Page 29 - Metalorming Magazine January/February 2023

Special Section
FABRICATION
26 MetalForming/January/February 2023 www.metalformingmagazine.com
Welding
Fasteners
to Hot Stamped Boron Steel Made Easy
While challenges certainly lie in wait when welding fasteners to hot-stamped boron steels, including variations in AlSi coating thickness and hardness differences between the steel and fasteners, these and other issues can be overcome by switching from MFDC to CD welding power supplies.
BY ALLEN M. AGIN AND BOB KOLLINS
Welding of fasteners to hot- stamped AlSi-coated stamp- ings first came to the fore- front in the early 2000s when the automotive industry ushered in hot-
Allen Agin is sales manager at Weld Sys- tems Integrators Inc.; www.wsiweld.com. Bob Kollins is principal, Technical Sales & Solutions LLC; www.tsssales.com.
stamped boron steel (HSBS) for increased strength and lighter weight to meet stricter vehicle fuel-efficiency requirements. Problems immediately came to light when trying to make con- sistent, reliable welds of fasteners to HSBS stampings. Hopefully, when you finish this article, you will see that weld- ing fasteners to this material can be easy. We will examine several issues surrounding the welding of fasteners to HSBS, and present solutions to solve them. Up for discussion:
• How variability in AlSi-coating thickness creates inconsistent welds
• Hardness differences between HSBS and fasteners
• Traditional MFDC welding power supplies and challenges welding with constant current
• Use of capacitance-discharge (CD) welding power supplies superior destruct studies
• Mechanical considerations addressed using a fast-follow-up weld-head design.
AlSi-Coating Variability
HSBS arrives at the end user as an unfinished product, with a tensile strength of approximately 550 to 600 MPa. The forming process—5 to 10 min. in an oven at 900 to 950 C, forming in a press, and then rapid cooling—
transforms the steel into a finished product with a tensile strength of 1500 MPa.
During the heating process, the AlSi coating transforms into a Fe-Al-Si coat- ing, which prevents scaling and the issues that come with it, and avoids superficial decarburization and the need for post-process cleaning. How- ever, as beneficial as the AlSi coating is, it does present challenges with fas- tener resistance welding. Because the final transformation of the HSBS is completed onsite, coating thickness can vary and cause a significant change in contact resistance from part to part. A change in resistance affects heat gen- eration during welding (heat generated = I2R, where I is weld current and R is resistance).
We easily can spot these differences in HSBS resistance as the materials have a slight change in appearance (Fig. 1, parts pulled from the same bin are of four distinctly different colors). This creates a moving target with regard to resistance each time that we try to weld a fastener to these parts, a target difficult to hit when using conventional mid-frequency direct-current (MFDC) weld controls because they supply con- stant current and generate inconsistent heat at the weld interface. The result: