An analytical method to predict and compensate for residual stress-induced deformation in overhanging regions of internal channels fabricated using powder bed fusion

Powder bed fusion (PBF) is ideally suited to build complex and near-net-shaped metallic structures such as conformal cooling channel networks in injection molds. However, warpage occurring due to the residual stresses inherent to this process can lead to shape deviation in the internal channels and needs to be minimized. In this research, a novel analytical model based on the Euler-Bernoulli beam bending theory was developed to estimate the residual stress-induced deformation of internal channels printed horizontally using PBF. The model was used to predict the shape deviation for three different shapes of channel cross sections (circular, elliptical, and diamond-shaped), and showed very good agreement with the experimentally determined shapes of nine different internal channels (three cases per cross-sectional shape). Further, the model predictions were used to compensate for the shape deviation in the design stage, resulting in a reduction in root mean square (RMS) deviation of the circular channel by a factor of 2. The proposed approach is thus expected to be a useful tool to generate design-for-AM guidelines for the additive manufacturing of overhangs and internal channels

Prediction of thermal stresses and shape deviation of selective laser melted overhanging region with a coupled CFD-FEM model

Selective laser melting (SLM), also known as powder bed fusion (PBF), is a flexible approach to fabricate complex-shaped metal parts layer-by-layer, especially for parts with complex interior shapes that are difficult to be machined conventionally. One of its typical applications is to fabricate molds consisting of conformal cooling system in which cooling channels may have to be printed horizontally without any supports [1]. Moreover, the internal channel surface cannot be further finished after SLM due to structural limitations. Thermal stress-induced deformation and surface roughness of the overhanging region are two major contributors to shape deviation and are thus concerns that must be addressed. The simulation work presented in this abstract investigates the mechanisms of deformation and surface roughness on overhanging region induced by thermo-mechanical behavior of SLM process under different overhanging angles, laser power, and scan velocity. A 3D coupled CFD-FEM model is developed by considering the heat conduction, melting and solidification with latent heat, surface tension, as well as Marangoni convection. A quasi-randomly distributed powder bed is employed. The simulation results are validated with SLM printing experiments. The overhanging region is nonrigid and essentially a cantilever due to the unmelted powder below. The simulation result shows that the stresses in the SLMed overhanging region are much lower than the stresses in the solid region. The stresses in the overhanging region are released, however, leading to unwanted upward deflection. The surface roughness on the overhanging region is largely determined by the shape and size of the molten pool. It increases with increasing overhanging angle and energy input per volume (i.e. increase of laser power or decrease of scan velocity). This simulation work can thus be directly used to compensate for the shape deviation in the design stage, namely design-for-AM guidelines for the additive manufacturing of internal channels. It will also be helpful for process parameter optimization in the overhanging region to minimize surface roughness.<br/

Prediction of thermal stresses and shape deviation of selective laser melted overhanging region with a coupled CFD-FEM model

Selective laser melting (SLM), also known as powder bed fusion (PBF), is a flexible approach to fabricate complex-shaped metal parts layer-by-layer, especially for parts with complex interior shapes that are difficult to be machined conventionally. One of its typical applications is to fabricate molds consisting of conformal cooling system in which cooling channels may have to be printed horizontally without any supports [1]. Moreover, the internal channel surface cannot be further finished after SLM due to structural limitations. Thermal stress-induced deformation and surface roughness of the overhanging region are two major contributors to shape deviation and are thus concerns that must be addressed. The simulation work presented in this abstract investigates the mechanisms of deformation and surface roughness on overhanging region induced by thermo-mechanical behavior of SLM process under different overhanging angles, laser power, and scan velocity. A 3D coupled CFD-FEM model is developed by considering the heat conduction, melting and solidification with latent heat, surface tension, as well as Marangoni convection. A quasi-randomly distributed powder bed is employed. The simulation results are validated with SLM printing experiments. The overhanging region is nonrigid and essentially a cantilever due to the unmelted powder below. The simulation result shows that the stresses in the SLMed overhanging region are much lower than the stresses in the solid region. The stresses in the overhanging region are released, however, leading to unwanted upward deflection. The surface roughness on the overhanging region is largely determined by the shape and size of the molten pool. It increases with increasing overhanging angle and energy input per volume (i.e. increase of laser power or decrease of scan velocity). This simulation work can thus be directly used to compensate for the shape deviation in the design stage, namely design-for-AM guidelines for the additive manufacturing of internal channels. It will also be helpful for process parameter optimization in the overhanging region to minimize surface roughness.<br/

Prediction of thermal stresses and shape deviation of selective laser melted overhanging region with a coupled CFD-FEM model

Selective laser melting (SLM), also known as powder bed fusion (PBF), is a flexible approach to fabricate complex-shaped metal parts layer-by-layer, especially for parts with complex interior shapes that are difficult to be machined conventionally. One of its typical applications is to fabricate molds consisting of conformal cooling system in which cooling channels may have to be printed horizontally without any supports [1]. Moreover, the internal channel surface cannot be further finished after SLM due to structural limitations. Thermal stress-induced deformation and surface roughness of the overhanging region are two major contributors to shape deviation and are thus concerns that must be addressed. The simulation work presented in this abstract investigates the mechanisms of deformation and surface roughness on overhanging region induced by thermo-mechanical behavior of SLM process under different overhanging angles, laser power, and scan velocity. A 3D coupled CFD-FEM model is developed by considering the heat conduction, melting and solidification with latent heat, surface tension, as well as Marangoni convection. A quasi-randomly distributed powder bed is employed. The simulation results are validated with SLM printing experiments. The overhanging region is nonrigid and essentially a cantilever due to the unmelted powder below. The simulation result shows that the stresses in the SLMed overhanging region are much lower than the stresses in the solid region. The stresses in the overhanging region are released, however, leading to unwanted upward deflection. The surface roughness on the overhanging region is largely determined by the shape and size of the molten pool. It increases with increasing overhanging angle and energy input per volume (i.e. increase of laser power or decrease of scan velocity). This simulation work can thus be directly used to compensate for the shape deviation in the design stage, namely design-for-AM guidelines for the additive manufacturing of internal channels. It will also be helpful for process parameter optimization in the overhanging region to minimize surface roughness.<br/

Prediction of thermal stresses and shape deviation of selective laser melted overhanging region with a coupled CFD-FEM model

Selective laser melting (SLM), also known as powder bed fusion (PBF), is a flexible approach to fabricate complex-shaped metal parts layer-by-layer, especially for parts with complex interior shapes that are difficult to be machined conventionally. One of its typical applications is to fabricate molds consisting of conformal cooling system in which cooling channels may have to be printed horizontally without any supports [1]. Moreover, the internal channel surface cannot be further finished after SLM due to structural limitations. Thermal stress-induced deformation and surface roughness of the overhanging region are two major contributors to shape deviation and are thus concerns that must be addressed. The simulation work presented in this abstract investigates the mechanisms of deformation and surface roughness on overhanging region induced by thermo-mechanical behavior of SLM process under different overhanging angles, laser power, and scan velocity. A 3D coupled CFD-FEM model is developed by considering the heat conduction, melting and solidification with latent heat, surface tension, as well as Marangoni convection. A quasi-randomly distributed powder bed is employed. The simulation results are validated with SLM printing experiments. The overhanging region is nonrigid and essentially a cantilever due to the unmelted powder below. The simulation result shows that the stresses in the SLMed overhanging region are much lower than the stresses in the solid region. The stresses in the overhanging region are released, however, leading to unwanted upward deflection. The surface roughness on the overhanging region is largely determined by the shape and size of the molten pool. It increases with increasing overhanging angle and energy input per volume (i.e. increase of laser power or decrease of scan velocity). This simulation work can thus be directly used to compensate for the shape deviation in the design stage, namely design-for-AM guidelines for the additive manufacturing of internal channels. It will also be helpful for process parameter optimization in the overhanging region to minimize surface roughness.<br/

3D Printed Graphene-Coated Flexible Lattice as Piezoresistive Pressure Sensor

Piezoresistive sponges represent a popular design for highly flexible pressure sensors and are typically fabricated using templating methods. In this work, we used stereolithography (SLA) to 3D-print an elastomeric body-centered cubic (BCC) lattice structure with a relative density of 21% and an elastic modulus of 31.5 kPa. The lattice was dip-coated with graphene nanoplatelets to realize a piezoresistive pressure sensor with excellent performance (gauge factor = 3.25, sensitivity = 0.1 kPa-1), high deformability (up to 60 % strain), and repeatability. The novel approach outlined in this work offers greater control over the microstructure and can be used to fabricate sensors with tunable properties

Design and fabrication of conformal cooling channels in molds:Review and progress updates

Conformal cooling (CC) channels are a series of cooling channels that are equidistant from the mold cavity surfaces. CC systems show great promise to substitute conventional straight-drilled cooling systems as the former can provide more uniform and efficient cooling effects and thus improve the production quality and efficiency significantly. Although the design and manufacturing of CC systems are getting increasing attention, a comprehensive and systematic classification, comparison, and evaluation are still missing. The design, manufacturing, and applications of CC channels are reviewed and evaluated systematically and comprehensively in this review paper. To achieve a uniform and rapid cooling, some key design parameters of CC channels related to shape, size, and location of the channel have to be calculated and chosen carefully taking into account the cooling performance, mechanical strength, and coolant pressure drop. CC layouts are classified into eight types. The basic type, more complex types, and hybrid straight-drilled-CC molds are suitable for simply-shaped parts, complex-shaped parts, and locally complex parts, respectively. By using CC channels, the cycle time can be reduced up to 70%, and the shape deviations can be improved significantly. Epoxy casting and L-PBF show the best applicability to Al-epoxy molds and metal molds, respectively, because of the high forming flexibility and fidelity. Meanwhile, LPD has an exclusive advantage to fabricate multi-materials molds although it cannot print overhang regions directly. Hybrid L-PBF/CNC milling pointed out the future direction for the fabrication of high dimensional-accuracy CC molds, although there is still a long way to reduce the cost and raise efficiency. CC molds are expected to substitute straight-drilled cooling molds in the future, as it can significantly improve part quality, raise production rate and reduce production cost. In addition to this, the use of CC channels can be expanded to some advanced products that require high-performance self-cooling, such as gas turbine engines, photoinjectors and gears, improving working conditions and extending lifetime

PDMS Flow Sensors With Graphene Piezoresistors Using 3D Printing and Soft Lithography

This paper reports the fabrication and characterization of a flexible piezoresistive flow sensor comprising a polydimethylsiloxane (PDMS) cantilever with a serpentine graphene nanoplatelets (GNP) strain gauge embedded at the cantilever base. A facile and cleanroom-free processing work flow involving a combination of high-resolution powder bed fusion and soft lithography was used to fabricate PDMS cantilevers (aspect ratio 20) with 150 µm × 150 µm microchannels on its surface. A high gauge factor of 55 (up to 5 times higher than reported in comparable piezoresistive flow sensors) was achieved using drop-casted GNP ink as the piezoresistive sensing element in the aforementioned microchannels. Finally, the use of the PDMS-graphene cantilever as an airflow sensor with enhanced sensitivity (20 times more than comparable piezoresistive cantilever sensors), low hysteresis, good repeatability, and bidirectional sensing capability was demonstrated

Source Seeking Control of Unicycle Robots with 3-D-Printed Flexible Piezoresistive Sensors

We present the design and experimental validation of source seeking control algorithms for a unicycle mobile robot that is equipped with novel 3D-printed flexible graphene-based piezoresistive airflow sensors. Based solely on a local gradient measurement from the airflow sensors, we propose and analyze a projected gradient ascent algorithm to solve the source seeking problem. In the case of partial sensor failure, we propose a combination of Extremum-Seeking Control with our projected gradient ascent algorithm. For both control laws, we prove the asymptotic convergence of the robot to the source. Numerical simulations were performed to validate the algorithms and experimental validations are presented to demonstrate the efficacy of the proposed methods