Effect of a Distributed Heat Source on Melt Pool Geometry and Microstructure in Beam-Based Solid Freeform Fabrication

The ability to control geometric and mechanical properties of parts fabricated using laser-based manufacturing processes requires an understanding and control of melt pool geometry and microstructure. With the development of electron beam manufacturing or future beam-based deposition processes, the user may have more control over the distribution of incident energy, so that beam width becomes a potential process variable. As such, the focus of this work is the effect of a distributed heat source on melt pool geometry (length and depth) and the thermal conditions controlling microstructure (cooling rates and thermal gradients) in beam-based solid freeform fabrication. Previous work by the authors has employed the Rosenthal solution for a moving point heat source to determine the effects of process variables (laser power and velocity) on solidification cooling rates and thermal gradients controlling microstructure (grain size and morphology) in laser-deposited materials. Through numerical superposition of the Rosenthal solution, the current work extends the approach to include the effects of a distributed heat source for both 2-D thinwall and bulky 3-D geometries. Results suggest that intentional variations in beam width could potentially enable significant changes in melt pool geometry without affecting microstructure.Mechanical Engineerin

Thermal Process Maps for Controlling Microstructure in Laser-Based Solid Freeform Fabrication

The ability to predict and control microstructure in laser deposited materials requires an understanding of the thermal conditions at the onset of solidification. The focus of this work is the development of thermal process maps relating solidification cooling rate and thermal gradient (the key parameters controlling microstructure) to laser deposition process variables (laser power and velocity). The approach employs the well-known Rosenthal solution for a moving point heat source traversing an infinite substrate. Cooling rates and thermal gradients at the onset of solidification are numerically extracted from the Rosenthal solution throughout the depth of the melt pool, and dimensionless process maps are presented for both thin-wall (2-D) and bulky (3- D) geometries. In addition, results for both small-scale (LENSTM) and large-scale (higher power) processes are plotted on solidification maps for predicting grain morphology in Ti-6Al-4V. Although the Rosenthal results neglect temperature-dependent properties and latent heat effects, a comparison with 2-D FEM results over a range of LENSTM process variables suggests that they can provide reasonable estimates of trends in solidification microstructure. The results of this work suggest that changes in process variables could potentially result in a grading of the microstructure (both grain size and morphology) throughout the depth of the deposit, and that the size-scale of the laser deposition process is important.Mechanical Engineerin

Effect of Free-Edges on Melt Pool Geometry and Solidification Microstructure in Beam-Based Fabrication of Bulky 3-D Structures

The success of laser and electron beam-based fabrication processes for additive manufacture and repair applications requires the ability to control melt pool geometry while still maintaining a consistent and desirable microstructure. To this end, previous work by the authors has employed point heat source solutions to investigate the effects of process variables (beam power and velocity) on melt pool geometry and solidification microstructure (grain size and morphology) in beam-based fabrication of bulky 3-D structures. However, these results were limited to steady-state conditions away from free-edges. The current work extends the approach to investigate transient behavior in the vicinity of free-edges, and follows the authors’ recent work for 2-D thin-wall geometries [1].Mechanical Engineerin

Effect on Free-Edges on Melt Pool Geometry and Solidification Microstructure in Beam-Based Fabrication of Thin-Wall Structures

The success of both laser and electron beam-based fabrication processes for additive manufacturing and repair applications requires the ability to control melt pool geometry while still maintaining a consistent and desirable microstructure. To this end, previous work by the authors has employed point-heat source solutions to investigate the effects of process variables (beam power and velocity) on melt pool geometry and solidification microstructure (grain size and morphology) in beam-based fabrication of thin-wall structures. However, these results were limited to steady-state conditions away from free-edges. The current work extends the approach to investigate transient behavior in the vicinity of a free-edge.Mechanical Engineerin

Work in Progress: The WSU Model for Engineering Mathematics Education

This paper summarizes progress to date on the WSU model for engineering mathematics education, an NSF funded curriculum reform initiative at Wright State University. The WSU model seeks to increase student retention, motivation and success in engineering through application-driven, just-in-time engineering math instruction. The WSU approach involves the development of a novel freshman-level engineering mathematics course EGR 101, as well as a large-scale restructuring of the engineering curriculum. By removing traditional math prerequisites and moving core engineering courses earlier in the program, the WSU model shifts the traditional emphasis on math prerequisite requirements to an emphasis on engineering motivation for math, with a just-in-time structuring of the new math sequence. This paper summarizes the development to date of the WSU model for engineering mathematics education, including a preliminary assessment of student performance and perception during the initial implementation of EGR 101. In addition, an assessment of first-year retention results is anticipated in time for the conference

The WSU Model for Engineering Mathematics Education

The traditional approach to engineering mathematics education begins with one year of freshman calculus as a prerequisite to subsequent core engineering courses. However, the inability of incoming students to successfully advance through the traditional freshman calculus sequence is a primary cause of attrition in engineering programs across the country. As a result, the WSU model seeks to redefine the way in which engineering mathematics is taught, with the goal of increasing student retention, motivation and success in engineering. The WSU approach begins with the development of a novel freshman-level engineering mathematics course, EGR 101 Introductory Mathematics for Engineering Applications. Taught by engineering faculty, the course includes lecture, laboratory and recitation components. Using an application-oriented, hands-on approach, the course addresses only the salient math topics actually used in core engineering courses. These include the traditional physics, engineering mechanics, electric circuits and computer programming sequences. The EGR 101 course replaces traditional math prerequisite requirements for the above core courses, so that students can advance in the engineering curriculum without having completed a traditional freshman calculus sequence. This has enabled a significant restructuring of the engineering curriculum, including the placement of formerly sophomore-level engineering courses within the freshman year. The WSU model concludes with the development of a revised engineering math sequence, taught by the math department later in the curriculum, in concert with College and ABET requirements. The result has shifted the traditional emphasis on math prerequisite requirements to an emphasis on engineering motivation for math, with a "just-in-time" structuring of the new math sequence. Key components included with this resource are a lab activity, classroom activity, case study, self-guided student work, quiz/test, example problems, simulation and graphics/video.MERC Reviewers comments: The instructor has spent considerable amount of time and effort to develop this course. I really commend the instructor for the innovativeness in using MATLAB for solving problems and relating to practical applications. I very highly recommend that this course be offered to bring about the role of mathematics in engineering education

GT2009-59117 DETERMINING THE SCATTER IN FATIGUE CRACK GROWTH RATE BASED ON VARIATIONS IN BULK PROPERTY DATA

ABSTRACT A technique to predict the variability of the Paris regime fatigue crack growth rates in ductile materials based on bulk property (yield strength, hardening modulus, and fracture toughness) variation is presented. The prediction, based on the plastic dissipation in the reversed plastic zone ahead of the crack tip, is carried out for Ti-6Al-4V. The empirical distributions of the bulk properties of Ti-6Al-4V are characterized and directly used in the probabilistic assessment of the fatigue crack growth rate. Since computing the plastic dissipation is a computationally intensive procedure, a novel sampling scheme based on confidence interval minimization was used to generate the empirical distribution of fatigue crack growth rate. This technique also predicts correlation between fatigue crack growth rate and fracture toughness, which may be useful in probabilistic design of turbines