Capacitor Discharge Stud Welding Process: A Comprehensive Guide

Capacitor Discharge Stud Welding

Capacitor Discharge Stud Welding is a method that uses a capacitor bank as the power source. The energy stored in the capacitors is quickly discharged to generate an arc as the heat source. Compared to Arc Stud Welding, Capacitor Discharge Stud Welding has the following characteristics:

1) Protective measures such as ceramic rings, argon, or flux are not necessary. This is because the welding time (i.e., the arc burning time) is extremely short, only about 2-3ms. The air doesn’t have time to infiltrate the welding area before the joint is formed.

2) There’s no need to consider the weld allowance. The welding pool is small, approximately 2mm, and the joint is plastically connected. The shortening after stud welding can be ignored.

3) There are no melted weld seams in the joint. Forced molding stud legs by ceramic ring molding cavities don’t exist. Therefore, there’s no need for visual quality inspection of the Capacitor Discharge Stud Welding joint. There will be no defects such as porosity or cracks.

4) The ratio of the diameter ‘d’ to the workpiece wall thickness ‘δ’ can reach 8-10. For instance, a stud with a diameter of 3mm can be welded on a 0.3mm thin plate without burning through, while in Arc Stud Welding, the ratio of ‘d’ to ‘δ’ is only 3-4.

Workpiece Material and Stud:

(1) Welding material

The depth of melting in Capacitor Discharge Stud Welding is small, and there’s minimal mixing of the stud metal with the parent material. Hence, welding can be carried out on extremely thin plates (such as 0.25mm) without burning through, and a variety of dissimilar metal welding can be performed.

Table 7-7 lists the typical combinations of commonly used workpiece materials and stud metals.

Workpiece MaterialsStud Metals
Low Carbon SteelLow Carbon Steel, Stainless Steel (Austenitic), Copper Alloy, Brass
Stainless Steel (Austenitic and Ferritic)Low Carbon Steel, Stainless Steel (Austenitic)
Aluminum AlloyAluminum Alloy
Copper, Lead-Free BrassLow Carbon Steel, Stainless Steel, Lead-Free Copper Alloy
Zinc AlloyAluminum Alloy
Titanium and Titanium AlloyTitanium and Titanium Alloy

(2) The stud

The stud can be almost any shape, such as cylindrical (with or without threads), square, rectangular, conical, slotted, and other punched parts. However, it must be suitable for clamping, and the welding end must be round. The diameter of the stud ranges from 1.6 to 13mm, generally within the range of 3 to 10mm.

For studs used in pre-contact and pre-gap Capacitor Discharge Stud Welding, the welding end should be designed with a pointed top or a small platform. The standard platform is cylindrical, with a conical shape chosen for special occasions. The slightly conical shape of the welding end facilitates the expulsion of gases generated during the welding process.

Typically, the diameter of the bottom of the welding end is larger than the stud body, and it’s generally designed with a flange, or a shoulder, so that the seam area is larger than the cross-sectional area of the stud, ensuring the joint strength equals or exceeds the strength of the stud.

To increase productivity, the shape and dimensions of the stud should be as standardized as possible. The recommended shapes and sizes of the two types of studs are shown in Table 7-8.

Table 7-8: Recommended Stud Shapes and Sizes

Threaded Unthreaded
DL+0.2D1±0.2D2±0.08L1±0.05HMaximum NDL+0.6D1±0.1D2±0.08L1±0.05H
M36~304.50.650.550.7~1.41.5φ36~305.0 0.650.550.7~1.4
M56~456.50.750.80 2φ56~457.0 0.750.80 
M810~559.0 φ7.110~559.0 

Welding Parameters

The quality of capacitor discharge stud welding depends on the welding energy, which is determined by the discharge current and time during welding. The discharge current varies with the charging voltage, and the discharge time is given by the equipment itself, so the welding energy is determined by the charging voltage.

The process requirements are usually determined according to the stud material, diameter, and selected welding method, which then determines the charging voltage. The charging voltage is typically not allowed to exceed 200V and is generally between 40~200V. The larger the stud diameter, the greater the discharge current needed, and the higher the chosen charging voltage.

The principles for choosing the main parameters during capacitor discharge stud welding are as follows:

(1) Power Polarity

Typically, the stud is connected to the negative terminal of the power supply, and the workpiece to the positive. However, for stud welding of aluminum alloy and brass workpieces, it is beneficial to connect the workpiece to the negative terminal.

(2) Welding Current

Peak current is 1000~10000A, and the magnitude of the current depends on the charging voltage, capacitance, and the resistance and inductance of the welding output circuit.

(3) Welding Time

This is chosen based on the energy stored in the capacitor and the circuit inductance. Generally, the time for the capacitor tip discharge to ignite the stud for welding is 1~3ms. When welding on galvanized steel plates, the welding time can be appropriately extended.

(4) Load Power

The welding energy of capacitor tip ignition stud welding is output by the capacitor group, so its load power should equal the energy stored by the capacitor. Load power is proportional to the stud diameter; as the stud diameter increases, the charging voltage should be increased, or the capacity of the capacitor group should be increased.

(5) Immersion Speed

The speed at which the stud is immersed into the workpiece is determined by the welding torch spring and the mass of the stud, with an immersion speed of 0.5~1.5m/s. It, along with the stud tip length, determines the welding time, so the immersion speed must be kept stable within the limit to achieve stable welding quality.

Key Points of Welding Operation

Quality control of capacitor discharge stud welding is more difficult than arc stud welding because the arc cannot be seen or heard, and it is difficult to judge the quality of welding from the appearance of the weld after welding.

Currently, the best approach is to conduct process evaluation before production, i.e., destructive testing of studs welded to materials similar to the actual ones in use, such as bending, twisting, or tensile tests. Once a satisfactory welding process is evaluated, it is used in production.

Then, during production, the weld quality is checked at regular intervals. Additionally, the following points should be noted during welding:

1) The capacity of the power supply should meet the requirements of the studs to be welded.

2) The surface of the workpiece should be kept clean, free of defects, excessive oil stains, lubricating grease (liquid), etc., and the surface should not be overly rough.

3) When placing the stud and operating the welding torch, the axis of the stud must be perpendicular to the surface of the workpiece. This is the key to ensuring complete fusion of the joint. Keep the welding torch stable during welding; it should not sway.

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