Machine Design

Arc-Welded Studs

In stud welding, the heat of an electric arc drawn between the fastener and the work melts a quantity of metal, after which the two heated parts are brought together under pressure. Welded fasteners must be made of a weldable material, and one end of the fastener must be designed for welding. Stud welding makes leakproof, pressure-tight connections and is adaptable to either automatic or semiautomatic operation. Automatic welds can be made at rates of up to 60/min. Electric arc and capacitor discharge are the two main stud-welding processes.

Electric-arc stud welding: In this semiautomatic electric-arc process, the heat for end welding the studs is from motor-generator or transformer-rectifier supplied dc current passing through an arc from the stud (electrode) to the plate (work). Weld cycle depends on stud diameter and may vary from 0.1 to 1 sec.

A high-strength bond is obtained because the full cross-sectional area of the fastener or stud is fused to the base metal. Electric-arc stud welding is best used when the base plate is heavy enough to support the full strength of the welded fastener. However, lighter-gauge materials are often arc welded. To avoid burn-through, the plate thickness should be at least one-fifteenth the weld-base diameter. To develop full fastener strength, basic plate thickness should be a minimum of one-third the weld-base diameter.

Capacitor-discharge stud welding: The second basic stud-welding process derives heat from an arc produced by a rapid discharge of capacitor-stored electrical energy, with pressure applied to the stud during or immediately following the electrical discharge. Like arc-stud welding, the heat for end welding of studs is developed by passage of current through an arc from the stud (electrode) to the plate (work).

A prime advantage of the capacitor-discharge system is its ability to weld studs to thin materials without great distortion, burn-through, or discoloration. Also, weld penetration is slight, so many dissimilar metals can be welded without metallurgical problems. Plates may be as thin as 0.016 in. for steel and 0.040 in. for aluminum.

The three main capacitor-discharge stud-welding systems are initial contact, initial gap, and drawn arc. These processes vary primarily in the manner of arc initiation. Gap welding is suited to the fabrication of heat-sensitive components. It is preferred for welding to nonferrous metals, especially to aluminum, and thin-gauge metals, because it produces the least marking on the reverse side.

TAGS: Fasteners
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