TY - BOOK

T1 - Resistance Spot Welding of Third Generation Advanced High Strength Steels

AU - Stadler, Manfred

N1 - embargoed until null

PY - 2021

Y1 - 2021

N2 - Third generation advanced high strength steels (AHSS), such as transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) and medium-Mn steels, combine high strength with good formability and are therefore promising candidates for the use for automotive applications. However, due to their relatively high alloying content and the rapid cooling during resistance spot welding, which is the predominant joining technology for automotive steel sheets, they tend to form hard and brittle joints and their weldability is therefore restricted. The present work aims to enable basic knowledge concerning the microstructural evolution during resistance spot welding of third generation AHSS in order to subsequently improve their mechanical performance by means of a targeted modification of the microstructure. For the TBF steel, large-scale hardness mappings revealed a pronounced hardening and thus embrittlement of the fusion zone (FZ), which was counteracted with a temper pulse. Microstructural characterizations by means of light optical microscopy and scanning electron microscopy (SEM) illustrated that the temper pulse must be adjusted precisely to the first pulse in order to get acceptable results. In a second approach, the cast-like structure of the outer FZ was modified by a recrystallization pulse in order to obtain more globularly shaped prior austenite grains with a high crack deflection capability, which were characterized using electron backscatter diffraction (EBSD). With both concepts a significant improvement of the mechanical performance could be achieved. The inferior mechanical properties of medium-Mn steel welds were mainly attributed to the presence of severe manganese segregations in the outer FZ, as detected by energy-dispersive X-ray spectroscopy (EDX). The segregations were homogenized by means of a recrystallization pulse, which led to an improved mechanical performance of the welds. An approach to estimate suitable cooling times between the two pulses based on the electrical resistance curve was suggested in order to facilitate the implementation of double pulsing in production. The detailed characterization of the heat-affected zone (HAZ) by means of EBSD, SEM, EDX and magnetic saturation measurement showed that the stability of the austenite strongly depends on the temperature, resulting in highly position dependent mechanical properties of the HAZ. These findings serve as basis for future efforts to precisely adjust the microstructure of the HAZ in order to improve the mechanical performance of the entire weld.

AB - Third generation advanced high strength steels (AHSS), such as transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) and medium-Mn steels, combine high strength with good formability and are therefore promising candidates for the use for automotive applications. However, due to their relatively high alloying content and the rapid cooling during resistance spot welding, which is the predominant joining technology for automotive steel sheets, they tend to form hard and brittle joints and their weldability is therefore restricted. The present work aims to enable basic knowledge concerning the microstructural evolution during resistance spot welding of third generation AHSS in order to subsequently improve their mechanical performance by means of a targeted modification of the microstructure. For the TBF steel, large-scale hardness mappings revealed a pronounced hardening and thus embrittlement of the fusion zone (FZ), which was counteracted with a temper pulse. Microstructural characterizations by means of light optical microscopy and scanning electron microscopy (SEM) illustrated that the temper pulse must be adjusted precisely to the first pulse in order to get acceptable results. In a second approach, the cast-like structure of the outer FZ was modified by a recrystallization pulse in order to obtain more globularly shaped prior austenite grains with a high crack deflection capability, which were characterized using electron backscatter diffraction (EBSD). With both concepts a significant improvement of the mechanical performance could be achieved. The inferior mechanical properties of medium-Mn steel welds were mainly attributed to the presence of severe manganese segregations in the outer FZ, as detected by energy-dispersive X-ray spectroscopy (EDX). The segregations were homogenized by means of a recrystallization pulse, which led to an improved mechanical performance of the welds. An approach to estimate suitable cooling times between the two pulses based on the electrical resistance curve was suggested in order to facilitate the implementation of double pulsing in production. The detailed characterization of the heat-affected zone (HAZ) by means of EBSD, SEM, EDX and magnetic saturation measurement showed that the stability of the austenite strongly depends on the temperature, resulting in highly position dependent mechanical properties of the HAZ. These findings serve as basis for future efforts to precisely adjust the microstructure of the HAZ in order to improve the mechanical performance of the entire weld.

KW - Widerstandspunktschweißen

KW - Dritte Generation Advanced High Strength Steels (AHSS)

KW - Doppelimpulsverfahren

KW - Anlassimpuls

KW - Rekristallisationsimpuls

KW - Wärmeeinflusszone

KW - Resistance spot welding

KW - Third generation Advanced High Strength Steels (AHSS)

KW - Double pulsing

KW - Temper pulse

KW - Recrystallization pulse

KW - Heat-affected zone

M3 - Doctoral Thesis

ER -