Arc welding differs from the soldering process in that the metals to be joined are melted to a significant depth. The molten metals mix along with filler material provided by a consumable electrode and when properly done the finished joint is quite strong. The soldering process, in contrast, consists of added material, traditionally a lead-tin combination, that lays on top of the metals to be joined, penetrating only enough to provide adhesion.
Soldering is at its best for making highly conductive electrical joints, watertight joints in copper pipe, and high-quality automotive radiator repairs. It does not result in joinery as strong as in arc welding where deep penetration and steel filler material are the norm.
Forge welding was common in ancient times for joining first copper and bronze, later iron and finally steel. (Steel is a generic name for iron that has been processed or has had material added to improve the quality.) In forge welding, the materials to be joined were heated to an elevated temperature, then hammered vigorously until a seamless joint was achieved. High-quality welds are possible in this labor-intensive process.
Electrical energy became available for many purposes at the start of the nineteenth century when mechanically generated static electricity was no longer the only option. In 1800, Sir Humphry Davy developed short-pulsed electric arcs. Two years later a continuous electric arc was demonstrated by Vasily Petrov, a Russian physicist. Subsequently, Baron Auguste de Méritens, a French researcher, built a carbon-arc torch, used in manufacturing batteries having lead plate electrodes.
Many metals, after being welded, fall prey to hydrogen embrittlement. The intense heat that accompanies the welding process causes ambient water vapor to decompose into hydrogen and oxygen. Hydrogen is a problem because it contaminates the weld, entering the crystal lattice and causing brittleness. This manifests as severe after-the-fact cracking that runs along the welded bead.
Several methods are used to mitigate this difficulty, typically providing a shielding to isolate the weld from atmospheric water vapor. One method is to coat the stick electrode with a material that ends up as slag that floats on the molten pool and ends up deposited on the finished bead. It protects the hot weld and is manually chipped away so the welded area can be painted or additional welding passes can be made.
Wire-feed welders are widely used. The most widely used version is gas metal arc welding (GMAW), sometimes referred to as metal inert gas (MIG) welding or metal active gas (MAG) welding. But, because of the nature of the uncoiling process, electrode coating is not possible. Bottled inert gas is blown continuously across the weld, isolating it from atmospheric moisture. However, this type of arc welder is not suitable for outdoor use where any breeze would disperse the shielding gas.
Portable welders, gasoline or diesel powered, make for a high-quality weld because the welding current is dc. There is no off time as in ac where the waveform crosses the X-axis at zero volts making for an intermittent arc. DC line-powered welders, with internal rectification, produce high-quality work. Also, ac welders now output square-wave welding current, so that the off time is not long enough to destabilize the arc.
The post Basics of arc welding appeared first on Test & Measurement Tips.
Filed Under: Test & Measurement Tips