Eli5: What is the difference between soldering and welding?
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Soldering uses a low-melting point metal alloy. There are lots of different alloys that can be used with different melting temperatures, most of which are easily achievable with a propane torch flame or with handheld electronic soldering irons.
Solder is used almost like a metal glue. Solder can be bought by the spool, it’s usually a very ductile, malleable wire.
Soldering is used on copper water pipes to seal fittings. Plumbers use a propane torch to heat up a fitting and then touch the hot copper pipe with the end of a solder wire just like you might melt a hot glue stick if you didn’t have a hot glue gun. The solder has a lower melting point than copper, and the copper will be heated up hotter than that temperature, so the solder melts on contact. Conveniently, the solder tends to get “wicked” into the joint. It quickly cools off and re-solidifies, forming a water tight seal that will outlast the building it’s installed in.
Soldering is also commonly used for electronics. Most solder alloys are made specifically to be very electrically conductive while also melting at workable temperatures. Solder melts at a low enough temperature that it can be liquified and applied to circuit board connections without burning the board.
In a soldering connection, *only the solder melts*. The metals it is connecting to do not melt.
Welding also usually uses a filler material, which is selected to be compatible and similar to the base material to be welded. There are three main types of welding, all of which use high current electricity to form an arc which heats the workpiece in a controllable manner.
* Stick welding – pretty much the crudest form of welding. Sticks that are about 1′ long are clamped into the welder, a ground clamp is attached to the work piece, and when you hold the stick close to the metal work piece the electricity arcs from the stick into the metal, which gets hot enough to melt the stick and the base material together, forming your weld bead. The stick runs out pretty quickly, so you usually have to stop and change sticks every few minutes. The stick has a mixture of metals on it, but also has shielding compounds that vaporize and protect the liquid metal from oxygen in the air while you’re welding. Stick welding is rather low cost because the machine is pretty basic. It just pumps electricity through the metal, the sticks do the rest.
* TIG welding – Tungsten Inert Gas welding. The welding machine has a tungsten tip which is non-consumable, meaning it doesn’t melt while you’re welding, and the machine is also hooked up to gas bottle(s) that feed an inert gas like argon, helium, or nitrogen to protect the liquid metal from oxygen. The welding tip has to deliver electricity and gas to the weld. The filler material is a literal stick of metal held in the other hand that the person welding holds up to the weld site and melts in. It’s like stick welding, except the operator has much more control. TIG requires both hands to weld, and is tough to learn but with practice can be used for very precise welds. Precision allows welding on thinner materials. This is fancy pants welding. A TIG welder is more expensive than stick welding, and you need shielding gas. TIG welding labor is often expensive because it’s a specialty skill and takes longer to perform.
* MIG welding – Metal Inert Gas welding. This is kind of a hybrid between TIG and stick welding, and it brings the best of both worlds with some drawbacks. MIG welders use a long “wire spool” of filler material, and use a small motor to feed this down the cable through the welding tip. This wire acts as the electrode (instead of tungsten in TIG), meaning that the welding tip has to deliver electricity, gas, AND the wire to the weld site. This frees up the worker’s other hand though, making MIG welding a lot easier to learn. As long as your MIG welder is set up with the best settings for the material type and thickness, MIG welding is honestly not much different from using a hot glue gun. MIG welding typically has the most expensive equipment, because the welding machine has to do more, but doesn’t require as much skill or time to perform welds.
The key thing about each of those main types of welding:
* A filler material is used. Welding adds material to the joint.
* Electrical arcing is used to locally heat up the base material and filler. There are gas welding options I’ll mention below, but electrical arcing is much more common.
* The base material melts.
Another common type of welding is *spot welding*. Spot welding is used for joining two thin materials to each other. Spot welding uses a c-clamp looking electrode that reaches above and below a workpiece, clamps the material together, and pumps high current through the materials to heat up and melt them together in a small spot. Spot welding does not use an arc, and does not use filler material. Spot welding only works on thin materials, but it’s super fast and convenient for joining pieces of sheet metal together without using fasteners like bolts or rivets.
Welding doesn’t actually require electricity, it just requires heat. For heat, gas torches can be used, and it can be welded just like the TIG process. A propane torch wont get hot enough to melt steel, but specialty torches burning a mixture of oxygen and fuel like acetylene can get plenty hot to weld. The downsides to oxy-fuel “gas” welding is that it requires oxygen and fuel gas, which are dangerous and require safety precautions to use, transport, and store. An advantage is that no electricity is required, which means you can weld in more remote areas. However this advantage is more or less moot because tow-behind diesel generators deliver plenty of power for most welding applications.
Welding requires more power than soldering because welding is typically done on metals with far higher melting points, as well as just larger work pieces. When done properly, welding provides very high strength connections. Soldering on the other hand does not add much if any mechanical strength to the joint. Often solder is just there to fill the gap. In piping, it just provides a permanent seal, and in electrical connection it’s just attaching two conductors.
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