If I have two solid objects, like two iron blocks, and put them next to each other, they stay independent objects. I can move them back apart no matter how long they stay next to each other. They typically do not combine into a single object. But if I heat up the materials into a liquid state, then they will be a single solid after they cool down. Are there any solid materials which can achieve the same feat without needing to get into a liquid state? I mean traditional solid objects rather than semi-solids like putty or stuff like that. And I also don’t mean that they combine through some sort of external chemical reaction like corrosion or rusting. I’m wondering if there are any materials where their solid crystalline structures will merge with the solid crystalline structures of another piece of that same material so that two independent solids become one just by being placed in contact with each other.
I know there are things like ice which may appear to do this, but it’s really because of melting at the surfaces. The pressure of the two surfaces pressing together causes those surfaces to melt and then that thin layer of liquid solidifies. At a minimum, is that thin layer of liquid necessary to cause two solids to join together?
I am pressed for time, but it sounds like you are talking about “cold welding”. I am not well versed on the subject, so even if I had more tIme, I don’t know if I could explain it.
What is not intuitive is that when you cut a fresh solid surface at ambient conditions, there is a layer of air molecules on that surface within nano-seconds. More layers follow quickly. So when you see two solids touching each other, there are layers of air molecules in between them. You can remove these air layers by cold welding or explosion welding etc or by the process of melting.
Semiconductors like Silicon need the surface to be free of air particles too, so that the circuits can be “drawn” on them. To achieve this you need very very high vacuum. The high vacuum is usually attained by cryogenic methods where you cool the air (gas) to such low temperatures and low pressures that the time for mono layer formation is in the 10s of minutes.
I am a ChemE and maybe wrong with the “drawing” of circuits part, but am very familiar with the vacuum /cryogenic technology needed to get to ultra low pressures.
Yes, many materials can be joined directly to like materials without a liquid phase ever coming into play. As others have mentioned, this is often called “cold welding,” and it happens most readily to metals in a vacuum.
But some metals will readily stick to other chunks of similar metals under less exotic conditions. Stainless steels and titanium alloys are particularly prone to a phenomenon called “galling,” wherein tiny patches of one part become effectively welded to another. The Wikipedia page on galling is itself galling—it’s pretty bad. To the best of my knowledge, the fundamental mechanisms for both galling and cold welding aren’t well understood, and they may well be the results of a single phenomenon. (If anyone has better information, I’m all ears).
When a stainless steel part is secured with a stainless steel bolt, the threads usually gall, even if they’re coated in grease. It’s worse with fine threads, and anti-sieze compound (which effectively puts dissimilar metal particles between the stainless parts) is one of the few ways to mitigate the problem.
The phrase “cold welding” is most often used to describe a physical phenomenon, not an actual joining technique. But there’s such a thing as “explosion welding,” which can fuse even dissimilar metals. That’s impractical with conventional (“fusion”) welding techniques. But at very high pressures—like those generated by high explosives—the conventional distinction between solids and liquids begins to lose meaning, so it’s not obviously liquid-free in the sense of the original question.
ETA: ninja’d on explosion welding!
The links above don’t seem to stress it, but cold welding and galling don’t actually require high pressure, vacuum, or repeated motion. When they talk about those factors, they are talking about joining random bits of metal together.
If you start with incredibly flat, clean surfaces, it’s the opposite: you have to be careful you don’t leave them wrung together, or they will weld.
Friction stir welding is probably on the border of the OP’s requirement. The welded metal doesn’t melt, but does get hot enough to soften. A great example closer to the requirement is the manner in which gauge blocks are wrung together.
This. When you talk about putting two samples of a material “next” to each other, they’re actually thousands or tens of thousands of atoms apart on average, only touching where two irregularities happen to meet. The percentage of surface that is actually touching by molecular standards is ordinarily negligible.
Gauge block - Wikipedia s are an exception: steel blocks that are so flat and smooth that they will stick to one another.
I haven’t used gauge blocks for many years, but I don’t recall a concern that once wrung, there was a timelimit less they get permanently joined (per the OPs question)
Is this the case?
I think the presence of oil on the mating surfaces might prevent a strong, permanent bond from forming. I wonder what would happen if you cleaned the mating surfaces to completely eliminate all foreign material (including any oil) and then brought them together.
It sounds like in typical solids, the irregular surfaces are so small that whatever parts are touching are so small that they don’t have any holding force. On our macro scale, it seems like they didn’t combine since they come apart so easily. But if the surfaces were shaped such that they would mate perfectly, then it would weld together perfectly.
Is the creation of crystals in a liquid solution an example of this on a tiny scale? For example, crystals will form if salt or sugar is dissolved in water. I guess the molecules get stuck together when they happen to bump into other molecules. I’m not sure if a single molecule meets the definition of a solid.
The come (or at least used to) with explicit instructions not to leave them wrung togethor overnight.
That doesn’t mean they will automatically be impossible to get apart after one night, but it does mean that your supervisor/manager will be very very very pissed off with you. And the first step isn’t “impossible to get apart”, it’s “the surfaces were damaged where they welded together”
You always clean the oil off before using them, and oil them before putting them away. You clean them with a slightly oily solvent, to stop them from rusting on the exposed suffaces, but it’s not oily enough to stop them cold welding if you leave them stuck together.
Perhaps I should also mention solder intermetallics:
When two peices of metal (perhaps copper) are soldered together, they are joined by a liquid (mostly tin), which then freezes into place. While the tin is melted, some of the tin melts into the copper, and some of the copper melts into the tim. Then, for the rest of the life of the joint, the copper and tin continues to migrate. The rate of migration depends on how warm the parts are, and it almost entirely stops at ordinary operating temperatures.
For normal electronic manufacture, the solder alloy and the component coating and the circuit board coating are choosen so that thin intermetallic layers are formed st the time of manufacture, and then the process (effectively) stops. But that is because the materials have been selected for that behavior, which is the result of decades of experience.
Or, if the surfaces are allowed to rub against each other under great pressure, or repeatedly for a long time, they will eventually polish each other and end up mating perfectly. This is why moving parts and screws seize up if you don’t use proper lubrication.
While the solder itself must (obviously) melt, it is not necessary for the terminal plating material (usually pure tin) or the terminal base material (usually copper) to melt when soldering. In fact, when using eutectic (63% Sn / 37% Pb) solder for a circuit card assembly, the peak temperature of the reflow oven is around 220 °C, which is a lower temperature than the melting temperatures of tin and copper. Instead, the tin plating and a small amount of copper dissolve into the solder. For this type of soldering, the tin plating and copper won’t melt, nor is it necessary for them to melt.
For other types of soldering, the plating material will melt. As an example, if both the solder and the plating are eutectic, then the plating will obviously melt into the solder, and a small amount of copper will dissolve into the solder. Or if you’re using lead free (e.g. SAC305) solder, then the plating material (eutectic or pure tin) will melt into the solder since the peak temperature during reflow will be around 245 °C. A small amount of copper will also dissolve into the solder.
In theory, when the mating parts or threads are made of similar materials that might happen. But corrosion is the most common cause of seized threads, even when the male and female threads are made from similar materials.
As I mentioned before, stainless-on-stainless threads are notorious for galling prior to any mutual polishing—and galling is basically micro-seizing due to cold welding.
What you’re describing about mutual polishing leading to cold welding isn’t impossible and it probably happens occasionally. But most seized parts and fasteners got that way because one or both pieces corroded.
That said, I’d be interested to read about real-world examples of parts that are known to have seized due to mutual polishing followed by cold welding.
P.S. Many people are surprised to learn that stainless steel can rust quickly if its naturally protective oxide layer is removed by abrasion or other means. (Rust is iron oxide, but the protective layer is chromium oxide). A stainless bolt in a stainless part can absolutely rust and seize.
If you look at meteorites, once the original fusing of the original fragments takes place, the boundaries between the various components and clasts still lookpretty darn sharp after 4 1/2 billion years.