Lou Kren Lou Kren
Senior Editor

Resistance Welding—A Fastening Option for Hot Stamping

November 1, 2019



Attendees of MetalForming magazine’s Hot Stamping Experience and Tech Tour, held this past September in Michigan, had an up-close look at capacitor discharge (CD) resistance welding, courtesy of this CD welding machine. The process promises quality welds of softer fasteners to harder hot stamped steels.

As hot stamping gains a foothold in North America among automotive OEMs and their Tier suppliers, other technologies, such as capacitive discharge (CD) resistance welding, can be applied to optimize the effectiveness of the process.

Employing advanced and ultra-high-strength steel (AHSS, UHSS), hot stamping has been revealed as the process that most efficiently produces—and many times the only process that produces—complex, safety-critical components. Common hot stamped components include A and B pillars, and roof reinforcements, and as the technology becomes more ubiquitous, more part applications surface. 

During the hot stamping process, steel blanks feed into tunnel or stacked furnaces for heating to a temperature (above 1500 F or so) that makes the blanks malleable. Blanks move into a press capable of controlling stroke rate and dwell time for forming, followed immediately by in-die quenching for 3 to 10 sec.—in-die water channels often serve this purpose. The heating and quenching processes—the former shifting the steel to a full austenitic phase followed by the latter for transformation to a full martensitic phase—create the hardened material while not overly stressing the tooling, thus the material can be formed more easily prior to reaching its final hardened state. 


Here’s a sample of a fastener welded onto material courtesy of the CD welding process. Note the clean welds at the base of the nut on the lower left and right.

A major aspect of hot stamping concerns joining, including the projection welding of fasteners, as these parts make their way into larger components and assemblies. Installing fasteners on hot stamped aluminum-silicon (AlSi)-coated parts can be tricky business owing to a number of factors, according to officials from Weld Systems Integrators (WSI), which provides systems for CD resistance welding in hot stamping applications. Bob Kollins, senior application engineer with Technical Sales & Solutions, which works with end-users to set up and operate stand-alone Amada Miyachi-powered CD welding units from WSI, along with Allen M. Agin, WSI Midwest regional sales manager, described and demonstrated fastener-welding challenges and solutions at MetalForming magazine’s Hot Stamping Experience and Tech Tour, held this past September in Michigan.

Especially in hot stamping applications, fasteners are extremely soft as compared to the base material, causing the fastener projections to collapse before a good weld is achieved. In addition, hot stamped materials develop an AlSi coating in the furnace, with the resistive qualities of the coating inconsistent across the hot stamped part. On top of that, the short weld times called for in projection welding bring difficulty in delivering enough heat to the base material.

CD welding stores energy for rapid energy release with large peak currents, which means more energy into weld formation and less into heating the surrounding material, as well as a smaller heat-affected zone as compared to other welding processes. In metallurgy-critical hot stamping, these attributes prove advantageous, which is why, despite the above-listed challenges, projection welding of fasteners to hot stamped parts sees wide use.

“Since 2007, we’ve worked with Tier Ones and OEMs producing and processing hot stamped parts inhouse,” says Kollins. “Typically, materials arrive at a Tier supplier ready to be stamped. Hot stamping, on the other hand, involves inhouse processing, which varies from manufacturer to manufacturer. Variations exist across the industry and even across a part in terms of contact resistance, specifically at the interface where the fastener attaches to the part.”

CD welding technology, Kollins explains, can work with these variations by altering the weld current to match the current needed for resistance at that particular location on the part. Today, Kollins and Agin see a rise in hot stamping usage and applications, and continuously work on developing welding technology, such as a newly released machine and monitoring capability, to account for trends, including the use of thicker material. MF

Industry-Related Terms: Forming, Projection Welding, Quenching, Stroke, Surface
View Glossary of Metalforming Terms

 

See also: Technical Sales & Solutions Limited

Technologies: Welding and Joining

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