Resistance Welding Q&A: Welding Nuts to Hot-Stamped Parts - Part 1

Q: I weld nuts to hot-stamped parts and am having difficulty getting consistent results. This includes parts where the nuts appear welded to the part, but sometimes pop off in transit to our customer. We also get inconsistent push-out test results. How come? I have asked this question many times and have gotten many different answers.

A: This is a great question. It also raises additional questions and challenges to be considered when making consistently good fastener welds on stampings with an aluminum silicate (AlSi) coating. 

First, we should touch on two different types of steel processing: mill processing and in-plant processing. Understanding these are different processes is the key to some of the major reasons for inconsistent weld results.

Mill processing is the processing done at the steel mill, prior to shipment to the supplier or end user. Many conventional metals are stamped and welded just as they come in from the mill, with no additional treatment. Mill-processed steel is malleable and can be shaped using conventional stamping presses and processes. Hot-stamped boron steel is not malleable and requires in-plant processing to stamp it.

In-plant processing is done at an end-user plant, or sometimes at a supplier plant, where they have furnaces and stamping presses to form stampings after they come out of the oven. Hot-stamped or press hardened materials must be formed using in-plant processing. During in-plant processing, the metal blanks are heated beyond 900°C prior to being formed in a press with water-cooled dies. The blanks are heated to increase the ductility of the steel, reducing springback during the stamping process. In-plant processing changes the metallurgical composition, multiplying the hardness from 50 kilopounds per square inch (ksi) to around 200 ksi. This new material develops an AlSi coating in the process. Welding fasteners to this material is much more difficult than to conventional steels. Think of it like pressing a ripe banana into tempered glass.

Why are my stampings different colors?

At first glance, this may seem like an unusual question until you look in your tote bin. Figure 1 shows an obvious difference in surface color (AlSi coating). This color inconsistency exists due to the variability of in-plant processing with normal everyday work schedules and adjustments. The color difference is caused by changes in the coating thickness due to deviations in line speed, furnace temperature, or die cooling from the ideal process parameters.

Fig. 1 — Showing color variations.

Does the color affect weldability?

Absolutely! Normally the steel used in production is processed at a mill, and what you get for production is generally consistent and the same color. However, during in-plant processing, color variations can occur, indicating a change in the thickness of the AlSi layers (see an example of the change in microstructure in Fig. 2). This changes the resistivity of the surface, which directly affects weld quality and repeatability.

Fig. 2 — A (top) — Thin surface microstructure; B (bottom) — thick surface microstructure.

What can I do to get consistent weld results?

The “best” textbook answer when using in-plant processing is to make sure there is no deviation from the recommended parameters of time, temperature, and die cooling for the stamping blanks. Even a slight change in the process can create a problem due to the AlSi coating.

The best real-world answer, however, is that holding a process like this to exact settings only seems to occur in laboratory conditions. Like an endangered species, it is almost never found in the real world. This brings us back to your original question. 

What is the best way to weld nuts and studs to hot-stamped materials?

This question comes up quite often. The answer depends on a variety of factors, including the following: in-plant processing, thickness of material, AlSi coating, projection style, nut/stud size, and weld specifications. With variations of any of these, you can get different answers from different people.

Today, the most common welding processes, when it comes to AlSi coated stampings, are capacitive discharge (CD) and medium-frequency direct current (MFDC).

Many years of lab testing and production have identified that a combination of short weld times, high current, high force, and fast follow-up deliver the greatest consistency. An example of these parameters for welding a M6 flange nut is shown in Fig. 3. Figure 3 shows the current output, time, and displacement when welding a M6 nut onto a 2-mm ALSi-coated material.

Fig. 3 — A (top) — Weld measurements; B (bottom) — graphed results.

While a MFDC process can be ideal for the right application, it does require tight control of in-plant processing. A CD process allows for variations due to in-plant processing in real-world conditions, as described earlier. Figure 4 shows several examples of thickness differences in the multiple AlSi coating layers as slight process changes occurred due to variations in in-house processing control. 

Fig. 4 — Four different microstructure surface thicknesses.

This article is one of a two-part series. See Part 2 here. 

This article was written by ALLEN M. AGIN for the American Welding Society. Agin is a sustaining member of AWS and an active member in RWMA and WEMCO. He has been involved in the applications, training, and sales of resistance welding since 1968. Agin is also an author and co-author of articles on the welding of fasteners to hot-stamped and press-hardened steels with AlSi coatings. He has presented to the American Institute of Steel Engineers, AWS Sheet Metal Welding Conferences, and the PMA Hot Stamping Experience & Tech Tour. 

Presented by The Resistance Welding Manufacturing Alliance (RWMA), an industry partner of the American Welding Society, is an active network of industry professionals dedicated to the advancement of resistance welding standards and processes.