Fusion welding joins dissimilar metals in a flash

Oct. 25, 2001
Fusion welders, precision TIG and plasma welders, adjustable adhesive dispensing valves, and acrylic assembly adhesives, are just some of the topics covered in this Fastening and Joining Reference Guide.

Edited by Lawrence Kren & Victoria Reitz

Fusion welders bond small-diameter studs to base parts using controlled short circuits.

Here's a manufacturing challenge: attach a flat gold-plated copper connector to a massive gold-plated copper bus. The obvious approach, mechanical fasteners, tend to be costly and unreliable. Resistance welding won't work and brazing compromises the copper's mechanical integrity. One answer is capacitive-discharge fusion welding. It makes a metallurgical weld in a few millionths of a second without annealing, or the need for an inert atmosphere, filler material, or special welding skills.

"The ability of fusion welding to join both similar and dissimilar metals can lower assembly costs and make some otherwise difficult jobs simpler," says Geoff Reilly of Contract Fusion Inc., East Providence, R.I. Contract Fusion, as the name implies, performs the welding service for a wide range of applications. It welds up to 1 million studs/week sized 0.014 to 0.25-in. diameter, all to American Welding Society standards of tensile and torsional strength. For example, fusion welding installs threaded studs on copper wave-guide tubes without deforming the tube. It can also attach tiny stainlesssteel and titanium posts (stops) on tweezers and forceps. The same process instantly

welds 0.016-in. diameter, 0.75-in.-length tin-plated Kovar leads to iridium cans, housing preassembled motion detectors with Mil-spec reliability. Nearly instantaneous, heat-free welds make all this possible.

Fusion welding is best described as a controlled shortcircuit. The power source (charged capacitors) shorts through an initiation nib on the stud to be welded. This small nib, which is either formed, turned, or headed, serves as a high-resistance focus for the discharge current. During the welding process, the stud moves at a controlled velocity over a distance of about 0.75 in. to the base part surface. The stud ionizes instantly upon contact, creating
a conductive path for the subsequent electric current pulse. This high-intensity arc melts the stud and base surfaces as the two impact one another. Heat rise and dissipation are almost instantaneous, preventing annealing and distortion.

The energy, velocity, and nib size are precisely predetermined so the welding action is automatic and consistent and involves no special operator skills. Because fusion welding only penetrates surfaces to about 0.002 in. deep, base part thickness is not a factor and the reverse surface is left unmarred. This eliminates complex through-hole mounting or costly brazing and secondary refinishing operations.

Electrolytic capacitors charge and the driver cylinder accelerates to the base part surface.

The capacitors short when the initiation nib contacts the base part.

The nib instantly vaporizes, creating a conductive, ionic pathway.

The remainder of the stored energy forms a hightemperature plasma.

The arc spreads across the stud face, melting a thin layer of the stud and base.

Impact produces a metallurgical bond about 0.002 in. deep into the base material.

Completed weld cools. Nearly instantaneous heat rise and fall eliminates annealing or burning.

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