Threaded Stud Welding: A Comprehensive Guide

Threaded Stud Welding A Comprehensive Guide

Classification and Principle of Stud Welding

Stud Welding (SW) is a method of welding metal studs or other fasteners like bolts and screws onto a workpiece. The technology of stud welding originated during World War II, invented by an engineer aboard an American warship. Since then, due to its high quality, efficiency, low cost, and simplicity, it has been quickly adopted in aerospace, shipbuilding, vehicle manufacturing, construction, and other industries.

Based on the welding power source used and differences in the joint formation process, stud welding can typically be classified into Arc Stud Welding (also known as Standard Stud Welding), Capacitor Discharge Stud Welding (also known as Capacitor Storage Stud Welding) and Short Cycle Stud Welding (also known as Short Time Stud Welding).

1.Arc Stud Welding

During welding, a stable arc is generated between the end of the stud and the surface of the workpiece. The arc, acting as a heat source, forms a molten pool on the workpiece, while the stud end is heated to form a melting layer.

Under pressure (from a spring or other mechanical pressure), the stud end is immersed in the molten pool, forcing all or part of the liquid metal out of the joint and forming a recrystallized plastic connection or a combination of recrystallized and re-solidified connections.

Arc Stud Welding usually uses a decreasing characteristic direct current arc welding power source, maintaining a constant welding current during the welding process.

Figure 7-1 Schematic Diagram of the Arc Stud Welding Process

2.Capacitor Discharge Stud Welding

Capacitor Discharge Stud Welding utilizes the arc generated by the rapid discharge of a storage capacitor as a heat source to connect the stud to the workpiece. The power supply is a capacitor group, with the capacitor charged before welding, hence it is also known as Capacitor Storage Stud Welding.

The discharge process between the stud end and the workpiece surface is an unstable arc process, where the arc voltage and arc current are instantaneously changing, making the welding process uncontrollable. Depending on the method of arc ignition, Capacitor Discharge Stud Welding can be classified into pre-contact, pre-gap, and arc-drawing methods.

(1)Pre-contact Capacitor Discharge Stud Welding

The welding process of pre-contact Capacitor Discharge Stud Welding is as shown in Figure 7-2. A small convex platform must be designed on the end of the stud to be welded. During welding, align the stud with the workpiece, making the small convex platform contact the workpiece (Figure 7-2a), and then apply pressure to push the stud towards the workpiece.

Subsequently, the capacitor discharges, and a large current passes through the small convex platform. Due to the high current density, the small convex platform is instantly burnt and generates an arc (Figure 7-2b). During the arc burning process, the to-be-welded surface is heated and melted.

At this point, since the pressure is always present, the stud moves towards the workpiece (Figure 7-2c). When the stud end contacts the workpiece, the arc extinguishes, forming a weld (Figure 7-2d). The whole welding process is characterized by contact before electrification and pressurization before electrification.

Figure 7-2 Schematic Diagram of the Welding Process of Pre-contact Capacitor Discharge Stud Welding

(2)Pre-gap Capacitor Discharge Stud Welding

The welding process of pre-gap Capacitor Discharge Stud Welding is shown in Figure 7-3. A small convex platform also needs to be designed on the end of the stud to be welded. During welding, the stud is aligned with the workpiece, but they don’t touch, leaving a gap between the two (Figure 7-3a).

Then the power is turned on, and the charging voltage of the capacitor (no-load voltage) is added in the gap. At the same time, the stud is disengaged and moves towards the workpiece under the action of a spring, gravity, or cylinder thrust. The moment the stud contacts the workpiece, the capacitor discharges immediately (Figure 7-3b), and the large current melts the small convex platform, igniting the arc.

The arc melts the two surfaces to be welded (Figure 7-3c), and finally, the stud is inserted into the workpiece, the arc is extinguished, and the welding is completed (Figure 7-3d). The characteristic of this method is to leave a gap, electrify before contact, discharge and pressurize, and complete welding.

Figure 7-3 Schematic Diagram of the Welding Process of Pre-gap Capacitor Discharge Stud Welding

(3)Arc-drawing Capacitor Discharge Stud Welding

During Arc-drawing Capacitor Discharge Stud Welding, the end of the stud to be welded doesn’t need to have a small convex platform, but needs to be machined into a cone or slightly spherical shape. The arc ignition method is the same as in Arc Stud Welding and requires operation by an electronic controller. Its welding gun is similar to the Arc Stud Welding gun. The welding process is as shown in Figure 7-4.

During welding, first position the stud on the workpiece and make them contact (Figure 7-4a). Press the switch of the welding gun, connect the welding circuit and the electromagnetic coil inside the welding gun to pull the stud away from the workpiece, igniting a small current arc between them (Figure 7-4b).

When the lifting coil is disconnected, the capacitor discharges through the arc. The high current melts the stud and the surface of the workpiece to be welded. Under the action of spring force or cylinder force, the stud returns and moves towards the workpiece (Figure 7-4c).

When the stud is inserted into the workpiece, the arc is extinguished, completing the welding (Figure 7-4d). The characteristic of this method is that after contact, the arc is drawn, and the welding is implemented after the capacitor discharges.

Figure 7-4 Schematic Diagram of the Welding Process of Arc-drawing Capacitor Discharge Stud Welding

3.Short-Cycle Stud Welding

Short-Cycle Stud Welding uses an inverter or dual rectifier as the power source, and the welding arc combustion process exhibits phase stability. The welding process of Short-Cycle Stud Welding consists of short-circuiting, lifting arc ignition, welding, nailing, and energized top forging.

Its welding time is only one-tenth to one-hundredth of that of ceramic ring or arc-drawing stud welding, hence it is called Short-Cycle Stud Welding. The power source for this type of stud welding is generally two paralleled sources that supply power to the arc sequentially. It could be two arc welding rectifiers or a rectifier plus a capacitor group.

Only when an inverter is used as the power source can dual power sources be avoided. Short-Cycle Stud Welding uses current that has been modulated in waveform. Its features are that no protection is needed, the stud does not need to undergo special processing, and it is easier to achieve automation.

The welding process of Short-Cycle Stud Welding is as shown in Figure 7-5, and the specific program is as follows:

Figure 7-5 Welding Process of Short-Cycle Stud Welding
  • I – Welding Current (A)
  • UW – Arc Voltage (V)
  • TW – Welding Time (ms)
  • Td – Energized Top Forging Stage (ms)
  • Ip – Leading Current (A)
  • S – Stud Displacement (mm)
  • Tp – Leading Arc Time (ms)
  • TL – Nailing Time (ms)
  • P – Pressure of the spring in the welding gun on the stud (N)

1) The stud falls and shorts with the workpiece, activating the welding gun switch and electrifying between the stud and the workpiece.

2) The stud lifts, igniting a small arc, at which point the arc current is Ip.

3) After a delay of several tens of milliseconds, the high current automatically connects, a high current (welding arc) occurs, a weld pool forms on the workpiece, and a melting layer forms at the end of the stud.

4) The stud and stud end immerse into the weld pool, the arc extinguishes, and simultaneously, the electromagnet in the welding gun releases spring pressure on the stud.

5) The joint forms, the welding ends, and the entire welding process does not exceed 100ms.

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