Forge welding
[2] It may also consist of heating and forcing the metals together with presses or other means, creating enough pressure to cause plastic deformation at the weld surfaces.
[3] The process, although challenging, has been a method of joining metals used since ancient times and is a staple of traditional blacksmithing.
This generally makes forge welding more versatile than cold-diffusion techniques, which are usually performed on soft metals like copper or aluminum.
This squeezed out air pockets and melted slag, bringing the iron grains into close contact to form a solid block (billet).
Many items made of wrought iron have been found by archeologists, that show evidence of forge welding, which date from before 1000 BC.
Because iron was typically made in small amounts, any large object, such as the Delhi Pillar, needed to be forge welded out of smaller billets.
This method of pattern welding first appeared around 700 BC, and was primarily used for making weapons such as swords; the most widely known examples being Damascene, Japanese and Merovingian.
Although it is possible to forge weld copper-based alloys, it is often with great difficulty due to copper's tendency to absorb oxygen during the heating.
With iron or steel, the presence of even small amounts of copper severely reduces the alloy's ability to forge weld.
Originally powered by waterwheels, modern mechanical-hammers can also be operated by compressed air, electricity, steam, gas engines, and many other ways.
[20][21] Modern forge-welding is often automated, using computers, machines, and sophisticated hydraulic presses to produce a variety of products from a number of various alloys.
Flat stock is heated and fed through specially-shaped rollers that both form the steel into a tube and simultaneously provide the pressure to weld the edges into a continuous seam.
[23] Diffusion bonding is a common method for forge welding titanium alloys in the aerospace industry.
These are resistance forge-welding techniques where the press or die is electrified, passing high current through the alloy to create the heat for the weld.
[25] Shielded active-gas forge-welding is a process of forge welding in an oxygen-reactive environment, to burn out oxides, using hydrogen gas and induction heating.
Care must be taken to avoid overheating the metal to the point that it gives off sparks from rapid oxidation (burning), or else the weld will be poor and brittle.
To produce the right amount of hardness in the finished product, the smith generally begins with steel that has a carbon content that is higher than desired.
In some cases the forge-welded objects are acid-etched to expose the underlying pattern of metal, which is unique to each item and provides aesthetic appeal.
Larger objects required a bigger heat source, and size reduced the ability to manually weld it together before it cooled too much.
[32] Early examples of flux used different combinations and various amounts of iron fillings, borax, sal ammoniac, balsam of copaiba, cyanide of potash, and soda phosphate.
The 1920 edition of Scientific American book of facts and formulae indicates a frequently offered trade secret as using copperas, saltpeter, common salt, black oxide of manganese, prussiate of potash, and "nice welding sand" (silicate).
