Item2018 International Solid Freeform Fabrication Symposium Table of Contents(2018) Laboratory for Freeform Fabrication and University of Texas at Austin ItemmicroCLIP Ceramic High-Resolution Fabrication and Dimensional Accuracy Requirements(University of Texas at Austin, 2018) Ware, Henry Oliver T; Sun, ChengCeramics have been broadly used as structural and functional materials with a wide range of engineering applications. Recent introduction of Continuous Liquid Interface Production (CLIP) uses projection UV photopolymerization and oxygen inhibition to tremendously reduce fabrication time. In addition to 3D printing polymeric materials, it has demonstrated the feasibility of fabricating 3D ceramic parts using photo-curable ceramic resins. However, the associated ceramic particle light-scattering significantly alters the process characteristics of the CLIP process, resulting in broadening of the lateral dimensions in associated with the reduction in the curing depth. Varying the exposure conditions to accommodate the scattering effect further affects the deadzone thickness, which introduces a systematic defocusing error to further complicate the process control. In this work we show that careful characterization and balance of both effects yields an optimal set of process parameters (UV Power and stage speed) for high-resolution 3D fabrication with a given photo-curable ceramic resin. ItemStructurally Intelligent 3D Layer Generation for Active-Z Printing(University of Texas at Austin, 2018) Khurana, Jivtesh B.; Simpson, Timothy W.; Frecker, MaryActive-Z Printing offers the ability to deposit material along non-planar layers to control the mechanical behavior of parts produced by material extrusion additive manufacturing. These non-planar layers can be exploited to incorporate a part’s loading conditions into the slicing process by aligning deposited layers with predicted localized stress tensors. In this work, we demonstrate that superior structural performance can be achieved by taking advantage of layer shapes derived from principal stress trajectories. A slicing method incorporating stress field data is developed to generate 3D layers from principal stress trajectories. As a demonstration, a 3-point bend specimen is manufactured with 3D layers derived from principal stress trajectories developed in a deformed specimen. Mechanical tests are conducted and 3-point bend specimens are shown to have superior mechanical response. This novel approach introduces new capabilities to Additive Manufacturing for structurally intelligent fabrication. ItemZ-Pinning Approach for Reducing Mechanical Anisotropy of 3D Printed Parts(University of Texas at Austin, 2018) Duty, Chad; Failla, Jordan; Kim, Seokpum; Smith, Tyler; Lindahl, John; Roschli, Alex; Post, Brian; Love, Lonnie; Kunc, VlastimilThe mechanical strength of extrusion-based printed parts is often greatly reduced (25-50%) in the build direction (z-direction) compared to the in-plane strength due to poor bonding between successively deposited layers. This effect can be magnified (75-90% difference) when depositing fiber-reinforced materials or larger print areas with long layer times. Therefore, a patent-pending approach has been developed that deposits material into intentionally aligned voids in the z-direction, allowing continuous material to span multiple layers. The “z-pinning” approach can be applied to several concepts for improving the interlaminar strength of extrusion-based 3D printed parts as well as techniques for applying the technology across a broad spectrum of deposition platforms and material systems. Initial experimental results demonstrate a significant improvement (>3x) in mechanical strength and (>8x) toughness for fiber reinforced components. ItemFabrication of Aligned Nanofibers Along Z-Axis – A Novel 3D Electrospinning Technique(University of Texas at Austin, 2018) Tan, George Z.; Zhou, YinggeThis study presents a 3D fabrication technique of nanofibrous scaffold for tissue engineering. A divergence static electric field was introduced in an electrospinning system to induce a self-assembly of aligned nanofibers into a tunable 3D architecture with thickness ranging from 2-12 mm. The effects of collector configuration on polycaprolactone (PCL) nanofiber attributes were investigated. Human fibroblast cells were cultured on the nanofiber scaffold in vitro for 7 days. It was found that the width and inclination angle of the collector influenced the nanofiber density distribution. The cells proliferated on the scaffold and organized as a fibrous matrix which mimicked the microstructure of native musculoskeletal tissues. ItemNanoparticle Bed Deposition by Slot Die Coating for Microscale Selective Laser Sintering Applications(University of Texas at Austin, 2018) Behera, Dipankar; Roy, Nilabh K.; Foong, Chee S.; Cullinan, MichaelThe minimum feature size in most commercially available metal additive manufacturing (AM) processes is limited to ~100 microns which poses a fundamental challenge in fabricating complex 3D micro-components. The authors have developed a microscale selective laser sintering (µ-SLS) process with the goal of fabricating these microproducts with 1µm minimum feature size resolution. To achieve near-net shaped sintered features, the powder bed layer should not be more than one micron thick. This paper presents the development and testing of a powder bed deposition mechanism using a slot-die coater. Metallic nanoparticles uniformly dispersed in a solvent were used in this study. A viscocapillary coating model was used to predict the wet thickness of the powder bed based on the coating gap. Experimental results revealed that uniform sub-micron layer thicknesses were achieved by optimizing the process parameters such as flow rate, coating speed, coating gap, and die gap. The novel approach discussed in this paper enables the implementation of a robust coating mechanism for high throughput AM. ItemFrequency Response Inspection of Additively Manufactured Parts for Defect Identification(University of Texas at Austin, 2018) Johnson, Kevin; Blough, Jason; Barnard, Andrew; Hartwig, Troy; Brown, Ben; Soine, David; Collum, Tristan; Kinzel, Edward; Bristow, Douglas; Landers, RobertThe goal of this paper is to evaluate internal defects in AM parts using dynamic measurements. The natural frequencies of AM parts can be identified by measuring the response of the part(s) to a dynamic input. Different excitation methods such as a modal impact hammer or shakers can be used to excite the parts. Various methods exist to measure the parts’ responses and find the natural frequencies. This paper will investigate the use of Doppler lasers, accelerometers and Digital Image Correlation (DIC). The parts evaluated in this work include sets of parts that are still attached to the AM build plate, this makes the identification of a faulty part much more difficult as parts on a build plate interact with each other as well as the build plate complicating the responses. Several approaches to these issues will be presented based on the above listed response measurements. ItemResearch on Relationship between Depth of Fusion and Process Parameters in Low-Temperature Laser Sintering Process(University of Texas at Austin, 2018) Kigure, T.; Yamauchi, Y.; Niino, T.Model of low-temperature laser sintering, in which part warpage during process is prevented by anchoring of parts instead of high-temperature preheating, is discussed. Low-temperature laser sintering process allows powder bed temperature to be lower than those in normal laser sintering process which suppresses parts warpage by preheating powder bed above its recrystallization temperature. When we introduce a new process or material, many experimental examinations are required to decide adequate building conditions. To reduce this process, theoretical process modeling and simulations are carried out. In stereolithography, relationship between laser irradiance and cure depth is known as “working curve,” and used for fundamental equation for this technology. On the other hand, many theoretical models for laser sintering have been introduced, and most of them are thermal models dealing with heat transfer in powder bed. Contrarily, there are few reports concerning measurement and calculation of fusion depth though fusion depth can be obtained easily by experiment and working curve is a useful to determination of building parameters. In this study, working curve which represents relationship between part thickness obtained by monolayer scan and incident energy was investigated. As a result of normalizing the power by the minimum power that can melt the surface of the powder bed, all the plots lay on the same single line. This line, namely master curve, is unique for each powder and useful to finding various parameters. ItemMicrowave Assisted Selective Laser Melting of Technical Ceramics(University of Texas at Austin, 2018) Buls, Sam; Vleugels, Jef; Van Hooreweder, BrechtDirect processing of near fully dense technical ceramics is not possible with conventional additive manufacturing (AM) processes due to the very high temperatures that are required. Therefore, indirect AM approaches are often used. These indirect processes show great potential but require extensive post processing (e.g. debinding and sintering) leading to shrinkage, limited geometrical accuracy and eventually limiting overall part quality. To overcome these limitations, this paper presents a novel Microwave Assisted Selective Laser Melting process that enables direct processing of technical ceramics. ItemRealtime Control-Oriented Modeling and Disturbance Parameterization for Smart and Reliable Powder Bed Fusion Additive Manufacturing(University of Texas at Austin, 2018) Chen, X.; Wang, D.; Jiang, T.; Xiao, H.The vision of sustainable mass customization calls for additive manufacturing (AM) processes that are resilient to process variations and interruptions. This work targets to pioneer a system-theoretical approach towards such a smart and reliable AM. The approach is based on control-oriented modeling of the process variations and on closed-loop model-based controls that facilitate in-situ adjustment of the part quality. Specifically, one focused example is laser-aided powder bed fusion. Building on the in-layer precision heating and solidification, together with layer-by-layer iterations of the energy source, feedstock, and toolpath, we discuss mathematical abstractions of process imperfections that will not only understand the intricate thermomechanical interactions but are also tractable under realtime computation budgets. In particular, we develop and validate a surrogate modeling of in-process disturbances induced by the periodic in- and cross-layer thermomechanical interactions. This control-oriented disturbance modeling allows for the adoption of high-performance control algorithms to advance AM quality in a closed loop, and we show a first-instance study on the effect of repetitive controls in reducing melt-pool variations in the periodic energy deposition. ItemPredictive Iterative Learning Control with Data-Driven Model for Optimal Laser Power in Selective Laser Sintering(University of Texas at Austin, 2018) Nettekoven, A.; Fish, S.; Topcu, U.; Beaman, J.Building high quality parts is still a key challenge for Selective Laser Sintering machines today due to a lack of sufficient process control. In order to improve process control, we propose a Predictive Iterative Learning Control (PILC) controller that minimizes the deviation of the postsintering temperature profile of a newly scanned part from a desired temperature. The controller does this by finding an optimal laser power profile and applying it to the plant in a feedforward manner. The PILC controller leverages machine learning models that accurately capture the process’ temperature dynamics based on in-situ measurement data while still guaranteeing low computational cost. We demonstrate the controller’s performance in regards to the control objective with heat transfer simulations by comparing the PILC-controlled laser power profiles to constant laser power profiles. ItemLaser Metal Additive Manufacturing on Graphite(University of Texas at Austin, 2018) Azizi, Arad; Schiffres, Scott N.Metal powders are typically directly fused to a metal substrate of similar composition in metal powder-bed fusion additive manufacturing. This work presents a process for printing a metal alloy directly on graphite, rather than a metal platform. This technology has attractive applications to heat transfer, as this process can be used to directly print heat sinks or heat exchangers on pyrolytic graphite. The heat transfer applications of metal-pyrolytic graphite are enormous, as pyrolytic graphite has the second highest thermal conductivity (>1700 W/m-K at room temperature) of any bulk material, with only expensive diamond exceeding it. Bonding of common metal alloys used in additive manufacturing and graphite are relatively weak and possess high contact angles. However, by using the proper interlayer material, wettability and reactivity of the graphite substrate with metal powder can increase drastically. The alloys that can typically bond to graphite require extended times at elevated temperatures (minutes to hours), while this study demonstrates rapid bonding (~100 µs). ItemTwo-Dimensional Characterization of Window Contamination in Selective Laser Sintering(University of Texas at Austin, 2018) Sassaman, Doug; Hall, Peter; Fish, Scott; Beaman, JosephMost Laser Sintering machines suffer from an issue where it is hypothesized that hot gases produced during the laser sintering process collect on the Zinc selenide (ZnSe) window separating the build chamber from the environment. This contamination has previously been shown to reduce delivered laser power by up to 10%, and necessitate frequent cleaning and replacement of the windows. A power meter was constructed in order to perform ex-situ measurements of laser attenuation at various locations on the window. Identical builds were performed using fire-retardant nylon 11 on a DTM Sinterstation 2500, and the windows were measured before and after each build. Results indicate that contamination is not uniform on the window, and may cause a variation in laser attenuation up to 3.5%±0.25% depending on scanning location. It is also shown here that the contamination patterns are not repeatable from build to build, even if performed on the same machine. ItemLaser Heated Electron Beam Gun Optimization to Improve Additive Manufacturing(University of Texas at Austin, 2018) Edinger, R.Electron Beam Additive Manufacturing requires to improve electron gun characteristics to become a highly competitive manufacturing process. Our work targets the optimization of beam focusing to reduce the beam spot size, to improve the beam deflection system resulting in higher positioning accuracy, to refine thermal stability by minimizing heat induced drifting and to introduce a new powder delivery device which can be synchronized to beam parameters. Heisenberg's uncertainty principle states that if a position of a particle is precisely known, its momentum becomes less accurate and vice versa. Therefore, it will be required to conceive gun parameters optimizing the balance of opposing laws. Our goal is to deliver an open platform electron beam additive manufacturing machine which utilizes the results presented in this paper. ItemTowards High Build Rates: Combining Different Layer Thickness within One Part in Selective Laser Melting(University of Texas at Austin, 2018) Kniepkamp, Michael; Harbig, Jana; Seyfert, Christoph; Abele, EberhardAdditive manufacturing of metallic parts using powder bed based fusion processes like selective laser melting is increasingly used in industrial applications. With typical layer thicknesses of 20 – 40 µm good surface qualities and high geometrical accuracy can be achieved compared to other AM processes. However, low layer thicknesses are to the detriment of build rates as more layers are required. Increasing the layer thickness can significantly increase build rates at the cost of surface quality and accuracy. In this paper a new parameter set for a layer thickness of 60 µm is developed and combinations of different layer thicknesses within one part are investigated. Thus increased build rates can be achieved while a high accuracy can be maintained when locally required. Specimens with combination of different layer thicknesses in various build orientations are produced and mechanically tested. Micrographs of the layer transitions are examined and recommendations for their design are given. ItemDevelopment of Novel High Temperature Laser Powder Bed Fusion System for the Processing of Crack-Susceptible Alloys(University of Texas at Austin, 2018) Caprio, L.; Chiari, G.; Demir, A.G.; Previtali, B.In the industrial panorama, Laser Powder Bed Fusion (LPBF) systems enable for the near net shaping of metal powders into complex geometries with unique design features. This makes the technology appealing for many industrial applications, which require high performance materials combined with lightweight design or conformal cooling channels. However, many of the alloys that would be ideal for the realisation of these functional components are classified as difficultly weldable due to their cracking sensitivity. Currently, industrial SLM systems employ baseplate preheating to minimise these effects although this solution is limitedly effective along the build direction and often does not achieve high enough temperatures for the realisation of crack-free specimen. In this work, the design and implementation of a novel inductive high temperature LPBF system is presented. Furthermore, preliminary results regarding depositions of Titanium Aluminide alloy with and without preheating are reported, showing the potential of the solution developed. ItemOn the Influence of Thermal Lensing During Selective Laser Melting(University of Texas at Austin, 2018) Goossens, Louca R.; Kinds, Yannis; Kruth, Jean-Pierre; Van Hooreweder, BrechtMulti kilowatt single mode lasers are increasingly being used in Selective Laser Melting (SLM), typically with the aim of improving productivity. However, the high power densities present in the optical path lead to a thermally induced focal shift i.e. thermal lensing. Whilst thermal lensing has been studied for many processes, its impact on parts produced by SLM is currently unknown. Therefore this work discusses the characteristics of a thermally induced focal shift supplemented by a method for the compensation of this effect. In addition, SLM parts with and without thermal lensing compensation are compared in order to show the effect on final part quality. ItemEffects of Identical Parts on a Common Build Plate on the Modal Analysis of SLM Created Metal(University of Texas at Austin, 2018) Cullom, Tristan; Hartwig, Troy; Brown, Ben; Johnson, Kevin; Blough, Jason; Barnard, Andrew; Landers, Robert; Bristow, Douglas; Kinzel, EdwardThe frequency response of parts created with Additive Manufacturing (AM) is a function of not only process parameters, powder quality, but also the geometry of the part. Modal analysis has the potential to evaluate parts by measuring the frequency response which are a function of the material response as well as the geometry. A Frequency Response Function (FRF) serves as a fingerprint of the part which can be validated against the FRF of a destructively tested part. A practical scenario encountered in Selective Laser Melting (SLM) involves multiple parts on a common build plate. Coupling between parts influences the FRF of the parts including shifting the resonant frequencies of individual parts in ways that would correspond to changes in the material response or geometry. This paper investigates the influence of the build plate properties on the coupling phenomena. ItemDMP Monitoring as a Process Optimization Tool for Direct Metal Printing (DMP) of Ti6Al-4V(University of Texas at Austin, 2018) Ray, Nachiketa; Bisht, Manisha; Thijs, Lore; Van Vaerenbergh, Jonas; Coeck, SamMetal Additive Manufacturing (AM) has evolved as a production technique for rapid prototyping as well as high volume precision manufacturing. In this work, DMP Monitoring, a new feature of 3D Systems’ direct metal printer, ProX® DMP 320 has been used as a tool for process parameter optimization. The effect of the variations of process parameters like layer thickness, laser power, scan speed and hatch spacing on the physical and mechanical properties of the additively manufactured Ti-6Al-4V samples have been investigated. In addition to the conventional post-processing evaluation methods like Archimedes’ density, X-ray CT and tensile testing, new in-situ process monitoring tools are assessed and compared with the traditional evaluation methods. ItemUse of SWIR Imaging to Monitor Layer-to-Layer Part Quality During SLM of 304L Stainless Steel(University of Texas at Austin, 2018) Lough, Cody S.; Wang, Xin; Smith, Christopher C.; Adeniji, Olaseni; Landers, Robert G.; Bristow, Douglas A.; Kinzel, Edward C.This paper evaluates using in-situ SWIR imaging to monitor part quality and identify potential defect locations introduced during Selective Laser Melting (SLM) of 304L stainless steel. The microstructure (porosity, grain size, and phase field) and engineering properties (density, modulus, and yield strength) depend on the thermal history during SLM manufacturing. Tensile test specimens have been built with a Renishaw AM250 using varied processing conditions to generate different thermal histories. SWIR imaging data is processed layer-to-layer to extract features in the thermal history for each process condition. The features in the thermal history are correlated with resulting part engineering properties, microstructure, and defects. The use of SWIR imaging is then discussed as a potential for processes monitoring to ensure part quality and develop layer-to-layer control in SLM.