Precision LCVD System Design with Real Time Process Control

A Laser Chemical Vapor Deposition (LCVD) system was designed using a fixed 100 Watt C02 laser focused on a moveable substrate. Temperature and height measurement devices monitor the reaction at the point of deposition to provide feedback for controlling the process. The LCVD system will use rapid prototyping technology to directly fabricate fully threedimensional ceramic, metallic, and composite parts of arbitrary shape. Potential applications include high temperature structures, electronic/photonic devices, and orthopaedic implants.Mechanical Engineerin

Thermal, Fluid, and Mass Transport Modeling of a Gas-Jet Reagent Delivery System for Laser Chemical Vapor Deposition (LCVD)

A gas-jet reagent delivery system for laser chemical vapor deposition (LCVD) is modeled with respect to heat transfer, fluid flow, and mass transport. A commercial package was used to model the geometry and flow field surrounding an LCVD reaction zone. The deposition temperature was analyzed for various materials and flow conditions. The forced flow environment was compared against buoyancy-driven flow, which is more typical of a statically filled chamber. A finite difference code was also developed to analyze the effect of the gas-jet on the concentration gradients above the deposition zone.This work was supported by the National Science Foundation, the Engineering Research Program of the Office of Basic Energy Sciences at the U. S. Department of Energy and the Georgia Institute of Technology with significant input from Dr. Andrei Fedorov.Mechanical Engineerin

Fabrication of Advanced Thermionic Emitters Using Laser Chemical Vapor Deposition-Rapid Prototyping 498

Laser Chemical Vapor Deposition-Rapid Prototyping (LCVD-RP) is a relatively new manufacturing process. Its capabilities are ideally suited for the manufacturing of a type of electron emitter called an integrated-grid thermionic emitter. The integrated-grid thermionic emitter is composed of wagon wheel-like structures of alternating layers of boron nitride and molybdenum on tungsten. The goal of this paper is to determine the feasibility of using LCVDRP technology to manufacture advanced thermionic emitters.Mechanical Engineerin

Heat Transfer Analysis of a Gas-Jet Laser Chemical Vapor Deposition (LCVD) Process 461

This paper describes the development of a computer model used to characterize the heat transfer properties of a gas-jet LCVD process. A commercial software package was used to combine heat transfer finite element analysis with the capabilities of computational fluid dynamic software (CFDS). Such a model is able to account for both conduction and forced convection modes of heat transfer. The maximum substrate temperature was studied as a function of laser power and gas-jet velocity.Mechanical Engineerin

Understanding print stability in material extrusion additive manufacturing of thermoset composites

Over the last several years, rapid progress has been made in 3D printing of thermoset polymer resins. Such materials offer desirable thermal and chemical stability, attractive strength and stiffness, and excellent compatibility with many existing high-performance fibers. Material extrusion additive manufacturing (AM) is an ideal technology to print thermoset-based composites because fibers align during extrusion through the deposition nozzle, thereby enabling the engineer to design fiber orientation into the printed component. Current efforts to scale thermoset AM up to large-scale have shown promise, but have also highlighted issues with print stability. To-date, very little research has focused on understanding how rheological properties of the feedstock dictate the mechanical stability of printed objects. This talk will describe our first efforts in this area by printing tall, thin walls to characterize buckling and yielding due to self-weight. The talk will begin with an overview of thermoset material extrusion AM, including a brief history and the current state of the art in small and large-scale printing. The talk will then describe simple thin-walled test geometry and experimental setup that enable quantitative assessment and monitoring of geometric stability during the printing process using machine vision. Two feed stocks are investigated, each having different rheological properties, and the height at which buckling begins and the height at which full collapse occurs are identified as a function of wall thickness. Complementary rheological characterization shows that collapse of thin printed walls is well predicted by the classical self-weight, elastic buckling model, provided the recovery behavior of the feedstock is accounted for. These tests highlight the importance of understanding recovery in material extrusion AM feedstocks and could lead to the design of better resins and fillers, and could provide guidelines for the selection of successful print parameters for both small and large-scale thermoset AM. The talk will conclude with a brief discussion of next steps and outlook on the future of material extrusion AM of thermoset materials

Printing criteria for material extrusion of high temperature thermoplastic composites

Over the last decade, the popularity of 3D printing has increased dramatically. Material extrusion (ME) is the most common type of 3D printing, which typically involves extruding a molten thermoplastic material through a small orifice in a specific pattern. Once considered only a technique for making non-functional prototypes, a wide range of ME systems are now using high performance materials for a variety of functional applications. However, the process science underlying the extrusion of these materials is not well understood. Therefore, the authors have developed a “printability” framework for evaluating extrusion-based printing criteria for a wide range of thermoplastic materials based on fundamental viscoelastic and thermo-mechanical properties. The framework establishes processing boundary conditions for the four basic modes of the ME process: pressuredriven extrusion, extruded geometry definition, geometry stability, and component integrity. The governing equations for each of these modes have been applied to a variety of high performance materials across a number of ME-based printing platforms, including the large-scale 3D printing of carbon fiber reinforced composites. Please click Additional Files below to see the full abstract

CHARACTERIZING THERMOMECHANICAL PROPERTIES OF LARGE-FORMAT PRINTED COMPOSITE POLYMER STRUCTURES

Large-format additive manufacturing (LFAM) is a manufacturing technique where a high volume of material is extruded in a layer-by-layer fashion to form structures that typically measure several meters in scale. The LOCI-One system is an LFAM-type system operated by Loci Robotics, Inc. that features a high throughput extruder mounted on a 6-axis robot arm. This research used the LOCI-One system to print single bead walls of 20% by weight carbon fiber reinforced acrylonitrile butadiene styrene (CF-ABS) at various layer deposition methods, print speed, layer times, and bead widths. The coefficient of thermal expansion (CTE) of the printed structures was measured to quantify effects of print conditions on thermomechanical performance. The CTE of the LFAM printed walls was measured using a large-scale digital image correlation system to characterize the distortion of the fiber reinforced composite material in the x- (print direction) and z- (between layers) directions. This study determined that with varying print parameters the CTE measured in the x-direction was largely influenced by bead geometry with the CTE measured in the z-direction relatively unaffected by either the varying parameters or the method in which layer deposition occurred.Mechanical Engineerin

Additive Manufacturing of High Performance Semicrystalline Thermoplastics and Their Composites

This work investigates the use of two semi-crystalline high performance thermoplastics, polyphenylene sulfide (PPS) and poly (ether ketone ketone) (PEKK), as feedstock for fused filament fabrication process. Composites of PPS and PEKK are emerging as viable candidates for several components in aerospace and tooling industries and additive manufacturing of these materials can be extremely beneficial to lower manufacturing costs and lead times. However, these materials pose several challenges for extrusion and deposition due to some of their inherent properties as well as thermal and oxidative responses. To better understand the properties of such systems specific to 3D printing and determine the critical parameters that make them “printable”, various rheological and thermal properties have been studied for neat as well as short fiber reinforced PPS and PEKK systems. Attempts were also made to print these materials in a customized high temperature fused filament fabrication system.Mechanical Engineerin

Fabrication of Multi-Layered Carbon Structures Using LCVD

Others have used Laser Chemical Vapor Deposition (LCVD) to create 3-D fibrous structures and helical springs. Current research efforts focus on the creation of more advanced three-dimensional carbon objects through the use of multi-layered deposition. Multi-layered structures require an understanding of interlayer adhesion and the propagation of geometric anomalies through multiple layers. An important aspect in minimizing these shape anomalies is the implementation of closed loop temperature control. Several laminated carbon structures are presented with discussions and observations about the fabrication process and visual characteristics of each. The major issues in using LCVD to create multi-layer carbon structures are addressed.McDonald Observator

Neutron Characterization for Additive Manufacturing

Oak Ridge National Laboratory (ORNL) is leveraging decades of experience in neutron characterization of advanced materials together with resources such as the Spallation Neutron Source (SNS) and the High Flux Isotope Reactor (HFIR) shown in Fig. 1 to solve challenging problems in additive manufacturing (AM). Additive manufacturing, or three-dimensional (3-D) printing, is a rapidly maturing technology wherein components are built by selectively adding feedstock material at locations specified by a computer model. The majority of these technologies use thermally driven phase change mechanisms to convert the feedstock into functioning material. As the molten material cools and solidifies, the component is subjected to significant thermal gradients, generating significant internal stresses throughout the part (Fig. 2). As layers are added, inherent residual stresses cause warping and distortions that lead to geometrical differences between the final part and the original computer generated design. This effect also limits geometries that can be fabricated using AM, such as thin-walled, high-aspect- ratio, and overhanging structures. Distortion may be minimized by intelligent toolpath planning or strategic placement of support structures, but these approaches are not well understood and often "Edisonian" in nature. Residual stresses can also impact component performance during operation. For example, in a thermally cycled environment such as a high-pressure turbine engine, residual stresses can cause components to distort unpredictably. Different thermal treatments on as-fabricated AM components have been used to minimize residual stress, but components still retain a nonhomogeneous stress state and/or demonstrate a relaxation-derived geometric distortion. Industry, federal laboratory, and university collaboration is needed to address these challenges and enable the U.S. to compete in the global market. Work is currently being conducted on AM technologies at the ORNL Manufacturing Demonstration Facility (MDF) sponsored by the DOE's Advanced Manufacturing Office. The MDF is focusing on R&D of both metal and polymer AM pertaining to in-situ process monitoring and closed-loop controls; implementation of advanced materials in AM technologies; and demonstration, characterization, and optimization of next-generation technologies. ORNL is working directly with industry partners to leverage world-leading facilities in fields such as high performance computing, advanced materials characterization, and neutron sciences to solve fundamental challenges in advanced manufacturing. Specifically, MDF is leveraging two of the world's most advanced neutron facilities, the HFIR and SNS, to characterize additive manufactured components