Khershed P. Cooper

Layered Manufacturing: Challenges and Opportunities

Abstract : Layered Manufacturing (LM) refers to computer-aided manufacturing processes in which parts are made in sequential layers relatively quickly. Parts that are produced by LM can be formed from a wide range of materials such as photosensitive polymers, metals and ceramics in sizes from a centimeter to a few meters with sub-millimeter feature resolutions LM has found use in diverse areas including biomedical engineering, pharmaceuticals, aerospace, defense, electronics and design engineering. The promise of LM is the capability to make customized complex-shaped functional parts without specialized tooling and without assembly LM is still a few years away from fully realizing its promise but its potential for manufacturing remains high. A few of the fundamental challenges in materials processing confronting the community are improving the quality of the surface finish, eliminating residual stress, controlling local composition and microstructure, achieving fine feature size and dimensional tolerance and accelerating processing speed. Until these challenges are met, the applicability of LM and its commercialization will be restricted. Sustained scientific activity in LM has advanced over the past decade into many different areas of manufacturing and has enabled exploration of novel processes and development of hybrid processes. The research community of today has the opportunity to shape the future direction of science research to realize the full potential of LM.

Analysis of Railgun Barrel Material

The effect of the number of shots on multiple-launch railgun barrel material performance under significantly high current conditions was studied. Material analysis was conducted on rail samples from four sets of experiments involving increasing number of shots. Cross sections of samples from two locations in the copper rail were examined using a variety of metallographic techniques. During each shot a thin film of molten aluminum from the armature deposited on the rail surface. The deposit became thicker and multilayered with increasing number of shots and was thicker in the bottom half of the vertically oriented rails than in the top half. Preferential removal of material occurred along both rail edges, which resulted in the formation of grooves over the length of the rails. Grooves were deeper near the top edge of the rails compared to the bottom edge. A proposed mechanism for grooving is dissolution of the rail material into molten aluminum flowing toward the rail interior due to aerodynamic drag and over the rail edge due to surface tension effects, assisted by extremely high rail edge temperatures. Asymmetry in the armature-rail gap due to magnetic interactions with the support structure may be the cause of asymmetry in deposit thickness and groove depth

Cyber-enabled manufacturing systems for additive manufacturing

Purpose – The purpose of this paper is to study cyber-enabled manufacturing systems (CeMS) for additive manufacturing (AM). The technology of AM or solid free-form fabrication has received considerable attention in recent years. Several public and private interests are exploring AM to find solutions to manufacturing problems and to create new opportunities. For AM to be commercially accepted, it must make products reliably and predictably. AM processes must achieve consistency and be reproducible. Design/methodology/approach – An approach we have taken is to foster a basic research program in CeMS for AM. The long-range goal of the program is to achieve the level of control over AM processes for industrial acceptance and wide-use of the technology. This program will develop measurement, sensing, manipulation and process control models and algorithms for AM by harnessing principles underpinning cyber-physical systems (CPS) and fundamentals of physical processes. Findings – This paper describes the challeng...

Thermal Modeling of Direct Digital Melt-Deposition Processes

Additive manufacturing involves creating three-dimensional (3D) objects by depositing materials layer-by-layer. The freeform nature of the method permits the production of components with complex geometry. Deposition processes provide one more capability, which is the addition of multiple materials in a discrete manner to create “heterogeneous” objects with locally controlled composition and microstructure. The result is direct digital manufacturing (DDM) by which dissimilar materials are added voxel-by-voxel (a voxel is volumetric pixel) following a predetermined tool-path. A typical example is functionally gradient material such as a gear with a tough core and a wear-resistant surface. The inherent complexity of DDM processes is such that process modeling based on direct physics-based theory is difficult, especially due to a lack of temperature-dependent thermophysical properties and particularly when dealing with melt-deposition processes. In order to overcome this difficulty, an inverse problem approach is proposed for the development of thermal models that can represent multi-material, direct digital melt deposition. This approach is based on the construction of a numerical-algorithmic framework for modeling anisotropic diffusivity such as that which would occur during energy deposition within a heterogeneous workpiece. This framework consists of path-weighted integral formulations of heat diffusion according to spatial variations in material composition and requires consideration of parameter sensitivity issues.

An Algorithm for Inverse Modeling of Layer-by-Layer Deposition Processes

Metallic parts can be made by deposition of liquid metal in a layer-by-layer fashion. By this means, layered structures can be produced that are made up of overlapping reinforced droplets. In particular, prototypes, i.e., customized parts and tooling, can be produced in this way. In order that layer-by-layer fabrication techniques transition from prototyping to manufacturing, however, the processes must be made reliable and consistent. Accordingly, detailed microstructural and thermal characterizations of the product are needed to advance manufacturing goals based on layer-by-layer deposition processes. The inherent complexity of layer-by-layer deposition processes, characteristic of energy and mass deposition processes in general, is such that process modeling based on theory, or the direct-problem approach, is extremely difficult. A general approach to overcoming difficulties associated with this inherent complexity is the inverse-problem approach. Presented here is an algorithm for inverse modeling of heat transfer occurring during layer-by-layer deposition, which is potentially adaptable for prediction of temperature histories in samples that are made by layer-by-layer deposition processes.

EM Gun Bore Life Experiments at Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5-MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical finite element analysis models of railgun operation. Results of several experiments on rail wear will be discussed.

EM gun bore life experiments at the Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5 MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical Finite Element Analysis (FEA) models of railgun operation. Results of several experiments on rail wear will be discussed.

Laser-based additive manufacturing: where it has been, where it needs to go

It is no secret that the laser was the driver for additive manufacturing (AM) of 3D objects since such objects were first demonstrated in the mid-1980s. A myriad of techniques utilizing the directed energy of lasers were invented. Lasers are used to selectively sinter or fuse incremental layers in powder-beds, melt streaming powder following a programmed path, and polymerize photopolymers in a liquid vat layer-by-layer. The laser is an energy source of choice for repair of damaged components, for manufacture of new or replacement parts, and for rapid prototyping of concept designs. Lasers enable microstructure gradients and heterogeneous structures designed to exhibit unique properties and behavior. Laserbased additive manufacturing has been successful in producing relatively simple near net-shape metallic parts saving material and cost, but requiring finish-machining and in repair and refurbishment of worn components. It has been routinely used to produce polymer parts. These capabilities have been widely recognized as evidenced by the explosion in interest in AM technology, nationally. These successes are, however, tempered by challenges facing practitioners such as process and part qualification and verification, which are needed to bring AM as a true manufacturing technology. The ONR manufacturing science program, in collaboration with other agencies, invested in basic R&D in AM since its beginnings. It continues to invest, currently focusing on developing cyber-enabled manufacturing systems for AM. It is believed that such computation, communication and control approaches will help in validating AM and moving it to the factory floor along side CNC machines.

Nanomanufacturing: path to implementing nanotechnology

Research and development work in nanoscience and nanotechnology has generated new fundamental understanding of physical phenomena and material behaviour at the nano-scale. This knowledge has resulted in discoveries of new materials, structures and devices. It is believed that these basic research investments should now lead to new products and applications. The general feeling is that nanotechnology needs to move from fundamentals to practice, from the laboratory to the marketplace. The broad consensus is that we need accelerated research and development investments in nanomanufacturing science to step up transition of nanotechnology and to hasten technology transfer. Such an effort will fulfil nanotechnology’s promise, which to bring about economic and societal benefits. This paper will define and discuss our perspective on nanomanufacturing, describe our manufacturing science programmes ongoing research and development efforts, list manufacturing challenges and discuss how these are being met through our basic research programmes.

Synthesis of MgB/sub 2/ by exposure of polycrystalline boron to magnesium vapor

Polycrystalline boron in lump form was reacted with magnesium vapor inside steel tubes at 900 C to produce MgB/sub 2/ using four to five times the stoichiometric requirement of Mg. The boron lumps were encased inside Ta foil folded over to form a boat to isolate them from the liquid Mg. These materials were sealed inside steel tubes by arc welding. Reactions were allowed to proceed for times ranging from 2.5 to 837 hours. Upon opening a tube after a 42-hour reaction time a gray powder was removed which X-ray diffraction indicated was MgB/sub 2/. SEM examination of the surfaces of the powder particles revealed a dense layer of 1-2 micron diameter crystallites which appear to be small plates with hexagonal symmetry. Metallographic examination of sections cut through the particles indicated the presence of a significant volume fraction of unreacted boron. The reaction appears to have proceeded initially along the grain boundaries in the polycrystalline lumps breaking them up into particles corresponding to the grain size. Magnetic susceptibility measurements on the powder and resistivity measurements on a cold pressed pellet show a sharp transition at 39 K despite the presence of unreacted boron. Even after reaction times up to 837 hours there was still a significant amount of unreacted boron.