How to Perform Spot Welding on Coated Metals?

Spot welding is a popular process in modern manufacturing. It is fast and suitable for the high-volume production of strong and durable components. Additionally, spot welding is cost-effective and requires minimal operator skill. However, using the technique to join coated workpieces can be demanding because of the properties of this coating. Successful spot welding on coated metals requires special attention to produce durable welds while retaining the coating’s protective properties.

This article explains what spot welding is, how spot welding is performed on coated metals, and the best practices for quality and safety.

What is Spot Welding

Spot welding, also known as resistance welding, is one of the oldest techniques available. It relies on the resistance of the flow of electric current through an electrode to generate heat. Spot welding is perfect for joining about 3mm thick or less metal pieces. To carry out spot welding, you will clamp the workpieces between the copper electrodes. 

Copper is a good conductor of electricity. However, it experiences resistance when in contact with the workpiece. This resistance produces heat, and the pressure from the electrode fuses the parts to form a strong bond. Spot welding is the go-to technique when joining car body panels and battery packs. It also has applications in fabricating structural elements and household appliances.

How to Perform Spot Welding on Coated Metals?

Carrying out resistance spot welding on coated metals requires careful consideration because of the unique challenges the coatings pose. Below is an extensive discussion on how you can perform spot welding on coated metals. 

Step 1- Understand the Coating Material

Before carrying out the spot welding process, you should understand the type of coating. It can be galvanised, aluminized, or anodized. A thorough understanding will help detect incompatibility issues. The workpiece material and coating thickness must be suitable for resistance to prevent flaws and electrical issues.

Step 2- Preparation

Now, you have to prepare the machine and workpiece. Machine preparation entails selecting the appropriate equipment and matching the power capacity with the workpiece material requirements. To prepare the workpiece, carefully inspect and clean the coating using suitable agents. Avoid grinding or using harsh chemicals that can affect the integrity of the coatings. 

Step 3- Machine Setup

When it comes to equipment set-up, two things are involved: selecting the appropriate electrodes and optimizing the welding parameters. Go for a copper-based electrode that can handle the coating’s high resistance. Additionally, shape the electrode to ensure good contact. After that, essential variables such as current, pressure, and time should be adjusted.

Step 4- Perform a Test Weld

This trial and error step is optional but important. Here, you weld scrap materials of similar properties to the workpiece to help detect mistakes early on. Physically inspect the weld and adjust the parameters till you get the desired quality. 

Step 5-  Carry out the Spot Welding Process

This is the primary step in spot welding coated parts. Align the workpiece properly to prevent gaps for consistent contact. Position the electrodes on the weld location and apply uniform pressure. Pass electric current to melt and fuse the workpieces at a point. Carry out different cycles and maintain consistent timing. Allow the weld to cool naturally to form strong joints.

Step 6- Inspect and Test the Weld

Proper inspection and testing complete the resistance spot welding operation. Inspect the welds and look for flaws and damage to the surrounding coating. Non-destructive testing such as x-rays and ultrasonic testing are used to assess the weld quality.

Step 7- Post-Weld Treatment

Based on the desired application of the part, carry out appropriate post-weld treatment. Remove any residue and apply the required touch-up coating to restore corrosion resistance. 

Types of Coatings and their Impact on Weld Quality

Coating comes in various varieties and unique application methods. Their impact on the final weld quality varies. Let’s discuss the common types of coating and how they affect the resistance spot welding process.

Galvanized Coating 

This describes the application of a thin layer of zinc on steel or iron for superior corrosion resistance. Zinc provides sacrificial protection and will degrade when in contact with moisture or chemicals. Welding zinc-coated workpieces require special attention because of the higher tendency of spatter and porosity.

Aluminized Coating 

Here, an aluminum coating is applied to the workpiece to prevent overheating and rusting. This surface finish operation is typical in automotive exhaust systems. Welding these components increases the risk of expulsion. Aluminum has a low melting point and causes inconsistencies when it flows into the weld zone.

Electroplated Coatings 

Electroplating is the process of using electric current to deposit a layer of protective metals for decorative and functional purposes. The coating can be nickel, tin, copper or chromium. Electroplated coatings increase the risk of cracks and may create brittle welds.

Painted or Powder Coated Surfaces

Some workpieces are also painted or powder-coated to improve their appearance. There are different coatings types, each having a unique effect on resistance spot welding. Generally, painted surfaces are insulators and prevent the free flow of electric current. They can degrade and cause harmful fumes.

Spot Welding Coated Metals: Best Practices

Spot welding coated metals pose significant challenges. However, you can get remarkable results with the best practices in mind. Some resistance spot welding tips and considerations include

Use the Correct Welding Parameters

Using the proper welding parameter is key when spot-welding coated metal to create strong and defect-free joints. Essential parameters to focus on are welding current, time, and electrode pressure. Select the correct welding current based on the coating type and thickness. Additionally, reduce the time and increase the pressure to counteract higher contact resistance.

Select the Right Electrode

Selecting the right electrode makes all the difference in resistance spot welding. The electrode will determine how well current is transferred and pressure is applied. Go for electrodes made with copper material such as copper-chromium-zirconium. The shape and size of the electrode also matter. Use dome-shaped electrodes of the right size for precision welding.

Optimize the Process

You should also optimize the entire spot welding process. This will ensure efficiency and repeatability. Conduct trial welds to calibrate the parameters. Use the peeling technique to separate the test parts to assess strength if necessary. Prepare the workpiece without tampering with the coatings and maintain proper joint and lap design. Additionally, inspect the welds to identify defects early. 

Fume Management

Most coatings generate fumes that pose health and safety risks. Hence, it is essential to put an adequate fume management system in place. To do this, install fume extractors with high-efficiency particulate air filters near the welding area to maintain clean air. Furthermore, wear personal protective equipment and weld in places with good ventilation.

Conclusion

Spot welding is essential in many manufacturing industries because of its simplicity and high speed. It creates strong and durable welds for thin metal sheets. Carrying out this process on coated metals requires an in-depth understanding of the coating type. You should also adequately prepare the workpiece and optimize crucial welding parameters. This article has explained the types of coating and their impact on weld quality, how weld coated materials and key considerations for the best results.

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