Submerged Dissimilar Friction Stir Welding of AA6061 and AA7075 Aluminum Alloys: Microstructure Characterization and Mechanical Property

Akbar Heidarzadeh,Mousa Javidani,Mohammadreza Mofarrehi,X.-Grant Chen,Amir Farzaneh

doi:10.3390/met11101592

Abstract

The possibility of underwater dissimilar friction stir welding of AA6061 and AA7075 aluminum alloy was explored to overcome the problem of hardness loss in different microstructural zones. Optical microscopy and electron backscattered diffraction were employed to characterize the microstructure of the joint. Vickers hardness measurements were conducted on the cross-section of the joint to evaluate the mechanical strengths. The results showed that the microstructure of the AA7075 side had undergone the same mechanisms as those occurring during conventional friction stir welding. In the case of the AA6061 side, in addition to typical restoration mechanisms, the grain subdivision was observed. The AA7075 side had finer grains compared to the AA6061 side, which may be related to the different morphology and size of precipitates. Moreover, friction stir welding caused a reduction in the hardness values in all the microstructural areas compared to those of corresponding base materials. For example, it caused a reduction in the hardness of a thermomechanically affected zone from 105 HV to 93 HV in the AA6061 side, and from 187 HV to 172 HV in the AA7075 side. The underwater media improved the overall hardness values in thermo-mechanically affected zones (13% reduction in hardness) compared to those reported in literature (57% reduction in hardness).

Highlights

Friction stir welding (FSW) is a suitable solid-state process to join dissimilar metals and alloys
Similar to the other dissimilar FSWed Al alloys [1,34], the macrostructure was composed of the base metals (BMs) (AA6061 and AA7075), heat affected zones (HAZs), thermo-mechanically affected zone (TMAZ), and stirred zone (SZ)
Metals 2021, 11, x FOR PEER REVIEWFor the sake of brevity, this study focused on TMAZs and interfacial ar4eaosf i1n0 SZ to elaborate on the main microstructural mechanisms of grain structure evolution

Summary

Introduction

Friction stir welding (FSW) is a suitable solid-state process to join dissimilar metals and alloys. The microstructures of dissimilar materials subjected to FSW usually consist of five distinct zones, namely an onion structure-like stirred zone (SZ), thermo-mechanically affected zone (TMAZ), and heat-affected zone (HAZ), as well as base metals (BMs) [1,2]. As the material flow plays a major role in the dissimilar material welding, the microstructural evolution strongly depends on the welding process variables. It is worth noting that selecting the right welding parameters such as tool geometry [6] and welding media [7] plays a main role in determining the optimum final mechanical properties of the FSWed joints of metals and alloys

Methods

Results

Conclusion