Ask The Welding Engineer

March 14, 2017

Q: My Company has been nut and stud welding for many years using AC and MFDC welders with great success. We recently ran into some issues with a new hot stamped part that we couldn’t projection weld consistently. What’s going on?

Your question is somewhat complicated and as you read on shows how one question will lead to another and another with the answers to each as follows.

A: The automotive industry is trending to make structural components stronger and lighter and hot stamped steel can accomplish both of those. The down side: traditional AC and MFDC welders can’t projection weld it consistently. All hot stamped parts are not created equally – die to die, run to run, each part can be different, and therein lies the challenge.

Q: Okay, so each part is different. My AC and MFDC welders both have closed loop feedback that allow the welder to adjust the settings during the weld. Where am I going wrong?

A: Let’s start this one with some theory. To generate weld heat, the formula is:

Weld Heat = I2(R)(t) – Thermal Loss

Where: I= current, R= resistance and t=time

For the sake of this, let’s establish that t, time, and thermal loss are constant numbers and will not change. Going back to my first answer, each part will have a different R, resistance. Simply put, your closed loop system will always make sure that your I, current, stays the same. If your current stays the same and your time stays the same, your delta in resistance will result in a change of heat produced – producing inconsistent results.

Q: So, if I can’t use my AC or MFDC welders to projection weld hot stamped parts, what do I do?

A: It has been found Capacitive Discharge (CD) Welders can have great success in welding hot stamped parts. A CD welder combines super-fast rise times, high currents, high weld forces, short weld times and lack of feedback from an open loop system.

Q: I’ve been told my MFDC welder has the fastest rise time from any manufacturer. I am still having issues projection welding the hot stamped material. Why is that?

A: The fast rise time is just part of the equation for success. The lack of feedback from an open loop system is also imperative for a consistent process.

Q: I have two questions. How does what you are saying fit into the heat generation equation above? And, if you have no feedback from an open loop system, how do you know if you have a good weld?

A: Here’s where it all gets tied in. Ohm’s Law says that:

V=IR

Where: V= voltage, I=current and R= resistance

The secondary voltage is going to remain consistent from the transformer and when the resistance in the hot stamped part increases or decreases, the current will vary inversely proportionate. This is what makes the lack of feedback from the open loop system another key to success in CD welding. Referring back to the heat generation equation: the current, I, does not remain a constant value as it does with AC or MFDC. As the resistance varies, so can the current allowing for a more consistent heat generation – and consistent welding results.

As for not having a closed loop system within the weld control, we integrate state of the art weld checkers which we can program hi/low windows around the current, voltage, resistance, displacement, and weld time into our machine packages. Any value that is outside the window would result in a machine fault.

Q: I’ve heard a few rumors about CD Welders and I’m wondering if you could help me out. I’ve heard CD Welders are unproven, slow, and expensive. What can you say to that?

A: We manufactured our first CD Welder for an Automotive application back in 2007. That welder is still in production today making great parts. Through the years, we have had OEM’s, Tier 1 and Tier 2 suppliers bring parts to our Welding Lab and we have demonstrated consistent results to all of them.

The welding process itself is very fast. A “normal” CD weld might only take 6 milliseconds. Our standard CD Welding unit takes 4 seconds to recharge its capacitors 100% – and begins recharging almost immediately after the weld is complete. In most applications, the cycle time from weld completion to weld current starting is more than 4 seconds. This cycle time would include cylinder retraction, part handling, fastener loading and cylinder extension.

Expensive is qualitative term and needs to be fully reviewed. While true, the capital expense for a CD Welder could be more than an AC or MFDC welder, there are many cost savings benefits of CD Welding. First, our capacitor banks need only a 30 amp, single phase primary power feed to charge. Second, the electrode life extends far beyond AC or MFDC. We have some customers reporting changing electrodes once a week – compared to multiple times a shift with the AC and MFDC welders. Third, the water cooling needs drop to 1-2 gallons per minute, compared to 3-4 GPM for an AC press welder or even 8-12 GPM for an MFDC welder. Finally, the greatest cost-saving benefit of CD Welding comes from the consistency and not having safety weld your fasteners.

Allen Agin is the Midwest Regional Sales Manager for Weld Systems Integrators, Inc. (567.225.4594) and has been involved in the Welding Industry since 1968. He is currently a published co-author of articles and papers on Projection Fastener Welding to High Boron AlSi Coated Hot Stamped Materials. He has been involved in the development of processes for producing consistent projection welding of fasteners to Hot Stamped Parts for the Automotive industry and is a member of AWS and RWMA. Questions and comments can be sent to Allen Agin c/o Welding Journal, 8669 NW 36 st., #130 Miami, FL 33166, or via e-mail at allen@wsiweld.com.