scholarly journals Can Potential Defects in LPBF Be Healed from the Laser Exposure of Subsequent Layers? A Quantitative Study

Metals ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 1012
Author(s):  
Alexander Ulbricht ◽  
Gunther Mohr ◽  
Simon J. Altenburg ◽  
Simon Oster ◽  
Christiane Maierhofer ◽  
...  
Keyword(s):  
Cylindrical Specimen ◽  
Optical Tomography ◽  
Thick Layer ◽  
Laser Exposure ◽  
Powder Bed ◽  
X Ray ◽  
Laser Illumination ◽  
Melt Pool ◽  
X Ray Computed ◽  
Subsequent Layer

Additive manufacturing (AM) of metals and in particular laser powder bed fusion (LPBF) enables a degree of freedom in design unparalleled by conventional subtractive methods. To ensure that the designed precision is matched by the produced LPBF parts, a full understanding of the interaction between the laser and the feedstock powder is needed. It has been shown that the laser also melts subjacent layers of material underneath. This effect plays a key role when designing small cavities or overhanging structures, because, in these cases, the material underneath is feed-stock powder. In this study, we quantify the extension of the melt pool during laser illumination of powder layers and the defect spatial distribution in a cylindrical specimen. During the LPBF process, several layers were intentionally not exposed to the laser beam at various locations, while the build process was monitored by thermography and optical tomography. The cylinder was finally scanned by X-ray computed tomography (XCT). To correlate the positions of the unmolten layers in the part, a staircase was manufactured around the cylinder for easier registration. The results show that healing among layers occurs if a scan strategy is applied, where the orientation of the hatches is changed for each subsequent layer. They also show that small pores and surface roughness of solidified material below a thick layer of unmolten material (>200 µm) serve as seeding points for larger voids. The orientation of the first two layers fully exposed after a thick layer of unmolten powder shapes the orientation of these voids, created by a lack of fusion.

A simple and low cost USB webcam-driven optical tomography system suitable for teaching X-ray computed tomography

2021 ◽  
Author(s):  
Cristhian Ariel Vega Mernes ◽  
Isabela Grinberg Francelino ◽  
Edson Massayuki Kakuno ◽  
Marcelo G Honnicke
Keyword(s):  
Computed Tomography ◽  
Low Cost ◽  
Optical Tomography ◽  
X Ray ◽  
Tomography System ◽  
X Ray Computed

scholarly journals An investigation on the suitability of modern nondestructive testing methods for the inspection of specimens manufactured by laser powder bed fusion

2021 ◽  
Vol 3 (7) ◽  
Author(s):  
Cara G. Kolb ◽  
Katja Zier ◽  
Jan-Carl Grager ◽  
Andreas Bachmann ◽  
Tobias Neuwirth ◽  
...  
Keyword(s):  
Empirical Studies ◽  
Ex Situ ◽  
Powder Bed Fusion ◽  
Laser Powder Bed Fusion ◽  
Detection Capability ◽  
Powder Bed ◽  
X Ray ◽  
Active Infrared Thermography ◽  
X Ray Computed ◽  
Nondestructive Testing Methods

AbstractLaser powder bed fusion (L-PBF) is increasingly used to fabricate functional parts used in safety-relevant applications. To ensure that the sophisticated part specifications are achieved, 100% quality inspections are performed subsequent to the buildup process. However, knowledge about the detectability of defects in L-PBF parts using NDT methods is limited. This paper analyzes the suitability of NDT techniques in an ex situ environment, in particular active infrared thermography, neutron grating interferometry (nGI), X-ray computed tomography, and ultrasonic testing for the examination of L-PBF parts made from Inconel 718. Based on a test specimen with artificially inserted defects with varying dimensions and depths, these NDT techniques were compared in terms of their attainable resolution and thus defect detection capability. The empirical studies revealed that nGI shows the highest resolution capability. It was possible to detect defects with a diameter of 100–200 m at a depth of 60–80 $${\upmu } \hbox {m}$$ μ m . The results are discussed with regard to their relevance for the examination of L-PBF parts and thus not only contribute to a better understanding of the potential of the NDT techniques in comparison but also assist stakeholders in additive manufacturing in evaluating the suitability of the NDT techniques investigated.

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 Multi-Scale Modeling of Residual Stresses Evolution in Laser Powder Bed Fusion of Inconel 625

2021 ◽  
Vol 6 (1) ◽  
pp. 2
Author(s):  
Mohamed Balbaa ◽  
Mohamed Elbestawi
Keyword(s):  
Residual Stresses ◽  
Cooling Rate ◽  
Process Parameters ◽  
Laser Exposure ◽  
Powder Bed Fusion ◽  
Laser Powder Bed Fusion ◽  
Linear Temperature ◽  
Powder Bed ◽  
Multi Scale ◽  
Melt Pool

Laser powder bed fusion exhibits many advantages for manufacturing complex geometries from hard to machine alloys such as IN625. However, a major drawback is the formation of high tensile residual stresses, and the complex relationship between the process parameters and the residual stresses has not been fully investigated. The current study presents multi-scale models to examine the variation of process parameters on melt pool dimensions, cyclic temperature evolutions, cooling rate, and cyclic stress generation and how they affect the stress end state. In addition, the effect of the same energy density, which is often overlooked, on the generated residual stresses is investigated. Multi-level validation is performed based on melt pool dimensions, temperature measurements with a two-color pyrometer, and finally, in-depth residual stress measurement. The results show that scan speed has the strongest effect on residual stresses, followed by laser power and hatch spacing. The results are explained in light of the non-linear temperature evolution, temperature gradient, and cooling rate during laser exposure, cooling time, and the rate during recoating time.

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.

scholarly journals Validation of an XCT/fDOT System on Mice

ISRN Optics ◽  
2012 ◽  
Vol 2012 ◽  
pp. 1-13 ◽  
Author(s):  
Anne Koenig ◽  
Anne Planat-Chrétien ◽  
Kai Hassler ◽  
Martina Bucher ◽  
Jean-Guillaume Coutard ◽  
...  
Keyword(s):  
Computed Tomography ◽  
Imaging System ◽  
Ex Vivo ◽  
Optical Tomography ◽  
Diffuse Optical Tomography ◽  
Cylindrical Geometry ◽  
X Ray ◽  
Dual Modality ◽  
X Ray Computed

In this paper we present systems for dual modality imaging, combining fluorescence-enhanced diffuse optical tomography and X-ray computed tomography. Fluorescence diffuse optical tomography is carried out in a cylindrical geometry, which ensures optimal sampling and a straight forward integration with the X-ray modality. Specific acquisition protocols and reconstruction software have been developed to this end. The X-ray computed tomography serves two purposes. First, it provides the anatomical information in the registered dual modality images. Second, it provides the actual shape and boundaries of the animal as a priori input to the fluorescence reconstruction algorithm. To evaluate the performance of the optical imaging system, experiments have been conducted on phantoms, mice with inserted fluorescing capillaries, and finally on mice bearing tumors, ex-vivo and in-vivo. Experiments on mice with capillaries inserted in different region of interest, allow estimating the detection limits of fluorophore concentrations. The fluorescence reconstructions are shown to be geometrically consistent with the X-ray images. Finally we demonstrate the capability of the bimodal system to localize real tumours in mice in-vivo. These results show that dual modality fluorescence-enhanced diffuse optical tomography and X-ray computed tomography imaging in cylindrical geometry has a high potential for small animal tumour evolution studies.

Sign in / Sign up

Export Citation Format

Share Document