Effect of Parameters on Fatigue Properties and Crack Propagation Behavior of Friction Stir Crack Repaired Al2024

Through investigating the effect of parameters on the fatigue properties and crack propagation behavior of friction stir crack repaired 2024 aluminum alloy, we demonstrated that the fatigue strength of friction stir repaired Al2024 was sensitive to the repairing parameters and had a “V” type discipline with the rotating speed or advancing velocity. The fatigue crack growth rates of the repaired specimens were higher than the base material counterpart, and the crack propagation mechanism in the repaired zone was mainly intergranular. When the improper repairing parameters were adopted, the delamination defect would form at the interfaces between the repaired layer and base material


Introduction
Since reliability and safety have become increasingly important for engineering structures and equipment, cracking issues are considered to play a detrimental key role in the lifetime of engineering materials and structures [1,2]. It is well known that a crack usually initiates from the surface or sub-surface because of the plane stress state for materials near the surface, and then the crack can continue to propagate for a certain period under loading before the final fracture, which is also named as the life of the structure. Due to the time-based failure mode, it provides a possibility for crack repair before the final fracture in engineering structures.
Nowadays, several crack repair methods have been proposed and implemented in engineering including welding, laser deposition, electron-beam irradiation, electro-pulsing, and electro-healing by electrochemical processes, etc. [3][4][5]. Nevertheless, most of the aforementioned crack repair methods have intrinsic disadvantages such as poor repairing quality, the recurrence of cracks or high energy consumption, etc.
Friction stir welding (FSW) is a promising joining technology in high strength to weight ratio metals including, but not limited to aluminum alloys, copper alloys, magnesium alloys, titanium alloys, and dissimilar metallic materials, etc. [6][7][8][9]. A consequential material processing technique named friction stir processing (FSP) has been proposed to produce fine and super-fine grains [10][11][12]. Since being invented, FSW and FSP are basically used for material jointing and processing, although several studies have attempted to repair defects by FSW [13][14][15], but few research work on crack repairing using FSP-related techniques have been reported. The application of FS processed structures mainly depends on the mechanical performances, which are strongly affected by the processing parameters [16]. Regarding fatigue behavior, several investigations have been conducted on FSW joints of similar and dissimilar alloys [17][18][19][20][21][22][23]. Deng and Chao et al. focused on the very high cycle fatigue properties of Al7050 and Al7075 FS welds [24,25]. However, the FSCR is different from the FSW process because of the prefabricated crack and the absence of the pin, which will also lead to a different fatigue behavior of the repaired specimen.
In order to optimize the FSCR parameters for engineering application, the present study elaborates the effect of crack repairing parameters on the fatigue and crack propagation behaviors of a FSCRed 2024 aluminum alloy. Moreover, the fatigue fracture morphology will be discussed based on the fractographic analysis.

Experimental Procedure
In this investigation, a 2024-T4 aluminum alloy plate of 4 mm thickness was used. The chemical composition (wt%) of the base metal was: Si 0.5, Fe 0.5, Cu 3.9, Mn 0.4, Mg 1.4, Ni 0.1, Zn 0.3, Ti 0.15, Al bal, and the mechanical properties of Al2024 are presented in Table 1. The plate was pre-cracked by a wire-cutting machine (Heng Tian DK7750, Taizhou, China) on the surface, where the crack depth and width was about 1 mm and 0.5 mm, respectively. A FSW-3LM-4012 friction stir welding system (China FSW Center, Beijing, China) was utilized to prepare the repaired specimens. The columned repairing tool was made from tool steel with a 15 mm shoulder diameter and ringed grooves at the shoulder end, and a 2° tilt angle of the repairing tool with respect to the Z-axis of the machine was used to strengthen the forging action.
Different parameters including rotating speed and advancing velocity were adopted in order to study the effect of repairing parameters on the mechanical performance of repaired specimens. The rotating speeds ranged from 700 r/min to 1500 r/min, while the advancing velocities ranged from 70 mm/min to 150 mm/min. The exact parameter pairs are shown in Figure 2. Repairing tool Crack Figure 1. Outline of friction stir crack repairing principle.
The application of FS processed structures mainly depends on the mechanical performances, which are strongly affected by the processing parameters [16]. Regarding fatigue behavior, several investigations have been conducted on FSW joints of similar and dissimilar alloys [17][18][19][20][21][22][23]. Deng and Chao et al. focused on the very high cycle fatigue properties of Al7050 and Al7075 FS welds [24,25]. However, the FSCR is different from the FSW process because of the prefabricated crack and the absence of the pin, which will also lead to a different fatigue behavior of the repaired specimen.
In order to optimize the FSCR parameters for engineering application, the present study elaborates the effect of crack repairing parameters on the fatigue and crack propagation behaviors of a FSCRed 2024 aluminum alloy. Moreover, the fatigue fracture morphology will be discussed based on the fractographic analysis.

Experimental Procedure
In this investigation, a 2024-T4 aluminum alloy plate of 4 mm thickness was used. The chemical composition (wt%) of the base metal was: Si 0.5, Fe 0.5, Cu 3.9, Mn 0.4, Mg 1.4, Ni 0.1, Zn 0.3, Ti 0.15, Al bal, and the mechanical properties of Al2024 are presented in Table 1. The plate was pre-cracked by a wire-cutting machine (Heng Tian DK7750, Taizhou, China) on the surface, where the crack depth and width was about 1 mm and 0.5 mm, respectively. A FSW-3LM-4012 friction stir welding system (China FSW Center, Beijing, China) was utilized to prepare the repaired specimens. The columned repairing tool was made from tool steel with a 15 mm shoulder diameter and ringed grooves at the shoulder end, and a 2 • tilt angle of the repairing tool with respect to the Z-axis of the machine was used to strengthen the forging action. Different parameters including rotating speed and advancing velocity were adopted in order to study the effect of repairing parameters on the mechanical performance of repaired specimens. The rotating speeds ranged from 700 r/min to 1500 r/min, while the advancing velocities ranged from 70 mm/min to 150 mm/min. The exact parameter pairs are shown in Figure 2. Based on ASTM E466-07 and ASTM E647-11, fatigue and crack propagation specimens were respectively sectioned from the repaired specimen perpendicular to the repairing direction and the detailed sizes are shown in Figure 3. Fatigue tests were performed in accordance with ASTM E466 using a QBG-100 high-frequency fatigue testing machine (Qianbang, Changchun, China) under a constant amplitude axial load with a stress ratio of R = 0.1. The maximum stress was set at 180 MPa (sinusoidal loading) and the cycling rate was kept at a frequency of 80 Hz. Crack propagation tests were performed in accordance with ASTM E647 under stress ratio R = 0.1 and frequency 15 Hz using the sinusoidal loading form, and a traveling optical microscope (Wuguang, Changchun, China) was utilized to monitor the crack propagation behavior continuously. Duplicate fatigue crack propagation tests were performed for each parameter pair. For comparison, the crack propagation test of the base material was also carried out. A Hitachi S-3400N scanning electron microscope (Hitachi, Tokyo, Japan) was adopted for post-mortem fractography analysis.

Surface Morphology of Repaired Specimens
During the FSCR process, material around the crack was heated and plasticized because of the Based on ASTM E466-07 and ASTM E647-11, fatigue and crack propagation specimens were respectively sectioned from the repaired specimen perpendicular to the repairing direction and the detailed sizes are shown in Figure 3. Fatigue tests were performed in accordance with ASTM E466 using a QBG-100 high-frequency fatigue testing machine (Qianbang, Changchun, China) under a constant amplitude axial load with a stress ratio of R = 0.1. The maximum stress was set at 180 MPa (sinusoidal loading) and the cycling rate was kept at a frequency of 80 Hz. Crack propagation tests were performed in accordance with ASTM E647 under stress ratio R = 0.1 and frequency 15 Hz using the sinusoidal loading form, and a traveling optical microscope (Wuguang, Changchun, China) was utilized to monitor the crack propagation behavior continuously. Duplicate fatigue crack propagation tests were performed for each parameter pair. For comparison, the crack propagation test of the base material was also carried out. A Hitachi S-3400N scanning electron microscope (Hitachi, Tokyo, Japan) was adopted for post-mortem fractography analysis. Based on ASTM E466-07 and ASTM E647-11, fatigue and crack propagation specimens were respectively sectioned from the repaired specimen perpendicular to the repairing direction and the detailed sizes are shown in Figure 3. Fatigue tests were performed in accordance with ASTM E466 using a QBG-100 high-frequency fatigue testing machine (Qianbang, Changchun, China) under a constant amplitude axial load with a stress ratio of R = 0.1. The maximum stress was set at 180 MPa (sinusoidal loading) and the cycling rate was kept at a frequency of 80 Hz. Crack propagation tests were performed in accordance with ASTM E647 under stress ratio R = 0.1 and frequency 15 Hz using the sinusoidal loading form, and a traveling optical microscope (Wuguang, Changchun, China) was utilized to monitor the crack propagation behavior continuously. Duplicate fatigue crack propagation tests were performed for each parameter pair. For comparison, the crack propagation test of the base material was also carried out. A Hitachi S-3400N scanning electron microscope (Hitachi, Tokyo, Japan) was adopted for post-mortem fractography analysis.

Surface Morphology of Repaired Specimens
During the FSCR process, material around the crack was heated and plasticized because of the friction and stirring action between the shoulder and specimen surface, and the thermoplastic

Surface Morphology of Repaired Specimens
During the FSCR process, material around the crack was heated and plasticized because of the friction and stirring action between the shoulder and specimen surface, and the thermoplastic materials cooled down and recrystallized with the shoulder passing by. Consequently, some arc corrugations emerged at the repaired specimen surfaces, as shown in Figure 4. The distance between two adjacent arcs, which can be named as the advancing pitch (in FSW, it is called the weld pitch [26]) and denoted by λ, is almost equal to the advancing distance per rotating cycle of the tool shoulder ( Figure 5) and is proportional to the ratio of advancing velocity (ν) and rotating speed (ω), and inversely proportional to the energy input E of the FSCR process, which can be expressed by Equation (1).
(1) Figure 4 shows the surface morphologies of the repaired specimens under varying parameters. With the increase in ν when ω = 1100 r/min, λ increased as the advancing distance increased per rotating cycle according to Equation (1), and obvious plastic deformation marks like burrs could be found at the repaired surfaces ( Figure 4a,c), especially when a low ν was adopted, as shown in Figure 4e. With the increase of ω when ν = 110 mm/min, λ decreased, and consequently a smoother repaired surface could be obtained (Figure 4d). When a relatively lower ω was chosen, the repaired surface became rougher with some burrs, as shown in Figure 4b,c. From Figure 4, it can be found that the rotating speed ω has a greater effect on the surface morphologies of repaired surfaces than that of advancing velocity ν.
Metals 2020, 10, x FOR PEER REVIEW 4 of 11 materials cooled down and recrystallized with the shoulder passing by. Consequently, some arc corrugations emerged at the repaired specimen surfaces, as shown in Figure 4. The distance between two adjacent arcs, which can be named as the advancing pitch (in FSW, it is called the weld pitch [26]) and denoted by λ, is almost equal to the advancing distance per rotating cycle of the tool shoulder ( Figure 5) and is proportional to the ratio of advancing velocity (ν) and rotating speed (ω), and inversely proportional to the energy input E of the FSCR process, which can be expressed by Equation (1).
(1) Figure 4 shows the surface morphologies of the repaired specimens under varying parameters. With the increase in ν when ω = 1100 r/min, λ increased as the advancing distance increased per rotating cycle according to Equation (1), and obvious plastic deformation marks like burrs could be found at the repaired surfaces ( Figure 4a,c), especially when a low ν was adopted, as shown in Figure  4e. With the increase of ω when ν = 110 mm/min, λ decreased, and consequently a smoother repaired surface could be obtained (Figure 4d). When a relatively lower ω was chosen, the repaired surface became rougher with some burrs, as shown in Figure 4b,c. From Figure 4, it can be found that the rotating speed ω has a greater effect on the surface morphologies of repaired surfaces than that of advancing velocity ν. . Repaired specimen surfaces. (a) S1100 V150, (b) S700 V110, (c) S1100 V110, (d) S1500 V110, (e) S1100 V90. . Repaired specimen surfaces. (a) S1100 V150, (b) S700 V110, (c) S1100 V110, (d) S1500 V110, (e) S1100 V90.  Figure 6 shows the fatigue lives of the repaired specimens in different repairing parameters. It can be seen that there was no remarkable linear correlation, but almost a "V" type discipline between fatigue life and rotating speed or advancing velocity (i.e., with the increase in rotating speed or advancing velocity, the fatigue life of the repaired specimen decreased first, and increased afterward in the selected parameter intervals. With the increase in rotating speed when ν = 110 mm/min, the fatigue life dropped dramatically from over 632,000 at 700 r/min and 425,000 at 900 r/min to only 44,800 at 1100 r/min, and then increased to around 280,000 at 1300 r/min, but with the further increase in rotating speed to 1500 r/min, the fatigue life showed a descending trend again (Figure 6a). Compared with the rotating speed, the effect of advancing velocity on the fatigue lives of repaired specimens was relatively weaker. As shown in Figure 6b, the fatigue life decreased from 313,000 and 179,000 at 70 mm/min and 90 mm/min to 44,800 at 110 mm/min, and continued to recover to over 114,000 and 235,000 as advancing velocity further increased to 130 mm/min and 150 mm/min, respectively. From the post fracture analysis, it can be found that repairing defects such as tunnel defects and holes are more prone to appear inside the repairing zone under ν = 110 mm/min and ω = 1100 r/min, as shown in Figure 7. Bhattacharya et al. [26] reported that there was a relationship between weld pitch and energy input in FSW, and better static joint strength was achieved only within a certain range of energy input as both very high and very low energy input attributed to poor joint strength. Regarding the FSCRed specimens, it can be seen in Figure 8 that there was also no linear relationship between the advancing pitch and fatigue life in the selected parameter interval, but a quadratic polynomial curve was a better fit to the fatigue life data versus advancing pitch, which was inversely proportional to the energy input of the FSCR process. A minimum fatigue life occurred as λ was 0.1 mm/r and the maximum λ ν ω Figure 5. Schematic of the advancing pitch. Figure 6 shows the fatigue lives of the repaired specimens in different repairing parameters. It can be seen that there was no remarkable linear correlation, but almost a "V" type discipline between fatigue life and rotating speed or advancing velocity (i.e., with the increase in rotating speed or advancing velocity, the fatigue life of the repaired specimen decreased first, and increased afterward in the selected parameter intervals. With the increase in rotating speed when ν = 110 mm/min, the fatigue life dropped dramatically from over 632,000 at 700 r/min and 425,000 at 900 r/min to only 44,800 at 1100 r/min, and then increased to around 280,000 at 1300 r/min, but with the further increase in rotating speed to 1500 r/min, the fatigue life showed a descending trend again (Figure 6a). Compared with the rotating speed, the effect of advancing velocity on the fatigue lives of repaired specimens was relatively weaker. As shown in Figure 6b, the fatigue life decreased from 313,000 and 179,000 at 70 mm/min and 90 mm/min to 44,800 at 110 mm/min, and continued to recover to over 114,000 and 235,000 as advancing velocity further increased to 130 mm/min and 150 mm/min, respectively. From the post fracture analysis, it can be found that repairing defects such as tunnel defects and holes are more prone to appear inside the repairing zone under ν = 110 mm/min and ω = 1100 r/min, as shown in Figure 7.  Figure 6 shows the fatigue lives of the repaired specimens in different repairing parameters. It can be seen that there was no remarkable linear correlation, but almost a "V" type discipline between fatigue life and rotating speed or advancing velocity (i.e., with the increase in rotating speed or advancing velocity, the fatigue life of the repaired specimen decreased first, and increased afterward in the selected parameter intervals. With the increase in rotating speed when ν = 110 mm/min, the fatigue life dropped dramatically from over 632,000 at 700 r/min and 425,000 at 900 r/min to only 44,800 at 1100 r/min, and then increased to around 280,000 at 1300 r/min, but with the further increase in rotating speed to 1500 r/min, the fatigue life showed a descending trend again (Figure 6a). Compared with the rotating speed, the effect of advancing velocity on the fatigue lives of repaired specimens was relatively weaker. As shown in Figure 6b, the fatigue life decreased from 313,000 and 179,000 at 70 mm/min and 90 mm/min to 44,800 at 110 mm/min, and continued to recover to over 114,000 and 235,000 as advancing velocity further increased to 130 mm/min and 150 mm/min, respectively. From the post fracture analysis, it can be found that repairing defects such as tunnel defects and holes are more prone to appear inside the repairing zone under ν = 110 mm/min and ω = 1100 r/min, as shown in Figure 7. Bhattacharya et al. [26] reported that there was a relationship between weld pitch and energy input in FSW, and better static joint strength was achieved only within a certain range of energy input as both very high and very low energy input attributed to poor joint strength. Regarding the FSCRed specimens, it can be seen in Figure 8 that there was also no linear relationship between the advancing pitch and fatigue life in the selected parameter interval, but a quadratic polynomial curve was a better fit to the fatigue life data versus advancing pitch, which was inversely proportional to the energy input of the FSCR process. A minimum fatigue life occurred as λ was 0.1 mm/r and the maximum λ ν ω Figure 6. Fatigue lives of the repaired specimens: (a) ν = 110 mm/min, (b) ω = 1100 r/min. Bhattacharya et al. [26] reported that there was a relationship between weld pitch and energy input in FSW, and better static joint strength was achieved only within a certain range of energy input as both very high and very low energy input attributed to poor joint strength. Regarding the FSCRed specimens, it can be seen in Figure 8 that there was also no linear relationship between the advancing pitch and fatigue life in the selected parameter interval, but a quadratic polynomial curve was a better fit to the fatigue life data versus advancing pitch, which was inversely proportional to the energy input of the FSCR process. A minimum fatigue life occurred as λ was 0.1 mm/r and the maximum obtained as a highest was 0.16 mm/r. When the highest λ is adopted, this represents the lowest energy input. obtained as a highest was 0.16 mm/r. When the highest λ is adopted, this represents the lowest energy input.  As a result, the comparison of the fatigue lives of specimens repaired in different parameters contributed to the conclusion that the fatigue strength of FSCRed Al2024 is quite sensitive to the repairing parameters under investigation.

Fatigue Crack Propagation of Repaired Specimens
Figures 9 and 10 present changes of fatigue crack growth rates (FCGRs) with repairing parameters. The FCGRs in the base metal were also plotted for comparison. Basically, the FCGRs of the repaired specimens were higher than the base material counterpart, which was similar to that of the FSW joint [27]. As can be seen in Figure 9, threshold values ΔKth of repaired specimens were all lower than that of the base material, especially when the rotating speed equaled 900 r/min or 1100 r/min as a constant advancing velocity 110 mm/min was adopted. The FCGR of repaired specimens under 700 r/min was relatively lower and comparable with the base material counterpart, while with the increase in rotating speed, the FCGRs of repaired specimens became increasingly higher, especially when ΔK exceeded 15 MPa·m 1/2 . However, there was an exception for 1500 r/min, at which the FCGR of the repaired specimen was located between those of 900 r/min and 1100 r/min, and similar conclusions can be drawn from Figure 10 (i.e., cracks propagated faster and the ΔKth were all lower in the repaired specimens than the base material counterpart). When ΔK was lower, the FSCRs obtained as a highest was 0.16 mm/r. When the highest λ is adopted, this represents the lowest energy input.  As a result, the comparison of the fatigue lives of specimens repaired in different parameters contributed to the conclusion that the fatigue strength of FSCRed Al2024 is quite sensitive to the repairing parameters under investigation.

Fatigue Crack Propagation of Repaired Specimens
Figures 9 and 10 present changes of fatigue crack growth rates (FCGRs) with repairing parameters. The FCGRs in the base metal were also plotted for comparison. Basically, the FCGRs of the repaired specimens were higher than the base material counterpart, which was similar to that of the FSW joint [27]. As can be seen in Figure 9, threshold values ΔKth of repaired specimens were all lower than that of the base material, especially when the rotating speed equaled 900 r/min or 1100 r/min as a constant advancing velocity 110 mm/min was adopted. The FCGR of repaired specimens under 700 r/min was relatively lower and comparable with the base material counterpart, while with the increase in rotating speed, the FCGRs of repaired specimens became increasingly higher, especially when ΔK exceeded 15 MPa·m 1/2 . However, there was an exception for 1500 r/min, at which the FCGR of the repaired specimen was located between those of 900 r/min and 1100 r/min, and similar conclusions can be drawn from Figure 10 (i.e., cracks propagated faster and the ΔKth were all lower in the repaired specimens than the base material counterpart). When ΔK was lower, the FSCRs As a result, the comparison of the fatigue lives of specimens repaired in different parameters contributed to the conclusion that the fatigue strength of FSCRed Al2024 is quite sensitive to the repairing parameters under investigation.

Fatigue Crack Propagation of Repaired Specimens
Figures 9 and 10 present changes of fatigue crack growth rates (FCGRs) with repairing parameters. The FCGRs in the base metal were also plotted for comparison. Basically, the FCGRs of the repaired specimens were higher than the base material counterpart, which was similar to that of the FSW joint [27]. As can be seen in Figure 9, threshold values ∆K th of repaired specimens were all lower than that of the base material, especially when the rotating speed equaled 900 r/min or 1100 r/min as a constant advancing velocity 110 mm/min was adopted. The FCGR of repaired specimens under 700 r/min was relatively lower and comparable with the base material counterpart, while with the increase in rotating speed, the FCGRs of repaired specimens became increasingly higher, especially when ∆K exceeded 15 MPa·m 1/2 . However, there was an exception for 1500 r/min, at which the FCGR of the repaired specimen was located between those of 900 r/min and 1100 r/min, and similar conclusions can be drawn from Figure 10 (i.e., cracks propagated faster and the ∆K th were all lower in the repaired specimens than the base material counterpart). When ∆K was lower, the FSCRs of Metals 2020, 10, 1026 7 of 11 the repaired specimens showed no obvious difference, nevertheless, when advancing velocities were equal to 70 mm/min and 150 mm/min as ω = 1100 r/min, the FSCRs were lower than the other repaired specimens once the ∆K exceeded 10 MPa·m 1/2 . By comparing Figures 9 and 10, it can be found that the FSCR data of the repaired specimens in Figure 8 were more scattered than that in Figure 10, particularly at higher ∆K, indicating that the effect of rotating speed on crack propagation behavior of the repaired specimens was higher than that of the advancing velocity.
Metals 2020, 10, x FOR PEER REVIEW 7 of 11 of the repaired specimens showed no obvious difference, nevertheless, when advancing velocities were equal to 70 mm/min and 150 mm/min as ω = 1100 r/min, the FSCRs were lower than the other repaired specimens once the ΔK exceeded 10 MPa·m 1/2 . By comparing Figures 9 and 10, it can be found that the FSCR data of the repaired specimens in Figure 8 were more scattered than that in Figure 10, particularly at higher ΔK, indicating that the effect of rotating speed on crack propagation behavior of the repaired specimens was higher than that of the advancing velocity.   of the repaired specimens showed no obvious difference, nevertheless, when advancing velocities were equal to 70 mm/min and 150 mm/min as ω = 1100 r/min, the FSCRs were lower than the other repaired specimens once the ΔK exceeded 10 MPa·m 1/2 . By comparing Figures 9 and 10, it can be found that the FSCR data of the repaired specimens in Figure 8 were more scattered than that in Figure 10, particularly at higher ΔK, indicating that the effect of rotating speed on crack propagation behavior of the repaired specimens was higher than that of the advancing velocity.   Based on the analysis of the fatigue properties and crack propagation rates under different repairing parameters, it can be seen from Figure 2 that the best match of rotating speed and advancing velocity was locate in the third quadrant, and the parameter match in the first quadrant was also acceptable. Parameter matches in the second quadrant mean that a high rotating speed and low advancing velocity were adopted, which will result in excess heat input and consequently lead to local melting and abnormal grain growth in the repairing zone, while parameters in the fourth quadrant mean a low rotating speed match with high advancing velocity, therefore, the heat input would be insufficient to obtain adequate plastic material flow. Figure 11a,b correspond to the representative fracture morphologies near the crack sources of the base material and friction stir repaired specimen, respectively. It can be seen in Figure 11a that the crack initiated from the specimen surface due to cyclic slipping, and typical slipping features can be found inside the crack source, which was also observed by Deng in the 7075 aluminum alloy [24]. As shown in Figure 11b, the crack initiated from the repaired surface due to the stress concentration at the arc corrugation formed during the FSCR process (showed in Figure 4), rather than cyclic slipping, and there was a semicircular zone (plotted with white solid line in Figure 11b) surrounding the crack initiation site, which was quite similar to Chao and Okada's results in the 6061 and 2024 FSW joint [28,29]. In the base material, due to the transgranular fracture mechanism, the crack propagation area exhibited obvious crystallographic features, and many paralleled glide plans and some fatigue striations could be found at the fracture (Figure 11a). Nevertheless, the crack propagation mechanism in the repaired zone was mainly intergranular, as shown in Figure 11b; there were few striations or slipping features, but some intergranular facets exist at the fracture surface. Due to the significantly refined equiaxed microstructure achieved via dynamic recrystallization during the FSCR process [24,30], intergranular crack propagation in the repaired zone becomes easier compared to that in the base material with coarse grains. As a result, the crack growth rates of the repaired specimens were higher than that of the base material, as can be seen in Figures 9 and 10.

Fracture Morphology Analysis
Defects that are closely related to the parameters will greatly reduce the fatigue strength of friction stir welds [31,32], and the same was for the FSCRed specimen. In Figure 12, combined boundaries of the repaired layer and base material under different repairing parameters are depicted. Sound combination of the repaired layer and base material could be obtained if the proper parameters were adopted (Figure 12a,b), otherwise, delaminating defect will be formed at the boundary of the repaired layer and base material, as shown in Figure 12c,d, which results in "notch effects" and consequently lead to early fatigue failure and greatly shorten the fatigue life of the repaired specimen, as shown in Figure 6. Based on the analysis of the fatigue properties and crack propagation rates under different repairing parameters, it can be seen from Figure 2 that the best match of rotating speed and advancing velocity was locate in the third quadrant, and the parameter match in the first quadrant was also acceptable. Parameter matches in the second quadrant mean that a high rotating speed and low advancing velocity were adopted, which will result in excess heat input and consequently lead to local melting and abnormal grain growth in the repairing zone, while parameters in the fourth quadrant mean a low rotating speed match with high advancing velocity, therefore, the heat input would be insufficient to obtain adequate plastic material flow. Figure 11a,b correspond to the representative fracture morphologies near the crack sources of the base material and friction stir repaired specimen, respectively. It can be seen in Figure 11a that the crack initiated from the specimen surface due to cyclic slipping, and typical slipping features can be found inside the crack source, which was also observed by Deng in the 7075 aluminum alloy [24]. As shown in Figure 11b, the crack initiated from the repaired surface due to the stress concentration at the arc corrugation formed during the FSCR process (showed in Figure 4), rather than cyclic slipping, and there was a semicircular zone (plotted with white solid line in Figure 11b) surrounding the crack initiation site, which was quite similar to Chao and Okada's results in the 6061 and 2024 FSW joint [28,29]. In the base material, due to the transgranular fracture mechanism, the crack propagation area exhibited obvious crystallographic features, and many paralleled glide plans and some fatigue striations could be found at the fracture ( Figure 11a). Nevertheless, the crack propagation mechanism in the repaired zone was mainly intergranular, as shown in Figure 11b; there were few striations or slipping features, but some intergranular facets exist at the fracture surface. Due to the significantly refined equiaxed microstructure achieved via dynamic recrystallization during the FSCR process [24,30], intergranular crack propagation in the repaired zone becomes easier compared to that in the base material with coarse grains. As a result, the crack growth rates of the repaired specimens were higher than that of the base material, as can be seen in Figures 9 and 10.

Fracture Morphology Analysis
Defects that are closely related to the parameters will greatly reduce the fatigue strength of friction stir welds [31,32], and the same was for the FSCRed specimen. In Figure 12, combined boundaries of the repaired layer and base material under different repairing parameters are depicted. Sound combination of the repaired layer and base material could be obtained if the proper parameters were adopted (Figure 12a,b), otherwise, delaminating defect will be formed at the boundary of the repaired layer and base material, as shown in Figure 12c,d, which results in "notch effects" and consequently lead to early fatigue failure and greatly shorten the fatigue life of the repaired specimen, as shown in Figure 6.

Conclusions
From this research on the effect of parameters on fatigue property and crack propagation behavior in pre-cracked Al2024, the following conclusions can be provided.
(1) The repairing parameters can obviously affect the surface morphology of repaired specimens, which can be explained by the advancing pitch λ. With the increase in repairing velocity, λ increased and obvious plastic deformation marks could be found at the repaired surfaces, while with the increase in rotating speed ω, λ decreased, and consequently, a smoother repaired surface could be obtained. (2) The fatigue life of FSCRed Al2024 was quite sensitive to the repairing parameters under investigation, and there was a "V" type discipline between fatigue life and rotating speed or advancing velocity, which can be clarified by energy input during the repairing process (i.e., both very high and very low energy input attributed to poor joint strength). A quadratic polynomial curve was better to fit the fatigue life data versus advancing pitch. (3) FCGRs of the repaired specimens were higher than the base material counterpart, and the threshold values ΔKth of the repaired specimens were all lower than that of the base material.
The effect of rotating speed on the crack propagation behavior of repaired specimens was higher than that of the advancing velocity. Through analysis, it was found that the best match of rotating speed and advancing velocity was located in the third quadrant. (4) Cracks initiated from the repaired surface at the arc corrugation in FSCRed specimens rather than cyclic slipping in the base material, and the crack propagation mechanism in the repaired zone was mainly intergranular with few striations or slipping features. A delamination defect formed at the boundary of the repaired layer and base material if improper repairing parameters

Conclusions
From this research on the effect of parameters on fatigue property and crack propagation behavior in pre-cracked Al2024, the following conclusions can be provided.
(1) The repairing parameters can obviously affect the surface morphology of repaired specimens, which can be explained by the advancing pitch λ. With the increase in repairing velocity, λ increased and obvious plastic deformation marks could be found at the repaired surfaces, while with the increase in rotating speed ω, λ decreased, and consequently, a smoother repaired surface could be obtained. (2) The fatigue life of FSCRed Al2024 was quite sensitive to the repairing parameters under investigation, and there was a "V" type discipline between fatigue life and rotating speed or advancing velocity, which can be clarified by energy input during the repairing process (i.e., both very high and very low energy input attributed to poor joint strength). A quadratic polynomial curve was better to fit the fatigue life data versus advancing pitch. (3) FCGRs of the repaired specimens were higher than the base material counterpart, and the threshold values ∆K th of the repaired specimens were all lower than that of the base material. The effect of rotating speed on the crack propagation behavior of repaired specimens was higher than that of the advancing velocity. Through analysis, it was found that the best match of rotating speed and advancing velocity was located in the third quadrant. (4) Cracks initiated from the repaired surface at the arc corrugation in FSCRed specimens rather than cyclic slipping in the base material, and the crack propagation mechanism in the repaired zone was mainly intergranular with few striations or slipping features. A delamination defect formed at the boundary of the repaired layer and base material if improper repairing parameters were adopted, which would lead to early fatigue failure and greatly shorten the fatigue life of the repaired specimen.
Due to its good applicability to aluminum alloys for friction stir technology, the FSCR method has a broad application prospect in aerospace engineering, the shipbuilding industry, and the automobile and railway transportation fields where aluminum alloys are widely used. The reliability of mechanical structures can be improved at a lower cost. Particularly, in the rapid repair of military aircraft, FSCR can increase combat resilience, which is more significant. However, the crack studied in this research was a prefabricated straight crack, and for real cracks in engineering, the tracks may be random, so a laser vision system needs to be used to trace the path of the crack, and robot repairing equipment is suggested to repair the crack in a complex trajectory.

Conflicts of Interest:
The authors declare no conflict of interest.