Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Additive manufacturing of metallic components – Process, structure and properties

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The Theory of Industrial Organization

Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

Additive manufacturing (AM) is poised to bring about a revolution in the way products are designed, manufactured, and distributed to end users. This technology has gained significant academic as well as industry interest due to its ability to create complex geometries with customizable material properties. AM has also inspired the development of the maker movement by democratizing design and manufacturing. Due to the rapid proliferation of a wide variety of technologies associated with AM, there is a lack of a comprehensive set of design principles, manufacturing guidelines, and standardization of best practices. These challenges are compounded by the fact that advancements in multiple technologies (for example materials processing, topology optimization) generate a "positive feedback loop" effect in advancing AM. In order to advance research interest and investment in AM technologies, some fundamental questions and trends about the dependencies existing in these avenues need highlighting. The goal of our review paper is to organize this body of knowledge surrounding AM, and present current barriers, findings, and future trends significantly to the researchers. We also discuss fundamental attributes of AM processes, evolution of the AM industry, and the affordances enabled by the emergence of AM in a variety of areas such as geometry processing, material design, and education. We conclude our paper by pointing out future directions such as the "print-it-all" paradigm, that have the potential to re-imagine current research and spawn completely new avenues for exploration. The fundamental attributes and challenges/barriers of Additive Manufacturing (AM).The evolution of research on AM with a focus on engineering capabilities.The affordances enabled by AM such as geometry, material and tools design.The developments in industry, intellectual property, and education-related aspects.The important future trends of AM technologies.

The status, challenges, and future of additive manufacturing in engineering

Machining of hardened steel components has traditionally been the domain of grinding. A laser-assisted machining (LAM) process with a larger material removal rate is developed to replace the currently used hard turning and grinding operations for machining hardened steel shafts. A three-dimensional transient thermal model is developed to predict the temperature field in the hollow shaft of varying thickness undergoing laser-assisted turning. The laser-assist effect on surface quality and subsurface integrity is investigated in terms of surface finish, size control, microhardness, microstructures and residual stresses. LAM produces the parts of a good surface finish of Ra less than 0.3 μm, a more uniform surface hardness distribution and no microstructure change. The thermal expansion effect on the actual depth of cut is experimentally studied to achieve accurate size control in LAM. It is shown that LAM produces about 150 MPa more compressive surface axial residual stresses than hard turning and reduces the variation in hoop stress than those produced by hard turning.

Laser-assisted machining of hardened steel parts with surface integrity analysis

A new approach for machining of Inconel 718 is presented in this paper. It combines traditional turning with cryogenically enhanced machining and plasma enhanced machining. Cryogenically enhanced machining is used to reduce the temperatures in the cutting tool, and thus reduces temperature-dependent tool wear to prolong tool life, whereas plasma enhanced machining is used to increase the temperatures in the workpiece to soften it. By joining these two non-traditional techniques with opposite effects on the cutting tool and the workpiece, it has been found that the surface roughness was reduced by 250%; the cutting forces was decreased by approximately 30–50%; and the tool life was extended up to 170% over conventional machining.

Hybrid machining of Inconel 718

Molecular dynamics simulations are carried out to parameterize and obtain a traction–separation law for a ductile–brittle interface in tensile and shear loadings at high temperatures. Traditionally the interface is characterized by a cohesive zone model (CZM) with the traction–separation law assumed or parameterized through experiments. The experimental determination of the shape of the CZM has proved to be difficult. In this study a traction–separation law is thus obtained for an aluminum–silicon carbide composite system by conducting molecular dynamics simulations. The generated parameterized traction–separation law is then input into a finite element model to predict the stress–strain response of the metal matrix composite under high strain rate loading and compared with experimental results.

Molecular dynamics based cohesive zone law for describing Al–SiC interface mechanics

Laser-assisted machining (LAM) of silicon nitride (Si 3 N 4 ) is evaluated for its potential to become an economically viable process in fabricating precision ceramic parts. On-line measurements of cutting force and workpiece temperature are performed, and tool wear and surface integrity are examined. Tool wear characteristics are determined as a function of workpiece temperature, which is measured on-line using a laser pyrometer Tool wear/failure mechanisms are characterized using optical microscopy while application of scanning electron microscopy to heated and machined surfaces, as well as to chips, is used to infer material removal mechanisms and the extent of damage caused by LAM. The sub-surface damage of parts produced by LAM is compared with that of typical ground parts.

Experimental Investigation of Thermo-Mechanical Characteristics in Laser-Assisted Machining of Silicon Nitride Ceramics

This work presents the effects of bearing configuration on the thermo-dynamic behavior of high speed spindles using the comprehensive dynamic thermo-mechanical model. The dynamic thermo-mechanical model consists of a comprehensive bearing dynamic model, a shaft dynamic model and a thermal model. The thermal model is coupled with the spindle dynamic model through bearing heat generation and thermal expansion of the whole system based on the bearing configuration. Thus the entire model becomes a comprehensive dynamic thermo-mechanical model. The new thermo-mechanical model also considers a pertinent mapping between bearing stiffness and shaft stiffness matrices based on bearing configurations, so that more general cases of bearing configurations can be modeled. Based on this model, the effects of bearing orientation on the spindle dynamics are systematically described and experimentally validated. It is shown that bearing orientation has a significant effect on spindle stiffness. Finally, the effects of various bearing configurations on spindle thermal and dynamic behavior are illustrated through numerical analysis with three different spindles.

Yung C. Shin

Papers

Laser-assisted machining of hardened steel parts with surface integrity analysis

Hybrid machining of Inconel 718

Molecular dynamics based cohesive zone law for describing Al–SiC interface mechanics

Experimental Investigation of Thermo-Mechanical Characteristics in Laser-Assisted Machining of Silicon Nitride Ceramics

Analysis of bearing configuration effects on high speed spindles using an integrated dynamic thermo-mechanical spindle model