Multiscale Simulation of Shot-Peening-Assisted Low-Pressure Cold Spraying Based on Al-Zn-Al₂O₃ Coatings

Abstract

Low-pressure cold spraying has gained much significance for its good economy in recent years. However, compared with high-pressure cold spraying, the unsatisfactory performance of coatings prepared by this method is a key problem restricting its further development. To improve the properties of the coating without incorporating severe conditions, the paper proposed an original shot-peening-assisted low-pressure cold-spraying method (i.e., SP-LPCS). By proceeding with cold spraying and shot peening alternately, SP-LPCS was proved to enhance the mechanical property of the coating effectively. Mixed particles of Zn, Al, and Al₂O₃ were adopted as the coating powder. Effects of shot-peening pressure, flow rate, and shot size on the micromorphology and the microhardness variance were studied. Results shows that the thickness of the plastic deformation layer stabilizes as the impact time increases to 6. The microscopic simulation of the deformation shows that according to the different metal characteristics of the powder, brittle grains fracture while plastic grains go through deformation and refinement. Meanwhile, the porosity decreases greatly after the impacts, resulting in a higher denseness of the coating. Several factors mutually contribute to the performance improvement of the coating. The microhardness of the material was increased after SP-LPCS, and obvious strengthening belts were observed, with the highest microhardness being 90.93Hv.

1. Introduction

Cold spraying is a kind of technology with little thermal influence compared with thermal spraying. The deposition mechanism of cold spraying is the self-locking phenomenon of powder depositing at a supersonic speed [1,2,3]. Therefore, for heat-sensitive materials, cold-spraying technology is a favorable processing method and has wide application prospects. At present, cold spraying has been successfully applied to the processing of metals such as TC4, copper, aluminum alloy, etc. [4,5,6,7]. Moreover, substrates compatible with this method are not subject to metals, but to other materials such as carbon-fiber-reinforced polymer [8].

According to the gas pressure, cold spraying can be divided into high-pressure cold spraying and low-pressure cold spraying. Cold spraying can be divided into two categories depending on the gas pressure applied, with high-pressure cold spraying indicating a pressure over 2 MPa [9,10], and low-pressure cold spraying operating around 0.6 MPa [11]. In aggregate, higher gas pressure induces higher particle velocity, and to further improve the velocity before deposition, the powder is accelerated in nitrogen or helium in high-pressure cold spraying [12,13]. Previous research showed that the coating prepared with helium gas had fewer pores and better mechanical properties such as bonding strength compared with low-pressure cold spray [14]. However, these demanding spraying conditions have led to a higher cost and limited its popularization and application in small and medium-sized enterprises. Meanwhile, cold spraying, due to its lower cost and flexible operation, has become a more attractive alternative in recent years [15,16]. Due to the low particle velocity of low-pressure cold spraying, it enables the powder to mainly consist of soft materials such as aluminum, zinc, copper, etc. [17]; however, heated carrier gas is needed in the spraying process in order to achieve better deposition efficiency. Additionally, many scholars have made improvements based on traditional cold-spraying technology to improve the quality of cold-sprayed coatings. New technologies such as laser-assisted cold spraying [18], vacuum cold spraying [19], in situ microforging-assisted cold spray [20], electrostatic-assisted cold spraying [21], and pulse-assisted cold spraying [22] have been put forward, which greatly promote the development of cold-spraying technology. Properties of the coatings prepared by laser-assisted low-pressure cold spraying were evaluated by Kulmala et al. [23], and they found that the new method cannot eliminate the pores; however, the coating was denser compared with those processed by the traditional method. It is because although laser-assisted cold spraying can reduce the deposition conditions of particles, it may lead to the oxidation of particles, particularly in prolonged sprays [24]. Therefore, laser-assisted cold spraying was best sprayed under inert gases to avoid metal oxidation. Vacuum cold spraying adds a vacuum chamber based on traditional cold spraying, which can successfully prepare Al₂O₃ and TiO₂ coatings [25,26]. Takana et al. [21] proposed a method of electrostatic-force-assisted cold spraying. The results showed that particles can be further accelerated under the action of electrostatic force to obtain higher particle velocity. However, this method is mainly used for submicron spraying, whose coating property is easily affected by shock waves. Luo et al. [27] proposed a new method of cold spraying, which is assisted by in situ cold forging. This method mixes shot-peening particles in the traditional cold-spraying powder so that the prepared coating can be compacted. By mixing the powder and the peening shots, the coating structure can be divided into two regions, and the compacted area processed by shot-peening particles is denser than the simply cold-sprayed area [28]. Based on this idea, Fan et al. [29] used powder with a wide particle size for depositing and found that the mechanical properties of the deposits are comparable to those produced under high processing due to the dense effect caused by the larger particles. Lu et al. [30] prepared Al coatings on LA43M by cold spraying and shot peening. The results show that compared with the conventional cold-sprayed Al coating, the porosity of Al coating after shot peening is reduced from 12.4% to 0.2%. Daroonparvar et al. [31] showed that the strengthening phenomenon similar to shot peening also can be observed in composite coatings, such as Ta/Ti/Al coatings. This is because the further impact particles with higher hardness and kinetic energy can create dense and refined soft grains [32].

In order to improve the effect of low-pressure cold spraying and improve the performance of materials from the point of view of increasing powder deposition efficiency and reducing coating porosity, a new method of shot-peening-assisted low-pressure cold spraying (SP-LPCS) method is proposed in this paper. In addition, a multiscale simulation model of SP-LPCS is established to understand the strengthening mechanism of the coating brought by the peening shots. In this multiscale simulation, the change in residual stress of the coating and the bearing mechanism of different particles are well-studied.

2. Experimental and Simulation Approaches

2.1. Material and Experiments

2.1.1. Coating Material

The cold-spray powder used in the experiment was mainly composed of aluminum (Al), zinc (Zn), and a small amount of alumina (Al₂O₃) particles (Beijing Science and Technology Materials Science and Technology Co., Ltd., Beijing, China). Pure aluminum and zinc were mixed at a volume ratio of 3:1, and 10% Al₂O₃ particles were added to the powder because macromolecular particles could be used to improve the roughness of the former layer of coating, so that the metal powder became easier to deposit and the deposition efficiency of the spray powder could be improved. Powder morphologies of different magnifications observed by the scanning electron microscope (SEM) are shown in Figure 1. The sizes of Al powder range from 5 to 25 μm with an average value of 20 μm, and the sizes of Zn, which are smaller but smoother as shown in Figure 1b, range from 4 to 15 μm with a mean value of 10 μm. It can be seen from Figure 1a that Al₂O₃ particles are distinctively larger, and irregularly shaped with sharp edges.

2.1.2. Experimental Procedure of SP-LPCS

Shot-peening-assisted low-pressure cold spraying, or SP-LPCS, is a method to prepare a layer of coating on the surface of the substrate by low-pressure cold spraying, and then to further treat the new coating by employing the shot-peening process to enhance the mechanical properties of the coating. The flow chart of the experiment is shown in Figure 2. In this experiment, after the spray gun took a certain path to complete the coating, the same path was used for 100% coverage shot peening to compress the coating. Low-pressure cold spraying and shot peening are carried out alternately during the whole process. The thickness of each coating formed by low-pressure cold spraying is controlled between 0.2–0.4 mm to ensure the strengthening effect of layered shot peening.

2.1.3. SP-LPCS Method

To achieve the expected effect of shot-peening strengthening, the process parameters of low-pressure cold-spraying equipment (Beijing Technical Material Science and Technology Co., Ltd., Beijing, China) were set to a constant value, as shown in

Table 1. Detail parameters of cold spray used in the experiment.

Carried Gas	Gas Pressure	Gas Temperature	Substrate	Spray Length	Powder
Air	0.6 Mpa	400 °C	20 steel	8 mm	Al, Zn

. The gas temperature was set to 400 °C to improve the powder deposition efficiency and avoid the oxidization of the metal particles at the same time.

In the shot-peening part of the experiment, the effect of peening shot size on the coating was studied by using three kinds of ceramic shots: AZB150, AZB150, and B40. Compared with other materials, ceramic shots have higher hardness and thus are hard to break during a collision; therefore, no pollution of broken ceramic would be blended into the coating.

Table 2. The experimental scheme of shot peening.

Case	Pressure/MPa	Flow Rate/(kg·min−1)	Shot Size/mm	Shot Velocity/(m·s−1)
a	0.4	2.5	0.21 (AZB210)	56.28
b	0.5	2.5	0.21	62.19
c	0.6	2.5	0.21	67.86
d	0.5	1.5	0.21	64.13
e	0.5	2.5	0.21	62.19
f	0.5	3.5	0.21	60.82
g	0.5	2.5	0.15 (AZB150)	62.39
h	0.5	2.5	0.21	62.19
i	0.5	2.5	0.4 (B40)	61.57

summarizes the process parameters of the device (China Kunshan Jiaxing Precision Co., Ltd., Suzhou, China) in the shot-peening process. To study the effects of different pressure, flow rate, and shot size on the properties of the coating, three groups of control experiments, as shown in Table 2, were designed.

However, in the process of computer simulation, it is hard to directly set an actual shot-peening pressure or flow rate for the emulation, but the shot velocity can be used as an accurate input to simulate the process. According to previous studies, shot velocity is closely related to pressure, flow, and shot size; thus, its value can be calculated from Equation (1) [33], which is also a widely used method to measure the influence of gas pressure and flow rate in the shot-peening process.

$$v=\frac{16.350p}{1.530q_m+p}+\frac{29.500p}{0.598d+p}+4830p$$

(1)

where v is the average velocity of particles; m/s. p is the air pressure; 10⁵ Pa. q_m is the flow rate of shots; kg/min; d is the diameter of the shot, mm.

2.2. Numerical Simulation

2.2.1. Approach of Multiscale Simulation

The multiscale simulation framework of SP-LPCS is shown in Figure 3. First of all, the mechanical properties of low-pressure cold-sprayed coating were obtained by compression test, which obtained basic data for macroscopic shot-peening simulation. Secondly, after the shot-peening-assisted macroscopic cold-spraying model was established, a small area of the plate was extracted, and the effect of shot-peening parameters on the coating was studied. Then, part of the coating with a certain strain was selected, and the corresponding microscopic model of the coating was established by simulating the composition and properties of the spray powder.

Finally, the different strains obtained from the macroscopic model were assigned to microscopic models to further study differences in deformation and stress between different particles in the coating components under the stress state, and to explore the mechanism of SP-LPCS on the microlevel.

2.2.2. Material Models

The compression experiment was carried out in order to obtain the properties of the mixed coating material for shot-peening simulation. The substrate used for cold spraying is 20 steel with a thickness of 2.5 mm. The surface of the substrate was sandblasted with brown fused alumina for 5 min to roughen the surface so that the powder can be better deposited on the substrate and interlocked with each other. Then, the spray material was cut into two specimens with a size of 5 × 5 × 10 mm³ by wire electrical discharge machining (WEDM) (Figure 4a). According to the requirements of GB/T7314-2017, the mechanical properties of the sprayed materials were obtained by compression test at room temperature.

Figure 5 presents the stress–strain curve of the specimen under the compression test. When the strain is 0.03, the specimen leaves the elastic stage and enters the plastic stage. When the true strain is close to 0.15–0.17, the specimen breaks, showing a drastic turning point at the peak stress. Through the compression test, the material parameters are summarized as shown in

Table 3. The mechanical property parameters of Al-Zn coating.

Elastic Modulus/MPa	Poisson’s Ratio	Density/g·cm−3	Yield Strength/MPa	Compressive Strength/MPa
5923.2	0.24	4.1	190.87	238.39

.

Unlike the macroscopic simulation, the three kinds of particles that made up the coating were independent of each other in the mesoscopic simulation to obtain their separate contribution to the strengthening effect of shot peening. Therefore, in the microscopic simulation, it is necessary to set the parameters for the particles, including Al₂O₃, Al, and Zn.

Table 4. The material parameters of microscopic simulation.

Material Parameters (Units)	Al	Zn	Al2O3
Density, ρ (kg/m3)	2700	7140	3680
Thermal conductivity, λ (W/m·K)	220	116	/
Specific heat, c (J/kg·K)	920	377	/
Elastic modulus, E (MPa)	69,000	96,500	370,000
Poisson’s ratio, υ	0.27	0.22	0.22
Bulk speed of sound, C0 (m/s)	5330	0.22	/
Slope of Us versus Up, s	1.34	1.56	/
Grüneisen coefficient, Γ0	1.97	1.92	/
Yield stress, A (MPa)	148	82.51	930
Hardening constant, B (MPa)	346	288.34	310
Hardening exponent, n	0.183	0.1786	0.6
Strain rate constant, C	0.001	0.0202	/
Thermal softening exponent, m	0.86	0.843	/
Melting temperature, Tm (°C)	620	419.83	2054
Reference temperature, T0 (°C)	25	25	/

summarizes the material parameters of the microscopic simulation.

2.2.3. FE model for Macrosimulation

Figure 6 shows the finite element model of the multishot-peening process. In the simulation, a cube of coating with a size of 1.5 × 1.5 × 0.2 mm³ was used to reveal the effect of shot peening (see Figure 6a). Six degrees of freedom of the bottom face were fixed in the simulation, but the other faces were not constrained. In order to ensure the calculation accuracy, the meshing of the middle area of 0.8 × 0.8 mm² was densified, and the numerical simulation was carried out by C3D8R.

Throughout the simulation, the shots were set to be rigid bodies. The tangential friction coefficient was set to 0.2, and the normal behavior was set to hard contact. Meanwhile, the thickness of the plastic layer, which is the key target of shot-peening evaluation, was obtained after 6 shots of impact at the same spot in the simulation. The thickness of the plastic layer was extracted from the central point of the plate along the thickness, as shown in Figure 6b. In addition, in order to obtain the residual stress of the coating after shot peening, the simulation results of the coating model needed to be introduced into the static implicit analysis for further calculation.

2.2.4. FE Model for Microsimulation

Although the macroscopic simulation can directly illustrate the deformation of the coating after shot peening, it is necessary to observe the stress state of the coating particles after deformation from the microlevel in order to reveal the mechanism of shot-peening strengthening on the coating. Therefore, the use of a multiscale model is a very effective analysis method.

In the microscopic model, it is necessary to apply periodic boundary conditions to the microscopic model to obtain the constraints between the microscopic model and the surrounding materials. Therefore, the microscopic model should satisfy the following equation:

$${\mu }_i={\overline{\epsilon }}_{ik}{x}_k+{\mu }_i^{*}$$

(2)

where ${\mu }_i$ is the periodic displacement field; ${\overline{\epsilon }}_{ik}$ is the average strain of the microscopic model; x_k is the coordinate of any point in the microscopic model; and ${\mu }_i^{*}$ is the periodic displacement correction.

On a pair of boundary surfaces, the periodic displacement field can be written as:

$$\begin{array}{l}{u}_i^{j+}={\overline{\epsilon }}_{ik}{x}_k^{j+}+u_i^{*}\hfill \\ u_i^{j-}={\overline{\epsilon }}_{ik}{x}_k^{j-}+u_i^{*}\hfill \end{array}$$

(3)

where the superscript j+ and j- denote the positive and negative directions along the X_j axis, respectively, because $u_i^{*}$ is the same on the opposite side of the parallel phase. Therefore, Equation (3) can be rewritten as follows:

$$u_i^{j+}-u_i^{j-}={\overline{\epsilon }}_{ik}(x_k^{j+}-x_k^{j-})={\overline{\epsilon }}_{ik}\Delta x_k^j$$

(4)

When the ${\overline{\epsilon }}_{ik}$ is given, the displacement difference can be calculated according to Equation (4).

Figure 7 shows the microscopic model used in the simulation. As shown in Figure 7, a microscopic model of 0.1 × 0.1 × 0.1 mm³ was established to describe the interaction between particles in the coating under the shot-peening process. In order to be as close to the real coating condition as possible, the microscopic model not only set up three kinds of coating metals: Al, Zn, and Al₂O₃, but also made gaps between different particles to represent the residual pores generated during the process of cold spraying. The material parameters used in the simulation are summarized in

Table 5. The setting of each phase in mesoscopic model.

Material	Volume Fraction	Size Distribution	Number of Mesh
Al	0.612463	/	313,581
Zn	0.250098	0.008–0.012	128,050
Al2O3	0.1076	0.04–0.06	55,091
Air/Pores	0.0298398	0.005–0.01	15,278

.

3. Effect of Shot-Peening Process on Plastic Strain

3.1. Shot-Peening Impact Times

Figure 8 presents the effect of shot-peening times on the equivalent plastic strain (PEEQ) distribution along the thickness of the coating. No matter how the condition is, the PEEQ distribution curves on the coating hold the same trend, showing the characteristics of rising and decreasing, and slowly sliding to zero along the thickness.

Note that for the shot-peening process under each working condition, it can be seen from Figure 8 that with the increase in impact times, the peak of each curve—that is, the maximum equivalent plastic strain of the coating—increases accordingly. For the several initial impacts, the thickness of the plastic deformation layer increases with the increase in shot peening times, but after 5 or 6 impacts, the deformation layer remains stable, indicating that after repeated impacts by the peening shot at the same position, the surface of the material shows the strain-strengthening effect. It is believed that the strengthening effect reaches saturation after the sixth shot impact, and increasing the number of peening shots over this will not significantly improve the properties of the material.

3.2. Shot-Peening Parameters

The effects of different shot-peening parameters on the maximum plastic strain and plastic layer thickness were compared. An impact time of 6 was set in the simulation to ensure that the saturated shot-peening effect was obtained. Figure 9a–c shows the effect of pressure on PEEQ. With the increase in shot-peening pressure, the thickness of the plastic deformation layer is 0.162 mm, 0.166 mm, and 0.172 mm, respectively. When the pressure increases by 50%, the thickness of the plastic layer only increases by 6.17%, indicating that increasing the shot-peening pressure has a certain effect on improving the thickness of the plasticized layer of the coating, but not significantly.

Figure 9d–f illustrates the effect of flow rate on PEEQ. With the increase in flow rate, the thickness of the plastic layer is 0.168 mm, 0.166 mm, and 0.165 mm, respectively. This means that in actual industrial production, increasing the flow rate does not have much effect on strengthening the coating.

In addition, the effect of shot-peening size on the coating is also studied, as shown in Figure 9g–i. With the increase in shot-peening size, the thickness of the plastic layer is 0.134 mm, 0.166 mm, and 0.195 mm, respectively.

Compared with shot-peening pressure and flow rate, an increase in shot diameter has a significant effect on the strengthening effect of the coating. This is because the diameter of the particle directly affects its kinetic energy after acceleration. According to formula 2, the kinetic energy of three kinds of shots put is 0.849 mJ, 2.315 mJ, and 15.683 mJ respectively. When the diameter increases from 0.15 mm to 0.4 mm, the kinetic energy increases by 18.47 times. It can be seen that the obvious difference in kinetic energy is the key to the huge difference in Figure 9g–i. Therefore, from the aspect of maximum plastic strain and plastic layer thickness, shot-peening size has the most significant strengthening effect on the coating compared with pressure and flow rate.

4. Microscopic Property Changes of Coatings after SP-LPCS

4.1. Strain and von Mises Stress Distribution

In order to thoroughly understand the mechanism of the shot-peening-assisted strengthening effect and simulate the deformation process of the coating, three strains (0.003%, 0.03%, 0.3%) were applied to the microscopic model for analysis. As shown in Figure 10, as the strain of the coating increases, the strain of each particle in the interior increases.

At the initial stage of compression, the strain begins to appear sporadically in some parts of the model. According to the comparison with the original model (Figure 10a), zinc is the first particle to produce plastic strain in the three spray powder components due to the low yield strength of zinc. With the application of stronger deformation, a large area of higher strain begins to occur in the middle of the top surface, which indicates that the aluminum particles also gradually experience plastic deformation after Zn.

In the simulation of this microscopic model, the crushing equation of the material is not taken into account, so it can be observed that in the later stage of compression, even if the pores almost disappear and the volume of the whole simulation unit is greatly compressed, the strain of Al₂O₃ during the whole process is almost kept at zero. This is due to the hard and brittle characteristics of Al₂O₃, which is markedly different from Zn; thus, almost no plastic deformation occurs in Al₂O₃ particles. In addition, it can be seen from Figure 10b,c that plastic deformation occurs easily along the path of zinc and the pores, forming obvious plastic bands. When the strain is 0.3%, the porosity of the coating decreases substantially.

Figure 11 shows the von Mises stress distribution of the microscopic model under different loads. As shown in Figure 11, the overall von Mises stress increases gradually with the increase in strain. Under different strains, Al₂O₃ bears the maximum stress, followed by Al, and then the stress of Zn is the smallest. Such difference in stress distribution becomes more and more obvious with the increase in displacement. When the strain increases from 0 to 0.003, the stress of Al₂O₃ increases sharply, from 1043 to 2900. This extremely high stress also indicates that such particles may be easy to fracture under shot peening. However, when the strain increases to 0.03, the stress of the Al particles only increases from 67 to 203, which shows no great growth. According to Figure 5, the stress of the whole coating is about 200 when the real strain of the material reaches 0.03, which is similar to that of Al particles. Moreover, the main matrix materials of the coating are Al and Zn, which shows that the internal stress of the mixed coating mainly comes from Al and Zn particles, indicating that Al and Zn are the main stress-bearing materials.

4.2. Microstructure and Microhardness

In the simulation, the strain distribution of the specimen under SP-LPCS has been obtained, which is considered to be the fundamental reason for the strengthening effect on the coating. Now the coating is analyzed experimentally. Figure 12 shows the coating morphology corresponding to different working conditions under 200 times’ magnification.

It can be seen that compared with the microstructure of the coating without shot peening (Figure 12a), the morphology of the coating under SP-LPCS has changed significantly, which is embodied in the minification of the particles and the densification of the bonds between the microstructure. This is because of the cold forging of shot peening, which enhances the extrusion between the particles, and leads to the deformation, fragmentation, and refinement of metal grains. However, this phenomenon of grain refinement does not appear uniformly across the whole coating but focuses on several strengthening belts horizontally distributed in the coating (marked in red dotted lines in Figure 12b–i. The refined area is particularly noticeable in the strengthening belt, which is mainly due to the limited immersion depth of shot peening and the difficulty in accurate operation in reality. At the same time, it can be seen that the refined particles are limited to Al and Zn, while the black Al₂O₃ has almost no deformation, which is consistent with the result from the previous microsimulation.

From the comparison between morphologies under different working conditions, generally speaking, the greater the pressure is, the more obvious the grain refinement phenomenon is, and the wider the corresponding strengthening belt is. A similar conclusion can be made by reducing the flow rate or increasing the shot diameter, which is consistent with the macrosimulation results shown in Figure 9.

The application of SP-LPCS has a great influence on the macroscopic and microscopic properties of the coating. The strain distribution of the coating and the grain size of the microstructure changed significantly under the impact of peening shots, and further experiments are needed to explore to what degree can these changes improve the actual performance of the coating.

To distinguish between strengthening belts and other regions in Figure 12, the hardness tests for each specimen were divided into two regions, as shown in Figure 13. A digital microhardness tester (300MHVD-30AP) with a loading force of 200 g and a loading time of 15 s was adopted to study the microhardness increase in the coating material. It can be observed that the microhardness of the coating after shot peening is higher than the original microhardness (marked in black dashed lines). Meanwhile, the microhardness of the same specimen in the strengthening belt is higher than that in other areas. This shows that shot peening can increase the internal hardness of the coating, and this strengthening effect becomes more obvious in the grain refinement area. The overall hardness of each coating is represented by the average value of the strengthening belt and other areas. When the pressure increases from 0.4 MPa to 0.6 MPa, the average hardness of the corresponding coating is 67.47 Hv, 72.72 Hv, and 84.12 Hv. In other words, compared with the coating without the shot-peening process, the microhardness increases by 27.3%, 37.2%, and 58.71%, which is up to more than one-half of the original hardness. With the increase in flow rate, the average hardness of the coating is 78.57 Hv, 72.72 Hv, and 66.47 Hv, indicating an increased ratio of 48.24%, 37.2%, and 25.41%, respectively. With the increase in shot size, the average hardness of the coating is 68.95 Hv, 72.17 Hv, and 90.93 Hv, and the corresponding improvement ratio is 30.09%, 37.2%, and 71.57%, respectively.

From the above results, it can be observed that the use of larger shots can improve the microhardness of the coating greatly, and the change in pressure and flow rate has a relatively lower influence on the property enhancement.

Meanwhile, observing the error between the data points taken from the test (error bars in Figure 13), it can be found that when the shot-peening speed exceeds a certain value, it will increase the difference between the data specimens, which reflects the expansion of the nonuniformity of the internal microstructure of the coating.

5. Conclusions

In this paper, the effect of shot peening on the strengthening effect of the coating processed by low-pressure cold spraying was studied, and the mechanism of stress distribution and deformation between different particles in the coating was analyzed by establishing a macro–micro simulation model. The main conclusions are as follows:

(1) The surface of the coating has obvious plastic deformation after shot peening, and the thickness of the plastic layer becomes stable with the increase in shot-peening times and reaches saturation. The shot-peening parameter that most greatly influences the coating property is the shot diameter, followed by pressure and flow rate.

(2) The microscopic model can reflect the stress transfer and deformation mechanism of particles in the coating after shot peening, which overcomes the deficiency of no visualization of particle changes in macroscopic simulation. The stress of Al is the highest under shot peening, which means that Al is easy to fracture under shot peening, which is consistent with the observation by scanning electron microscope. In addition, the internal pores of the coating are reduced with the increase in strain.

(3) The microstructure of Al–Zn alloy coating prepared by SP-LPCS shows the phenomenon of grain deformation, refinement, and fragmentation, as well as the decrease in porosity, which is especially obvious in some specific areas (strengthening belt), attributing to the improvement of the coating’s properties. Compared with increasing the shot-peening flow rate, increasing the shot size and shot-peening pressure are more effective to improve the microhardness of the material.