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RESISTANCE SPOT WELDINGREPORT

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Table of Content

Definition (What is Resistance Spot Welding?)

Spot welding means joining two or more metals parts together with the help of heat and

pressure. It is considered one of the oldest welding processes. This type of welding can be

used to weld thin or thick foils but cannot be used for above about 6 mm thickness. High

quality welds can also be made in stainless steels, nickel alloys, aluminium alloys and

titanium.

Resistance spot welding produces little fumes and very less arc rays. Danger of flying sparks

is less when compared to other processes. But tongs and linkages can cause higher risk of

mechanical hazards such as pinching and crushing the fingers and hands.

Resistance is the opposition that a substance offers to the flows of electric current. If the metal

has lesser resistance, less heat is generated when current passes through it. The higher the

resistance of a metal, the more heat is generated when the same current passes through it. If

you want the best results in resistance welding you should have detailed understanding of the

materials being welded, careful control of the heat and pressure at the weld point, and

consideration of numerous other factors.

Equipment and tool

Types of Electrodes Used

Electrodes vary by shape (called electrode geometry). The right electrode is selected in order to improve electrical-thermal-mechanical performance. As the cross-sectional area increases rapidly with distance from the workpiece, thereby providing a good heat sink.

The diameter of the electrode contact area is a consideration; if the area is too small, it will produce undersized welds with insufficient strength; if the diameter of the electrode is too large, it will cause inconsistent and unstable weld growth characteristics.

The electrode must be able to:

conduct current to the workpiece mechanically constrain the workpiece conduct heat from the workpiece

The materials used to construct the electrode has to sustain high loads at high temperatures, while maintaining adequate electrical and thermal conductivity.

A range of copper-based or refractory-based electrode materials are used based on the application.  Three groups of electrode materials are outlined below.  Within each group, the Resistance Welding Manufacturers Association (RWMA) sorts electrode materials into classes. 

Group A contains copper-based alloys.  Common examples are:

Class 1: (99% copper, 1% cadmium; 60 ksi UTS (forged); conductivity  92% IACS)  Specifically recommended, because of its high electrical and thermal conductivity, for spot welding aluminum alloys, magnesium alloys, brass and bronze.

Class 2: (99.2% copper, 0.8% chromium; 62 ksi UTS (forged), 82% IACS)  General purpose electrode material for production spot and seam welding of most materials.

Group B contains refractory metals and refractory metal composites.  

Group C contains specialty materials such as dispersion-strengthened copper. 

In general, higher levels of resistance occurs when the power supply overcomes the level of resistance. To produce higher levels of resistance dissimilar parts are used.

Conductive electrodes such as copper are used to weld resistive materials like nickel or stainless steel

Resistive electrodes such as those made from molybdenum are used for welding conductive metals such as gold or copper

Welding operation

How the operation/ how this welding function:

Bulk Resistance: Metals have what is called a PTC or Positive Temperature

Coefficient. This means that their resistance increases as temperature increases.

Contact Resistance: When two surfaces come in contact, microscopically the

surfaces are rough, where some points come in contact on the surface and some do

not. At the points where contact is made and assuming the two pieces of metal are

pressed together with some pressure, the oxide layer breaks forming a limited

number of metal-to-metal bridges. The weld current is then passed over a large

area as it moves through the bulk metal. Since the current is forced through a

limited number of bridges this "necking down" increases the current density,

generating heat which causes melting. As melting occurs, new bridges are formed.

Molten metal also has higher resistance than non-molten metal, forcing the current

toward newer bridges. The process proceeds until the entire surface is molten.

When the electrical current is turned off, the electrodes cool first, which then cools

the molten metal. When everything solidifies the weld is formed.

When to use

Why use this welding compare to others

Advantages and disadvantages

Advantages

Comparatively low cost

Resistance Spot Welding (RSW) method doesn't need highly skilled worker.

Distortion or warping of parts is eliminated though it leaves some depressions or

indentation

The joint made is highly uniform.

Automatic or semi-automatic operation both can be done

There is no need for edge preparation. 

Welding can be done in quick succession. It just needs a few seconds to make the

joint.

Disadvantages

The equipment cost is high so it can has an effect on the initial cost.

Skilled welders or technicians are needed for the maintenance and controlling.

Some metals need special surface preparation for making the RSW a success.

The thick jobs are not easy to weld.

Example of welding application in industry ( Min 2 Video)

Application of Resistance Spot Welding

Spot welding of thick steel plates has been done and it has replaced the need for riveting.

The welding of two or more sheet metals can be joined by mechanical means more economically by using the spot welding methods. We don't need gas tight joints.

Spot welding can be used for attaching braces, pads or clips with cases, bases and covers which are mainly product of sheet metal forming.

Automobile and aircraft industries relies greatly of spot welding these days.