Microstructural Development in a TRIP-780 Steel Joined by Friction Stir Welding (FSW): Quantitative Evaluations and Comparisons with EBSD Predictions

Abstract

The present work describes the effect of FSW on the result microstructure in the stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and base metal (BM) of a TRIP-780 steel. X-ray diffraction (XRD), optical microscopy (OM) and EBSD were used for determinations retained austenite (RA) in the SZ, It was found that the amount of RA developed in SZ was relatively large, (approximately 11% to 15%). In addition, recrystallization and the formation of a grain texture were resolved using EBSD. During FSW, the SZ experienced severe plastic deformation which lead to an increase in the temperature and consequently grain recrystallization. Moreover, it was found that the recrystallized grain structure and relatively high martensite levels developed in the SZ lead to a significant drop in the mechanical properties of the steel. In addition, microhardness profiles of the welded regions indicated that the hardness in both the SZ and TMAZ were relatively elevated confirming the development of martensite in these regions. In particular, to evaluate the mechanical strength of the weld, lap shear tensile test was conducted; exhibited the fracture zone in the SZ with shear fracture with uniformly distributed elongation shear dimples.Key-words: AHSS; TRIP steel; XRD; EBSD; FSW. Microstructural Desenvolvimento em um Aço TRIP-780 Unido por Soldagem por Atrito com Pino Não Consumível (FSW): Comparações e Avaliações Quantitativas com EBSD PrediçõesResumo: O presente trabalho descreve o efeito do processamento FSW na microestrutura resultante na zona soldada, zona termomecanicamente afetada (TMAZ), zona afetada pelo calor (ZAC) e no metal base (MB) de um aço TRIP-780. Difração de raios-X (DRX), microscopia óptica (MO) e EBSD foram utilizados para a determinação de austenita retida (RA) na SZ. Verificou-se que a quantidade de RA desenvolvida na SZ foi relativamente grande, (cerca de 11% a 15%). Além disso, a recristalização e a formação de textura de grão foram resolvidas usando EBSD. Durante friction stir welding, FSW (Soldagem por Atrito com Pino Não-consumível), a SZ experimentou severa deformação plástica que conduziu a um aumento na temperatura e, consequentemente, recristalização dos grãos. Verificou-se que a estrutura do grão recristalizado e níveis relativamente elevados de martensita desenvolvidos na SZ, conduziram a uma redução significativa nas propriedades mecânicas do aço. Além disso, os perfis de microdureza das regiões soldadas indicam que a dureza, tanto na SZ como na TMAZ foi elevada, confirmando o desenvolvimento de martensita nestas regiões. Em particular, foi visualidade na zona fundida trincas alongadas em perfil y, distribuída uniformetente.

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“…This unique microstructure is usually produced using a quenching and partitioning heat treatment process. The volume fraction of 15.4% RA was calculated using X-Ray diffraction according to the methodology of [11]. The carbon equivalent of 0.68 was calculated using the Yurioka et al [12] formula.…”

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“…Like induction seam welding, friction stir welding (FSW) is a solid-state welding process as well. Medina et al [31] conducted FSW trials on a TRIP780 (ferrite-bainite-retained austenite BM) alloy and characterized the weld microstructure to show a predominantly martensitic stir zone (SZ) and a HAZ which nearly matched the BM microstructure. They also found an increase in retained austenite content within the SZ and HAZ when compared to the BM.…”