Improving process control in electron beam welding using the enhanced modified Faraday cup

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 1/e(2) 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 18 month 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.