They seem to involve connecting pieces of metal together by way of heat, but that’s the extent of my knowledge. I’m guessing that welding involves higher heat and doesn’t require a “filling” metal.
Short story, in welding the base metal is melted and a filler rod or wire of the same base metal is added to fill the joint and create a bead. In soldering, or brazing, the base metal is not heated to a molten state.
Source: http://www.epemag.wimborne.co.uk/solderfaq.htm
Soldering only heats the base metals sufficiently for the solder to flow into a joint and make an electrical connection. Soldering is not used to make a strong physical connection. OTOH, a weld actually melts the base metal to make a strong physical joint.
I’m no welding expert, but I have done a little soldering in my day.
There are many differences: the degree of heat, the types of materials that can be joined, and the reasons for joining.
Soldering is (generally) a much lower-heat process. The most common application for soldering is connecting wires and/or components to make an electrical connection, but you also solder lead strips when making stained glass windows, for instance.
In soldering, you heat the materials to be connnected with a soldering iron (a pen-like device with a tip that gets very hot) until they are hot enough to melt the solder, which is mostly lead. The molten solder flows into the join, and when it cools the pieces are firmly joined with an electrically conductive connection.
Welding is a very different thing. A real expert will be along shortly to provide more detail, but one difference is that welding, AFAIK, is never used for electrical purposes, like soldering. If it is, it is only in very specialized applications. Welding is almost always for structural purposes: joining two big pieces of metal together.
There are many different forms of welding, virtually all of which are done at very high temperatures: arc (electrical), heliarc (electrical in a flow of helium), oxy-acetylene, and on and on. Some, but not all, require the use of a filler material (analogous to solder) to make the join.
And for what it’s worth, silver soldering makes a relatively high strength joint. It’s right in between soldering and brazing in terms of temperature and comes in right handy because you can then solder other components onto the piece with a lower temperature solder without fear of the silver soldered joint coming apart.
Plus, commasense, I think most low temp solders are primarily tin.
It is? Really?
Hang on, let me see if I can dig my old mass spectrometer out of the closet. Just a sec… it’s around here somewhere…oh, so that’s where the torque wrench got to! There’s the mass spec. Now where’s that f***ing manual?
Okay, drop in the solder.
Well, what do you know! You’re right. 60% tin/40% lead. Thanks.
Bitch-slap me if I’m wrong, but isn’t the key difference this: With soldering, you use molten metal to “glue” two things together, whereas with welding you melt the two things into each other?
Not quite. The type of welding you’re referring to is yet another type of welding that hasn’t yet been mentioned in this thread. It’s called spot or pressure welding.
It’s usually done on sheet metal, like body panels on a car. The two sheets are forced together with a clamp, while they are heated using an electrical current passed through the sheets via the clamp. The welding robots you may have seen on car assembly lines are using this process.
It’s also possible to do this type of welding using explosives.
There’s no filler material, or “glue” involved.
With the other types of joining, the difference is in the type of “glue”, and how far the “glue” penetrates into the surface of the materials being joined.
With arc welding, the penetration is deep, as much as 10 mm.
At the other end of the spectrum, solder will alloy with copper only at the surface of the copper, with a penetration depth of the order of a few atoms.
Consider yourself bitchslapped. But not very hard. What you say is almost correct. The only thing you have wrong is that with welding, quite ofthen you are not just melting the two things into each other, but using a filler that melts into the pieces you are joining together.
In other words, with soldering the solder is the only thing that melts. In welding, the pieces you are joining and the filler (if any) melt.
Consider yourself bitchslapped. But not very hard. What you say is almost correct. The only thing you have wrong is that with welding, quite ofthen you are not just melting the two things into each other, but using a filler that melts into the pieces you are joining together.
In other words, with soldering the solder is the only thing that melts. In welding, the pieces you are joining and the filler (if any) melt.
In the USA, the solder sold for plumbing no longer contains any appreciable amount of lead. I think it contains NO lead at all. But solder used for sheetmetal and general electrical work is tin and lead. Some circuit boards are soldered with silver solder.
Ta. That’s pretty much what I meant, but I didn’t express it right in a single pithy phrase. I was vaguely aware that in welding you (may) also use some additional filler as well as the bits you’re sticking together.
The nearest I’ve come to welding was brazing copper and constantan wires together in the physics lab under the technician’s direction, and that really was just a case of heating 'em both red hot and touching them together. Quick and dirty, and strong enough for what I needed at the time.
Another analogy, I guess: When you make balsa-wood model planes, the glue you use for that penetrates into both pieces of wood and solidifies. But when you use a plastic kit, the glue solvent melts the surfaces of the plastic, which then congeal when the solvent evaporates again. Or something.
From what I understand from my brief foray into jewelry making, silver soldering is more like welding than other types of soldering. You use a torch not an iron and the bond is quite strong. My father says that he has used silver soldering to hold together brass pieces of equipment he was repairing.
I think I’ve read that even fresh-out-of-the-box American kitchen faucets still contain traces of lead.
Why? No idea. Can’t a 100 percent lead-free faucet and solder be made?
Silver brazing is strong enough to make a bicycle frame. Though it’s only good for attaching metal tubes to lugs. Welding allows lugless joints (one tube bonded to another tube directly), which is how most frames are made these days.
Desmostylus, what you described is more generally called “resistance welding.” For a long time it was the only form of welding that didn’t involve a filler material. Now certain types of laser welding don’t use filler material, but most do.
As for solder, I’m fairly certain the plumbing solder contains no lead, the downfall of Rome and all, you know. Plus there’s the further distinction that the purpose of the solder joint isn’t electrical in nature.
Also, we do a lot of soldering at work that doesn’t use a soldering iron or gun. It uses a high-frequency inductive heater, that is, it magnetizes the parts and solder and makes it hot that way. The solder paste (a mix of solder and flux) melts and fills the joint via gravity. This application is is used to hold filler necks and emissions tubes to fuel tanks.
The biggest distinguishing difference between soldering and welding is whether or not only the filler melts. If it’s just the filler, then it’s soldering or brazing. If it involves the joined parts also melting, then it’s welding. So what’s the difference between soldering and brazing? Melting point – if it’s less than 800 degrees F it’s soldering. Greater than that it’s brazing. Exactly 800 F? Uh, I don’t know.
In Sn/Pb soldering you usually see a flux used. This serves as a lubricant and helps the solder flow where you want it. Strictly speaking, though, there’s nothing inherint in soldering that demands a flux, meaning, just because there’s no flux doesn’t mean it’s not soldering.
Sometimes you’ll want to solder or braze rather than weld for the express intent of not wanting to heat treat the joined materials. Melting and resolidification change them structurally.
Brazing or soldering also makes removal and replacement of the joined parts easier. You can appy heat and pull them apart rather than having to cut them apart. Think of replacing a faucet in your water system. You and apply heat and remove the old part. Clean it up a little and solder in a new one. If it was welded you’d have to cut off the old piece and now the pipe is shorter. So you might need to weld in a new pipe as well as a faucet.
No, it isn’t generally called “resistance welding”, especially when as I pointed out, you can do it with explosives, and as you pointed out you can do it with lasers.
Yeah, it is, but it was my fault for not being clear about what particular aspect of your message I was referring to:
Just to be clear for everyone else’s sake, the “explosives” part doesn’t apply to resistance welding on a car assembly line 
Also spot welding is anything that results in a welded spot. In general parlance it usually means resistance spot welding (say a 3-6mm diameter weld), but the actual welding method is “resistance welding.” On a lot of Chrysler pickups you can see spot welds all over the bed. Makes me shudder.
Lasers aren’t generally used for spot welding. In the auto industry they’re especially used for welding blanks, i.e., pieces of metal you run through a press. Instead of a big sheet of homogenous metal, you laser weld the exact pieces of metal (different thicknesses and/or coatings) into a sheet, and stamp it all in one shot.
Yeah, I’d agree with most of that. If you’re still searching for the common link, it’s the “pressure” part.