Abstract
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.
Highlights
As a representative of the third generation of advanced high strength steels (AHSS), medium-Mn steels, with an increased but still moderate amount of manganese of 4–10 wt%and a carbon content of 0.2 wt% or less, are considered suitable to fulfill the requirements for body components in the automotive industry [1,2,3,4]
Because of the high operating speed and the suitability for automation, resistance spot welding (RSW) is the dominant technology for sheet metal joining in the automotive industry [11], and research regarding the weldability of medium-Mn steels is decisive
Due to the rapid cooling combined with the relatively high alloying content of medium-Mn steels, they tend to form a hard and brittle martensitic fusion zone (FZ), which leads to a poor tensile-shear strength of the welds [12] and to insufficient mechanical properties during the cross-tension strength (CTS) test with low maximum force and an interfacial failure (IF)
Summary
As a representative of the third generation of advanced high strength steels (AHSS), medium-Mn steels, with an increased but still moderate amount of manganese of 4–10 wt%and a carbon content of 0.2 wt% or less, are considered suitable to fulfill the requirements for body components in the automotive industry [1,2,3,4]. First introduced in 1972 by Miller [5], research on this steel grade has been intensified in recent years [4,6,7,8,9]. Their superior mechanical properties are achieved by a ferritic microstructure with a high amount of retained austenite of usually more than 30%, which is stabilized by manganese enrichment at intercritical annealing [4,6,8,10]. Due to the rapid cooling combined with the relatively high alloying content of medium-Mn steels, they tend to form a hard and brittle martensitic fusion zone (FZ), which leads to a poor tensile-shear strength of the welds [12] and to insufficient mechanical properties during the cross-tension strength (CTS) test with low maximum force and an interfacial failure (IF)
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