Effect of ultrasonic impact time on microstructure and properties of 7A52 aluminum alloy tandem MIG welded joint

7A52 (Al-Zn-Mg-Cu) alloy is a high-strength aluminum alloy; its welded joints are often accompanied by defects such as poor wear resistance and low fatigue strength. Herein, we try to optimize the welded joint of 7A52 aluminum alloy by using ultrasonic impact treatment (UIT). Generally, the mechanical properties such as microhardness and fatigue strength of the welded joint after UIT will be improved. 7A52 aluminum alloy tandem metal inert gas (MIG) welded joints with UIT time per unit area of 2.5 min, 5 min, 10 min, 15 min, 30 min, and 75 min were studied. Through the surface topography, microstructure observation, and mechanical properties test, the time parameters of excessive treatment, lack of treatment, and proper treatment were selected, and the effects of UIT, excessive treatment, lack of treatment, and proper treatment on fatigue strength were analyzed. Test results show that the mechanical properties of welded joints after UIT are improved. The proper treatment time is 15min and its fatigue strength is 37.86MPa, respectively, under the stress ratio of 0.1. Compared to the original welding condition with a fatigue strength of 28.61MPa, the fatigue strength of the welded joints of 7A52 aluminum alloy increased by 32.33%. The largest percentage of grain size reduction occurs when the UIT is 15 min. Moreover, excessive treatment and lack of treatment will not further refine the grains and optimize the mechanical properties.


Introduction
As a lightweight metal, aluminum alloy has met the needs of aircraft and sophisticated equipment at different times. Lightweight is the eternal goal pursued in aerospace [1,2]. With the continuous innovation of aircraft design ideas, advanced aircraft component manufacturing has put high requirements on materials, and thus, demand on greater performance of aluminum alloy is also increasing [3]. 7A52 (Al-Zn-Mg-Cu) aluminum alloy is a high-strength aluminum alloy produced in China, which is widely used in aerospace and other departments [4][5][6]. However, when welded, 7A52 aluminum alloy exhibits welding cracks, welding porosity, and low weld strength. The welded joint has poor wear resistance and low fatigue strength [7]. Therefore, improving the fatigue properties of 7A52 armored aluminum alloy has become an urgent problem to be solved in popularizing its practical application in welded components engineering.
As a new welding technology to achieve high efficiency, tandem metal inert gas (MIG) welding has the advantages of good mechanical properties of joints and high welding efficiency [8]. Zhang et al. [9] found that the comparison between single and double wire welding show that the distortion of the double wire welded joint was smaller, the microstructure of double wire welded metal was tighter than the single one, the heat-affected zone was narrower, and the microhardness was higher. Moreover, tandem MIG welding can effectively reduce the probability of occurrence of welding pore defects, refine the grain of the weld zone, and obtain high-quality fish scale weld [10]. Hence, tandem MIG welding is more suitable for the welding of 7A52 aluminum alloy thick plate. However, due to the mutual interference of the electromagnetic field between the two arcs during tandem MIG welding, some problems arise such as poor welding stability and poor fatigue performance.
Ultrasonic impact treatment (UIT) is used to impact the surface of the metal component with high frequency, which makes the surface of the impacted components produce large compressive plastic deformation [11][12][13]. It can effectively eliminate the residual tensile stress on the surface of welded joint and weld zone, and achieve the effect of reducing the residual stress in welded parts [14,15]. Chen et al. [16] found that the nanometer grain layer can be successfully prepared by UIT and A-UIT on the surface of aluminum alloy welded joint. However, there was no research on the fatigue properties of welded joints. After UIT, the wear and fatigue properties of tandem MIG welding can be effectively improved [17].
At present, only a simple attempt has been made related to this research. Although the properties of welded joints have been improved, excessive impact will lead to the failure of metallurgical combinations between the strong plastic deformation layer and the undeformed layer, generating superimposed defects, resulting in a new stress concentration area, and leading to fatigue cracking [18]. In engineering practice, we still do not know how to deal with improper handlings such as excessive treatment and lack of treatment. The UIT of tandem MIG welded aluminum alloy specimens was carried out at different times and lengths, so as to obtain better UIT process. These studies are of great significance to the engineering application of 7A52 aluminum alloy, and this will lay a theoretical and experimental foundation for the wide application of tandem MIG welded joint of 7A52 aluminum alloy in the future.

Experimental
The base material with 10mm used in this investigation is 7A52-T5 (Al-Zn-Mg-Cu) aluminum alloy plate with the chemical composition and mechanical properties of the wire and substrate represented in Tables 1 and 2, respectively. Aluminum-magnesium (Al-Mg) ER5356 alloy welding wire with a size of 1.6 mm was selected in the experiment.
Welding was carried out using the digital pulse tandem MIG welding machine Fronius TPS5000 and the matching welding robot. The two-wire feeding tubes of the tandem MIG welding machine send two welding wires into the welding gun at the same time. Two welding wires were melted in the double arc to form a molten pool, and two welding machines communicate with each other through a special device to ensure that the two arcs do not interfere with each other. Under the action of double arc, the forming of the welded joint can be optimized, and the welding efficiency becomes higher. Figure 1 shows the tandem MIG welding process during welding; multi-layer and multi-pass welding are adopted. The groove form of welding is butt V-groove with a groove gap of 2 mm between two plates and a groove angle of 30 on one side. After welding, the appearance with fish scale pattern is formed and no obvious splash, undercut, and other defects were observed (see Fig. 2). Table 3 lists the welding parameters of tandem MIG welding.
UIT-125 ultrasonic impact equipment was used to treat 7A52 aluminum alloy welded joint with the UIT process shown in Fig. 1. The impact degree is characterized by the ultrasonic impact time per unit area (10 ×10 mm). The following parameters are all expressed by ultrasonic impact time, which are 0 min, 2.5 min, 5 min, 10 min, 15 min, 30 min, and 75 min. Ultrasonic impact process parameters are shown in Table 4.
In this experiment, the surface morphology of the joint after UIT at different times was observed using a FEI-QUANTA 650 scanning electron microscope, and the longitudinal section morphology after UIT treatment was observed by a metallographic optical microscope. HXD-1000TM digital microhardness was used to measure the microhardness of the welded joint along the depth direction in different times of ultrasonic treatment. The X-ray stress analyzer of X-stress3000 measured the residual stress distribution of the welded joint at a different time of UIT, and the grain size of the joint surface after UIT was analyzed and calculated. The high frequency fatigue testing machine controlled by MTS Landmark was used for the fatigue test with a stress cycle ratio of R=0.1 and a loading frequency of 121Hz-137Hz. The specimens were cut by wire cut electrical discharge machining (WEDM); three test specimens were cut for each parameter. The  fatigue specimen size of the welded joint is shown in Fig. 3. The specimen was placed on the fatigue testing machine for multiple alternating loads, and the fatigue strength of the specimens was measured. Relevant studies shown that UIT has no significant effect on the tensile strength of the welded joint [19]. Therefore, only fatigue strength tests were carried out.

Surface Morphology Analysis
The surface morphology of 7A52 aluminum alloy tandem MIG welded joint with different UIT time are shown in Fig. 4. Figure 4a shows the surface morphology without UIT, while Fig. 4b shows the surface morphology after UIT for 2.5 min. It can be seen that the surface before UIT is not flat and there are many interlaced, disordered, narrow, and deep strip gullies. After UIT, the morphology with sharp and

Surface roughness analysis
The laser confocal microscope was used to measure the samples with different time of UIT for 5 times, and the average value of 5 times was taken (see Fig. 5).
With the increase of ultrasonic impact time, the roughness decreases first and then increases as shown in Fig. 5. The roughness of the original weldment significantly reduces from 44.45 to 40.94μm, 37.08μm, and 31.63μm for impact times of 2.5 min, 5 min, and 10 min, respectively. The specimens treated for 15 min exhibited the least roughness and the best surface quality. When the ultrasonic impact time is 30 min, the surface roughness of the specimen did not reduce further. When the treatment time is 75 min, the roughness value is 45.23μm, which is greater than the surface roughness of the specimen without UIT. Moreover, the surface quality deteriorates, which shows that excessive UIT cannot further improve the surface quality of the specimen. Compared with the treatment for 30 min, the increase of the roughness value is not significant. This means that after the ultrasonic impact time reaches a certain value, further increase of the treatment time cannot significantly increase the roughness value, and the change gradually tends to be gentle.

Microstructure analysis
The surface structure of 7A52 aluminum alloy tandem MIG welded joint after different UIT at 100 times magnification is shown in Fig. 6. Figure 6a shows the original weld surface structure. It can be seen that the weld surface structure without UIT is still in an equiaxed crystal form with uniform size. No obvious difference from the matrix structure is observed and the surface is relatively flat with good surface quality. Figure 6b shows the surface structure of the weld with the ultrasonic impact time of 2.5 min. After the UIT, the surface structure begins to undergo plastic deformation with a deformation layer thickness of about 130μm. The surface grains are refined and the grain boundaries are not obviously entangled with each other. Obvious differences with the matrix structure are observed. The surface roughness has slight changes and the surface quality is observed to be better.
It can be seen that after UIT, the surface structure has obvious plastic deformation with the thickness of about 160μm, 190μm, and 210μm, for treatment of 5 min, 10 min, and 15 min as shown in Fig. 6c, d, and e, respectively. The surface grains are obviously refined and the increase of grain boundaries deepens the color of the plastic deformation layer, which is related to the matrix group. With the increase of the distance from the matrix, the grains gradually become large, and finally, the structure becomes the same as that of the matrix. The flatness of the surface layer changes slightly and a better surface quality is observed. Figure 6f and g show the surface structure of the weld with the treatment of 30 min and 75 min with the thickness of the plastic deformation layer of about 230μm and 260μm, respectively. This time, the surface plastic deformation degree is severe and a plastic flow trend appears. The local is in the form of a vortex and the appearance of equiaxed crystal disappears completely. The surface structure produces spatter and burr and the surface quality is poor.
In summary, with the increase of UIT time, the thickness of plastic deformation layer of aluminum alloy welded joint increases continuously. However, according to the results of Sections 3.1 and 3.2, when the UIT time exceeds 15min, the surface of the specimen will become rough, which is unfavorable to the mechanical properties of the welded joint. Therefore, 15 min of UIT is the better parameter.

XRD analysis
It can be seen from Fig. 7 that with the increase of ultrasonic impact time, the diffraction peak is widened, which is caused by grain refinement, instrument broadening, and micro-stress interaction. The grain size of the joint surface is calculated according to the Scherrer formula (see Equation (1)).
The value of constant K is related to the definition of β. When β is a half width height, K is 0.89, while when β is the integral width, K is taken as 1.0. The calculated grain size (D) is shown in Fig. 8. Figure 8 shows that after UIT, the grain size of the sample surface is refined to the nanometer level. Moreover, with the increase of UIT time, the surface grain size decreases. Under the same chemical composition, the strength and hardness of the material increase with the decrease of grain size D. When the ultrasonic impact time increased from 2.5 to 75 min, the grain size decreased by 2.12nm, 0.7nm, 1.34nm, 0.25nm, and 0.14nm. Therefore, with the increase of UIT time, the grain  size is constantly decreasing the degree of reduction becoming less and less obvious, especially after the ultrasonic impact time of 15 min. The grain refinement process caused by plastic deformation is controlled by many factors, such as material properties, deformation amount, and deformation rate. The grain size decreases with the increase of deformation amount, but when the deformation amount reaches a certain critical value, the change amount of grain size decreases and tends to saturate [20]. With the development of UIT, dislocations form dislocation wall and dislocation tangle through slip, accumulation, interaction, annihilation, and rearrangement, which hinders further movement of dislocations and makes it difficult for the grain fragmentation to reach the "saturation" state.

Microhardness analysis
Microhardness data in Fig. 9a reveals that the microhardness of the weld area without UIT has little change and fluctuates from 80 to 90HV, with an average value of 82.9HV. In addition, after UIT, the microhardness value of the weld surface increases significantly and decreases gradually with the increase of distance from the ultrasonic impact surface, showing a monotonous downward trend. On the other hand, the microhardness value of the part beyond the deformation layer still continues to increase and finally tends to be gentle in the area greater than 1 mm. After UIT, a plastic deformation layer was formed on the surface of the welded joint, and the grain size showed a gradient distribution, as shown in Fig. 9b. This is the main reason for the microhardness distribution on the surface of the welded joint; i.e., the increase of microhardness is caused by grain refinement and work hardening.

Residual stress analysis
In this paper, the X-ray diffraction method is used to determine the residual stress of the original welding state and the specimens at different times of UIT. The measurement results are shown in Fig. 10a and the average residual stress at different times of UIT is shown in Fig. 10b.
It can be seen from Fig. 10a and b that the residual stress of the as-welded 7A52 aluminum alloy welded joint is mainly tensile stress. The maximum residual tensile stress in the weld area is 166.6MPa and the average value is 82.6MPa. Compared with the distribution of residual stress in aswelded specimens, it can be found that the welded joints of 7A52 aluminum alloy changed from residual tensile stress to residual compressive stress after UIT. With the increase of the distance from the weld center, the residual compressive stress decreases gradually and the curve shows an upward trend [21]. With the increase of UIT time from 2.5, 5, 10, 15, and 30 to 75 min, the average value of residual pressure stress is as follows: −63.8MPa, −101.8MPa, −66.1MPa, − 141.9MPa, −119.8MPa, and −116.5MPa, respectively. It can be seen that the value generally decreases first and then does not change. When the ultrasonic impact time is 15 min, the maximum residual pressure stress is 141.9MPa. The "saturation" of residual stress with the increase of ultrasonic impact time is due to dislocation packing caused by the dislocation slip and rearrangement after UIT. Different dislocation distribution corresponds to different residual stress distribution; i.e., high dislocation density means a more concentrated stress, which is shown as the transformation of tensile and compressive stress [22]. When the treatment time is too long, the dislocation density increases and the dislocation wall hinders further movement of the dislocation to produce a saturation Fig. 9 Microhardness change and test position of weld area after UIT at different times. a Average microhardness distribution of welded joints. b SEM image of plastic deformation layer of welded joint phenomenon. Therefore, a certain amount of residual compressive stress is introduced into the surface of 7A52 aluminum alloy welded joint after treatment, which effectively improves the fatigue performance of 7A52 aluminum alloy welded joint.

Fatigue strength of tandem MIG welded joints
The median S-N curve of 7A52 aluminum alloy tandem MIG welded joint at different time of as-welded and UIT is shown in Fig. 11. The fatigue strength results at 2×10 6 cycles are listed in Table 5.
Looking at Fig. 11 and Table 5, it is difficult to visually evaluate whether the different times of ultrasonic impact have a significant impact on the fatigue strength due to the small difference of the curves of the processed samples. It is necessary to calculate the characteristic S-N curve equation according to the median S-N curve equation to accurately evaluate the fatigue strength of different UIT time. Under a stress ratio of 0.1 and with cycle times of 2×10 6 , the fatigue strength of the as-welded specimen of tandem MIG welded joint is only 28.61MPa. However, the fatigue strength of the undertreated joint is 29.56MPa, which is 3.32% higher than that of the aswelded joint. The fatigue strength of the better treated joint is 37.86MPa, which is 32.33 higher than that of the as-welded joint, and the fatigue strength of the excessive treated joint is 27.78MPa and decreased by 2.9 compared with that of the aswelded joint. From literature [14], the better treatment can increase the fatigue strength most, while the lack of treatment can improve the fatigue strength.
On the other hand, the excessive treatment has a negative impact on the fatigue strength. As known, fatigue performance depends on the competition between beneficial factors and harmful factors, which are mainly affected by the following four factors: the degree of surface grain nanocrystallization, the size of introduced residual compressive stress, the size of near-surface microhardness, and weld toe repair. The measurement results acquired from the toe radius measurement method are shown in Fig. 12. For 30min UIT, the size of nanocrystals is small, the residual compressive stress and microhardness are large, but the UIT time is too long and the toe radius is small, which does not form a good arc transition.   Compared with the lack of treatment r=2.46 mm and the proper treatment r=3.11 mm, the excessive treatment r=2.03 mm and the toe radius are small. It can be seen from the Fig. 12 that the depth is large. To a certain extent, it appears to be a kind of macro defect with increased stress concentration. Moreover, the surface cracks produced during treatment make the material undergo premature failure and the fatigue performance becomes poor.
According to the existing data, the mathematical equation is fitted by origin as follows: For tandem MIG welded joints, the equation between ultrasonic impact time t and grain size D is as follows: The equation between fatigue strength Δσ and grain size D is as follows: The equation between fatigue strength Δσ and ultrasonic impact time t is as follows: In Equation (2), the function equation has a minimum point, and the grain size of the tandem MIG welded joint is smaller than that of the tandem MIG welded joint under the same UIT time. In Equation (3), the fatigue strength of welded joint is larger than that of the tandem MIG welded joint under the same grain size. In Equation (4), there are maximum points in both functions and the fatigue strength of the tandem MIG welded joint is larger than that of the tandem MIG welded joint under the same UIT.

Conclusions
In this study, the tandem MIG welded joint of 7A52 aluminum alloy was subjected to UIT at different times. By testing the surface morphology, microstructure, and mechanical properties of the specimens with different treatment times, the UIT time that yielded a better performance was analyzed. The conclusions are as follows: 1) When the ultrasonic impact time is 15 min, the surface of the welded joint is smooth without obvious gully and the best morphology is achieved. When the ultrasonic impact time is 5 min, 10 min, and 15 min, the better plastic deformation layer appears on the weld surface and the grain refinement is obvious. 2) Results show that the grain size decreases with the increase of ultrasonic impact time. The largest percentage of grain size reduction occurs when the UIT is 15 min. Subsequently, the grain is crushed to saturation.
3) The surface microhardness of welded joint after ultrasonic impact increases with the increase of impact time. When the impact time increases from 2.5 to 15 min, the microhardness increases the most. UIT will transform the residual stress from tensile stress to compressive stress. When the ultrasonic impact time is 15 min, the average residual compressive stress of welded joints is the largest. 4) Compared with the original welding condition, the fatigue strength of the welded joints of 7A52 aluminum alloy is the highest when the ultrasonic impact time is 15 min with an increase rate of 32.33%.
As revealed by the above results, the proper treatment of UIT was 15 min. UIT can improve the surface microhardness and fatigue strength of welded joint, which is suitable for practical engineering application.
Funding This research was supported by National Natural Science Foundation of China (51165026) (51765053) and Natural Science Foundation of Inner Mongolia Autonomous Region (2015MS9537).
Data availability The data presented in this study are available on request from the corresponding author.

Declarations
Ethics approval We are sure that the manuscript does not violate ethics approval.
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