Support-Free Infiltration of Selective Laser Sintered (SLS) Silicon Carbide Preforms

Non-metallic objects are often difficult to manufacture due to high melting temperatures, poor sinterability, limited ductility, and difficulty in machining. Freeform fabrication techniques coupled with liquid infiltration offer a cost-effective and rapid manufacturing mechanism for composite parts with complex geometry and adequate properties. Selective laser sintered (SLS) silicon carbide (SiC) preforms infiltrated with liquid silicon develop localized infiltrant overextrusions onto surfaces and at surface irregularities. Several shrinkage mechanisms including densification were studied as possible causes of these overfilling extrusions, and the results are discussed below. This research was supported by NSF Grant Number DMI-0522176.Mechanical Engineerin

IMPROVING NAVAL AVIATION MAINTENANCE OPERABILITY IN SUPPORT OF CONUS DETACHMENTS

Naval Aviation currently operates as complete, internally supported squadrons responsible for their own maintenance equipment for operations. In this capacity, a squadron conducting training detachments away from its home station is required to transport all imperative equipment and personnel via contracted ground and government air transport. Because there is no additional equipment capacity to draw from for detachments, flight operations at the home station are reduced during the ground transportation period. This proposal assesses if it is beneficial for the Naval Aviation enterprise to continue the current transportation procedures of aviation maintenance equipment to detachments within the continental United States.Outstanding ThesisCommander, United States NavyLieutenant, United States NavyApproved for public release. Distribution is unlimited

Video Microscopy of Selective Laser Sintering

This paper presents the design and implementation of a video microscopy system that enables real time observation and archival of selective laser sintering of polymer and metal materials. The design objectives and selection of system components are discussed in the first section of this paper. Experimental results from preliminary experiments conducted on polycarbonate, wax and nylon powders are also presented.Mechanical Engineerin

Selective Laser Sintering of Quartz Powder

This research describes the feasibility of fusing quartz powder by Selective Laser Sintering (SLS). SLS is a method of rapid prototyping three-dimensional objects from a computer-aided design database. The effects of different processing parameters, including powder size, laser power and scan mte were explored. Single and multiple layer specimens have been made. The resulting structures were evaluated using SEM and the density of the multiple-layer structure was detected by a geometrical mass/volume technique. It was determined that particle size was the dominant variable affecting part quality. Smaller and spherical unifonn particles are preferred. Future work will concentrate on optimizing powder size and shape and higher laser power.Mechanical Engineerin

Densification of Selective Laser Sintered Metal Part by Hot Isostatic Pressing

Metal matrix alloy composite parts were made from powders by Selective Laser Sintering (SLS). In this study, partially dense (60%-80%) metal parts made by SLS were densified to full density (>98%) by hot isostatic pressing (HIPping) without any loss of shape. HIPping was done by vacuum sealing SLS samples in glass capsules. HIPping parameters, such as, temperature, pressure, and time, were studied with respect to density, linear shrinkage, and microstructures. Anisotropy in linear shrinkage was correlated to the SLS processing parameters. Densification resulting from HIPping was correlated to microstructures and theoretical HIP densification maps. A detailed analysis of such maps is presented.Mechanical Engineerin

Effect of In-Plane Voiding on the Fracture Behavior of Laser Sintered Polyamide

The primary contributors to poor mechanical properties in polyamide materials used during Selective Laser Sintering® are qualified. Methods to quantify the decreased mechanical properties, including Scanning Electron Microscopy (SEM) of fracture surfaces, are compared against each other and against mechanical properties of components fabricated using multiple process parameters. Of primary interest are Ultimate Tensile Strength (UTS) and Elongation at Break (EOB) of tensile specimens fabricated under conditions that produce varying degrees of ductile and brittle fracture.Mechanical Engineerin

RSRM Segment Train Derailment and Recovery

On May 2, 2007, a freight train carrying segments of the space shuttle's solid rocket boosters derailed in Myrtlewood, Alabama, after a rail trestle collapsed. The train was carrying Reusable Solid Rocket Motors (RSRM) 98 center and forward segments (STS-120) and RSRM 99 aft segments (STS-122). Initially, it was not known if the segments had been seriously damaged. Four segments dropped approximately 10 feet when the trestle collapsed and one of those four rolled off the track onto its side. The exit cones and the other four segments, not yet on the trestle, remained on solid ground. ATK and NASA immediately dispatched an investigation and recovery team to determine the safety of the situation and eventually the usability of the segments and exit cones for flight. Instrumentation on each segment provided invaluable data to determine the acceleration loads imparted into each loaded segment and exit cone. This paper details the incident, recovery plan and the team work that created a success story that ended with the safe launch of STS120 using the four center segments and the launch of STS122 using the Aft exit cones assemblies

Multiple material systems for selective beam sintering

A method and apparatus for selectively sintering a layer of powder to produce a part comprising a plurality of sintered layers. The apparatus includes a computer controlling a laser to direct the laser energy onto the powder to produce a sintered mass. The computer either determines or is programmed with the boundaries of the desired cross-sectional regions of the part. For each cross-section, the aim of the laser beam is scanned over a layer of powder and the beam is switched on to sinter only the powder within the boundaries of the cross-section. Powder is applied and successive layers sintered until a completed part is formed. Preferably, the powder comprises a plurality of materials having different dissociation or bonding temperatures. The powder preferably comprises blended or coated materials.Board of Regents, University of Texas Syste

Preferences and skills of Indian public sector teachers

With a sample of 700 future public sector primary teachers in India, a Discrete Choice Experiment is used to measure job preferences, particularly regarding location. General skills are also tested. Urban origin teachers and women are more averse to remote locations than rural origin teachers and men respectively. Women would require a 26-73 percent increase in salary for moving to a remote location. The results suggest that existing caste and gender quotas can be detrimental for hiring skilled teachers willing to work in remote locations. The most preferred location is home, which supports decentralised hiring, although this could compromise skills