Why do two pieces of metal get welded together when they touch in space?

This phenomenon is not mysterious, known as cold welding, has been widely used in industrial production. Although it is called cold welding, it is not because the space is too cold, the two pieces of metal are frozen together.

Humans have learned a lesson from this phenomenon when exploring space. NASA launched the Galileo Jupiter probe in 1989. All inspections were normal before launch, but when the probe entered space, it found that the main antenna could not be fully opened. Fortunately, there was a backup antenna, otherwise the entire mission could only be declared over.

But the power of the backup antenna is too small, which greatly reduces the transmission efficiency. To this end, the Galileo Jupiter probe had to abandon several scheduled exploration missions, such as the detection of Io.

Later, scientists found through research that the reason for this failure was that the folded main antenna of Galileo was partially stuck together due to the phenomenon of cold welding, which prevented the main antenna from being fully opened after lift-off. After this lesson, subsequent spacecraft will pass special treatment to deal with the possible cold welding phenomenon.

So why does cold welding occur?

We usually heat two objects in contact and then fuse them together. Cold welding means welding objects together without the need for heat.

Cold welding occurs based on the phenomenon of diffusion. As we all know, matter is composed of atoms, and particles such as atoms are constantly doing irregular thermal motion, which is also called Brownian motion. When objects are in contact with each other for a long time, the particles on the contact surface will spread due to Brownian motion, such as flattening the surface of two lead blocks and pressing them tightly together. After a long time, they will stick together.

Although cold welding may occur between non-metallic substances and different metals, the occurrence of cold welding also requires certain conditions.

In fact, if two pieces of metal are in contact in space, if they are only in contact, cold welding will not occur, and certain conditions are required, such as applying pressure and rubbing each other. Ultrasonic welding uses the friction generated by high-frequency vibration to fuse two objects together. In short, it is necessary to provide enough energy to make the atoms between the contact surfaces hold hands smoothly, so that the cold welding phenomenon occurs. In addition, the compatibility of the two materials also determines the probability of cold welding. For example, under suitable conditions, cold welding is more likely to occur between two pieces of gold.

Space is a high vacuum environment. When two pieces of smooth metal are in contact, and there is no oxide layer on the contact surface, the contact will be very close because there is no air obstruction. The pieces of metal stick together and cold welding occurs. Moreover, space is an oxygen-free environment. Even if the oxide layer is worn off, the metal surface will not be oxidized again, which further increases the probability of cold welding.

Cold welding is not easy to occur on the earth, because the metal is exposed to the air, most of the surface has an oxide layer, and there is usually an air layer between the two contact surfaces.

The cold welding of the folded antenna of the Galileo Jupiter probe was caused by improper handling of the active interface and subsequent vibration and friction, resulting in cold welding.

Cold welding can easily cause some moving parts on the spacecraft to fail, such as accelerated bearing wear, making it difficult for solar cell wings to stretch. In order to deal with this hazard in the aerospace field, it is possible to choose mating materials that are not prone to cold welding, and to coat the contact surface, or apply a little lubricant, which can reduce the friction between metals and effectively reduce the probability of cold welding.

Although the phenomenon of cold welding is harmful in one respect, it also has a good side. Since high temperature heating is not required during the welding process, the material has almost no shrinkage rate, so the welding port is not easy to produce tiny cracks, so the strength after welding is high, and it is not easy to crack and fall off.