Shot welding is a specific type of spot welding

Spot welding

Spot welding is a process in which contacting metal surfaces are joined by the heat obtained from resistance to electric current flow. Work-pieces are held together under pressure exerted by electrodes. Typically the sheets are in the thickness range...

 used to join two pieces of metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 together. This is accomplished by clamping

Clamp (tool)

A clamp is a fastening device to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure...

 the two pieces together and then passing a large electric current

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

 through them for a short period of time. Assuming the right amount of current for the right time, this will weld

Weld

Weld most commonly refers to a joint formed by welding.Weld may also refer to:-People:* Weld family, an extended family of New England** Theodore Dwight Weld** Tuesday Weld* Weld-Blundell family* Cecil Weld-Forester, 1st Baron Forester...

 the two pieces of metal together. Shot welding was invented by Earl J. Ragsdale, a mechanical engineer at the Budd Company

Budd Company

The Budd Company is a metal fabricator and major supplier of body components to the automobile industry, and was formerly a manufacturer of stainless steel passenger rail cars during the 20th century....

, in 1932 for the purposes of welding stainless steel

Stainless steel

In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

. This welding method was used to construct the Pioneer Zephyr

Pioneer Zephyr

The Pioneer Zephyr is a diesel-powered railroad train formed of railroad cars permanently articulated together with Jacobs bogies, built by the Budd Company in 1934 for the Chicago, Burlington and Quincy Railroad , commonly known as the Burlington...

.

The method

The E. G. Budd Company of Philadelphia recognized the important metallurgical characteristics of stainless steel (18-8) and developed a spot welding machine to take advantage of the oxidized layer of stainless steel. Heat treating the 18-8 stainless steel leaves a metal with non-magnetic and ductile

Ductility

In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...

 properties. Reheating the metal to 1000–1100 °C repeatedly impairs the characteristics of the metal. The metal becomes susceptible to corrosion

Corrosion

Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

 and carbide migration, and loses fatigue resistance. The important factor in controlling the metal's properties is the dwell time at those temperatures. Using a controlled time element and recorder, a power supply with smooth current, and very brief high currents, a satisfactory spot weld may be produced.

The corona

Corona

A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...

 of the shot weld should not exist on the metal, and the equipment used produces satisfactory welds with a smaller than normal diameter. Sufficient pressure is applied to hold the two sheets of metal together and the peak current rapidly melts the interface, producing a small nugget of weld metal, which when cooled results in a shear resistant metal interface. Good shotwelds have twice the shear strength

Shear strength

Shear strength in engineering is a term used to describe the strength of a material or component against the type of yield or structural failure where the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is...

 of a rivet

Rivet

A rivet is a permanent mechanical fastener. Before being installed a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole, and the tail is upset, or bucked A rivet...

 of similar diameter and can be placed 50 percent closer together to each other. Distortion

Distortion

A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...

is eliminated, a problem in gas welding processes.