Scott Fish

Battery–Inductor Parametric System Analysis for Electromagnetic Guns

Growing interest in battery performance and cost reduction for hybrid and electric vehicles has restimulated interest in the United States in the use of high-power batteries as a potential source of pulsed power. Recent progress in high-power density lithium-ion batteries and high-power semiconducting switches has suggested that a battery-inductor-based pulsed power system could become a viable option to pulsed alternators for electromagnetic (EM) launchers and other pulsed loads in the megajoule range. Approximate system sizing and a parametric study are presented, showing the effects of battery and inductor parameters on the overall efficiency and system size for a conceptual 2-MJ muzzle energy EM launch system utilizing the STRETCH circuit topology. The results show the relationship between potential increases in future component performance on overall system size reduction and efficiency.

In situ process monitoring in selective laser sintering using optical coherence tomography

Abstract. Selective laser sintering (SLS) is an efficient process in additive manufacturing that enables rapid part production from computer-based designs. However, SLS is limited by its notable lack of in situ process monitoring when compared with other manufacturing processes. We report the incorporation of optical coherence tomography (OCT) into an SLS system in detail and demonstrate access to surface and subsurface features. Video frame rate cross-sectional imaging reveals areas of sintering uniformity and areas of excessive heat error with high temporal resolution. We propose a set of image processing techniques for SLS process monitoring with OCT and report the limitations and obstacles for further OCT integration with SLS systems.

In-situ thermal image correlation with mechanical properties of nylon-12 in SLS

Purpose This paper aims to establish a method to verify in real time the quality of a part being built using Selective Laser Sintering (SLS). Design/methodology/approach A SLS build of 30 Nylon 12 tensile bars was done while using an infrared camera to record the thermal history of each bar. The thermal history was then compared to the ultimate tensile strength (UTS) of each bar. In addition, an attempt was made to identify where the fracture of each bar occurred based on its thermal history. Findings Several analysis techniques were used to compare the thermal history of each bar to its UTS. The strongest correlation found was 0.746. In addition, multiple strategies for predicting the break location where used, with the most successful making a correct prediction on 46 per cent of the bars. Originality/value This paper studies the feasibility of in-situ build verification, a technique that if successful would greatly help the further adoption of SLS as a method of manufacturing.

Multi-layer computational modeling of selective laser sintering processes

Selective laser sintering (SLS) is an additive manufacturing technique able to rapidly create parts directly from a CAD model using a laser to selectively fuse successive layers of powder. However, defects can arise in SLS parts due to incomplete fusion of the powder layers or thermal stresses introduced by large temperature gradients during the part build. Accurate models of the SLS process are needed to ensure that high quality parts are produced and to allow new materials and designs to be used without requiring extensive experimentation. Most existing models of the SLS process are very narrowly focused, predicting the temperature history of a single powder layer after a single laser pass or examining the impact of a few processing parameters on the properties of the produced part. A model capable of predicting a complete temperature history during an entire part build does not yet exist. Therefore, a new thermal model able to simulate multiple powder layers is proposed.A transient, three-dimensional, finite volume model is developed and implemented in ANSYS Fluent. A domain of cells representing multiple layers of an SLS build is initialized, some with the properties of air and some with the properties of powder, depending on cell location. A Gaussian heat source representing the laser is applied to the top layer of powder cells. The center of the Gaussian is varied with time along an established path to simulate the motion of the laser along the powder bed. At all times the three-dimensional heat equation is solved to produce a temperature profile of the powder bed. When the laser completes a full scan of the powder layer, the air cells directly above the powder layer are re-initialized as powder cells and re-set to an initial temperature, representing the addition of a new powder layer. The process is repeated for each new layer. Temperature history results from the model are validated against experimental data available in the literature and good agreement is obtained. As the model accounts for multiple powder layers, it can be used to simulate an entire part build and predict the impact of any of the SLS processing parameters on part quality and thus enable better control and optimization of the SLS process.Copyright

An Automated Laser Control Technique for Improving Powder Bed Temperature Uniformity in Selective Laser Sintering

Selective Laser Sintering (SLS) of polymers is a common technique for creating structural plastic components in additive manufacturing. As a result, the mechanical strength of SLS parts is often of concern. Testing has revealed a large variation in mechanical properties of components within the same build. It is believed that this variation is a result of poor temperature control during the sintering process. The proposed control method involves using an infrared camera to measure initial powder temperature before computing an optimal laser power profile to sinter the powder. By controlling the laser energy, it is expected that a more uniform post lasing temperature can be achieved and variation in mechanical properties can be reduced. The feed-forward control method is presented along with preliminary results.