In industrial applications, joining two metal bodies is the most critical process. As load concentration is maximum at the welded joints, a proper welding process needs to be followed. Even though different welding techniques are available, all of them work on the same principle of fusing material at higher temperatures.
Resistance welding is one such welding type that uses a combination of thermal and electrical processes. As the name suggests, the electrical resistance property of a material helps in creating favorable temperatures for welding. There is a lot to learn about this unique style of welding, which we will try to explain in this article.
Resistance Welding
Working Principle
Every metal has an electrical resistance property that affects its current carrying capacity. This property can be used to create high temperatures at the contact points. Pressure is applied at the welding spots, which helps in completing the electrical circuit. The heat released at this spot is high enough to melt the metals and bond them.
Since its inception before World War 1, resistance welding has proved to be a good alternative to other welding processes. By using the electrodes, this wedding technique eliminates the need for consumable materials for welding. Like other welding techniques, resistance welding has its advantages and disadvantages too:
Advantages and Disadvantages Of Resistance Welding
The resistance welding process has its upper hand and limitations. These factors help in comparing the resistance welding process with other options. Studying these factors can also help in dealing with shortcomings by using advanced technologies
Advantages
- Resistance welding takes a very short time to weld a joint resulting in a high welding rate
- Thin sheets of metal can be easily welded with this technique
- The welding process does not require extra filler material
- The generation of fumes and gasses is very low
- It is compatible with automation and can be used for multi-axis welding
- Heat affected area is small and does not compromise the structural strength of the material
- Complex structures can be welded with a constant quality of welded joint
Disadvantages
- This technique is not suitable for thick metallic structures
- In most cases, the resistance welding is bulky and not easily movable
- The electric power consumption in this welding process is high
- Multiple welding joints are needed since a single welded joint has a low tensile strength
Resistance Welding Of Resistance Welding
The use of resistance welding dates back to the pre-world war era. Due to the increase in the mass production process, resistance welding adapted to industrial applications. Some of these applications are discussed below:
- Due to its compatibility with thin metal sheets, this welding technique is used for joining the automobile chassis structure.
- Another application of resistance welding is seen in the aircraft manufacturing industry. This welding technique is a good alternative to the riveting process of the aircraft body.
- The critical parts like fuel tanks and underbelly covers of automobiles are made with a resistance welding process.
- The joining of metallic pipes of an exhaust system is done by this technique. This also helps in welding the protective bodies of pressure vessels like boilers.
- A seamless joint can be achieved by this technique to join longer sections of metal sheets. This is achieved at a higher production rate making it suitable for mass production.
Working Process Of Resistance Welding
1. Equipment
Resistance welding solely depends on various equipment. The equipment controls the welding process to ensure a constant rate of metal joining. Some of these pieces of equipment are discussed below.
a. Electrical Circuit: The electrical circuit contains the power supply and a series of voltage-regulating devices. Resistance welding uses electrical resistance and hence proper output voltage is needed. A step-down transformer is used to convert voltage and current into suitable levels.
b. Electrodes: Electrodes are the most important factor in resistance welding. These conductors are responsible for the transfer of electricity from the circuit to the workpiece. These electrodes are explained separately.
c. Operating mechanism: A mechanical arrangement is used to initiate the welding process. This mechanism helps in clamping the workpiece during welding. Using a pneumatic circuit makes the whole operation faster. The clamps are provided with an inlet for compressed air and are controlled by switches.
2. Working
The preparation for welding is done before starting the actual process. The workpiece is marked for the welding spot. Since two workpieces are placed simultaneously, proper clamping is done to lock them in the desired position. The workpieces are then moved between the two electrodes. The welding marking must match the fixed electrode’s position. After ensuring the placement of both workpieces, the power supply is checked for proper setting.
When everything is ready, the switch/lever can be operated. The currents start flowing through the workpiece. When the movable electrode comes in contact with the workpiece, the electrical current passes from one electrode to the other. This creates high temperatures at the contact points resulting in the melting of metal.
After the process is completed, the movable electrode is lifted. The hot welded joint is allowed to cool naturally. In the end, an inspection is done to trace any visible cracks on the welded spot.
Factors Affecting The Welding Process
For a successful welding process, certain parameters play an important role. The parameters or factors govern the welding process and help in keeping it effective over a long period. These factors are explained below.
1. Weldability
Weldability is the property of a material to allow welding with other materials. The welding process may be with a similar material or a dissimilar material. The weldability of metal is further affected by sub-factors. The resistivity of metal helps in resisting the current flow. This helps in generating heat suitable for welding.
Thermal conductivity then plays an important role in conducting heat through the metal. Good thermal conductivity indicates a faster rate of heating. Lastly, the melting temperature has to be lower enough to allow the melting of the metal. All these sub-factor work in a balanced way to improve the weldability of a material. A higher weldability number means the material is easily weldable.
2. Heat And Current
The heat and current always work in pairs to create favorable conditions for welding. The heat has a direct proportionality relationship with the square of the current. Whenever the current is increased, the resultant heat is four times more. This relationship is used to heat the material at the expense of a low current supply. This leads to a faster welding process.
3. Metals
Every metal has a different limit of electrical resistance. Therefore choosing a suitable combination of metal is important to get a good welded joint. Some common metals are mentioned below along with their compatibility with resistance welding.
| Metal | Compatibility |
| Stainless Steel | Yes |
| Aluminum | Yes |
| Steel alloy | Yes |
| Nickel | Yes |
| Low carbon steel | Yes |
| Medium and high-carbon steel | Yes |
| Magnesium | Yes |
| Aluminum and Copper | No |
| Steel and Aluminum | No |
| Steel and Titanium | No |
| Carbon steel and Aluminum | No |
Resistance Welding Electrodes
As discussed earlier, the electrodes are the most important part of the resistance welding setup. They are made up of a mixture of high-conducting materials. As they have to undergo high temperatures and endure heavy loads, electrodes must maintain constant performance. Their shape is made in a tapered form with cross-section areas decreasing towards the contact point.
This shape helps in dissipating heat away from the electrodes. The contact area has an optimal size to allow good welds. The most popular used materials are copper and refractory materials. There are numerous combinations of these materials classified under groups. Group A has copper alloys, Group B uses refractory metal composites, and group C contains a special combination of copper electrodes.
Welding Bonds
When two metals are fused in welding, the welded spot creates bonds. Bonding is a molecular-level phenomenon that tells us about the type of welding process that led to the melting and joining of the material. Some welding bonds are explained below to clear any confusion related to the process.
1. Thermo-compression Bond: This bonding happens between two dissimilar metals due to the high welding energy. The grain structures of these dissimilar metals combine in a very short time. Hence the overall bonding process does not need the melting of the metals. This bond has good shear and tensile strength.
2. Fusion Bond: The fusion bond can happen between similar as well as dissimilar metals. The common factor is the grain structure of these metals. This process requires proper heating of the metals up to the melting point. Melting leads to the formation of large grains and creates alloys of both metals. The bond possesses good peel, tensile, and shearing strength.
3. Reflow Bond: This bond usually happens with the brazing materials at lower temperatures. These brazing materials are used with other materials to form a reflow bond. The low melting pout helps in the mixing of the brazing material with the other metals. This bond has good tensile strength.
Certification Program For Resistance Welding
Resistance welding has many sub-types. It is a fabrication process that involves high temperatures and powerful currents. As a result, this welding technique is limited to skilled professionals. Hence, various Welding Societies offer certification programs to develop a skilled labor force. These programs help in understanding the resistance welding process along with its subtypes like spot welding, sea, welding, etc.
Conclusion
Resistance welding is a unique type of welding. In this technique, welding temperature is achieved by using the electrical resistance of materials. The welding process has seen various upgrades over the years yet the working principle remains the same. In this article, we discussed resistance welding and its working in detail. We also got to know about the governing factors of this welding technique. With this information, understanding other welding techniques have therefore become easier.
