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Wang DW, Han HL, Sa B, Li KL, Yan JJ et al. A review and a statistical analysis of porosity in metals additively manufactured by laser powder bed fusion. Opto-Electron Adv 5, 210058 (2022). doi: 10.29026/oea.2022.210058
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Review Open Access
A review and a statistical analysis of porosity in metals additively manufactured by laser powder bed fusion
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Abstract
Additive manufacturing (AM), or 3D printing, is an emerging technology that “adds” materials up and constructs products through a layer-by-layer procedure. Laser powder bed fusion (LPBF) is a powder-bed-based AM technology that can fabricate a large variety of metallic materials with excellent quality and accuracy. However, various defects such as porosity, cracks, and incursions can be generated during the printing process. As the most universal and a near-inevitable defect, porosity plays a substantial role in determining the mechanical performance of as-printed products. This work presents a comprehensive review of literatures that focused on the porosity in LPBF printed metals. The formation mechanisms, evaluation methods, effects on mechanical performance with corresponding models, and controlling methods of porosity have been illustrated and discussed in-depth. Achievements in four representative metals, namely Ti−6Al−4V, 316L, Inconel 718, and AlSi10Mg, have been critically reviewed with a statistical analysis on the correlation between porosity fraction and tensile properties. Ductility has been determined as the most sensitive property to porosity among several key tensile properties. This review also provides potential directions and opportunities to address the current porosity-related challenges. -
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References
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Wang DW, Han HL, Sa B, Li KL, Yan JJ et al. A review and a statistical analysis of porosity in metals additively manufactured by laser powder bed fusion. Opto-Electron Adv 5, 210058 (2022). doi: 10.29026/oea.2022.210058
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Figure 1.
Schematic illustration of LPBF processing.
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Figure 2.
Representative cross-sections and 3D reconstructions of (a) gas porosity and (b) LOF porosity detected in LPBF printed metals. The dashed circles in (b) indicate unmelted feedstock powder. Figure reproduced with permission from (a) ref.37,43, Elsevier; (b) ref.43, Elsevier, under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
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Figure 3.
Synchrotron X-ray images of pores generated from keyholes. The time marks represent the delays of each graph after the first shot. Figure reproduced with permisson from ref.29, The American Association for the Advancement of Science.
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Figure 4.
Porosity detected in LPBF printed Ti−6Al−4V alloy, with respective AR and sphericity values. Figure reproduced with permission from ref.84, under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
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Figure 5.
Representative 3D reconstructions of (a) the distribution of porosity in a Ti−6Al−4V sample and (b) views of a specific pore. Figure reproduced with permission from: (a) ref.101, Springer Nature. (b) ref.44, Elsevier.
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Figure 6.
Comparative illustration of the limitations of X-ray CT. (a) Microscopic image of a LOF pore; (b) comparison of pores detected using X-ray CT and the Archimedes method; and (c) comparison of the same cross-section in a 316L sample detected by CT and by confocal microscopy (CM). Figure reproduced with permission from: (a, b) ref.76, Elsevier; (c) ref.104, under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
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Figure 7.
1D ultrasonic A-scan diagrams of (a) conventional UT and (b) phased array UT; and 2D ultrasonic C-scan images of (c) conventional UT and (d) phased array UT. Figure reproduced from ref.117, under a Creative Commons Attribution 4.0 International License.
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Figure 8.
Detriments of porosity to the mechanical performance. (a) Schematic illustration of uniaxial tensile load applied to a sample with inner porosity; (b) fractographic image of a LPBF printed Ti−6Al−4V tensile sample. Figure reproduced with permission from (b) ref.123, Elsevier.
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Figure 9.
Simulated stress distribution profile of two pores in a LPBF printed Ti−6Al−4V sample under identical uniaxial tensile load. Figure reproduced from ref.90, under a Creative Commons Attribution 4.0 International License.
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Figure 10.
Void growth and coalescence under tensile load. (a−f) High-resolution transmission electron microscopic images presenting the void growth and coalescence during in-situ tensile loading of an Al–Cu–Mg sample, with (g) corresponding schematic illustration. Figure reproduced with permission from ref.136, Elsevier.
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Figure 11.
FEM simulation of uniaxial tensile tests and corresponding comparison between experimental and predicted data of a 22NiMoCr37 steel sample; the mesh is installed with cylindrical voids. Figure reproduced with permission from ref.146, Springer Nature.
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Figure 12.
Fatigue crack propagation detected by CT in three LPBF printed Ti−6Al−4V samples. The red regions indicate the pores as the initiation sites of fatigue cracks. Figure reproduced with permission from ref.150, under a Creative Commons Attribution 4.0 International License.
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Figure 13.
Experimental and predicted fatigue limits of silicon carbide samples against defect size. PM, LM, and LEFM are acronyms of point method, line method, and linear elastic fracture mechanics, respectively. Figure reproduced with permission from ref.154, Elsevier.
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Figure 14.
Designation of the testing directions.
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Figure 15.
Summarized tensile properties of LPBF printed Ti−6Al−4V versus porosity fraction, with corresponding phenomenological models. The red data spots are outliers that excluded from fitting.
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Figure 16.
Summarized tensile properties of LPBF printed 316L SS versus porosity fraction, with corresponding phenomenological models. The red data spots are outliers that excluded from fitting.
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Figure 17.
Summarized tensile properties of LPBF printed Inconel 718 versus porosity fraction, with corresponding phenomenological models. The red data spots are outliers that excluded from fitting.
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Figure 18.
Summarized tensile properties of LPBF printed AlSi10Mg versus porosity fraction, with corresponding phenomenological models. The red data spots are outliers that excluded from fitting.
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Figure 19.
The influence of LPBF processing parameters. (a) Cross-sections of single scan tracks of LPBF printed AlSi10Mg; (b) schematic illustration of the porosity formation due to inappropriate overlap rates. Figure reproduced with permission from: (a) ref.232, Taylor & Francis; (b) ref.244, Elsevier.
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Figure 20.
Comparison between online monitoring and offline CT images. (a) The image acquired through online optical monitoring and (b) the offline CT slice at the same layer of a LPBF printed AlSi10Mg sample. Figure reproduced with permission from ref.265, Springer Nature.
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Figure 21.
Challenges and potential opportunities for the study of porosity in LPBF printed metals.
