Inert noble gas helps protect the hot metal during welding.
The welding process heats metal to high temperatures. While the heat is essential to melt the metal, it also has the unwanted side-effect of accelerating oxidation reactions. Oxygen and water vapor can react with the hot metal to create flaws and weaken the weld. Noble gases are often employed as "shielding gases" to avoid this undesirable outcome. Does this Spark an idea?
Function
Shielding gases temporarily surround the metal and displace the air around the weld, removing oxygen and other gases that could initiate unwanted reactions. The shielding gas must be inert or chemically nonreactive; otherwise it might react with the metal itself. The three most important shielding gases are helium, argon and carbon dioxide. Out of these three, helium and argon are classified as noble gases, meaning they are found in the last column of the periodic table.
Arc Welding
In arc welding, the high voltage on two electrodes ionizes the gas separating them so that current can flow between them and create an arc. Helium is a much smaller atom than argon and its outer electrons are much more difficult to remove, so when helium is used as a shielding gas instead of argon, the voltage is higher given an equal amount of current. The higher voltage results in a greater amount of available heat.
Properties
Helium is also a better conductor of heat than argon, so welding with helium tends to produce a more even distribution of heat in the fusion zone. Welding with argon, by contrast, tends to yield a hot core surrounded by a cooler zone. Because argon is easier to ionize, it also tends to provide a more stable arc, meaning it's easier to maintain a steady and continuous arc current.
Uses
Another consideration is the flow rate. Argon is much more dense and heavy than helium, so it provides better protection at a low flow rate. That's why welding with helium typically requires a flow rate two to two and a half times the flow rate for a comparable argon weld. Since helium is lighter than air, however, welders can reduce the flow rate somewhat if they're welding the metal from overhead. Given the different properties of these two noble gases, welders must weigh the advantages and disadvantages of each when choosing the shielding gas for a specific application.
Tags: flow rate, helium argon, much more, noble gases, react with, react with metal