The magic of cold metal welding

In everyday life, when we put two pieces of metal of the same material close together, nothing strange will happen. But in the space environment, two pieces of metal might fuse into one piece and never separate.

The Galileo probe’s high-gain antenna was unable to fully open due to the cold welding effect
The antenna won’t open, disrupting the Jupiter mission

On October 9, 1989, NASA launched Galileo, a probe to Jupiter. To ensure Galileo can receive command signals from Earth, which is so far away from Jupiter, NASA has specially equipped the spacecraft with a “high-gain antenna” capable of transmitting 80 kilobytes per second. In order to reduce the transport volume, the high-gain antenna is designed in an umbrella shape that opens only when it is needed to receive a signal.
After a year and a half of space travel, Galileo arrived near Jupiter on April 10, 1991. But scientists discovered that Galileo’s high-gain antenna couldn’t open anyway. They put forward thousands of possible reasons for this, and they were all rejected. Eventually, they suspected that the reason was that the high-gain antenna’s umbrella structure would not separate because of the phenomenon of cold welding. As a result, NASA has resorted to Galileo’s low-gain antenna, which transmits at a rate 1/10,000th that of its high-gain antenna. Galileo’s mission to IO had to be canceled because the transmission speed was so compromised that it was only able to send back images of Jupiter every few minutes instead of every few weeks. This is very regrettable.
Cold welding, because the atoms are spreading

When it comes to welding, the first thing that comes to mind is high temperature: whether it’s gas welding, electric welding, laser welding, or friction welding, the principle is to melt metal with enormous heat and resolidify it into a complete part, the essence of which is the diffusion of metal atoms between the solder and the solder. However, high temperatures are not necessary for metal atoms to spread, and the phenomenon of cold welding is a prime example.

Feynman, American physicist

The atoms pass through the contact between the two metals and spread out

Cold welding usually only happens in space, because when two pieces of metal are in space, there is no air between them to separate them. Feynman, an American scientist known as one of the funniest scientists, once explained the phenomenon of cold welding in this way: In a vacuum, when two metals come into contact with each other, they can’t tell which side of the metal they are from because there is nothing between the atoms on either side of the contact surface. The atoms spread out from each other, and gradually the atoms in the two metals get mixed up and eventually the two metals merge. But if there are other non-homogeneous atoms, such as air or an oxide layer, the metal atoms will realize that they belong to different parts and will not fuse together.
What are the conditions for cold welding?

In real life, cold welding of metal rarely happens. Some people have been more prone to cold welding of gold foil as the object, do a finger pinch gold foil experiment, the results of the two gold foil and not connected together. You might think that the cold welding didn’t happen, perhaps because the experimenter’s fingers weren’t strong enough. So, is it possible to cold weld metals if the pressure is high enough?
At the Bank of England’s vault in London, nearly 5,000 tonnes of gold bullion, each weighing 12.4kg, are neatly stacked. The overall mass of the gold was so great that the bank’s vault was split into eight vaults to distribute the weight, to keep it from sinking underground. Each nugget in the lowest pile is weighed down by 348 kilograms of other nuggets above it. However, even the lowest nugget was not cold welded. Why is that?

You can’t cold-weld two sheets of gold, even if you have a mighty diamond finger

The gold bricks in the London vault did not cold weld under the weight

Most metals are crystals held together by metallic bonds, so you can imagine metal atoms holding hands with each other. If another piece of metal comes in, you have to break the old metal bond and make a new metal bond. To break existing metal bonds, extra energy is needed. Scientists speculate that the cold welding of the antenna on Galileo derives its energy from the vibration of the engine and gyroscope during the rocket launch and from the heat generated by friction.
When space equipment is operating on the ground, friction from vibration wears away the oxide layer on the surface of the metal, but because of the oxygen in the ground air, the exposed metal is quickly reoxidized. Under the action of oxidation layer, oil stains and other factors, even if the contact of metal is close, it is difficult to occur cold welding. In space, however, the metal exposed by friction can’t form a protective oxide film any further. Friction heats up the interface between the two metals and briefly breaks the metal bond, creating the conditions needed for cold welding.

Metal oxide film will prevent metal cold welding phenomenon

To avoid similar situations in the future, NASA simulated cold welding on the ground. The simulations showed that Galileo’s antenna produced only a very weak cold weld, with a bonding force of 0.3 newtons, which still exceeded the separation force generated by the antenna’s spring structure.
The existence of cold welding will bring many problems to the normal operation of space equipment, such as accelerating bearing wear, or making it difficult to fully open the solar cell wing. In order to avoid cold welding, the most direct solution is to use less prone to cold welding metal, or to apply lubricants to the surface of the moving parts. On the one hand, the lubricant can isolate the metal, on the other hand, it can reduce the friction between the metals, to avoid cold welding after the metal bond is damaged.

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