9414 Arc Cutting

Arc cutting is not a joining process, but rather a cutting process, but is included in this section on joining because it is similar to welding in that an arc from an electrode is used. Plasma arc cutting is the most common arc cutting process used for aluminum. It takes the place of flame cutting (such as oxyfuel gas cutting) used for steel, a method unsuited to aluminum because aluminum's oxide has such a high melting point relative to the base metal that flame cutting produces a very rough severing.

In plasma arc cutting, an arc is drawn from a tungsten electrode and ionized gas is forced through a small orifice at high velocity and temperature, melting the metal and expelling it and in so doing cutting through the metal. To cut thin material, a single gas (air, nitrogen, or argon) may act as both the cutting plasma and to shield the arc, but to cut thick material, two separate gas flows (nitrogen, argon, or, for the thickest cuts, an argon-hydrogen mix) are used. Cutting can be done manually, usually on thicknesses from 0.040 to 2 in. (1 to 50 mm) or by machine, more appropriate for material \ to 5 in. (6 to 125 mm) thick.

Arc cutting leaves a heat-affected zone and microcracks along the edge of the cut. Thicker material is more prone to cracking, since thick metal provides more restraint during cooling. The cut may also have some roughness and may not be perfectly square in the through thickness direction. The specification for aluminum structures requires therefore that plasma-cut edges be machined to a depth of | in. (3 mm). The quality of the cut is a function of alloy (6xxx series alloys cut better than 5xxx), cutting speed, arc voltage, and gas flow rates.

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