Page 36 - MetalForming July 2019

Fabrication: Welding Well
Forge Force (Optional)
Upslope (Optional)
Force
Current Downslope
(Optional)
Pulsing (Optional)
Temper Current (Optional)
Squeeze Time
Weld Time
Quench Time (Optional)
Temper Time (Optional)
Hold Time
Spot Welding Cycle
at the edge of the nugget during a peel or chisel test.
In the end, conventional peel and chisel testing of spot welds in AHSS sheet likely will produce interfacial or partial interfacial failure modes (Fig. 4). And, these failure modes can occur even when weld strength is acceptable for the intended application. Even full interfacial failures can exhibit high strength, although it may sometimes be challenging to differentiate between an interfacial failure and a “stuck” weld condition (which refers to an unfused bond of unacceptable strength).
Improving AHSS Welds
To improve nugget failure modes for AHSS, the hard martensite must be softened. Adding a temper cycle at the end of the weld (Fig. 5) offers a simple and effective approach to accomplish- ing this. This requires including a suf- ficient amount of quench time prior to tempering, to allow the complete
Fig. 5—To improve nugget failure modes for AHSS, by softening the martensite, fabrica- tors add a temper cycle at the end of the RSW process.
transformation to martensite; the amount of tempering time and current dictates how much softening occurs.
Of course, the quench time adds cycle time to each weld, so production requirements mandate that this time
be kept as short as possible. In addition, there are other approaches to reduce the cooling rate, including current puls- ing and current sloping, and use of longer weld times and shorter hold times. MF
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34 MetalForming/July 2019
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