Tips for Choosing the Right Weld Head for Micro Resistance Spot Welding

Successful resistance welding is achieved at the intersection of material choice, process development, and equipment selection. In this blog post, we will review available options and impart some guidance on how to choose the right weld head for your micro welding application.

A typical resistance welding setup (Figure 1) consists of a power supply, a weld head, electrodes, and, optionally, an external monitor. This is the same whether they are implemented for manual production or installed in an automated line. The power supply creates the energy, the weld head exerts the force, the electrodes make contact with the parts, applying the force and delivering the energy, and a monitor ensures the parameters set in the weld schedule are met.

Let’s look specifically at the weld head and the various options to consider when selecting one for your resistance welding application.

Selecting a Suitable Weld Head

The weld head applies the force onto the parts to be welded, and, combined with the electrodes, provides the current path to the material. Weld head types include manual, pneumatic, motorized servo and electromagnetic, classified based on their actuation technology. Figure 2 offers an overview of these types.

Proper weld head selection is essential to ensuring a successful resistance welding process. When choosing a weld head, consider its force, dynamics, electrode configuration and weld head configuration. We will cover each of these in more detail in the following sections.

Force

Force is the amount of pressure the electrodes will exert on the parts and is one of the primary factors determining success of a weld. Too little force – the parts will not have sufficient contact and can result in sparks and spitting of material. Too much force – and the contact resistance between the parts will be low, resulting in a cold weld. And of course, once the right force is dialed in, you want to have consistency to ensure a consistent process. The SI unit of force is Newton – which is kg m/s^2. In the US, the unit of lbf (pound force) is often used.

The amount of force needed depends on the parts themselves – materials, geometry. For micro welding, necessary force ranges from 0.39 N (40 gf) (0.09 lbf) to 490 N (50 kgf) (110 lbf).

Weld head dynamics

“Weld head dynamics” refers to the way the head moves and interacts with the parts. Regardless of actuation type, there are several phases of the motion as highlighted in Figure 3.

• Approach – the approach phase brings the electrodes from their home position until they make contact with the part(s). Typically, the motion from home position to initial contact will be <25 mm (1 inch).
• Impact Force – the impact force is the initial force applied after coming in contact with the parts. Depending on the approach speed and force mechanism, this often has a “ringing” profile that is dampened as the force is continued to be applied.
• Squeeze – once it dampens enough, the weld head enters the ”squeeze” phase. This phase is relatively stable and ensures a consistent force is applied prior to firing the welding current. In some weld heads, once the force reached, the welding power supply is fired. This is aptly termed “force fired”.
• Follow-up  – once the energy is fired and the materials start to melt, acceleration of the head is required to maintain contact with the materials. This phase is called the “follow-up” phase.
• Hold – following the weld current, the parts need to be held in position as they cool and re-solidify. This is called the “hold time.”

After that time, the electrodes retract and return to their home position.

Electrode configuration

The electrode configuration describes the way the electrodes actually contact the parts to be welded. Figure 4 shows the three primary types of electrode configuration: direct, indirect, and parallel/series.  Part design and weld accessibility are key parameters in helping you determine the correct electrode configuration for your process.

• As the name suggests, a direct electrode configuration provides a direct current path from the positive to the negative electrode; the parts are placed between the electrodes with one electrode on each part and the weld nugget is formed between the electrode faces. Materials, part thickness, and polarity of the current flow all affect the position of the nugget. Our webinar on Resistance Welding Troubleshooting shows this effect and how to adjust it to the desired location.
• In an indirect electrode configuration, both electrodes are on the same side of the assembly, and the current moves from the positive electrode, through the two parts where the weld nugget is formed, and then over to the negative electrode. This is a common configuration when welding thicker materials.
• Parallel/series electrode configurations are used when the parts to be joined are different thicknesses. They are similar to indirect configurations in that the electrodes are again placed on the same side of the assembly. The electrodes are in contact the thinner part on top while a thicker part is underneath. The current is pushed from one part to the other. In parallel configurations, the electrodes are close enough to each other that the weld nugget forms between the two, whereas in a series configuration, the electrodes are far enough apart that two distinct nuggets form.

Weld head configuration – inline or offset (Figures 5a and 5b) – refers to the way that the electrodes are configured in relation to the actuation. Factors to consider in determining the right weld head configuration for your application include part access, geometry (complex or simple), and landing area.  Inline provides the best performance because the force is directly along the axis. Offset configuration provides a deeper throat depth for larger parts.

Ensuring Welding Project Success

The blog looked at weld heads and various configurations. But this is only one component of a typical resistance welding setup.

Since many factors go into choosing the right resistance welding equipment, it is important to work with an experienced team of product and process engineers who can evaluate the specific welding application.

It is also important to note that once the right equipment is chosen for the process it can be validated and remain under control. However, continuous system monitoring is necessary to ensure successful welds even when using the correct power supply and weld head. For more information read Resistance Weld Monitoring Ensures Quality and Provides Traceability and Why Weld Monitoring? 3 Reasons: Analysis – Stability – Yield