Cold welding
Cold welding is known as a phenomenon to be able to connect predominantly metallic workpieces of the same material with each other at room temperature so that the connection to the "normal " welding is very close; therefore stirred the term. This phenomenon was discovered in the 1940s.
Cause
For parts with high surface quality ( contact surfaces extremely flat and smooth design ) touch each other as compared to those with poor surface quality a lot of the metal atoms at the two interfaces and the attractive forces Selbiger each other connect them to a stable atomic lattice. You are then not separate again without damaging the surface finish. The phenomenon intensifies when pressure is applied or the absence of air (vacuum).
Examples
The effect occurs, for example with gauge blocks, if they are " wrung ", ie are very close and long time can not be separated again.
It also occurs on the very close contact of galvanized surfaces (eg screws made of stainless high-grade steel in threads ) with the use of pressure ( and resulting friction). If the over the air oxygen passivating oxide layer on the surface does not have similar deformability as the metal underneath, they will be damaged, which can then cold welding occur. If one tries to separate the two parts again, further surface damage and deformation can occur due to abrasion, to the breaking of the workpiece or tool. Remedial action is taken here by lubrication or coating with a very fine non-metallic lubricating film. Counter cold welding during thread cutting must be paid to permanent lubrication during machining and tool downtime can be avoided.
Also in the manufacture of so-called winding connections makes you look this principle advantage.
The cold welding is also a problem in the production of dry nanoparticles. You can reduce it by grinding aids are added during comminution (grinding ) of the particles, the mix between the particles to be ground and prevent sticking together. As a grinding aid, for example, in the solid state befindliches carbon dioxide is used.
The Kaltverschweißungs effect advantage makes itself, however, the powder metallurgy. In powders occur large surfaces. This promotes the effect of the cold welding process in the compaction of the metal powder under high pressure to the so-called green sheets. During the subsequent heat treatment, the so-called sintering, the powder particles together at their contact surfaces by diffusion of metal atoms into a solid compound.
" Cold welding" of plastics
The combination of plastics ( especially PVC ) at ambient temperature is defined, although often referred to as " cold welding", but the correct term for this, based on an entirely different basis method is solvent welding, since in this case the plastic polymers with the aid of a liquid welding agent ( " foil glue" tetrahydrofuran) are " swollen " at the wetted interface. A common application is the welding of PVC sheeting, such as the plant of garden ponds.