Stud welding refers to a method where an arc is generated between the end face of a metal or similar material and the surface of another metal workpiece. When the mating surfaces melt, pressure is quickly applied to complete the welding. The stud welding method was invented in 1918, and due to its advantages such as speed, reliability, simplified processes, and cost reduction, it has attracted widespread attention globally. After continuous improvements and refinements, especially following World War II, it developed rapidly.
So, what type of welding machine is required for stud welding? Here are three steps to help you understand the relevant welding machines.
Step 1: Understand the Classification of Stud Welding Machines
First, stud welding requires a corresponding stud welding machine for the process.
Stud welding machines are divided into two major categories:
1. Arc Stud Welding Machine: Uses an arc welding rectifier as the power source for welding.
2. Capacitor Discharge Stud Welding Machine: Uses the energy stored in a capacitor and discharges it instantaneously for welding.
The characteristics and applications of these two welding methods can be referenced in Table 1.
| Welding Method | Welding Time (tw) ms | Stud Diameter (d) mm | Welding Current (I) A | Protection Method | Minimum Plate Thickness | |
| Arc Stud Welding | Ceramic Ferrules Protection | ＞100 | 3～25 | 300～3000 | Ceramic Ferrules | 1/4d (but not less than 1mm) |
| Gas Protection | ＞100 | 3～16 | 300～3000 | Gas | 1/4d (but not less than 1mm) | |
| Short Cycle Welding | ≤100 | 3～12 | ≤1500 | No protection or gas protection | 1/8d (but not less than 0.6mm) | |
| Capacitor Discharge Stud Welding | ＜10 | 3～10 | ≤3000 (peak) | No protection | 1/10d (but not less than 0.5mm) | |
*Note: Minimum plate thickness refers to the thickness to avoid burn-through.
Then, let's move on to Step 2.
Step 2: Understand the Features
1.1 Arc Stud Welding Machines
The welding power supply is generally a thyristor-controlled or inverter-type arc welding rectifier.
The DC welding power supply used for stud welding should have the following characteristics:
a. The welding power supply should have a falling static external characteristic. Only with this feature can arc stability be maintained, ensuring high-quality welds.
b. The welding power supply should have an arc-starting current (40-50A) and a high open-circuit voltage (70-100V) to ensure a 100% arc initiation success rate. For welding large-diameter studs, the open-circuit voltage may even exceed 100V. This is necessary to meet the requirements for larger lifting heights.
c. The welding power supply should have a high load voltage. According to the definition of arc welding power supply’s falling characteristics, when the welding current is ≥600A, the load voltage should remain constant at 44V. In construction sites, the welding cables are often long, sometimes up to 50 meters, which can cause significant voltage drop. If the load voltage is not increased to compensate for this, the welding capability will decrease. If the welding cables are not sized according to ISO14555 specifications, the issue may be more severe, even making welding impossible. This is one of the main reasons why welding machines of the same current rating from different manufacturers can have significant differences in the maximum diameter of studs they can weld.
d. The welding current should have a steep rise at the leading edge. A key feature of stud welding is the instant high current, so it is required that the welding current reaches its peak within 32ms after the machine is turned on. For short-cycle stud welding, the rise time of the welding current should be even shorter. Otherwise, the welding current may not reach its peak by the time the welding cycle ends, making it difficult to ensure the welding quality.
The only way to increase the welding current rise speed is to reduce the inductance of the reactor. In regular arc welding rectifiers, the reactor is used not only for filtering but also to limit the rise speed and peak value of short-circuit currents, reducing arc striking impact currents, minimizing spatter and arc pits, and preventing burn-through of the workpiece. In stud welding, however, the logic sequence is based on preset parameters such as arc striking, stud lifting, and activation of the main power supply. That means the main power supply is only connected when there is a certain gap between the stud and workpiece, avoiding spattering during arc striking. In fact, the largest spatter in stud welding occurs when the stud is inserted into the molten pool, resulting in sudden spattering.
e. The power supply should have a small internal resistance. The electrical insulation of the welding power supply's main circuit is rated for H-class or B-class heat resistance. H-class insulation, compared to B-class, has the advantage of being smaller and lighter in volume. However, a deeper analysis shows it is not without drawbacks. According to GB11021, the maximum temperature for H-class and B-class insulation is 180°C and 130°C, respectively, with H-class insulation allowing about 40% higher temperatures. This means the current density of the coils can be significantly increased in the main circuit design to reduce the wire cross-sectional area, which in turn increases the wire resistance or the overall circuit impedance. This is a fatal flaw for stud welding machines that operate at high currents. For example, if the main circuit insulation of the welding power supply is upgraded from B-class to H-class, and the reduction in the cross-sectional area of all wires in the secondary circuit increases the total resistance by just 0.006Ω, the power loss at a 2500A welding current will increase by 37.5kW. The total power loss, including that in the main transformer’s primary circuit, will be substantial. Increased power loss in the main circuit of the welding power supply will inevitably reduce the welding power output, lowering the welding capability. This is the trade-off for reduced size and weight. In other words, to weld studs of the same diameter, H-class insulation machines need higher power to achieve the same results, resulting in reduced efficiency. The ST-3150 arc stud welding machine developed by IKING Group, with B-class insulation, can weld studs with a diameter of 30mm, which is difficult to achieve with H-class insulated arc stud welding machines of the same rating.
f. The power supply cabinet or power box should have sufficient capacity. The load duty cycle of arc stud welding machines is very low, typically less than 15%, so the average power consumption is relatively low. However, the instantaneous power can be very high. When welding large diameter studs, the instantaneous power can exceed 300kW. Setting up dedicated power lines, increasing the capacity of the power supply cabinet, or staggering peak electricity usage are effective solutions.
The above outlines the main characteristics of arc stud welding machines. For more information, you can check the product details of arc stud welding machines or contact us for further assistance.
Next, we will look at the main features of Capacitor Discharge Stud Welding Machines. Please continue reading.
1.2 Capacitor Discharge Stud Welder
There are two types of capacitor discharge stud welding: arc striking and arc drawing. Arc drawing capacitor discharge stud welding is similar to short-cycle arc stud welding, with a welding time of about 3 to 10 ms (as shown in Table 1). Arc striking capacitor discharge stud welding has an arc starting point on the end face of the stud to be welded, which can be further divided into contact and gap types. Contact-type stud welding has a welding time of ≤3 ms, while gap-type stud welding has a welding time of approximately 1 ms. The use of gap-type capacitor discharge stud welding allows for welding aluminium and its alloys without the need for gas protection. The welding time for capacitor discharge stud welding is fixed and cannot be adjusted.
The welding energy for capacitor discharge stud welding depends on the capacitor's capacitance and charging voltage, which can be calculated using the following formula:
W = C*U²
Where:
W is the rated stored energy of the welder (J)
C is the total capacitance of the capacitor bank (F)
U is the charging voltage (V)
The peak welding current for capacitor discharge stud welding is typically between 1000 and 10,000 A, depending on the capacitance of the capacitor, charging voltage, and the inductance and resistance of the welding circuit. For safety reasons, the charging voltage is generally limited to 200 V.
Capacitor discharge stud welders should be equipped with current-limiting protection, constant current charging devices, and automatic discharge devices to ensure the safety of both personnel and equipment.
For more information about capacitor discharge stud welders, you can check the capacitor discharge welding product details or contact us for further information.
It’s worth noting that besides the welder's characteristics, understanding welding polarity is also important.
1.3 Welding Polarity
In general, "DC positive polarity" is used to weld ferrous metals, that is, the stud (or welding gun) is connected to the negative pole of the welding power supply, and the workpiece is connected to the positive pole. In this way, the anode temperature is higher than the cathode, which can increase the penetration depth; "DC reverse polarity" is used to weld copper, aluminium and their alloys, that is, the stud is connected to the positive pole, and the workpiece is connected to the negative pole. Positive ion bombardment is used to remove the oxide layer on the surface of the workpiece and improve the welding quality. This takes advantage of the bombardment of positive ions to remove the oxide layer from the surface of the workpiece, improving the welding quality. This polarity should be used when welding with arc stud welders or capacitor discharge stud welders.
Finally, the welding process parameters for arc stud welding are provided to assist in effective welding during practical operations. The welding process parameters for arc stud welding include welding current, welding voltage, welding time, lift height, protrusion length, and insertion speed.
a) The welding current is mainly adjusted based on the diameter of the stud and ranges from approximately 300 to 3000 A. For non-alloy steel, the welding current can be estimated using the following formula when the stud diameter (d) is known:
I (A) = 80 × d (mm) for d ≤ 16 mm
I (A) = 90 × d (mm) for d > 16 mm
For alloy steels, the welding current is about 10% lower than the calculated value above. The welding current for short-cycle arc stud welding (600–1500 A) is fixed and depends on the power source, so the welding energy depends solely on the welding time.
b) The arc voltage is determined by the static characteristics of the power source and is mainly influenced by the lift height and welding current. It typically ranges from 20 to 40 V. The surface oil or grease on the workpiece will increase the arc pressure, while inert gases will reduce the arc voltage.
c) For flat welding (where the workpiece is parallel to the ground), the welding time can be estimated using the following formula:
tw (s) = 0.02 × d (mm) for d ≤ 12 mm
tw (s) = 0.04 × d (mm) for d > 12 mm
For vertical welding (where the workpiece is perpendicular to the ground), the welding time should be reduced. The short-cycle welding time is less than 100 ms and depends not only on the stud diameter but also on the current strength.
d) The lift height of the stud is proportional to the stud diameter, ranging from about 1.5 to 7 mm. The lift height is primarily designed to prevent short-circuiting when molten droplets transition, which could affect the arc stability and weld quality. Maintaining a stable arc is crucial to providing sufficient energy for welding, as the temperature of the arc column is much higher than that of the anode or cathode. In penetration welding, the lift height should be increased to allow the high-temperature arc to burn through the galvanized sheet quickly to achieve a satisfactory joint. However, increasing the lift height also has drawbacks: it lengthens the arc, making it more susceptible to magnetic field effects and arc deflection, and it can increase the occurrence of porosity in the weld.
e) The protrusion length of the stud is proportional to its diameter, generally ranging from 1 to 8 mm. When ceramic rings are used to protect the molten pool, the protrusion length is also related to the required weld toe size around the weld. If larger weld toes are required, the protrusion length should be increased; otherwise, it can be reduced. The protrusion length is slightly greater than the stud’s melting length. If it is designed too long, the distance between the stud end face and the workpiece after lifting may be too short to form a stable arc, resulting in excessive metal spatter and slag inclusion defects. Conversely, if the protrusion length is too short, the molten metal will be insufficient, leading to poor weld formation.
f) The insertion speed of the stud is controlled by extrusion, which pushes out harmful substances before the weld formation to create a good welding joint. However, the insertion speed should not be too fast, as this could cause excessive spatter. For studs with a diameter (d) ≤ 14 mm, the insertion speed is about 200 mm/s, and for studs with a diameter (d) > 14 mm, the speed is 100 mm/s. Welding guns typically have adjustable damping devices to meet these requirements. When welding larger-diameter studs, the molten metal volume increases, so the insertion speed should be reduced to minimize spatter.
Welding current, welding time, lift height, and protrusion length are the four main process parameters for arc stud welding. These parameters should be set based on the stud diameter and workpiece material. When welding studs of the same diameter with equipment from different manufacturers, the welding process parameters may differ, so multiple test welds should be conducted. After evaluating the appearance, formation, stud height, and mechanical properties of the weld (such as tensile, hammering, bending, and torsion tests, the optimal set of process parameters can be selected for welding.