2014 International Solid Freeform Fabrication Symposium

Permanent URI for this collectionhttps://hdl.handle.net/2152/88475

Proceedings for the 2014 International Solid Freeform Fabrication Symposium. For more information about the symposium, please see the Solid Freeform Fabrication website.

The Twenty-Fifth Annual International Solid Freeform Fabrication (SFF) Symposium – An Additive Manufacturing Conference, held at The University of Texas in Austin on August 4-6, 2014, was attended by 334 researchers from 16 countries. This is a significant increase from the record-setting 2013 attendance of 218, over 50% growth in one year. The organizers are pleased that 90 of the attendees were students, representing 27% of the audience. The number of oral and poster presentations increased as well, from 118 in 2013 to 196 this year. In anticipation of the increase, the conference organizers relocated the meeting from the University of Texas Thompson Conference Center, the venue for the meeting’s first 24 years. The AT&T Executive Education and Conference Center was able to handle the increased number of attendees.

The meeting consisted of plenary and parallel technical sessions. To celebrate the 25th anniversary of the conference, the morning plenary sessions on the first two days were given over to looking back at the first 25 years of additive manufacturing and looking forward to the next 25 years, respectively. The Monday morning session was an inventor’s forum with talks by the technology inventors/founders: Chuck Hull (vat polymerization), Michael Cima (Binder Jetting), Lisa Crump (Material Extrusion) and Carl Deckard (Powder Bed Fusion). The session was opened by Harris Marcus, founder of the SFF Symposium, who gave a brief history of the conference. Terry Wohlers followed with perspectives on the origins of additive manufacturing. Terry deserves special credit for giving his entire presentation the way we did presentations 25 years ago: completely using 35 mm slides which were produced and archived by him in the 1980s-1990s. For a significant number of attendees, this was the first time they had seen a 35 mm presentation. The Tuesday morning plenary session was a series of short presentations contrasting how we did research 25 years ago with how we do research today. The topic areas and speakers were Process Development (Phill Dickens, Then; Brent Stucker, Now), Computational Methods (Rich Crawford, Then; Jack Beuth, Now), Materials (Gideon Levy, Then, Tom Starr, Now), Design (David Rosen, Then, Carolyn Seepersad, Now). Gideon Levy then chaired a short panel on the subject of the next 25 years of additive manufacturing to transition from the past and present to the future. While these presentations do not have associated papers in this conference Proceedings, the organizers have arranged for many of the presentations themselves to be included in the flash drive. These appear in the folder “Plenary Presentations”.

Tuesday evening, Hod Lipson gave an outstanding plenary presentation on the future of additive manufacturing. This was followed by the International Digital Sculpture and Engineered Forms Exhibit, curated by Mary Visser of Southwestern University. The art show included digital art by 20 digital artists from around the world. Also shown were 10 engineered pieces that have an artistic quality. The attendees were invited to enjoy the pieces and to consider the amorphous interface between art and engineering.

This year’s best oral presentation was entitled, “A Survey of Sensing and Control Systems for Machine and Process Monitoring of Directed-Energy, Metal-Based Additive Manufacturing”, authored by E.W. Reutzel and A.R. Nassar from Pennsylvania State University. Selection is based on the overall quality of the paper, the presentation and discussion at the meeting, the significance of the work and the manuscript submitted to the proceedings. Selected from 171 oral presentations, the associated manuscript appears on Page 309. The best poster presentation selected from 25 posters was given by Y. Bai and C.B. Williams from Virginia Polytechnic Institute and State University. Titled, “An Exploration of Binder Jetting of Copper”, the paper is included in the Proceedings on Page 793.

Posters are judged based on the quality and organization of the poster as well as the discussion of the poster by the author during the poster session.

The recipient of the International Outstanding Young Researcher in Freeform and Additive Manufacturing Award was Dr. Adam T. Clare from The University of Nottingham. Dr. Joseph Beaman won the International Freeform and Additive Manufacturing Excellence (FAME) Award. He holds the Earnest F. Gloyna Regents Chair in Engineering at The University of Texas at Austin.

The editors would like to extend a warm “Thank You” to Rosalie Foster for her detailed handling of the logistics of the meeting, as well as her excellent performance as registrar during the meeting. We would like to thank the Organizing Committee, the session chairs, the attendees for their enthusiastic participation, and the speakers both for their significant contribution to the meeting and for the relatively prompt delivery of the manuscripts comprising this volume. We look forward to the continued close cooperation of the additive manufacturing community in organizing the Symposium. We also want to thank the Office of Naval Research (N00014-14-1-0691) and the National Science Foundation (CMMI-1433422) for supporting this meeting financially. The meeting was co-organized by The University of Connecticut at Storrs, and the Mechanical Engineering Department/Lab for Freeform Fabrication under the aegis of the Advanced Manufacturing and Design Center at The University of Texas at Austin.

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Processing and Characterizations of Eucalyptus-PA12 Composite by Laser Sintering
(University of Texas at Austin, 2014) Jiang, Kaiyi; Guo, Yanling; Bourell, David L.
To meet the existing requirements to make Laser Sintering (LS) technology more economical and environmentally friendly, a new type of low cost sustainable material (eucalyptus-polyamide 12 (EPA12) composite) was developed. This paper presents initial research into the LS PA12 with wood powder additions. EPA12 mixed in a ratio of 1:2 by volume has been shown to be extremely processable by LS. Before sintering experiments, thermal conductivity (which is an important component in understanding and optimizing the processing of laser sintering EPA 12) was measured. During LS processing, a variety of laser powers were chosen to investigate the effect of the energy input on the densification, mechanical properties and forming accuracy of the material. The dispersion of eucalyptus in the LS specimens of the composite powder was examined by scanning electron microscopy (SEM). The microstructure of sintered EPA12 was observed. By comparing the microstructures, observable differences based on varying levels of laser power were also present. The maximum tensile strength and the flexure strength of prototypes are 3.7 MPa and 38 MPa, respectively. These values increased from the minimum with increased energy input. On the contrary, the forming accuracy was high at a relatively low laser power.
Simulation of the Surface Topography on Laser Sintered Polymer Parts
(University of Texas at Austin, 2014) Delfs, Patrick; Herale, Ashrith Ananthraj; Li, Zhiyuan; Schmid, Hans-Joachim
One barrier of laser sintering (LS) to become the main process for Direct Manufacturing (DM) is the surface quality of LS parts. Hence, the property which has to be improved is the rough surfaces of LS parts due to the layered structure. Another additional effect is the incomplete melting of powder particles on the surface due to the high process temperature. In this paper we demonstrate our approach of a theoretical model for the topography of LS part surfaces. We investigated the surface roughness as a function of surface orientation. Considering that the model involves further variables as layer thickness, particle density and particle size distribution to describe the topography precisely. Experimental results were used to optimize and check the results of the model.
Wire+Arc Additive Manufacturing of Aluminum
(University of Texas at Austin, 2014) Gu, Jianglong; Cong, Baoqiang; Ding, Jialuo; Williams, Stewart W.; Zhai, Yuchun
Wire+Arc Additive Manufacturing is very suitable for the production of large scale aluminium parts. However implementation is currently limited by issues such as porosity and low mechanical properties. We have studied the utilization of new deposition processes such as pulsed advanced cold metal transfer which allows modification of the thermal profile resulting in refined equiaxed microstructure and elimination of porosity. Standard and new feedstock compositions are being evaluated and developed with ultimate tensile strengths of up to 260 MPa with 17% elongation being obtained in the as-deposited condition. Post build heat treatments compositional changes and high-pressure inter-pass rolling are being investigated in order to increase the strength further.
Mechanisms for Post Processing Deformation without Part-Specific Tooling
(University of Texas at Austin, 2014) Nussbaum, J.; Crane, N.B.
Post processing deformation of additive manufactured components offers many advantages from reduced printing time and material usage to simplified multimaterial fabrication. For thermoplastic components, controlled deformations can be achieved by locally heating to temporarily soften the material. This paper investigates multiple methods of selectively heating and deforming desired locations to form the final component. The deformation location can be controlled by geometry (thickness), surface properties (reflectivity) or by local addition of a secondary material to control the heating. Demonstrations are provided for each of these deformation methods and then an example is provided to show how this approach could produce customized geometries from a standard feature.
The BCC Unit Cell for Latticed SLM Parts; Mechanical Properties as a Function of Cell Size
(University of Texas at Austin, 2014) Maskery, I.; Aremu, A.O.; Simonelli, M.; Tuck, C.; Wildman, R.D.; Ashcroft, I.A.; Hague, R.J.M.
The existing framework describing the mechanical properties of lattices places strong emphasis on one important property, the relative density of the repeating cells. In this work, we explore the effects of cell size, attempting to construct more complete models for the performance of lattices. This was achieved by examining the elastic modulus and ultimate tensile strength of latticed parts with a range of unit cell sizes and fixed density. The parts were produced by selective laser melting (SLM). The examined cell type was body-centred-cubic (BCC), a cell of high relevance for SLM because of its self-supporting structure. We obtained power law relationships for the mechanical properties of our latticed specimens as a function of cell size, which are similar in form to the existing laws for the density dependence. These can be used to predict the properties of latticed column structures comprised of BCC cells, and may be easily amended for other situations. In addition, we propose a novel way to analyse the elastic modulus data, which may lead to more general models, applicable to parts of varying size. Lastly, our general methodology may be of use in future studies which explore the other parameters that determine lattice performance; the choice of cell type, the global shape of the lattice structure and the type of stress.
Adaptive Slicing for Multi-Axis Hybrid Plasma Deposition and Milling
(University of Texas at Austin, 2014) Xiangping, Wang; Haiou, Zhang; Guilan, Wang; Lingpeng, Wu
Hybrid Plasma Deposition and Milling (HPDM), a five-axis manufacturing system integrated material additive and subtractive processes, can be used to create overhang metallic components directly without the usage of sacrificial support structure. Different from conventional slicing methods, a new slicing algorithm with changeable direction and thickness is proposed in this paper. Minimal overhang length is selected as the objective function to optimize the build direction. The thickness is adjusted to meet allowable overhang length and allowable cups height. The input mesh is first decomposed into non-uniform thickness segment meshes and then each segment is cut into uniform thickness slices. The output slices consist of split slices between two adjacent segment meshes and inner slices for each segment mesh. Examples and analyses confirm the feasibility and effectiveness.
Three-Dimensional In-Process Temperature Measurement of Laser Sintered Part Cakes
(University of Texas at Austin, 2014) Josupeit, Stefan; Schmid, Hans-Joachim
An uneven temperature distribution and varying cooling rates at different positions within the part cake are two of the most important challenges regarding the part quality and reproducibility of the polymer laser sintering process. In the presented work, a temperature measurement system is implemented within an EOSINT P395 laser sintering system. It allows the determination of a three dimensional temperature distribution and history during the full build and cooling process. The influence of important job parameters, for example the packing density, job height and layer thickness, can be figured out. In combination with a finite element simulation of the cooling process, the temperature measurement will be the basis for optimized process controls.
Object Shaping of Polystyrene with a Sonotrode
(University of Texas at Austin, 2014) Tilli, J.; Fantoni, G.; Currenti, S.; Razionale, A.V.
Rapid Prototyping methods are often based on additive manufacturing strategies. Depending on the application, it can also be considered the use of a removal manufacturing rapid prototyping methodology. In this paper the authors investigate the possibility of using the ultrasound technology for the shaping of materials that can be considered as a final object, or as a rapid manufactured mold for the creation of prototypes. In this case, free-form objects can be manufactured starting from foam like materials, and model the foam through the use of a ultrasonic horn thus shaping the desired features. The authors tested the possibility of implementing the technology onto automated systems, allowing considering the use of this system on automated manufacturing lines. Geometrical deviations from the 3D model are measured and material morphology before and after machining is analyzed.
Investigation of Advanced Process Control Methods for Exposure Controlled Projection Lithography
(University of Texas at Austin, 2014) Zhao, Xiayun; Rosen, David W.
The DMD based Exposure Controlled Projection Lithography (ECPL) process has promising applications in fabrication of microfluidics and micro optics components. Unlike a conventional layer-stacking projection stereolithography process, ECPL cures a 3D feature by projecting radiation through a stationary, transparent substrate by varying exposure patterns and durations implemented by a sequence of DMD bitmaps. Due to the unavailability of an in situ metrology for cured part dimensions, unmeasurable time-varying disturbances such as oxygen inhibition and light source fluctuations, and the complex chemical & physics interactions in photopolymerization, a common practice in stereolithography process planning is to use experimental characterization and statistics models in an open-loop mode, which yields poor accuracy. This paper reviewed existing process control methods for ECPL and defined a need for advanced control methods. As a first proposal for advanced control methods to mask projection stereolithography, the paper surveyed relevant processes and put forward a hierarchical framework of advanced control methods for ECPL, including evolutionary cycle-to-cycle (EC2C) and adaptive neural network (ANN) backstepping control methods. The goal is to identify some advanced control methods, which are capable of tracking the process dynamics by online updating the model parameters with real-time measurement feedback. Such closed-loop control methods are promising to be able to improve the process precision and robustness.
Towards a Sustainable and Economic Selection of Part Candidates for Additive Manufacturing
(University of Texas at Austin, 2014) Lindemann, Christian; Jahnke, Ulrich; Reiher, Thomas; Koch, Rainer
Additive manufacturing has a high potential for improving time and cost efficiency as well as functionality of products in many branches. Today potential users struggle with the integration of this technology in their businesses. The production costs of this technology often seem too high compared to traditionally manufactured parts and many users seem disappointed with the performance of the technology. One of the crucial points for bringing the technology to new users and new industries is the appropriate selection of feasible part candidates. A systematic selection of parts is crucial for the sustainable and successful use and integration of additive manufacturing into existing businesses. The selection needs to be based on technical, economical and strategic aspects. This paper presents a methodology to help end users to find appropriate part candidates, which are capable of bringing AM into their businesses. The concept furthermore includes approaches for redesigning current available parts and helps to estimate the economic implications of the use of the technology.
The GE Aircraft Engine Bracket Challenge: An Experiment in Crowdsourcing for Mechanical Design Concepts
(University of Texas at Austin, 2014) Carter, W.T.; Emo, D.J.; Abbott, D.H.; Bruck, C.E.; Wilson, G.H.; Wolfe, J.B.; Finkhousen, D.M.; Tepper, A.; Stevens, R.G.
An emerging international engineering design trend has resulted from widespread use of social media: a large number of people are engaged in collaborative engineering design activities to build their design expertise through interaction with other designers, to compete for recognition or prizes, or simply for the enjoyment of doing so. The term “crowdsourcing” was introduced in 2005 and implies soliciting contributions from a large group of people, usually an online community, in order to get a broad perspective from various points of view. This community-generated creativity is contrary to conventional practice in most manufacturing companies, which prefer tight control of engineering designs and practices because they represent key intellectual property and know-how. Recognizing that crowdsourcing represents a potential resource, GE embarked on an experiment to see how a for-profit company might benefit from soliciting new design approaches from this non-traditional source. The design of a specific part, an aircraft engine bracket, was released to an online community of engineers with an invitation to submit improved designs in an open competition. Entrants were encouraged to consider additive manufacturing as the fabrication method. Hundreds of designers submitted concepts and some achieved 80% reduction in weight.
Effect of Process Parameters and Heat Treatment on the Microstructure and Mechanical Properties of SLM-built HY100 Steel
(University of Texas at Austin, 2014) Dilip, J.J.S.; Stucker, Brent; Starr, Thomas L.
HY100 is a high strength low alloy steel used for naval and pressure vessel applications. In general, the alloy is used in the quenched and tempered condition. In the present work, fully dense metallic samples were produced from HY100 pre-alloyed powders using selective laser melting (SLM). Test samples were built with varying process parameters (scan speed and laser power). The SLM-built samples were given direct tempering treatment and a standard quench and temper heat treatment. Tensile properties of the samples were evaluated in direct temper, and quench and temper conditions. The study investigates the influence of process parameters and heat treatment on the microstructure and mechanical properties of SLM-built HY100 steel.
In Vitro Assessment of Laser Sintered Bioactive Glass Scaffolds with Different Pore Geometries
(University of Texas at Austin, 2014) Kolan, Krishna C.R.; Thomas, Albin; Leu, Ming C.; Hilmas, Gregory E.
The pore geometry of bioactive glass scaffolds intended for use in bone repair or replacement is one of the most important parameters that could determine the rate of bone regeneration. The pore geometry would also affect the mechanical properties of the scaffolds and their rate of degradation. Scaffolds with five different architectures, having ~50% porosity, were fabricated with silicate (13–93) and borate (13–93B3) based bioactive glasses using a laser sintering process. An established, late-osteoblasts/early-osteocytes cell line was used to perform cell proliferation tests on the scaffolds. The results indicated that the cells proliferate significantly more on the scaffolds which mimic the trabecular bone architecture compared to traditional lattice structures.
Microstructural Characterization and Modeling of Beam Speed Effects on Ti-6Al-4V by Electron Beam Additive Manufacturing
(University of Texas at Austin, 2014) Gong, X.; Lydon, J.; Cooper, K.; Chou, K.
In this study, the influence of the beam scanning speed to the microstructure of Ti-6Al4V parts processed by EBAM is investigated. EBAM parts were fabricated with 4 different scanning speeds, ranging from 214 mm/s to 689 mm/s, and the microstructures were studied. In addition, the volume fractions of different phases were obtained by thermal and phase transformation modeling. The microstructure is characterized by columnar structures of prior β grains along the build direction, and fine Widmanstätten (α+β) structure and α′ martensites are presented inside of the prior β. Both the prior β grain size and α-lath thickness decrease with the increase of the scanning speed. For the phase constitution, the volume fraction of α′ increases with the scanning speed while the volume fraction of α decreases due to the increase of cooling rate during solid phase transformation. The results from the analytical phase transformation model are consistent with the microstructure characterization from the experiment.
Characterization of Acoustic Softening of Aluminum 6061 Within a Plasticity Framework
(University of Texas at Austin, 2014) Mao, Q.; Coutris, N.; Fadel, G.
Ultrasonic additive manufacturing (UAM) is a rapid prototyping technology that features a metal joining process through ultrasonic welding. The bonding mechanisms and mechanics of UAM have been investigated for decades. Meanwhile, the plastic deformations of metals were extensively studied by many researchers for their significant roles in bond formation. However, most of these research efforts considered solely the surface frictional effects on plastic deformation whereas the volumetric effects of ultrasound were rarely considered. This paper investigates the effects of ultrasound on deformation of Aluminum 6061 through experimental studies and highlights the volumetric effects of ultrasound, i.e. the “acoustic softening”: a stress reduction on the stress-strain relation of Aluminum 6061 upon application of ultrasounds. Based on observations obtained from a designed experimental setup, a phenomenological model is proposed to characterize the acoustic softening effects in terms of the ultrasonic intensities. Additionally, by modifying Hockett’s plasticity model, a plasticity frame work is established to characterize the deformation of Aluminum 6061 in UAM. The acoustic softening model is then incorporated into the plasticity framework. The complete model is then validated by comparing its predictions with experimental measurements.
Development of Novel EBSM System for High-Tech Material Additive Manufacturing Research
(University of Texas at Austin, 2014) Guo, C.; Lin, F.; Ge, W.J.; Zhang, J.
Electron beam is more appropriate for metal additive manufacturing (AM) than laser because of its high energy converting efficiency and high absorption for various materials. It becomes a preference for AM study of high-tech material with high melting point, high brittleness or graded material. A novel electron beam selective melting (EBSM) system with dual-material processing capability has been developed in Tsinghua University to meet the wide high-tech material AM research requirement. A vibration driven powder supplier was developed and the supplier had a high compatibility to various powders. A stable supplying rate and a supplying accuracy less than 7.5% were obtained with the supplier. Two powders can be supplied individually to obtain a mixture with tailored proportion for each powder layer. The mixture is homogenous and the actual proportion is close to the desired value. In order to prolong the spreading comb’s lifetime and avoid tooth breaking, a low deformation powder spreading device was designed based on dual inclined combs and a one-way scraping mechanism. The system provides exchangeable building tanks with sizes of 100 ×100 ×100 mm3 and 250 × 250 × 250 mm3, which can save powder when the part is small and the powder is expensive. The novel EBSM system is capable of building parts with single material and has a potential of dual-material processing.
A Comparison of the Computational Speed of 3DSIM Versus ANSYS Finite Element Analyses for Simulation of Thermal History in Metal Laser Sintering
(University of Texas at Austin, 2014) Zeng, Kai; Teng, Chong; Xu, Sally; Sublette, Tim; Patil, Nachiket; Pala, Deepankar; Stucker, Brent
A new simulation infrastructure for predicting the effects of changes in process parameters on mechanical properties, residual stress/strain, crystal structure, and other micro & macro features of components made using metal-based AM techniques has been developed at the University of Louisville (UofL) and is being commercialized by 3DSIM, LLC. Based upon its MatLab and Fortran code, UofL personnel predicted their multi-scale, multi-physics finite element solvers should solve for thermal history and residual stress evolution many orders of magnitude faster than competing tools while achieving better solution accuracy. In order to test this contention, a series of computational experiments were designed to benchmark the performance of the code being commercialized by 3DSIM against a well-respected simulation tool, ANSYS. The results of these initial studies indicate the 3DSIM architecture is significantly faster than ANSYS for simulating metal-based AM processes.
3D Printing Thin Skinned Composites to Achieve the Strength-to-Weight Ratio of Aluminum
(University of Texas at Austin, 2014) Fly, David E.
Kevlar and stainless steel mesh reinforcements were added using epoxy to 3D printed ABS-M30 thin skins, thereby making a composite structure with significantly improved mechanical properties over that of the 3D printed plastic alone. These additive manufactured composites have a strength to weight ratio that is comparable to solid aluminum. Flexural 3-point bend tests and Charpy Impact tests were conducted. Experiments were conducted that were designed to characterize the influence of adding Kevlar to the composite structure and also the influence of pre-mixing glass microspheres into the epoxy. These new additive manufactured (AM) composites are an attractive choice to designers attempting to reduce weight because any 3D printed shape can be reinforced in this manner. Additionally, actual production time is less than 3D printing a fully solid component.
Comparing Mechanical and Geometrical Properties of Lattice Structure Fabricated Using Electron Beam Melting
(University of Texas at Austin, 2014) Park, Sang-in; Rosen, David W.; Duty, Chad E.
To design lattice structure, a uniform voxel based approach is widely used which divides a part into unit volumes (e.g., cubes) and maps lattice topology into those volumes. In contrast, conformal lattice structures represent a second design method for constructing lattices in which unit cells are constructed parallel to the surface to be reinforced and are deformed in a manner that enables them to conform to the surface. In this paper, the strength of lattice structures designed using these two methods (uniform voxel based and conformal) are compared based on additive manufacturing (AM) process effects. For this purpose, spheres filled with three types of lattice structure are fabricated using electron beam melting technology and tested in compression. Effects of AM processes are studied in two ways – volumetric and structural performance equivalence. Struts in lattice structures are observed through a microscope to examine volume-equivalence and tests are simulated numerically and compared to identify structural equivalence.
Design for Additive Manufacturing of Kinematic Pairs
(University of Texas at Austin, 2014) Pareek, Shrey; Sharma, Vaibhav; Rai, Rahul
While additive manufacturing processes are better suited for fabrication of parts with complex geometries, they face serious challenges whilst fabricating parts that require relative motion with respect to each other. The primary challenge in additive manufacturing of mechanisms is preventing the mating parts from bonding with each other during the fabrication process. In this paper the authors investigate design and additive fabrication of kinematic pairs that can move relative to each other. The paper outlines fabrication of kinematic pairs based on optimal clearance value for three basic lower order kinematic pairs, viz. revolute pair, prismatic pair, and cylindrical pair. Using empirical testing functional relationships between extractive force and clearance, and between moment and clearance have been developed. These functional relationships can be used by users to fabricate kinematic pairs using FDM based 3D printing processes. The efficacy of the proposed approach is demonstrated on 3D printed kinematic pairs and experimental validation studies.

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