What is the resistance welding?

Learn more about the electric and other type of resistance welding machine 

electric resistance welding

Although it usually requires large,generally immobile,and expensive machines,electric resistance welding,also know as ERW,is a clean and efficient method of joining thin metal parts to one another quickly and solidly.Electric resistance welding is used for some extremely crucial manufacturing processes,such as the seam welding used for fuel tanks, large pipes, and other containers.The principle behind electric resistance welding,and hence behind electric resistance welding machines,is straightforward,but can be applied in several different ways.

Resistance welding is a group of welding processes in which coalescence is produced by the heat obtained from resistance of the work piece to electric current in a circuit of which the work piece is a part and by the application of pressure.There are at least seven important resistance-welding processes.These are flash welding, high-frequency resistance welding, percussion welding, projection welding, resistance seam welding, resistance spot welding, and upset welding. They are alike in many respects but are sufficiently different.

Resistance welding machines work by clamping the two pieces of metal that are to be joined between two copper electrodes.A strong electric current is passed through the electrodes and the metal pieces generally,a current with fairly low voltage but high amperage.The metal’s resistance to the current converts the electric energy into heat energy,which melts both parts at the place where the electrodes are creating a circuit through them.The pressure of the electrodes then squeezes the two parts into each other,literally combining their substance at that point, and creating a weld.

The Resistance Welding Process

The two most common forms of resistance welding are spot welding and seam welding. Each of those type uses a different type of machine with different electrodes, and is used for very different fabrication processes. Both are most useful with thin metal sheets or plates, however.

Spot welding

Spot welding process

Spot welding is a type of electric resistance welding that is carried out with two clamping electrodes that complete an electric circuit through the two sheets of metal.The maximum level of resistance is at the point where the two sheets of metal are pressed against each other,so maximum heating occurs here as well.Heat increases the resistance,which causes more heat,and so on.To prevent the electrodes from sinking into the other side of the sheets,the electrodes are water-cooled,so the heat is concentrated at the place where the sheets touch and not elsewhere.

The  two areas of the sheets touching each other become molten and flow together,forming a bond.As the electrodes continue to clamp down,the heat flows back towards them,then is drained away by their water cooling, hardening the weld.There is a possibility of damaging the weld at this point if the heat has been built up too quickly,however,and cannot be drained off in time to prevent the electrodes from squeezing out molten metal from the weld point.Spot welding is quick and cheap, but not very strong.It is used frequently in building cars.

Resistance spot welding (RSW) is a resistance welding process which produces coalescence at the faying surfaces in one spot by the heat obtained from resistance to electric current through the work parts held together under pressure by electrodes.

The size and shape of the individually formed welds are limited primarily by the size and contour of the electrodes. The equipment for resistance spot welding can be relatively simple and inexpensive up through extremely large multiple spot welding machines. The stationary single spot welding machines are of two general types: the horn or rocker arm type and the press type.

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The horn type machines have a pivoted or rocking upper electrode arm, which is actuated by pneumatic power or by the operator`s physical power. They can be used for a wide range of work but are restricted to 50 kVA and are used for thinner gauges. For larger machines normally over 50 kVA, the press type machine is used. In these machines, the upper electrode moves in a slide. The pressure and motion are provided on the upper electrode by hydraulic or pneumatic pressure, or are motor operated.

For high-volume production work, such as in the automotive industry, multiple spot welding machines are used. These are in the form of a press on which individual guns carrying electrode tips are mounted. Welds are made in a sequential order so that all electrodes are not carrying current at the same time.

Projection welding (RPW) is a resistance welding process which produces coalescence of metals with the heat obtained from resistance to electrical current through the work parts held together under pressure by electrodes.

The resulting welds are localized at predetermined points by projections,embossments, or intersections. Localization of heating is obtained by a projection or embossment on one or both of the parts being welded.There are several types of projections: 
  • the button or dome type, usually round
  • elongated projections
  • ring projections
  • shoulder projections
  • cross wire welding
  • radius projection

The major advantage of projection welding is that electrode life is increased because larger contact surfaces are used. A very common use of projection welding is the use of special nuts that have projections on the portion of the part to be welded to the assembly.

Seam welding

Resistance Seam Welding (RSEW)

Seam welding is similar to spot welding in general principle,but replaces the stationary, clamp-like electrodes of spot welding with a pair of electrodes in the form of copper wheels or rollers.These wheels heat the metal plates with an electric current in the same way as a spot welder,but are in constant motion,simultaneously rolling the sheets forward between them,heating the metal to the molten point with electric resistance,and squeezing them into each other to form a bond.Seam welding forms an unbroken seam between the plates a strong,uniform,continuous weld from one end of the workpiece to the other.Unlike spot welding,seam welding is very strong and is used for such important items as fuel tanks. 

Whether it is built to carry out spot welding or seam welding,a resistance welding machine takes advantage of the characteristics of electrical resistance to heat and weld two pieces of metal together with a minimum of byproduct heat,fumes,and other peripherals of ordinary welding.Although limited to applications that use thin sheet metal,resistance welding is still an important and useful part of the welding repertoire.

Resistance seam welding (RSEW) is a resistance welding process which produces coalescence at the faying surfaces the heat obtained from resistance to electric current through the work parts held together under pressure by electrodes.

The resulting weld is a series of overlapping resistance spot welds made progressively along a joint rotating the electrodes. When the spots are not overlapped enough to produce gaslight welds it is a variation known as roll resistance spot welding. This process differs from spot welding since the electrodes are wheels. Both the upper and lower electrode wheels are powered. Pressure is applied in the same manner as a press type welder. The wheels can be either in line with the throat of the machine or transverse. If they are in line it is normally called a longitudinal seam welding machine. Welding current is transferred through the bearing of the roller electrode wheels. Water cooling is not provided internally and therefore the weld area is flooded with cooling water to keep the electrode wheels cool.

In seam welding a rather complex control system is required. This involves the travel speed as well as the sequence of current flow to provide for overlapping welds. The welding speed, the spots per inch, and the timing schedule are dependent on each other. Welding schedules provide the pressure, the current, the speed, and the size of the electrode wheels.

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This process is quite common for making flange welds, for making watertight joints for tanks, etc. Another variation is the so-called mash seam welding where the lap is fairly narrow and the electrode wheel is at least twice as wide as used for standard seam welding. The pressure is increased to approximately 300 times normal pressure.The final weld mash seam thickness is only 25% greater than the original single sheet.

Source: http://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&NM=76

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