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WELDING RESEARCH

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Fig. 15 — Secondary electron images of a spot weld produced using unusual process parameters (24-kA current, 5-cycle weld time, 1200-lb weld force). button formation appeared to be promoted by zinc ingestion.

eld

sion with weld times of 20 and 30 cycles. For currents of 12 kA and less, zinc expul- sion was prevented by reasonably low- weld forces. In contrast, the currents over 12 kA were found to be impractical as the required forces to prevent expulsion were then too high for many industrial welding machines. Despite this new constraint, we

occasionally selected currents of 12 kA and higher, as justified later.

Weld Joint Properties

Spot Weld Morphology

In addition to preventing zinc expulsion,

the compressive force applied by the elec- trodes forges the fused region to a thinner gauge. Such thinning caused by indenta- tion can alter spot weld properties, as quantified in a recent study (Ref. 21), and was therefore another factor guiding the selection of the process parameters. For a weld force of 1200 lb (5.3 kN), Fig. 4 de-

178-s NOVEMBER 2005

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