Resistance Welding Q&A: Welding Nuts to Hot-Stamped Parts - Part 2
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. ...
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.
This article is one of a two-part series. See Part 1 here.
What other factors do I need to consider?
You may want to take the following factors into account:
-Life cycle cost
Expanding on a few of these factors, let’s start with power supply. A MFDC system, including the latest “fast-rise time” type of transformer, can require up to 1000 ampere (A) of three-phase power and a 2200 A inverter. CD processes require a small fraction of that power. For example, one manufacturer’s CD process only requires 30 A of single-phase 480 volts alternating current. Installation costs of the primary power supply, as well as power cost over the life cycle of the capital equipment, are substantial factors that should be considered.
Supplying chilled water can also be a significant cost to consider. For example, MFDC processes require significant water cooling for the weld control and transformer. A CD welding process that does not require water cooling for the control or transformer saves at least 8 gal per min (30 L per min). In high-duty- cycle CD applications, a small chiller might be recommended to cool the tooling. There are capital cost savings in buying a much smaller chiller, or not buying a chiller at all, not to mention the ongoing power savings from not running a large chiller.
Another often overlooked factor is floor space. Some capacitive storage banks are simply more space efficient. United States-built CD banks are typically much smaller than those sourced overseas. Recently, a large stamping company purchased CD welding machines based on price alone to keep the project cost down. The CD banks were three to five times the size of the domestic units they didn’t purchase. The overall footprint of each machine was almost double what it could have been. Put together with the smaller chiller requirements mentioned earlier, the savings in facility investment would have been huge.
Rework is another major cost to consider. In another case, also due to upfront cost-saving measures, two automotive suppliers found themselves reworking lots of parts, and paying more than they had saved on the equipment purchase. They had chosen the MFDC process to weld stampings with AlSi coatings based on equipment price. They didn’t recognize a variety of issues with their in-plant processing (some of which I’ve touched on above), so production welding yielded inconsistent results. Both companies were forced to implement “safety” gas metal arc welds (GMAW) to their fasteners, adding more weight to the vehicle and much greater production cost due to longer processing time, as well as additional personnel, welding gas, and consumables.
There are at least several other factors that I don’t have the space to mention here. It is best to discuss all of these with a resistance welding machine builder who has a proven track record on hot-stamped materials. Designing your process for success should also include doing lab welding, testing, and performing a Design of Experiment (DoE) on your stampings before making a major investment.
I’ve been hearing about the heat-affected zone (HAZ). Why does this matter?
The HAZ is a very important, often critical, factor in all resistance welds. The ideal resistance weld utilizes the highest possible amount of heat for the shortest possible amount of time. Using a process that cannot deliver weld current in the shortest amount of time can end up heating an area far outside the weld zone. This may cause a change in hardness in the base material, which can lead to a material failure in the HAZ. Figure 5 shows what appears to be a good weld based on the number of weld nuggets, but with severe overheating leading to a weakened HAZ. In this case, the end-user was using a MFDC power supply, and experiencing much lower push-out values than expected. Simply put, the metallurgy changed and the part was weakened due to excessive heating. Had the HAZ been much smaller, push-out values would have been much higher.
Fig. 5 — Overheated weld showing a large HAZ.
On the other hand, Fig. 6 shows examples of the desired HAZ and excellent push-out test results when using high currents and very short weld times. There is no visible HAZ around the projections, or around where they pulled.
As you can see, answering your original question is not simple. It raises many additional concerns that should be taken into account to create consistent and reliable welds.
Fig. 6 — A (top) — Showing almost no HAZ; B (middle) — showing bottom side again with almost no HAZ; C (bottom) — showing nugget pull with almost no HAZ.
In conclusion, one of the most important exercises you should consider is partnering with a reputable resistance welding machine builder who has the expertise, experience, and ability to demonstrate a variety of different processes when welding AlSi-coated stampings. While this demonstration can take place in the machine builder’s welding lab, this demonstration should use your production parts and a range of their production welding equipment. This equipment should include both CD and MFDC welding machines with all necessary production features. Following this process will supply you with the absolute correct answers for your particular application, enabling you to consistently produce the strongest possible welds.
I am very grateful for the assistance of the following people in answering your question in more depth: Bob Kollins, at Technical Sales and Solutions, for support with the measurements and waveforms shown in Fig. 3; and Min Kuo, PhD, at ArcelorMittal Global R&D, for the microstructure imaging on our parts shown in Figs. 2 and 4.
This article is one of a two-part series. See Part 1 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.