Microstructural Evolutions of 2N Grade Pure Al and 4N Grade High-Purity Al during Friction Stir Welding.

Tomoya Nagira,Xiaochao Liu,Kohasaku Ushioda,Hidetoshi Fujii

doi:10.3390/ma14133606

Abstract

The grain refinement mechanisms along the material flow path in pure and high-purity Al were examined, using the marker insert and tool stop action methods, during the rapid cooling friction stir welding using liquid CO2. In pure Al subjected to a low welding temperature of 0.56Tm (Tm: melting point), the resultant microstructure consisted of a mixture of equiaxed and elongated grains, including the subgrains. Discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX), and geometric dynamic recrystallization are the potential mechanisms of grain refinement. Increasing the welding temperature and Al purity encouraged dynamic recovery, including dislocation annihilation and rearrangement into subgrains, leading to the acceleration of CDRX and inhibition of DDRX. Both C- and B/-type shear textures were developed in microstructures consisting of equiaxed and elongated grains. In addition, DDRX via high-angle boundary bulging resulted in the development of the 45° rotated cube texture. The B/ shear texture was strengthened for the fine microstructure, where equiaxed recrystallized grains were fully developed through CDRX. In these cases, the texture is closely related to grain structure development.

Highlights

Friction stir welding (FSW) has been recognized as a promising candidate for a solid-state joining technique, which offers several potential advantages including the avoidance of both crack formation and high distortion [1,2,3,4]
Numerous studies have investigated the microstructural features of Al alloys, which are a result of complex plastic deformation phenomena at high temperatures, including recovery, recrystallization, and grain growth during FSW [6,7,8,9,10,11,12,13,14,15,16]
To obtain a better understanding of the microstructural evolution during FSW, the stop action method, where the FSW machine is emergently stopped and the tool is immediately lifted from the work piece, followed by rapid cooling using a mixture of methanol and dry ice or water, was adopted by Fonda et al [12], Prangnell et al [13], Su et al [14], and Suhuddin et al [15]

Summary

Introduction

Friction stir welding (FSW) has been recognized as a promising candidate for a solid-state joining technique, which offers several potential advantages including the avoidance of both crack formation and high distortion [1,2,3,4]. The material flow path around the tool can be visualized by the marker insert method Based on these techniques, Liu et al simultaneously combined the marker insert and stop action methods during more rapid cooling FSW using liquid CO2 [17] in our previous studies [18,19,20,21]. In our previous studies [22,23,24], the effect of the welding temperature on the recrystallization behavior of high-purity Ag with a low SFE (22 mJ·m−2) was examined using the stop action and marker insert methods during rapid cooling FSW. The microstructural evolution of pure Al during FSW along the material flow path was examined through a simultaneous combination of the marker insert and tool stop action methods while rapidly cooling using liquid CO2. The effects of the welding temperature and Al purity on the grain refinement mechanism and texture evolution, with respect to dynamic recrystallization during FSW, were examined

Materials and Methods

High-Purity Al

Findings

Texture Development