Overhang Angles Research Articles

Material extrusion 3D printing of ceramic structures with overhangs: rheological aspect and introduction of powder-on-demand supporter

Additive manufacturing (AM), represented as a manufacturing process, which is easily applicable to intricate designs, is widely considered in a variety of industries as a simple and cost-effective solution that complements or replaces traditional manufacturing processes. However, due to the demanding properties of ink materials employing ceramic materials, more stringent considerations are required in selecting an appropriate printing technique and implementing the rheological properties of the ceramic ink suitable for the selected printing technique. Here, material extrusion (ME) was selected as a 3D technique applicable to ceramic materials, and the rheological requirements of ceramic pastes that maximize their printability in ME were scrutinized in detail. It was found that the viscoelastic properties of the ceramic pastes played an important role in determining the flowability, cumulative height, and overhang angles that can be allowed in ME. A new supporter system for overhangs, powder-on-demand supporter (PODS), was introduced to ME and demonstrated to understand if PODS can further extend the limits in cumulative height and overhang angles allowed in ME.

Pure niobium manufactured by Laser-Based Powder Bed Fusion: influence of process parameters and supports on as-built surface quality

Niobium (Nb) is a transition metal commonly used as an alloying element for increasing strength, toughness, corrosion resistance, and other properties of steel and superalloys. Pure Nb, however, is a very interesting metal for its excellent superconductivity. This makes it suitable for producing superconducting magnets and devices for particle acceleration systems and particle physics research (e.g., superconducting resonant cavities). In this work, the production of Nb by the Laser-Based Powder Bed Fusion (PBF-LB/M, also known as Laser Powder Bed Fusion or LPBF) process was examined. Manufacturing parameters were investigated to achieve additively manufactured parts with a relative density higher than 99.5% and showing a down-skin surface roughness in the range of 20–70 μm, depending on the inclination angle. Studies related to the limiting angle of self-supported Nb parts were also conducted, and innovative non-contact supporting structures were successfully developed. These allowed to creation of parts with very small overhang angles, without compromising the downward-facing surfaces; indeed at the same time, the as-built surface finish was improved.

Prevention of enclosed voids in topology optimization using a cumulative sum flood fill algorithm

Topology optimization has seen increased interest with the rise of additive manufacturing (AM) as a fabrication method, because of its ability to exploit the geometric complexity that AM offers. However, AM still imposes some geometric restrictions on the design, most notably on minimum feature size, overhang angles, and enclosed voids. Enclosed voids are problematic because for many AM methods it is impossible to remove supports, unmelted powder or uncured liquid from them. This paper introduces a filter based on a cumulative sum flood fill algorithm to alleviate this issue in a flexible manner. This filter produces a density field where every enclosed void element is rendered solid. It successfully eliminates enclosed voids in both 2D and 3D problems, with low computational cost due to its geometric nature. In addition we demonstrate direct control over the location, amount, and size of powder removal features by varying boundary conditions for the filter, running additional flood fills, and adding morphology operators, respectively.

Topology Optimization of Self-supporting Porous Structures Based on Triply Periodic Minimal Surfaces

With the advent of additive manufacturing (AM), topology optimization (TO) has become increasingly important in structural design. A key component of structural design is generating the interior shape of an object under predefined force conditions and specific constraints. Fabricating such models by layer-based AM suffers from the problem of adding and removing interior supporting structures, which can produce artifacts in the final product. In this paper, we propose an algorithm to design a self-supporting porous structure by integrating overhang constraints into the topology optimization framework. A minimum thickness constraint is also built into the optimization process to automatically enforce printability of the resulting structures. We utilize triply periodic minimal surfaces (TPMSs) to represent interior structures which can be analyzed, optimized and stored directly using analytical functions. We apply several acceleration methods including super element strategy, multigrid algorithm and GPU-based solver to handle models comprising of several million of elements efficiently. Numerical results indicate that the optimized interior structures obtained by our approach exhibit improved printability as they largely satisfy manufacturing requirements on overhang angles and minimal thicknesses.

3D Claying: 3D Printing and Recycling Clay

Clay is of great interest as a 3D printing material thanks to its ease of use, recyclability and reusability. This paper analyses the technical aspects of the whole printing process. The behaviour of 3D printing clay is studied with respect to the environment and its specific application as a temporary or definitive formwork system for cement parts. The study addresses the performance of clay and the loss of its properties and characteristics according to the type of protection, whether it is in direct contact with air or cement, or protected with plastics, metal sheets, or combinations of both. A 3D printing system with various printers and 3D models has been considered, observing a direct relationship between the prototype shape, extrusion process and resulting material. The most important variables in 3D printing have been considered: layer height, line thickness, base definition, total model height, overhang angles, overlap between layers, etc. The main technical aspects have been analysed such as raw material properties, kneading, process control, post-treatments and material hardening. As a natural material, clay can be reused indefinitely under certain conditions to be part of a circular economy with low energy consumption and minimal resources. It is concluded that the option of using ceramics in 3D printing for very diverse uses in the architecture, engineering & construction (AEC) sector is very promising due to their ease of implementation, recycling capability and suitability to different environments.

Robotic laser directed energy deposition-based additive manufacturing of tubular components with variable overhang angles: Adaptive trajectory planning and characterization

The present work reports on an adaptive trajectory planning to build tubular components with variable overhang angles using a robotic Laser Directed Energy Deposition (LDED) based Additive Manufacturing process without utilizing support structures. The proposed technique uses a non-parallel slicing methodology to build complex components (e.g., bent pipes) and deploys adaptively varying scanning speed and tool orientation. The variation in scanning speed aids in having point-to-point variable layer height enabling non-parallel deposition while changing the tool orientation during deposition permits the manufacturing of support-free bent pipe parts. The bent pipes with 45° and 90° bents were built using an in-house developed LDED system and the built parts are characterized for geometry, density, microstructure, and microhardness. The geometrical analysis indicates a deviation in the range of + 0.5 mm to − 0.5 mm, with minimal roundness deviation at different sections. The density analysis of the segments extracted from the bent pipe reveals a density > 98%, with the presence of a few lack of fusion pores and gas pores at isolated locations. The microstructure and microhardness studies show that regions built with higher scanning speeds have a finer grain structure and higher average hardness. The study paves a path to build defect-free and dimensionally stable complex-shaped components with varying overhang angles using LDED.

Self-Supporting Structures Produced through Laser Powder Bed Fusion of AlSi10Mg Alloy: Surface Quality and Hole Circularity Tolerance Assessment

In the context of the Design for Additive Manufacturing (DfAM), the elimination and/or reduction of support structures for the parts is a key issue for process optimization in terms of sustainability and surface quality. In this work, the assessment of the surface quality of overhanging thin walls and unsupported holes with different diameters (4, 6, 8 mm) was carried out through confocal microscopy, SEM-EDS analysis and CMM measurements. To this aim, two different types of AlSi10Mg alloy parts were produced with the L-PBF technology, having self-supporting features such as thin walls and holes with different overhang angles. The results showed that (i) unsupported, down-facing surfaces can be printed consecutively without supports up to a 30° overhang angle and with a surface roughness (Sa) ranging from 3 to 40 µm; (ii) unsupported holes can be produced as well, having a mean circularity tolerance ranging from 0.03 to 0.55 mm, regardless of the diameter value; (iii) density and microstructure analysis both revealed that the parts’ integrity was not affected by the design choices.

Adaptive Process Planning for Additive/Subtractive Hybrid Manufacturing of Overhang Features

Abstract Additive/subtractive hybrid manufacturing (ASHM) combines the selective laser melting (SLM) with precision milling for a better surface finish as well as geometric and dimensional accuracies of a part. In the SLM-based ASHM, the inherent characteristic of the powder bed determines that the AM and milling process are both three-axis. To produce a complex part with overhang features, alternations between SLM and milling are needed. The alternating sequence is of significance in the overall process: a large slab height of continuous SLM may cause tool/part collision; while a small slab height could reduce the fabrication efficiency. In addition, the remelting of the material during SLM causes the distortion of substantial machined surface, and thus deteriorates the surface finish of a part. However, there have little published reports on process planning for three-axis SLM-based ASHM with the consideration of alternation planning and surface finish improvement. This paper proposed a new adaptive process planning method, in which maximum slab heights of continuous SLM were calculated adaptively, and the shielding height was also employed to protect the machined surface. The overhang angles at different locations of a typical complex part were calculated by a triangular facet method. A collision detection method to justify the interference of the tool and as-built part was proposed, and the maximum slab heights were then determined. ASHM experiment was conducted to fabricate small samples with different overhang angles, and the shielding height was determined based on the surface quality. A propeller with overhang features was manufactured using this new planning method. The result shows that the proposed method improved the fabrication efficiency, reduced the number of process alternations as well as improved the surface finish.

Geometric deviation and compensation for thin-walled shell lattice structures fabricated by high precision laser powder bed fusion

With the continuous improvement in the precision of the laser powder bed fusion (LPBF) process, printing parts with highly complex geometries and fine details has been realized. Given the large design freedom provided by LPBF, various shell lattice structures have been designed and fabricated successfully. However, few works focus on predicting and minimizing the discrepancy between as-designed and as-printed structures, which is crucial for the geometric fidelity and resulting structural performance. This is partially due to the complexity of structures with thin-walled features and continuously changing overhang angles. This paper proposed a new method and workflow to predict the printing deviation and conduct the compensation for the thin-walled shell lattice structures fabricated by the high-precision LPBF (hp-LPBF). Numerical simulation, which was used to capture dynamic interaction behavior between the rake, powder particles, and samples, was performed using the discrete element method (DEM) to identify the key factors affecting the deviation prediction. Subsequently, a semi-analytical model was proposed to predict the as-printed thicknesses, in which the molten depth-induced staircase effect, compression-induced thickening effect, and scrape-induced thinning effect were considered. Both inclination angles and wall thickness were observed to have significant influences on the roughness and dimensional accuracy, of which three types of deviation were determined: (1) non-compensable deviation; (2) large but compensable deviation; (3) negligible deviation under 5 %. Experimental results show that, by pre-compensation, the geometric deviation of thin walls with thicknesses of 135 µm and 185 µm was controlled to less than 10 µm. The effectiveness and generality of this method for the thin-walled shell structures were further demonstrated by a case study of Schwarz Primitive triply periodic minimal surface lattice structure, which is characterized by micro-CT using the proposed workflow to predict and modify the lattice thickness and generate the compensated geometry for printing.

Geometry-Based Radiation Prediction of Laser Exposure Area for Laser Powder Bed Fusion Using Deep Learning

Laser powder bed fusion (LPBF) is a promising technique used to manufacture complex geometries in a layer-wised manner. Radiation during the LPBF process is influenced by the part geometry, e.g., the overhang angle and the wall thickness. Locally varying radiation can cause deformation of the product after manufacturing. Thus, the prediction of the geometry-caused radiation before the manufacturing can support the evaluation of the design printability to achieve first-time-right printing. In this paper, we present a framework to predict the geometry-based radiation information using a deep learning (DL) algorithm based on the part geometry from computer-aided design (CAD). The algorithm was trained using data from an LPBF-print job consisting of parts with varying overhang angles. Image data, which include the information of radiation, were captured with an optical tomography (OT) camera system that was installed on a LPBF machine used in a laboratory environment. For the DL algorithm, a U-Net based network with mean absolute error (MAE) loss was applied. The training input was binarized OT data representing the contour of the designed geometry. Complementary, the OT data were used as ground truth for the model training. For the application, the design contours of multiple layers were extracted from the CAD file. The result shows the applicability to predict the OT-like radiation by its contour, which has the possibility to show the anomaly due to the part geometry.

Electron Beam-Melting and Laser Powder Bed Fusion of Ti6Al4V: Transferability of Process Parameters

Metal powder bed-based Additive Manufacturing (AM) technologies, such as Electron Beam-Melting (EBM) and Laser Powder Bed Fusion (LPBF), are established in several industries due to the large design freedom and mechanical properties. While EBM and LPBF have similar operating steps, process-specific characteristics influence the component design. The differences in the energy coupling lead to differing solidification conditions, microstructures, and, thus, mechanical properties. The surface finish and geometrical accuracy are also affected. As opposed to LPBF, EBM powder layers are preheated prior to selective melting. In this study, similar volume energy densities in LPBF and EBM were used to manufacture Ti6Al4V test geometries to assess the process transferability. Since the energy coupling of LPBF and EBM differ, heat source absorption was considered when calculating the volume energy density. Even when a similar volume energy density was used, significant differences in the component quality were found in this study due to specific respective process constraints. The extent of these constrains was investigated on voluminous samples and support-free overhanging structures. Overhang angles up to 90° were manufactured with LPBF and EBM, and characterized with regard to the relative density, surface roughness, and geometric compliance.

Structural stability of thin overhanging walls during material extrusion additive manufacturing of thermoset-based ink

Recent developments have enabled material extrusion additive manufacturing of thermoset-based composite inks on the large scale. In addition, printing out-of-plane components is of broad interest to the polymer material extrusion community. This work addresses some of the challenges associated with both large-scale and out-of-plane thermoset material extrusion additive manufacturing by studying the height at which thin overhanging walls collapse. Walls at a range of overhang angles were printed until they collapsed. An optical camera captured the profile of each wall throughout the print, allowing the collapse height to be identified and the geometric fidelity to the programmed angle to be evaluated. Using previously measured rheological properties, predictive models were generated to approximate the collapse height and profile of the deflected walls. First, an analytical model was created to predict the height at which the walls would yield. The analytical model assumes the walls exhibit a perfectly linear profile; however, experiments proved this assumption to be false. Therefore, a finite element simulation was developed to account for the elastic deflection that occurs during printing. The finite element simulation predicts both the yield height and the deflected profile after the deposition of each layer. For the properties of the thermoset ink used here, the yield height predicted by the analytical model and finite element simulation are virtually identical. These predictions match experimental data reasonably well, but minor errors are observed. Accounting for the fully plastic moment appears to explain the small mismatch between experimental data and predictions. Additionally, the finite element simulation provides an excellent prediction of the deflected profile before the wall begins to collapse. By demonstrating that the collapse height and deflected profile of thin overhanging walls can be predicted, this work illustrates how the soft viscoelastic properties of thermoset-based composite inks limit the scale of a key feature required to print some nonplanar components. It also provides a basis to tailor in-process curing systems to suppress deflection and collapse of thin overhanging walls.

Calibration Dependencies and Accuracy Assessment of a Silicone Rubber 3D Printer

Silicone rubbers are relatively new in additive manufacturing, with only a few commercial printing services and reports on custom-built printers available. Publications and standards on calibration and accuracy assessment are especially lacking. In this study, the printhead calibration process of a custom-built silicone printer is explained, and a set of test objects is proposed and evaluated. The printer in use is based on an open-source filament printer, capable of multi-material printing with silicone rubbers and thermoplastic polymers. Three different high-viscosity single-component liquid silicone rubbers and one polylactic acid thermoplastic filament were used as printing materials. First, the calibration process of the silicone printhead was conducted, and the dependency of the dosing accuracy on silicone viscosity, nozzle diameter and extrusion speed was evaluated. Second, various test specimens were proposed and printed to characterize the accuracy and geometric limitations of this printer. These test parts contained features such as thin walls, slender towers, small holes and slots, unsupported overhangs and bridges. It was concluded that silicone viscosity strongly affects geometric inaccuracies. Design recommendations were deducted from the results, advising for wall thicknesses above 1 mm, slenderness ratios below 2, bridging lengths below 2 mm and unsupported overhang angles below 30°.

Process planning for additive manufacturing of geometries with variable overhang angles using a robotic laser directed energy deposition system

In the present work, a novel Laser Directed Energy Deposition (LDED) process planning methodology is proposed to build a dome structure with variable overhang angles. Overhang structures with different overhang angles were built where the maximum angle of 35° can be used to build overhang structures without the process and structure compromise. The thin-wall hemispherical dome built using the developed methodology shows the maximum deviation of 2% with respect to the diameter of the original CAD model data. The study paves a way for building high-value, lightweight thin-walled structures with complex cylindrical-based shape (e.g., storage tanks, nozzles, combustion chambers) for engineering applications.

Anisotropic deformation of 316L stainless steel overhang structures built by material extrusion based additive manufacturing

Metal components with complex structures can be fabricated via additive manufacturing (AM) technologies benefited from the geometrically free of design. A fused filament fabrication (FFF)-debinding-sintering method has emerged within the past five years owing to the low cost, moderate energy consumption, and promising scalability. However, recent studies failed to address the capability and limitations of FFF for the fabrication of metals with complex structures, such as foam, lattice, and other functional hollow structures. Herein, we comprehensively revealed the dimensional change (shrinkage and change of angle) of the FFF-built overhang pillars using analytical modeling, finite element (FE) simulation, and experimental verification approaches. Six groups of pillars with overhang angles from 40° to 90° were designed and fabricated. The effect of gravity on the bending of overhang pillars was involved in the modeling and simulation. The gravity accelerated the shrinkage in the z-direction and results of shrinkage and deflection ratio exhibited a good consistency within the analytical, numerical, and experimental approaches. An extended application to predict the shrinkage of a lattice structure was also provided. This study provides the fundamental understanding of the sintering behavior of the metallic overhang structure fabricated by AM, and will significantly contribute to the future research efforts of FFF-built three-dimensionally complex metallic parts without support structures.

Process Planning for Additive Manufacturing of Geometries with Variable Overhang Angles Using a Robotic Laser Directed Energy Deposition System

Process Planning for Additive Manufacturing of Geometries with Variable Overhang Angles Using a Robotic Laser Directed Energy Deposition System

Topology Optimization of Self-Supporting Structures for Additive Manufacturing with Adaptive Explicit Continuous Constraint

The integration of topology optimization (TO) and additive manufacturing (AM) technologies can create significant synergy benefits, while the lack of AM-friendly TO algorithms is a serious bottleneck for the application of TO in AM. In this paper, a TO method is proposed to design self-supporting structures with an explicit continuous self-supporting constraint, which can be adaptively activated and tightened during the optimization procedure. The TO procedure is suitable for various critical overhang angles (COA), which is integrated with build direction assignment to reduce performance loss. Besides, a triangular directional self-supporting constraint sensitivity filter is devised to promote the downward evolution of structures and maintain stability. Two numerical examples are presented; all the test cases have successfully converged and the optimized solutions demonstrate good manufacturability. In the meanwhile, a fully self-supporting design can be obtained with a slight cost in performance through combination with build direction assignment.

Self-supporting structure design with feature-driven optimization approach for additive manufacturing

In this work, a topology optimization approach is developed for additive manufacturing (AM) of 2D and 3D self-supporting structures. Three important issues, i.e., overhang angle control, avoidance of the so-called V-shaped areas and minimum length scale control are addressed. 2D solid polygon and 3D polyhedron features are introduced as basic design primitives that are capable of translations, deformations and intersections to drive topological changes of the structure. The overhang angle control is realized in a straightforward way only by imposing upper bounds to related design variables without introducing any nonlinear constraint. The V-shaped area is avoided by simply limiting the positions of solid features. Minimum length scale control is controlled by a robust formulation. Numerical examples in 2D and 3D demonstrate the effectiveness of the proposed approach for various build directions, critical overhang angles and minimum length scales considered in AM.

Influence of Pulsed Exposure Strategies on Overhang Structures in Powder Bed Fusion of Ti6Al4V Using Laser Beam

Manufacturing structures with low overhang angles without support structures is a major challenge in powder bed fusion of metals using laser beam (PBF-LB/M). In the present work, various test specimens and parameter sets with continuous wave (cw) and pulsed exposure are used to investigate whether a reduction of downskin roughness and overhang angle can be achieved in PBF-LB/M of Ti6Al4V. Starting from cw exposure, the limits of overhang angle and surface roughness at the downskin surface are investigated as a reference. Subsequently, the influence of laser power, scanning speed, and hatch distance with fixed pulse duration (τpulse = 25 µs) and repetition rate (υrep = 20 kHz) on surface roughness Ra is investigated. Pulsed exposure strategies enable the manufacturing of flatter overhang angles (≤20° instead of ≥25°). Furthermore, a correlation between the introduced volume energy density and the downskin roughness can be observed for pulsed exposure. As the reduction in volume energy density causes an increase in porosity, the combination of pulsed downskin exposure and commercial cw infill exposure is investigated. The larger the gap in volume energy density between the infill area and downskin area, the more challenging it is combining the two parameter sets. By combining cw infill and pulsed downskin exposure, flatter overhang structures cannot be manufactured, and a reduction in roughness can be achieved.

Influence of Part Geometry and Feature Size on the Resulting Microstructure and Mechanical Properties of the Case Hardening Steel 16MnCr5 processed by Laser Powder Bed Fusion

Additive manufacturing, especially laser powder bed fusion (LPBF), allows the build-up of complex parts. However, the resulting microstructure is not only influenced by the manufacturing parameters but also by the part geometry due to the influence on heat dissipation. At first, six geometric features are identified which can affect heat dissipation. Geometric samples of a case-hardening steel are produced via LPBF. Analysis of the microstructure and the mechanical properties show that the cross-sectional area and overhang angles have significant influence resulting in a hardness differences of more than 90 HV while increasing the ferrite content with increasing overhang angles. For further investigation, the results are analytically linked to the part density.