What is cold welding?

00:00:08
Speaker 1: Hey, Kelly, how are things going on the science farm?

00:00:11
Speaker 2: You know, they're going pretty well, but we have some old pipes and they've been sailing and so, you know, renovating the bathroom in the kitchen. We're pretty low on why to do list, but suddenly they went to the top and so there's been lots of renovating projects happening.

00:00:26
Speaker 1: Ooh, does that mean lots of opportunities to show your kids how to use power tools? Circular saws and arc welders and all that good stuff.

00:00:34
Speaker 2: Uh, there's lots of opportunities for me to play with power tools, many different kinds of saws. No arc welders yet, but generally the kids are not playing with the power.

00:00:45
Speaker 1: Tools, not until they hear this episode and then they start asking for their turn.

00:00:50
Speaker 2: This is why my kids don't get to listen to your podcast.

00:00:52
Speaker 1: Man, how can you consider yourself a good parent if you don't share this podcast with them.

00:00:58
Speaker 2: I'm pretty sure that the number one rule of parenting is keep the kids alive, and I'm pretty good at that, and that just seem to be tied to them not listening to your podcast.

00:01:10
Speaker 1: Fair point. Hi, I'm Daniel, I'm a particle physicist and a professor at UC Irvine, and I want everybody's kids to stay happy and healthy.

00:01:33
Speaker 2: I'm Kelly Wieder Smith. I'm a parasitologist. I'm adjunct at Rice University, and I also want kids to stay happy and healthy, which probably means no power tools before the age of ten.

00:01:45
Speaker 1: Ooh, you hear that Kelly's kids tenth birthday, you're getting power tools.

00:01:49
Speaker 2: Oh no, that's only a couple months away from one of them. I meant twenty twenty.

00:01:56
Speaker 1: Wow, that was pretty quick backpedaling right there. You know, eventually they got to learn some useful skills, so arc welding could be in their future.

00:02:04
Speaker 2: You know, she's really good with the screwdriver, and I feel like I've done my part. She could also make a little circuit. She did that for the Talent Show and it had little light up eyes and the mascot of her school and it was pretty cool. I'm doing a great job, Daniel.

00:02:20
Speaker 1: You're doing a bang up job for sure, and so welcome to the podcast. Daniel and Joor explain the universe, in which we try to do a bang up job of welding your brain to the universe, making those connections between everything that's happening inside your skull and all the physics of the universe, the black holes, the tiny particles, the galaxies, the arc welding, everything that's going on out there. We want you to understand it. No topic is too big, no topic is too small, no question is too weird, no idea is too bunkers for us to consider, because our goal is to make sense of the whole universe. My usual co host he can't be here today, but I'm very happy to be joined by Kelly to talk about today's amazing and fun topic.

00:03:05
Speaker 2: I'm excited to be here. You know how, I think you and I just had a podcasting aniversary, didn't we. It's been like two and a half years or two, I don't know, it's been a while. I'm excited.

00:03:18
Speaker 1: Oh no, I'm embarrassed. I haven't been keeping tracks. I'm totally unaware of our anniversary. I apologize. I should have gotten you something.

00:03:26
Speaker 2: Well, that's you got me a really interesting topic to talk about. Oh, thank you.

00:03:30
Speaker 1: Yes, that must have been my subconscious plan of what's what it was the whole time.

00:03:35
Speaker 2: Yes, the two year anniversary is the cold welding anniversary.

00:03:38
Speaker 1: M Well, congratulations on two years of being on the podcast. We love all your contributions.

00:03:44
Speaker 2: Oh, I love being here.

00:03:45
Speaker 1: Thanks awesome. And sometimes on the podcast we talk about quantum mechanics. Sometimes we talk about black hole, sometimes we talk about planetary orbits in the future of our star. And sometimes we talk about the everyday physics and science that is around us, stuff that makes up our lives, how it works, and how hard it is to understand even the everyday normal stuff in our universe.

00:04:07
Speaker 2: It never gets easier.

00:04:09
Speaker 1: It's sort of amazing the complexity of things that we see around us, rocks and blueberries and ice cream, and it's incredible that we can understand how that stuff all comes together from the tiny little particles that are inside it. The buzzing and tuing and frowing of all those little bits somehow weave themselves together into this incredible experience we have where things seem solid or liquid or gaseous, that we know that that actually all just bubbles up from the tiny little particles inside them.

00:04:35
Speaker 2: I'm glad you're confident that we understand the world, because as a biologist, I'm way less confident. But we'll go with it.

00:04:41
Speaker 1: You know, I was recently doing college interviews, and one young lady told me she was really interested in biology because she thought it was cool how you could explain everything using the smallest bits around us, the cell. And I was like, well, it is very cool how the cell comes together to make complex biology. But you know you can go further than that, right, you can dig deeper.

00:05:04
Speaker 2: You leave her alone. She's on the right track.

00:05:06
Speaker 1: I almost convinced her to do particle physics, but she was like, nah, she wanted to study stuff that was more relevant to our lives. Good for her, but there are still lots of deep mysteries remaining about this fairly pedestrian idea. Everything is made of particles. Those particles follow simple rules, and how those bits bang up against each other and stick or don't stick and repel makes up our whole world. Why some things are shiny, why some things are sticky. We think that those are all emergent properties from those little particles and the rules that they follow. But it's complicated. It's not like if you give me the location and velocity of ten to twenty nine particles, I can tell you exactly how they're going to behave, Oh, this thing's going to flow or this thing's going to conduct electricity because it's a lot of particles. It's too many for us to understand, which is why chemistry is still a thing, right, which is a reason, the reason, the reason we don't just have particle physics as the only science in the universe.

00:06:00
Speaker 2: I mean, that would not be as interesting. But yeah, I remember I took stochastic differential equations in college, and I was just like, how do we know anything about the universe? Ever, this is also complicated, and.

00:06:15
Speaker 1: It is incredible because the universe seems chaotic. All those tiny little particles are so dependent on other tiny little particles. It's incredible that anything emerges. But one of the great philosophical mysteries of science is that simple stuff does emerge. Balls seem to follow simple trajectories, even if we can't understand the tiny particles inside them. You can make chicken soup without understanding quantum gravity. The universe is understandable at all these different layers, which means there are fascinating questions and interesting mysteries at every level of science. You know, studying galaxies, studying people, studying cells, studying funguses, and studying the tiny little particles questions to be asked at every level.

00:06:56
Speaker 2: Not sure we'll ever understand people, but well we'll do our best and keep.

00:07:00
Speaker 1: And forward exactly. And so one of the topics we love thinking about is how materials work. Why protons and electrons come together to make one thing conduct electricity and something else be transparent and something else be opaque. How does that all emerge from the tiny little particles that are inside them? And there's so many different aspects of these materials we can explore, like how you stick them together to build stuff? How does the Eiffel Tower come together from all those individual bits of steel?

00:07:28
Speaker 2: Gorilla glue?

00:07:32
Speaker 1: No, wow, you just invented an entire country over there.

00:07:38
Speaker 2: I mean, it's a really good glue. You can get it in a can and spray it.

00:07:41
Speaker 1: Are you saying if they were going to build the Eiffel Tower today, they would use guerrilla glue instead of welding.

00:07:47
Speaker 2: Probably, I don't know. You'll have to tell us about welding. Then I'll make my decision at the end.

00:07:51
Speaker 1: Well, today in the podcast, we are going to talk about welding, how it works, how you can stick stuff together, and how you can possibly do it without even heating things up in a way that could be helpful for space industry and could be safe for even Kelly's children.

00:08:06
Speaker 2: Oh all right, you've got my attention. Maybe my oldest can listen to the you know, second part of this discussion.

00:08:14
Speaker 1: So today in the podcast, we'll be asking the question what is cold welding? So this is a topic a bunch of listeners wrote in to ask me about what is the physics or the chemistry or just the science of cold welding. How could it be possible to stick stuff together without melting it down first, and could this potentially be a way to build things in space?

00:08:43
Speaker 2: So, you know, when you proposed the topic of cold welding to me, I could not remember a time when I had heard this phrase, And so my first thought was was it like sticking like pieces of gum together? And then I'm like, no, welding, welding specifically refers to metal, right, It's like putting two pieces of met together, is that right?

00:09:01
Speaker 1: Yeah?

00:09:02
Speaker 2: Yeah, yeah, And so then I was like, I don't know, but but I hear it has to do with space, So I'm gonna have to I'm gonna have to go with that. So let's see if your listeners are more clued into the world of welding than I am. I bet they are, so.

00:09:16
Speaker 1: Thanks very much to everybody who answers these questions for the podcast. If you would like to contribute your uninformed speculation for the episode, please don't be shy. Write me to questions at Danielandjorge dot com. Everybody's welcome, So think about it for a moment before you hear these answers. What do you think cold welding is and how does it work? Here's what listeners had to say.

00:09:39
Speaker 3: Well, cold welding can't work by heating up two metal sources and melting them together, but it could create a chemical reaction that causes them to melt together. So my guess is it's not actually fire, but it's some sort of chemical.

00:09:53
Speaker 4: Maybe it is using a chemical reaction to fuse two things together to rearrange them molecules. It reminds me of the old farmers in Vermont would attach wrought iron gate latches to granite posts by boring a hole in the granite and sticking the gate latch in there, and then pouring some sort of a powder that would create a reaction and fuse the two together. I've tried to cut them out many many, many years later, and it's impossible. It's sort of just one thing.

00:10:25
Speaker 2: Well, those were two great answers, And to be honest, I hadn't heard about farmers attaching iron latches using a powder to connect it to granite. Like that's awesome. I've got to find an excuse for how to do that on the farm, even though right now I don't think I need to.

00:10:42
Speaker 1: I think that sounds like farm magic, you know. I think he'd run into some magicians and they have some like spells and powders, and they're able to like fuse things together. I think we need to dig into this. This could be like a whole revolution in science.

00:10:54
Speaker 2: So you hadn't heard of that before either.

00:10:56
Speaker 1: Vermont farmer magic. Noila had not heard of that before.

00:11:01
Speaker 2: I can imagine it being pretty obscure Vermont farmer magic. But that's not that far away from the Salem witches.

00:11:10
Speaker 1: Those farmers. You gotta be careful. You gotta be careful using your powers out there where everybody can see you. Yeah, but this doesn't seem to be something a lot of people have understood. And frankly, I was a little surprised, Kelly that you didn't come across it in all of your deep, deep research about space settlements.

00:11:27
Speaker 2: You know, I have literally thousands of pages of notes, and my mind retains one percent of what I write down, because I figure, why memorize it if I could just you know, control what it control f for my search term and then see what I wrote down in the past. So I bet I read about cold welding at some point, but it did not stay in my internal hard drive.

00:11:50
Speaker 1: Well maybe it turns out it won't be crucial for a space settlement, after all, we'll find out.

00:11:56
Speaker 2: So I asked a silly I won't say stupid, a silly question at the beginning. You know, could it be like sticking two pieces of gum together? And you told us no, it requires metal. What else are the very very basic facts of welding.

00:12:12
Speaker 1: Yeah, so before we talk about specialized cold welding, let's just talk about normal vanilla hot welding. I guess, and this is basically sticking two pieces of metal together. You want to build the Eiffel Tower, you want to build a bicycle, you want to build a tank, you want to build an airplane, all this kind of stuff. You got to use welding. Welding is basically the way that we corillatly stuff together in the modern world.

00:12:38
Speaker 2: So I'm doing a bathroom renovation and luckily when I opened up the wall, there were no copper pipes, because then I was gonna have to solder, and I don't know how to solder, but I would have learned. Is soldering welding because it's like that little silvery thing that you unroll and you heat up and you put on the copper. That's metal too, right, So does that count?

00:13:03
Speaker 1: Yes, so that does not count as welding. Soldering and welding are two different things. So welding is when you melt two pieces of metal, so the metals themselves join become like one piece. So you got two objects you want to stick them together. You actually melt the edges of both of them. Soldering is different soldering. You have a lower temperature filler, something which melts at a much lower temperature that basically acts as glue and bonds to the two surfaces. But you don't actually melt the two things you're sticking together. So you got two pipes you want to stick together, you can solder them together. By melting some filler material which sticks them together, or you can actually weld them by melting the two pipes and sticking them together when they're liquid, so when they cool, you get those chemical bonds between them.

00:13:50
Speaker 2: But soldering is still cool because you get you used a bload. I was kind of torn when I opened up the walls and there was pets. Part of me wanted to solder, part of me did, But anyway, okay, I'm getting a soft track.

00:14:01
Speaker 1: And soldering somebody. You can do an electrical laborate. You have that special solder material which you touch to the tip of the soldering iron. It just like instantly liquifies, right, super cool stuff, and that stuff you use because it liquifies very very easily, and then it cools very quickly and becomes solid again. So that's soldering. But welding is different, right. Welding is when you really want to join these two materials and you do it by some combination usually of heat and pressure.

00:14:27
Speaker 2: Okay, and I'm guessing that we've been welding. It's got to be after we get access to fire. How much after we get access to fire before we're welding.

00:14:39
Speaker 1: Yeah, welding technically is something we've been doing for thousands of years. Essentially, blacksmiths invented welding. They notice you got two pieces of metal, you heat them up and pound them hard enough. They will join a lot of the stuff that's been connected together to make basic swords and tools for farms and all this stuff that blacksmiths have been making for thousands of years. They called that forge welding. Basically, heat it up and hammer it hard and you will get that fusion, that connection between the two materials that tells you that they are now one.

00:15:11
Speaker 2: Okay, So they're using like a furnace, Is that the most typical way or a blowtorch? Is that the most typical way to get the heat that you need to combine two metals.

00:15:22
Speaker 1: Yeah, exactly. And when my dad retired from Los Almo's National Labs, he actually took up blacksmithing as a hobby. I was pretty sure he was like preparing for the end times. He thought, like, what would be a useful skill to have if civilization collapsed, and he decided blacksmithing would be useful, And so he got a little furnace and he would go to the dump and like pick up scrap metal, you know, like truck suspensions and all sorts of stuff, and he would melt it down and hammer it into shapes. And he made all sorts of stuff from like towel hooks to his own spears and stuff like that.

00:15:57
Speaker 2: Whoa, that's awesome.

00:15:59
Speaker 1: It was pretty cool until half the town burnt down in a forest fire, which is a real tragedy. And then the neighbors were a lot less keen on him having like a thousands of degrees furnace in his garage spewing up sparks into the dry woods. So he got shut down. Yeah, reasonable, reasonable, yep.

00:16:18
Speaker 2: But said, hey, do you know what turneski?

00:16:21
Speaker 1: I don't. Who's that?

00:16:22
Speaker 2: I think he's the current blacksmith that loaves Alabos and he's a friend of mine. Oh, right, back and off, okay, okay, So I was trying to get you to talk about electricity.

00:16:31
Speaker 1: Yeah, but blacksmithing is an ancient form of welding, right, You heat it up, you squish it together, and what's happening there is that the metals on one side and the metals on the other side are bonding together, and so there's really no distinction there. And that makes a very very strong connection. Right. But in the late nineteenth century we invented arc welding. We had electricity now, and see we could more efficiently deliver energy than just like sticking the whole thing in an oven, which also isn't possible if you don't have a big enough oven. You want to weld together two I beams, you don't want to have to stick the whole thing in the oven and then hammer them together. You want to only heat at the spot that between them that's touching.

00:17:09
Speaker 2: So what is more dangerous a giant furnace operating at intense heat or electrifying metals and working with fads. Kid one is not allowed to listen to the episode up to this point yet, let's see where we get to.

00:17:24
Speaker 1: But our welding is actually really cool because it basically makes a circuit. You're running electricity through the metals, and because the metals are not perfect conductors, there's some resistance to them, and resistance means heat resistance. You're turning electrical energy into thermal energy. Like the way a light bulb works is you're running electrical current through something which is a resistorance. It's gonna heat up and then it's going to glow. So the reason the metals get hot is the same reason that a light bulb glows.

00:17:54
Speaker 2: Ah, And so we just don't let it get out of control because you don't want your light bulbs burning or melting.

00:18:01
Speaker 1: That's right. And so in arc welding, what you're doing is creating a circuit, and then you're passing that electricity from a stick which arcs onto the spot that you're trying to weld, passes energy through that and then completes the circuit. And so you just power that electricity back and forth. You can be DC, you can be acy, all sorts of varieties of it. It was meant that in the eighteen hundreds when people were learning about the power of electricity.

00:18:27
Speaker 2: You know, you've got me wondering now, fun tangent. So the first the first space walk ever conducted by a woman was a sptlanasub Atskaya, and she welded in the vacuum of space for the first time. So do you think that she welded with heat or electricity?

00:18:47
Speaker 1: I suspect that was an arc welding, right, because the heat generation from like a blowtorch might require combustion with the atmosphere respect that was probably arc welding.

00:18:57
Speaker 2: Yeah, oh I should have known that. As soon as you started saying it, I was like, oh, of course. But anyway, all right, so fun, Yeah, there you go.

00:19:04
Speaker 1: But hold on, that sounds super dangerous. You have this like arc welding in space. I mean that could easily rupture a suit or cause some damage. So that seems crazy to me.

00:19:15
Speaker 2: Well, you know what's infuriating she is this like hypercompetent, incredible cosmonaut. And there's this movie called Solute seven that came out in Russia and it's sort of like Solute seven is almost their equivalent to our Apollo thirteen. So Solute seven like they had a problem I think it was with the solar panels and the power ran down, so they had to send somebody up there to try to fix it and get it back online. But at the beginning of the movie they create this scene where Setlanasovitskaya is welding in space, which happens, but she gets a welding burr in her glove and she's starting to lose pressure and her male colleague needs to pull her back into the station because she just like I don't know. It's like losing her composure. And that didn't happen. She did a great job of welding. She didn't get a burr in her glove. She didn't need to be saved by a man. But that's how the movie starts, and that made me very frustrated. Tangent done. Oh and the rest No, no, almost done, Tangent almost done. The other big lie in the movie is that they need to get up there to get the station started again because the shuttle, the Americans are going to set off the Shuttle and they want our technology, so they're going to put the Salute seven space station in the shuttle's cargo bay and take it back to Earth to steal our technology, which it wouldn't fit. That didn't happen, but that's like a huge and at the very end you've got the Space Shuttle driving past Salute and the Space Shuttle people who are driving Salute the Solute cosmonauts for doing such a good job of getting it fixed before they could go there to steal it. And it was like what anyway, a classic film, Like all.

00:21:00
Speaker 1: The research you did for your space settlement book may have ruined you for science fiction Kelly.

00:21:05
Speaker 2: Ah, yeah, yeah, that's probably true.

00:21:09
Speaker 1: That's the downside of knowing a bunch of stuff.

00:21:11
Speaker 2: Better not to know.

00:21:14
Speaker 1: So in the eighteen hundreds we developed this new technique like basically the first advance since blacksmithing for thousands of years in how to join metals. And this got really fast forwarded in the World Wars in the early nineteen hundreds because people wanted to make lots and lots of tanks and airplanes and all sorts of stuff, and arc welding can be a little bit painful, it's kind of slow, and it can be inefficient, and so they developed lots more techniques to do this much more quickly, to do it much more efficiently. So fast welding really was born in the first part of last century because of the World Wars, humans wanted to kill each other more efficiently, so they had to be quick at building those killing machines.

00:21:56
Speaker 2: So depressing. Same goes for rockets, right, that's the rockets took us to the Moon. We're used to drop bombs on London without great aim, I'd say, But anyway, Yeah, it's.

00:22:06
Speaker 1: A conflict I feel deeply internally. I mean a lot of advances in physics come from wanting to build weapons of mass destruction, and so that's you know, also where a lot of fund incomes from. But it does, in the end reveal a lot about the nature of the universe. So it's a tough situation, yep. But it turns out that you could apply a lot of these welding techniques not just to metals. You can also weld things like glass. Right, you can take two pieces of glass and you can have a glass rod, and you can use a blowtorch to melt the glass and stick them together. And so you can weld pieces of glass together. You can even weld glass to ceramic or to steal all sorts of stuff. So welding is not just something you have to do with metals.

00:22:47
Speaker 2: I didn't realize that. I guess I did think welding was metal specific.

00:22:51
Speaker 1: Yeah, it's basically just joining stuff together by melting them. And it makes a lot of sense because you melt it together and that the atoms are all jiggling and they can and jiggle together and when they cool, it's like they're one again. What sort of mysterious is doing this without heat? And so cold welding is a much stranger, much more interesting and maybe potentially useful in space kind of process.

00:23:15
Speaker 2: And we're going to learn more about it after the break. All right, So, hot welding uses heat to combine metal, glass, plastic, and steal. But you have told me that we can do it without heat, and so maybe my daughter can listen to this part of the episode. So how do we do that? Do we just bang them together really hard and hope they stick.

00:23:52
Speaker 1: That's the long and short of it. Yes, essentially you can do cold welding if you have pressure and if the materials are very, very very clean. Think about what's happening inside of metal. Metal is essentially a crystal. You have all of these atoms of iron or whatever, and they're lining up in a crystal, and the reason they're so strong is all those bonds are holding them together. How do you make a bigger crystal? Well, you could like rejigger all the atoms and make them liquid again and then stick them together so they cool back down to make a crystal. But what if you just have like half of a crystal here and half of a crystal there, and you just like stick them together so that the two bits of crystal like link together, sort of like sticking two puzzle pieces together.

00:24:34
Speaker 3: Right.

00:24:34
Speaker 1: You don't have to melt them down to liquid cardboard to make to stick them together. You just sort of fit the nibs into the holes and they click together into one big picture. Okay, So that's the sort of idea behind cold welding. Like, if you have a material that's very regular and you have two pieces of it, if you just get them together close enough and apply a little bit of pressure, they will all of a sudden act like they were always one thing. They will just sort of like stick together.

00:25:02
Speaker 2: All right. So I'm having a little trouble wrapping my head around it. So I'm imagining like, all right, so you put two pieces of metal together, and very very very slowly they start what like acting like liquid and sort of mixing together or yeah, why so why does that? Why does that happen?

00:25:21
Speaker 1: It's not very very slow. The whole thing can happen in seconds or less, and there's no liquid involved. This is just two pieces of metal clicking together. I read this fun quote by Richard Feinman. He says, quote, the reason for this unexpected behavior is that when the atoms in contact are all of the same kind. There's no way for the atoms to know they are in different pieces of copper. Essentially, you just have like a bunch of copper here and a bunch of copper there. You bring them close together, they will stick together. They do like to bond to each other. Think about what happens between two atoms of copper. There are electrons that are like flowing between them, bonding them together. That's what the bond is, right, The actual nuclei are and stuck together. It's thee electrons between them. And so if you have two atoms you bring them near each other, they will bond. They don't have to become a liquid in order to bond together.

00:26:14
Speaker 2: So why do metals do this? And like the I've got a book sitting next to me with lots of pages, but the pages are staying distinct. Why why doesn't everything start just sort of blending together.

00:26:30
Speaker 1: Exactly? Why isn't the whole world just fuse the one lump right exactly? Yes, this is something metals can do because the electrons in a metal are very mobile. Like some materials, the electrons mostly stick around their own atom or the neighboring atom. But when metals come together the energy levels of those electrons are such that the electrons can really flow freely through the metal. It's more like an ocean of electrons. And this all depends on how the energy levels translate. Like you have an individual atom, we know that the electrons around that atom have energy levels. Like you remember from your high school chemistry that there's like the one p orbital and the four f orbital, and there's all these energy levels for an electron.

00:27:12
Speaker 2: We shouldn't assume I remember anything from high school chemistry, but keep going.

00:27:16
Speaker 1: I don't remember it either. I probably just remember it because my son is taking high school chemistry, so I have to hear about all this stuff. Anyway, Electrons around atoms have energy levels, right, Well, what happens when you have two atoms and now your electrons are sort of like zigging back and forth between those two atoms. Well, the energy levels get more complicated because you're now responding to the positive charge of two atoms. Now add another atom, and another atom and another atom. You get a whole crystal lattice. And so now you get a bunch of different energy levels for the electrons. And metals and insulators have different kinds of energy levels there. In metals, it's very easy for the electrons to jump up to the higher energy levels and then move around the whole atom. In insulators is like a big gap, then the electrons can't easily get up there, so they're sort of like trapped around individual atoms more So, the short answer is, metals have a lot of electrons that are easy to flow around, and so if you bring two pieces of metal together, their electrons would just sort of like start flowing back and forth, and then the copper atoms will be like, hey, you're my neighbor. I'm your neighbor. Let's share electrons, and boom, they're bonded.

00:28:23
Speaker 2: So all right, So I've got these copper pipes m in a different part of my house, and and if you get this electron c and these metals can all like come together. Why do my copper pipes retain their shape? Why doesn't the like copper you know that's above or that's at the top of the pipe sort of like start moving into the copper that's at the bottom of the pipe, and it all just sort of like becomes a glob.

00:28:51
Speaker 1: Well, copper is pretty strong, right, it's crystal structure preserve itself, but when it comes next to another piece of that crystal structure, then they will link together. It's just like the puzzle pieces, right. Puzzle pieces hold themselves together into a shape, but if you bring them nearby into something else where where the nibs and the holes all match up, they will click in the place. The real question is, like you've touched copper together before, you didn't notice it, like cold welding together. Right. It's not like every time two copper pennies touch that they cold weld. It's not like every time you have change in your pocket you pull it out and it's like one single blob. Right, That would be pretty weird. And so why doesn't this happen all the time. The answer is that these materials have to be really really clean. Essentially, you need like really bare copper to touch really bare copper, or bar iron to touch bear iron, and that doesn't happen very often because we're in an atmosphere, and so the copper in your pipes interacts with the atmosphere and the oxygen in the atmosphere and forms like thin layers of oxides. So if you take two copper pipes and you bang them together, the copper is not actually touching. What's touching are these thin layers of sides and greases and all sorts of other stuff on the surface, So you're not really getting that clean contact.

00:30:06
Speaker 4: Is it?

00:30:07
Speaker 2: Also like a lot of the metals that you encounter that we encounter in our day to day lives, are they pure metals or are they also metals that have like other stuff mixed in.

00:30:16
Speaker 1: Yeah, a lot of the stuff we interact with are alloys, right, steel for example, there's all sorts of stuff mixed into it, and a lot of stuff we interact with are not pure metals. So this requires sort of a special situation. You need pure metals and you need them to be super duper clean, And so if you wanted to accomplish cold welding, what you got to do is like really carefully clean the surface. Sometimes you can actually just rub the two surfaces together in order to scrape them clear of these oxides, press them together, and they will make a metallic bond.

00:30:47
Speaker 2: So like if I rub my copper pipes, I know I'm stuck on the copper pipes. I rubbed the copper pipes together to clean them off and press them together. Is that going to be enough or know where we back to. Your copper pipes are probably not pure copper.

00:31:01
Speaker 1: It depends on the purity of your copper. But this is something you can actually accomplish like down here on Earth without special materials. It was first demonstrated in the mid seventeen hundreds by a guy in England who just took two lead balls and he pressed them together and twisted them and rubbed them together and the two pieces would join. This seemed like magic at the time. It's like without any heat, he's somehow turning two lead balls into one single thing of lead.

00:31:28
Speaker 2: Did they burn him at the state.

00:31:31
Speaker 1: Him and all the Vermont farmers, Yeah, exactly. This was in the or mid seventeen hundred, so seventeen twenty four by a guy named Reverend Dsagular I'm not sure if he's actually French, and it's like Disagulae, but he demonstrated this phenomenon to the Royal Society and published the details in a scientific journal at the time. So this is something you can accomplish with pretty normal materials, and then archaeologists discover the humans have actually been doing this for thousands of years. You don't need heat in order to accomplish welding as long as you have pressure and you have two clean materials. So people have found like gold boxes made by cold welding that date to like seven hundred BC.

00:32:18
Speaker 2: Okay, so this is going to be potentially another silly question. So these boxes were gold, and so if having them together is enough for them to weld, how do we know that they hadn't like that this gold wasn't like lining a wooden box and over time it cold welded on its own, Like, how do we know that they did it purposefully?

00:32:41
Speaker 4: Oh? Yeah?

00:32:41
Speaker 2: Or how do we know they didn't use hot welding.

00:32:44
Speaker 1: If you do hot welding, you can see the effects at the intersection, like you can see that it's been melted. It changes the chemical composition a little bit, and then there's a transition there between the heated part and the not heated part. So cold welding does look different from hot welding, so we can tell that this was not hot welded gold. Obviously, people have been doing goldsmithing also for thousands of years using heat and hammering, but these are cold welded materials. But you're right, it could have been accidental and probably the discovery of it was accidental. Somebody for some reason accidentally squeezed two pieces of gold together and discovered they had welded. And you know, I hope that was a happy accident for whoever was doing that.

00:33:24
Speaker 2: Oh, happy accident.

00:33:26
Speaker 1: But it's sort of amazing that you can do this, you know. On one hand, it seems sort of crazy, like you stick two things together and they like talk to each other, and they like intermingle themselves and click together. It seems kind of impossible to get everything lined up right. On the other hand, it seems kind of obvious because copper is copper. Like you make copper crystal over here and you make copper crystal over there. They're gonna have the same atomic spacing. It's basically the same stuff. It's ready to get clicked together. You push on it a little bit, and those atoms are just gonna happily line up and interact with their new neighbors. It doesn't really matter that they used to be separate. When to push them together, they're ready to grab onto each other.

00:34:07
Speaker 2: I feel like once you explain the concept to me, it's less surprising to me that it happens and more surprising to me that it doesn't happen, Yes, all the time. Are there examples of it happening when it's not when it wasn't intended to happen.

00:34:22
Speaker 1: Oh, there's lots of examples of it happening when it wasn't intended, especially in the space program, which we'll get into in a little bit. It's a little bit complex to make it happen for more complex materials, like if you imagine something that's an alloy or an ionic solid. These things really are more complex because they have lots of different components to them, and so getting everything lined up is more tricky. I think it is possible in principle to cold well things that are not like pure copper or pure gold. It's just harder. You basically need more pressure and more time and a little bit more luck to make sure everything actually does line up against itself. But we have used cold welding, and there's been examples of on purpose and accidental cold welding in the environment of space.

00:35:06
Speaker 2: All right, Well, before we get to welding in space space space, let's take a commercial break. Okay, we're back, all right, So you promised me stories about cold welding happening when we didn't want it to happen. But you told me that I had to wait till we got to the space part of the episode, which is almost certainly, you know, always my favorite part of any episode. So tell me about how space poses unique challenges and benefits for welding.

00:35:48
Speaker 1: Yeah, so space is sort of an obvious place you might want to do cold welding because there's no atmosphere, which means there's nothing to oxidize your materials. You have a piece of bare pure cop or whatever, it's gonna stay bare pure copper. You don't need to like scrub the surface to get the oxides off because there's no oxygen to make any oxides, So there's no like gas in the middle there to interfear, no impurities, Your bare stuff stays bear stuff.

00:36:15
Speaker 2: Have we used that to our advantage yet? Like, do we are there things that we decide we're just gonna put together in space because we're gonna send clean stuff up there and then assemble it with cold welding while it gets up there.

00:36:26
Speaker 1: You know, it seems like an obvious application, but I couldn't actually find an example of times this had been done intentionally and It seems to me like a great idea because having any sort of like intense heat in space seems pretty dangerous. Like the story you told us was terrifying to me, because you know, on Earth, you accidentally bring your heat too close to something, it melts or maybe you get a burn or something. But in space you destroy your spacesuit. Right, everything is so much more dangerous out there in space. It seems like it would be awesome to do welding without the danger of heat or like super high electrical arcing.

00:37:02
Speaker 2: Yeah, space sucks.

00:37:03
Speaker 1: Space sucks exactly. I did find a couple of examples of accidental space welding go on. In nineteen sixty five, Gemini four was the first American spacewalk. So they sent the guy out there, They open the hatch. He goes out there, he space walks all around, He's having a great time. They asked him to come back in. He actually didn't want to come back in because he was having so much fun. And when he came back in and they couldn't close the hatch again, the hatch was like stuck in the open position. They had to like really yank on it, which is not a situation you want to be in when you're out in space and you like can't close the door.

00:37:40
Speaker 2: No, no, you know. Apparently apparently the same thing happened during the first Soviet space walk. Are you familiar with this story?

00:37:46
Speaker 1: No, what happened.

00:37:47
Speaker 2: It doesn't have to do with cold welding. But Alexei Leonov went out to do the first space walk ever because I think they beat us to that too, and he couldn't get back in, and he's known to exaggerate his stories a bit like astronauts. That's right, Well, I would too if I was an astronaut. But I think, you know, like his suit. You know, he got out into the vacuum of space and his suit sort of like puffed up a little bit because you know, you pressurize your suit, and after it got a little like puffy. Or My understanding is either he couldn't get through the hatch or he couldn't like bend the way he needed to, so for a second there he couldn't get back in. But I guess in both cases they managed to get the astronaut back in and bring them back home safely, thank goodness.

00:38:26
Speaker 1: Who Well, in this case, NASA was trying to figure out like why did the hatch get stuck? Kind of an important thing to understand, And one of the initial suggestions was cold welding. That maybe, like the outside of the hatch cold welded itself to the surface of the spaceship. And you know, this is the kind of thing we're not used to worrying about. On the surface of the Earth. You don't worry about like your keys cold welding themselves to your car and stuff, because everything's covered, right, this pint or this oxidation or just plain dirt is keeping stuff from cold welding. But in space this is maybe something we had to worry about, Like anytime two metals touch, are we gonna worry about them like spot welding themselves together? Oh my god, I know. And actually, if you look online, there's a lot of lore about how this is an example of cold welding. But you dig a little bit deeper and it turns out it's actually not cold welding at all. When it came back down to Earth, the engineers dug into it and they found out it was just a stuck spring that didn't compress right. So like very normal door not working.

00:39:24
Speaker 2: Okay, so you gave us like a psych moment could be cold welding, but it's not. Is there are there any actual cold welding examples in space or are you just gonna keep messing with us?

00:39:36
Speaker 1: No, there is one real example, and this is the Galileo probe. And so this is a probe which went out to explore the Solar System and take pictures of Jupiter, for example, but it got delayed.

00:39:48
Speaker 2: Wait, NASA, there were delays in a NASA project.

00:39:50
Speaker 1: I know, shocking, right, And the spacecraft was like moved across the country multiple times, and during those transports, like shaking a bunch, and the lubrication that was supposed to protect the metals from cold welding together eroded away. And so when they launched this thing, and then they were supposed to unfurl the high gain antenna, the thing that was going to send us data back with images of Jupiter on it, it turns out that several of the metal struts had become cold welded together, and so it couldn't unfurl.

00:40:23
Speaker 4: No.

00:40:24
Speaker 2: Oh, and they even planned for it, and that did planning didn't work. Oh my gosh, I'm getting stressed just thinking about it. They did they find a way around it, or were they just stuck.

00:40:34
Speaker 1: They never got the high gain intended to work. Fortunately, there's always like redundancies on these craft, and they had the low gain antenna which was not designed to be used to like send data. It was more like for control systems. But they had to reprogram it and use it to send the actual pictures, which like delayed people seeing these images of Jupiter. But of course you know, we got them. It's just more like downloading on one of those slow modems rather than your high speed internet.

00:40:58
Speaker 2: Yeah, but still, oh my goodness, those engineers they figured it out, Thank goodness. I'm sure there were a lot of PhD students who were like, I can stay in grad school for another year or two, that's fine, but at least I'm gonna get my images eventually.

00:41:09
Speaker 3: I know.

00:41:10
Speaker 1: It's like the old days when you're down the neck picture and you're seeing one row of pixels at a time come across the screen and you're like, what am I looking at? I download the right picture or not a worry?

00:41:19
Speaker 2: And you go do my computer freeze or should I shut it down? Or should do I wait? Yeah?

00:41:24
Speaker 1: Fun, So in practice, it's not really that big a problem. It's not like when we build big space stations or space settlement, we're gonna have to worry about cold welding all the time. You really need like perfectly clean materials and they have to be pressed together for long enough for this to happen. So not really a huge concern in.

00:41:41
Speaker 2: Space, all right, So are there any other applications that we should chat about.

00:41:46
Speaker 1: People have discovered that cold welling can also be really helpful for nano circuits, Like you want to build really really small electronics in these days, the trend is to build like tinier and tinier microchips, which use less power and can be squeezed into all sorts of scenarios, so you can have like microchips in your cereal or whatever you need, you know exactly, you know, they can count your calories for you, right exactly. Anyway, nanoscale fabrication is tricky because spot welding is very hard. When it's really really tiny, you know, basically need like tiny sources of heat or you need this electricity to arc perfectly. Well, what they've discovered is that you can do cold welding for nanofabrication. I read this paper that came out of nature like ten years ago where they had ultra thin gold nano wires and you just bring them together and if the edges touch, boom, they cold well together within seconds. You don't even need a lot of pressure. Whoa yeah. And these wires once they've cold welded together essentially as like a near perfect bond. They did all these tests of like the electrical conductivity and a crystal orientation, and it's as if it was always just one wire. So this means, Kelly, do need your children, with like their delicate little fingers to do tiny little arc welding on our nano circuits?

00:43:06
Speaker 2: Oh? Good, good, because that was gonna be a thing.

00:43:10
Speaker 1: But it might be that we can send your kids down the space to do space construction jobs without arc welding. Right, how do you.

00:43:16
Speaker 2: Feel about that any of the episode? Nope, none of the episode. None.

00:43:21
Speaker 1: If your kids grow up and they want to work construction in space, you're gonna say no, yes, no. You didn't think about that deeply at all. It was just like a straight up no.

00:43:32
Speaker 2: I hope they don't go that route, but if they do, I will support them grudgingly.

00:43:38
Speaker 1: Well, I think cold welding is super fascinating, and what it tells me is that there's a lot more to understand about how these particles weave themselves together to make the sort of macroscopic materials that we're familiar with, and that there's still sort of magic left. I mean, I'm not talking about Vermont Farmer magic. I'm talking about accomplishing things that seem impossible to us, you know, just like sticking two pieces of metal together and make them into one metal. Every time there's an advancement in material science, it feels almost like we're creating real magic.

00:44:06
Speaker 2: I agree, but I still feel like the main takeaway for me is one more thing to worry about in space because cold welding going wrong. We should have gotten that in the book.

00:44:17
Speaker 1: I think you already had like a thousand reasons not to build in space in your book. You don't need another reason. Yeah, I guess beat that dead horse. And as much as I'm jokingly negative about chemistry, it does really give us access to the microscopic nature of materials. It's the easiest way to see these emergent properties, to see how something can be sticky or shiny or brittle because of the way the particles inside it interact and cold welding is just another manifestation of that that our world really is dominated by the microscopic rules that somehow bubble up to make this incredible variety of behaviors and phenomena.

00:44:56
Speaker 2: Yeah, I gotta give it to you chemistries pretty much.

00:44:58
Speaker 1: It is kind of magic, dang it. Yeah.

00:45:02
Speaker 2: Yeah, my chemistry professors kept telling me that, but but now I believe it.

00:45:09
Speaker 1: Finally, thirty years later, I've learned it too.

00:45:13
Speaker 2: Thirty Oh, why you might be on it?

00:45:17
Speaker 1: All right? Well, thanks Kelly for joining us on this tour of chemistry and magic and this dive into how cold welding works.

00:45:24
Speaker 2: Thanks for having me.

00:45:25
Speaker 1: All right, Everyone, keep wondering about how the universe works and send us your questions. This episode was inspired by listeners who wanted to understand how cold welding works. If you have ideas for things you'd like to hear us explore, please don't be shy. Write to me two questions at Danielandhorge dot com. Tune in next time. Thanks very much. For more science and curiosity, come find us on social media, where we answer questions and post videos. We're on Twitter, Discord, Instant, and now TikTok. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.