Numerically Based Phase Transformation Maps for Dissimilar Aluminum Alloys Joined by Friction Stir-Welding

Carter Hamilton,Aleksandra Węglowska,Adam Pietras,Stanisław Dymek,Mateusz Kopyściański

doi:10.3390/met8050324

Carter Hamilton, Aleksandra Węglowska + Show 3 more

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https://doi.org/10.3390/met8050324

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Abstract

Sheets of aluminum 2017A-T451 and 7075-T651 were friction stir-welded in a butt-weld configuration. An existing computational model of the welding process for temperature distribution and material flow was adapted to estimate the phase transformations that occur across the weld zone. Near the weld center, process temperatures are sufficient to fully dissolve the equilibrium η phase in 7075 and partially dissolve the equilibrium S phase in 2017A. Upon cooling, Guinier–Preston (GP) and Guinier–Preston–Bagaryatsky (GPB) zones re-precipitate, and hardness recovers. Due to the more complete dissolution of the equilibrium phase in 7075, the hardness recovery skews toward whichever side of the weld, i.e., the advancing or retreating side, represents the 7075 workpiece. Phase transformation maps generated by the numerical simulation align not only with the hardness profiles taken across the weld zone, but also with positron lifetimes obtained through positron annihilation lifetime spectroscopy (PALS). Boundaries between the aluminum matrix and the secondary phases provide open volumes to trap positrons; therefore, positron lifetimes across the weld correspond with the phase transformations that occur in 7075 and 2017A during processing.

Highlights

Friction stir-welding (FSW) is a common technique utilized to join metallic materials and produce high-quality welds
Kumar et al [1] and Mishra et al [2,3] have published a book series on friction stir-welding/processing that thoroughly details the current state of knowledge for this technology
The Instytut Spawalnictwa (Institute of Welding) in Gliwice, Poland, friction stir-welded the aluminum 2017A-T451 and 7075-T651 sheets utilizing a conventional milling machine modified for the FSW process

Summary

Introduction

Friction stir-welding (FSW) is a common technique utilized to join metallic materials and produce high-quality welds. Since the friction stir-welding process takes place at lower temperatures than traditional welding methods, i.e., below the melting and solidus temperatures of the alloys, FSW generally circumvents the detrimental effects that can arise from melting and re-solidification. Numerous manufacturing sectors, such as the aerospace and automotive industries, require dissimilar-metal welds in order to produce structures that are lightweight and mechanically sound. Common materials for these structures are aluminum alloys from the 2xxx and 7xxx series, which are classified as “non-weldable” with traditional fusion methods. Because joining occurs in solid-state, FSW mitigates the chemical and Metals 2018, 8, 324; doi:10.3390/met8050324 www.mdpi.com/journal/metals

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