Effect of microstructure and microtexture modified by magnetic field on as-weld notch bending performance

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Abstract

A 10 mm 316 L stainless steel, filled by 1.2 mm 316 LSi, was joined using multi-layer laser-GMAW in the presence of a constant magnetic field. Results suggested that the addition of a 18 m T magnetic field resulted in an optimized weld bead formation with asymmetrical fusion line and grain growth by stabilizing the arc morphology and droplet transfer in the narrow gap. The interaction of arc current and magnetic field differed in central and columnar zones with a moving heat source such that refinement was relatively obvious in the columnar zone. These two zones were dominated by high angle grain boundaries (HAGBs: 15°-60°) and the magnetic field potentially strengthen both the fraction and density. Due to the induced rotation and deflection behavior in austenite (γ) grain growth, the applied magnetic field progressively decreased the texture density of (010)[101] in columnar zones while altered (051)[61-5], (032)[62-3] and (011)[32-2] to (001)[31-0] and (001)[12-0] in central zones. Refinement of γ grains was attributable to the fragment, rotation and deflection of ferrite and associated γ assemblies under axial torques induced by the magnetic field and arc current. Also, γ grain refinement and texture alternation of the central and columnar zones decrease the notch sensitivity of the weld bead. These results and findings suggested a novel means for improving the weld quality of thick plates with favorable microstructure and microtexture using an external magnetic field.

Introduction

Over the past several decades, laser-arc hybrid welding (LAHW) technology has proven to be an excellent method for joining plates of considerable thickness in systematic conclusions of (Churiaque et al., 2019). Compared with traditional arc welding of thick plates, LAHW has a great number of advantages. (Fernandes et al., 2020) proposed that LAHW can substantially reduce filling amount and corresponding heat input, resulting in less residual stress and thus less deformation of an as-weld component. Besides, welding speed is also significantly increased, shortening the production cycle for forming parts. To date, single pass butt welding of thick plates of 24 mm has been accomplished by (Wahba et al., 2016), using parameters of 20 kW, 340A, 1 m/min. (Michael et al., 2009) even achieved 32 mm butt weldments utilizing 20 kW laser power. However, limited by the threshold of laser power available in practice, narrow gap multi-layer laser-arc hybrid welding (NGMHW) technology has risen in importance in manufacturing the thick plate component. In such case, coarse and inhomogeneous microstructure continues to present as a problem in NGMHW weld beads. The performance of as-welded joints is thus limited by undeserved outcomes.

Previous experimental research confirmed the beneficial effect of an applied magnetic field on microstructure and performance in laser-welded seams. (Bachmann et al., 2013) demonstrated that employing a magnetic field had a “dissipating effect” on the dynamic behavior of the weld pool. The mechanism proved to be suppression of the weld pool flow speed, resulting in an alteration of the weld shape. (Chen et al., 2018a) discovered that a reduction in velocity of the melt flow near the surface and the change of seam morphology resulted in formation of a flat solid/liquid interface. Further, (Gatzen and Tang, 2010) reported a steady magnetic field also altered the flow direction of liquid metal, which can be attributed to the heterogeneity of the electromagnetic brake. During laser welding of aluminum die casting, (Fritzsche et al., 2018) employed a magnetic field to achieve a significant reduction in porosity. Surface smoothing resulted from melt convection along depth direction. (Avilov et al., 2015) pointed out that the application of an alternating magnetic field can also prevent gravity drop-out of the weld pool in laser welding of 15 mm thick plates. (Üstündag et al., 2018) also acquired the same conclusions. (Yi and Chen, 2018) used transverse current and a magnetic field to generate a vertical force equal to the hydrostatic pressure in high power laser welding of 16 mm thick plates. By this means, root defects could be eliminated. (Tang and Gatzen, 2010) demonstrated that, in the presence of a magnetic field, element diffusion was more manifest along the welding direction and accelerated as the depth increased. (Liu et al., 2018) demonstrated that electromagnetic stirring disrupted the morphological stability and continuity of Laves phase, enhancing the tensile strength and high cycle fatigue resistance of as-weld joints. Moreover, in solder joints, (Yamanaka et al., 2016) found Sn exhibited a tendency to align along the direction of the magnetic field and this phenomenon was enhanced by magnetic induction intensity. Among these studies, the effect of a magnetic field is governed by the strength and direction of thermal induced current. The functions can be summarized as an alternation of the weld bead appearance, solute elements redistribution and the solidification structure modification.

During LAHW, however, the influence of a magnetic field is achieved by the Lorentz force F=J×B, also identified as electromagnetic force (EMF). In this formulation, J represents the radial component of current passing through conductive melt and B the vertical component of magnetic induction intensity. (Malinowski-Brodnicka et al., 1990) found that EMF causes the annular flow of liquid metal and concurrent microflow at the solid/liquid interface on macro and micro scales, changing the seam formation and solidified microstructure. In addition, (Li et al., 2016) reported that the addition of a magnetic field effectively eliminated the hump of weld bead in high speed gas metal arc welding. (Lin et al., 2016) used tracer particles to record the flow characteristic of the weld pool. Results suggested that the elimination of the characteristic hump was caused by the suppression of backward flow. (Jeng et al., 2018) determined that electromagnetic stirring could cause various orientations of the ferrite (δ) colonies and change the precipitates from Al-Mn-Si oxides to Si-rich particles during arc welding cast austenitic stainless steel. During laser-MIG butt welding of 316 L thin sheets by (Chen et al., 2017), results showed that the use of a magnetic field induced a decrease in δ content, random distribution of orientation and refinement of the microstructure. (Chen et al., 2018b) conducted a series of fatigue tests of welds influenced by a magnetic field. The results demonstrated that the microstructural optimization accelerated the formation of strain-induced martensite at the crack tip. We conclude from the evidence above that external magnetic field is a feasible tool to optimize both the microstructure of grain characteristics and texture.

Based on the previous study conducted by (Zhengwu et al., 2020), weld seams created with magnetic induction intensity of 0 m T and 18 m T were chosen as subjects for the investigation. The considerable differences between columnar zone, central zone and interfacial zone in multi-layer laser-gas metal arc welding (GMAW) welds were examined in the two samples. Detailed post experiment analysis of the two specimens was conducted using advancing detecting techniques for examination of microstructures, microtextures and resultant performances of the as-weld joints with different magnetic field.

Section snippets

Materials

In this study, SUS 316 L stainless steel plates of 10 mm thickness, served as base metal (BM), and ER316 LSi filler wire of 1.2 mm diameter were used. The nominal chemical compositions of primary elements were listed in Table 1. The dimension of the experimental plate was 150 mm × 80 mm×10 mm, with a 20° groove angle, 4 mm thickness of the root face and a 2 mm gap width.

Experimental method

The setup for the magnetic field-assisted multi-layer laser-GMAW experiment was illustrated in Fig. 1. A three-dimensional

γ grains of upper hybrid welds

As can be seen from OM pictures in Fig. 3, the specimens, after being shallow etched, displayed distinguished characteristics of interlamellar boundaries, fusion lines (FL) and γ grains. We concluded that the presence of 18 m T magnetic field altered the macrostructure of the transverse section in two hybrid layers. For the lower layer in Fig. 3a, a twisted FL indicated an unstable metal flow resulted from undeserved droplet transfers in the narrow gap. This was primarily due to side wall

Conclusions

A 10 mm 316 L stainless steel, filled by 1.2 mm 316 LSi, was joined using multi-layer laser-GMAW subjected to a constant magnetic field. The influence of magnetic induction intensity on the microstructure, microtexture and residual stress of γ grains was investigated. In addition, the function mechanism of the magnetic field and resultant notch bending performance are also studied. The main results and findings are listed as follows.

  • 1

    An external constant magnetic field profoundly controlled both

CRediT authorship contribution statement

Zhengwu Zhu: Formal analysis, Writing - original draft, Writing - review & editing. Xiuquan Ma: Project administration. Chunming Wang: Funding acquisition. Gaoyang Mi: Conceptualization. Chongjing Hu: Visualization. Shuye Zheng: Investigation.

Declaration of Competing Interest

None.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 51705173), National Natural Science Foundation of China (Grant No. 51861165202) and Fundamental Research Funds for the Central Universities (HUST: 2018JYCXJJ033). We would like to express our deep gratitude to the Analysis and Test Center of HUST (Huazhong University of Science and Technology) and the State Key Laboratory of Material Processing and Die & Mold Technology of HUST, for their friendly

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