The effects of electron beam rotation upon electron beam welded copper - 304 couples

The United States Air Force, Arnold Engineering Development Center, has been using copper to 304 stainless steel couples made using the electron beam welding process during the fabrication of intrusive gas-path diagnostic probes for over five years. Only a limited physical analysis of the resulting welds had been done. The purpose of this investigation was to determine the effects that varying the rotation frequency of the electron beam had upon the mechanical characteristics of the copper to 304 stainless steel couples.

During welding of the samples, all controllable weld process parameters were held constant with the exception of the electron beam rotation frequency; the rotation frequency was varied from 20 Hz to 180 Hz in steps of 20 Hz. Samples welded without electron beam rotation provided a baseline for comparison.

Microhardness distributions showed that weld region homogeneity as evidenced by microhardness maps and optical microscopy was a function of the electron beam rotation frequency. As the electron beam rotation frequency was increased so did the uniformity of the microhardness distribution.

There was no correlation between electron beam rotation frequency and weld tensile strength since each test coupon failed in the Cu base material outside of the weld region. The weld region material was stronger than the Cu base material regardless of the electron beam rotation frequency.

The top weld bead width and bottom weld bead width decreased only slightly with increases in the electron beam rotation frequency.

The welds made at all electron beam rotation frequencies used for this study contained cracks within the weld region, heat affected zone (HAZ), or both. The relative number, length, and location of the individual cracks changed with electron beam rotation frequency. Cracking in the HAZ due to liquid metal embrittlement (LME) was not evident in those samples welded with the electron beam rotation frequencies below 100 Hz. Cracking due to LME outside of the weld region in the HAZ was observed to increase with the electron beam rotation frequency above 80 Hz.

The relationship between weld region cracking and residual stress within the weld region was shown to be dependent on the electron beam rotation frequency. Cracking in the weld region was not observed in samples welded with the electron beam rotation frequency above 80 Hz. Within the weld region the maximum crack length was 0.5 mm with most crack lengths approximately 0.2 mm or less. Cracking in the baseline sample, welded without electron beam rotation, was limited to a 0.01 mm crack in the 304 SS HAZ.

Divorced spherical structures similar to those found in documented monotectic systems were observed. These structures were found to include regions of planar front growth that transitioned into perturbed cellular growth forms.