Welding conjures a certain allure of intense temperatures and white-hot flying sparks mixed with a rugged charm. It is a process that requires a very skillful balance of strength and strict precision. Before the welder even fires his flame, having the right gas mixture is already half the battle won.
Why is the right gas mixture so important?
Shielding protects molten metal from reacting with atmospheric gases like oxygen, carbon dioxide, nitrogen and water vapour. A smooth welding process requires a careful selection of gases based on their properties. The wrong choice of gas can result in flawed and irregular welds.
What are Shielding Gases?
Shielding gases are inert or semi-inert gases that protect the weld from damage as a result of contact with atmospheric gases. They affect the amount of heat produced by the arc and the appearance of the resultant weld bead.
Inert Gases in Welding
Inert Gases (also known as noble gases) are colourless, odourless and chemically unreactive.
Pure argon is often used with aluminum and nonferrous metals. This gas is good for shielding flatly-positioned welds and welds in deep grooves.
Argon is suitable for easier starts, alternating current (AC) applications and for longer arcs at lower voltages. The addition of helium improves argon’s heat transfer properties and argon mixed with carbon dioxide or oxygen can help stabilise the arc.
Pure helium is ideal for welding magnesium, copper and aluminium. However, it creates an erratic arc which can result in spatter when working with steel. Helium is effective for mechanised welding but allows less room for error in manual welding.
A helium-argon mixture can be used to clean cathodes.
Semi-Inert Gases in Welding
Gases with low chemical reactivity are referred to as semi-inert gases. The right mixture of semi-inert gases at the right quantities can improve the quality of a weld.
Hydrogen can produce cleaner weld surfaces and better bead profiles for grades of stainless steel that are sensitive to oxygen. However, if used incorrectly, trapped gas can cause weld porosity and cracking under the bead in carbon and low-alloy steels.
Adding hydrogen to argon deepens the penetration and increases the speed of welding.
Nitrogen increases weld penetration and arc stability. Nitrogen mixtures can increase the mechanical properties of alloys containing nitrogen and prevent pitting corrosion as well as nitrogen loss from the metal.
Oxygen mixtures are also commonly used to shield welds. They stabilise arcs, minimise spatters and improve metal transfer.
Oxygen’s oxidising properties make it unsuitable for working with copper, aluminium and magnesium. The gas should be used sparingly as too much may cause products to become brittle.
Carbon dioxide improves weld speed, penetration and mechanical properties that make it the most suitable for steel in metal inert gas (MIG) welding.
However, this gas also causes a shakier arc, spatter losses and a lot of smoke fumes. Mixing carbon dioxide with argon minimises the spatter.
Carbon dioxide should not be used with thin metals such as aluminium that cannot sustain its high temperatures.
Basic Knowledge of Gases in Welding
An understanding of the roles these gases play in welding is crucial to the welding craft where each fine detail influences the overall process. Practising with the right tools and guidance of an instructor is the best way to determine the best gases and their applications to your welding projects.
Arc welding - Joining two or more metal pieces into a single part via electricity
Cathode - Positive terminal in electrolysis, prone to accumulating an oxidation layer
Weld bead - Single deposit of filler material
Weld penetration - The distance of fusion extending into the base metal, correlated to strength
Weld porosity - The amount of trapped gas inside a weld bead, which result in round holes
Weld speed - Slower weld speeds give deeper weld penetration
Metal inert gas (MIG) welding - A continuous solid wire electrode and shielding gas is fed through a welding gun and into the weld pool, joining two base materials together
Nitrogen loss - Gives rise to larger grains
Nonferrous metals - Metals containing little or no iron
Pitting corrosion - Hard-to-detect cavities in materials
Spatter - Unwanted droplets of molten material
Spatter loss - Material lost as spatter