Exploring the Influence of Vibration Impact and Electric Spark on Aluminum Alloy Stress Fields

: Vibration Impact Compound Electric Spark (VIES) is a technology that combines the advantages of vibration impact and electric spark surface enhancement. Due to its simple process, cost-effectiveness, and superior efficiency, it has attracted considerable attention in industrial applications. Through finite element simulation analysis, this study designed orthogonal experiments to explore the impact of VIES on the stress field distribution of aluminum alloy plates. The results indicate that factors such as impact frequency, impact height, and electric current have a certain influence on the distribution of longitudinal residual pressure stress in aluminum alloy plates. The experiments indicate that increasing the impact height within a certain range leads to an increase in residual stress at the impact site, but the depth of the compression stress layer decreases. Meanwhile, increasing the impact frequency increases the residual stress at the impact site, while the depth of the compression stress layer remains constant. In summary, both impact height and frequency have an impact on the stress in aluminum alloy plates. Additionally, orthogonal experimental results suggest that electric current has no significant effect on the stress field.


Introduction
The 2024-T3 aluminum alloy, composed of elements like aluminum, copper, magnesium, and manganese, is renowned for its exceptional strength and toughness.As a highperformance alloy, it is particularly well-suited for handling the cyclical stresses in aircraft structures, such as in the fuselage and wing sections.Due to its outstanding strength and lightweight characteristics, it has become the ideal choice for manufacturing key components of military and commercial aircraft.[1][2][3]。 However, it is noteworthy that under high temperature and pressure conditions, the corrosion resistance of the 2024-T3 aluminum alloy will be significantly impacted.[4][5][6] Therefore, to adapt to such special conditions, additional anti-corrosion treatment is required for it.In response to the 2024-T3 aluminum alloy, to further enhance its surface corrosion resistance, surface strengthening treatments are required.The commonly used surface technologies include cold spraying, micro-arc oxidation, laser cladding, Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), high-energy beam surface strengthening, chemical heat treatment, thermal spraying, and shot peening, etc.These technologies can significantly improve the surface hardness and wear resistance of the material, thereby extending its service life.7-12。However, these traditional methods often involve complex operations, long processing cycles, special environmental requirements, and high costs.To address this challenge, this study proposes an innovative surface strengthening technology-Vibration Impact and Electrical Spark Surface Strengthening (VIES).This technology integrates the advantages of vibration impact and electrical spark surface strengthening.Through precise process design and parameter adjustment, it significantly improves the surface quality of the workpiece.This technique not only enhances the material's hardness and wear resistance but also optimizes the surface microstructure to improve fatigue resistance and corrosion resistance [13][14][15].
Due to its simple operation, cost-effectiveness, and high efficiency, VIES technology demonstrates great potential and broad prospects for industrial application.This study aims to delve into the working principles and performance of Vibration Impact and Electrical Spark Surface Strengthening (VIES) technology, employing advanced continuous coupled thermo-stress analysis methods.This approach can intricately showcase how the stress field changes during the vibration impact and electrical spark surface strengthening processes.This analysis provides a solid theoretical foundation and data support for the future improvement of VIES technology and its implementation in real-world application scenarios.

Geometric Model
In the ABAQUS finite element simulation software, a three-dimensional geometric model was established based on the actual VIES process.This model includes a ball made of hard alloy and a 2024-T3 aluminum alloy plate.The ball is a regular sphere with a diameter of 15mm; the plate is square with a side length of 16mm and a height of 2mm.A schematic diagram of the geometric model is shown in Figure 1.

Mechanical Model
Many ultrasonic peening experiments use equipment that mainly operates around 20 kHz.During ultrasonic peening, the impact needle head applies static pressure (generally the weight of the ultrasonic peening equipment) and the output end ultrasonic impact amplitude to the material surface.The vibration frequency of the impact needle head changes with the actual contact conditions and is not fixed at its no-load output frequency of 20 kHz.Moreover, the amplitude of the impact needle is just theoretical data and cannot objectively reflect the actual impact conditions on the specimen.This is because collisions between the impact head and the workpiece create energy barriers that prevent the impact needle from moving forward or returning along its original path.The collision frequency of the impact needle with the specimen surface during the experiment is much lower than the output frequency of the amplitude rod.Considering that the experiment involves vibrational impact, not highfrequency ultrasonic impact, the vibration impact frequency in numerical simulations can be consistent with reality.When the static pressure and amplitude are determined, the dynamic impact force of the impact needle on the specimen is a constant amplitude and varies according to a sine function.Therefore, this means that the dynamic impact force applied by the needle on the specimen is predictable and regular, following a sinusoidal pattern.This predictability allows for a more controlled and uniform treatment of the material surface during ultrasonic peening.The sinusoidal nature of the force ensures that the impact is delivered in a consistent manner, which is crucial for achieving desired material properties like improved fatigue life or surface hardening.The variation in the impact force due to the actual contact conditions and collisions is an important consideration in the design and operation of ultrasonic peening equipment, as it affects the efficiency and effectiveness of the treatment.Understanding these dynamics is key to optimizing the process for specific materials and applications.

Figure 1. VIES Geometric Model
This is achieved by applying a periodically varying sinusoidal force at the reference point RP of the impact ball (see Figure 1) along the Z direction (i.e., the thickness direction of the plate), and this periodic sinusoidal force is applied through a Fourier function, which is as follows: In the formula: F0 represents the static pressure; n is the number of harmonics; an, bn are the amplitudes of the harmonics; ω is the angular frequency of the shock wave.For a sinusoidal wave, n =1, an =0, and the remaining parameters are determined based on different process parameters of ultrasonic impact.

Johnson-Cook Constitutive Relationship
During the vibratory impact process, the high-frequency action of the impact on the specimen causes deformation in the material's surface layer, resulting in a high strain rate.At this point, the yield strength and yield limit of the material undergoes significant changes under different strain rates.The Johnson-Cook model can address this issue, and the constitutive relationship expression of this model is as follows: in this model, the Johnson-Cook constant parameters A, B, C, m, and n are 369 MPa, 684 MPa, 0.0083, 1.7, and 0.73, respectively.

Force Boundary Conditions
During the process of vibration-impact compound electric spark impact, to reduce the energy loss caused by the vibration of the specimen, the bottom surface of the specimen is usually attached to a rigid mold.In this model experiment, to prevent specimen vibration, the displacement in all directions at the bottom of the specimen was restricted.During the impact process, the impact ball is treated as a rigid body, meaning the ball will not experience any loss during the impact, and friction between the specimen and the ball is inevitable.In conventional shot-peening models, many scholars have analyzed the role of friction in the impact process.In this model experiment, the friction coefficient between the impact needle and the specimen is set to 0.5.The collision contact between the impact needle and the treated surface adopts the contact pair algorithm.In the calculation process, each contact pair consists of a master surface and a slave surface, and the contact direction is always the normal direction of the master surface.Still, nodes on the slave surface will not penetrate the master surface.In this model experiment, the impact surface of the rigid impact ball is set as the master surface, and the upper surface of the specimen is set as the slave surface.The main factors affecting the performance of the 2024-T3 aluminum alloy are electric current, impact frequency, and impact height.These three factors were selected for an orthogonal experiment.Based on the actual experimental conditions and equipment parameter constraints, three levels were determined for each of electric current, impact frequency, and impact height.The three levels of electric current are 30A, 40A, and 50A; the three levels of impact frequency are 20Hz, 25Hz, and 40Hz; the three levels of impact height are 0.06mm, 0.008mm, and 0.01mm.Detailed experimental parameter information is shown in Table 1.

Analysis of the Impact of Impactor Height on Stress Field
In this study, a single impact was used to impact the aluminum alloy plate, and there was no movement in the non-thickness direction.Therefore, to observe the distribution of the surface stress field of the aluminum alloy plate, it is not necessary to separately analyze the X and Y directions.Both are rotationally symmetric forces, so the focus is on studying the residual stress S22 in the Y direction.The distance between point A and point B in Figure 2 is defined as the diameter of the longitudinal residual stress S22 interval, and the distance between point A and point C in Figure 3 is defined as the depth of the longitudinal residual stress layer.As shown in Figure 4(a), this is the longitudinal residual stress distribution graph on the surface of the aluminum plate from the starting point (0,3.2,2) to the endpoint (0,-3.2,2)at impact heights of 0.010mm, 0.006mm, and 0.008mm under a frequency of 20Hz/s and t=5.0s.It can be observed that under a constant frequency, the longitudinal residual compressive stress at the midpoint A (0,0,0) is -107MPa, -131MPa, and -146MPa, respectively.With an increase in the impact height, the compressive stress at point (0,0,2) shows a trend of initially increasing and then decreasing.According to the elastoplastic constitutive model, which considers both elastic and plastic behaviors, when the material is subjected to force, it first undergoes the elastic stage.During this stage, the stress and strain follow Hooke's Law, i.e., a linear relationship.If the material reaches its yield point, it enters the plastic stage.In the plastic stage, the relationship between stress and strain is no longer linear.After removing the externally applied yielding force, the elastic force is released, leaving behind plastic deformation.These processes result in non-uniform deformation within the material, thereby generating residual stress, which is the source of residual compressive stress.Theoretically, with an increase in impact height, the residual compressive stress at the midpoint should continuously increase because the impact leaves behind more plastic deformation.Combined with Table 2, it can be observed that when the impact height increases, the diameter of the residual compressive stress range synchronously increases.This implies that within a certain range, increasing the impact height indeed effectively increases the longitudinal residual compressive stress at the midpoint.However, the depth of the compressive stress layer decreases.More longitudinal residual compressive stress is stored within the longitudinal range rather than the depth range.When exceeding a certain range, the increase in impact height also means an increase in the contact area between the impact ball and the plate.This results in a larger diameter of the compressive stress range and an increase in the depth of the compressive stress layer.
More longitudinal residual compressive stress is distributed within a farther longitudinal range and deeper compressive stress layer depth.As a consequence, the compressive stress value at the midpoint decreases to some extent.
In summary, within a certain range and with constant impact frequency, as the impact height increases, the compressive stress at the impact site significantly increases, and the resulting compressive stress range continues to enlarge.However, the depth of the compressive stress layer decreases.Beyond a certain range, with an increase in impact height, the compressive stress at the impact site slightly decreases, but the decrease is small.Meanwhile, the compressive stress range and compressive stress layer depth continue to increase.

Analysis of the Impact of Impact Frequency on
Single Impact Stress Field As shown in Figure 5(a), this is the longitudinal residual stress distribution diagram of the aluminum alloy plate surface from the starting point (0,3.2,2) to the endpoint (0,-3.2,2)at t=5.0s, with an impact height of 0.010mm, under impact frequencies of 20Hz/s, 25Hz/s, and 40Hz/s.It can be observed that under a certain impact height, the longitudinal residual pressure stresses at the midpoint A (0,0,0) are -104MPa, -105MPa, and -107MPa, respectively.With the increase in impact frequency, the compressive stress at point (0,0,2) shows an increasing trend.The compressive stress values are consistent with the longitudinal stress distribution pattern, while the depth of the compressive stress layer remains constant.
In summary, with the increase of vibration impact frequency, the residual compressive pressure stress at the impact point will also increase, but the magnitude of the increase is small.Additionally, the generated compressive stress interval will widen, while the depth of the compressive stress layer remains unchanged.

Analysis of Orthogonal Experimental Results
The stress field of the combined vibration impact and electrical spark composite impact generally undergoes three steps: Impact Stage: Under the given impact conditions, the 2024-T3 aluminum alloy plate forms a molten pool region with significant contact stress under the combined action of mechanical impact and thermal stress.
Release Stage: After the impact process is complete, the contact failure between the small ball and the aluminum alloy plate occurs, and the contact stress between the small ball and the plate is released.
Cooling Stage: Cooling conditions are applied to the workpiece, allowing it to cool to room temperature.The release of thermal stress stabilizes the 2024-T3 aluminum alloy plate.
In the actual ultrasonic impact combined with the electrical spark process, a dense and well-bonded composite layer is eventually formed on the surface of the substrate.Due to the inconsistent material properties and good adhesion between the substrate and the composite layer, specimens with the composite layer exhibit higher residual stress on the surface and a deeper residual stress depth compared to specimens without the composite layer.In this study, due to the inability of ABAQUS to simulate the ultimately generated aluminumnitride ceramic composite layer with inconsistent material properties, there is a discrepancy between the simulated residual stress of the VIES specimens and the actual residual stress.However, the simulation process remains consistent with the actual ultrasonic impact combined with the electrical spark process, with the only modification being the substitution of vibration for a small vibration frequency.This adjustment provides a reference for the surface generated by vibration impact combined with an electrical spark after a cooling period.Considering the above, the residual stress characteristic point (0,0,2) on the surface after cooling for a period following vibration impact combined with an electrical spark is marked.According to the principle that the smaller the residual stress at this point, the higher the score, specific details are presented in Table 3.In summary, based on the analysis, the results of the orthogonal experimental design are as follows: the optimal process parameters are in Scheme 3; the intermediate process parameters are in Scheme 9; the least favorable process parameters are in Schemes 1 and 8.

Summary
This paper primarily investigates the influence of combined vibration impact and electric spark impact(VIES) on the stress field distribution of aluminum alloy plates.The experiment considers factors such as impact frequency, impact height, and electric current, and conducts orthogonal experiments.Through finite element simulation analysis, longitudinal residual stress distribution diagrams under different impact heights and frequencies are obtained.The experimental results indicate that within a certain range, with an increase in impact height, the residual stress value at the impact site significantly increases, but the depth of the compression stress layer decreases.Meanwhile, with an increase in impact frequency, the residual stress value at the impact site also increases, but the increase is relatively small, and the depth of the compression stress layer remains unchanged.In summary, both impact height and frequency have a certain impact on the stress field distribution of the aluminum alloy plate.Among the experimental results obtained, Scheme 3 represents the optimal process, while Schemes 1 and 8 represent the least favorable processes.

Figure 4 .
Figure 4. Vertical Stress Distribution at Different Impact Frequencies

Figure 5 .
Figure 5. Vertical Stress Distribution at Different Impact Frequencies

Table 1 .
Orthogonal test table of finite element analysis

Table 2 .
Distribution range of longitudinal residual compressive stress under different experimental schemes."

Table 3 .
Residual stress scores of vibration and shock combined EDM with different parameters.