Microstructure and tensile properties of the laser welded TWIP steel and the deformation behavior of the fusion zone

Highlights

• Successful welded joints with austenitic structure are obtained by CW CO₂ laser.

• Inclusions and granular divorced eutectic phases form in FZ during welding.

• The FZ exhibits a good combination of strength and ductility.

• Strength and ductility of FZ are improved by the formation of mechanical twins.

Abstract

Microstructure and tensile properties of the laser welded joint of Fe–18.8Mn–0.6C TWIP steel were investigated in this research. The microstructure of fusion zone (FZ) was characterized by means of X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). TEM and in-situ SEM observation were employed to investigate the microstructural evolution and strengthening mechanism of FZ during deformation. The welded joint with a fully austenitic structure was obtained by the laser welding. The granular divorced eutectic phases (Fe, Mn)₃C and inclusions formed in the interdendritic regions during the solidification of FZ. The fully austenitic structure and coarse dendrite grains were responsible for the fracture at the weld seam. The FZ exhibited a good combination of strength (e.g. tensile strength up to 1000 MPa) and ductility (e.g. total elongation up to 73%). The microstructural evolution revealed that dislocation slip was the main deformation mechanism at low strains of FZ, while at relatively high strains, mechanical twinning was the domain deformation mechanism and played an important role in improving the strength and ductility as well as the work-hardening effect of FZ.

Introduction

TWinning Induced Plasticity (TWIP) steels have attracted increasing interests in the recent decade due to their excellent mechanical properties, superior formability and high energy-absorption, which have a potential application in lightweight automobile [1], [2], [3]. The outstanding properties of TWIP steels originate in the constituent element manganese (Mn) which has a remarkable influence on the phase composition and microstructure of TWIP steels [4], [5], [6], as well as the deformation mechanism by controlling the stacking fault energy (SFE) [7].

There is no doubt that weldability is a key factor for structural applications of TWIP steels. Owing to the austenitic stabilization effect of Mn element, welded joints with austenitic structure can be obtained for TWIP steels, while other high strength steels in low content of Mn element tend to obtain a martensitic weld seam during the welding process. This makes welding characteristics of TWIP steels a worthy research subject. Meanwhile, with the increasing requirement of automobile safety and structural integrity of car body, laser welding has been widely used in the process of car manufacturing, which offers the benefits of low heat input per unit volume, narrow weld seam, small heat affected zone (HAZ) and low thermal distortion of the work piece. Therefore, the studies on laser welding performance are essential for industrial applications of TWIP steels.

Several studies have concentrated on the effect of welding conditions on the mechanical properties of welded joints of TWIP steels with different chemical composition. Mújica et al. [8] studied the microstructure and mechanical properties of laser welded Fe–25Mn–3Si–3Al TWIP steel with a thickness of 3 mm, and found that FZ with a fully austenitic structure could be strengthened by grain refinement. Moreover, these authors also characterized laser welded dissimilar joints of Fe–24Mn–0.6C TWIP and TRIP800 steels. They reported that the heterogeneous microstructure of FZ was attributed to dilution of alloy elements especially Mn element, and the bands of martensite in FZ were responsible for the rupture at the weld seam [9], [10]. Yoo et al. [11] demonstrated that the hot cracking susceptibility of Fe–18Mn–0.6C TWIP steel welded by Gas Tungsten Arc Weld (GTAW) was mainly affected by the continuous γ-M₃C eutectic phases along the grain dendrite boundaries in FZ. In addition, Behm et al. [12] evaluated the mechanical characteristics of laser welded dissimilar joints of Fe–15Mn–0.7C–2.5Al–2.5Si TWIP and ferrite steels by using the welded KS-2 specimens, and found that the weld metal composition and distribution had an influence on the joint strength, ductility and fatigue life. Thus it can be seen that complicated microstructure of FZ formed during the welding process could significantly affect the mechanical performance of welded joints. However, there are still not many reports up to date focusing on the metallurgical reaction products during the laser welding process and strengthening mechanism of laser welded joints of high-Mn TWIP steels during deformation.

The present work aims to investigate the microstructure and tensile properties of laser welded joints of Fe–18.8Mn–0.6C TWIP steel. A detailed microstructural study on the secondary phase particles formed in FZ during the laser welding process was carried out by means of TEM combined with carbon extraction replica technique. In addition, TEM and in-situ SEM observation were employed to investigate the microstructural evolution and strengthening mechanism of FZ during deformation.