Cold or contact welding

Cold or contact welding was first recognized as a general materials phenomenon in the 1940s. It was then discovered that two clean, flat surfaces of similar metal would strongly adhere if brought into contact under vacuum. According to this phenomenon of the 1940's two clean, flat surfaces of similar metal would strongly adhere if brought into contact under vacuum. Its lately known that by pressing the metals tightly together, increasing the duration of contact, raising the temperature of the workpieces, or any combination of the above could add on to the first contact. As per research, even for the very smooth metals, only the high points of each surface, called 'asperites,' touch the opposing piece which means that as little as a few thousandths of a percent of the total surface is involved in the adhesion and these small areas of taction develop strong molecular connections. If the surfaces are sufficiently smooth the metallic forces between them ultimately draw the two pieces completely together and eliminate even the macroscopic interface. Electron microscope investigations of contact points reveal that an actual welding of the two surfaces takes place after which it is impossible to separate the former asperitic interface. the cold welding effect is eliminated or reduced when exposed to oxygen or certain other reactive compounds which produces surface layers, for example, a metal oxide which has mechanical properties similar to those of the parent element (or softer), here surface deformations will not crack the oxide film.

Applications

Powders in powder metallurgy present large surface areas over which vacuum contact can occur and they use cold welding to best advantage. For instance, a 1 cm cube of metal comminuted into 240–100 mesh-sieved particles (60–149 μm) yields approximately 1.25×106 grains having a total surface area of 320 cm2. This powder, reassembled as a cube, would be about twice as big as before since half the volume consists of voids.

it is important to obtain minimum porosity (that is, high starting density) in the initial powder-formed mass if a sturdy final product is preferred. Minimum porosity results in less dimensional change upon compression of the workpiece as well as lower pressures, decreased temperatures, and less time to prepare a given part. Careful vibratory settling reduces porosity in monodiameter powders to less than 40%. Large increase in net grain area will enhance the contact welding effect and noticeably improve the 'green strength' of relatively uncompressed powder whereas decrease in average grain size does not decrease porosity. In space applications moulds may not even be required to hold the components for subsequent operations such as sintering because cold welding in the forming stage is adequate to produce usable hard parts
Hard monodiameter spheres packed like cannonballs into body-centered arrays gives a porosity of about 25%, much lower than the ultimate minimum of 35% for vibrated collections of monodiameter spheres. (The use of irregularly shaped particles produces even more porous powders.) Selected range of grain sizes, typically 3–6 carefully chosen gauges in most terrestrial applications reduces porosity. Such mixtures theoretically permit less than 4% porosity in the starting powder but it becomes 15–20% more with binary or tertiary mixtures. powders containing particles in a wide range of sizes can approach 0% porosity as the finest grains are introduced. Even the introduction of vibration or shaking to free movement cannot induce closest configuration in powder mixtures. Sizeable theoretical and practical analyses exist to assist in understanding the packing of powders. Its seen that gravitational differential settling of the mixture tends to segregate grains in the compress, and some degree of cold welding occurs immediately upon formation of the powder compress which generates internal frictions that strongly impede further compaction. Moderate forces applied to a powder mass immediately cause grain rearrangements and superior packing. Specifically, pressures of 105 Pa (N/m2) decrease porosity by 1–4%; increasing the force to 107 Pa adds only an additional 1–2%. greater force is required mechanically to close all remaining voids by plastic flow of the compressed metal and distinct physical effects of particle deformation and mass flow become significant at still higher pressures or by the application of heat.