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RESEARCH PAPERS: Heat Transfer in Manufacturing

Modeling and Experimental Verification of Transient/Residual Stresses and Microstructure Formation in Multi-Layer Laser Aided DMD Process

[+] Author and Article Information
S. Ghosh, J. Choi

Department of Mechanical and Aerospace Engineering, University of Missouri-Rolla, 1870 Miner Circle, Rolla, MO 65409

J. Heat Transfer 128(7), 662-679 (Dec 12, 2005) (18 pages) doi:10.1115/1.2194037 History: Received May 02, 2005; Revised December 12, 2005

Despite enormous progress in laser aided direct metal/material deposition (LADMD) process many issues concerning the adverse effects of process parameters on the stability of variety of properties and the integrity of microstructure have been reported. Comprehensive understanding of the transport phenomena and heat transfer analysis is essential to predict the thermally induced residual stresses and solidification microstructure in the deposited materials. Traditional solidification theories as they apply to castings or related processes, assume either no mass diffusion in the solid (Gulliver-Scheil) or complete diffusion in the solid (equilibrium lever rule) in a fixed arm space. These are inappropriate in high energy beam processes involving significantly high cooling rates. The focus of this paper is the solute transport in multi-pass LADMD process, especially the coupling of the process scale transport with the transport at the local scale of the solid-liquid interface. This requires modeling of solute redistribution at the scale of the secondary arm spacing in the dendritic mushy region. This paper is an attempt toward a methodology of finite element analysis for the prediction of solidification microstructure and macroscopic as well as microscopic thermal stresses. The computer simulation is based on the metallo-thermo-mechanical theory for uncoupled temperature, solidification, phase transformation, and stress/strain fields. The importance of considering phase transformation effects is also verified through the comparison of the magnitudes of residual stresses with and without the inclusion of phase transformation kinetics. The simulation has been carried out for H13 tool steel deposited on a mild steel substrate.

Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Metallo-thermo-mechanical coupling during processes involving phase transformation

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Figure 2

Flow chart showing the microstructure computation and net residual stress calculation procedure with the inclusion of phase transformation

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Figure 3

(a) Schematic of a double-pass laser aided DMD process, (b) section X-X chosen for stress analysis, (c) locations of the points on section X-X chosen for transient stress analysis, (d) section Y-Y chosen for solidification microstructure, and (e)Y1-Y1 at the interface of the first layer and substrate along the direction of laser travel and Y2-Y2 at the interface of the third and fourth layers along the direction of laser travel

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Figure 4

Temperature contour on section X-X at time (a)t=1.0s (Single-pass on the first layer) and (b)t=3.0s (Double-pass on the first layer). All temperature values are in degrees kelvin (K).

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Figure 5

Cooling rate contour on section X-X at time (a)t=1.0s (Single-pass on the first layer) and (b)t=3.0s (Double-pass on the first layer). Cooling rate values are in degrees kelvin/second (K/s).

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Figure 6

Time-temperature transformation (TTT) diagram of H13 Tool Steel. (Boyer, H.E., and Gray, A.G., 1977, Atlas of Isothermal Transformation and Cooling Transformation Diagrams,” American Society for Metals, Metals Park, OH).

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Figure 7

Comparison of (a) transverse (S11) and (b) longitudinal (S22) transient stresses at point “a” with and without phase transformation effects

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Figure 8

Comparison of (a) transverse (S11) and (b) longitudinal (S22) transient stresses at point “b” with and without phase transformation effects

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Figure 9

(a) Cooling rate, (b) temperature gradient, (c) solidification velocity, (d) primary dendrite arm spacing, λ1, and (e) secondary dendrite arm spacing, λ2 at Y1-Y1 at time t=3.125s

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Figure 10

(a) Cooling rate, (b) Temperature gradient, (c) solidification velocity, (d) primary dendrite arm spacing, λ1, and (e) secondary dendrite arm spacing, λ2 at Y2-Y2 at time t=15.125s

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Figure 11

Schematic of the experimental setup for double-pass single layer LADMD process

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Figure 12

Time-temperature variation at (a) point “a,” (b) point “b,” and (c) point “c” of Fig. 1

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Figure 13

(a) Transverse (S11) and (b) longitudinal (S22) thermal stresses at point “b” of Fig. 3

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Figure 14

(a) Transverse (S11) and (b) longitudinal (S22) thermal stresses at point “c” of Fig. 3

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Figure 15

SEM micrograph of deposited sample showing the microstructure: (a) on a surface along the laser travel and (b) on a surface lateral to the laser travel. Laser traverse speed is 12.7mm∕s.

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Figure 16

SEM micrograph of deposited sample showing the microstructure: (a) on a surface along the laser travel and (b) on a surface lateral to the laser travel. Laser traverse speed is 19mm∕s.

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Figure 17

X-ray diffraction data for point “b” (diffraction intensity versus 2θ)

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