Spot welding is among the earliest welding procedures. It’s employed in a vast selection of businesses but especially for the welding of sheet steel automobile bodies.
So what is spot welding? Spot welding is considered to be one of the most efficient welding processes and is typically used to merge two or more metal sheets into one. To make this possible, the welder has to apply not only immense pressure and heat but also an electrical current through the sheets to eliminate resistance and make them fuse.
The method is used in several industries, which we will get into at the end, but it is notable for welding low carbon steel sheets for vehicle bodies. The size and shape of the welds are dependent on the size of the electrodes that are used, which, in turn, depends on the thickness of the base metal and the skill of the welder.
How spot welding works
The best way to understand how spot welding works is to follow the steps that go into it. Here is a brief overview:
Step 1 – workpieces and metallic sheets are aligned
First, the workpieces or metal sheets are lined up precisely. This is an essential step because once the sheets are welded together, there is no going back. If they aren’t aligned correctly, the weld will be incorrect, and the step will have to be repeated with new sheets.
Step 2 – electrode pressing
In the next step, suitable electrodes are chosen for welding the two workpieces together. Instead of pure copper resistance electrodes, pure refractory ones are used because of their ability to retain heat and resist oxidation. It also contains fractions of Molybdenum (MO), which also has a melting point that is high enough to prevent delamination.
Step 3 – current is made to flow through the electrodes
Once the electrodes are brought into position, a leg pedal is used to generate a high voltage current that flows through the rods and into the workpieces. Squeezing the electrodes to the metal sheets with the appropriate amount of pressure is vital when it comes to quality welds.
The heat required to melt the metal is generated from the resistance of the base metal, and it is limited to the point where the electrode and the weld area meet. The current is switched off in the holding stage while pressure is maintained, and the nugget is allowed to cool down.
The amount of time the current flows through the workpieces and the type of current used is dependent on the thickness and the type of material you are working with. Once the sheets are joined accurately, the electrodes are removed and repositioned in the next point to be welded.
Materials that work for spot welding
Several types of materials can be used for spot welding and besides metallic materials; this includes wire mesh as well. Steel is preferred, especially in the automotive industry, because it has a high electrical resistance and isn’t as strong a conductor as other materials.
Similarly, low carbon steel is used more often than the high-carbon content variety because of its resilience to cracks. Stainless steel welds are also considered to be less hard than mild steel and mixed steel welds, but nickel alloys and titanium are also popular choices.
In contrast, spot welding can be difficult with metals such as aluminum and galvanized steel since both require higher levels of electrical current for weld formation. However, even the best material can be difficult to weld if it is not of the appropriate thickness.
As a rule of thumb, the sheets have to be at least 3 mm thick. And both have to be of the same size for optimum results. On the other hand, if the sheets are of different sizes, a 3:1 ration should be maintained for a quality weld.
Parameters of spot welding
As one of the broadest used variants of the resistance welding process, spot welding is dependent on several parameters, each with its uses and benefits. These include:
The electrode force
By electrode force, we mean the energy that is required to merge the metal sheets that are to be fused. The stronger the force, the more heat is generated, so engineers have to be careful to ensure the quality of the weld is not compromised.
The weld current has to be increased to increase force as well. However, if it is cranked up too high, the spatter generated between the sheets and electrode will also be high. The result? The electrodes will adhere to the sheets and possibly get ruined.
Additionally, if the pressure is too high, you will end up creating a small spot weld as the current, and the heat that is generated from the sheets and electrodes are spread over a larger area, thus reducing the penetration of the weld.
Squeeze time refers to the period between the initial application of the electrode force on the workpieces and the first time the current is applied. Also known as programmed squeeze time, the parameter stabilizes the tip fore before the current application. If this step is skipped, it could lead to premature electrode wear, interfacial expulsion, and arcing.
If the squeeze duration is prolonged, it may give you a quality weld, but it can cost you in operations since it will generate fewer welded parts per shift. Similarly, a shorter squeeze time can result in inconsistent quality and metal expulsion.
Generally, squeeze time should be programmed in such a way into the power supply to allow time for the force of the electrode to stabilize.
Weld duration or weld time
Weld time refers to the duration that the current is applied to the workpieces or metal sheets during the welding process. This is calculated in line voltage cycles. However, the weld time is difficult to accurately determine because it is dependent on what needs to be done for the weld spot.
However, you need to follow some rules. For instance:
- The weld time has to be brief as possible.
- The current should produce optimum weld quality.
- The duration of the weld should be enough to ensure that the indentation made by the weld is as minuscule as possible. The smaller the weld time, the smaller the indentation.
- The weld time for welding thick sheets should produce a nugget that has a large diameter.
- The parameters that are chosen for the weld should wear down the electrodes as less as possible.
- The duration of the weld should be changed in case of automatic tip dressing. In this case, the contact surface of the electrode is maintained at a constant value.
In other words, the best weld time for quality welding is one that is as short as possible.
As the name implies, hold time refers to the duration after welding when the electrodes remain in contact with the sheet. This is done to allow the weld area to cool down. The weld chunk then solidifies before the welded parts are released.
If the hold time is stretched out, the heat in the weld spot may spread to the electrode and heat it, which will start to wear out. Additionally, if the material has high carbon content and the hold time is high, the weld may become brittle and develop cracks. However, if you remove the electrodes before the nugget becomes solid, it may pull apart, resulting in a weak weld.
Applications of spot welding
Spot welding is a popular welding method in the automobile industry because it is a cheap but efficient way to merge sheet metal. As such, it can be utilized either with trained rudimentary staff or robots since it requires little skill. The bottom line is that the welding process is fast and effective, which means more cars developed per shift.
This welding method is used liberally in manufacturing electronics such as circuit boards, gas sensors, and even solar panels. This method is known as electronic resistance welding and is also used to create complex and delicate electrical components from cables to switches and hand tools.
Most people are unaware of this, but spot welding is also used to join rolls of nails. Steel wires are welded to nails at high speeds by using pneumatic nail gun magazines. If you use seam welding wheels, you can get 1200 nails/minute.
Of course, to ensure the nails are secured to the wires accurately, the current that is applied has to be precise as possible at the point where the point of the nail is under the welding wheel.
Is spot welding easy?
Spot welding is among the earliest welding procedures and is very well-known and easy to do for even beginner welders. The procedure is totally defined and readily applies to a lot of thin metals like stainless steel, nickel alloys and titanium. It is often utilized in aerospace and automotive businesses, but improvements are highly desired.
How thick can you spot weld?
Spot welding is largely employed for linking parts which are normally around 3 millimeters in thickness. Thickness of those components to be welded ought to be equivalent or the proportion of thickness ought to be less than 3:1. The strength of this joint is dependent upon the quantity and size of this welds. Spot-weld diameters range from 3 mm to 12.5 mm.
Can you spot-weld aluminum?
Spot welding aluminum is getting a lot more common as aluminum replaces steel in numerous applications where weight is important, like cars. You can take advantage of three-phase electric power to spot-weld lightweight aluminum. Spot welding typically supplies current for 0.1 seconds or much less, so the current needs to be extremely significant.