196,009 research outputs found
The use of a ns-pulsed, high repetition rate green laser for SLM of 99.9% pure Cu
No full textThe most common laser type used in selective laser melting (SLM) machines is continuous fiber laser emitting at 1 m. The low optical absorptivity of Cu to ~1 m wavelength renders pure Cu a highly demanding material for SLM . The low optical absorptivity along with high thermal conductivity causes unstable processing conditions when standard SLM machine are used with pure Cu. Conversely, Cu has a much higher optical absorptivity at the green wavelength. Until recently, high power green lasers have not been available for material processing. This work investigates the use of a novel ns-pulsed fiber laser operating at the second harmonic (532 nm) for SLM of 99.9% pure Cu powder. In particular, the laser source operates at 30 MHz repetition rate providing ns regime and up to 110 W average power. The green laser is implemented to a bespoke open SLM platform. Results show that cubic specimens with densities >99.5% could be achieved
Limits and solutions in processing pure Cu via selective laser melting using a high-power single-mode fiber laser
No full textThe demand for additive manufacturing (AM) of Cu and its alloys shows an increased trend from the energy and heat transfer–related applications. Selective laser melting (SLM) is amongst the key AM processes for metals, providing high geometrical accuracy and design flexibility. The technology is most commonly employed using high-brilliance fibre lasers operating at 1 μm. However, the elevated reflectivity of Cu at this wavelength, combined with its high thermal conductivity, is the cause for a highly unstable process, whereby pore-free products are difficult to obtain. Accordingly, the present work explores the limitations in processing pure Cu powders with a 1-kW single-mode fibre laser providing solutions and different strategies for improving part quality. The process parameters were studied for single and multi-pass melting strategies. The power level requirements, as well as the build plate material, are assessed through an analytical model. The results demonstrate that a correct sequence of multi-pass strategies can improve the part density up to 99.1% ± 0.2% with an industrially acceptable build rate of 12.6 cm3/h
Selective Laser Melting di componenti in rame puro per applicazioni ad alta conducibilità termica ed elettrica
No full textSelective laser melting of pure Cu with a 1 kW single mode fiber laser
No full textComplex geometries and fine critical features enabled by the powder bed fusion technologies are highly appealing for heat exchanger manufacturing. Copper is the main metal of choice for such applications. However, its processability using Selective Laser Melting (SLM) systems is limited by its high reflectivity at 1 μm, the emission wavelength of the laser sources commonly employed. Furthermore, the high conductivity of the pure copper, desirable for the final use of the products, causes instability and difficulties in the powder bed solidification. Resultantly, high density components are difficult to obtain. In this work, a 1 kW single mode fiber laser is used to process pure Cu powder with 99.9% purity. The high power is required to overcome the low efficiency of the process due to the high reflectivity. A prototype SLM machine is employed allowing for a flexible manipulation of the process parameters. The densification phenomenon is discussed as well as the causes that lead to porosity
Arteriopatia occlusiva perlferica: Ruolo dell'angio-RM
No full textPurpose. Magnetic resonance angiography (MRA) has recently become instrumental in the diagnosis of arterial disease in various body districts and is gaining an increasingly important role in the study of peripheral vascularisation. The aim of our study was to evaluate the reliability of MRA using digital subtraction angiography (DSA) as the reference standard. Materials and methods. Between November 2003 and August 2004, 30 patients with known peripheral arterial disease were studied by MRA and DSA. MRA was performed with a Philips Intera 1.5 T, with acquisitions from the coeliac trunk to the feet. For acquisitions of the feet and ankles we used unenhanced time-of-flight (TOF) sequences with a head coil. The angiographic sequence was acquired in three volumes of 40-45 cm after administration of paramagnetic contrast material. Results. In the patients with peripheral arterial disease, the technique provided a precise evaluation of the stenosis (mild, moderate, severe) or obstruction of the peripheral district as well as the detection of other diseases, such as stenosis of the renal arteries or aneurysms. Conclusions. Total-body three-dimensional (3D) MRA allows a fast, safe, and accurate assessment of the arterial system in patients with arteriosclerosis and can be considered an alternative to DSA in the management of patients with steno-obstructive disease of the peripheral arteries
Dr. Duane M. Jackson, Morehouse College, July 2011
No full textThis video is a conversation with Dr. Duane M. Jackson. Dr. Jackson talks about his paper, "Recall and the Serial Position Effect: The Role of Primacy and Recency on Accounting Students' Performance." Jackie Daniel, AUC Woodruff Library, is the interviewer
"Reflections on the subject of Emigration from Europe with a view to Settlement in the United States" By M. Carey.
No full text"Reflections on the subject of Emigration from Europe with a view to Settlement in the United States: containing bried sketches of the moral and political character of those states.
By M. Carey, member of the American philosophical, and of the American Antiquarian Society, and author of The Olive Branch, Cindiciae Hibernicae, essays on banking, on political economy, and on internal improvement.
To which are now added the English editor's comments on the subject; together with Important Advice to Emigrants, and Cautions Against Impositions Practiced in the Outports
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Dr. Glendon Swarthout
No full textHosted by Roger M. Busfield, MSU Assistant Professor of Speech and Theater, Meet the Author is designed to introduce a general audience to a contemporary author and their work through in-depth interviews. This episode features a conversation between Dr. Glendon Swarthout, prolific author and English professor at MSU, and assistant professors Sam S. Baskett and Theodore B. Strandness
High speed videography of gap bridging with beam oscillation and wire feeding during the laser welding of stainless steel and aluminum alloys
No full textLaser beam welding is known for its quality and speed. Given its susceptibility to gaps, the technology is applied in the industrial field with hard automation and dedicated fixtures rather than small-batch production. The latter cannot always guarantee the strict conditions on the fit-up of joints, especially with complex geometries. Gap-bridging techniques may be exploited to overcome these inaccuracies. The present work investigates the simultaneous use of low frequency circular beam oscillation and wire feeding as means to produce a continuous weld seam in the presence of constant air gaps. Lap joint welding of 2 mm-thick AISI301LN and butt joint welding of 3 mm-thick AW6005A-T6 alloy were conducted with gaps up to 1mm. High-speed imaging at 10kHz provided an insight in the dynamics of the oscillating weld pool and spatter formation. Optical inspection and metallographic analyses were used to verify the gap-bridging capability as well as the resulting seam quality
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