Page 48 - MetalForming-Apr-2018-issue

Welding Well
Fig. 4—A "sausage-shaped" projection design on the fastener is recommended for welding to high-strength steel.
test the welds every hour or so. A push- out-, tensile- or torque-testing device can be used to verify the strength of welded fasteners.
Given the proper settings, accept- able weld strength can be obtained from standard AC and MFDC press- type projection welders. However, a growing trend is to use capacitor-dis- charge (CD) resistance welders, which produce an almost instantaneous, high-amperage burst of power. Our lab
Fig. 5—A weld nut with a ring pro- jection also works well when
welding to high-strength steel, especially when a gas- or liquid- tight joint is needed.
tests show that using a CD welder and high force setting can result in welds with approximately twice the strength of other methods, and joints that approach the strength of the base material. Other newer technologies include hybrid CD/MFDC resistance- welding machines and a new “fast-rise” MFDC power-supply design, both of which offer better control of output.
With a suitable welder identified, the proper fastener geometry will make it much easier to successfully weld nuts and studs to hard, high-strength alloys. According to Ron Foreman, welding- lab manager for Buckeye Fasteners, a supplier of nuts and studs designed for resistance welding, the pointed “upside-down-pyramid” projection designs used for many years in mild- steel applications tend to collapse
before a strong joint forms with high-strength
alloys.
To prevent blowout of the
projections before they sink into the high-strength sheet, he recommends choosing a fastener with three rounded “sausage-shaped” projections (Fig. 4). Also, when welding fasteners to thick material, Foreman recommends six projections positioned well away from the fastener’s self-piloting ridge. In addition, he reports that rounded 360- deg.-ring projection designs work well (Fig. 5), especially when the joint must be gas- or liquid-tight.
In conclusion, since the requirement to resistance weld nuts and studs to high-strength steel is the new normal, stampers must understand the process and make the necessary changes to avoid expensive rejects. MF
46 MetalForming/April 2018
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