Dr. Livingstone, I Presume WELDWISE 2400 - Manual (Page 9)

0428-INS-400 Rev. E
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Overview of Resistance Welding
Introduction
In simplest terms, welding is a process by which two or more pieces of metal are joined by applying
heat and pressure. Back in the good old days, blacksmiths and other crafty people would heat metals in
a furnace and then weld them by hammering the red-hot metals together. By hammering the metals as
they cooled, the weld would be made stronger. This heating-and-hammering method is known as forge
welding. While forge welding worked quite well for most of the welding done back then, today's
welding requirements are a bit more advanced. After all, it would be pretty difficult to heat all the
metal needed to build an automobile in a big factory furnace and expect workers to hammer together
each specific part used in the manufacturing process. We'd all still be riding horses to work!
Fortunately, there are always a handful of brilliant people throughout history who are kind enough to
invent newer, faster, and better ways of doing things. One of these people was a professor by the name
of Elihu Thompson. Sometime in the year 1885, Professor Thompson invented a process called electric
resistance welding. He discovered that to weld metals together, one could fire an electric current
through the metals while they were tightly clamped together. When the current passed through the
metals, it would create such a high heat that the metals would melt and run together and a weld would
be made. Many times, the welded metal would be even stronger than the original metals used in the
welding process.
Today's resistance welders work almost exactly the same way they did when Thompson invented the
process. The current is generated by a transformer, and is fired through electrodes, which hold the
metal pieces in place. These electrodes also apply force to the metal pieces, usually before, during, and
after the firing of the electric current. This method is called resistance welding because it is the
resistance between the contact surfaces of the metals being welded that generates the heat to fuse them
together.
Resistance is the opposition that a substance offers to the flow of electric current. The less resistance a
metal has, the less heat is generated when current passes through it. Conversely, the higher the
resistance of a metal, the more heat is generated when that same current passes through it. This
behavior can be paraphrased as follows: the heat is where the resistance is, and the resistance is where
the heat will be. Obtaining the best results in resistance welding requires a thorough understanding of
the materials being welded, careful control of the heat and pressure at the weld point, and consideration
of numerous other factors. This chapter will deal with the basics of resistance welding, the variables
involved, and why they're so important to the welding process.