Analysis of Weak Zones in Friction Stir Welded Magnesium Alloys from the Viewpoint of Local Texture : A Short Review

Friction stir welding (FSW) is a promising approach for the joining of magnesium alloys. Although many Mg alloys have been successfully joined by FSW, it is far from industrial applications due to the texture variation and low mechanical properties. This short review deals with the fundamental understanding of weak zones from the viewpoint of texture analysis in FSW Mg alloys, especially for butt welding. Firstly, a brief review of the microstructure and mechanical properties of FSW Mg alloys is presented. Secondly, microstructure and texture evolutions in weak zones are analyzed and discussed based on electron backscatter diffraction data and Schmid factors. Then, how to change the texture and strengthen the weak zones is also presented. Finally, the review concludes with some future challenges and research directions related to the texture in FSW Mg alloys. The purpose of the paper is to provide a basic understanding on the location of weak zones as well as the weak factors related to texture to improve the mechanical properties and promote the industrial applications of FSW Mg alloys.


Introduction
As the lightest structural materials, magnesium and its alloys have excellent specific strength, high specific rigidity, good cast-ability, hot formability, excellent machinability, good electromagnetic interference shielding and recyclability [1,2].Mg alloys are emerging as important engineering materials, especially in the automotive, rail transit, marine, and aerospace industries [1][2][3].Welding is essential for the development of manufactured products.Currently, riveting, fusion welding and hybrid welding are common methods used to join Mg alloys [4].However, welding defects such as hot cracks, pores, the loss of alloying elements as well as coarse grains are commonly presented in fusion welding of Mg alloys [5].Most hybrid welding methods are mixing fusion welding and other joining methods [6,7].Welding defects also exist in these joints.The hybrid welding process is more complicated.Self-piercing riveting (SPR) is another effective method to join Mg alloys [8].Liu et al. [9] found that equivalent strength of dissimilar joints of AA7075/AZ31 by friction SPR was higher than the yield strength of AZ31 Mg alloy.Durandet et al. [10] found that laser preheating can enhance the dynamic recrystallization (DRX) and prevent cracking, which causes an acceptable strength of the joint of AZ31 alloy prepared by SPR.Haque et al. [11] suggested that assistive technologies help to improve the quality of SPR joints, but some important issues remain to be resolved, such as the corrosion resistance of Mg joints welded by SPR [12].Therefore, the joining of Mg alloys, a key factor for the industrial application of Mg alloys, is still restricted.

A Brief Review of Microstructure and Mechanical Properties of FSW Mg Alloys
During FSW, intense plastic deformation occurs in the weld zone (WZ).Dynamic recrystallization causes dramatic changes of microstructure in the WZ and seriously affects the mechanical properties of joints.Therefore, it is meaningful to explore the relationships between microstructure and mechanical properties of FSW Mg alloys.Indeed, there are many published papers on the subject [27][28][29][30].The grains in WZ can be significantly refined during FSW, especially for cast Mg alloys [22,[31][32][33][34][35][36].The average grain size in WZ for AZ (Mg-Al-Zn) and AM (Mg-Al-Mn) series of Mg alloys is usually about 8-12 µm [1,[37][38][39].For some special Mg alloys, such as ZK60 alloy, due to the presence of Zr-rich precipitates, grain size can be reduced to 3-5 µm in WZ.In addition, the grain size can be further reduced by using some special methods.For example, nano-grains are obtained in FSW AZ31 alloy by liquid nitrogen cooling [34].Refined grains in WZ would have beneficial effects on the mechanical properties of FSW Mg alloys.
Precipitation is another factor affecting the mechanical properties of FSW Mg alloys.Precipitates in Mg alloys are commonly broken up during FSW [40][41][42].In addition, precipitates would be diffuse and uniformly distributed in WZ.For example, precipitates in the cast and aged ZK60 alloy are segregated along grain boundaries in base metal (BM) and are broken up and changed into a uniform distribution after FSW [42,43].Therefore, regardless of grain size or precipitates, the mechanical properties of Mg alloys would be enhanced by FSW.Statistical results about alloy types, thickness, welding parameters and tensile properties of FSW Mg alloys are presented in Table 1.It should be stated that the data are mainly about friction stir butt welding Mg alloys.It is found that the joints usually show a lower tensile strength than that of the BMs, especially for wrought Mg alloys [27,[37][38][39][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57].Table 1 presents that yield strength (YS) of the joints is only 40-80% of BM.The ultimate tensile strength (UTS) of joints is much lower than that of the BMs.Joint efficiency is calculated as the UTS of joint divided by that of the   1 shows the joint efficiency of FSW Mg alloys usually in the range of 50-95%.There is no doubt that defects such as voids, lack of penetration, etc. have an impact on the mechanical properties of the welds.However, more scholars attributed the phenomenon to other factors related to texture.The low mechanical properties may be the main reason for limiting industrial applications of FSW Mg alloys.
For the hexagonal close-packed Mg, due to the asymmetry of crystal structure, texture has an impact on the plastic deformation and strength of materials.It is well known that the YS is mainly determined by the critical resolved shear stress (CRSS) and SF values, while the UTS is largely affected by work hardening as well as the fracture behavior in weak zones.In most cases, FSW/FSP Mg alloys fractured near the interface of WZ and transition zone (TZ) on advancing side (AS) [27,37,61,[82][83][84].Therefore, the WZ/TZ interface is speculated to be a weak zone [29,61].It should be noted that weak factors in these weak zones of FSW Mg alloys may significantly differ from such defects as voids, lack of penetration, channels, etc.However, it can accelerate fracture and reduce the mechanical properties of FSW Mg alloys.In fact, there are several weak zones related to the texture in FSW Mg alloys [82,84,85], including the WZ/TZ interface, stir zone (SZ)-side, triple junction region and SZ-center.The rough locations of these weak zones are provided in Figure 1.Different colors in Figure 1

Interface of WZ/TZ
As mentioned above, DRX causes grain refinement in Mg alloys after FSW (in most cases) [31,34,86], which leads to large differences in grain size between the WZ and TZ.Therefore, the interfaces of WZ/TZ are clearly observed by macroscopic observation, especially for cast Mg alloys [61,87,88].The clear interface indicates a dramatic variation of microstructure on two sides of the interface, implying that the interface of WZ/TZ would be a weak zone of FSW Mg alloys.For some special alloys, little difference in grain size is observed between the WZ and TZ.The interfaces of WZ/TZ are less clear.However, significantly different characteristics of the interface are observed in AS and retreating side (RS) [37,64,87,[89][90][91]. Figure 2 shows that the interface of WZ/TZ is clearly observed in AS.However, that in RS is less clear.Steuwer et al. [89] deeply studied the different formations of WZ/TZ interfaces in AS and RS.Material flows forwards in AS experiencing a shear stress, which leads to a narrow deformation zone adjacent to the WZ/TZ interface.Then, a clear interface is presented in AS [89].However, for the RS, material flows are trapped and extruded during the transferring from AS to RS, which leads to a wider, less severe strain gradient in RS [89,91].Then, a less clear interface of WZ/TZ is formed in RS (see Figure 2c)."-" indicates the data are not provided in the reference.

Interface of WZ/TZ
As mentioned above, DRX causes grain refinement in Mg alloys after FSW (in most cases) [31,34,86], which leads to large differences in grain size between the WZ and TZ.Therefore, the interfaces of WZ/TZ are clearly observed by macroscopic observation, especially for cast Mg alloys [61,87,88].The clear interface indicates a dramatic variation of microstructure on two sides of the interface, implying that the interface of WZ/TZ would be a weak zone of FSW Mg alloys.For some special alloys, little difference in grain size is observed between the WZ and TZ.The interfaces of WZ/TZ are less clear.However, significantly different characteristics of the interface are observed in AS and retreating side (RS) [37,64,87,[89][90][91]. Figure 2 shows that the interface of WZ/TZ is clearly observed in AS.However, that in RS is less clear.Steuwer et al. [89] deeply studied the different formations of WZ/TZ interfaces in AS and RS.Material flows forwards in AS experiencing a shear stress, which leads to a narrow deformation zone adjacent to the WZ/TZ interface.Then, a clear interface is presented in AS [89].However, for the RS, material flows are trapped and extruded during the transferring from AS to RS, which leads to a wider, less severe strain gradient in RS [89,91].Then, a less clear interface of WZ/TZ is formed in RS (see Figure 2c).

Interface of WZ/TZ
As mentioned above, DRX causes grain refinement in Mg alloys after FSW (in most cases) [31,34,86], which leads to large differences in grain size between the WZ and TZ.Therefore, the interfaces of WZ/TZ are clearly observed by macroscopic observation, especially for cast Mg alloys [61,87,88].The clear interface indicates a dramatic variation of microstructure on two sides of the interface, implying that the interface of WZ/TZ would be a weak zone of FSW Mg alloys.For some special alloys, little difference in grain size is observed between the WZ and TZ.The interfaces of WZ/TZ are less clear.However, significantly different characteristics of the interface are observed in AS and retreating side (RS) [37,64,87,[89][90][91]. Figure 2 shows that the interface of WZ/TZ is clearly observed in AS.However, that in RS is less clear.Steuwer et al. [89] deeply studied the different formations of WZ/TZ interfaces in AS and RS.Material flows forwards in AS experiencing a shear stress, which leads to a narrow deformation zone adjacent to the WZ/TZ interface.Then, a clear interface is presented in AS [89].However, for the RS, material flows are trapped and extruded during the transferring from AS to RS, which leads to a wider, less severe strain gradient in RS [89,91].Then, a less clear interface of WZ/TZ is formed in RS (see Figure 2c).In addition to the large variation of grain size between WZ and TZ, different textures are observed on two sides of the WZ/TZ interface.EBSD orientation maps of the SZ/TZ interfaces of FSW AZ31 Mg alloy are shown in Figure 3 and the inverse pole figure (IPF) is color-coded according to the transverse direction (TD) [92].It is noted that the difference in grain size between the SZ and TZ is not evident.However, clear interfaces of SZ/TZ with different colors are observed in both the AS and RS (see Figure 3), indicating different grain orientations in the TZ and SZ.Woo et al. [73] reported that significant texture variations between the TZ and SZ can cause inhomogeneous deformation and lead to the fracture near the SZ/TZ interface.There is no doubt that large texture variations at the SZ/TZ interface can accelerate fracture and reduce the mechanical properties of FSW Mg alloys.Therefore, the interface of WZ/TZ, especially for that in AS, is a significant weak zone for FSW Mg alloys.Indeed, there are many FSW Mg alloys fractured at or near the interface of WZ/TZ in AS during transverse tensile tests [1,27], confirming that the interface of WZ/TZ in AS is a significant weak zone.
the SZ and TZ is not evident.However, clear interfaces of SZ/TZ with different colors are observed in both the AS and RS (see Figure 3), indicating different grain orientations in the TZ and SZ.Woo et al. [73] reported that significant texture variations between the TZ and SZ can cause inhomogeneous deformation and lead to the fracture near the SZ/TZ interface.There is no doubt that large texture variations at the SZ/TZ interface can accelerate fracture and reduce the mechanical properties of FSW Mg alloys.Therefore, the interface of WZ/TZ, especially for that in AS, is a significant weak zone for FSW Mg alloys.Indeed, there are many FSW Mg alloys fractured at or near the interface of WZ/TZ in AS during transverse tensile tests [1,27], confirming that the interface of WZ/TZ in AS is a significant weak zone.

SZ-Side
In addition to grain refinement, a strong texture is formed in FSW Mg alloys [28,31,32,73,93,94].Textures in typical regions of FSW AZ31 plate are presented in Figure 4 [82].The corresponding positions are shown in Figure 4a.Significantly different textures are found in various regions (see Figure 4b).BM shows a typical characteristic of hot-rolled Mg-alloy plate with the c-axis parallel to the normal direction (ND).SZ-center (position of a1 in Figure 4a) shows the c-axis of most grains perpendicular to the TD, while SZ-side (position of a2) presents a texture with the c-axis nearly parallel to the TD.Basal slip and extension twinning are most easily activated at room temperature for Mg alloys due to the low CRSS [95,96].The activation of basal slip and extension twinning can be predicted by SF values based on EBSD data.Figure 4c shows that SZ-side has the highest SF value (~0.4), while SZ-center presents the lowest SF value (~0.14).The results indicate that basal slip is easily activated in SZ-side and hardly activated in SZ-center.Therefore, SZ-side would be a weak zone for FSW Mg alloys, compared with the SZ-center as well as the BM.

SZ-Side
In addition to grain refinement, a strong texture is formed in FSW Mg alloys [28,31,32,73,93,94].Textures in typical regions of FSW AZ31 plate are presented in Figure 4 [82].The corresponding positions are shown in Figure 4a.Significantly different textures are found in various regions (see Figure 4b).BM shows a typical characteristic of hot-rolled Mg-alloy plate with the c-axis parallel to the normal direction (ND).SZ-center (position of a 1 in Figure 4a) shows the c-axis of most grains perpendicular to the TD, while SZ-side (position of a 2 ) presents a texture with the c-axis nearly parallel to the TD.Basal slip and extension twinning are most easily activated at room temperature for Mg alloys due to the low CRSS [95,96].The activation of basal slip and extension twinning can be predicted by SF values based on EBSD data.Figure 4c shows that SZ-side has the highest SF value (~0.4), while SZ-center presents the lowest SF value (~0.14).The results indicate that basal slip is easily activated in SZ-side and hardly activated in SZ-center.Therefore, SZ-side would be a weak zone for FSW Mg alloys, compared with the SZ-center as well as the BM.To deeply understand the weak factors from the viewpoint of local texture in SZ-side, SF maps of extension twinning and basal slip in SZ-side of FSW AZ31 alloy are studied based on EBSD data.Figure 5 shows that a narrow region in SZ-side close to the SZ/TZ interface presents the highest SF value for extension twinning (the blue zone in Figure 5a).A relatively wide region, which is slightly away from the SZ/TZ interface, shows the highest SF value for basal slip (the blue zone in Figure 5b).The results indicate that dominant deformation mechanisms in SZ-side are changed from extension twinning to basal slip as the region moving away from the SZ/TZ interface [84].
Based on the SF values, SZ-side can be divided into two micro-regions.One is the Easy to Activate Extension Twinning region (EAET region), which is close to the SZ/TZ interface.The other is slightly away from the SZ/TZ interface, named as the Easy to Activate Basal Slip region (EABS region).{0001} pole figures of various micro-regions are presented in Figure 5c, indicating that the To deeply understand the weak factors from the viewpoint of local texture in SZ-side, SF maps of extension twinning and basal slip in SZ-side of FSW AZ31 alloy are studied based on EBSD data.Figure 5 shows that a narrow region in SZ-side close to the SZ/TZ interface presents the highest SF value for extension twinning (the blue zone in Figure 5a).A relatively wide region, which is slightly away from the SZ/TZ interface, shows the highest SF value for basal slip (the blue zone in Figure 5b).The results indicate that dominant deformation mechanisms in SZ-side are changed from extension twinning to basal slip as the region moving away from the SZ/TZ interface [84].
Based on the SF values, SZ-side can be divided into two micro-regions.One is the Easy to Activate Extension Twinning region (EAET region), which is close to the SZ/TZ interface.The other is slightly away from the SZ/TZ interface, named as the Easy to Activate Basal Slip region (EABS region).{0001} pole figures of various micro-regions are presented in Figure 5c, indicating that the c-axis is inclined about 10 • towards TD in EAET region, while it is inclined about 40 • towards TD in EABS region.It confirms that it is easy to activate extension twinning in the EAET region, while it is easy to activate basal slip in the EABS region during transverse tensile tests [84].In addition, it is found that EAET region is very narrow, only about 100 µm wide along the TD (see Figure 5).The activation of different deformations in such a narrow zone of SZ-side may cause plastic deformation incompatibility and initiate fracture.The results fully confirm that SZ-side is a weak zone of FSW Mg alloys.

Triple Junction Region
A special region is presented adjacent to the WZ/TZ interface and near the transfer region of the crown zone (CZ) and SZ in AS.Because it is near the junction region among the TZ, CZ and SZ, it was named as triple junction region [85].In fact, the triple junction region widely exists in FSW Mg alloys.However, it may be hardly observed in optical micrographs, because the variation of grain size in triple junction region is not evident.In addition, triple junction region can be observed in AS, but is hardly identified in RS.EBSD maps of the triple junction region of FSW AZ31 alloy are provided in Figure 6.It is confirmed that the variation of grain size in the region is not obvious.However, clear interfaces with different colors are observed in the orientation map, indicating different textures presented in this region [85].
Based on the texture distributions, the triple junction region may be divided into four parts:

Triple Junction Region
A special region is presented adjacent to the WZ/TZ interface and near the transfer region of the crown zone (CZ) and SZ in AS.Because it is near the junction region among the TZ, CZ and SZ, it was named as triple junction region [85].In fact, the triple junction region widely exists in FSW Mg alloys.However, it may be hardly observed in optical micrographs, because the variation of grain size in triple junction region is not evident.In addition, triple junction region can be observed in AS, but is hardly identified in RS.EBSD maps of the triple junction region of FSW AZ31 alloy are provided in Figure 6.It is confirmed that the variation of grain size in the region is not obvious.However, clear interfaces with different colors are observed in the orientation map, indicating different textures presented in this region [85].
Metals 2018, 8, x FOR PEER REVIEW 8 of 16 the triple junction region.It was confirmed by a sharp corner appearing close to the triple junction region for the tensile and compressive samples interrupted at 5% strain [97].Moreover, the authors recently studied different fracture behaviors of FSW AZ31 Mg alloy during Surface and Base bending tests and found that triple junction region has a significant effect on fracture behavior as it is subjected to tensile state, and the effect is much smaller in compression state [98].Based on the texture distributions, the triple junction region may be divided into four parts: Regions I-A, I-B, II and III (see Figure 6b).Figure 6c shows that the c-axis is inclined about 19 • towards TD in Region I-A, while it is inclined about ~56 • towards TD in Region I-B.The angles between the c-axis and TD in Regions II and III are ~22 • and ~78 • , respectively.It is noted that the c-axis of grains in Region I-B has rotated toward ND compared with Region I-A.Therefore, it is speculated that the formation of triple junction region has a close relationship with screw threads of stir tool [83].The screw threads extrude the materials in Region I-B with a vertical stress during FSW, which causes the c-axis of grains in Region I-B rotated toward the ND.However, the detailed formation mechanism of triple junction region is still unknown.
To explore the plastic deformation mechanisms in the triple junction region, SF values of basal slip and extension twinning in the region are studied.Figure 7 shows that all micro-regions in triple junction region have a high SF value of basal slip (>0.3).It means that basal slip is easily activated in the triple junction region.Moreover, large differences in SF values of extension twinning are observed in the region.The highest SF value of extension twinning is ~0.37 in Region I-A, while the lowest one is 0.11 in Region III.The results indicate that the competition of basal slip and extension twinning in triple junction region is very complex.An incompatible deformation easily occurs in the triple junction region.It was confirmed by a sharp corner appearing close to the triple junction region for the tensile and compressive samples interrupted at 5% strain [97].Moreover, the authors recently studied different fracture behaviors of FSW AZ31 Mg alloy during Surface and Base bending tests and found that triple junction region has a significant effect on fracture behavior as it is subjected to tensile state, and the effect is much smaller in compression state [98].Triple junction region is also observed at the dissimilar joint of ZK60/AZ31 by FSW [83].In addition to the texture variation, grain size shows large differences in various micro-regions of the triple junction region [83].Owing to the presence of triple junction region in AS, the joint is fractured close to ZK60 alloy, the "hard" material side in AS, during the transverse tensile tests.Moreover, a sharp corner is observed in the fracture line of the fractured sample of ZK60/AZ31 joint [83].The location of sharp corner is near close to the triple junction region.The results confirmed that triple junction region has an impact on the fracture behaviors of FSW Mg alloys, which is a significant weak zone for FSW Mg alloys.

SZ-Center
The fracture of FSW Mg alloys usually starts at the SZ/TZ interface, and it is propagated toward SZ-center with an inclined angle of ~45 • [29,37,78].Therefore, SZ-center may be another weak zone of FSW Mg alloys.Yang et al. [29] studied the activation of twinning in SZ-center during the transverse tensile tests.They found that twinning is hardly activated in SZ-center as the tensile stress was lower than 75% UTS.However, many compression twins and double twins are detected in SZ-center by EBSD as the tensile stress reached 95% UTS.
In general, compression twinning and double twinning are hardly activated at room temperature due to the high CRSS.However, it may occur in the later stage of deformation due to the high stress.Compression twinning can be activated as the grains are in compression along the c-axis, with an average CRSS value of 112 MPa [96,99].As uniaxial tensile tests were carried out on tensile samples, there is a compression along the thickness and width directions.The grains in SZ-center exactly present the c-axis nearly paralleling to the thickness direction (i.e., the WD, see Figure 4b).Therefore, there are opportunities to activate compression twinning and double twinning in SZ-center during the transverse tensile tests.The formation of compression twins and double twins may produce a highly localized strain field and induce strain incompatibility and the failure at the twin interfaces [29].Therefore, SZ-center may be another weak zone of FSW Mg alloys.
Nevertheless, the authors believe that several weak zones related to texture exist in FSW Mg alloys, including the WZ/TZ interface, SZ-side, triple junction region and SZ-center.The approximate locations of weak zones are presented in Figure 1.The weak zones related to texture have an impact on the mechanical properties and fracture behavior of FSW Mg alloys.

How to Modify the Local Texture and Strengthen the Weak Zones in FSW Mg Alloys
Researchers tried to modify welding parameters and refine the grains to improve the mechanical properties of FSW Mg alloys [29,37,75,88].However, Wang et al. [32] found that the texture of Mg alloys can significantly reduce the effect of grain size on the strength of the welds.The main factor affecting the mechanical properties of FSW Mg alloys is not grain size but the local texture in WZ [32]. Lee et al. [100] found that for FSW AZ61 sample, a subsequent compression along the ND can change the texture by inducing deformation twins.The YS can significantly be raised from 140 to 260 MPa.However, it should be stated that the tensile specimen is a pocket one and sampling only from SZ-center.Many weak zones, such as the SZ/TZ interface, SZ-side and triple junction region, are excluded in the test specimen.Therefore, the results cannot represent the entire mechanical properties of the joint after subsequent compression.
To modify the texture and strengthen the weak zones in FSW Mg alloys, subsequent rolling and tensile along the TD were employed in FSW AZ31 alloy [27,101].Microstructure evolution and twinning behaviors at the SZ/TZ interface (a weak zone) after subsequent deformations are evaluated by EBSD. Figure 8a shows that many extension twins are formed in SZ-side after post-rolling along the TD [101].The formed twins in SZ-side present a low SF value, especially for extension twinning (see Figure 8b,c).The results confirm that subsequent rolling can change the grains from soft orientations into hard orientations.In addition, SF value of basal slip in SZ-side is always higher than that of extension twinning after post-rolling.It means that the EAET region disappears in SZ-side after post-rolling.The dominant deformation mechanism in SZ-side is basal slip and not changed after post-rolling (see Figure 8d).disappears in SZ-side after post-rolling.The dominant deformation mechanism in SZ-side is basal slip and not changed after post-rolling (see Figure 8d).
During the subsequent rolling process, fine-grain strengthening and texture strengthening enhance the weak zones in FSW AZ31 alloy.It reduces the inhomogeneous deformation between SZ-center and SZ-side and improves the mechanical properties of the joints.It is reported that the YS is raised from ~87 MPa in the as-welded state to ~171 MPa in the specimens with a rolling strain of ~7%.In addition, subsequent rolling cause the fracture position of FSW Mg alloys changed from SZ-side to BM [27,101].Therefore, subsequent deformation along specific directions can significantly enhance the weak zones and improve the mechanical properties of FSW Mg alloys by inducing deformation twins.Moreover, some relationships between welding parameters and texture distributions are found in FSW Mg alloys [102,103].However, it is difficult to control the texture in WZ by changing the welding parameters so far.Therefore, how to modify the textures in weak zones through a simple and quick method needs further systematic research.

Conclusions and Further Works
In this paper, weak zones in FSW Mg alloys and the weak factors from the viewpoint of texture are reviewed.Current situation and some critical scientific problems in the research field are summarized as follows: (1) Various textures in WZ cause different plastic deformations and several weak zones related to the texture form in FSW Mg alloys.Weak factors in these weak zones significantly differ from such defects as voids, lack of penetration, channels, etc.However, they can accelerate fracture and reduce the mechanical properties of FSW Mg alloys.(2) The WZ/TZ interface is a weak zone of FSW Mg alloys.Drastic changes of microstructure and During the subsequent rolling process, fine-grain strengthening and texture strengthening enhance the weak zones in FSW AZ31 alloy.It reduces the inhomogeneous deformation between SZ-center and SZ-side and improves the mechanical properties of the joints.It is reported that the YS is raised from ~87 MPa in the as-welded state to ~171 MPa in the specimens with a rolling strain of ~7%.In addition, subsequent rolling cause the fracture position of FSW Mg alloys changed from SZ-side to BM [27,101].Therefore, subsequent deformation along specific directions can significantly enhance the weak zones and improve the mechanical properties of FSW Mg alloys by inducing deformation twins.Moreover, some relationships between welding parameters and texture distributions are found in FSW Mg alloys [102,103].However, it is difficult to control the texture in WZ by changing the welding parameters so far.Therefore, how to modify the textures in weak zones through a simple and quick method needs further systematic research.

Conclusions and Further Works
In this paper, weak zones in FSW Mg alloys and the weak factors from the viewpoint of texture are reviewed.Current situation and some critical scientific problems in the research field are summarized as follows: (1) Various textures in WZ cause different plastic deformations and several weak zones related to the texture form in FSW Mg alloys.Weak factors in these weak zones significantly differ from such defects as voids, lack of penetration, channels, etc.However, they can accelerate fracture and reduce the mechanical properties of FSW Mg alloys.(2) The WZ/TZ interface is a weak zone of FSW Mg alloys.Drastic changes of microstructure and texture exist near the WZ/TZ interface, especially for that in AS, which can accelerate fracture and reduce the mechanical properties of the joints.Thus, how to reduce the changes of microstructure and texture in the WZ/TZ interfaces would be an interesting research direction.(3) SZ-side has large SF values of basal slip and extension twinning during the transverse tensile tests.Dominant deformation mechanisms in SZ-side are transferred from extension twinning to basal slip with greater distance from the SZ/TZ interface.SZ-side can be divided into EAET region and EABS region.FSW Mg alloys could fracture in SZ-side on AS.However, due to the narrow EAET region, the fracture actually occurrs in EAET region or EABS region is still unknown.It is meaningful to confirm whether fracture is initiated in EAET region or EABS region.(4) Significant differences of microstructure and texture are presented in triple junction region.
The complex competition of basal slip and extension twinning results in incompatible deformations in the triple junction region, which has a significant promoting effect on the fracture of FSW Mg alloys.However, the detailed formation of triple junction region is still unclear.How to control the formation of triple junction region needs to be studied.(5) Subsequent deformation along specific directions can significantly enhance weak zones related to the texture and improve the mechanical properties of FSW Mg alloys by inducing deformation twins.There are some relationships between welding parameters and texture distributions of FSW Mg alloys.However, it is difficult to control the texture in WZ by changing the welding parameters.Therefore, how to modify the textures in weak zones through a simple and quick method needs further systematic research.

4 Figure 1 .
Figure 1.Schematic diagram of weak zones in FSW Mg alloys from the viewpoint of local texture.

Figure 1 .
Figure 1.Schematic diagram of weak zones in FSW Mg alloys from the viewpoint of local texture.

Figure 1 .
Figure 1.Schematic diagram of weak zones in FSW Mg alloys from the viewpoint of local texture.

Figure 2 .
Figure 2. Interfaces between the TZ and WZ of FSW Mg alloys: (a) macro photo; (b) the interface in AS; and (c) the interface in RS [83].

Figure 4 .
Figure 4. EBSD orientation maps of various regions in FSW AZ31 alloy: (a) measurement positions; (b) {0001} pole figures; and (c) SF maps of basal slip in various regions during the transverse tensile tests [84].

Figure 4 .
Figure 4. EBSD orientation maps of various regions in FSW AZ31 alloy: (a) measurement positions; (b) {0001} pole figures; and (c) SF maps of basal slip in various regions during the transverse tensile tests [84].

Figure 5 .
Figure 5. SF maps at the SZ/TZ interface of FSW AZ31 alloy: SF maps for extension twinning (a) and basal slip (b); and {0001} pole figures in micro-regions near the SZ/TZ interface [84] (c).

Figure 5 .
Figure 5. SF maps at the SZ/TZ interface of FSW AZ31 alloy: SF maps for extension twinning (a) and basal slip (b); and {0001} pole figures in micro-regions near the SZ/TZ interface [84] (c).

Figure 7 .
Figure 7. SF maps of triple junction region in FSW AZ31 alloy during the transverse tensile tests: (a) SF map for basal slip; and (b) SF map for extension twinning.

Figure 7 .
Figure 7. SF maps of triple junction region in FSW AZ31 alloy during the transverse tensile tests: (a) SF map for basal slip; and (b) SF map for extension twinning.

Figure 7 .
Figure 7. SF maps of triple junction region in FSW AZ31 alloy during the transverse tensile tests: (a) SF map for basal slip; and (b) SF map for extension twinning.

Figure 8 .
Figure 8. EBSD orientation maps of the SZ/TZ interface in FSW AZ31 alloy with a subsequent rolling strain of 2.5%: (a) band contrast map with extension twin boundaries indicated by red lines; SF map for basal slip (b) and extension twinning (c); and (d) variation of SF values as a function of the distance from the SZ/TZ interface [101].

Figure 8 .
Figure 8. EBSD orientation maps of the SZ/TZ interface in FSW AZ31 alloy with a subsequent rolling strain of 2.5%: (a) band contrast map with extension twin boundaries indicated by red lines; SF map for basal slip (b) and extension twinning (c); and (d) variation of SF values as a function of the distance from the SZ/TZ interface [101].

Table 1 .
Summary of tensile properties of FSW Mg-alloys.