How to Achieve High-Quality Nut Spot Welding

With the advancement of industrial technology, nut spot welding has found extensive applications in the automotive and cabinet sheet metal industries. However, achieving high-quality nut spot welding is no easy feat. If you are an operator or manager involved in nut spot welding, reading this article is essential to ensure the safety of your products and to learn how to achieve high-quality nut welding.

What are the main challenges in nut spot welding?

To perform stud welding, holes are first cut or punched into the sheet metal. The stud nut is then positioned concentrically over the hole, and a firm weld is achieved by applying downward pressure with an electrode to create an electrical discharge. In actual mass production, three problems most commonly arise: First, cold soldering occurs, leading to insufficient strength between the nut and plate, causing nut detachment or loosening. Second, thread failure occurs where threads fail to pass gauges, primarily due to excessive extrusion material clogging threads or nut deformation. Achieving high-quality stud welding requires resolving or avoiding these two issues. Additionally, post-weld nut deformation is another significant problem in stud welding. The following sections address how to resolve these issues and achieve high-quality nut welding.

Resolving Poor Welds

Protruding nut welding is a high-specification process involving short duration, high current, and high pressure. It rapidly melts the protruding points while heating the sheet contact area to a molten or plastic state, ultimately forming a weld nucleus under pressure. High-quality welding requires several key elements:

1. Higher peak current, particularly with DC output, necessitates higher-power equipment to ensure full current delivery. DC’s superior penetration also enhances fusion efficiency.
2. Shorter discharge time to release current instantaneously, preventing stud softening without melting while fully heating the plate area.
3. Greater pressure, which essentially means better followability. Since the melting process of nut studs typically occurs within 5 milliseconds, even high pressure can lead to pressure loss if followability is inadequate during this brief window. This ultimately causes spatter in the weld pool and results in poor bonding. Current approaches to resolving follow-up issues include: 1.  Selecting more suitable cylinders. For example, smaller cylinders are needed for welding small nuts, as larger cylinders exhibit reduced follow-up capability under low pressure. Since a single machine often accommodates multiple nut specifications—a common workshop scenario—larger cylinders are chosen for compatibility, prioritizing current capacity over the reduced force requirements for small nut welding. Second, optimize followability by selecting servo-electric cylinders or electromagnetic pressure cylinders, or adding leaf springs.

Preventing Extrudate Clogging in Threads

Excessive extrudate results from over-energy—high current or prolonged welding time causes melted projections to extrude excessively toward both sides. Inner extrudate flows into thread bores, causing blockages. Three solutions exist:

1. Reduce welding current and shorten welding time to ensure a molten core forms within a shorter duration;
2. Increase air blowing to burn off or blow away molten extrusion from the metal surface during welding, preventing it from adhering to the screw threads and causing blockage;
3. Customize contour-matching locating pins. These pins constrain the bottom holes of the plate and the inner hole of the nut. The flow direction of molten or plastically deformed metal during welding determines the final shape of the extrusion. Contour-matching pins effectively align with the inner holes of the plate and nut, sealing the deformed metal outside the nut hole and significantly reducing its impact on the nut’s internal passage.

Preventing Nut Deformation

Nut deformation, especially during welding of small nuts, frequently results in post-weld failure to pass thread gauges. Three solutions exist:

1. Reduce welding pressure. As nuts soften from heat during welding, excessive pressure readily causes deformation;
2. Reduce welding time. Discharge duration simultaneously melts the stud and heats the nut. Shortening discharge time effectively prevents thermal plastic deformation of the nut.
3. Select appropriate power sources. Medium-frequency DC power supplies or capacitor-storage power sources provide superior DC current characteristics while reducing welding time. This minimizes nut heating without compromising fusion integrity, thereby reducing deformation.

Apart from the above-mentioned issues related to nut welding, in the nut processing, we also have requirements for the sealing performance of the nut welding. At this time, we need to precisely control the welding parameters. Not only should the nut be firmly welded to the sheet, but also its sealing performance should be guaranteed. We can use a leak testing machine to test the sealing performance of the welded products during the parameter adjustment and sample testing stages. After multiple tests, we can obtain the most suitable parameters, which can ensure the quality and stability of the welded products.

Summary

The above outlines methods for achieving high-quality stud welding of nuts. Understanding these principles and techniques will enable you to resolve existing issues in nut welding and enhance the quality of products manufactured using this process. For further questions regarding nut welding, consult the AGERA team. Our technical specialists will assist you in addressing your current challenges. In addition, AGERA offers a wide range of nut spot welding machines for your selection, including MFDC spot welding machines, capacitor energy storage spot welding machines, and you can also customize an automated nut projection welding system.