INTRODUCTIONStud welding is a general term for joining a metal stud or similar part to a workpiece. Welding can be done by a number of welding processes including arc, resistance, friction, and percussion. Of these processes, the one that utilizes equipment and techniques unique to stud welding is arc welding. This process, known as stud arc welding (SW), will be covered below. The other processes use conventionally designed equipment with special tooling for stud welding.
In stud arc 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 dc power sources similar to those used for shielded metal arc welding. 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 commonly known as arc stud welding and capacitor discharge stud welding, respectively.
ARC STUD WELDING
Arc stud welding, the more widely used of the two major stud welding processes, is similar in many respects to manual shielded metal arc welding. The heat necessary for welding of studs is developed by a dc arc between the stud (electrode) and the plate (work) to which the stud is to be welded. The welding current is supplied by either a dc motor-generator or a dc transformer-rectifier power source, similar to those used for shielded metal arc welding. 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 one 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. A ferrule is not used with some special welding techniques, nor with some nonferrous metals.
CAPACITOR DISCHARGE STUD WELDING
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, or by drawing an arc as the stud is lifted away from the workpiece. In the first type, arc times are about three to six milliseconds; in the second type, they range from six to fifteen 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 sized studs.
For either process, a wide range of stud styles is available. They include such types as threaded fasteners, plain or slotted pins, internally threaded fasteners, flat fasteners with rectangular cross section, and headed pins with various upsets. Studs may be used as holddowns, standoffs, heat transfer members, insulation supports, and in other fastening applications. Most stud styles can be rapidly applied with portable equipment.
CAPABILITIES AND LIMITATIONS
Because stud arc 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. The local heat input may be harmful when studs are welded onto medium and high carbon steels. The unheated portion of the stud and base metal will cool the weld and heat-affected zones very rapidly, causing these areas to harden. The resulting lack of weld joint ductility may be detrimental under certain types of loading, such as cyclic loads. On the other hand, when stud welding precipitation hardened aluminum alloys, a short weld cycle minimizes overaging and softening of the adjacent base metal. Metallurgical compatibility between stud material and the base metal must also be considered.
Studs can be welded at the appropriate time during construction or fabrication without access to the back side of the member. Drilling, tapping, or riveting for installation is not required.
Using this process, designers need not specify thicker materials nor provide heavy bosses and flanges to obtain required tap depths for threaded fasteners. With stud welded designs of lighter weight, not only can material be saved but the amount of welding and machining needed to join parts can be reduced.
Small studs can be welded to thin sections by the capacitor discharge method. Studs have been welded to sheet as thin as 0.03 in. (0.75 mm) without melt-through. They have been joined to certain materials (stainless steel, for example) in thicknesses down to 0.01 in. (0.25 mm). Because the depth of melting is very shallow, capacitor discharge welds can be made without damage to a prefinished opposite side. No subsequent cleaning or finishing is required.
Capacitor discharge power permits the welding of more dissimilar metals and alloys than arc stud welding. While both can join steel to stainless steel, only the capacitor discharge welding system can join brass to steel, copper to steel, brass to copper, aluminum to die-cast zinc, and similar combinations.
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.
Most studs applied by capacitor discharge power require a close tolerance projection on the weld base to initiate the arc. Stud diameters that can be attached by this method generally range from 1/8 to 3/8 in. (3.2 to 9.5 mm). Above this size, arc stud welding is more economical.
A welding power source located convenient to the work area is required for stud welding. For arc stud welding, 230 or 460 V ac power is required to operate the dc welding power source. For most capacitor discharge welding, a single phase 110 V ac main supply will serve, but high production units require three phase ac, 230 or 460 V, for operation.
ARC STUD WELDING - PRINCIPLES OF OPERATION
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.
The most basic equipment arrangement consists of the stud gun, a control unit (timing device), studs and ferrules, and an available source of dc welding current. Equipment is now available in which the power source and gun timing device are integrated into one unit. The stud is loaded into the chuck, the ferrule (also known as an 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 de-energized 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. For example, typical weld time is about 0.13 seconds for a 10 gage (0.135 in. or 3.4 mm diameter) stud, and 0.92 seconds for a 7/8 in. (22 mm) diameter stud. An average rate is approximately 6 studs per minute, although a rate of 15 studs per minute can be achieved for some applications.
The equipment involved in stud welding compares with that of manual shielded metal arc welding with regard to portability and ease of operation. The initial cost of such equipment varies with the size of the studs to be welded.
DESIGNING FOR ARC STUD WELDING
When a design calls for stud type fasteners or supports, arc stud welding should be considered as a means for attaching them. Compared to threaded studs, the (work) material thickness required to obtain full strength is less for arc stud welding. The use of arc welded studs may reduce the thickness of bosses at attachment points or may eliminate them. Cover plate flanges may be thinner than those required for threaded fasteners. Thus, there is potential weight savings when the process is used.
The weld base diameters of steel studs range from 1/8 to 1-1/4 in. (3.2 to 32 mm). For aluminum, the range is 1/8 to 1/2 in. (3.2 to 13 mm), and for stainless steels it is1/8 to 1 in. (3.2 to 25 mm). For design purposes, the smallest cross-sectional area of the stud should be used for load determination, and adequate safety factors should be considered.
To develop full fastener strength, the plate (work) thickness should be a minimum of approximately one third the weld base diameter. A minimum plate thickness is required for each stud size to permit arc stud welding without melt through or excessive distortion. For steel, a 1:5 minimum ratio of plate thickness to stud weld base diameter is the general rule.
Fasteners can be stud welded with smaller edge distances than those required for threaded fasteners. However, loading and deflection requirements must be considered at stud locations.
The most common stud materials welded with the arc stud welding process are low carbon steel, stainless steel, and aluminum. Other materials are used for studs on a special application basis. Typical low carbon steel studs have a chemical composition as follows (all values are maximum): 0.23 percent carbon, 0.90 percent manganese, 0.040 percent phosphorus, and 0.050 percent sulfur. They have a minimum tensile strength of 55 000 psi (380 MPa) and a minimum yield strength of 50 000 psi (345 MPa). The typical tensile strength for stainless steel studs is 85 000 psi.
High-strength studs, meeting the SAE steel fastener Grade S tensile strength of 120 000 psi (825 MPa) minimum, are also available. These studs are basically carbon steels that are heat treated to meet the tensile strength requirement.
Low carbon and stainless steel studs require a quantity of welding flux within or permanently affixed to the end of the stud. The main purposes of the flux are to deoxidize the weld metal and to stabilize the arc.
Aluminum studs do not use flux on the weld end. Argon or helium shielding is required to prevent oxidation of the weld metal and stabilize the arc. The studs usually have a small tip on the weld end to aid arc initiation.
Most stud weld bases are round. However, there are many applications which use a square or rectangular shaped stud. With rectangular studs, the width-to-thickness ratio at the weld base should not exceed five to obtain satisfactory weld results. In addition to conventional straight threaded studs, they include eye-bolts, J-bolts, and punched, slotted, grooved, and pointed studs.
Stud designs are limited in that (1) welds can be made on only one end of a stud; (2) the shape must be such that a ferrule (arc shield) that fits the weld base can be produced; (3) the cross section of the stud weld base must be within the range that can be stud welded with available equipment; and (4) the stud size and shape must permit chucking or holding for welding. A number of standard stud designs are produced commercially. The stud manufacturers can provide information on both standard and special designs for various applications.
One important consideration in designing or selecting a stud is to recognize that some of its length will be lost due to welding, since the stud and the base metal melt. The molten metal is then expelled from the joint. Part of the material from the length reduction appears as flash in the form of a fillet around the stud base. This flash must not be confused with a conventional fillet weld because it is formed in a different manner. When properly formed and contained, the flash indicates complete fusion over the full cross section of the stud base. It also suggests that the weld is free of contaminants and porosity. The stud weld flash may not be fused along its vertical and horizontal legs. This lack of fusion is not considered detrimental to the stud weld joint quality.
The dimensions of the flash are closely controlled by the design of the ferrule, where one is required. Since the diameter of the flash is generally larger than the diameter of the stud, some consideration is required in the design of mating parts. Flash size and shape will vary with stud material and ferrule clearance. Therefore, test welds should be made and checked.
Ferrules are required for most arc stud welding applications. One of them is placed over the stud at the weld end where it is held in position by a grip or holder on the stud welding gun. The ferrule performs the following important functions during welding:
Concentrating the heat of the arc in the weld area
Restricting the flow of air into the area, which helps to control oxidation of the molten weld metal
Confining the molten metal to the weld area
Preventing the charring of adjacent non-metallic materials
The ferrule also shields the operator from the arc. However, safety glasses with No. 3 filter lenses are recommended for eye protection.
Ferrules are made of a ceramic material and are easily removed by breaking them. Since ferrules are designed to be used only once - their size is minimized for economy, and their dimensions are optimized for the application. A standard ferrule is generally cylindrical in shape and flat across the bottom for welding to flat surfaces. The base of the ferrule is serrated to vent gases expelled from the weld area. Its internal shape is designed to form the expelled molten metal into a cylindrical flash around the base of the stud. Special ferrule designs are used for special applications such as welding at angles to the work and welding to contoured surfaces. Ferrules for such applications are designed so that their bottom faces match the required surface contours.
SPECIAL PROCESS TECHNIQUES
There are several special process techniques that employ the basic arc stud welding process, but each is limited to very specific types of applications.
One special process technique, referred to as gas-arc, uses an inert gas for shielding the arc and molten metal from the atmosphere. A ferrule is not used. This technique is suitable for both steel and aluminum stud welding applications, but its primary use is with aluminum. It is usually limited to production type applications because a fixed setup must be maintained, and also the welding variables fall into a very narrow range. Without a ferrule, there is greater susceptibility to arc blow and poorer control of the fillet around the base.
Another special process technique, which again does not use a ferrule, is called short cycle welding. It uses a relatively high weld current for a very short time to minimize oxidation and nitrification of the molten metal. Short cycle welding is generally limited to small studs, 0.25 in. (6.4 mm) diameter and under, where the amount of metal melted is minimal. One application is the welding of studs to thin base materials where shallow penetration is required and backside marking is not a consideration.
AWS Welding Handbook, Eigth Edition Volume 2 Welding Processes