Study on the effect of high energy ultrasonic wave on MIG welding deformation and welding joint performance of LC52 aluminum alloy plate (2025)

As a medium to high strength aluminum alloy material, LC52 aluminum alloy has superior performance and is widely used in the manufacturing of large-scale equipment in aerospace and weapons such as aircraft, spacecraft, armored vehicles, etc. As an Al–Mg-Zn alloy, LC52 aluminum alloy is commonly used in MIG welding due to its large heat input, high oxidation and thermal conductivity, which results in uneven temperature field and local high temperature, leading to significant welding residual stress and stress concentration^1,2,3,4, and even defects such as thermal cracks, pores, and welding slag and resulting in poor welding joint performance^5,6,7,8,9,10, which seriously threaten the stability and safety of welded components during service. Frequency conversion or broadband critical refracted longitudinal wave (LCR wave) non-destructive testing methods can be used to accurately obtain the distribution of welding residual stress gradients and locate the above-mentioned defects. However, how to effectively quantify and control the MIG welding residual stress and deformation of LC52 aluminum alloy components becomes a challenging problem.

In recent years, researchers have begun to extensively study the ultrasonic impact welding treatment method, which mainly improves the performance of welded joints from the perspective of eliminating welding residual stress^11,12. The surface of aluminum alloy welded joints is caused by ultrasonic impact to become nanosized¹³, significantly improving the surface hardness of welding. However, this method has a poor effect on stress homogenization of components and can also cause impact damage to welds, even leading to the generation of cracks. In addition, the ultrasonic impact method cannot change the microstructure of the weld and the penetration depth of the welded joint, nor can it suppress the generation of defects during the welding process. Essentially, it cannot achieve the goal of improving welding defects and enhancing the mechanical properties of welded joints. Meanwhile, some research results indicate that weld grains are effectively regenerated by using electromagnetic field assisted welding technology during the welding process, therefore improving weld hardness, but this method will significantly weaken some important mechanical properties of the weld, including strength and toughness^14,15.

Wider weld seam and melt width can be obtained by injecting high-energy ultrasound into the interior of the welded metal base material, effectively eliminating stress concentration and significantly improving the microstructure of the weld seam. However, no analysis and research have been found on the deformation control of LC52 aluminum alloy plates by applying high-energy ultrasound during the welding process. In order to obtain LC52 aluminum alloy welding joints with high welding efficiency and good performance, high-energy ultrasound was injected into the aluminum alloy base material to control the welding process of the plate in this paper. The influence of high-energy ultrasound on the welding deformation and welding joint performance of LC52 aluminum alloy plate during the welding process is also systematically studied. By comparing with traditional welding processes, injecting high-energy ultrasonic waves during the welding process can effectively control the welding deformation of LC52 aluminum alloy and improve its welding performance, providing important support for the improvement of welding quality of aluminum alloy plates and the stability and safety of their service.

Macroscopic basic principles and strategies of high energy acoustic beam control for welding residual stress

The most effective process for controlling welding residual stress is low stress welding technology, which involves in-situ welding residual stress reduction and homogenization of constrained welded components during the welding process. Reducing and homogenizing residual stress is to achieve plastic yield of those elastic stresses under the action of external energy macroscopically speaking, thereby releasing the stress. High-energy elastic waves are sequentially injected into the welding heat affected zone during the welding process according to a certain timing control strategy. The uncured welding melt pool area is controlled or not controlled by a low-power acoustic beam to prevent defects such as vacuum. High-power acoustic beam control is used for welded and solidified area. When the energy of the elastic waves in the solid component is greater than the potential energy composed of residual welding stresses inside the component, the distribution state of the welding residual stress field in the elastic component can be reduced and homogenized. The purpose of controlling the deformation and cracking of welded components can be achieved after high-energy sound beams control.

In addition, residual stress control can be carried out based on a certain strategy or method. The directionality of high-energy ultrasound is used to form a directional sound beam in order to reduce and homogenize residual stress in a certain area of the welded component. By increasing the excitation intensity of the welding residual stress concentration area (such as increasing control power, amplitude, control time, etc.), the welding residual stress can be gradually driven to the periphery of the concentration area or areas far away from the weld and heat affected zone, and then the working intensity of the high-energy sound beam transducer corresponding to the periphery can be regulated to control the residual stress at the relevant positions.

Microscopic mechanism of residual stress controlled by high-energy acoustic beams

In order to improve the phenomenon of stress concentration, the high-energy ultrasonic method is used to regulate the internal stress of LC52 aluminum alloy material. The purpose of residual stress control based on high-energy ultrasound is to readjust the distorted lattice arrangement and restore it to a stable and orderly arrangement state. The stress relief process is to provide sufficient power to dislocation atoms, overcome their resistance, slide them out of the crystal interior, and reduce lattice distortion. This process has the effect of refining grains and improving material mechanical properties. Lattice dislocations in aluminum alloy materials are a typical defect in crystals. When high-energy sound beams propagate through aluminum alloy plates, the fracture point of nonlinear forced vibration is the “pinning point” of lattice dislocations. Due to excessive vibration energy, the “pinning point” is likely to be detached, resulting in an increase in the length of dislocation lines. Assuming the number of dislocations in the Saiji group is R, it is represented as follows.

N represents the number of dislocations in the formula, the total length of dislocations is defined as the length L of the plug product group, and the Berger vector of dislocations is represented as b; the total length of the bilateral product group is 2a, and μ is expressed as the Poisson’s ratio.

According to Eq.(1), it can be inferred that an increase in the length of dislocation lines L will lead to a decrease in the number of dislocations N within the plug plot. Combined with the principle that the degree of stress concentration is directly proportional to the number of dislocations N within the plug plot^16,17, it can be theoretically concluded that a decrease in the number of dislocations will reduce the stress concentration of aluminum alloy materials under the action of high-energy sound beams.

The propagation of high-energy ultrasound in aluminum alloy materials containing dislocations is accompanied by the superposition of elastic strain and additional strain caused by dislocations, resulting in the following relationship.

In Eq.(2), u_e is defined as the elastic displacement of the crystal, E1 and E2 are represented by Eqs.(3) and (4) respectively, and a represents the Lagrangian coordinate of the propagation direction of high-energy ultrasound in the aluminum alloy component.

where λ and μ are Lame constants; C₁₁₂ and C₁₈₅are third-order acoustic elastic coefficients.

The dislocation lines with a length of L₀ are distributed along the y-direction and anchored at y = 0 and y = L₀, respectively. Therefore, the additional strain caused by the dislocation line length L (0 ≤ L ≤ L0) should satisfy the following relationship, as shown in Eq.(5).

where u_d is defined as the additional displacement caused by dislocations, q is the conversion factor between tangential stress and normal stress, and N is the dislocation density (i.e. the number of dislocations).

Dislocations can absorb high-energy ultrasound waves through the displacement and elastic displacement of crystal dislocations under the action of high-energy sound beams, presenting higher-order harmonics externally and causing the detachment of pinning points and elongation of dislocation lines internally¹⁸. The nonlinear oscillation equation of the dislocation line is shown in Eq.(6).

The strain and stress generated by u_d and u_e are represented as ξ and σ In the formula, respectively; A is the mass of dislocations per unit length; B is the dislocation damping coefficient; C, C,, R are the different acoustic elastic coefficients and dislocation characteristic parameters of aluminum alloy component materials, respectively. The stress caused by dislocation additional displacement and crystal elastic displacement can be obtained from Eqs. (2), (5) and (6), which can be combined with the high-energy vibration theory of dislocation strings to analyze the effect and influence of high-energy ultrasonic wave on controlling residual stress.

Materials

Hot rolled plate withLC52 aluminum alloy was used as the metal base material in this test, as shown in Fig.1, and the micro grain structure of plate is in the shape of directional strip.

Two LC52 aluminum alloy plates with a size of (300 × 110 × 20) mm were selected and the experiment of butt welding for the aluminum alloy plates was carried out using the melting electrode argon arc welding (MIG welding) process.

Metal base material with an overall size of (300 × 220 × 20) mm was formed after welding. The bottom end of the single-sided V-shaped groove (angle of 45°) had a root gap, and the root gap and blunt edge length were both 2mm, as shown in Fig.2. The welding wire adopted ER5356 aluminum wire with a diameter of 1.6mm, where magnesium and iron content were accounted for 5.5% and 0.4% respectively. The main chemical composition of the welding wire was shown in Table 1.

Testing system

The welding test system was mainly consisted of a welding system, a high-energy ultrasonic control system, an auxiliary clamping device, and power cables, as shown in Fig.3.The welding system was mainly consisted of a robotic welding gun (i.e. Funis TPS5000MIG welding machine) and a welding workbench. A welding gun protective cover was equipped around the front of the welding plate during the welding process of aluminum alloy plates due to the fast-welding speed of MIG, which enables the welding process to have inert gas protection welding function. The protective cover was moved with the movement trajectory of the welding gun.

The high-energy ultrasound control system was composed of a central controller (upper industrial computer), a power ultrasonic power supply, and an ultrasonic oscillator (including high-energy beam exciter and amplitude rod), as shown in Fig.3. Wired or remote wireless upper control of multiple power ultrasonic power supplies and corresponding transducers was achieved by using the central controller based on certain control constraints and through the 485-communication module. The maximum output power of the power ultrasonic power supply was 2600 W (the maximum output power was 1000W according to actual requirements of this experiment), and the operating frequency range was 0–40kHz; The diameter of the PZT4 piezoelectric ceramic high-energy acoustic beam transducers in Fig.3 were 70mm with a resonant frequency of 14.7kHz. It was equipped with a protective shell on the outside to prevent welding slag from splashing onto the piezoelectric chip of the transducer. The lower end was equipped with a titanium alloy ultrasonic amplitude rod, which played a role in focusing and insulation of sound energy. A clamping fixture (i.e. auxiliary clamping device) was used to fix the aluminum alloy plate on the workbench before welding to prevent deformation during the welding process. The ultrasonic oscillator was tightly attached to the aluminum alloy plate by using a fixed sleeve fixture structure, and the coupling force between the ultrasonic oscillator’s working surface and the plate was determined by the spring force on the plug bolt. Simulation testing was carried out on the fixture structure of fixed sleeve. It can be seen from Fig.4 that the fixed sleeve was hardly deformed after applying a compression load of 180 N (30 N × 6 threaded holes), indicating that the fixture structure had stable strength and met the requirements for normal welding test operation.

Experiment method

Ultrasonic residual stress detection method

According to the national standard GB/T 32,073–2015^19,20, the broadband harmonic method was used for stress gradient detection with the detection frequency range of transducer from 1.0MHz to 10.0MHz and the detection depth range of transducer from 0.7mm to 6.3mm (three frequencies 1.0MHz, 2.5MHz and 5.0MHz was selected in this experiment, corresponding to detection depths of 6.3mm, 2.6mm and 1.3mm). The residual stress distribution gradient of the welded part was detected on both sides of the weld seam when the detection probe was perpendicular to the direction of the weld seam at positions 6mm, 30mm, 60mm and 90mm away from the weld toe. The reference zero stress test block was used for numerical calibration before the detection, and the pre- tensile specimen was loaded using an electronic universal testing machine to obtain a stress coefficient (4.6MPa/ns).

The schematic diagram of the ultrasonic stress detection system is shown in Fig.5. The broadband stress testing equipment was used through the ultrasonic stress measurement system independently developed by the Testing and Control Research Institute of Beijing Institute of Technology with a detection accuracy of ± 25MPa and a detection range from − σ_S to + σ_S (σ_S represents the yield strength of the test material). The design of the detection wedge should ensure effective fit with the surface profile of welding residual stress detection area of the component. This system can accurately achieve rapid non-destructive testing of residual stress gradients in metal processing components.

Welding test method

In order to investigate the effects of high-energy ultrasonic wave on the residual stress distribution, welding deformation, microstructure and mechanical properties of LC52 aluminum alloy plates during welding, three sets of experiments were designed. The first group was a conventional welding test without the application of high-energy ultrasound (i.e., comparative test, defined as CW), the second group was a low stress welding test with the application of high-energy ultrasound during the welding process (defined as LSW), and the third group was a test with high-energy ultrasound applied to the welded aluminum alloy plates (defined as SRAW).

The oxide film and oil stains on the surface of the metal base material were removed by sanding with sandpaper and wiping with acetone before welding²¹. The welded component was placed on a workbench with a certain size, and ultrasonic vibrators coated with high-temperature resistant blue oil on the contact surface with the plate were distributed on the lower surface of the aluminum alloy plate to be welded, as shown in Fig.3. The high-energy ultrasonic control system was turned on for welding testing according to the welding parameters shown in Table 2 based on the national standard GB/T 38811-2020 after ensuring that the ultrasonic oscillator was in close contact and well coupled with the LC52 aluminum alloy plate. The high-energy ultrasonic control system was kept in working state after the welding work of the robotic arm was completed for the LSW control test. The control system was turned off when the weld was completely cooled to room temperature after a certain period of time. The process flow for controlling residual stresses in LC52 aluminum alloy plate welding was shown in Fig.6.

The parameters of high-energy ultrasonic wave control for LSW and SRAW experiments were shown in Table 3.

The method of sample preparation

After completing the welding test on the aluminum alloy plate, samples of a certain size are cut at the corresponding positions of the CW and FSW plates using wire cutting. The sampling positions and sizes of various samples are shown in Fig.9. Obtain three tensile specimens, three weld zone impact specimens, three fusion zone impact specimens, three heat affected zone impact specimens, and three metallographic specimens from the corresponding plates of CW and LSW. According to the standard ISO 148-1:2016, the charpy pendulum impact test specimen should be subjected to impact testing at room temperature using the SANS PTM2200-D1 tester; According to the standard ISO 4136:2001, a tensile test was conducted using SANS SHT4305 equipment at a testing speed of 2mm/min. After polishing the cross-section of the metallographic sample, the sample was subjected to metallographic corrosion using a prepared 4% nitric acid ethanol solution. The corroded sample was then placed under an Olympus GX51 metallographic microscope for observation and analysis of its microstructure. In order to better understand the fracture behavior of the tensile specimen, the fracture surface of the tensile specimen was observed and analyzed using a QUANTA FEG450 field emission scanning electron microscope (SEM).

After completing the control of residual stress in aluminum alloy plate welding, three sets of testing residual stress gradient distribution states were obtained. The degree of residual stress reduction and homogenization in CW and LSW welding was calculated. Dedicated coordinate measuring instrument was used to detect and compare welding deformation of CW and LSW plates after cooling to room temperature, mainly measuring the external dimensional deformation of two sets of metal base materials after the welding test.

SARW result analysis

According to the residual stress detection method in section 1.3, SARW control test results of aluminum alloy welded plates were shown in Fig.7.

As seen in Fig.8, the average welding residual stress values with detection frequencies of 1.0MHz, 2.5MHz, and 5.0MHz decreased to 82MPa, 74MPa, and 56MPa, respectively after the SARW testing. Meanwhile, there was a significant decrease in welding residual stress values within the range of the second to fifth control cycles. The residual stress values at different depths after control basically reached below 17% σ_S (σ_S is the yield strength of LC52 aluminum alloy material).

The difference between the ultrasonic energy injected into the LC52 aluminum alloy material and the bound potential energy caused by dislocations increased due to the accumulation of control cycles. As a result, the welding residual stress in the material was more significantly released, resulting in faster stress reduction during the aforementioned control cycles. In addition, it can be seen from the last two control cycles that the state of balanced distribution of residual stress was begun to emerge, and the stress release was significantly decreased, therefore, the stress value was basically remained in a stable state. More than eight control cycles was carried out to achieve residual stress reduction rate of over 65.0% in the welding of LC52 aluminum alloy plates with a thickness of 20mm under the corresponding control system mode (100 W/30min PWM).

Comparative analysis of CW and LSW results

A control process table was compiled based on the trajectory of the robotic arm welding gun and the arrangement of the six-channel high-energy sound beam excitation unit array. The start stop status and working parameter status of each high-energy sound beam excitation unit were controlled by using the central controller and 485-communication module during the welding process based on this process table, achieving the effect of reducing and homogenizing welding residual stress. The specific control process table was shown in Table 4.

The comparison of residual stress distribution in the welding of aluminum alloy plates under CW and LSW was shown in Fig.9 (the white line in the middle of the figure represents the weld seam).

As shown in Fig.9, the stress values obtained by LCR wave detection became larger with the increase of the detection depth. The residual stress values of LSW welding were significantly reduced compared to CW, and the distribution fluctuations were also significantly reduced. There was no significant difference in the magnitude of residual stress at different detection depths, indicating that the residual stress distribution of LSW welding was more balanced than CW welding.

The welding residual stress reduction rate and homogenization rate of LSW compared to CW were calculated based on below formulars (7) and (8) (homogenization rate should be obtained by selecting the same location area).

where RS₀ is the average welding residual stress without regulation (CW), and RS₁ is the average welding residual stress after regulation (LSW).

where σ₀ is the standard deviation of welding residual stress without control (CW), and σ₁ is the standard deviation of welding residual stress after control (LSW).

The obtained reduction rate and homogenization rate of residual welding stress were shown in Table 5.

The deformation of the welded tested plate can be measured using dedicated coordinate measuring instrument. The comparison of welding deformation between CW and LSW was shown in Table 6. As seen in Table 6, the welding deformation of LSW was reduced by 68.8% compared to CW. Due to the change of stress causing strain, which in turn inevitably leads to deformation or cracking of the component, the deformation of the component can be controlled by uniform stress distribution. In summary, the welding deformation of LC52 aluminum alloy plate was controlled by the LSW regulation process, resulting in good flatness welded components, thereby improving their shape retention ability during service.

The influence of high-energy ultrasonic wave on the microstructure of welded joints

The microstructure of the weld seam under CW and LSW control processes was shown in Fig.10.

The CW weld zone was represented in Fig.10a, c, while the LSW weld zone was shown in Fig.10b, d. It can be seen that there were a large number of inter-granular defects such as pores and cracks in the weld zone of Fig.10a from markings in the red area, while there were almost no obvious bubble defects in Fig.10b. Although a small number of cracks are shown in Fig.10b, the overall lattice gap in the weld zone was smaller than that in the CW weld zone. In addition, there were many coarse columnar grains in Fig.10c, while Fig.10d was mainly composed of fine equiaxed grains. The LSW weld zone shows significantly smaller grains than the CW weld zone, with no obvious directional characteristics and more uniform grain distribution. The softening effect of ultrasonic wave was an important reason for this phenomenon²², which indicates that the solidification process of the grain structure at the welding joint was improved by the application of high-energy ultrasonic wave during the welding process, effectively enhancing the strength and stability of the welding joint.

High-energy ultrasound had effect on welding temperature and heat transfer through related studies, ultimately affecting the heat affected zone near the weld seam. The microstructure of the transition zone was shown in Fig.11.

The grain shape and direction in the upper left corner area of Fig.11a, b were different from other areas and should be the base metal area (i.e. BMZ area). As shown in Fig.11, the grains of BMZ under LSW were no obvious directionality compared to those under CW, indicating that the strength and hardness of the BMZ lattice structure were further improved and the material strength and toughness of the base material were enhanced under the influence of ultrasonic regulation.

As seen in Fig.11a, b, the heat affected zone (HAZ) was relatively narrow, mainly manifested as the fusion zone (FZ). Heat was quickly diffused to the BMZ due to low heat input and high thermal conductivity of LC52 aluminum alloy during the MIG welding process of this material. Therefore, a narrow HAZ was formed between the BMZ and FZ (near the metal base material side), which was basically located at the fusion junction of the weld seam. The microstructure of the HAZ area that was mainly composed of columnar grains was similar to that of the BM area, but the overall grain size in this area is smaller than that of the BMZ. The area close to the HAZ was the FZ of welded joint. FZ was the transition zone between the weld seam and the base metal, which is the liquid metal area immediately adjacent to the weld seam after melting. This area was mainly composed of small equiaxed grains. As shown in Fig.11, the microstructure of the HAZ and FZ in LSW was uniform and fine compared to the CW transition zone, with a higher density of lattice structure., the range of welding energy field was expanded due to the acoustic flow effect generated by high-energy ultrasonic control, and the input heat was uniformly diffused to the surrounding area, forming a wide and symmetrical FZ. Therefore, LSW welding sites had wider FZ and more and wider equiaxed grains compared to CW welding sites., the fragmentation of dendrites at the weld seam position helped to promote the generation of small equiaxed grains under the application of high-energy ultrasound, resulting in a small amount of equiaxed grains appearing in the HAZ under LSW compared to the same area under CW, which indicate that the material had better processing and using performance under LSW control process.

Furthermore, the microstructure of the heat affected zone under LSW was closer to the dense grain structure of FZ than under CW. The grain structure of HAZ was originally coarse, however, it showed no significant difference from the microstructure near the center area of the component (mainly FZ), indicating that ultrasonic stress control significantly improved the texture of HAZ after ultrasonic control, resulting in a higher degree of fine grain strengthening in the microstructure of HAZ compared to BMZ, that is, FZ and HAZ have greater strength and toughness compared to BMZ. Ultrasonic excitation was used to suppressed the coarsening of the original HAZ grains, resulting in finer and more evenly distributed grains, indicating that applying high-energy ultrasonic excitation around the weld seam during the welding process was effective and feasible for improving the lattice structure state and stress reduction homogenization.

The effect of high-energy ultrasound on the tensile properties of welded joints

The strength of the base metal zone (BMZ) and heat affected zone (HAZ) is much higher than that of the weld zone (WZ) in tensile testing, so the fracture location of the tensile specimen is generally WZ. The actual tensile curve (stress–strain form) of the sample was shown in Fig.12. It can be seen from Fig.12 that the tensile curves of both conventional welding (CW) and low stress welding (LSW) testing samples showed an upward trend. The maximum load of about 600MPa (60KN loading) was reached at strain of 0.06 in the LSW state, and the maximum load of about 350MPa (35KN loading) is reached at strain of 0.04 in the CW state. After that, the curves of both states sharply decreased, which indicate that the tensile duration of the tensile specimen under the LSW process was longer than that under the CW process, presenting that the tensile specimen based on the LSW process had stronger tensile ability than that based on the CW process.

The comparison of tensile strength between LSW and CW welds was showed in Fig.13. It can be seen from the above graph that the average tensile strength of LSW welded joints is about 24.0% higher than that of CW welded joints, which indicate that the microscopic grain state of the welded joints were improved and welding quality defects were effectively reduced as the result of ultrasonic flow effect. The internal temperature field of the welding pool of aluminum alloy plates became more uniform after injecting high-energy ultrasound, and the local temperature at the welding position of the component and the temperature gradient of the weld solidification were reduced, resulting in the reduction of welding residual stress caused by heat input, thereby improving the tensile strength of each tensile sample. The average tensile strength of LSW welded joints was about 19.0% higher than that of CW welded joints, indicating that LSW control technology had better effect on improving the tensile performance of welds and even the overall mechanical properties of the plate.

The elongation of welded joints under conventional welding (CW) and low stress welding (LSW) control processes was shown in above Fig.14. The elongation rate in the LSW state increased by 46.7% compared to the CW state, and there was no significant difference in the elongation rate of each tensile sample in the LSW state, indicating that the toughness of the welded joint was improved after the application of high-energy ultrasonic wave during the welding process and the tensile performance of the weld was more uniform, which was consistent with the increase in tensile strength of welding joint.

In order to further analyze the fracture mode and microstructure of the tensile specimens, one CW and one LSW tensile specimen were selected each based on the same cutting position, and the fracture condition of the CW and LSW welded joints was observed by SEM, as shown in Fig.15.

As seen in Fig.15, the fracture surfaces of (a) CW and (b) LSW tensile specimens were both ductile fracture morphology. The fracture of the CW tensile specimen was occupied by cleavage planes and larger micro pore defects, in addition to exhibiting larger toughness dimples; the fracture of LSW tensile specimens presents fine dimples and relatively small and few micro pore defects and cleavage planes, which are consistent with the results of the tensile performance experiment mentioned above, indicating that the mechanical properties of LSW controlled process are better than those of CW process. The refinement and uniform grain distribution of the microstructure result in finer and more uniform toughness dimples on the weld fracture surface compared to conventional welding processes due to high-energy ultrasonic excitation. It indicated that while the welding residual stress was reduced and homogenized, the tensile performance of the welded joint under LSW control process was also improved. However, there were still a small number of cleavage planes with shapes similar to those in CW and smaller micro hole defects in the fracture of LSW welds, which to some extent led to a decrease in the tensile strength of the welded joint. It indicated that the application of high-energy ultrasonic wave during the welding process will not fundamentally change the brittleness of the welded joint.

The influence of high-energy ultrasonic wave on the impact performance of welded joints

The impact resistance of welded joints includes the overall welding quality of the heat affected zone (HAZ), fusion zone (FZ), and welding zone (WZ). The greater the impact energy or impact absorption energy, the higher the toughness and impact resistance of the welding material. The impact resistance of LC52 aluminum alloy is crucial to the overall performance of the product equipment, and relevant impact tests need to be carried out. The impact test results were shown in Fig.16. As shown in this graph, the average impact absorption energy of FZ, HAZ, and BMZ in CW state was 18.0J, 29.9J, 27.8J respectively. The impact absorption energy of FZ and HAZ in the welded joint increased after applying high-energy ultrasonic wave for stress control during the welding process. The impact resistance of FZ and HAZ in the LSW state of the welded joint increased by 71.7% and 33.6% respectively compared to the CW state, and its impact strength was significantly improved. The grains in LSW state are finer and more uniform, resulting in better impact resistance compared to the presence of coarse grains in the CW state, which is consistent with the previous results of metallographic microstructure analysis.

This article was based on the residual stress control method of high-energy sound beam, and compared the LC52 aluminum alloy plate subjected to conventional welding (i.e. CW) and high-energy ultrasonic wave (i.e. LSW) during the welding process from the aspects of welding residual stress distribution, welding deformation, microstructure, impact strength, tensile strength, and elongation. The control effect and effectiveness of high-energy ultrasound on welding deformation and welding joint performance were analyzed. The specific conclusion is as follows.

(1)The residual stress reduction rate of LC52 aluminum alloy plate welding reached over 65.0% in the ultrasonic control system mode (100 W/30min PWM) and with more than 8 control cycles based on the SARW control process. The residual stress values were significantly reduced and stress distribution had more balanced based on LSW control process through certain temporal constraints compared to the CW process. The stress reduction rate and homogenization rate at each detection depth reached over 70.0% and 50.0%, respectively, resulting in a 68.8% reduction in LSW welding deformation compared to the CW process. It indicates that LSW method can effectively control the welding deformation of LC52 aluminum alloy plates, thereby improving the shape retention ability of the plates during service.

(2)FZ became wider based on the effect of high-energy ultrasonic wave on heat transfer and diffusion. LSW welded joints exhibited significant grain refinement and uniform distribution compared with CW, especially in the microstructure changes of the weld zone (WZ), fusion zone (FZ), and heat affected zone (HAZ). It indicates that applying high-energy ultrasonic excitation in the surrounding area of the weld was effective in improving the lattice structure and stress reduction or homogenization during welding. A small amount of equiaxed grains appeared in the HAZ under the action of high-energy ultrasound, showing that LC52 aluminum alloy material had better processing performance under LSW control process.

(3)The tensile specimens based on the LSW process had stronger tensile strength compared to the CW process. The tensile strength and impact strength of LSW welded joints presented a certain degree of improvement compared with CW welded joints. Where the overall tensile strength of LSW welded joints increased by about 24.0%, and the impact strength of the weld fusion zone and heat affected zone increased by 71.7% and 33.6%, respectively. The elongation of LSW welded joints increased by about 46.7% compared to CW state, indicating that the toughness of the welded joints was improved after the application of high-energy ultrasound during the welding process, which was consistent with the increase of their average tensile strength.

(4)The fracture of LSW tensile specimens had fine dimples and relatively small and fewer micro pore defects and cleavage planes compared with the fracture of CW tensile specimens, which was consistent with the results of tensile tests mentioned above. Although the brittleness of LC52 aluminum alloy plate welded joints were not fundamentally changed by using the LSW control process, the micro grain size and distribution state were improved, resulting in more uniform and fine toughness dimples in the fracture of the welded joint and the suppression of micro pore defects, which indicates that the LSW control process showed better mechanical properties compared to the CW process.

Authors and Affiliations

1. Key Laboratory of Fundamental Science for Advanced Machining, Beijing Institute of Technology, Beijing, 100081, China

   Cong Jin,Chunguang Xu,Peilu Li&Peng Yin

2. China North Vehicle Research Institute, Beijing, 100072, China

   Cong Jin

3. Inner Mongolia North Heavy Industry Group Co., Ltd., Baotou, 014033, China

   Yunxi Gao&Yongqiang Wang

4. State Key Laboratory of Smart Manufacturing for Special Vehicles and Transmission System, Inner Mongolia First Machinery Group Co., Ltd., Baotou, 014032, China

   Peilu Li

5. Ningbo Branch of Chinese Academy of Ordnance Science, Ningbo, 315103, China

   Peng Yin

Contributions

Writing-original draft, data analysis, checking & supervision, C.J.; writing-review, project administration, C.X.; investigation, P.L., W.S., W.Z., W.L., P.Y. and W.Z. All authors have read and agreed to the published version of the manuscript.

Corresponding author

Correspondence to Cong Jin.

Competing interests

The authors declare no competing interests.

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