|MLA Citation:||Bloomfield, Louis A. "Question 707"|
How Everything Works 19 Jan 2018. 19 Jan 2018 <http://howeverythingworks.org/print1.php?QNum=707>.
An arc welder passes an electric current through the air from the pieces to be joined to a welding rod. The rod becomes so hot as the result of this arc that it melts and joins with the other pieces of metal, binding them together permanently. This scheme only works with relatively non-flammable metals such as steel. Aluminum or titanium will burst into flames when the arc starts. To joint these flammable metals, the arc has to be protected by a shroud of an inert gas such as argon or helium. TIG and MIG welding are based on this inert gas approach (the "IG" part of the names). In Tungsten-Inert-Gas (TIG) welding, an arc passes from the pieces being joined to a tungsten electrode. Tungsten has such as high melting point that it survives this arc and another piece of metal, the welding rod, is fed into the arc where it melts to form the joint. In Metal-Inert-Gas (MIG) welding, the arc passes from the pieces to a metal welding rod. This system resembles normal arc welding, in that the welding rod melts to form the joint, however now the arc is shrouded by a flow of inert gas so that there is no oxygen around to support combustion. Flammable metals can be welded with TIG or MIG welding and so can non-flammable metals.
As for oxygen-acetylene welding, here a very hot flame is used to heat the pieces involved to very high temperatures. A welding rod that melts at a slightly lower temperature than the pieces themselves is then used to join the pieces. The advantage to using this system is that it doesn't pass a current through the pieces and doesn't rely on their electric properties. The current of an arc welder could damage thin materials but an oxygen-acetylene flame should not (assuming they are relatively non-flammable metals). I'm sure that the metallurgical characteristics of the joints vary from system to system, but I can't make any useful statements about this. For a more detailed discussion of when and where to use each technique, you'll need a more experienced person than me.