever beam by 17.9% and improve the density by as much as 99.97%. Originality/value The present study proposes a computational framework, which could find the parameters that not only yield the lowest distortion but also produce fully dense components in the LPBF process.

scholarly journals A processing diagram for high-density Ti-6Al-4V by selective laser melting

2018 ◽  
Vol 24 (9) ◽  
pp. 1469-1478 ◽  
Author(s):  
Yinmin (Morris) Wang ◽  
Chandrika Kamath ◽  
Thomas Voisin ◽  
Zan Li
Keyword(s):  
Mechanical Properties ◽  
Energy Density ◽  
Processing Parameters ◽  
High Density ◽  
Low Density ◽  
Laser Powder Bed Fusion ◽  
Content Type ◽  
Powder Bed ◽  
Melt Pool ◽  
Hatch Spacing

Purpose Density optimization is the first critical step in building additively manufactured parts with high-quality and good mechanical properties. The authors developed an approach that combines simulations and experiments to identify processing parameters for high-density Ti-6Al-4V using the laser powder-bed-fusion technique. A processing diagram based on the normalized energy density concept is constructed, illustrating an optimized processing window for high- or low-density samples. Excellent mechanical properties are obtained for Ti-6Al-4V samples built from the optimized window. Design/methodology/approach The authors use simple, but approximate, simulations and selective experiments to design parameters for a limited set of single track experiments. The resulting melt-pool characteristics are then used to identify processing parameters for high-density pillars. A processing diagram is built and excellent mechanical properties are achieved in samples built from this window. Findings The authors find that the laser linear input energy has a much stronger effect on the melt-pool depth than the melt-pool width. A processing diagram based on normalized energy density and normalized hatch spacing was constructed, qualitatively indicating that high-density samples are produced in a region when 1 < E* < 2. The onset of void formation and low-density samples occur as E* moves beyond a value of 2. The as-built SLM Ti-6Al-4V shows excellent mechanical performance. Originality/value A combined approach of computer simulations and selected experiments is applied to optimize the density of Ti-6Al-4V, via laser powder-bed-fusion (L-PBF) technique. A series of high-density samples are achieved. Some special issues are identified for L-PBF processes of Ti-6Al-4V, including the powder particle sticking and part swelling issues. A processing diagram is constructed for Ti-6Al-4V, based on the normalized energy density and normalized hatch spacing concept. The diagram illustrates windows with high- and low-density samples. Good mechanical properties are achieved during tensile tests of near fully dense Ti-6Al-4V samples. These good properties are attributed to the success of density optimization processes.

scholarly journals Effect of Hot Isostatic Pressing on Porosity and Mechanical Properties of 316 L Stainless Steel Prepared by the Selective Laser Melting Method

Materials ◽  
2020 ◽  
Vol 13 (19) ◽  
pp. 4377
Author(s):  
Tomas Cegan ◽  
Marek Pagac ◽  
Jan Jurica ◽  
Katerina Skotnicova ◽  
Jiri Hajnys ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Stainless Steel ◽  
Hot Isostatic Pressing ◽  
Microstructural Analysis ◽  
Scanning Speed ◽  
High Yield ◽  
Laser Method ◽  
Preparation Conditions ◽  
Lower Porosity ◽  
316 L Stainless Steel

The manufacturing route primarily determines the properties of materials prepared by additive manufacturing methods. In this work, the microstructural features and mechanical properties of 316 L stainless steel prepared by the selective laser method have been determined. Three types of samples, (i) selective laser melted (SLM), (ii) selective laser melted and hot isostatic pressed (HIP) and (iii) selective laser melted and heat treated (HT), were characterized. Microstructural analysis revealed that SLM samples were formed by melt pool boundaries with fine cellular–dendritic-type microstructure. This type of microstructure disappeared after HT or HIP and material were formed by larger grains and sharply defined grain boundaries. The SLM-prepared samples contained different levels of porosity depending on the preparation conditions. The open interconnected LOF (lack of fusion) pores were observed in the samples, which were prepared with using of scanning speed 1200 mm/s. The blowhole and keyhole type of porosity were observed in the samples prepared by lower scanning speeds. The HIP caused a significant decrease in internal closed porosity to 0.1%, and a higher pressure of 190 MPa was more effective than the usually used pressure of 140 MPa, but for samples with open porosity, HIP was not effective. The relatively high yield strength of 570 MPa, tensile strength of 650 MPa and low ductility of 30–34% were determined for SLM samples with the lower porosity content than 1.3%. The samples after HIP showed lower yield strengths than after SLM (from 290 to 325 MPa) and relatively high ductility of 47.8–48.5%, regardless of the used SLM conditions.

Experimental Study on Pore-Defect by Selective Laser Melting of 316L Stainless Steel

2019 ◽  
Vol 801 ◽  
pp. 239-244
Author(s):  
Xin Yu Liu ◽  
Lu Pan ◽  
Wen Hao Wang ◽  
Si Yao Li
Keyword(s):  
Stainless Steel ◽  
Energy Density ◽  
Laser Power ◽  
316L Stainless Steel ◽  
Scanning Speed ◽  
Micro Structure ◽  
Technological Parameters ◽  
Product Density ◽  
Air Bubble ◽  
Test Verification

With different process parameters (laser power, scanning speed and scanning distance),the low-time defects of forming part were studied by microscope,including air bubble, pore, micro-crack and macro-crack. The formation mechanism of pore-defect was analyzed. The micro-structure and composition of the pore-defect were studied by means of SEM and EDS. The results showed that the porosity mainly included circular air porosity, irregular process porosity and oxide inclusion.Linear energy density (E=P/v) was introduced as synthetic parameter.According to analysis and test verification, the optimum technological parameters of 316L stainless steel were laser power 190-210KW, laser speed 800-1000mm/s and scanning interval 0.9-0.11mm,and the linear energy density was about 200J/m. There were no cracks, no bubbles, a small amount of porosity, and the product density reached 99.7%.

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