Process Maps for Laser Deposition of Thin-Walled Structures

In solid freeform fabrication (SFF) processes involving thermal deposition, thermal control of the process is critical for obtaining consistent deposition conditions and in limiting residual stress-induced warping of parts. In this research, nondimensionalized plots (termed process maps) are developedJrom numerical models of laser-based material deposition of thin-walled structures that.map out the effects of changes in laser power, deposition speed and part preheating on process parameters. The principal application of this work is to the Laser Engineered Net Shaping (LENS) process under development at Sandia Laboratories; however, the approach taken is applicable to any solid freeform fabrication process involving. a moving heat source. Similarly, although thinwalled structures treated in the current work, the same approach could be applied to other commonly fabricated geometries. A process map for predicting and controlling melt pool size is presented .and numerically determined results are compared against experimentally measured melt poollengthsfor stainless steel deposition in the LENS process.Mechanical Engineerin

Bound

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Could Low Vitamin D Status Explain the Increased Rates of Hypertensive Disorder in Pregnancy in the US Population and in Non-Hispanic Black Women? An Examination of NHanes 2001-2006

Background: The incidence of Hypertensive Disorders in Pregnancy (HDP) is increasing in the US and is linked to serious long and short-term health problems for both mother and fetus. Vitamin D has been shown to have direct influence on molecular pathways involved in pregnancy. However a link between vitamin D status and HDP in Pregnant women has not been established. Objectives: The purpose of this study is to determine (1) the association between vitamin D deficiency and the occurrence of (HDP) and (2) whether non-Hispanic Black women (NHB) are at greater risk for HDP due to low vitamin D status. Methods: Pregnant females in the National Health and Nutrition Examination Survey (NHANES) study from 2001 to 2006 were used in this study. Participant’s response to interview questions and laboratory results were taken into account to determine HDP status. Logistic regression was used to determine the association between vitamin D status and HDP. Results: Pregnant women with low vitamin D status (25(OH)D \u3c 20ng/ml) were 1.123 (95%CI: 0.808-1.56) times more likely to have HDP compared to women who were vitamin D sufficient. This association was not significant. NHB women did not show a significant increased risk for HDP. Conclusions: Low vitamin D status during pregnancy may lead to an increased risk for Hypertensive Disorders in Pregnancy. However more research on larger sample size is needed to determine the true extent of the association of vitamin D status with HDP in the general population and that of non-Hispanic Black women

Ultraviolet Curing of Highly Loaded Ceramic Suspensions for Stereolithography of Ceramics

Ceramic green bodies can be created using stereolithography methods where a ceramic slip consisting of 45-55 vlo ceramic powder is dispersed within an ultraviolet-curable aqueous acrylamide solution. Two ceramic materials were investigated: silica [Si02] for investment casting purposes, and alumina [AI203] for structural parts. After mixing the powders in the curable solution, the ceramic slip is tape cast onto a substrate for cure under a high intensity ultraviolet lamp (220-450 nm) at different exposure times. The materials systems were evaluated at different solids loadings (10-50 v/o) for cure thickness and viscosity control. Silica had a cure depth of 330 f.lm at a solids loading of 55 vlo, and at 50 vlo, alumina had a cure depth of 300 f.lm. Preliminary work utilizing scattering theory revealed the cure depth is controlled by the particle size and the refractive index difference between the ceramic and ultraviolet solution. The refractive index difference is the dominating factor. Two particle size distributions of alumina were used to more accurately determine the effect of particle size.Mechanical Engineerin

Process Maps for Controlling Residual Stress and Melt Pool Size in Laser-Based SFF Processes 200

Thermal control of solid freeform fabrication processes is critical for obtaining consistent build conditions and in limiting residual stress-induced tolerance losses. In this paper, thermomechanical models are presented for the building of thin-walled structures by laser-based SFF processes. The simulations are used to develop two non-dimensional plots (termed process maps) that quantify the effects of changes in wall height, laser power, deposition speed and part preheating on melt pool size (for consistent build conditions) and thermal gradients (for limiting residual stresses). Mechanical simulations are used to demonstrate the link between thermal gradients and maximum final residual stresses. Models are applied to the Laser Engineered Net Shaping (LENS) process; however, the general approach, insights and conclusions are applicable to most SFF processes involving a moving heat source. The two process maps described herein can be used together to determine optimal process variables for obtaining consistent melt pool length while limiting residual stress in the part. Results from the residual stress simulations also identify two important mechanisms for reducing residual stresses and quantify maximum stress reductions that can be achieved through manipulation of all process variables.This research has been supported by the National Science Foundation under grant DMI9700320 and by Sandia National Laboratories under grant BE-0792.Mechanical Engineerin

Images, Silences, and the Archival Record: An Interview with Michelle Caswell

Dr. Michelle Caswell is an Associate Professor of Archival Studies in the Department of Information Studies at the University of California, Los Angeles, where she is also an affiliated faculty member with the Department of Asian American Studies and the Center for Southeast Asian Studies. Her book, Archiving the Unspeakable: Silence, Memory, and the Photographic Record in Cambodia (2014), which explores the role of archives and records in the construction of memory about the Khmer Rouge in Cambodia through a collection of mug shots taken at Tuol Sleng prison, won the 2015 Waldo Grifford Leland award for Best Publication from the Society of American Archivists. Caswell is also the co-founder of the South Asian American Digital Archive, an online repository which documents and provides access to the diverse stories of South Asian Americans

Residual Stress Control Issues for Thermal Deposition of Polymers in SFF Processes 209

Controlling residual stress-induced warping and other tolerance losses is important for accurately creating parts by solid freeform fabrication (SFF). In this paper, results are presented from warping experiments on plate-shaped acrylonitrile butadiene styrene (ABS) specimensmcreated by an extrusion process used in Shape Deposition Manufacturing (SDM). Experimental results are compared to predictions from both one- and two-dimensional types of residual stress models. In addition to SDM, methods and results from this study are applicable to a number of other solid freeform fabrication processes involving extrusion of polymers or polymer slurries. Results from polymer extrusion are compared with those from existing work on thermal deposition of metals. Unlike metals, polymer deposition shows essentially no stress reduction due to preheating by the deposition process. Due to a greater number of deposited rows, directionality of warping is also greater than in metals. Polymer deposition experiments show that a preheat temperature near the glass transition temperature is needed for essentially no warping. Comparison of predicted and measured curvatures show that a simple 1-D thermomechanical model does not predict warping magnitudes well, but does provide insight into trends in warping as a function of preheat temperature. The effects of successive material deposition are substantial in this process and a 2-D model that includes the effects of successively deposited rows can provide much more accurate curvature predictions.The authors gratefully acknowledge financial support from the Gintic Institute of Manufacturing Technology and from the National Science Foundation, under grant DMI9700320. A software grant from SDRC allowed use of their software for finite element pre- and post- processing.Mechanical Engineerin

Scattering of ultraviolet radiation in turbid suspensions

A Beer’s law expression for the penetration depth of ultraviolet radiation in a concentrated suspension of scattering particles is used to model the depth of cure for a suspension of ceramic particles in a medium of photocurable monomers. The cure depth is predominantly controlled by the square of the refractive index difference between the ceramic particles and the medium, Δn2 = (np−n0)2Δn2=(np−n0)2. A secondary effect on the cure depth is the ratio of the interparticle spacing to the ultraviolet wavelength. Theoretical results agree with experimental data for 0.40–0.50 volume fraction ceramic-filled suspensions. © 1997 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71270/2/JAPIAU-81-6-2538-1.pd

Melt Pool Size and Stress Control for Laser-Based Deposition Near a Free Edge

Thermomechanical models developed in this research address two experimental observations made during the deposition of thin-walled structures by the LENSTM process. The first observation (via thermal imaging) is of substantial increases in melt pool size as a vertical free edge is approached under conditions of constant laser power and velocity. The second observation (via neutron diffraction) is of large tensile stresses in the vertical direction at vertical free edges, after deposition is completed and the wall is allowed to cool to room temperature. At issue is how to best control melt pool size as a free edge is approached and whether such control will also reduce observed free edge stresses. Thermomechanical model results are presented which demonstrate that power reduction curves suggested by process maps for melt pool size under steady-state conditions can be effective in controlling melt pool size as a free edge is approached. However, to achieve optimal results it is important that power reductions be initiated before increases in melt pool size are observed. Stress simulations indicate that control of melt pool size can reduce free-edge stresses; however, the primary cause of these stresses is a constraint effect which is independent of melt pool size.This research was supported by the National Science Foundation Division of Design, Manufacture and Industrial Innovation, through the Materials Processing and Manufacturing Program, award number DMI-0200270.Mechanical Engineerin