Friction Stir Welding/Processing of Mg-Based Alloys: A Critical Review on Advancements and Challenges

Abstract

Friction stir welding (FSW) and friction stir processing (FSP) are two of the most widely used solid-state welding techniques for magnesium (Mg) and magnesium alloys. Mg-based alloys are widely used in the railway, aerospace, nuclear, and marine industries, among others. Their primary advantage is their high strength-to-weight ratio and usefulness as a structural material. Due to their properties, it is difficult to weld using traditional gas- or electric-based processes; however, FSW and FSP work very well for Mg and its alloys. Recently, extensive studies have been carried out on FSW and FSP of Mg-based alloys. This paper reviews the context of future areas and existing constraints for FSW/FSP. In addition, in this review article, in connection with the FSW and FSP of Mg alloys, research advancement; the influencing parameters and their influence on weld characteristics; applications; and evolution related to the microstructure, substructure, texture and phase formations as well as mechanical properties were considered. The mechanisms underlying the joining and grain refinement during FSW/FSP of Mg alloys-based alloys are discussed. Moreover, this review paper can provide valuable and vital information regarding the FSW and FSP of these alloys for different sectors of relevant industries.

Keywords: friction stir processing; friction stir welding; magnesium-based alloys; mechanical properties; microstructure; severe plastic deformation; texture.

Figure 1

(a) Schematic representation of FSW technique, and FSP technique (Adapted from [27]). (b) Lap joint configuration of the 2024-T3 AlClad FSW assembly (Reprinted with permission from ref. [10]. Copyright 2020 Elsevier Ltd.). (c) Rotation tool in FSW/FSP technique (Adapted from [27]). (d) IR image of FSP tool during processing (Reprinted with permission from ref. [7]. Copyright 2016 Elsevier B.V.). (e) Different FSP tool pin profiles (Reprinted with permission from ref. [7]. Copyright 2016 Elsevier B.V.).

Figure 2

Joint configurations for friction stir welding: (a) square butt, (b) edge butt, (c) T butt joint, (d) lap joint, (e) multiple lap joint, (f) T lap joint, and (g) fillet joint (Reprinted with permission from ref. [91]. Copyright 2005 Elsevier B.V.).

Figure 3

Schematics of several grain refinement mechanisms during the CSA-FSP of the AZ31B magnesium alloy including (a) strain-induced CDRX, (b) micro-shear band induced CDRX, (c) strain-induced DDRX, and (d) twinning-induced DRX (Reprinted with permission from ref. [57]. Copyright 2019 Elsevier B.V.).

Figure 4

Schematic of the grain refinement process during FSP of the QE22 alloy (Reprinted with permission from ref. [99]. Copyright 2018 Elsevier Inc.).

Figure 5

Retardation mechanism in grain growth during composite manufacturing in FSW/FSP technique: (a) grains nucleation and (b) pinning of grain boundaries due to reinforcements (Reprinted with permission from ref. [95]. Copyright 2015 Elsevier B.V.).

Figure 6

Applications of FSW/FSP technique in railway, aerospace, automotive, renewable energy, shipbuilding and marine, and defense industries (Reprinted with permission from ref. [28]. Copyright 2020 Elsevier Ltd.).

Figure 7

(a) Optical macrograph of the transverse cross-section of the Al-Mg joint welded in air with FSW (Reprinted with permission from ref. [14]. Copyright 2020 Elsevier B.V.), and EBSD maps of the grain structures of the FSP stir zone magnesium alloy samples after (b) 1 pass, (c) 2 passes, (d) 4 passes and (e) 6 passes of FSP (Reprinted with permission from ref. [102]. Copyright 2015 Elsevier Ltd.).

Figure 8

Microstructure of the as-cast AZ91 alloy: (a,b) optical microscopy (OM) images, and (c,d) scanning electron microscopy (SEM) images (Reprinted with permission from ref. [15]. Copyright 2020 Elsevier B.V.).

Figure 9

Microstructure of AZ91 alloy friction stir processed: (a) OM image of various zones, (b) SEM image of various zones, (c) SEM image of center of SZ, (d) OM image of TMAZ, and (e,f) SEM images of SZ at higher magnification (Reprinted with permission from ref. [15]. Copyright 2020 Elsevier B.V.).

Figure 10

Schematic illustration of the grain refinement process of the two-pass FSP AZ31 Mg specimens (Reprinted with permission from ref. [54]. Copyright 2008 Acta Materialia Inc.).

Figure 11

TEM images of dislocations distribution at SZ of the friction stir processed AZ31 alloy: (a) rotation speed of 1000 rpm—processing speed of 25 mm/min and (b) rotation speed of 5000 rpm—processing speed of 125 mm/min (Reprinted with permission from ref. [3]. Copyright 2020 Elsevier B.V.).

Figure 12

TEM images of precipitations distribution at SZ of AZ31 alloy friction stir processed: (a) rotation speed of 1000 rpm—processing speed of 25 mm/min, (b) rotation speed of 5000 rpm—processing speed of 125 mm/min, and (c) AZ31 alloy unprocessed (Reprinted with permission from ref. [3]. Copyright 2020 Elsevier B.V.).

Figure 13

TEM image and SAED patterns at SZ of the friction stir processed LZ91 alloy: (a) bright field image, (b) SAED pattern of A zone, and (c) SAED pattern of B zone (Reprinted with permission from ref. [24]. Copyright 2018 Elsevier B.V.).

Figure 14

Pole figures of (0001) derived from EBSD for various locations on the cross-sections of NZ30K alloy friction stir processed in different rotation speeds and processing speeds (Reprinted with permission from ref. [19]. Copyright 2016 Elsevier Ltd.).