An arc welding process that uses an arc between a metal stud, or similar part, and the other workpiece. The process is used without filler metal, with or without shielding gas or flux, with or without partial shielding from a ceramic or graphite ferrule surrounding the stud, and with the application of pressure after the faying surfaces are suficiently heated.  See STANDARD WELDING TERMS.

In arc stud welding, the base end of the stud is joined to the other work part by heating the stud and the work with an arc drawn between the two. When the surfaces to be joined are properly heated, they are brought together under low pressure. Stud welding guns are used to hold the studs and move them in proper sequence during welding. There are two basic power supplies used to create the arc for welding studs. One type uses d-c power sources similar to those used for shielded metal arc welding (SMAW). The other type uses a capacitor storage bank to supply the arc power. The stud arc welding processes using these two types of power sources are known as arc stud welding and capacitor discharge stud welding, respectively.

Arc stud welding, the more widely used of the two major stud welding processes, is similar in many respects to manual SMAW. The heat necessary for welding of studs is developed by a d-c arc between the stud (electrode) and the plate (work) to which the stud is to be welded. Welding time and the plunging of the stud into the molten weld pool to complete the weld are controlled automatically. The stud, which is held in a stud welding gun, is positioned by the  operator, who then actuates the unit by pressing a switch. The weld is completed quickly, usually in less than a second. This process generally uses a ceramic arc shield, called a ferrule. It surrounds the stud to contain the molten metal and shield the arc.

Capacitor discharge stud welding derives its heat from an arc produced by the rapid discharge of electrical energy stored in a bank of capacitors. During or immediately following the electrical discharge, pressure is applied to the stud, plunging its base into the molten pool of the workpiece. The arc may be established either by rapid resistance heating, and vaporization of a projection on the stud weld base (arc time: 3-6 milliseconds), or by drawing an arc as the stud is lifted away from the workpiece (arc time: 6-15 milliseconds). The capacitor discharge process does not require a shielding ceramic ferrule because of the short arc duration and small amount of molten metal expelled from the joint. It is suited for applications requiring small to medium studs.

For either process, a wide range of stud styles is available. They include such types as threaded fasteners, plain or slotted pins, and internally threaded fasteners. Most stud styles can be rapidly applied with portable equipment.

Capabilities-

Because arc stud welding time cycles are very short, heat input to the base metal is very small compared to conventional arc welding. Consequently, the weld metal and heat-affected zones are very narrow. Distortion of the base metal at stud locations is minimal.

Studs can be welded at the appropriate time during construction or fabrication without access to the back side of the base member. Drilling, tapping, or riveting for installation is not required.

Small studs can be welded to thin sections by the capacitor discharge method. Studs have been welded to sheet as thin as 0.75 mm (0.03 in.) without meltthrough. They have been joined to certain materials (stainless steel, for example) in thicknesses down to 0.25 mm (0.01 in.). Because the depth of melting is very shallow, capacitor discharge welds can be made without damage to a refinished opposite side. No subsequent cleaning or finishing is required.

Limitations-

Only one end of a stud can be welded to the workpiece. If a stud is required on both sides of a member, a second stud must be welded to the other side. Stud shape and size are limited because the stud design must permit chucking of the stud for welding. The stud base size is limited for thin base metal thicknesses.

Studs applied by arc stud welding usually require a disposable ceramic ferrule around the base. It is also necessary to provide flux in the stud base or a protective gas shield to obtain a sound weld.

The arc stud welding process involves the same basic principles as any of the other arc welding processes. Application of the process consists of two steps:

(1) Welding heat is developed with an arc between the stud and the plate (work).

(2) The two pieces are brought into intimate contact when the proper temperature is reached.

Applications

Arc stud welding has been widely accepted by all the metalworking industries. Specifically, stud welding is used extensively in the following fields: automotive, boiler and building and bridge construction, farm and industrial equipment manufacture, railroads, and shipbuilding. Defense industry applications include missile containers, armored vehicles, and tanks.

Some typical applications are attaching wood floors to steel decks or framework; fastening linings or insulation in tanks, boxcars, and other containers, securing inspection covers, mounting machine iiccessories; securing tubing and wire harnesses; and welding shear connectors and concrete anchors to structures.

Equipment

The most basic equipment arrangement consists of the stud gun, a control unit (timing device), studs and ferrules, and an available source of d-c welding current. In terms of portability and ease of operation, the equipment involved in stud welding compares with that of manual SMAW.

Guns-

There are two types of stud welding guns, portable hand-held and fixed production types. Automatic stud feeding systems are available for both.

Power Sources-

A direct-current power source is used for arc stud welding. Alternating current is not suitable. The three basic types of d-c power sources that can be used are: transformer-rectifier; motor-generator, (motor or engine driven), and battery. The following are general characteristics desired in a stud welding power source:

(1) High open-circuit voltage, in the range of 70 to 00 V.

(2) A drooping output volt-ampere characteristic

(3) A rapid output current rise to the set value

(4) High current output for a relatively short time.

The current requirements are higher, and the dutycycle is much lower for stud welding than for other types of arc welding.

Duty Cycle-

The basis for rating special stud welding power sources is different from that of conventional arc welding machines. Because stud welding requires a high current for a relatively short time, the current output requirements of a stud welding power source are higher, but the duty cycle is much lower than those for other types of arc welding.

The duty cycle for stud arc welding machines is based on the formula:

Percent duty cycle = 1.7 x number of one-second loads per minute, where the one-second load is the rated output.

Thus, if a machine can be operated six times per minute at rated load without causing its components to exceed their maximum allowable temperatures, then the machine would have a 10% duty cycle rating.

Power Control Units-

The control unit consists fundamentally of a contactor suitable for conducting and interrupting the welding current, and a weld timing device with associated electrical controls. Once set, the control unit maintains the proper time interval for the size of stud being welded.

Procedure-

The mechanics of the process are illustrated in Figure A-7. The stud is loaded into the chuckthe ferrule (arc shield) is placed in position over the end of the stud, and the gun is properly positioned for welding. The trigger is then depressed, starting the automatic welding cycle. A solenoid coil within the body of the gun is energized. This lifts the stud off the work, and at the same time, creates an arc. The end of the stud and the workpiece are melted by the arc.

When the preset arc period is completed, the welding current is automatically shut off and the solenoid is deenergized by the control unit. The mainspring of the gun plunges the stud into the molten pool on the work to complete the weld. The gun is then lifted from the stud, and the ferrule is broken off. The time required to complete a weld varies with the cross-sectional area of the stud. An average rate is approximately 6 studs per minute, although a rate of 15 studs per minute can be achieved for some applications.

Inspection-

The latest edition of ANSUAWS D1.l, Structural Welding Code-Steelcontains provisions for the installation and inspection of steel studs welded to steel components. Quality control and inspection requirements for stud welding are also included. ANSUAWS C5.4, Recommended Practices for Stud Weldinglatest edition, briefly covers inspection and testing of both steel and aluminum stud welds.

 

Capacitor Discharge Stud Welding

Capacitor discharge stud welding is a stud arc welding process in which d-c arc power is produced by a rapid discharge of stored electrical energy with pressure applied during or immediately following the electrical discharge. The process uses an electrostatic storage system as a power source in which the weld energy is stored in capacitors of high capacitance. No ferrule or fluxing is required.

There are three different types of capacitor discharge stud welding: initial contact, initial gap, and drawn arc. They differ primarily in the manner of arc initiation. Initial contact and initial gap capacitor discharge stud welding studs have a small, specially designed projection (tip) on the weld end of the stud. Drawn arc stud welding creates a pilot arc as the stud is lifted off the workpiece by the stud gun. That version is similar to arc stud welding.

Initial Contact Method

In initial contact stud welding, the stud is placed against the work. The stored energy is then discharged through the projection on the base of the stud. The small projection presents a  high resistance to the stored energy, and it rapidly disintegrates from the high current density. This creates an arc that melts the surfaces to be joined. During arcing, the pieces to be joined are being brought together by action of a spring, weight, or an air cylinder. When the two surfaces come in contact, fusion takes place, and the weld is completed.

Initial Gap Method

To begin, the stud is positioned off the work, leaving a gap between it and the work.

The stud is released and continuously moves toward the work under gravity or spring loading. At the same time, open-circuit voltage is applied between the stud and the work. When the stud contacts the work, high current flashes off the tip and initiates an arc. The arc melts the surfaces of the stud and work as the stud continues to move forward. Finally, the stud plunges into the work, and the weld is completed.

Drawn Arc Method

Arc initiation is accomplished in a manner similar to that of arc stud welding. The stud does not require a tip on the weld face. An electronic control is used to sequence the operation. Weld time is controlled by an electronic circuit in the unit. The welding gun is similar to that used for arc stud welding.

The stud is positioned against the work; the trigger switch on the stud welding gun is actuated, energizing the welding circuit and a solenoid coil in the gun body. The coil motion lifts the stud from the work, drawing a low amperage pilot arc between them. When the lifting coil is de-energized, the stud starts to return to the work. The welding capacitors are then discharged across the arc. The high amperage from the capacitors melts the end of the stud and the adjacent work surface. The spring action of the welding gun plunges the stud into the molten metal to complete the weld.

Applications

Some industrial applications of capacitor discharge stud welding are aircraft and aerospace, appliances, building construction, maritime construction, metal furniture, stainless steel equipment, and transportation.

It is possible to weld studs to dissimilar metals with capacitor discharge stud welding because the penetration into the work from the arc is so shallow that there is very little mixing of the stud metal and work metal. A few of the combinations that may be welded are steel to stainless steel, brass to steel, copper to steel, brass to copper, and aluminum to die cast zinc.

The process can be used on parts that have had the face surface painted, plated, polished, or coated with ceramic or plastic, because postweld cleaning or finishing operations on the side of the base metal opposite to the stud attachment are eliminated.

Safety Precautions

Personnel operating stud welding equipment should be provided with face and skin protection to guard against bums from spatter produced during welds. Eye protection in the form of goggles or a face shield with a No. 3 filter lens should be worn to protect against arc radiation. Before repairs to equipment are attempted, electrical power should be turned off and electric switch boxes locked out. Capacitors used in capacitor discharge equipment should be completely drained of electrical charge before attempting repairs.

Reference: American Welding Society, Welding Handbook, 8th Edition, Vol. 2. Miami, Florida: American Welding Society, 1991.

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