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Spot weldability of TRIP steel with high carbon, high aluminium content

Spot weldability of TRIP steel with high carbon, high aluminium content
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Recent trends towards the integration of strong steels into the automotive industry have accentuated demands on their resistance spot weldability, a technique most widely used for bonding during automotive assembly. However, strong steels are inherently rich in alloying elements. TRIP steels are not exceptions because they require certain level of carbon content and cementite inhibitors such as silicon to maintain austenite stability. The resulting high hardenability makes it difficult to avoid the formation of brittle martensite in the heat affected zone of resistance spot welds. Recent research on TRIP-assisted steel alloy with relatively high carbon and aluminium concentration showed a possible way forward. In this so called δ-TRIP steel, δ-ferrite is stabilized by aluminium and persists in the microstructure at all temperatures after solidification. Soft δ-ferrite can be retained in the martensite that forms. Since it is assumed that avoiding fully martensitic weld nugget may be helpful for better weldability and weld toughness, this idea is promising. Previous research done by Yi et al. reports that δ-ferrite retained after welding process is indeed good for better spot weldabilty, but it is not clear whether the properties achieved are adequate. Expanding on this notion, the effectiveness of δ-ferrite in terms of spot weldability has been qualitatively investigated in this work with alloys newly designed for the purpose. The thesis can be divided into two parts, pre-weld studies and an understanding of the weld. The initial part deals with the optimization of heat treatment conditions itself to acquire the best possible mechanical properties for the alloys. Specimens were tensile tested and analyzed using optical and electron microscopes. The weld study covers microstructure, joint property evaluations and fracture surface analysis. Relations between weld microstructure and fracture mechanisms have been obtained. To further enhance spot weldability, additional heating processes after spot welding with different conditions were applied to assess whether the tempering of martensite in the weld nuggets is advantageous. The procedure was expected to soften the weld nugget and result in better weld integrity.
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