28 research outputs found
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Fundamentals of Liquid Phase Sintering During Selective Laser Sintering
One of the advantages of the Selective Laser Sintering (SLS) process is that a variety of materials
can be processed. However, the goal of being able to produce fully dense metal parts with no post
processing has been elusive. Using Selective Laser Sintering to produce metal parts with full
density without post processing poses a challenge since both the processing conditions and the
metal system must be controlled. This article describes two metallurgical mechanisms by which
loose metal powder beds could be sintered to nearly full density using a scanning laser beam. The
mechanisms are particle rearrangement during liquid phase sintering (LPS) and in-situ infiltration.
Some of the particles, when heated by the laser radiation, melt and form a liquid. Ifthis liquid has
certain physical properties (e.g., low viscosity and high surface tension) and wets the other solid
particles, then the SLS process can in theory produce dense layers by either mechanism. The
purpose of this study is to determine the process and material selection parameters required to
achieve fully dense parts during direct Selective Laser Sintering of metal.Mechanical Engineerin
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Fundamentals of Liquid Phase Sintering Related to Selective Laser Sintering
Liquid phase sintering is one of the underlying principles that must be
modeled and understood when the Selective Laser Sintering (SLS) process is
used. This paper describes the initial studies being conducted to measure
surface tension of metal alloys used for SLS. A low melting point solder was
used to verify the wetting balance and pendant drop techniques and equipment
for determining surface tension. The liquid-solid, liquid-vapor, and solid
vapor surface tension of 80 Sn - 20 Pb solder on mild steel was determined to
be 245, 417, and 662 dynes/cm.Mechanical Engineerin
Evidence for circadian influence on human slow wave sleep during daytime sleep episodes
The occurrence of slow wave sleep within spontaneously initiated daytime sleep episodes was studied to examine hypothesized associations with prior wakefulness and circadian factors. There was a strong relationship between measures of slow wave sleep and the proximity of sleep episodes to the maximum of body core temperature. Those sleep episodes that began within 4 hours of the maximum in body core temperature contained significantly more slow wave sleep than did all other daytime sleep periods, approximating proportions typical of nocturnal sleep. Multiple regression analysis revealed no relationship between measures of slow wave sleep and prior wakefulness. These findings are consistent with an hypothesized approximately-12-hour rhythm in the occurrence of slow wave sleep and they underscore the influence imposed on human sleep by the endogenous circadian timing system