Local changes in the microstructure, mechanical and electrochemical properties of friction stir welded joints from aluminium of varying grain size

Marta Orłowska,Florian Pixner,Andreas Hütter,Agnieszka Kooijman,Cezary Jasiński,Yaiza Gonzalez-Garcia,Norbert Enzinger,Małgorzata Lewandowska

doi:10.1016/j.jmrt.2021.11.057

Abstract

The present study shows results of friction stir welded (FSW) samples after different plastic deformation routes. The welds were made of coarse-grained and ultrafine-grained commercially pure aluminium. As a plastic deformation method a new hybrid process has been chosen, which resulted in obtaining samples with different characteristics of microstructure, which also differed in dependance of the examined plane. Microstructure observations showed that, regardless of the base material, due to continuous dynamic recrystallization a stir zone was characterized by equiaxial grains with an average size of 3.5–5.0 μm. However, significant differences in the changes of the microstructure in thermomechanically affected and heat affected zones have been obtained between welds. Microhardness profiles revealed a decrease in the stir zones in comparison with the initially deformed samples, but an increase for the annealed samples. Tensile tests showed differences between the samples. In the deformed samples, the rupture occurred in a stir zone, while in the undeformed samples in the base material. In addition, due to the application of 3D digital image correlation, it was possible to observe deformation and local changes between the weld zones during the tensile test. Additionally, local electrochemical measurements were performed with two sizes of working electrode, which included the application of microcapillary technique. The results showed higher corrosion resistance in 3.5% NaCl in the stir zones.

Highlights

Friction stir welding (FSW) [1] is a solid-state welding technique by which metallic plates or sheets can be joined together
Application of plastic deformation caused a significant reduction in grain size and changes in the fraction of high angle grain boundaries (HAGB)
The shape of the grains is a direct consequence of the upsetting process, which causes them to be flattened into ‘pancakes’, and causes a further grain refinement

Summary

Introduction

Friction stir welding (FSW) [1] is a solid-state welding technique by which metallic plates or sheets can be joined together. FSW provides a unique combination of large strain, high temperature and high strain rate that results in both unique performance and complex microstructural changes such as grain structure, texture, phase transformation and dispersion of intermetallic compounds [2]. The welds obtained are characterized by four specific zones in which different changes in the microstructure occur: (i) base material (BM); (ii) heat-affected zone (HAZ); (iii) thermomechanically affected zone (TMAZ) and (iv) in the center of the weld, nugget or stir zone (SZ). Due to plastic deformation at elevated high temperature, dynamic recrystallization (DRX) may occur in this area, resulting in equiaxed grains a few microns in size. Applied process parameters determine the microstructure and properties of welds, as was summarized in details in a review paper [3]

Methods

Results

Discussion

Conclusion