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       Different from shot blasting, shot peening mainly uses projectiles to remove foreign matter from metal surfaces. Shot blasting processing is divided into air blowing type and impeller impact type. The latter is widely used in industrial large-scale shot blasting. In this study, a new control cage with concave or convex shape is proposed to improve the coverage and uniformity of impeller impact shot blasters. The effectiveness of the proposed control cage is verified using discrete element methods and experiments. In addition, the optimal designs in terms of mass flow rate, coverage and uniformity are confirmed. In addition, the distribution of marks on the surface is analyzed through experiments and simulations. In addition, by using the new impact model in the control cage, the projectiles are projected onto a wider surface area. In summary, we have confirmed that the concave control cell provides approximately 5% higher coverage than the traditional model and produces uniform projectile trails when using low mass flow rates.
       The use of pellets in metal surface treatment and processing has been actively studied since the 1950s. In the steel industry, shot blasting is actively used to remove oxide scale and other foreign substances on the surface of stainless steel metal to prevent deterioration of the surface quality. According to the material and shot projection method, shot blasting can be divided into blowing and impeller. Shot blasting and shot peening are both metal surface treatment methods. But they are classified according to their functional purpose. Shot blasting is used to throw granules of metal materials onto the surface to remove foreign substances or improve the surface roughness by removing the sharp edges of the product. In contrast, shot peening is a processing method that creates residual stress on the surface by ejecting shot particles at high speed and improves the surface strength and fatigue life.
       The mechanical shot blasting machine consists of a distributor, a control cage and knives. First, when the pellets are fed into the rotating distributor, they are ejected from the blast machine through the holes in the control cage. Then, the shot blasting is performed when the high-speed rotating blades collide with the pellets and eject them onto the surface. After shot blasting, coverage and uniformity are measured, which indicate the effectiveness of the treatment. Coverage can be calculated as the ratio of the sum of the areas of the bullet scars to the total surface area. In addition, uniformity is measured, indicating how evenly the pellets are distributed over the surface. Coverage and uniformity are improving, but further improvements are needed.
       The calculation method for shot peening usually focuses on residual stress, mainly using the finite element method (FEM). Tange and Okada7 repeated such simulations using randomly generated projectiles until 100% coverage was achieved. They established a relationship between coverage and fatigue strength through experiments and FEM methods. However, if all bullet scars are assumed to have the same shape, bullet scars that hit the same location are excluded from the coverage calculation. Meguid et al.8 analyzed residual stresses as a function of projectile shape, impact velocity, and work hardening of the base material. As in previous studies, repeated shocks were excluded. Gangaraj et al.9 predicted the overall coverage using an axisymmetric model assuming a uniform projectile size distribution and a uniform bullet scar. Meguid et al.10 and Schwarzer et al.11 measured the variations in residual compressive stress with projectile size and velocity and measured the coating by regularly repeating rather unrealistic hexagonal bullet marks. Bagherifard et al.12 used the same assumptions but took pellet rebound into account in their coating calculations. A combined CFD/FEM analysis by Nguyen et al.13 showed that shot peening significantly affected the coating coverage by pellet size and peening angle. Kirk and Abyaneh14 performed numerical calculations of the coating by randomly placing pellets. Taro et al.15 successfully applied this method assuming uniform pellet size. Marini et al.16 used FEM and radiation penetration techniques to study the effects of pellet size and velocity on surface roughness and residual stress during micro shot peening.
       Since the finite element method cannot use a large number of particle contacts, most studies use only a small number of particles for analysis. When the finite element method is used to calculate the contacts between multiple particles, each particle is discretized using multiple meshes and analyzed. Therefore, using a large number of particle contacts in the finite element method is very time consuming. However, in shot peening or shot blasting, the coating and uniformity are more important than the residual stress, so many shots should be used. Therefore, there are limitations in using only the finite element method to analyze shot peening and shot peening.
       To address the limitations of the above studies, many recent studies have been conducted using the discrete element method (DEM). Bhuvaragan et al.17 accurately predicted the residual stress and plastic deformation using the finite element method and DEM. Thus, by applying the contact force calculated by DEM to the FEM model, the residual stress can be accurately measured. Similarly, Hong et al.18 used a combination of the finite element method and DEM to conduct an in-depth analysis of the shot peening impact force that recovers after a single impact on a metal surface. They identified the quality control parameters that should be most carefully controlled in shot peening and which have the greatest influence on the residual stress. Murugaratnam et al.19 implemented a new algorithm to significantly adjust the recovery coefficient for repeated shot peening at the same point in DEM. In addition, the combined effects of the initial velocity, mass flow rate, and clamping angle on the compressive residual stress are also analyzed. Hou et al.20 used 3D DEM to analyze the dynamic behavior of shot particles inside a shot blasting machine. They observed that the faster the impeller rotates, the higher the rate of change of pellet velocity. Ahmad et al.21 accurately calculated the induced compressive stress by performing a DEM/FEM coupling analysis on the Johnson-Cook material model. In addition, Marini et al.22 showed that the residual stress field measured at the V-notch edge was in good agreement with the finite element method thermal field corrected using experimental data. Furthermore, Choi et al.23 proposed a new impeller blade design using DEM and conducted experiments to improve the coating and uniformity.
       Similar to the analysis of the collision process between the metal surface and shot particles in shot blasting, Dong et al.24 proposed a numerical model based on the SPH method to simulate and model the droplet impact process on the elastic beam, and adopted the continuous surface force (CSF) method to simulate the surface tension effects caused by the droplet impact. The droplet impact is mainly observed in inkjet printing, deicing, pesticide spraying, etc. Van Dam and Le Clercq25 experimentally studied the impact of inkjet droplets on a solid substrate, measured the shape of the impact interface, and experimentally measured the volume of fine bubbles of water droplets in the early stages of impact, and compared the results with the equations . Zhang et al.26 used the SPH method to conduct a shot peening study by simulating the impact of particles on a metal surface covered with a rust layer and analyzed the effects of the impact behavior of different particle shapes on the deformation and damage of the rust layer on the substrate under different initial conditions and impact angles.
       However, despite a significant number of related studies, there are few reports on the control cage, one of the main components of the impeller shot blasting machine, in both experiments and simulations. In particular, the design of control cages has not changed since the 1960s, which means that the relationship between the design of control cages and coverage or uniformity has not been further studied. Therefore, this study shows that the newly proposed design of control cage can improve coverage and uniformity through DEM simulations that consider multiple particles and interparticle collisions. Moreover, the experimental results are consistent with the simulation results.
       This study analyzes the impeller shot blasting process used by steel companies to remove scale during iron making. The shot blaster used in this study consists of a distributor that produces a certain number of pellets, a control cage that controls the direction of the pellets, and a blade that accelerates the pellets。