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Research Papers

Improving Process Control in Electron Beam Welding Using the Enhanced Modified Faraday Cup

[+] Author and Article Information
T. A. Palmer, J. W. Elmer

 Lawrence Livermore National Laboratory, Livermore, CA 94550

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J. Manuf. Sci. Eng 130(4), 041008 (Jul 10, 2008) (15 pages) doi:10.1115/1.2950061 History: Received March 01, 2007; Revised April 08, 2008; Published July 10, 2008

Process control in electron beam welding is typically based on control of machine settings, such as accelerating voltage, beam current, focus coil current, and vacuum level. These settings, though important, provide little insight into the characteristics of the beam used to make the weld. With the enhanced modified Faraday cup (EMFC) diagnostic tool, these beam characteristics, including the peak power density, full width at half maximum, and full width at 1e2 values, can be quantified. The use of this diagnostic tool in an extended production run at Lawrence Livermore National Laboratory (LLNL) is described. Results show that machine performance, in terms of these measured beam characteristics, varies over time when the EMFC is not used to adjust the machine settings. Testing has shown that the variability of the beam characteristics can be measurably decreased with the use of the EMFC diagnostic tool. With the implementation of this diagnostic tool in the process control procedures, every electron beam weld, which encompassed approximately 90 welds over an 18month time frame, met all of the requirements defined in the weld process specification and passed all of the postweld quality control checks. The results also show that variations in each of the measured beam parameters can be controlled at levels below ±2.2%, which is smaller than the 5% tolerance band suggested by ASME for other welding parameters. Such an enhanced level of control allows product throughput to be increased by decreasing the number of rejected parts through the elimination of unexpected variations in beam characteristics. The benefits of integrating this diagnostic tool into future process control regimes are also discussed.

Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Overview of desired weld characteristics and geometries

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Figure 2

Reconstructions of the beams made during the weld development activities for (a) the sharp focus setting, (b) the +5 defocus setting used to make the full-penetration welds, and (c) the +10 defocus setting used for the cosmetic pass. These beams are representative of the beams used throughout the production run.

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Figure 3

Reconstructions of the beams made at the operator-determined sharp focus setting for welds made after individual pump downs of the vacuum chamber

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Figure 4

Reconstructions of the beams made at the operator determined sharp focus setting for welds made with two different filaments. Focus coil current settings at sharp focus on Filament 1 varied between 0.607A and 0.612A and on Filament 2 between 0.610A and 0.618A.

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Figure 5

Plot showing the variation in the measured peak power density at the operator-determined sharp focus settings, prior to making adjustments with the EMFC, as a function of (a) the focus coil current and (b) over the course of the 18month long production run. The shaded bands in each plot indicate the ±5% tolerance for the measurements.

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Figure 6

Plots showing the variation in the FWHM values for the operator-determined sharp focus setting, prior to making adjustments with the EMFC, as a function of (a) the focus coil current setting and (b) the number of welds made over the course of the 18month long production run. The shaded bands in each plot indicate the ±5% tolerance for the measurements.

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Figure 7

Plots showing the variation in the peak power density for the EMFC determined sharp focus setting as a function of (a) the focus coil current setting and (b) the number of welds made over the course of the 18month long production run. The shaded bands in each plot indicate the ±5% tolerance for the measurements.

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Figure 8

Plots showing the variation in the FWHM values for the EMFC determined sharp focus setting as a function of (a) the focus coil current setting and (b) the number of welds made over the course of the 18month long production run. The shaded bands in each plot indicate the ±5% tolerance for the measurements.

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Figure 9

Plots showing comparison of (a) peak power density, (b) FWHM, and (c) FWe2 values measured at the operator sharp focus and the diagnostic-determined sharp focus setting. The shaded bands in each plot indicate the ±5% tolerance for the measurements.

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Figure 10

Plot showing the variation of the relative focus difference between the operator and diagnostic-determined sharp focus, which is indicated by the horizontal line, for welds made over the course of the production run

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Figure 11

Plots showing the variation in the (a) peak power density and (b) FWHM for the weld parameters over the course of the production run. The shaded bands in each plot indicate the ±5% tolerance for the measurements.

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Figure 12

Plots showing comparison of (a) peak power density, (b) FWHM, and (c) FWe2 values measured for the beams used to make each of the welds. In the plot, the difference between welds made at the fixed defocus setting (+5) and those made with a flexible defocus setting is shown. The shaded bands in each plot indicate the ±5% tolerance for the measurements.

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Figure 13

Plot showing variations in the relative defocus required to achieve the required full-penetration weld pass beam parameters. The relative defocus settings are shown with respect to the diagnostic-determined sharp focus, which is indicated by the horizontal line.

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Figure 14

Plots showing the variation in the (a) peak power density and (b) FWHM for the cosmetic pass parameters over the course of the production run. The shaded bands in each plot indicate the ±5% tolerance for the measurements.

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Figure 15

Plot showing variations in the relative defocus required to achieve the required cosmetic weld pass beam parameters. The relative defocus settings are shown with respect to the diagnostic-determined sharp focus, and the horizontal line represents the nominal defocus setting.

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Figure 16

Plots showing comparison of (a) peak power density, (b) FWHM, and (c) FWe2 values measured for the beams used to make the cosmetic pass on each of the welds. In the plot, the difference between welds made at the fixed defocus setting (+10) and those made with a flexible defocus setting is shown. The shaded bands in each plot indicate the ±5% tolerance for the measurements.

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