Popular Additive Manufacturing Research Articles

A route to improving elongation of high-temperature laser sintered PEKK

Abstract Laser sintering (LS) is one of the most popular additive manufacturing (AM) techniques as it produces parts of complex geometry with high dimensional accuracy and good mechanical strength. However, the nature of the LS process often leads to brittle behavior characterized by a low elongation at break if compared to conventional polymer processing techniques, e.g., injection molding (IM). For poly(ether ketone ketone) (PEKK), such elongation is currently below 3%. This study determines and then optimizes the relationship between cooling time and crystallization of PEKK during LS and the resulting elongation at break. The elongation at break of PEKK was successfully improved by using shorter cooling times. The combination of the slow crystallization kinetics of PEKK and a short cooling time of 1 h increased elongation at break to 14%; this is a striking result never achieved for PAEKs in LS before. A calibration curve was developed that can be used to correlate PEKK structure and mechanical properties to cooling conditions according to the application. This methodology can also be applied to select and optimize the mechanical properties of other LS polymers sharing similar kinetics of crystallization and processing temperatures. This work suggests a great potential for a wide range of “post-processing” heat treatments to be used in AM to tailor the ultimate mechanical properties.

Effects of laser scanning speed on surface roughness and mechanical properties of aluminum/Polylactic Acid (Al/PLA) composites parts fabricated by fused deposition modeling

Although fused deposition modeling (FDM) has gradually become one of the popular additive manufacturing technologies in different industries, high surface roughness has always been an inevitable disadvantage of FDM parts. Laser polishing represents a recent and novel application of laser surface irradiation that can be used for precise, post-process smoothing of the rough surfaces commonly encountered on FDM parts. The influence of laser polishing on surface modification and mechanical properties of aluminum/Polylactic Acid (Al/PLA) composites has been investigated. Laser scanning speed was varied to evaluate its effect on the surface quality and mechanical properties of the experimental samples. The results indicated that laser polishing could enable reductions in surface roughness of over 86.6% (from 23.27 μm to 3.11 μm Sa). The tensile strength of specimen also increased from 41.01 MPa to 51.31 MPa with increasement of 25.1%. The dynamic mechanical analysis (DMA) results showed that there was a remarkable improvement in the storage modulus and glass transition temperature of Al/PLA composite specimens after laser polishing, which suggested that the laser polishing treatment could play a role in decreasing the porosity inside the FDM parts and improve interfacial adhesion between the PLA matrix and Al fibers. Moreover, the fracture morphologies were observed to investigate the possible strengthening mechanism. These results demonstrate how laser polishing can simultaneously smooth and modify the surface characteristics of a FDM part surface.

Toolpath planning for multi-gantry additive manufacturing

Additive Manufacturing (AM), specifically Fused Filament Fabrication (FFF) is revolutionizing the production of many products. FFF is one of the most popular AM processes because it is inexpensive, requires little maintenance, and has high material utilization. Unfortunately, long cycle times are a significant drawback that prevents FFF from being more widely implemented, especially for large-scale components. In response to this, printers that employ multiple independent FFF printheads simultaneously working on the same part have been developed, and multi-gantry configurations are now commercially available; however, there is a dearth of formal research on multi-gantry path planning, and current practices do not maximize printhead utilization or as-built mechanical properties. This article proposes a novel methodology for generating collision-free toolpaths for multi-gantry printers that yields shorter print times and superior mechanical properties compared with the state of the art. In this, a metaheuristic approach is used to seek near-optimal segmentation and scheduling of each layer while a collision checking and resolution algorithm enforces kinematic constraints to ensure collision-free solutions. Simulation is used to show the resulting makespan reduction for various layers, and the proposed methodology is physically implemented and verified. Tensile testing on samples printed via the current and proposed methods confirm that the proposed methodology results in superior mechanical properties.

Surface chemical analysis of copper powder used in additive manufacturing

Additive manufacturing (AM) has during years gained significant interest owing to its endless component design possibilities. One of the most popular AM techniques is laser powder bed fusion (LPBF), which selectively melts metal powder layer‐by‐layer in a chamber with protective argon atmosphere. This technique is attractive for realizing Cu‐based products in which the high electrical conductivity of Cu is combined with component design possibilities. The successful use of Cu powder not only poses challenges owing to the high reflectivity and thermal conductivity of Cu but also involves the important concern of controlling the powder surface chemistry since the powder surface constitutes the main source of oxygen. It is of crucial importance to control the oxygen level in order to maintain good electrical conductivity and brazing ability of the AM‐fabricated Cu‐part. In LPBF, fine spherical powder with size of 10–60 μm is used, providing significant specific surface area, and this powder is also usually recycled several times, and hence, the role of powder surface chemistry is evident. Two kinds of copper powder with purities 99.70 and 99.95 wt% were analysed in both virgin and in used conditions after numerous printing cycles using LPBF. The powder was analysed by X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). A clear difference between the two powder grades in terms of surface chemistry was observed. The oxide thickness and bulk oxygen content increased for both powder grades after recycling. The surface oxides under different conditions are identified and the effect of powder purity on the oxide formed is discussed.

Effect of initial microstructure on the deformation heterogeneities of 316L stainless steels fabricated by selective laser melting processing

The selective laser melting (SLM) is a popular additive manufacturing (AM) technique used for the fabrication of metal parts. In the present study, two 316L stainless steel specimens (SLM-I and SLM-II) with different microstructures were fabricated with different levels of energy density by changing the laser power and scanning speed, which are the main SLM process conditions. The deformation and fracture behavior of miniature SLM specimens under uniaxial tension were experimentally measured via optical microscopy (OM), field emission scanning microscopy (FE-SEM), and an electron back-scattered diffraction (EBSD) technique. In order to analyze the deformation heterogeneities under uniaxial tension, the inverse pole figure (IPF) map, kernel average misorientation (KAM) map, Taylor factor (TF) map, grain boundaries (GBs), Σ3 twin boundaries (TBs), and melt pool boundaries (MPBs) developed in deformed SLM specimens were analyzed at different strain levels. The effect of microstructural factors on the deformation heterogeneities of SLM specimens was explained by the evolution of KAM, GBs, MPBs, and Σ3 TBs. The initial microstructures of the SLM specimens significantly influenced the generation and propagation of cracks under uniaxial tension.

Plateau-Rayleigh instabilities in pore networks of additively manufactured polymers: A modeling perspective

Fused Filament Fabrication (FFF) additive manufacturing (AM) is a popular and widespread polymer AM method that provides rapid part production with relatively low cost prototyping. Polymeric structures made through FFF AM typically have internal pore structures which result from the manufacturing process. A Monte Carlo algorithm is implemented to simulate pore morphology evolution from build-path networks to spheroidal pores in AMed post-annealed structures. The model demonstrated that the evolution of these pore networks phenomenologically follows stages associated with the Plateau-Rayleigh instability and that thermal gradients result in migration of the spherical pores, as was observed experimentally.

Stable layer-building strategy to enhance cold-spray-based additive manufacturing

Cold spray (CS) has recently become one of the popular additive manufacturing (AM) processes for its advantages: high-forming efficiency, low temperature, and no phase changing of materials. These advantages may make CS able to form large volume objects and possibly directly iterate with material-removing processes to become a hybrid AM process. Current research proposes using a bulk-based volume-forming strategy (e.g. a tessellation-based method) for volume building. Although it can form 3D volumes, the control of the process is difficult and it has limitations in forming complex 3D near-net-shapes with acceptable accuracy. This also conflicts with the basic principle of AM, where the volume forming is via the manner of layer by layer. To solve this problem for easy process control, this paper proposes a new spray strategy, one that considers the characteristics of cold spray and kinematic parameters, to enhance stable layer building for 3D shape forming. Both simulation and experiments are conducted for method verification. The benchmarking test on some basic objects shows better shape accuracy than existing methods, and the proposed process is easier to control, repeat and follow. This implies that the proposed method makes CS a real and ready layer-by-layer AM process for 3D shape forming.

Processing-structure-property relationships of bisphenol-A-polycarbonate samples prepared by fused filament fabrication

Fused filament fabrication (FFF) is one of the most popular additive manufacturing processes. However, structural applications of FFF are still limited by unwanted variations in mechanical strength and structural dimensions of printed parts. To obtain a fundamental understanding of these issues, we focused on the interlayer bonding region of bisphenol-A-polycarbonate samples. The samples were prepared by a low-cost open-source FFF 3D printer, and full three-dimensional (3D) geometrical characterizations were performed on them using X-ray micro computed tomography (micro-CT). The results showed significant geometry variation depending on different printing conditions, including print speed, layer height, and nozzle temperature. Based on the results, we demonstrated the effects of reducing layer height and increasing nozzle temperature as well as compensating material extrusion rate to improve geometric precision. Moreover, uniaxial tensile and Mode III tear tests results showed that there are linear relations between bonding zone geometry and bonding strength. In addition, from the 3D geometry of the resulting printed part, we could estimate the Young’s modulus in the extrudate stacking direction using finite element method, which showed good agreement with the measured value. We envision that our findings can contribute to providing guidelines for the selection of printing parameters to improve or customize printing quality. Our experimental data may also serve as benchmark data for future multi-physics simulation models.

Prediction of initial transient behavior with stationary heating during laser powder bed fusion processes

Laser powder bed fusion (L-PBF) has become a popular additive manufacturing process. To minimize the need to perform experimental trial-and-error based searching, physical models can be used to determine process parameters to achieve desirable process conditions. In this paper, a physics-based predictive model is presented to describe the underlying physics of L-PBF during the initial highly transient process of molten pool and keyhole formation during spot heating, i.e., zero-velocity, finite-duration laser energy addition to a Ti-6Al-4V/Ti-6Al-4V powder bed system. The simulation presented herein uses a computationally-efficient volumetric energy addition method to describe energy transfer to the particles during melting and ray-tracing to account for attenuated beam energy reaching the substrate through the powder bed. Validation of the model was performed against imagery captured from synchrotron measurements of a static L-PBF process which showed a keyhole welding mode for each of two power settings. The model presented herein was able to describe the laser energy absorption, molten bead formation, keyhole welding mode, and showed excellent correlation of keyhole and molten bead dimensions for each of the cases presented.

Experimental characterization and micromechanical-statistical modeling of 316L stainless steel processed by selective laser melting

Additive manufacturing (AM) is regarded as one of the breakthrough innovations since the 19th century, whose impact on the manufacturing world is recognized as the same with the impact of airplane on the transportation industry [1]. Selective laser melting (SLM), one of the most popular AM techniques for processing metallic materials, has demonstrated its great potentials in medical, aerospace and automotive applications. The SLM technique itself is a complex metallurgical process, involving multi-physics phenomenon in laser scanning, powder melting and bonding procedure. Such a process may introduce a variety of defects and flaws such as pores and cracks, significantly reducing the mechanical performance of the final products. In this paper, we present experimental studies and micromechanical modeling to investigate the effects of the internal pores on the mechanical properties of 316L stainless steel (SS316L) processed by the SLM technique. Specifically, the internal pore characteristics such as size, morphology, spatial distribution and porosity (defined as the total volume of the pores divided by the total volume of the material) of the SLM processed SS316L is experimentally characterized and correlated with the mechanical properties. More importantly, a micromechanical model taking into account the statistical pore characteristics from the XCT analysis is developed to predict the elastic properties of the SS316L product. The numerical prediction results show good agreement with two analytical models and the experimental characterization of the mechanical properties. The present study provides future designers a methodology in predicting the mechanical properties using the XCT analysis results, which is a promising possibility of saving the expensive cost on destructive testing.

Calibration and Validation Framework for Selective Laser Melting Process Based on Multi-Fidelity Models and Limited Experiment Data

Abstract There are significant quality and reliability problems for components/products made by additive manufacturing (AM) due to various reasons. Selective laser melting (SLM) process is one of the popular AM techniques and it suffers from low quality and reliability issue as well. Among many reasons, the lack of accurate and efficient models to simulate the SLM process could be the most important one because reliability and quality quantification rely on accurate models; otherwise, a large number of experiments should be conducted for reliability and quality assurance. To date, modeling techniques for the SLM process are either computationally expensive based on finite element (FE) modeling or economically expensive requiring a significant amount of experiment data for data-driven modeling. This paper proposes the integration of FE and data-driven modeling with systematic calibration and validation framework for the SLM process based on limited experiment data. Multi-fidelity models are the FE model for the SLM process and a machine learning model constructed based on the FE model instead of real experiment data. The machine learning model, after incorporation of the learned physics from the FE model, is then further improved based on limited real experiment data through the calibration and validation framework. The proposed work enables the development of highly efficient and accurate models for melt pool prediction of the SLM process under various configurations. The effectiveness of the framework is demonstrated by real experiment data under 14 different printing configurations.

Quantification of additive manufacturing induced variations in the global and local performance characteristics of a complex multi-stage control valve trim

Control valves that are used in severe service applications have trim cages that are geometrically quite complex. Most of these trims are manufactured using traditional manufacturing methods which are expensive and time-consuming. In order to reduce manufacturing costs and shorten the product development cycles, Additive Manufacturing (AM) methods have been gaining popularity over the traditional manufacturing methods. Selective Laser Melting (SLM) is one of the most popular AM techniques. In this paper, the effect of the conventional Electron Discharge Machining (EDM) method and the SLM method on the performance characteristics of a complex multi-stage disc stack trim is investigated. Experimental tests conducted on the SLM trim showed that the flow capacity reduced in comparison to the EDM manufactured trim. Surface profile measurements indicated that the surface roughness of the SLM trim was significantly higher than the EDM trim. In order to evaluate the effect of surface roughness on performance in detail, well validated numerical simulations were conducted to compare the local performance of the valve trims manufactured by the two methods. The simulation results showed that the wall shear stress increases by 1.9 times on the trim manufactured by the SLM method due to the increased roughness.

Bitmap generation from computer-aided design for potential layer-quality evaluation in electron beam additive manufacturing

PurposeElectron beam additive manufacturing (EBAM) is a popular additive manufacturing (AM) technique used by many industrial sectors. In EBAM process monitoring, data analysis is focused on information extraction directly from the raw data collected in-process, i.e. thermal/optical/electronic images, and the comparison between the collected data and the computed tomography/microscopy images generated after the EBAM process. This paper aims to postulate that a stack of bitmaps could be generated from the computer-aided design (CAD) at a range of Z heights and user-defined region of interest during file preparation of the EBAM process, and serve as a reference image set.Design/methodology/approachComparison between that and the workpiece images collected during the EBAM process could then be used for quality assessment purposes. In spite of the extensive literature on CAD slicing and contour generation for AM process preparation, the method of bitmap generation from the CAD model at different field of views (FOVs) has not been disseminated in detail. This article presents a piece of custom CAD-bitmap generation software and an experiment demonstrating the application of the software alongside an electronic imaging system prototype.FindingsResults show that the software is capable of generating binary bitmaps with user-defined Z heights, image dimensions and image FOVs from the CAD model; and can generate reference bitmaps to work with workpiece electronic images for potential pixel-to-pixel image comparison.Originality/valueIt is envisaged that this CAD-bitmap image generation ability opens up new opportunities in quality assessment for the in-process monitoring of the EBAM process.

Post-Process Effects of Isothermal Annealing and Initially Applied Static Uniaxial Loading on the Ultimate Tensile Strength of Fused Filament Fabrication Parts.

Fused filament fabrication (FFF) is one of the most popular additive manufacturing (AM) techniques used to fabricate polymeric structures. However, these polymeric structures suffer from an inherent deficiency of weak inter-laminar tensile strength. Because of this weak inter-laminar strength, these parts fail prematurely and exhibit only a fraction of the mechanical properties of those fabricated using conventional means. In this paper, we study the effect of thermal annealing in the presence of an initially applied static uniaxial load on the ultimate tensile strength of parts fabricated using FFF. Tensile specimens or dogbones were fabricated from an acrylonitrile butadiene styrene (ABS) filament with a glass transition temperature (Tg) of 105 °C; these specimens were then isothermally annealed, post manufacture, in a fixture across a given range of temperatures and static loads. Tensile testing was then performed on these specimens to gauge the effect of the thermal annealing and static loading on inter-laminar tensile strength by measuring the ultimate tensile strength of the specimens. A design of experiments (DOE) approach was followed to calculate the main and interaction effects of the two factors (temperature and static loading) on the ultimate tensile strength, and an analysis of variance was conducted. Cross-sectional images of the specimens were studied to observe the changes in the mesostructure of the specimens that led to the increase in inter-laminar strength of the parts. The results show that temperature plays a dominant role in increasing the ultimate tensile strength and an 89% increase in the average ultimate tensile strength was seen corresponding to an annealing temperature of 160 °C. A change in the mesostructure of the parts is seen, which is characterized by an increase in bond length and void coalescence. These results can be helpful in studying the structural strength of 3D printed parts, and thus could eventually guide the fabrication of components with strength comparable to those of conventional manufacturing techniques.

Mechanical and Thermo-mechanical Properties of Carbon fiber Reinforced Thermoplastic Composite Fabricated Using Fused Deposition Modeling Method

Abstract Additive manufacturing technologies have been successfully applied in various engineering applications because advantages of low cost, minimal wastage and ease of material change. Fused deposition modeling is one of the most popular additive manufacturing techniques used for fabricating thermoplastic parts. Due to the intrinsically limited mechanical properties of thermoplastic materials, there is need to improve it by addition of specific filler material. In present paper chopped Carbon fibers and Polylactic Acid is used as reinforcing material and thermoplastic matrix for the preparation of fiber reinforced plastic composites by fused deposition modeling. Mechanical and thermomechanical properties are studied by varying the percentage of chopped carbon fiber into thermoplastic matrix. Carbon fiber reinforced plastic feedstock filaments are fabricated with varying weight percentage like 12%, 15% and 20% of chopped carbon fiber. Mechanical and thermo-mechanical properties of composite studied as per ASTM standard. It is observed that at 15 weight percent of carbon fiber reinforcement shows 32% and 22% enhancement in tensile strength and flexural strength as compared to pure Polylactic acid sample. As compared with pure Polylactic sample impact strength and hardness value increases by 53% and 85% with the reinforcement of 20 weight percent of carbon fiber. Dynamic mechanical analysis results shows enhancement in storage modulus and glass transition temperature by 18% and 3oC with reinforcement of 15 weight percentage of carbon fiber compared with pure Polylactic acid sample. Thermogravimetric analysis shows composite with 15 weight percentage carbon fiber has higher thermal stability

Effect of Process Parameters and Thermal Annealing on Mechanical Properties of Fused Filament Fabricated Specimens

Fused filament fabrication (FFF) is the most popular additive manufacturing technique to produce three-dimensional complex structures. It is a layer by layer additive manufacturing technique in which parts are created by the addition of one layer over another by using melted thermoplastic polymers. Fused filament fabricated parts have broad applications in automotive, medical, art and adornments fields, etc. The mechanical and thermal properties of FFF printed parts mainly depends on process parameters like infill pattern, layer thickness, infill speed, Nozzle temperature, infill density. The only way to improve the mechanical and surface properties of a part after printing is to make use of the post-processing techniques. Thermal annealing is one of the best post-process treatment for fused filament fabricated parts to improve mechanical properties. The objective of this paper is to provide an overview of the effect of different process parameters on the mechanical properties of fused filament fabricated parts and also investigate the influence of thermal annealing on the mechanical properties of printed parts. Mechanical properties of printed parts are analyzed by conducting different mechanical tests like tensile strength, compressive strength, and impact strength tests, etc. These mechanical tests are conducted on the standard specimens of ASTM or ISO standards. Thermal annealing should be possible by heating the printed part in a temperature range between the glass change temperature (Tg) and the melting point temperature (Tm) of the material used. It is found that the mechanical properties like tensile strength, compressive strength, and impact strength, etc. depend more on the process parameters like infill density, layer thickness, nozzle temperature, etc. It is observed that thermal annealing caused an improvement in flexural strength and tensile strength of fused filament fabricated parts. The tensile test for high tensile PLA showed that the maximum tensile strength furthermore, modulus for the non-heat-treated examples was acquired 65.75 MPa and 4.9 GPa individually, at 250°C. The tensile test results for heat-treated examples exhibited that the maximum tensile strength and modulus were gained 67.4 MPa and 5.65 GPa, separately, at 250°C.

Hybrid Manufacturing and Experimental Testing of Glass Fiber Enhanced Thermoplastic Composites

Additive Manufacturing (AM) is gaining enormous attention from academic and industrial sectors for product development using different materials. Fused Deposition Modelling (FDM) is a popular AM method that works with thermoplastics. This process offers benefits of customisation both in terms of hardware and software in the case of desktop-based FDM systems. Enhancement of mechanical properties for the traditional thermoplastic material is a widely researched area and various materials have been added to achieve this goal. This paper focuses on the manufacture of glass fiber reinforced plastic (GFRP) composites using Hybrid Fused Deposition Modelling (HFDM). Commonly available polylactic acid or polylactide (PLA) material was inter-laced with 0.03 mm thick glass fiber sheets to manufacture GFRP products followed by tensile testing. This was done to investigate whether adding more layers increases the tensile strength of the GFRP products or not. Furthermore, the maximum number of glass fiber layers that can be added to the 4 mm thick specimen was also identified. This was done to demonstrate that there is an optimal number of glass fiber layers that can be added as after this optimal number, the tensile strength start to deteriorate. Microstructural analysis was undertaken after tensile testing followed by ultrasonic testing to assess the uniformity of the GFRP composites.

3D printed polycarbonate reinforced acrylonitrile–butadiene–styrene composites: Composition effects on mechanical properties, micro-structure and void formation study

3D printing is one of the most popular additive manufacturing technique due to its usage in vast applications. The process of 3D printed polycarbonate (PC) reinforced acrylonitrile-butadiene-styrene (ABS) composite increases the mechanical properties and yields higher strength for 3D printed structures/products. In this paper, a comparative study was conducted on PC/ABS polymer composites developed using fused deposition modeling (FDM) and conventional compression molding (CM). The proposed study aims at analyzing 3D printed PC/ABS in terms of their processibility, microstructure, and mechanical performance. Three different specimens were prepared with weight percentages (10 wt%, 20 wt%, and 30 wt%) of PC reinforcement in ABS. Mechanical properties of the specimens are used to find the best composition of the composite using FDM and CM. Similarly, the microstructure of specimens is studied to identify the variations in the strength of the polymer composites. This study proves the compatibility of the two polymers. With an increase in the PC content in the sample, the hardness and strength are improved and can provide an excellent amount of strength to the product at a required concentration of PC reinforcement. This phenomenon was explained based on changes in the void formation using micro-structural study. Knowing the appropriate polymer composition, it contributes to printing complex 3D printed with better rational, aesthetic and economic benefits for different applications such as automotive, marine, and several other fields.

Characterization of the anisotropic thermal conductivity of additively manufactured components by fused filament fabrication

Recently, additive manufacturing (AM) has been successfully employed to fabricate heat exchangers due to its ability to create complex geometrical structures with high volumetric-to-area ratio, which can be designed to increase convective heat transfer from surfaces. Fused filament fabrication (FFF) is one of the most popular AM methods due to it is accessible and low-cost hardware. The effect of process parameters on the mechanical properties of FFF 3D-printed parts has been studied extensively. However, there are limited reliable data for the thermal conductivity of 3D-printed components which has impeded the development of additively manufactured heat exchangers. In the current study, the effect of the layer height and raster width have been investigated experimentally and numerically to characterize the effective thermal conductivity of 3D-printed components and investigate the thermal anisotropic nature of unidirectional printed parts. The results showed that increasing the layer height and width causes deterioration in the effective thermal conductivity of up to 65% of the pure polymer. In addition, the thermal conductivity was measured for a range of PLA composites and it was found that their anisotropic ratio can be as high as 2. The unidirectional effective conductivity model was subsequently modified to characterize the common cross-hatched layer fill configuration, and the influence of fill ratio on the effective thermal conductivity was investigated. Finally, the effective thermal conductivity of several commercially available PLA composite filaments was characterized experimentally.

Improving the filament weld-strength of fused filament fabrication products through improved interdiffusion

Fused Filament Fabrication (FFF) is a popular additive manufacturing technique where molten polymer filament is applied in a raster pattern, layer by layer, to obtain the work piece. A necessary consequence of this method is a pronounced mechanical anisotropy of the product; the interface between the filaments is weaker compared to the filament itself. The strength of this interface is governed by the reptation theory which postulates a more efficient interpenetration of polymeric surfaces with decreasing polymer viscosity. This relationship was utilized in this work to modify a polycarbonate-acrylonitrile butadiene styrene polymer blend to produce FFF work pieces with less mechanical anisotropy, independent of printer settings. The tensile strength ratio of the printed interface to bulk tensile strength could be increased from 41% to 95%. Though the absolute bulk tensile strength decreases slightly, this method presents an easy and effective way to address the mechanical problems inherent in the FFF-method.