Page 30 - Metalorming Magazine January/February 2023

Special Section: FABRICATION WELDING FASTENERS
Fasteners may fail pushout requirements and potentially fall off. Also noteworthy: Even though the coating is very incon- sistent, the base metal holds its desired integrity.
Hardness Caries from Steel to Fastener
Metal formers also typically overlook and fail to account for the difference in the hardness of the HSBS and the fastener itself. According to the best data we can obtain from fastener manu- facturers, fastener tensile strength ranges from 400 to 450 MPa, a stark contrast from the 1500-MPa tensile strength of formed HSBS. This means that traditional destructive testing can be dif- ficult to gauge; we have seen fasteners fail before the base metal, which can result in anomalies in the final pushout-test data.
Why MFDC Welding Doesn’t Fit
Fig. 1
Although MFDC can maintain a programmed current output within ±2 percent, traditional MFDC weld controls have had limited success when welding fasteners to HSBS, which we attribute to the inconsistency of the post-processed AlSi coating. When the weld current is held at a steady state and the resistance rises above the baseline, increased energy is created at the fastener interface. This results in excessive heating and reduced weld strength. The same problem can occur inversely when resistance drops below baseline and results in an insufficient amount of heat generated at the pro- jection interface, causing a weak weld.
So, as good as MFDC performs when welding HSBS material in a normal flat-to-flat configuration, MFDC is not recom- mended for fastener welding to HSBS.
Alternatives to MFDC
Knowing that traditional MFDC weld controls cannot over- come the challenges created by inconsistent AlSi-coating thick- ness, we have a few choices:
• Perform 100-percent inspection of each fastener and reweld any that fail.
• Tack weld (using gas-metal-arc welding) each nut to act as a safety weld.
• Use a capacitor-discharge (CD) fastener welding machine, which will overcome the resistance inconsistencies and deliver a repeatable process.
CD welding uses the energy stored in a bank of capacitors to deliver energy to the weld. A capacitor bank of a specific capacity (C) is charged to a high voltage (V), where the energy available to the weld secondary, E:
E=1/2CV2.
Because C is fixed, the voltage is adjusted to change the energy available to the weld. For each weld to form using the same energy level, the capacitors are charged to the same voltage. That fixed amount of energy stored in the capacitors then is applied to the primary side of the weld transformer, and the transformer steps down the voltage to a level appropriate for welding.
Note, however, that the current delivered to the weld on the secondary side of the transformer is not fixed or controlled. Ohm’s law tells us that V=IR; the current on the secondary
Fig. 2
Fig. 1 shows four parts (labeled 1-4) each with a different color yet pulled from the same part bin. The corresponding cross-sections (Fig. 2, supplied by Min Kuo, Ph.D. at Arcelor- Mittal Global R&D) illustrates the different AlSi-coating thick- ness corresponding to each part.
Fig. 3—Microhardness of a fastener welded to HSBS using a WSI CD welding machine.
Fig. 4—Here’s what can happen during destructive testing of weldments where the fastener and base material have sig- nificantly different tensile strengths.
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