Spot welding is a common method used for sheet metal because it’s a straightforward process. But it is possible to make mistakes if you have gaps in your welding knowledge and experience. In this article, we will examine the materials, applications and common problems that most welders experience.
The first question that many less experienced welders may ask is what is spot welding? This is a resistance welding process that is typically used to weld at least two metal sheets together. Pressure and electric current is applied to the spot-weld location and the necessary heat is generated by the internal resistance of the metal to that electric current.
The pressure and electric current are applied to the metal with copper alloy electrodes. The tips of these electrodes are placed on opposing sides of the metal pieces that will be spot welded. The heat melts the metal, and the electrode pressure squeezes that molten metal into a weld. The spot-welding moniker refers to the small size and circular dot shape of the weld.
To create the spot weld, the sheet metal or wire meshes must be electricity conducting. The typical application is thin metal sheet welding but welding above 1” thickness is possible if specialized and heavy-duty equipment is used.
Spot welding is common in automobile manufacturing processes and a vehicle may have more than 1000 of them. This welding task is usually performed by robots in a few seconds, but sheet metal shops will use spot welds for certain manufacturing and repair processes.
Some other industries that use resistance spot welding include construction, electronics, aerospace, manufacturing, battery construction and more. In an industrial setting spot welding robots are common because they improve productivity and efficiency.
But, a spot welder is a good addition for a welding shop because it’s possible to make complex shapes easily when compared to MIG or TIG welding. If you need to make resistance spot welds to complete a job, it will be difficult without a manual spot-welding machine.
Now that you have a basic ground in the spot-welding process, let’s take a closer look at how it works. The process uses low voltage and high-current electrical pulses that are delivered to the electrodes. The metal becomes molten quickly and the pressure is applied to fuse the metals together. Due to the application of pressure, this resembles a cold-welding process but, in that case, no electrical current is used.
The initial stage is to overlap the work-pieces or metal sheets and then set them in place. The required pressure may be applied manually, with springs, pneumatically or via a hydraulic system.
The next stage is the application of a heavy electrical current to the weld area between the welding electrodes. As the current reacts with the internal resistance of the metal the formation of molten metal occurs. The temperature of the metal may reach approximately 2000ºC which is above the fusion point.
The copper alloy electrodes must have a higher melting point than the work-pieces or they can melt too. There are other spot welding electrode materials, including tungsten, molybdenum and more.
The final stage of the spot weld process is the cooling and solidification of the weld or nugget. The electrodes are still in contact during this stage to support the weld joint until the metals are cooled and hardened. If wire meshes are welded, the contact points are positioned in a perpendicular position to create connections that are stronger and more durable.
- Any conductive metal can be spot welded.
- Spot welding is relatively easy to learn.
- Resistance welding is simple to control and repeatable.
- The electricity used is efficient.
- Spot welding delivers high energy to a precise spot quickly.
- This is the optimum method for strong welds on thin metals without burn through.
- This is a fast and efficient welding process.
- There are many welding electrodes types to weld different alloys.
- The electrodes dissipate heat from the spot weld quickly.
- This is a tried and tested welding process with lots of support.
- A spot weld cannot be created if one side of the workpiece is not accessible.
- The nugget and surrounding area can be too hard which creates cracks.
- Frequent repairs may be required depending on the material and thickness.
- The output is 1-20 volts, minor fluctuations can degrade the quality of the spot weld.
- The physical and chemical properties of the workpiece may be altered which can compromise the corrosion resistance qualities of aluminum, stainless steel, and other metals.
There are four variables to adjust and check during the welding cycle to create welds that are satisfactory:
1. The Pressure of the Welding Electrode
The correct amount of electrode force or pressure must be applied to create quality spot welds. If the pressure is insufficient the weld joints will be small and too weak. If the pressure is too high, there may be cracking around the spot weld. The electrode depth should not exceed 25% of the thickness to avoid these problems.
2. The Tip Diameter
The diameter of the welding tip will control the spot weld size. If the tip is too large, this can cause overheating on the work piece leading to voids and gas pockets which degrade the weld joint. If the tip is too small, the spot weld nugget will be smaller and weaker.
3. The Current
The heat that’s generated by the electrical current will depend on the thermal conductivity and the electrical resistance of the metal. It’s important to consider the classification of the metal work-pieces to determine the proper current for the welding machine.
4. The Welding Time Cycle
The results of spot welding can be improved and imperfections may be avoided if the exposure to the electrode pressure and weld current is adjusted as follows:
- Downslope: This is the electrical current span where the cut is made from the peak value.
- Hold Time: This is the resting time where the molten metal solidifies under continuing electrode pressure to conduct heat away from the weld efficiently.
- Squeeze Time: This is the time when the electrode welding tip is used to apply pressure to the overlapping metal materials.
- Upslope: The time it takes for the electrical current to reach the peak value.
- Off Time: The delay of the electrical current from the completion of the spot-welding sequence to the start of the next one.
- Weld Time: The duration of the electrical current between the electrode tips when the heat is generated, and the fusion occurs.
What Metals Can You Spot Weld?
There are four metals and alloys that can be spot welded with varying degrees of success:
Steel has high electrical resistance and low thermal conductivity which makes it easy to spot weld successfully. Low carbon steel can be spot welded easily, but high carbon steels can be prone to weld cracks due to the formation of brittle micro-structures. Both austenitic and ferric grade stainless steel is commonly spot welded.
2. Galvanized Steel
Galvanized steel is zinc coated as this requires a slightly higher electrical welding current than uncoated steel work-pieces. Copper welding electrodes tend to degrade the surface of zinc alloys which reduces the weld quality. Spot welding zinc coated steel requires frequent electrode tip exchanges or the tip is dressed to restore the clean copper tip surfaces.
Copper and copper alloys can be fused together with resistance spot welds. But, copper electrodes cannot be used because the heat generation between them and the work-piece is too similar. To weld copper an electrode made with an alloy that has a higher electrical resistance and melting temperature above copper (1080ºC) is required. Two common electrode materials used for copper spot welds are: tungsten and molybdenum.
The electrical resistance and thermal conductivity of aluminum is close to copper. But the melting point is lower and this makes spot welding viable. Aluminum has low resistance, and a high electrical current is required for spot welding. The surface of copper welding electrodes is degraded by aluminum after a relatively low number of spot welds. So, high quality stable welds are hard to create, and this is why aluminum spot welding is a specialized application.
Spot Welding In Conclusion:
We hope that this primer has answered the question, what is spot welding? To summarize, resistance spot welding can be a fast and cost-efficient process to fuse sheet metals together. It’s a common target for robotic automation to make the spot welds precise and to improve efficiency. But, these qualities make spot welding on a smaller scale an attractive option for many manufacturing and repair applications.