Microstructure Characterization of Massive Ferrite in Laser-Weldments of Interstitial-Free Steels

Benjamin Pei-Herng Liu,Shing-Hoa Wang,Ming-Chin Tsai,Tsai-Fu Chung,Chih-Yuan Chen,Ching-Yuan Huang,Jianxun Fu,Jer-Ren Yang

doi:10.3390/met10070898

Abstract

Laser welding, which is known for its precision and high welding rate, offers a high-cooling-rate processing environment. In this study, laser welding was applied to interstitial-free steel plates with phosphorous additions of 0.002 and 0.009 wt%. After laser welding was performed and the weldment was cooled by protective gas, massive ferrite was produced. The base metal, heat affected zone, and weld region were observed and compared by optical microscopy (OM). It was found that increasing the phosphorous content led to refinement of the grain size of the massive ferrite. In addition, the allotriomorphic ferrite in the base metal and the massive ferrite in the weld were characterized and analyzed under scanning electron microscopy–electron backscatter diffraction (SEM-EBSD). The substructures of massive ferrite in OM can be resolved to be low-angle sub-boundaries in kernel average misorientation (KAM) analysis conducted on SEM. Furthermore, TEM analysis revealed that the substructures of massive ferrite were associated with the dislocation cell structures; it is presumed that during the growth of massive ferrite, the rapid migration of incoherent boundaries generated a high dislocation density, and subsequent cooling led to auto-tempering.

Highlights

Nowadays, in the fabrication of metal structures, metal joining is an important process
Welding is shown in Figure which presents weld region, (HAZ), and A
After the weld region dropped to a temperature sufficient to nucleate massive ferrite during the cooling of the laser weld process, to a temperature sufficient to nucleate massive ferrite during the cooling of thethe laser weld process, massive ferrite grains grew along the temperature gradient, which followed moving direction massive ferrite grains grew along the temperature gradient, which followed the moving direction of of the laser beam, from top to bottom, as presented in the middle region of Figure 1

Summary

Introduction

In the fabrication of metal structures, metal joining is an important process. When a high energy source is provided, the welded parts melt and join together, and the variety of the microstructures in the weld region and the heat affected zone deserve much study [1,2,3,4,5]. One popular method to join metals is laser welding, for it offers stable and high-quality results [6,7], which are highly desirable in the automotive industry. Laser welding, which has a concentrated heat source containing a high power density, offers a high welding rate, and, brings about rapid heating and cooling rates. Due to this advantage, the weld region can be narrow, and the heat affected zone very small. The precise control of laser welding has made it a key process in the automotive industry [8,9]

Methods

Results

Conclusion