Sometimes, welding structures made of folded sheet metal can be a real hassle. You can’t reach the right spots, and you wish you could fuse them in the middle and keep them flush instead of sealing around the edges. That’s where spot welding comes into the picture.
Sure, you may have tackled these issues with a less elegant non-welding solution, such as screws or studs. However, that’s time-consuming and doesn’t produce the smooth and secure result of spot welding.
Unlike studs and welded corner joins, spot welds won’t catch on anything or wear out over time.
But what exactly is spot welding?
How Does Spot Welding Work?
Spot welding is a type of resistance welding and one of the oldest methods of welding metal sheets together without filler material. It works by applying heat and pressure to a specific spot using shaped electrodes made from a copper alloy.
As the electrical current runs between the electrodes, the metal sheets melt where the electrodes meet. When you remove the current, the electrodes keep pushing the solidifying metal together to fuse it.
Some robots can spot weld things, for example in car factories. However, manual spot welding stations are every bit as convenient and more versatile.
Also, there are spot welding guns for portable use with mobile resistance welding gear. These are very useful for vehicle repairs and other fixes around the home and workplace. They come in the form of pliers with cables feeding them the current, and some have the transformer mounted onto the pliers while others keep them separate.
The technical details
Since metals differ in electrical resistance and thermal conductivity as well as hardness, the time, pressure, and electrical intensity required will vary.
If you want to get technical, we can sum it up with the following equation: Q = I^2Rt. “Q” is the thermal energy, “I” refers to the current, “R” represents electrical resistance, and “t” is the amount of time under electricity.
Spot welding can use alternating current or direct current, although the latter is rare.
However, modern medium-frequency invert welding is gaining a strong standing. This form of spot welding lets you control the current and time by phase shifting with thyristor controllers.
Therefore, some call it phase lag welding and others prefer Medium-Frequency Direct Current (MFDC) welding. Frequencies range from one kiloHertz to a few.
What you need
You don’t need much equipment for spot welding, other than a decent selection of electrodes to accommodate a variety of tasks and metals. However, you do need protective gear, because there will be sparks flying around. These items are what you need:
- Safety goggles
- Flame-resistant gloves
- Long-sleeve cotton shirt
- Sturdy pants
- Protective shoes
While spot welding doesn’t expose you to ultraviolet flash burns or a rain of molten metal like most welding methods do, covering your skin with clothing that won’t melt will save you some pain and potential scarring. While work shoes aren’t essential, there are two reasons why you’d want them while spot welding.
Firstly, you’ll spare your ordinary shoes from damage. Secondly, the electromagnetic current may move things around, and you may drop something on your feet.
You may also want a bucket. In case an electrode breaks and sprays coolant water, it’ll save you some trouble.
What Are the Uses of Spot Welding?
There are applications for spot welding in all industries that involve construction using metal sheets. Examples include electronics, furniture, aerospace, and automotive plants. In fact, it’s the gold standard for car construction for over 100 years.
Spot welding allows for both easy automation using robots and easy training of new workers. There’s a reason why it’s so prevalent in high-volume manufacturing situations.
While spot welding is excellent in many circumstances, it does have its limitations. For example, you can only make localized weld joints, and those may not always be that strong. Plus, it’s often tough to work with complex shapes or anything other than sheet metal.
image source: pexels.com
What materials can you spot weld?
Steel is ideal for welding in general, and it has been the primary use for spot welding for ages. However, high-carbon steel is a bit tricky, because the brittle microstructures give it poor fracture toughness and a potential for cracks in welded spots.
Spot welding is also great on galvanized steel; you just need a somewhat higher current. There is a slight problem with this and other zinc-coated metals and zinc alloys, and that is electrode degradation.
As contaminants accumulate on the copper electrode, weld quality decreases. However, it’s easy to fix this issue by cutting or sanding the surface to expose the copper again.
Stainless steel, titanium, and nickel alloys work fine too.
While aluminum comes close to copper in terms of electrical resistance and thermal conductivity, the lower melting point makes it suitable for spot welding. However, it requires a powerful current in a very short time to avoid overheating the very conductive aluminum. Like zinc, it degrades copper electrodes pretty fast.
You can also spot weld copper sheets and copper alloys, but this is practically impossible with ordinary copper electrodes. If you are spot welding copper and aluminum regularly, you’ll want to get some electrodes made of tungsten or molybdenum for these tasks. They have much higher melting points and electrical resistance.
Understanding these materials can take some time. This cheat sheet will help you understand.
Spot welding advantages and disadvantages
Spot welding is a unique and very versatile method of fusing metal together, but it has its limitations. Let’s summarize the main strengths and weaknesses of spot welding.
- Simple and quick
- No need for special training
- Doesn’t require flux or fillers
- No open flame or flash
- Easily welds different metals together
Spot welding is very safe and easy when compared to other welding methods. Plus, it has a much smoother result than most solutions including welding and screwing. Also, it works perfectly on both wafer-thin and thick sheets of various metals.
- Sometimes it’s hard to reach the right spot
- May degrade the metal somewhat
Spot welding only works when the two electrodes can reach the right place and clamp the plates together with sufficient pressure, heat, and current. This limits the types of welds you can do with it, as each machine has a fixed thickness range and can’t move much.
Plus, it’s essential to use the right size and shape of electrodes and keep them in good condition across welds. Otherwise, the join may be weak.
Also, there may be slight warping or cracks around the weld spot. This can look bad and reduce fatigue strength and corrosion resistance in some cases.
How to Spot Weld
Are you still wondering whether or not spot welding is the right choice for you? Let’s look into the process to give you a better understanding.
First, you insert the correct electrodes for the task, paying attention to the shape and temperature capacity. Then, you place the workpieces and align them precisely. Once you’ve fused a spot, it’s tough to adjust or undo it, so be careful.
Next, you choose the right settings and turn on the current. The required time and intensity vary depending on the material and thickness, so it’s a bit of an art. Hitting the weld switch clamps down on the workpieces and liquefies the contact spot. When the electrodes release the workpiece, the weld is complete, and you can move on to the next spot.
If you did it right, the join will be very strong due to the heavy compression and intense electricity. A good join crater should be no deeper than 20 percent of the plate’s thickness, and the outer ring shouldn’t be too discolored or cracked.
Some blue or brown is fine and indicates a good weld without overheating. It’s generally wise to try the strength by tugging on the workpieces after the spot welding.
Keep in mind that spot welders can produce a powerful electromagnetic field. Therefore, you should place your phone and wallet at a safe distance before beginning.
If you need strong spot welds on bigger items at a smaller facility, you’ll want a select capacitor discharge welder. By storing electricity in capacitors, they can deliver powerful currents without blowing your fuses or requiring an electrical supply upgrade.
Final Thoughts on Spot Welding
image source: pixabay.com
Now you know what spot welding is, what it’s for, and how to use it. A spot welder makes a useful addition to just about any steel mill or workshop, and they’re very safe and easy to use compared to other types of welders.
Do you have more questions or comments on spot welding? Leave a comment below or contact us here.