Cold welding

from Wikipedia, the free encyclopedia

When cold welding to refers to the phenomenon, mainly metallic workpieces of the same material so connect at room temperature that the compound is the "normal" welding comes very close; hence the name comes from. This phenomenon was discovered in the 1940s.

Cold pressure welding

Cold pressure welds are made under high pressure and, in contrast to other welding methods, below the recrystallization temperature of the individual parts. The two parts remain in the solid state, but plastic deformation with a strong approach of the contact surfaces is necessary. The intensive contact between the two contact surfaces creates a stable connection between the two workpieces. For a good connection you need materials with sufficient cold formability (e.g .: copper and aluminum with and with each other). In individual cases, a connection is also suitable for permanent power lines. Here, prior degreasing and tearing of the surface oxide helpful.

root cause

In the case of workpieces with a high surface quality (contact surfaces extremely flat and smooth), compared to those with poor surface quality, many of the metal atoms touch each other at the two interfaces and the forces of attraction between them connect them to form a stable atomic lattice . They cannot then be separated from one another again without destroying the surface quality. The phenomenon increases when pressure is applied or in the absence of air (vacuum).

Examples

The effect occurs z. As in gauge blocks when they wrung are so very close lie together and longer time can not be separated again.

It also occurs when metal surfaces come into very close contact (e.g. screws made of stainless steel in threads) when pressure is applied (and the resulting friction). If the chromium oxide layer on the surface, which is passivating against atmospheric oxygen, does not have similar deformability properties as the metal underneath, it will be damaged, which can then lead to cold welding. If you try to separate the two parts again, further surface damage and deformation can occur through abrasion , up to and including the breakage of the workpiece or tool. This can be remedied by lubrication or coating with a very fine, non-metallic sliding film. To prevent cold welding during thread cutting, constant lubrication must be ensured during machining and tool downtimes avoided.

This principle is also used when creating so-called wound connections .

Cold welding is also a problem in the dry production of nanoparticles . It can be reduced by adding grinding aids during the comminution (grinding) of the particles, which mix between the particles to be ground and thus prevent them from sticking together. As a grinding aid z. B. used in the solid state of carbon dioxide.

Powder metallurgy , on the other hand, makes use of the cold welding effect . Large surfaces occur with powders. This favors the cold welding effect when the metal powder is compacted under high pressure into so-called green compacts. During the subsequent heat treatment, known as sintering , the powder particles are firmly bonded to their contact surfaces by diffusion of the metal atoms.

"Cold welding" of plastics

Cold welding is also understood to mean the diffusion bonding of thermoplastics with solvent-based wet adhesives .

The joining of plastics (especially PVC ) at ambient temperature is often referred to as "cold welding", but the correct term for this process, which is based on a completely different basis, is solvent welding, because the plastic polymers are made using a liquid welding agent ("film adhesive"). , Tetrahydrofuran ) "swell" at the wetted interface. A common application is the welding of PVC film webs, e.g. B. in the creation of garden ponds.

When solvent welding PVC floor coverings, employees are exposed to tetrahydrofuran. The BG / BGIA recommendations evaluate the use of PVC cold welding agents with a tetrahydrofuran content of a maximum of 90%. Only one of the measurements on which the recommendations are based showed an exposure above the limit value, all other measurement results were below half the occupational limit value . Accompanying measurements with a direct display photoionization detector showed that the short-term requirements were met. No respiratory protection is required when used properly.

literature

Individual evidence

  1. German statutory accident insurance e. V. (DGUV): DGUV Information 213-719 - BG / BGIA recommendations for risk assessment according to the Hazardous Substances Ordinance: Use of cold welding agents for PVC floor coverings. Retrieved November 25, 2019 .