Engineering

Part 11 - Welding 2

Passing an electric current through two parts to cause melting of the materials at the interface, known as resistance welding, was patented by Elihu Thomson in 1885.

Invented in 1893, exothermic or thermite welding requires no external source of heat or current. Instead it uses a chemical alumino-thermic reaction between aluminum powder and a metal oxide to produces sufficient heat to melt two metal object together.

Electric arc welding became more common with the development of electrode coatings known as fluxes. These shields the welding area from impurities while also adding alloying components to stabilize the arc and make a stronger, more reliable weld.

World War I spurred the development of welding for military tanks and ships as it was faster and cheaper than traditional rivetted construction. The British constructing the "Fullagar," a ship, with an entirely welded hull, in 1920. She was classed with the note "Electrically welded – subject to annual survey. Experimental."

Automatic welding, with continuously fed electrode wire, was developed in 1920 and hydrogen, argon, and helium were tested to shield welds from atmosphere oxygen and nitrogen which caused porosity and brittleness. 

The world's first welded road bridge, the Maurzyce Bridge, was built n near Łowicz, Poland in 1928 and the first all-welded merchant vessel, M/S Carolinian, was launched in 1930.

In the 1930's, developments in automatic welding, alternating current, and fluxes led to a major expansion of arc welding and to the welding of reactive metals like aluminum and magnesium. Arc welding was first use to construct aircraft during the second world war although most airframes were rivetted.

Kyle Taylor developed direct-current arc, stud welding, in 1930. The stud is typically joined to a flat plate by using the stud as one of the electrodes. The polarity used in stud welding depends on the metal. Welding aluminum uses a positive electrode while welding steel requires a negative electrode. It also requires a flux tip and a ceramic ring (ferrule) to concentrate the heat, prevent oxidation and retain the molten metal in the weld zone. The ferrule is broken off the fastener after the weld is completed.

Submerged arc welding, patented in 1935, uses a consumable solid or tubular electrode and the arc zone is "submerged" under a blanket of flux consisting of lime, silica, manganese oxide and calcium fluoride. When molten, the flux provides a current path between the electrode and the work. The thick layer of flux, completely covering the molten metal, prevents spatter and sparks as well as blocking the intense ultraviolet radiation and fumes that are characteristic of arc welding. 

In Russia, Konstantin Khrenov developed the first underwater electric arc welding system. Gas/tungsten arc welding, also known as tungsten inert gas (TIG) welding, uses a non-consumable tungsten electrode. The weld area and electrode are protected from atmospheric contamination by an inert shielding gas (argon or helium). When helium is used, this is known as heliarc welding. A filler metal is typically used, though some welds do not require it. A constant-current passes across the arc through a column of highly ionized gas and metal vapours known as a plasma. 

It is typically used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The technique provides strong, high quality welds but it is relatively difficult to master and slower than most other welding techniques. A related process, plasma arc welding, uses a different welding torch to create a more focussed welding arc allowing it to be automated.

Spot welding (see top video) is a type of electric resistance used to join sheet metal parts (such as automobile body panels). Two shaped copper alloy electrodes simultaneously clamp the sheets together and pass a welding current through a small "spot". Energy can be delivered to the spot in a very short time (approximately 10–100 milliseconds) so that welding does not cause excessive heating of the remainder of the sheet.

In the 1950's shielded metal arc welding, using a flux-coated consumable electrode, became the most popular metal arc welding process. In 1957, a flux-cored arc welding process that used a self-shielded wire electrode permitted automatic welding. The same year, Robert Gage invented plasma arc welding. 

Electroslag welding was introduced in 1958 as a way to join thick plate but its use was restricted because of poor weld quality. 

In 1953, the Soviet scientist N. F. Kazakov proposed the diffusion bonding method, a solid-state welding technique capable of joining similar and dissimilar metals. This requires temperature, 50-75% of the absolute melting temperature of the materials and high pressure so that the molecules of each surface diffuse into the other surface. The process is typically used to weld alternating layers of thin metal foil, and metal wires or filaments and high-strength and refractory metals.

Electron beam welding, capable of making deep and narrow welds possible was available from 1958. 

Magnetic pulse welding, invented in 1967, is caused by a very short electromagnetic pulse (<100 µs (microseconds)) having a very high amplitude and frequency (500 kA and 15 kHz). This produces a high-density magnetic field, creating an eddy current in one of the work pieces. The high magnetic pressure is much higher than the material yield strength as one of the work pieces impacts the other with a collision velocity up to 500 m/s (1,100 mph) causing a solid state weld. 

Friction stir welding, invented in 1991 by Wayne Thomas at The Welding Institute, Britain. A non-consumable tool generates heat by friction against the workpiece and, as the tool moves along the joint line, it forges the hot and softened metal by mechanical pressure.

Laser beam welding and cutting uses the beam as a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume applications using automation, as in the automotive industry.