Journal
METALS
Volume 11, Issue 2, Pages -Publisher
MDPI
DOI: 10.3390/met11020270
Keywords
advanced high strength steels; AHSS; medium-Mn steel; resistance spot welding; double pulsing; dynamic resistance curves
Funding
- Austrian BMK [846933]
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The cooling time greatly influences the mechanical performance and microstructural features of medium-Mn steel in double pulse resistance spot welding. Short cooling times lead to in-process heat treatment, while excessively long cooling times result in deteriorated mechanical properties due to insufficient re-heating of the fusion zone.
In the present work, the influence of the cooling time on the mechanical performance, hardness, and microstructural features of a double pulse resistance spot welded medium-Mn steel are investigated. Curves of the electrical resistance throughout the welding revealed that the cooling time strongly influences the heat generation during the second pulse. A second pulse after a short cooling time re-melts the center, and heat treats the edge of the primary fusion zone. This desired in-process heat treatment leads to a modification of the cast-like martensitic structure by recrystallization illustrated by electron backscatter diffraction measurements and to a homogenization of manganese segregations, visualized by energy-dispersive X-ray spectroscopy, which results in an enhanced mechanical performance during the cross tension strength test. In contrast, during excessively long cooling times, the resistance drops to a level where the heat generation due to the second pulse is too low to sufficiently re-heat the edge of the primary FZ. As a consequence, the signs of recrystallization disappear, and the manganese segregations are still present at the edge of the fusion zone, which leads to a deterioration of the mechanical properties.
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