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Posted on Dec 13, 2020
When spot welding thin metal, the main objective is to avoid warping, burn-through, and excessive heat affected zone (HAZ) while ensuring the weld produces enough mechanical strength for the operation.
Spot welding sheet metal involves the use of the electric resistance welding technique to bring parts of sheet metal together into one fabrication assembly. When spot welding thin metal, the main objective is to avoid warping, burn-through, and excessive heat affected zone (HAZ) while ensuring the weld produces enough mechanical strength for the operation. The heat-affected zone is basically a physical witness during the spot welding process which may consist of small burn marks on the sheet metal. It would also sometimes be visible as a small indent on the surface of the sheet metal fabrication in which the sheet metal has melted and fused together under the pressure of spot welding tip. These indents however can remain untouched on components that do not require a cosmetic surface.
The equipment used in the spot welding of sheet metal fabrication consists of tool holders and electrodes. The tool holders act as a means to hold the electrodes in place firmly while supporting optional water hoses that cool the electrodes during the welding process. The methods of tool holding include light-duty, paddle-type, universal, and regular offset. The electrodes are typically made of low resistance alloy, most notably copper, which are generally designed into many different shapes and sizes depending on the applications. The two pieces of sheet metals being welded together are referred to as the workpieces and must conduct electricity. The width of the workpiece normally spans from 5 to 50 inches by the throat length of the welding apparatus. The thickness may range from 0.008 to 1.25 inches.
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The spot welding sheet metal process is not too complicated. It basically makes use of the heat generated by the resistance of electric current at the joint of two separate pieces of sheet metal. The sheet metal to be welded can typically be of a variety of different materials, such as mild steel, galvanized steel, stainless steel, etc., each of which may have varying thicknesses. To perform a spot welding process, a spot welding machine is required.
During the process, two copper electrodes are used to concentrate the flow of electric current through the sheet metal and clamp the workpiece in place as the weld is formed. The actual welding of an individual spot only requires a fraction of the section to be fused. Variables that can affect the quality of the spot weld between two sheets of metal include surface coating, material gauge, cleanliness, and the shape of the electrode tip. Generally speaking, the smaller the copper electrode tip, the higher quality of the spot weld will be. But an indent may be more likely to occur, and you may have to consider a cosmetic treatment depending on the sheet metal components you are welding. Conversely, the larger the electrode tip, the cleaner the spot weld will be on the sheet metal fabrication. The resulting spot weld may however be weaker because the energy is dissipated over a larger area.
Besides sheet metal, spot welding is also used for welded wire mesh or wire mesh. Thicker metal is harder to spot weld because the heat is dissipated to the surrounding metal more easily. Sheet metal goods, like buckets, that have undergone a spot welding process can easily bear physical witness as we mentioned earlier. Depending on the metal, the extent to which the spot weld can be identified on the surface can vary. The number of welding currents may also vary. For instance, sheet metal fabrication of aluminum alloys requires higher welding currents because of the substantially higher thermal conductivity and electrical conductivity. This then requires a larger, more powerful welding transformer.
The most common application of sheet metal spot welding is perhaps the fabrication of car components in the automobile manufacturing industry. In fact, the majority of operations to form a car lies in the welding of sheet metal. To facilitate the workflow, spot welding can be completely automated, which are largely deployed in many of the industrial robots found in assembly lines. The spot welding of sheet metal is also used in medical settings, where smaller spot welders are used to fabricate medical components in orthodontics. Another application of spot welding is to make batteries, more specifically the battery cells made of nickel-cadmium, nickel-metal hydride, or lithium-ion.
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To prevent warping and burn-through, it is advised to not weave or whip the torch. The longer the arc remains on an area of sheet metal, the hotter it becomes. When welding, always have the tip traveled in a straight line at a speed fast enough to maintain a good bead profile. Another method to prevent burn-through during welding is to apply a “push technique” of a MIG gun. This allows the wire to be directed to the cooler edge of the sheet metal compared to pulling the MIG gun. Pulling the gun may lead to an increased chance of blowouts because the wire is directed toward the hotter area of the metal.
Alternatively, you can use backing bars to complement the spot welding process to dissipate heat faster to reduce heat-affected zones and the likeliness of burn-through. Place a backing or chill bar on the top of the heat-affected zone, or you can simply use a metal bar that is made of aluminum or copper because it can dissipate heat better.
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