A Study on Fatigue Crack Propagation for Friction Stir Welded Plate of 7N01 Al-Zn-Mg Alloy by EBSD.

Wenyu Liu,Tao Wang,Dongting Wu,Yong Zou,Shuwei Duan

doi:10.3390/ma13020330

Abstract

EBSD (electron backscattered diffraction) was used to study the fatigue crack propagation mechanism in a friction stir welding joint of a 15 mm-thick 7N01 aluminum alloy plate. Crack tips with detailed features were clearly characterized by EBSD images. The plastic zone caused by crack was small in the stir zone. Due to the fine grain strengthening in the stir zone, there were several lattice distortion regions which were observed in the BC (band contrast) map but disappeared in the SEI (secondary electron image). In the stir zone, fatigue crack tends to awake and grow along grain boundaries, and propagate with little deformation of the grains. When the crack tries to grow across a boundary, the deformation of the plastic zone at the crack tip shows little correlation to the cyclic loading direction. However, the plastic zone in base metal, the rolled plate, is large and continuous, and no obvious lattice distortion region was found. According to Schmidt factor, the base metal near crack is fully deformed, lots of low angle boundaries parallel to the cyclic force can be observed. The base metal showed a better ability for fatigue crack propagation resistance.

Highlights

IntroductionSince FSW (friction stir welding) was invented by TWI (The British Welding Institute) in 1991 as a solid phase joining technology, it has been a method for the welding of nonferrous metals and alloys
Since FSW was invented by TWI (The British Welding Institute) in 1991 as a solid phase joining technology, it has been a method for the welding of nonferrous metals and alloys
This paper shows detailed EBSD results with relatively large region for fatigue crack propagation stage propagation stage under higher ∆K

Summary

Introduction

Since FSW (friction stir welding) was invented by TWI (The British Welding Institute) in 1991 as a solid phase joining technology, it has been a method for the welding of nonferrous metals and alloys. As a thermo-mechanical coupling process, FSW connects materials on atom level by plastic deformation [1]. For those metals with low melting temperatures, such as aluminum and magnesium, FSW joints usually show high performance without defects created during fusion welding, and it has been widely used in the area of aerospace and transportation [2,3]. A majority of failures of engineering materials and structures are a result of fatigue. Our understanding of fatigue of materials and structures has grown significantly over the years, failure from metal fatigue is still an ongoing engineering problem. The fatigue performance of FSW joints and the mechanism of fatigue have become more important with the wide usage of FSW [4].

Methods

Results

Conclusion