Resistance-Spot Welding - Basic Application Setup

Resistance welding comes in many forms. Spot, seam, and projection are commonly used resistance weld processes. This article is going to focus on spot welding.

If you are concerned with a consistent quality of spot welding production then you need to understand that the three key principles are the time, the electrical current, and the pressure (electrode force - mechanical pressure). These three factors are closely related and need to be balanced in order for the operator to produce a quality bond between two pieces of metal with consistency. Electrodes conduct the current to the two pieces of metal to be fused as they are forged together.

The weld cycle must first build an adequate amount of heat through the electrical current and the time the current is applied to raise a small volume of molten metal from the work pieces to be bond. This molten metal is then allowed to cool while under the pressure of the electrodes until it has set at the maximum strength that the metal will allow to hold the parts together. This is referred to as the hold time on the welder.

If one of these parameters is changed without the proper adjustments to another, it may result in a poor bond.

Example: If the current is increased without an adequate amount of pressure it will result in an excessive weld splash. Conversely, a change in pressure without the corresponding changes in time and current may result in poor welds also.

The kva ratings of resistance welders are greatly impacted by various aspects of the machine set-up. When a part is very large and cannot be fit into a short set of spot welder arms - the arms may have to be extended or changed in order to accommodate the depth of the part. A spot weld weld machine with long arms for a deep entry of a product to be bonded will have a much greater electrical impedance than the same machine kva with a short set of arms. The resistance welder efficiency is greatly reduced by the electrical impedance.

Electrical impedance is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In terms of resistance welding it is the increased resistance due to the greater distance and obstacles that the current needs to flow through to get to its destination (the welding electrodes).

How to Select and Prepare Tungsten Electrodes For Your Application

To those of us who may not be aware, tungsten is an uncommon metallic element that is used in the manufacturing of TIG (or GTAW) electrodes. The TIG electrodes demand the type of hardness and high temperature resistance that tungsten alone can provide. This is because tungsten electrodes are meant to carry the welding current to the arc. It is to be noted that tungsten has an inconceivably high melting point when compared with other metals.

These electrodes come in different sizes and lengths and are available either as pure tungsten or a hybrid of tungsten mixed with other rare earth elements and oxides. Choosing the right electrode depends on the type and thickness of the base material and whether you are going to weld with alternating or direct current.

Please know that pure tungsten electrodes are expected to contain nearly 100% tungsten and have the maximum consumption rate of all electrodes. Tungsten electrodes are strangely less costly than its alloyed counterparts. These electrodes form a balled tip when heated and provide great arc stability for AC welding with balanced wave. Pure tungsten also provides good arc stability for AC sine wave welding - for aluminum and magnesium welding.

Thoriated tungsten electrodes (Color Code: Red) contain a minimum of 97.30% tungsten and balance thorium. These electrodes are the most widely used electrodes and are known for longevity and easy use. Thorium is highly useful in increasing the electron emission qualities of the electrode. Thus, it improves arc starts and enables a higher current carrying capacity. It also results in a lower level of weld contamination than other electrodes.

Ceriated tungsten electrodes (Color Code: Orange) contain a minimum of 97.30% tungsten and balance cerium. These electrodes are known to perform remarkably well in DC welding at low current settings. With its excellent arc starts at low amperages, ceriated tungsten are widely used as orbital tube and pipe manufacturing, thin sheet metal work or jobs where small and delicate parts are welded.

Lanthanated tungsten electrodes (Color Code: Gold) contain a minimum of 97.80% tungsten and balance lanthanum. These electrodes are known for their excellent arc starting as well as arc stability, low-burn-off rate and quick re-ignition capabilities. The fact is, in select cases, lanthanated tungsten electrodes can replace 2% thoriated without having to make significant welding program changes.

Zirconiated tungsten electrodes (Color Code: Brown) contain a minimum of 99.10% tungsten and balance zirconium. Zirconiated tungsten electrode is capable of stable arc and it resists tungsten spitting. The added feature is its current carrying capability which is equal if not more than thoriated tungsten. Zirconiated electrodes are unsuited for DC welding.

Rare earth tungsten electrodes (Color Code: Gray) contain unspecified additions of rare earth oxides or a mixture of different oxides. However, manufacturers will indicate each additive and its percentage on the outer label. Depending on the additives, you can obtain a stable arc in both AC and DC processes as also more longevity.

Finally, it may be said that the arc quality and welding performance you achieve will largely depend on the type and shape of tungsten you use. Each of these six types of tungsten has its own merits and shortcomings. It is therefore critically important you choose the right tungsten appropriate for each application.